U.S. patent number 3,848,273 [Application Number 05/327,803] was granted by the patent office on 1974-11-19 for shank for bone implants.
This patent grant is currently assigned to Sulzer Brothers Ltd.. Invention is credited to Otto Frey.
United States Patent |
3,848,273 |
Frey |
November 19, 1974 |
**Please see images for:
( Certificate of Correction ) ** |
SHANK FOR BONE IMPLANTS
Abstract
The shank is provided with texturized zones in the external
surface to insure uniform contact between the bone cement and the
shank when the shank is implanted. The texturized zones allow the
shank to be withdrawn readily from a bone and can be formed to
depths of 10 to 100.mu..
Inventors: |
Frey; Otto (Winterthur,
CH) |
Assignee: |
Sulzer Brothers Ltd.
(Winterthur, CH)
|
Family
ID: |
4213024 |
Appl.
No.: |
05/327,803 |
Filed: |
January 29, 1973 |
Foreign Application Priority Data
Current U.S.
Class: |
623/23.29 |
Current CPC
Class: |
A61F
2/30771 (20130101); A61F 2/3804 (20130101); A61F
2002/30838 (20130101); A61F 2002/30904 (20130101); A61F
2230/0004 (20130101); A61F 2002/3631 (20130101); A61F
2/4261 (20130101); A61F 2002/30906 (20130101); A61F
2002/4631 (20130101); A61F 2002/30136 (20130101); A61F
2/3094 (20130101); A61F 2/3662 (20130101) |
Current International
Class: |
A61F
2/30 (20060101); A61F 2/00 (20060101); A61F
2/42 (20060101); A61F 2/38 (20060101); A61F
2/46 (20060101); A61F 2/36 (20060101); A61f
001/24 () |
Field of
Search: |
;3/1
;128/92C,92CA,92B,92BA,92BC ;32/1A |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Vitallium Surgical Appliances, (catalog) by Austenal Co., New York,
N.Y. 1964, page 23, F. R. Thompson, Hip Prosthesis Relied Upon.
.
"Surgical Implants-The Role of Surface Porosity in Fixation to Bone
& Acrylic" by R. Peter Welsh et al., The Journal of Bone &
Joint Surgery, Vol. 53-A, No. 5, July 1971..
|
Primary Examiner: Gaudet; Richard A.
Assistant Examiner: Frinks; Ronald L.
Attorney, Agent or Firm: Kenyon & Kenyon Reilly Carr
& Chapin
Claims
What is claimed is:
1. A solid shank for implanting in a bone cement to anchor a bone
implant in a bone, said shank having a narrowing cross-sectional
area towards one end and an external non-porous surface having
discrete unconnected depressions therein between raised portions of
said shank, said depressions being between 10 microns and 100
microns depth.
2. A shank as set forth in claim 1 wherein said depressions are
separated by rounded portions to merge together smoothly.
3. A shank as set forth in claim 1 wherein said depressions extend
into said surface for a depth of between 20 microns and 30
microns.
4. A shank as set forth in claim 1 having a longitudinal axis and
wherein said surface includes at least two load bearing zones
having said depressions therein, each zone having a regular
toothing extending transversely of said longitudinal axis to form
said depressions.
5. A shank as set forth in claim 4 wherein said regular toothing
includes transverse rows of spaced apart teeth with the spacing
between discrete teeth increasing toward said one end of said
shank.
6. A shank as set forth in claim 1 wherein said zones includes a
plurality of regularly disposed trough-shaped depressions.
7. A shank as set forth in claim 1 having a plurality of
longitudinal side walls defining a generally trapezoidal
cross-sectional area, at least two of said side walls having said
depressions therein.
8. A shank as set forth in claim 10 wherein said depressions are
each formed as a single shell of a length (u) to width (v) to depth
(t) ratio of 12:4:1.
Description
This invention relates to a shank for a bone implant.
Bone implants, such as joints for the hip, elbow and wrist, usually
have shanks which are engaged in corresponding passages in the
particular bone concerned and are anchored in the passage by means
of a quick-setting bone cement such as methylmethacrylate. It has
been found that the bone cement, while curing, often changes volume
either by increasing or, and more frequently, by contracting.
Because of this, and particularly where the volume changes occur
lengthwise of the shank, the volume changes are very often the
cause of the bone cement becoming detached from some parts of the
shank surface. The parts affected then cease to be able to transmit
any load from the shank to the bone. This, in turn, leads to
recession of the bones in the parts concerned, so that the shank
starts to work loose. The load also tends to become increasingly
concentrated in the remaining unloosened parts of the shank, often
to such an extent that the shank ruptures.
A main reason for the above problems is that present-day bone
cements cure rapidly and are relatively highly viscous plastics
with poor contact properties, i.e., the cements do not readily form
an accurate "negative" of the shank surface.
Daily practice with bone implants and their shanks has also shown
the need for the implant and the anchorage of the implant to be so
devised that the implant can be withdrawn readily from the bone at
any time without damage to the bone surrounding the shank.
Accordingly, it is an object of the invention to provide a shank
for an implant which is capable of compensating for changes in
volume in a bone cement during setting.
It is another object of the invention to preclude the loosening of
bone implant shanks in a bone due to changes in the volume of a
bone cement.
Briefly, the invention provides a shank having texturized zones of
depressions in an external surface for anchoring a bone implant in
a bone. The zones are generally disposed in the load bearing zones
of the surface of the shank while the shank narrows in
cross-section toward one end.
In use, the shank is secured in a bone by a suitable bone cement so
that the cement fills the spaces within the texturized zones of
depressions.
Advantageously, the depth of the depressions in each zone is
between 10 and 100 microns (.mu.). Shallower depths lead to
unreliable adhesion between the cement and the shank surface, and
depths of more than 100 microns (.mu.) readily result in air
bubbles remaining when the shank is driven into the bone cement,
the bubbles impairing adhesion. Also, the possibility of
withdrawing the shank from the bone is impaired in increasing
proportion as the depressions are made deeper. The optimum depth is
governed very largely by the particular bone cements used, more
particularly their viscosity, changes in volume and their ability
to make good contact, i.e., the extent to which they can mate
accurately with a texture on the shank surface to provide a
faithful copy "in reverse" of the texture.
In order to ensure satisfactory adhesion, at least the bearing and
load-taking surfaces are provided with the texturized zones of
depressions. For example, in one embodiment these surfaces are
provided with regular toothing extending transversely of the
direction in which the implant is introduced into and withdrawn
from the bone. In this event, to facilitate withdrawal, the
distance between discrete teeth can increase towards the narrow end
of the shank.
Textures in the form of recesses of a shape resembling shells or
troughs have proved satisfactory in other embodiments of the
invention. Both kinds of texture can, of course, be used with
advantage simultaneously. For instance, in the case of a multiple
polygonal shank, the load-taking and the bearing surfaces can have
toothing while the other surfaces can have shell-like recessings.
To further improve adhesion, the texturized shank surface can also
be roughened, e.g. by sand blasting with 0.05 to 0.2 millimeters
(mm) diameter silica pellets.
Conveniently, to improve the fatigue strength of the shank, the
edges and corners at junctions between the various surfaces which
make up a multi-surface polygonal shank, and the raised and
recessed junctions between the various teeth and between and in the
troughs, are very rounded and free from projections.
These and other objects and advantages of the invention will become
more apparent from the following detailed description and appended
claims taken in conjunction with the accompanying drawings in
which:
FIG. 1 illustrates a sectional view through a femur fitted with a
prosthetic head having a shank according to the invention;
FIG. 2 diagrammatically illustrates a sectional view through an arm
fitted with an artificial elbow joint using shanks in accordance
with the invention;
FIG. 3 diagrammatically illustrates a sectional view of a
prosthetic wrist joint fitted in a forearm and a carpal bone using
shanks according to the invention;
FIG. 4 illustrates a considerably enlarged and diagrammatic plan
view of a texturized zone of depressions in the form of regularly
disposed trough-like recesses;
FIG. 5 illustrates a vew taken on line V--V of FIG. 4;
FIG. 6 illustrates a side elevational view of a cutting bit for
forming the depressions shown in FIGS. 3 and 4; and
FIG. 7 illustrates a plan view of the bit of FIG. 6.
Referring to FIG. 1, a prosthetic femur head 1 has been implanted
in a femur 2 which has been appropriately prepared by surgery. The
head 1 includes a shank 3 anchored by means of a bone cement 4,
e.g. methyl methacrylate, in a passage or recess 5 in the femur
bone 2 whose relatively compact cortical substance is shown more
darkly dotted than the porous spongy substance. In the part near
the head 1, the shank 3 has a polygonal large-area shape with
rounded corners and edges. In order to prevent accidental turning,
the shank cross-sectional shape resembles a kite (FIG. 6) and in
the end-distal from the exposed end merges, on the assumption that
the shape narrows on all sides and continuously, into a
substantially trapezoidal cross-section having rounded edges.
The shank loosening previously referred to occurs, in the
artificial hip joint shown, more particularly in region A of
bearing or support surface 6, to spread in the course of time more
particularly to region B of load-taking surface 7.
The surface of the shank 3 which is non-porous has a texturized
zone which is, indicated in FIG. 1 by trough-like discrete
unconnect recesses or depressions between teeth of a regular or
uniformly formed toothing. An endeavor is also made to show, in
diagrammatic and sketch form, the toothing formed as texturizing on
the surfaces 6, 7 which appear just as section lines. Also
indicated is an increase in tooth spacing towards the exposed end
of shank 3.
Referring to FIG. 2 as another example of implant shanks having
texturized surfaces, an elbow joint 8 has a shank 9 on one part
which shank 9 narrows on all sides. The shank 9 is introduced into
a recess or passage or the like in a humerus 10 and retained by
bone cement 4. A shank 11 on the other part of the joint 8 is
anchored similarly in an ulna 12.
Referring to FIG. 3, for a wrist joint 13, one shank 14 is retained
by bone cement 4 in a radius bone 25 and another shank 26 is
anchored by bone cement 4 in a carpal bone and/or metacarpal bone
27.
Referring to FIGS. 4 and 5 which are views to a considerably
enlarged scale, the texturizing can take the form e.g. of
trough-like or shell-like recesses or depressions 15 disposed
regularly in rows a, b, c. If the raised parts between the discrete
shells 15 are also removed inside the various rows a, b, c, the
resulting toothing is very advantageous for the surfaces 6 and 7.
The advantages of toothing are that, in cooperation with a shank
shape that narrows on all sides and continuously, a fitted shank
can be removed simply by applying a force sufficient just to
release the shank by one toothing step in the withdrawal direction.
Thereafter, the shank can readily be withdrawn from the cement
bed.
Referring to FIG. 5, the shank material 16 is usually one of the
known metal alloys which are conventionally used for bone implants.
As already emphasized, the depth t of the depressions in the
texturized zones can be between 10 and 100 .mu.. From 20 to 30
microns (.mu.) has been found a very good value for the depth t in
existing shanks where methyl methacrylate is used as the bone
cement. Experiments by the Applicant has also shown that
satisfactory dimensions for a single shell of shell-like
texturizing are given by a ratio of length u to width v to depth t
of 12 : 4 : 1.
The surface 17 of the texturized shank 3 has the texturizing
roughened as well, to further improve adhesion between the cement 4
and the shank 3. That is, the portions between the depressions 15
which are rounded so that the depressions merge together smoothly,
can be roughened. As mentioned, this roughness is produced by
blasting with silica pellets of from 0.05 to 0.2 millimeters (mm)
diameter.
A technique which has proved satisfactory for producing the
shell-like texturing, is the cutting or milling of the periphery of
the shank transversely of a longitudinal direction. A suitable tool
20 for this can be seen in FIGS. 6 and 7. The cutting bit has, at
the junction between the cylindrical portion and the circular end
face, a radius r of curvature which is adapted to the bit diameter
d and to the required trough size. Also, the bit has a central
plane surface s which is also of a particular diameter. As FIG. 7
shows, the bit 20 has six lips 21 a, 21 b. Two lips 21 a are
disposed opposite one another to cover the whole bit along a
diameter, whereas the other four lips 21 b extend only to the edge
of the plane surface. While being texturized, the shank 3 is
positioned relatively to the cutter bit 20 in the manner visible in
FIG. 6.
As already mentioned, the trough depth t arising from the cutting
or milling operation is mainly governed by the cutting radius r.
The length u can be controlled to some extent by cutter speed
and/or the rate of feed, and the width v can be varied within
limits by varying the "line spacing" of the cutting operation. The
same tool 20 can be used to produce toothing, but instead of
cutting being performed by the cutting edges of radius r, the
cutting is performed by the side walls of the cutting edges.
As an example, for a texturizing of the kind specified with the
ratio of u to v to t .apprxeq. 12 : 4 : 1 and with t to .apprxeq.
20 microns (.mu.), the cutter 20 has the diameter d of 8
millimeters (mm), the radius r of curvature of approximately 3.25
millimeters (mm) and the diameter of the plane surface s of
approximately 1.5 millimeters (mm). The feed in this case is 250
mm/min. and cutter speed is approximately 8 meters/minute
(m/min).
The texturizing can also be formed in the shank surface e.g. by
means of forming punches.
* * * * *