U.S. patent application number 14/984146 was filed with the patent office on 2016-04-28 for head clamp for imaging and neurosurgery.
This patent application is currently assigned to RENISHAW (IRELAND) LIMITED. The applicant listed for this patent is RENISHAW (IRELAND) LIMITED. Invention is credited to Hugo George Derrick, Matthew David Frederick Stratton.
Application Number | 20160113719 14/984146 |
Document ID | / |
Family ID | 40862786 |
Filed Date | 2016-04-28 |
United States Patent
Application |
20160113719 |
Kind Code |
A1 |
Stratton; Matthew David Frederick ;
et al. |
April 28, 2016 |
HEAD CLAMP FOR IMAGING AND NEUROSURGERY
Abstract
A head clamp for neurosurgery is described that includes a
member for at least partially encircling the head of a subject. The
first end and second end of the head clamp include first and second
skull attachment portions for attaching the member to the head of a
subject. A position setter, such as an indexing mechanism, is also
provided that allows the member to be moved or indexed between at
least two repeatable relative positions.
Inventors: |
Stratton; Matthew David
Frederick; (Stroud, GB) ; Derrick; Hugo George;
(Stroud, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
RENISHAW (IRELAND) LIMITED |
Swords |
|
IE |
|
|
Assignee: |
RENISHAW (IRELAND) LIMITED
Swords
IE
|
Family ID: |
40862786 |
Appl. No.: |
14/984146 |
Filed: |
December 30, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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13320953 |
Nov 17, 2011 |
9254177 |
|
|
PCT/GB2010/001003 |
May 20, 2010 |
|
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|
14984146 |
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Current U.S.
Class: |
128/845 |
Current CPC
Class: |
A61B 17/60 20130101;
A61B 2090/067 20160201; A61B 17/64 20130101; A61F 5/01 20130101;
A61F 5/3707 20130101; A61B 17/58 20130101; A61B 17/56 20130101;
A61F 5/37 20130101; G06F 3/00 20130101; A61B 17/6458 20130101; G06F
3/01 20130101; A61B 90/14 20160201; A61F 5/00 20130101; A61B
2090/374 20160201; A61B 17/6433 20130101; A61B 2090/3983 20160201;
G06F 3/016 20130101; G01D 13/24 20130101; G01D 13/22 20130101; A61B
2090/3954 20160201 |
International
Class: |
A61G 15/00 20060101
A61G015/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 21, 2009 |
GB |
0908787.5 |
Claims
1. A method of performing neurosurgery, comprising the steps of;
(i) attaching a head clamp comprising a moveable member to the head
of a subject, (ii) moving the member into a first position and
acquiring at least one image of the head, (iii) moving the member
into a second position and performing a neurosurgical procedure,
wherein the relative difference in position between the first
position and the second position of the member is known.
2. A method according to claim 1, wherein the head clamp comprises
an indexable member and steps (ii) and (iii) comprise indexing the
member into the first position and second position
respectively.
3. A method according to claim 1, wherein at least one datum
feature is provided on the head clamp that allows images acquired
during step (ii) to be related to neurosurgical procedures
conducted during step (iii).
4. A method according to claim 1, wherein step (i) comprises
attaching the head clamp to the head with a predetermined
engagement force such that deformations of the member are
known.
5. A method according to claim 1, wherein step (i) comprises
applying a clamping force substantially along a clamping axis,
wherein the clamping axis is aligned, at least approximately, with
the cantho-meatal plane of the subject's head.
6. A method according to claim 1, wherein the member comprises a
c-shaped member, wherein the first position of step (ii) is a
position in which the c-shaped member substantially extends around
the top of the head thereby allowing imaging of the head and the
second position of step (iii) is a position in which the c-shaped
member substantially extends around a side of the head thereby
providing access the head to allow the neurosurgical procedure to
be performed.
Description
[0001] This is a Continuation of U.S. application Ser. No.
13/320,953 filed Nov. 17, 2011, which in turn is a National Phase
of International Application No. PCT/GB2010/001003 filed May 20,
2010, which claims the benefit of British Application No. 0908787.5
filed on May 21, 2009. The disclosures of the prior applications
are hereby incorporated by reference herein in their
entireties.
[0002] The present invention relates to a clamp for attachment to
the head of a subject, and in particular to an indexable head clamp
suitable for use in both magnetic resonance imaging (MRI) of the
head and stereotactic neurosurgery procedures.
[0003] It is known to provide various types of head clamp to grip
and stabilise the head of a subject during neurosurgical
procedures. One known type of head clamp comprises a generally
c-shaped frame member for partially encircling the head of a
subject with a pin or pins provided at each end of the frame member
to engage the skull. Such head clamps are placed in the desired
position relative to the head before being firmly attached in
place. Once attached, the head clamp can be secured to an operating
table. Various examples of such head clamps are described in U.S.
Pat. No. 4,169,478, U.S. Pat. No. 5,318,509, U.S. Pat. No.
6,381,783 and U.S. Pat. No. 7,229,451.
[0004] In stereotactic neurosurgery, a neurosurgeon will often have
to insert instruments into the brain with millimetre accuracy. In
particular, the success of stereotactic neurosurgery is highly
dependent on the accuracy with which neurosurgical instruments,
such as electrodes or catheters, can be guided to a predetermined
target site within the brain. Stereotactic head frames for use in
precision guided neurosurgery are known. For example, the Leksell
(Registered Trade mark) frame produced by Elekta AB, Sweden
includes a base ring attachable to the base of the skull. Once the
base ring is attached to the subject's head and has been imaged it
acts as a platform of known position relative to the head from
which instruments can be stereotactically guided to the required
target site or sites in the brain. The base ring can also be locked
to an operating table to immobilise the head. It has also been
proposed previously to mount MRI coils directly to the base ring of
a Leksell type frame. For example, WO2006/134357 describes
apparatus in which RF coils for MRI can be directly attached to
such a base ring.
[0005] According to a first aspect of the present invention, a head
clamp for neurosurgery is provided that comprises a member for at
least partially encircling the head of a subject, and at least
first and second skull attachment portions for attaching the member
to the head of a subject, characterised in that the head clamp
comprises a position setter that allows the member to be moved
between at least two repeatable relative positions.
[0006] The present invention thus provides a head clamp for
neurosurgery comprising a member that is attachable to a head using
first and second skull attachment portions that engage the skull of
a subject. The member may be of any shape but it is preferred that
the member comprises a c-shaped member. Advantageously, such a
c-shaped member partially encircles the head. It should be noted
that the term c-shaped member is used herein to encompass members
that may be v-shaped, u-shaped, G-shaped, arced, bent or a variant
shape thereof. Conveniently, the first and second skull attachment
portions are located at the first and second ends of the c-shaped
member. Advantageously, the first and second skull attachment
portions are locatable at substantially opposite sides of a skull.
As described in more detail below, the skull attachment portions
may include one or more bone pins or screws that can be secured
directly to the skull bone of a subject. A force applicator may
also be provided, e.g. as described below, to force such pins into
contact with the skull. Means may also be provided to prevent
rotation of the first and/or second skull attachment portions
relative to the member during the attachment or removal of the head
clamp.
[0007] The present invention also includes a position setter that
allows the member to be moved between at least two repeatable
relative positions. The position setter may comprise a position
encoder (e.g. a rotary encoder) or positional markings that
measure, or allow to be measured, an angle between the member and a
part of the head clamp that has an invariant position relative to
the skull to which it is attached. Advantageously, the position
setter comprises an indexing mechanism that allows, when the head
clamp is attached to a head, the member to be indexed between two
or more repeatable relative positions.
[0008] Providing a head clamp with a position setter in accordance
with the present invention allows the member to be moved between at
least two repeatable positions that have a known (predetermined)
position relative to each other. This allows, for example,
measurements (e.g. MRI images) of the head that are tied to a
unique reference or datum position on the member when that member
is in a first position to be linked back to that reference position
when the member has been moved into the second position. In this
manner, MRI images of the brain acquired with the member in a first
position can be used, after applying appropriate co-ordinate
transformations, to precisely guide neurosurgical devices even when
the member has been moved into another position. This ability to
move the member into a plurality of repeatable relative positions
is particularly advantageous because it enables the member to be
moved between different positions that allow optimised imaging
(e.g. using MRI) or provide the necessary access to the head for
the insertion of neurosurgery devices or instruments.
Advantageously, the position setter allows the member to be moved
whilst the clamping force for holding the head clamp on the head is
applied.
[0009] None of the known head clamp arrangements mentioned above
provide a position setter according to the present invention. The
c-shaped members of known head clamps can be finely adjusted during
attachment to the head, but are not moveable between two or more
repeatable relative positions. Such known head clamps are simply
adjusted into a preferred position and then used as a means for
securing the head to an operating table during neurosurgery.
Similarly, the base ring of a Leksell frame is secured in position
and then used as a reference platform after being imaged; the base
ring is therefore specifically designed to be as immobile as
possible after attachment to the skull. In contrast to these prior
art devices, the present invention offers, by virtue of the
provision of a position setter such as an indexing mechanism, the
ability to optimise the position of the member relative to the head
for different applications without loosing the ability to then use
any positional information that has been obtained using the member
as a reference or base position.
[0010] Advantageously, the member can, when the head clamp is
attached to the head of a subject, be moved (e.g. indexed) into a
first position. Conveniently, the first position comprises a
position in which the member substantially extends around the top
of the head (e.g. in a so-called sagittal position) thereby
allowing imaging (e.g. MRI) of the head. In such a first position,
MRI coils and/or MRI visible fiducial markers may be attached
(directly or indirectly) to the member. Orientating the member so
that it substantially extends around the top of the head allows
such fiducial markers and associated RF coils to be placed in close
proximity to the side of the head and allows such coils to extend
along the whole side of the head thereby allowing more of the head
to be imaged. In other words, the first position of the member may
be selected to optimise imaging.
[0011] Preferably, the member can, when attached to the head of a
subject, be moved (e.g. indexed) into a second position.
Advantageously, the second position comprises a position in which
the member substantially extends around a side of the head thereby
enabling neurosurgical procedures (e.g. stereotactic neurosurgery)
to be performed on the head. Such a second position thus allows the
member to be moved away from the top of the head to one side of the
head. This allows the surgeon to have improved access,
substantially unhindered by the head clamp, to the required part of
the head through which the brain will be accessed. In other words,
the second position of the member may be selected to optimise
access for neurosurgery.
[0012] The first and second positions into which the member can be
moved are repeatable relative positions. In other words, the
relative difference or change in position of the member between the
first and second positions is known. For example, the head clamp
may be designed such that there is a certain positional difference
between the first and second positions and/or any positional
difference may be measured. Knowing the relative difference in the
first and second positions means that any position measurements
made relative to the member with the head clamp in the first
position (e.g. using MRI) can be tied to positions on that member
with the head clamp in the second position. In this manner,
neurosurgical instruments guided by a stereoguide attached to the
member of the head clamp may be accurately guided to target sites
in the brain identified using MRI, even though the member may be
indexed into different positions during imaging and surgery.
[0013] The head clamp may be attached to a skull by only the first
and second skull attachment portions. Alternatively, the head clamp
may include further skull attachment portions. Advantageously, the
first and second skull attachment portions are locatable on
substantially opposite sides of a head. Preferably, the first and
second skull attachment portions exert a clamping force on the head
substantially along a clamping axis. The head clamp, when attached
to the head, is conveniently arranged so that the clamping axis is
substantially parallel to the anterior commisure posterior
commisure (ACPC) plane of the brain. Alignment with the ACPC, which
is an internal landmark of the brain, may be achieved sufficiently
accurately by aligning the clamping axis with external landmarks
such as the cantho-meatal plane. The first skull attachment portion
is, in use, conveniently attached to the forehead of the
subject.
[0014] The member (e.g. a c-shaped member) is advantageously
rotatable about the clamping axis. For example, the member may be
moveable (e.g. indexable) between two or more rotary positions. The
member may thus adopt a first position in which the member
substantially extends around the top of the head; this may be
defined as a 0.degree. position. A second position in which in
which the member substantially extends around a side of the head
may then be defined as a +90.degree. or a -90.degree. position
(depending on which side of the head it is positioned). Preferably,
a plurality of repeatable positions between -135.degree. and
135.degree. are provided. The position setter may, of course,
provide other repeatable relative angular positions if
required.
[0015] In order to provide the positional link between different
positions, it is preferable that the user is able to unambiguously
establish the particular repeatable relative position into which
the member has been placed. This may be achieved by providing
markings or the like on the head clamp that allows the selected
position to readily determined. If an indexing mechanism is
provided, it is also preferred that the member can only be indexed
into relatively few different positions thereby ensuring there is
no confusion over the position in which the member is located.
Preferably, such an indexing mechanism allows the member to be
indexed in to fewer than twenty, more preferably fewer than ten,
more preferably fewer than five, or more preferably three or fewer,
or more preferably fewer than three different repeatable relative
positions. If the member is rotatable about an axis as described
above, it is preferred that the repeatable relative positions are
separated by an angle of at least 5.degree., more preferably at
least 10.degree., more preferably at least .degree., more
preferably at least 45.degree. and more preferably at least
75.degree..
[0016] The position setter may comprise an indexing mechanism of
any suitable type. Conveniently, the indexing mechanism allows the
member to be indexed when the clamping force for engaging the head
clamp with the skull (e.g. as produced by the force applicator) is
being applied. Preferably, each indexed position provided by such
an indexing mechanism can be selected with no, or negligible,
backlash. Advantageously, the indexing mechanism comprises first
and second mating parts that may adopt a plurality of different,
repeatable, indexed positions relative to one another. The indexing
mechanism may comprise parts that provide a v-tooth, Curvic or
Hirth coupling. Alternatively, the first part may comprise a
plurality of pins that mate with a plurality of tapered holes
provided in the second part. In a further alternative embodiment,
each part may comprise a set of balls arranged to engage each other
to define the plurality of indexed positions. Each indexed position
may be provided by a kinematic connection. Advantageously, the
first mating part of the indexing mechanism may be rotatable
relative to a second mating part of that mechanism; e.g. the first
mating part may be located at least partially within the second
mating part. A locking screw may be provided to lock the first and
second parts into the selected indexed position. The locking screw
may engage a feature of the first part or second part.
[0017] The first and second skull attachment portions may include
any suitable elements for engaging the head. Advantageously, any
such elements pass through openings in the skin and directly engage
with the skull bone of the subject. The first skull attachment
portion and/or the second skull attachment portion may conveniently
comprise one or more pins for direct engagement with the skull bone
of a subject. Advantageously, the first skull attachment portion
comprises the same number of pins as the second skull attachment
portion. Preferably, the first and second skull attachment portions
each include two pins for direct engagement with the skull bone of
a subject. Such an arrangement, in particular providing the first
and second skull attachment portions with the same number of pins,
is preferred as it allows the member to be loaded with the required
clamping force without being substantially distorted or
twisted.
[0018] Advantageously, each pin has an outer, soft, coating. For
example, metal (e.g. titanium) pins may be provided that are coated
with an outer layer of soft material (e.g. rubber, plastic etc).
Such a coating provides protection against damage to soft tissue
during attachment of the head clamp to a subject. It should be
noted that a bone attachment pin of this type may also be used in
different applications. A pin is thus described herein that
comprises a core (e.g. a metal or ceramic core) with a sharp tip,
wherein a soft coating is provided on the tip.
[0019] The first skull attachment portion may conveniently comprise
a pin carrying member that is pivotally attached to the member. For
example, the pin carrying member may be pivotally attached to the
first end of a c-shaped member. The second skull attachment portion
may also comprise a pin carrying member that is pivotally attached
to the member. For example, the pin carrying member may be
pivotally attached to the second end of a c-shaped member. Each pin
carrying member may carry, or be arranged to carry, one or more
pins for direct engagement with the skull bone of a subject.
Attaching pins to the end of the c-shaped member using such a
pivotally attached pin carrying member allows the applied clamping
force to be evenly distributed between the pins and minimises out
of plane distortions of the member. Advantageously, each pin
carrying member is arranged to pivot relative to the member in a
single plane.
[0020] The head clamp may comprise a force applicator to urge the
first and second skull attachment portions into engagement with the
skull. The force applicator may urge the first and second skull
attachment portions into engagement with the skull with a variable,
e.g. user definable, force. Preferably, the force applicator urges
the first and second skull attachment portions into engagement with
the skull with a predetermined amount of force. For example, the
force applicator may be arranged to apply a certain force (e.g. 200
N) to the skull per skull attachment pin. Advantageously, the force
applicator may include a force indicator for indicating when a
preferred engagement force has been applied. The force indicator
preferably comprises a tactile indicator, such as a recessed rod
that becomes flush with a reference surface when the required force
is applied. The force applicator may also comprise a force limiter
for ensuring that a certain force limit is not exceeded. For
example, the force limiter may ensure the predetermined amount of
force is not exceeded.
[0021] The force applicator may be located adjacent to only one of
the first and second skull attachment portions. For example, the
force applicator may be located at only one end of a c-shaped
member (e.g. at the opposite end to the position setter or indexing
mechanism). This, however, obviously results in the application of
an equal and opposite force at the first and second ends of the
c-shaped member. It can also be seen that the clamping force is
transmitted between the skull attachment portions via the member.
Applying a predetermined clamping force in this manner also ensures
that the member is, in use, subjected to the same (predetermined)
force and hence deforms, at least approximately, by a known amount.
The member does not, therefore, have to be sufficiently rigid to be
undistorted by the clamping force. Instead, it is preferred that
the member deforms by at least approximately the same amount when
the predetermined clamping force is transmitted through it.
[0022] The member preferably comprises at least one datum feature.
Each such datum feature preferably provides a positional reference
point on or in a fixed positional relationship to the member. Each
datum feature may be a marking (e.g. a visible or MRI visible
marking) or a physical feature or set of features. A single datum
feature may be provided. Advantageously, a datum feature is
provided on each side (e.g. on opposing faces) of the member.
[0023] Advantageously, the at least one datum feature is located
substantially on a neutral axis of distortion of the member; the
neutral axis being an axis about which the member deforms when the
clamping force is applied. If two datum features are provided on
opposite sides or faces of the member, each datum feature may be
located substantially on a neutral axis of distortion of the
member. Any variations in the applied clamping force will thus have
no substantial effect on the position of the datum feature(s)
relative to the first and second skull attachment portions or the
head to which the clamp is attached. Associated apparatus may then
include a feature that is complementary to the datum feature such
that the associated apparatus may be repeatedly placed in the same
defined position relative to the datum feature of the head
clamp.
[0024] Advantageously, a datum feature is provided on the member
that comprises the first part of a kinematic joint. In other words,
the datum feature may comprise a kinematic datum feature. A
kinematic datum feature may thus be arranged so that it uniquely
constrains an associated, complementary, kinematic feature in each
of the 6 degrees of freedom. For example, the datum feature may
comprise three v-grooves radially spaced apart from one another by
120.degree. and extending along directions that intersect at a
common point. In such an example, the common point may be located
substantially on the neutral axis of the member. Associated
apparatus (e.g. stereoguides, fiducial markers for MRI etc) may
then comprise the second part of the kinematic joint, such as three
balls (or at least partly spherical features) spaced in a circle
and separated from each other by 120.degree.. The first and second
parts of the kinematic joint may then be arranged to provide a
highly repeatable kinematic link in which there is only one
constraint on each degree of freedom of movement between the first
and second parts.
[0025] Advantageously, the head clamp comprises at least one
attachment feature that allows the head clamp to be secured to
associated apparatus. Preferably, the at least one attachment
feature is located substantially on a neutral axis of distortion of
the c-shaped member. The at least one attachment feature may thus
provide a mechanical linkage by which the head clamp can be secured
to associated apparatus such as an operating table bed or an MRI
headcoil assembly. A common feature may provide both a datum
feature and an attachment feature. Advantageously, the attachment
feature is separate from the datum feature. Preferably, a datum
feature of the head clamp is accessible when the attachment feature
is secured to the associated apparatus. This allows, for example,
the at least one attachment feature to be used to secure the head
clamp to an operating table bed whilst a feature of a stereoguide
is mated with the datum feature of the head clamp. Supplementary
attachment features may also be provided on the member if
required.
[0026] Advantageously, the head clamp is substantially MRI
compatible. For example, the member may be formed from a polymer, a
reinforced plastic, a filled ceramic or a glass-filled polymer. The
head clamp may also include ceramic and/or plastic components. The
head clamp may contain some metal components (e.g. springs), but
the amount of metal is preferably not enough to cause the
generation of eddy currents that are sufficient to cause
substantial heating of the structure or induce magnetic field
distortions that effect the MR image.
[0027] The present invention also provides a head clamping kit that
comprises a head clamp as described above and a secondary clamping
device that is attachable to the head clamp. Advantageously, the
secondary clamping device provides, when attached to the head
clamp, additional mechanical support to the member. Conveniently,
the secondary clamping device provides such mechanical support to
the member only in the event of mechanical failure of that member.
In other words, the secondary clamp is normally non-load bearing
and only provides mechanical support if the member fails. This
backup or support clamp may be used during surgical procedures in
which the force exerted on the head clamp may momentarily exceed
safe tolerances.
[0028] Also described herein is a kit comprising a head clamp for
neurosurgery and a secondary clamping device attachable to the head
clamp. The head clamp may comprise a member for at least partially
encircling the head of a subject. For example, the head clamp may
comprise a c-shaped member for partially encircling the head of a
subject. The head clamp may include first and second skull
attachment portions; for example provided at the first and second
ends of a c-shaped member. The secondary clamping device provides,
when attached to the head clamp, additional mechanical support to
the member (e.g. the c-shaped member) in the event of mechanical
failure of that member.
[0029] The present invention also extends to apparatus for imaging
the head of a subject that comprises a head clamp as described
above and apparatus (e.g. MRI or CT apparatus) for imaging a head.
Preferably, the head clamp is releasably attachable to the
apparatus. For example, the MRI apparatus may include a clamping
mechanism for engaging an attachment feature of the head clamp
and/or one or more fiducial markers that can be repeatably attached
to a datum feature of the head clamp.
[0030] The apparatus for imaging the head of a subject may comprise
a housing for at least partially surrounding a body part, and a
first fiducial marker assembly retained at least partially within
the housing that comprises one or more fiducial markers and a datum
feature, the position of the datum feature being fixed relative to
the one or more fiducial markers, wherein the first fiducial marker
assembly is moveable with respect to the housing and the datum
feature is accessible from outside of the housing. Such apparatus
may include one or more RF coils for magnetic resonance imaging
that are located within the housing. The housing may comprise a
first housing part and a second housing part, wherein the first and
second housing parts can be moved, e.g. pivoted, into a closed
position that defines an imaging space in which a human head can be
located. Fiducial marker assemblies may also be provided in each
housing part. Such apparatus is described in more detail in
Applicant's co-pending PCT patent application, the contents of
which are hereby incorporated by reference, that has the same
filing date as the present application and claims priority from UK
patent application No. 0908784.
[0031] The present invention also extends to apparatus for
neurosurgery, comprising a head clamp as described above and a
stereoguide device for retaining and guiding neurosurgical
instruments. Preferably, the stereoguide device is releasably
attachable to the head clamp. For example, the neurosurgery
apparatus may comprise a clamp for engaging an attachment feature
of the head clamp and/or the stereoguide device may be repeatably
attached to a datum feature of the head clamp. The member of the
head clamp may be moveable into repeatable relative positions that
provide an axis of movement or rotation of the stereoguide.
[0032] According to a second aspect of the invention, a method of
performing neurosurgery is provided that comprises the steps of (i)
attaching an head clamp comprising a moveable member to the head of
a subject, (ii) moving the member into a first position and
acquiring at least one image of the head, (iii) moving the member
into a second position and performing a neurosurgical procedure,
wherein the relative difference in position between the first
position and the second position of the member is known.
[0033] Preferably, the head clamp comprises an indexable member and
steps (ii) and (iii) comprise indexing the member into the first
position and second position respectively. Advantageously, at least
one datum feature is provided on the head clamp that allows images
acquired during step (ii) to be related to neurosurgical procedures
conducted during step (iii). Preferably, step (i) comprises
attaching the head clamp to the head with a predetermined
engagement force such that deformations of the member are known.
Conveniently, step (i) comprises applying a clamping force
substantially along a clamping axis, wherein the clamping axis is
aligned, at least approximately, with the cantho-meatal plane of
the subject.
[0034] Preferably, the member of the head clamp comprises a
c-shaped member, wherein the first position of step (ii) is a
position in which the c-shaped member substantially extends around
the top of the head thereby allowing imaging of the head and the
second position of step (iii) is a position in which the c-shaped
member substantially extends around a side of the head thereby
providing access the head to allow the neurosurgical procedure to
be performed.
[0035] Also described herein is a head clamp for neurosurgery,
comprising; a c-shaped member for partially encircling the head of
a subject, and first and second skull attachment portions provided
at first and second ends of the c-shaped member, wherein the head
clamp comprises at least one fiducial marker (e.g. at least one MRI
visible fiducial marker). At least one fiducial marker may be
formed integrally with the head clamp. For example, a fiducial
marker may form part of, or be fixed to, any part of the head
clamp. At least one fiducial marker may be attachable (e.g.
releasably attachable) to a part of the head clamp in a known
position relative to that part of the head clamp. The head clamp
may also comprise a datum feature (e.g. provided on the c-shaped
member) that allows associated apparatus to be located in a known
position relative to the head clamp. Such a datum feature may also
allow the fiducial marker to be attached to the head clamp in a
known relative position.
[0036] A fiducial assembly may also be provided that comprises at
least one fiducial marker (e.g. at least one MRI visible fiducial
marker) and a datum feature having a fixed position relative to the
at least one fiducial marker. The datum feature may be combined
with one or more fiducial markers. The datum feature preferably
allow associated apparatus to be located in a known position
relative to the assembly. The fiducial assembly may include means
for attachment to the skull of a subject. For example, such means
may comprise a clamp (e.g. a c-clamp) for securing the fiducial
assembly to a skull.
[0037] The invention will now be described, by way of example only,
with reference to the accompanying drawings in which;
[0038] FIG. 1 illustrates a head clamp of the present
invention,
[0039] FIG. 2 shows an exploded view of the force control mechanism
of the head clamp of FIG. 1,
[0040] FIG. 3 is a view of the force control mechanism prior to the
required force being applied,
[0041] FIG. 4 is a view of the force control mechanism when the
required force is being applied,
[0042] FIG. 5 is an exploded view of the indexing mechanism of the
head clamp of FIG. 1,
[0043] FIG. 6 illustrates a rubber coated attachment pin,
[0044] FIG. 7 shows the kinematic datum feature of the head clamp
in more detail,
[0045] FIG. 8 shows a frontal view of the head clamp when attached
to a head and indexed into a first (imaging) configuration,
[0046] FIG. 9 shows a side view of the head clamp when attached to
a head and indexed into a first (imaging) configuration,
[0047] FIG. 10 shows a frontal view of the head clamp when attached
to a head and indexed into a second (surgical) configuration,
[0048] FIG. 11 shows a side view of the head clamp when attached to
a head and indexed into a second (surgical) configuration,
[0049] FIG. 12 shows MRI imaging apparatus for receiving a head
clamp of the type shown in FIGS. 1 to 11 in an open
configuration,
[0050] FIG. 13 shows a head clamp retained by the MRI
apparatus,
[0051] FIG. 14 shows the MRI imaging apparatus in a closed
configuration,
[0052] FIG. 15 shows one part of the housing of the MRI imaging
apparatus in more detail,
[0053] FIG. 16 provides a more detailed view of how the fiducial
marker assembly is attached to the housing,
[0054] FIG. 17 provides an exploded view of how the fiducial marker
assembly is attached to the housing,
[0055] FIG. 18 shows an optional support or back-up clamp for the
head clamp,
[0056] FIG. 19 is an exploded view of a Hirth coupling that can
provide an alternative indexing mechanism for the head clamp of
FIG. 1,
[0057] FIG. 20 shows the Hirth coupling of FIG. 19 in an unlocked
configuration,
[0058] FIG. 21 shows the Hirth coupling of FIG. 19 in a locked
configuration,
[0059] FIG. 22 shows an anti-rotation device for locking the force
applicator mechanism of a head clamp as shown in FIG. 1,
[0060] FIG. 23 shows the anti-rotation device of FIG. 22 in an open
configuration when engaging the head clamp, and
[0061] FIG. 24 shows the anti-rotation device of FIG. 22 in a
closed configuration that prevents rotation of the force applicator
mechanism.
[0062] Referring to FIG. 1, a head clamp 2 of the present invention
is illustrated. The head clamp 2 comprises a generally c-shaped
member 4 having a first end 6 and a second end 8.
[0063] At the first end 6 of the c-shaped member 4, a first pair of
skull attachment pins 10 and 12 are mounted to a first v-shaped pin
carrying member 14. The first v-shaped member 14 is pivotally
connected at its apex to a force applicator mechanism 16 which is
in turn attached to the first end 6 of the c-shaped member 4. The
first v-shaped member 14 is pivotable about the axis P1 such that
the pins 10 and 12 can rotate in a single plane. The force
applicator mechanism 16 comprises a profiled portion 18 which can
be rotated to drive the first v-shaped pin carrying member 14 along
the clamping force axis 20 thereby allowing the head clamp to be
secured to the head of a subject. More details of the force
applicator mechanism 16 are outlined below with reference to FIG.
2.
[0064] At the second end 8 of the c-shaped member 4 there is
provided a second pair of skull attachment pins 22 and 24 that are
mounted to a second v-shaped pin carrying member 26. The second
v-shaped member 26 is pivotally connected to an indexing mechanism
28 that is in turn mounted to an aperture provided at the second
end 8 of the c-shaped member 4. The second v-shaped member 26 is
pivotable about the axis P2 such that the pins 22 and 24 can rotate
in a single plane. The indexing mechanism 28 comprises a locking
screw 29 for locking the selected indexed position. More details of
the indexing mechanism 28 are provided below with reference to FIG.
3.
[0065] The c-shaped member 4 is approximately mechanically
symmetrical about the plane 30 which provides a neutral axis of
distortion. A neutral axis of distortion is thus the axis of
symmetry of the c-shaped member 4 that is not subject to
compression or tension (i.e. is not distorted) when the head clamp
exerts a clamping force on a head. The c-shaped member 4 also
comprises ribs 31 to improve stiffness and in particular to reduce
the possibility of the c-shaped member 4 twisting. In this example,
the c-shaped member 4 is formed from a glass-filled polymer
material, such as Ryton R7, which is light, stiff and MRI
compatible. It should be noted that it would also be possible to
use other materials (e.g. ceramic) to form the c-shaped member. For
example, a number of different polymer materials could be used and
it would also be possible to use a ferrous metal (such as steel) if
MRI compatibility was not required.
[0066] On both faces of the c-shaped member 4 there are provided
identical datum features that lie on a neutral axis. The datum
feature shown in FIG. 1 comprises three sub-features in the form of
v-grooves 32a-32c that are radially spaced apart from one another
by 120.degree. and extend along directions that intersect at a
common point. As explained in more detail below, the v-grooves
32a-32c of the datum feature provide a repeatable, kinematic,
mechanical link with associated apparatus such as a stereoguide or
MRI fiducial marker assemblies. The c-shaped member 4 also
comprises an attachment feature on each face. The attachment
feature is, in this example, provided in the form of three recesses
34a-34c and a circular rim portion 35.
[0067] The three recesses 34a-34c and/or the circular rim portion
35 can be used to provide a pseudo-kinematic link that allows the
head clamp to be secured to apparatus such as a surgical bed or MRI
apparatus in an approximate position. In this example, attachment
features are provided on both faces that are co-axial with the
datum features and therefore also lie substantially on the neutral
axis. It would, of course, also be possible to space apart the
attachment and datum features and/or provide either feature on only
one face. In this example, supplementary attachment features are
also provided by the protrusion 37 and the apertures 38 provided in
the c-shaped frame 4. Apparatus may be secured to the head clamp 2
using some or all of the attachment features and supplementary
attachment features as appropriate; this is described in more
detail below.
[0068] More details of the various parts of the head clamp of FIG.
1 will now be described with reference to FIGS. 2 to 7.
[0069] Referring to FIG. 2, the force applicator mechanism 16 is
illustrated in an exploded view. The force applicator mechanism 16
comprises a force control member 40 that comprises the profiled
portion 18 at its proximal end, a force indication rod 42, a
helical spring 44, a hollow cylindrical sleeve 48 that fits within
the aperture at the first end 6 of the c-shaped member 4 and a
shaft portion 50. An optional clamping element 46 may also be
provided to lock the force control member 40 to the first end 6 of
the c-shaped member 4. The distal end of the shaft portion 50
comprises a yoke 52 to which the v-shaped pin carrying member 14
can be attached using a pair of pivot bolts 56 and 58 and a
threaded loading stud 54. Skull attachment pins 10 and 12 can be
attached to the tapered apertures of the v-shaped pin carrying
member 14.
[0070] The force applicator mechanism 16, when assembled, allows a
predetermined force to be applied to the skull via the skull
attachment pins 10 and 12. In particular, rotation of the force
control member 40 causes the spring 44 to urge the shaft portion 50
along the clamping axis 20 thereby moving the pins 10 and 12 into
engagement with the skull. As the clamping force applied to the
skull is increased, the spring 44 compresses which in turn causes
the force indication rod 42 to move relative to the force control
member 40. The face 60 of the force indication rod 42 become flush
with the face 62 of the profiled portion 18 when the required,
preset, clamping force is being applied. Although not shown, an
optional force limiter may be provided to prevent the preset
clamping force being exceeded.
[0071] Referring now to FIG. 3, the tactile indicator of the force
applicator mechanism 16 is shown before the predetermined clamping
force is applied. It can be seen that the face 60 of the force
indication rod 42 is sub-flush or recessed with respect to the face
62 of the profiled portion 18 of the force control member 40.
[0072] Referring to FIG. 4, the tactile indicator of the force
applicator mechanism 16 is shown when the required, predetermined,
clamping force is applied. It can be seen that the face 60 of the
force indication rod 42 is flush with respect to the face 62 of the
profiled portion 18 of the force control member 40. This provides
the user with a simple, tactile, indication that the desired
clamping force is being applied.
[0073] Referring to FIG. 5, the indexing mechanism 28 is shown in
an exploded view with insets providing alternative views of several
components. The indexing mechanism comprises a detent insert 70, an
indexer body ring 72, a locking or indexing screw 29 and an
indexer-head shaft 74. The distal end of the indexer-head shaft 74
comprises a yoke 84 to which the v-shaped pin carrying member 26
can be attached using a pair of pivot bolts 88 and 90 and threaded
loading stud 86. Skull attachment pins 22 and 24 can be attached to
tapered apertures of the v-shaped pin carrying member 26.
[0074] The indexer-body ring 72, when assembled, is moulded into an
indexer-eye 73 provided at the inwardly facing side of the aperture
provided at the second end 8 of the c-shaped member 4. The proximal
end 76 of the indexer-head shaft 74 can, when the locking screw 29
is unscrewed, freely rotate within the indexer body ring 72. The
indexing mechanism is configured to operate when the head frame is
clamped to the head and hence, in use, there will be a force of
around 400 N urging the indexer-head shaft 74 into engagement with
the indexer-body ring 72.
[0075] The locking screw 29 has a conical tip 79 and the
indexer-head shaft 74 comprises a chamfered collar 80 in which are
formed a plurality of conical depressions 75. The indexer-body ring
72 comprises three chamfered buttresses. The required indexed
position is locked when the conical tip 79 of the indexer screw 29
emerges through the hole 73 in the indexer-body ring 72 and engages
with one of the conical depressions 75 of the indexer-head shaft
74. When the screw 29 is fully engaged with a conical depression
75, the indexer-head shaft 74 is displaced fractionally away from
its rotation axis so that its chamfered collar 80 is pushed into
and supported by two chamfered buttresses 78 of the indexer-body
ring 72. This three point support provides backlash free and
accurate positioning.
[0076] The detent insert 70 is a moulded thermoplastic component
that is held in place in the outwardly facing side of the aperture
provided at the second end 8 of the c-shaped member 4 by a pair of
sprung lugs 71. The purpose of the detent insert 70 is to guide the
indexer-head shaft 74 into angular positions relative to the
indexer-body ring 72 that are close enough to the locked or indexed
positions to enable the indexer screw 29 to engage the selected one
of the conical depressions 75. The detent insert 70 also forms the
function of providing tactile feedback to a user that an indexed
position has been achieved by clicking into place. The detent
insert 70 comprises four protruding sprung elements 73 that provide
this positional click by engaging with complementary slots 81
located at the index positions that are provided at the proximal
end 76 of the indexer-head shaft 74.
[0077] The indexer-head shaft 74 is made in a single piece. This is
not essential but has been found to aid positioning between the
indexing components (e.g. the chamfered collar 80 and conical
depressions 75) and the yoke 84 whilst also simplifying assembly
and cleaning. Assembly of the indexer-head shaft 74 and the
indexer-body ring 72 is made possible by the inclusion of slots 82
in the chamfered collar 80 which can be aligned with and pass over
the chamfered buttresses 78.
[0078] The indexing mechanism 28 thus allows a plurality of
discrete and repeatable indexed positions to be provided. It
should, however, be noted alternative indexing mechanisms could be
used to provide the same function. For example, the indexing
mechanism may be implemented using a v-tooth (e.g. Curvic or Hirth)
coupling, a plurality of pins that mate with a plurality of tapered
holes or complementary sets of balls. Instead of the indexing
mechanism of the present example, different types of position
setter could be provided; for example, a rotary position encoder
could be provided instead of the indexing mechanism.
[0079] Referring to FIG. 6, a pin 100 for engaging the skull of a
subject is shown. Such a pin may be used in apparatus as described
above or as part of any other appropriate surgical apparatus. The
pin 100 comprises a titanium core having a tapered portion 122
(e.g. that can be pushed into the tapered bore on the arm of the
v-shaped member) and a sharp skull engaging tip 124. Although a
tapered connection between the pin and member is shown, other
connections (e.g. threaded connections) may be provided. The skull
engaging tip 124 is coated with a ball of a soft, e.g. rubber,
material 126. This soft coating or material 126 helps prevent
unwanted damage to soft tissue (e.g. skin) whilst a head clamp is
being positioned but it can be readily pierced by the titanium tip
when the pins are located in the required position and a clamping
force is applied. It should be noted that the soft coating,
although preferred, is by no means essential. Similarly, the pins
may be formed from many materials other than titanium (e.g. other
metals or ceramic etc). The pin 100 may be permanently attached to
a part of the head clamp but it is preferably provided as
disposable (e.g. single use) item that can be easily attached to
and detached from the head clamp.
[0080] Referring to FIG. 7, the datum and attachment features of
the head clamp are shown in more detail. The datum feature
comprises three v-grooves 32a-32c that are radially spaced apart
from one another by 120.degree.. The attachment feature comprises
three recesses 34a-34c and the circular rim portion 35. Apertures
38 that are provided as part of the supplementary attachment
features are also shown.
[0081] A number of potential uses of the head clamp described with
reference to FIGS. 1 to 7, along with methods of attaching the head
clamp to a subject, will now be described with reference to FIGS. 8
to 11.
[0082] In use, the head clamp 2 is placed over a subject's head so
that the c-shaped member 4 partially encircles the head. The pins
10 and 12 provided at the first end 6 of the c-shaped member 4 are
aligned with the centre of the forehead and the pins 22 and 24
provided at the second end 8 of the c-shaped member 4 are located
on the opposite side (rear) of the head. The clamping axis 20 of
the head clamp is also approximately aligned with the cantho-meatal
plane. An alignment aid, for example a band attached to the head or
an alignment aid mounted to the head clamp, may be used to
facilitate the required alignment. The first and second v-shaped
members 14 and 26 are also respectively arranged such that the pins
10 and 12 pivot in substantially the same plane as the pins 22 and
24. Once aligned, the profiled portion 18 of the force applicator
mechanism 16 is turned thereby advancing the pins 10 and 12 towards
the head. The applied force is then increased until the pins 10,
12, 22 and 24 cut through the skin and engage the underlying skull
bone with the required, predetermined, amount of force. In this
example, a total force of 400 N is applied which results in each
pin applying a 200 N force to the skull bone. Pivoting of the first
and second v-shaped members 14 and 26 ensures that the force is
evenly applied to the skull through each pin.
[0083] Applying a predetermined (preset) amount of force evenly
distributed through the pins has the advantage that the c-shaped
member 4 will deform by substantially the same amount each time it
is attached to a head. Furthermore, locating the datum and
attachment features substantially on the neutral axis ensures that
the position of such features is substantially invariant even if
the c-shaped member is subjected to slightly different distortion
forces.
[0084] FIGS. 8 and 9 show the head clamp 2 attached to a head 140
and indexed into a first, imaging, position. In this first
position, the c-shaped member extends around the top of the head
140. This position is particularly suited for imaging applications,
such as MRI, because it allows MRI coils and/or fiducial markers to
be placed in close proximity to both sides of the head without any
interference from the c-shaped member 4.
[0085] FIGS. 10 and 11 shows the head clamp 2 attached to the head
140 and indexed into a second position in which the c-shaped member
extends around the side of the head. This second position provides
the access to the top of the head that a surgeon typically requires
to perform stereotactic neurosurgery.
[0086] Although two different indexed positions are shown in FIGS.
8-11, it should be noted that the c-shaped member of the head clamp
could be indexable between more positions. For example, the
c-shaped member could also be indexable into a position on the
other side of the head and/or into any one or more other (e.g.
intermediate) indexed positions. It is, however, preferably to
provide only a few index positions to ensure that there is no
potential for confusion over the index position that has been set.
Markings may also be provided, if required, to help indicate the
indexed position that has been adopted.
[0087] The positional differences between the various indexed
positions adopted by the head clamp are preferably known or
measured. For example, the head clamp may be designed so that there
is a predetermined positional change in the position of one or more
datum features between each of the indexed positions.
Alternatively, a calibration procedure may be performed prior to
use (e.g. during manufacture of the head clamp) in which the
position of the datum feature is measured for different indexed
positions. The head clamp may thus be supplied with a set of
co-ordinate transformations that provide such position mapping
information.
[0088] It should be noted that the head clamp may be formed from
the glass filled polymer material mentioned above or any other
suitable alternative material(s). The various parts of the head
clamp may be single use (disposable), multi-use or re-useable. If
any part of the head clamp can be used more than once, it is
preferred that such a part can be sterilised in an autoclave.
[0089] Referring now to FIGS. 12 to 17, there will be described
apparatus for imaging the head of subject that is designed for use
with a head clamp of the type described with reference to FIGS. 1
to 11. Although the apparatus may be used with any imaging
technique, the following examples describe its use with MRI
apparatus.
[0090] FIG. 12 illustrates head imaging apparatus suitable for use
in MRI that is placed in its open position. The apparatus comprises
a housing formed from a first housing part 160 that is connected to
a second housing part 162 by a pivot joint 164. As will be
described in more detail with reference to FIG. 14-16 below, each
housing part comprises a hollow plastic shell that contains RF coil
assemblies for MRI, various electronic control circuitry and a
floating fiducial marker assembly. The fiducial marker assemblies
of the first and second housing parts are retained in their
respective housing parts and have inwardly facing datum features
166 and 168 that are externally accessible through an aperture
formed in the housing. A clamp mechanism in the form of a toggle
clamp 170 is provided on the base 171 of the apparatus between the
two housing parts 160 and 162.
[0091] As illustrated in FIG. 13, the toggle clamp 170 is designed
such that activation of lever 172 causes the clamp to engage and
hold the circular rim portions 35 of the two attachment features
provided on the opposed faces of the c-shaped member 4 of the head
clamp 2 that is described above with reference to FIGS. 1 to 11.
The head imaging apparatus also comprises a channel 173 having
angled walls for engaging the protrusion 37 of the c-shaped member
4 to provide additional mechanical support. The toggle clamp 170 is
also arranged such that, when engaging the circular rim portions 35
of the head clamp 2, the datum features of the head clamp 2 are
still accessible. In particular, the datum features 166 and 168 of
the fiducial marker assemblies are arranged such that they can be
brought into contact with the complementary datum features of the
head clamp when held by the toggle clamp 170.
[0092] FIG. 14 illustrates the apparatus described above with
reference to FIGS. 12 and 13 in its closed position. A two-part
locking latch 174 is provided to hold the housing parts together in
the closed position. In this closed position, the datum features
166 and 168 of the fiducial marker assemblies are biased into
engagement with the complementary datum features of the head clamp
2. The fiducial markers of the fiducial marker assemblies are thus
accurately held in a known, repeatable, position relative to the
datum features of the head clamp. These fiducial markers thus act
as highly accurate and repeatable reference position markers in any
acquired MRI images.
[0093] Referring to FIG. 15, the components located within the
first housing part 160 of the apparatus described with reference to
FIGS. 12 to 14 will be described. It should be noted that the
second housing part 162 houses similar components and is therefore
not shown.
[0094] The first housing part 160 comprises a plastic shell that
holds the various sets of RF coils 180 that can be used in
conjunction with MRI apparatus to obtain high resolution images of
the head. These RF coils 180 are secured to the housing and
connected to electronic control circuitry provided on circuit
boards 182. The RF coils 180 and circuitry 182 typically handle
electrical signals during use and the first housing part therefore
also acts as an insulating shield that prevents patients and
operators being exposed to the electrical voltages. Cables to and
from the electrical circuitry 182 are routed via the pivot joint of
the housing.
[0095] The first housing part 160 also contains a first floating
fiducial marker assembly 190. The fiducial marker assembly 190
comprises a fiducial marker 192 in the form of a square frame 194
with a diagonal cross member 196. The fiducial marker 192 comprises
or contains a material that is MRI visible, such as copper sulphate
solution. The fiducial marker assembly 190 also comprises a
circular datum portion 198 having a surface protruding through an
aperture in the housing that provides the externally accessible
datum feature 166. A rigid right angled framework section 200 is
also provided to connect the datum feature 166 to the fiducial
marker 192.
[0096] As will be explained in more detail below, the fiducial
marker assembly 190 is retained within the housing part 160 but is
free to move relative to that housing part (i.e. it can be said to
be floating or substantially unconstrained). This prevents any
distortions of the housing being passed to the fiducial marker
assembly and also permits the datum feature of the fiducial marker
assembly to always adopt the same position relative to the
complementary datum feature of the head clamp even if the housing
parts do not adopt repeatable relative positions. A biasing
mechanism 202 is, however, provided to retain the fiducial marker
assembly and to bias the datum feature of that assembly into
engagement with the head clamp during use.
[0097] Referring to FIGS. 16 and 17, the biasing mechanism 202 of
the first housing part 160 described above is shown in cut-away and
exploded cut-away views respectively.
[0098] The biasing mechanism 202 comprises an x-shaped flexible
member 204 having four legs with ends that are each secured to the
housing part 160 by separate screws 206. The centre of the x-shaped
flexible member 204 engages a spherical protrusion 208 provided at
the centre of the circular datum portion 198 of the fiducial marker
assembly 190; the spherical protrusion 208 thus provides a single
point of contact with the substantially flat flexible member 204.
The biasing mechanism 202 also comprises four flexible loops 210.
Each loop 210 is captured between the housing part 160 and one of
the legs of the x-shaped flexible member 204 and also engages a
protruding feature 212 provided on the circular datum portion 198.
The loops are held in slight tension and, because they are equally
spaced around the circumference of the circular datum portion 198,
they maintain the circular datum portion 198 in a substantially
central position within the aperture of the housing. The
flexibility of the loops 210 and x-shaped member 204 does, however,
allow movement of the fiducial marker assembly in all 6 degrees of
freedom when necessary so as to enable the datum feature 166 to
adopt the necessary position relative to a complementary datum
feature provided on a head clamp. It should also be noted that the
biasing mechanism 202 provides the only mechanical connection
between the housing and the fiducial marker assembly and
consequently any distortions of the housing will not be transmitted
to that assembly.
[0099] The head clamp and imaging apparatus described above are
designed to be used together, but it is important to note that each
could be used separately for different purposes. For example, the
head clamp could be used purely for clamping a head during surgical
or other procedures. Similarly, the imaging apparatus could be
arranged to engage and image other body parts to which other
fixtures are attached.
[0100] It should be remembered that the head clamp described above
with reference to FIGS. 1 to 11 is also designed to be used in
surgical procedures. In particular, once an MRI image of the head
has been obtained, the head clamp may be indexed to at least one
further position that is suitable for conducting a surgical
procedure. The, or each, datum feature provided on the head clamp
may thus be used to locate other apparatus in a known position
relative to the head clamp. For example, a datum feature could be
used to accurately position, relative to the head clamp,
retro-reflective surgical navigation instruments, surgical robots
such as the Renishaw-Mayfield neuromate (Registered Trade mark)
robot, and targeted radiotherapy devices such as the Leksell
gamma-knife (Registered Trade mark) apparatus. Position information
acquired from the MRI images can then be tied back to the datum
feature on the head clamp and can therefore be used to precisely
target regions or points in the brain.
[0101] Although the mechanical strength of the c-shaped member 4 of
the above described head clamp is designed to be more than adequate
for all expected mechanical loads to which it will be subjected in
use, it is noted that the head clamp may, on very rare occasions,
be subjected to large mechanical impulses that could exceed safe
design limits. For example, a surgeon may have no choice other than
subjecting the head clamp to large forces to perform an emergency
procedure (e.g. resuscitation etc). In the case of a c-shaped
member formed from a rigid material, such as a glass filled
polymer, any failure may be catastrophic; this can obviously have
severe consequences if a neurosurgical procedure is in
progress.
[0102] Referring to FIG. 18, a supplementary support or backup
clamp 230 (e.g. formed from metal) is shown that can be attached to
the head clamp 2 during surgery. The backup clamp 230 may, for
example, attach to the apertures 38 (not visible in FIG. 18 but
shown in FIG. 1) formed in the c-shaped member 4 of the head clamp
2. The supplementary clamp 230 is arranged so that access to the
datum feature of the head clamp 2 is maintained. In normal use, the
supplementary clamp 230 takes none of the mechanical load of the
head clamp 2 but it does, however, act as a backup device that
takes the full mechanical load of the head clamp 2 if that head
clamp 2 was to fail. The supplementary clamp 230, which is by no
means essential, thus mitigates the unwanted consequences that
might occur with a catastrophic failure of the head clamp 2.
[0103] Referring to FIG. 19, an exploded view is shown of an
indexing mechanism 328 suitable for inclusion in a head clamp as
described above. The indexing mechanism 328 may, for example, be
provided instead of the indexing mechanism 28 described in detail
with reference to FIG. 5.
[0104] The indexing mechanism 328 comprises a so-called Hirth
coupling in which a series of concentric features 330 are provided
around the aperture at the second end 308 of the c-shaped member
304. An indexable part 300 is also provided that has a face plate
302 comprising a plurality of concentric features 306 that
complement the concentric features 330 of the c-shaped member 304.
In particular, the complementary features 330 and 306 are
configured such that, when biased into engagement, the indexable
part 300 can adopt (i.e. can be indexed into) any one of multiple
different orientations relative to the c-shaped member 304.
[0105] An indexing adjuster 310 and release lever 340 are also
provided. The indexing adjuster 310 is insertable into the aperture
at the second end 308 of the c-shaped member 304 and includes
flexural elements to provide a preload bias (i.e. to bias the
complementary features 330 and 306 into engagement even when the
head clamp is not loaded) and stops to prevent the part being
overloaded. The associated release lever 340 comprises a cam 344
and an axle 342. The axle 342 can be attached (clipped into)
corresponding slots 346 provided at the second end 308 of the
c-shaped member 304.
[0106] Rotation of the release lever 340, when the axle 342 is
clipped into the slots 346, allows the cam 344 to engage and
disengage the back surface of the indexing adjuster 310. Rotation
of the release lever 340 can thus be used to force the cam 344
against the back surface of the indexing adjuster 310 to separate
the complementary features 330 and 306. Once these features are
separated, the indexable part 300 can be indexed into a different
orientation and locked in that orientation by rotating the release
lever 340 to reduce the force applied by the cam 344. In other
words, the indexing adjuster 310 and release lever 340 allow a user
to axially separate the complementary features 330 and 306 to
enable the indexable part 300 to be rotated into a different
orientation relative to the second end 308 of the c-shaped member
304.
[0107] The indexable part 300 also comprises a yoke 384 to which a
v-shaped pin carrying member (not shown in FIG. 19 but included in
FIGS. 20 and 21 below) can be pivotally mounted in a similar manner
to that described above with reference to FIG. 5.
[0108] Referring next to FIG. 20, the indexing mechanism 328 of
FIG. 19 is shown when assembled. FIG. 20 shows the release lever
340 in the vertical or unlocked orientation in which the cam 344
and indexing adjuster 310 (not visible in FIG. 20) have forced the
indexable part 300 out of engagement with the features 330 provided
at the second end 308 of the c-shaped member 304. In this unlocked
configuration, the c-shaped member 304 may be indexed into the
required position.
[0109] Referring to FIG. 21, the indexing mechanism 328 of FIG. 19
is again shown when assembled but with the release lever 340 in the
horizontal or locked orientation in which its cam 344 (not visible
in FIG. 21) does not apply a disengaging force via the indexing
adjuster 310 (also not visible in FIG. 21). The features of the
indexable part 300 thus engage the features 330 provided at the
second end 308 of the c-shaped member 304. In this locked
configuration, the c-shaped member 304 is securely held in the
required indexed position.
[0110] A Hirth coupling of this type has the advantage of providing
accurate metrology whilst allowing multiple, repeatable, angular
orientations to be selected. In particular, the arrangement does
not necessarily require a separate locking piece because of the
angle-defining features 330 and 306 provided at the second end 308
of the c-shaped member 304 and the face plate 302 respectively.
These angle-defining features may, for example, be formed (e.g.
ground) in the frame and/or face plate or may be formed in a
component that is subsequently over-moulded into the end of the
frame or into the face plate. The Hirth coupling offers low levels
of backlash and self-centres. A further benefit of the Hirth
coupling is that it is self-locking and resistant to accidental
release when under load; the dimensions of the interlocking
elements are also selected such that the mechanism can only engage
in the allowed orientations (e.g. 0.degree. and).+-.90.degree..
Preferably, all of the components of the indexing mechanism 328 are
MRI benign.
[0111] A head clamp of the type described above comprises a
c-shaped member having an indexing mechanism at one end and a force
applicator mechanism at the other end. Each of these mechanisms
preferably carry one or more pins for engaging the skull of a
subject. Although these pins may be coated with a soft material,
they can still present a sharps risk to the patient when the head
clamp is being attached or removed.
[0112] The indexing mechanism of the head clamp is preferably
lockable in various indexed positions when not loaded (i.e. when
not attached to the skull of a subject). The various skull
attachment pins carried by the indexing mechanism are thus
prevented from rotating about the clamping axis when the head clamp
is being attached or removed. In contrast, the pins carried by the
force applicator mechanism are typically free to rotate about the
clamping axis until they engage the skull. This free rotation can,
in some instances, pose a sharps risk to the patient who may have
their forehead scored from a rotating yoke and associated skull
mounting pins during fitting or removal of the head clamp.
[0113] Referring to FIGS. 22 to 24, a yoke hold or anti-rotation
device 400 is illustrated for reducing the risk of skin injury
resulting from the free rotation of skull attachment pins that may
be permitted by the force applicator mechanism.
[0114] FIG. 22 shows the anti-rotation device 400 in an open
configuration. The anti-rotation device 400 comprises a first
portion 402 linked to a second portion 404 by a hinge 406. The
first and second portions 402 and 404 have a generally half
cylindrical shape. The first portion 402 comprises a cradle grab
408, a slot 410, a clasp 412 and a circumferential boss 417. The
second portion 404 comprises a circumferential boss 414, a
protrusion 415 and a pair of ledges 416 (only partially visible in
FIG. 20) that can mate with the clasp 412 of the first portion. The
anti-rotation device 400 may be formed as a single piece from, for
example, a suitable polymer. The anti-rotation device 400 may be a
single use item or a multi-use item that is suitable for
sterilisation.
[0115] As shown in FIGS. 23 and 24, the anti-rotation device 400
shown in FIG. 22 may be attached to the force applicator mechanism
16 described above with reference to FIGS. 1 to 4.
[0116] FIG. 23 shows the anti-rotation device 400 in its open
configuration with the first portion 402 placed into contact with
the force applicator mechanism 16.
[0117] FIG. 24 shows the anti-rotation device 400 after it has been
closed around the force applicator mechanism 16. The pair of ledges
416 and clasp 412 cooperate to provide a snap fit lock that retains
the anti-rotation device 400 in position. The slot 410 receives the
c-shaped member 4 and prevents rotation of the anti-rotation device
400 relative to that c-shaped member 4. The cradle grab 408 and
protrusion 415 of the anti-rotation device 400 engage the yoke 52
of the force applicator mechanism 16 and prevent it from rotating.
In this manner, the yoke 52 is locked in a single orientation
relative to the c-shaped member 4.
[0118] It should be noted that the anti-rotation device 400 can
travel back and forth along the clamping axis with the yoke 52. In
other words, the anti-rotation device 400 does not interfere with
the normal operation of the force applicator mechanism 16 (i.e.
which drives the yoke 52 back and forth). The anti-rotation device
400 can thus be used when attaching and/or detaching the head clamp
to a subject.
[0119] Removal of the anti-rotation device 400 (e.g. by unclipping
it) when the head clamp is secured to a subject permits the
c-shaped member to be indexed into the required orientation(s). In
this manner, the anti-rotation device 400 reduces the sharps risk
associated with using the head clamp without an adverse effect on
its function.
[0120] It should be noted that the anti-rotation device 400 is by
no means essential for operation of the head clamp. It should also
be noted that alternative means for preventing rotation of the yoke
carrying the skull attachment pins may be provided. For example,
the force applicator mechanism 16 may comprise an integral
anti-rotation component. Although the anti-rotation device 400 is
described for use with the force applicator mechanism 16, the
skilled person would also appreciate a similar device may be used
to prevent any unwanted rotation of other force applicator
mechanisms and/or the indexing mechanism. For example, a similar
anti-rotation device may be used for alternative indexing
mechanisms that have a freely rotatable yoke prior to engagement of
the head clamp with the skull.
[0121] It should again be noted that the above embodiments are
merely examples of the present invention. The skilled person would
be aware of the many variations and alternative embodiments that
would be possible.
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