U.S. patent application number 11/828423 was filed with the patent office on 2008-01-24 for implant.
This patent application is currently assigned to AstraZeneca AB. Invention is credited to Stig HANSSON.
Application Number | 20080020344 11/828423 |
Document ID | / |
Family ID | 20412114 |
Filed Date | 2008-01-24 |
United States Patent
Application |
20080020344 |
Kind Code |
A1 |
HANSSON; Stig |
January 24, 2008 |
IMPLANT
Abstract
Implant (10) having a shaft which is adapted in use to be
embedded in bone tissue and which has an outer surface provided
with a circumferentially-oriented roughness. The
circumferentially-oriented roughness has first and second axial
sections (19, 21) with each section comprising a series of
circumferentially-oriented peaks which have a crest and which are
axially spaced apart by troughs. The axial spacing (d) between the
crests of adjacent peaks in the first axial section (19) is less
than the axial spacing (3d) between the crests of adjacent peaks in
the second axial section (21). Although the axial spacing between
the crests of adjacent peaks in the first and second axial sections
of circumferentially-oriented roughness differs, the first and
second axial sections of circumferentially-oriented roughness are
adapted in use to provide the same or substantially the same
pitch.
Inventors: |
HANSSON; Stig; (Askim,
SE) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT & DUNNER;LLP
901 NEW YORK AVENUE, NW
WASHINGTON
DC
20001-4413
US
|
Assignee: |
AstraZeneca AB
|
Family ID: |
20412114 |
Appl. No.: |
11/828423 |
Filed: |
July 26, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10361383 |
Feb 10, 2003 |
7264470 |
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11828423 |
Jul 26, 2007 |
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09402918 |
Oct 13, 1999 |
6547564 |
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PCT/SE99/01272 |
Jul 14, 1999 |
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10361383 |
Feb 10, 2003 |
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Current U.S.
Class: |
433/173 ;
433/174; 623/17.17 |
Current CPC
Class: |
A61C 8/0022 20130101;
A61B 17/863 20130101; A61C 8/0025 20130101 |
Class at
Publication: |
433/173 ;
433/174; 623/017.17 |
International
Class: |
A61C 17/00 20060101
A61C017/00; A61F 2/02 20060101 A61F002/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 17, 1998 |
SE |
9802571-1 |
Claims
1-28. (canceled)
29. A method of manufacturing a dental implant, comprising:
providing a shaft having a coronal end, an apical end, and an outer
surface; creating a circumferentially-oriented roughness on a first
axial section of the outer surface of the implant; and creating a
circumferentially-oriented roughness on a second axial section of
the outer surface of the implant, the first axial section being
located coronally of the second axial section, wherein the
circumferentially-oriented roughness of each of the first and
second axial sections has the same or substantially the same pitch,
wherein the circumferentially-oriented roughness of each of the
first and second axial sections comprises a series of
circumferentially-oriented peaks being axially spaced apart by
troughs, wherein the axial spacing between adjacent peaks in the
first axial section is less than the axial spacing between the
adjacent peaks of the second axial section, wherein the dental
implant is configured to support one or more artificial teeth.
30. The method of claim 29, wherein the circumferentially-oriented
roughness of the first axial section has a height, from trough to
crest of the peak, of about 0.2 mm or less.
31. The method of claim 30, wherein the circumferentially-oriented
roughness of the first axial section has a height, from trough to
crest of the peak, less than that of the second axial section.
32. The method of claim 30, wherein the height, from trough to
crest of the peak, of the second axial section ranges from about
0.15 to about 1 mm.
33. The method of claim 30, wherein the circumferentially-oriented
roughness of at least one of the first and second axial sections is
a screw thread profile.
34. The method of claim 30, wherein the circumferentially-oriented
roughness of at least one of said first and second axial sections
is a series of axially spaced-apart circumferential lines of
beads.
35. The method of claim 30, further comprising creating a blind
bore that extends apically into the shaft from the coronal end to a
surface in-between the apical and coronal ends of the shaft.
36. The method of claim 35, further comprising internally threading
the blind bore.
37. The method of claim 36, wherein the internally-threaded section
of the blind bore terminates at a position disposed apically of the
first axial section.
38. The method of claim 30, further comprising creating at least
one of the circumferentially-oriented peaks such that it is
continuous between the first and second axial sections.
39. The method of claim 30, wherein the pitch is a predetermined
distance, the ratio of the predetermined distance to the axial
spacing between the crests of adjacent peaks in the first axial
section is a first multiple integer, the ratio of the predetermined
distance to the axial spacing between the crests of adjacent peaks
in the second axial section is a second multiple integer and the
first multiple integer is greater than the second multiple
integer.
40. The method of claim 30, wherein the crests of substantially all
of the peaks in the first axial section lie in an axial plane
parallel to the main axis of the shaft.
41. The method of claim 30, wherein the crests of substantially all
of the peaks in the first second section lie in an axial plane
parallel to the main axis of the shaft.
42. The method of claim 30, wherein the peaks in the first axial
section are at a common angle to a main axis of the implant.
43. The method of claim 30, wherein the troughs of substantially
all of the peaks in the first axial section lie in an axial plane
parallel to the main axis of the shaft.
44. The method of claim 30, wherein the troughs of substantially
all of the peaks in the first or second section lie in an axial
plane parallel to the main axis of the shaft.
45. The method of claim 30, wherein the first axial section has a
substantially cylindrical configuration.
46. The method of claim 30, wherein substantially all of the crests
in the first axial section and the second axial section are
configured to be screwed into bone tissue.
47. The method of claim 30, wherein the first axial section is
configured to substantially inhibit marginal bone resorption.
48. The method of claim 30, wherein the first axial section and the
second axial section are configured to substantially prevent
fractures from being formed in bone tissue.
49. The method of claim 30, wherein each of the peaks includes a
coronal flank and an apical flank, wherein the coronal flank and
the apical flank form an angle of about 80 degrees.
50. The method of claim 30, wherein each of the peaks includes a
coronal flank, an apical flank, and a curved portion between the
coronal flank and the apical flank.
51. The method of claim 50, wherein the curved portion is disposed
between the coronal flank and the apical flank of one peak.
52. The method of claim 30, wherein a height of the peaks, as
measured from trough to crest of the peak, of the second axial
section is greater than about 0.2 mm.
53. The method of claim 30, wherein the height of the peaks, as
measured from trough to crest of the peak, of the second axial
section is about 0.3 mm.
54. The method of claim 30, wherein each of the peaks includes a
coronal flank and an apical flank, wherein a portion between the
coronal flank and the apical flank of substantially all of the
peaks substantially lack sharp edges.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to an implant having a shaft
which is adapted in use to be embedded in bone tissue and which has
an outer surface provided with a circumferentially-oriented
roughness. This will hereinafter be referred to as an "implant of
the type defined".
BACKGROUND OF THE INVENTION
[0002] Implants of the type defined are known for use as the
anchoring members of dental and orthopaedic prostheses. To this
end, the implant is inserted into a bore-hole drilled into the bone
tissue of a bone tissue structure at a site where a prosthesis is
required, ordinarily by screwing of the implant into the bore-hole.
The convention in the art is for the circumferentially-oriented
roughness to take the form of a screw thread and in this case the
bore-hole will ordinarily be (i) provided with internal threads in
advance, or (ii) left un-tapped with the implant being provided
with a self-tapping capacity, e.g. by the provision of one or more
axially-extending cutting recesses or notches in the screw
thread.
[0003] A superstructure having the prosthetic part of the
prosthesis is then secured to the implant. In the case of a dental
prosthesis, the superstructure will typically consist of a spacer
or transmucosal component which engages to the implant to bridge
the gingiva overlying the maxilla or mandible at the implant site
and the prosthetic part, e.g. a crown, bridge or denture, is then
secured to the spacer. There are various other forms that the
superstructure can take as is known in the art. For instance, the
prosthetic part may be secured directly to the implant.
[0004] The long-term integrity of the prosthesis is highly
dependent on the successful osseointegration of the implant with
the bone tissue structure, that is to say, the remodelling of the
bone tissue in the bone tissue structure into direct apposition
with the implant. A study on the factors which affect the
osseointegration of implants was undertaken by Professor Per-Ingvar
Branemark and co-workers and the results were published in a book
entitled "Osseointegrated Implants in the Treatment of the
Edentulous Jaw: Experience from a 10-Year Period". Almqvist &
Wiskell International, Stockholm, Sweden, 1977. It was found by
Branemark et al that successful osseointegration depends upon inter
alia the use of biocompatible materials for the implant, for
example titanium and alloys thereof, and the surgical procedure
adopted, for example leaving the implant unloaded for several
months before adding the superstructure.
[0005] Implants of the type defined are not necessarily always used
as part of a prosthesis, in some instances they can be a "stand
alone" structure. As an example, implants of the type defined are
known for use as bone fixation screws. The success of these "stand
alone" implants is also highly dependent on their successful
osseointegration.
[0006] Implants of the type defined have some notable advantages in
promoting successful osseointegration with the adjacent bone
tissue, a major one being as a result of the fact that the main
loads on the implant in the clinical situation are axial loads.
These implants are very well suited to support axial loads and this
may be particularly important in the initial stages of the
osseointegration process in which it is important that the implant
is fully stable and as immovable as possible in the borehole
(primary fixation). One can consider this to be due to the bone
tissue growing into the troughs between adjacent peaks of the
circumferentially-oriented roughness on the implant.
[0007] The Applicant has also identified that it is advantageous
for an implant of the type defined to transmit the axial loading
thereon evenly to the adjacent bone tissue to prevent high stress
concentrations occurring in the adjacent bone tissue and
concomitantly marginal bone tissue resorption. If marginal bone
tissue resorption occurs this will reduce the anchorage of the
implant and may undermine the long-term stability of the implant
resulting in due course in failure of the prosthesis. In the
particular case of dental prostheses, the aesthetic appeal is also
undermined by marginal bone tissue resorption, an important
drawback since dental prosthetics forms part of the field of
cosmetic surgery.
[0008] The present invention proposes to provide an implant of the
type defined having features which promote its maintenance in a
bone tissue structure whilst at the same time facilitating its
insertion into the bone tissue structure in the first place.
SUMMARY OF THE INVENTION
[0009] According to the present invention there is provided an
implant of the type defied in which the circumferentially-oriented
roughness has first and second axial sections each comprising a
series of circumferentially-oriented peaks which have a crest and
which are axially spaced apart by troughs, the axial spacing
between the crests of adjacent peaks in the first axial section is
less than the axial spacing between the crests of adjacent peaks in
the second axial section and the first and second axial sections of
circumferentially-oriented roughness are adapted in use to provide
the same or substantially the same pitch.
[0010] The larger inter-peak spacing in the second axial section of
circumferentially-oriented roughness acts to promote primary
fixation of the implant in the bone tissue during the early phases
of osseointegration since each trough between adjacent peaks can
capture a relatively large volume of bone tissue to interlock the
implant with the bone tissue. The smaller inter-peak spacing in the
first axial section, on the other hand, enables the stiffness of
the implant to be increased thereby improving the ability of the
implant to transmit loads more evenly to the bone tissue to inhibit
marginal bone resorption. Adapting the first and second axial
sections to have the same or substantially the same pitch means
that both axial sections produce the same or substantially the same
axial displacement into the bone tissue on one rotation thereof
thus ensuring that the provision of the two different axial
sections of circumferentially-oriented roughness does not
complicate insertion of the implant into the bone tissue. If the
first and second axial sections of circumferentially-oriented
roughness did not have the same or substantially the same pitch
then a greater force would need to be applied to insert the implant
resulting in fractures being formed in the bone tissue.
[0011] In an embodiment of the invention such as the one
hereinafter to be described the pitch is a predetermined distance,
the ratio of the predetermined distance to the axial spacing
between the crests of adjacent peaks in the first axial section is
a first multiple integer and the ratio of the predetermined
distance to the axial spacing between the crests of adjacent peaks
in the second axial section is a second multiple integer which is
less than the first multiple integer. The first multiple integer
may be a multiple integer of the second multiple integer.
[0012] In an embodiment of the invention such as the one
hereinafter to be described the peaks in the first and second axial
sections are circumferentially-oriented at a common inclined angle
to the main axis of the implant.
[0013] In an embodiment of the invention such as the one
hereinafter to be described the shaft has a coronal end and an
apical end and the first axial section is located coronally of the
second axial section.
[0014] In an embodiment of the invention such as the one
hereinafter to be described the first and second axial sections are
contiguous.
[0015] In an embodiment of the invention such as the one
hereinafter to be described the first axial section extends from
the coronal end of the shaft to a position coronally of the apical
end and the second axial section extends from the first axial
section towards the apical end of the shaft. The implant may have a
coronal end which is spaced coronally from the coronal end of the
shaft by a smooth coronal portion of the implant, as in the
embodiment of the invention hereinafter to be described, in which
case the smooth coronal portion is preferably no more than 4% of
the total length of the implant, more preferably in the range
1.5-3.7% of said total length.
[0016] In an embodiment of the invention such as the one
hereinafter to be described the axial extent of the first axial
section is greater than the axial extent of the second axial
section. Alternatively, the axial extent of the first axial section
may be less than the axial extent of the second axial section or
the axial extents of the first and second axial sections may be the
same or substantially the same.
[0017] In an embodiment of the invention in which the first axial
section is disposed coronally of the second axial section, such as
the one hereinafter to be described, a blind bore extends apically
into the shaft from the coronal end thereof to an end surface
in-between the apical and coronal ends of the shaft for a
superstructure to be secured to the implant, the blind bore
comprising an internally-threaded section having a coronal edge and
an apical edge for screw connection of the superstructure to the
implant with the apical edge terminating at a position which is
disposed apically of the first axial section. Alternately, the
apical edge of the internally-threaded section of the blind bore
may terminate at a position which is disposed coronally of the
second axial section. The internally-threaded section may be an
apical section of the blind bore, as in the embodiment of the
invention hereinafter to be described.
[0018] In an embodiment of the invention such as the one
hereinafter to be described all or substantially all of the crests
of the peaks in the first and second axial sections lie on an axial
plane parallel to the main axis of the shaft. Expressed another
way, the major transverse dimension of the implant at the first and
second axial sections is uniform.
[0019] In an embodiment of the invention such as the one
hereinafter to be described the height of the peaks, as measured
from the troughs to the crests, in the first axial section differs
from that in the second axial section. To advantage, the height of
the peaks in the first axial section is less than that in the
second axial section. This feature further enables the stiffness of
the implant to be increased.
[0020] In an alternative embodiment of the invention the height of
the peaks, as measured from the troughs to the crests, in the first
axial section is the same or substantially the same as in the
second axial section.
[0021] In an embodiment of the invention such as the one
hereinafter to be described the ratio of the height of the peaks,
as measured from the troughs to the crests, to the axial spacing
between the crests of adjacent peaks in the first axial section is
the same or substantially the same as in the second axial
section.
[0022] In an embodiment of the invention such as the one
hereinafter to be described the height of the peaks, as measured
from the troughs to the crests, in the first axial section is no
greater than 0.2 mm, for example in the range 0.02-0.20 mm, and the
height of the peaks, as measured from the troughs to the crests, in
the second axial section is greater than that in the first axial
section, for instance in the range 0.15 mm to 1 mm. Such heights
complement the primary fixation and stiffness characteristics of
the implant provided by the different inter-peak spacings of the
first and second axial sections.
[0023] In an embodiment of the invention such as the one
hereinafter to be described the peaks in the first and second axial
sections are bounded by flank surfaces and the angle between the
opposed flanks of adjacent peaks in the first and second axial
sections is the same.
[0024] In an embodiment of the invention such as the one
hereinafter to be described the troughs in at least one of the
first and second axial sections are a continuous curved
surface.
[0025] In an embodiment of the invention such as the one
hereinafter to be described the circumferentially-oriented
roughness in the first and/or second axial section is presented by
a screw thread profile with the circumferentially-oriented peaks
being defined by thread elements of the screw thread profile.
[0026] Typically, the screw thread profile of the first and/or
second axial section will be formed by a screw thread structure. In
such case, the screw thread structure of the first axial section
may be formed by a first set of independent screw threads each
having turns; the turns of each independent screw thread in the
first set defining thread elements in the first axial section and
being sequentially arranged with the turns of the other independent
screw threads in the first set with adjacent turns of one of the
independent screw threads of the first set being axially-spaced
apart by a predetermined spacing distance which is the same for
adjacent turns of the other independent screw threads in the first
set; and the screw thread structure of the second axial section may
be formed by (i) an independent screw thread having turns which
define the thread elements of the second axial section and are
axially-spaced apart by the predetermined spacing distance or
essentially the predetermined spacing distance, or (ii) a second
set of independent screw threads numbering less than in the first
set each having turns, the urns of each independent screw thread in
the second set defining thread elements in the second axial section
and being sequentially arranged with the turns of the other
independent screw threads in the second set with adjacent turns of
each independent screw thread of the second set being
axially-spaced apart by the predetermined spacing distance or
essentially the predetermined spacing distance.
[0027] In an embodiment of the invention one or more of the
independent screw threads of the first and second axial sections
are shared by the first and second axial sections.
[0028] In an embodiment of the invention such as the one
hereinafter to be described the or each independent screw thread of
at least one of the first and second axial sections is a
microthread, that is to say, a thread having a height which is no
greater than 0.2 mm.
[0029] In an embodiment of the invention only the screw threads of
the first axial section are microthreads. It could be the case,
though, that the screw threads of both the first and second axial
sections are microthreads.
[0030] In an embodiment of the invention the
circumferentially-oriented roughness in at least one of the first
and second axial sections is formed by a series of axially
spaced-apart circumferential lines of beads. The beads in each line
may be circumferentially spaced-apart.
[0031] In an embodiment of the invention such as the one
hereinafter to be described the implant is a dental implant adapted
for implantation in the maxilla or mandible of an edentulous
patient for supporting a superstructure which presents one or more
artificial teeth.
[0032] According to the invention there is further provided a
method of implanting an implant into a bone tissue structure
comprising the steps of providing an implant according to the
invention, providing a bore-hole in the bone tissue structure and
screwing the implant into the bore-hole so that the shaft is
embedded in the bone tissue.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] By way of example, a self-tapping endosseous screw-type
dental implant in accordance with the present invention will now be
described with reference to the accompanying Figures of drawings in
which:
[0034] FIG. 1 is a side view of the dental implant;
[0035] FIG. 2 is a perspective view of the dental implant;
[0036] FIG. 3 is a cross-sectional side view of the dental
implant;
[0037] FIG. 4 is a plan view of the dental implant;
[0038] FIG. 5 is an underneath view of the dental implant;
[0039] FIG. 6 is an exploded view of a first section of the
external screw threading on the dental implant; and
[0040] FIG. 7 is an exploded view of a second section of the
external screw threading on the dental implant.
DESCRIPTION OF EXEMPLARY EMBODIMENT OF THE INVENTION
[0041] In the accompanying Figures of drawings there is shown
various views of a self-tapping endosseous screw-type dental
implant 10 of a dental prosthesis in accordance with the present
invention. The implant 10 is for insertion into a bore-hole drilled
into a toothless-site in a maxilla or mandible of a partially or
fully edentulous patient to anchor to the maxilla or mandible a
superstructure of the prosthesis which comprises a prosthetic part,
namely one or more artificial teeth. The implant 10 is made from
commercially pure titanium, a titanium alloy, another biocompatible
metal or metal alloy or a ceramic to promote osseointegration of
the implant with the bone tissue of the boundary walls of the
bore-hole.
[0042] Referring to FIG. 1, the implant 10 has an apical end 1
which is presented by a first conical section 3 to ease insertion
of the implant 10 into the bore-hole, a coronal end 5 presented by
a second conical section 6 and an intermediate section 7 of
constant diameter which extends between the first and second
conical sections 3, 6.
[0043] The length of the implant may be in the range 8-19 mm,
depending on the clinical situation, and have a maximum outer
diameter of 3.5 mm or 4.0 mm. The axial extent of the second
conical portion 6 is preferably small compared to the total length
of the implant 10, as an example no more than 4.0% perhaps in the
range 1.5%-3.7%.
[0044] Turning to FIGS. 2 to 4, a socket 9 having an open end 11 in
the coronal end 5 extends apically into the implant 10. The socket
9 is for receiving an abutment structure (not shown) which will
bridge the gingiva overlying the bore-hole and support/present the
prosthetic part. The socket 9 consists of a conical coronal section
13, an internally-threaded apical section 15 and a cylindrical
intermediate section 17. The abutment structure will have an apical
section which is able to be screw retained in the implant socket 9
for releasably securing the abutment structure to the implant
10.
[0045] As shown in FIGS. 1 to 3, 6 and 7, the outer surface of the
implant 10 over the major part of its length is provided with screw
threading which is divided into coronal and apical sections 19, 21
having different thread heights h1, h2. As shown most dearly in
FIG. 1, the coronal section 19 of screw threading is positioned on
the intermediate cylindrical section 7 of the implant 10 whereas
the apical section 21 of the screw threading bridges the
intermediate cylindrical section 7 and the first conical section
3.
[0046] Referring to FIG. 6, the screw threading in the coronal
section 19 is composed of a series of axially spaced-apart screw
thread elements each having the same height h1. These screw thread
elements are formed by the turns of three separate microthreads
(triple microthread) which are sequentially arranged. This means
that a screw thread element formed by a first turn of one of the
microthreads is axially spaced from a screw thread element formed
by the next turn of that microthread by two other screw thread
elements, each being respectively formed by a turn of the other two
microthreads. A screw thread element belonging to one of the
microthreads is therefore axially spaced from the next adjacent
screw thread element formed by the same microthread by screw thread
elements from each of the other two microthreads. By "microthread"
is meant a screw thread having a height which is no greater than
0.2 mm. Accordingly, the screw thread elements in the coronal
section 19 have a height h1 which is no greater than 0.2 mm,
preferably 0.1 mm.
[0047] Referring to FIG. 7, the screw threading in the apical
section 21 is composed of a series of axially spaced-apart screw
thread elements which, other than those in the first conical
section 3, each have the same height h2. The screw thread elements
of the apical section 21 are formed by the turns of a single
macrothread. By "macrothread" is meant a screw thread having a
height greater than 0.2 mm. Accordingly, the screw thread elements
of the apical section 21 on the intermediate section 7 have a
height greater than 0.2 .mu.m, preferably 0.3 mm.
[0048] The angle formed between the coronal and apical flanks of
adjacent screw thread elements is the same in both the coronal and
apical sections 19, 21. Preferably the angle formed is 80.degree..
It will also be noted from FIGS. 6 and 7 that the coronal and
apical flanks of adjacent screw thread elements in the coronal and
apical sections 19, 21 are connected by a curved surface, that is
to say, there is no axial straight part in-between adjacent screw
thread elements in the coronal and apical sections 19, 21.
[0049] As can be seen particularly from FIGS. 1 and 3, the tips of
the screw thread elements of the coronal section 19 and the tips of
the screw thread elements of the apical section 21 positioned in
the intermediate cylindrical section 7 of the implant 10 all lie on
a common plane when viewed in side section and circumscribe the
circumference of the cylindrical intermediate section 7. In other
words, the major diameter of the intermediate cylindrical section 7
is constant.
[0050] As shown in FIGS. 6 and 7, as well as the screw thread
elements in the coronal and apical sections 19, 21 having different
heights from one another the crest-to-crest spacing between
adjacent screw thread elements in the coronal section 19 is
different from the crest-to-crest spacing between adjacent screw
thread elements in the apical section 21. The crest-to-crest
spacing in the coronal section 19 is d whereas the crest-to-crest
spacing in the apical section 21 is 3d. As an example, d may be
0.22 mm. In the case where h1 is 0.1 mm and h2 is 0.3 mm the ratio
of the inter-crest spacing to the height would thus be the same for
both the coronal and apical threaded sections 19, 21, namely
d/h1=2.2=3d/h2.
[0051] It follows from the above that the crest-to-crest spacing
between adjacent screw thread elements of each microthread is the
same as that between adjacent screw thread elements of the
macrothread, namely 3d. The fact that the crest-to-crest spacing
between adjacent screw thread elements per se in the coronal
section 19 is less than that in the apical section 21 is, of
course, due to adjacent turns of each microthread being
interspersed with a turn from each of the other two microthreads.
It will also be noted from FIG. 1 that the turns of the
macrothreads and the macrothreads are aligned parallel with one
another at an inclined angle to the rotational axis of the implant
10.
[0052] It will gathered from the above that the pitch of the
coronal and apical threaded sections 19, 21 will be the same, again
being 3d. For this reason, the pitch of the implant 10 remains
uniform along its length notwithstanding the difference in
crest-to-crest spacing in the apical and coronal threaded sections
19, 21, that is to say, the coronal and apical screw threaded
sections 19, 21 will both produce the same axial displacement of
the implant 10 in the apical direction on one rotation or
revolution of the implant 10 when being screwed into the borehole
provided therefor at the toothless site in the maxilla or mandible.
If the coronal and apical sections 19, 21 did not have constant
pitch then a greater force would need to be applied to insert the
implant 10 into the bore-hole resulting in the bone threads formed
in the boundary wall of the bore-hole being fractured.
[0053] As a rule, a constant pitch for two threaded sections having
different crest-to-crest spacings between the adjacent screw thread
elements thereof will result where the first threaded section is
formed by the sequential arrangement of the turns of a first set of
screw threads each having the same pitch and the second threaded
section is formed by (i) a single screw thread having the same
pitch as the screw threads in the first threaded section, or (ii)
the sequential arrangement of the turns of a second set of screw
threads numbering less than in the first set each having the same
pitch as the screw threads in the first threaded section. The
number of screw threads in the first threaded section does not need
to be a multiple integer of the number of screw threads in the
second threaded section, as in the illustrated embodiment of the
invention. For example, there could be five microthreads in the
coronal section 19 and two macrothreads in the apical section
21.
[0054] As shown in FIGS. 1 to 3 and 5, the implant 10 has three
cutting recesses or grooves 23a, 23b, 23c positioned symmetrically
about the circumference of the apical end 1 of the implant 10 for
self-tapping of the implant to when being screwed into the
bore-hole provided therefor in the maxilla or mandible.
[0055] In use, the implant 10 is screwed into the bore-hole
provided at the toothless-site in the maxilla or mandible such that
the coronal and apical sections 19, 21 are embedded in bone tissue
with the second conical section 6 protruding from the maxilla or
mandible. The screw thread elements of the macrothreads in the
apical section 21 of the implant 10 act to provide primary fixation
of the implant in the bore-hole. The screw thread elements of the
microthreads in the coronal section 19 also act to provide fixation
for the implant 10 in the bore-hole. As a result of the screw
threads in the coronal section 19 being microthreads, though, the
implant 10 is stiffer than it would be if the screw threads were
macrothreads as in the apical section 21. This enables the implant
10 to transfer loads more evenly to the bone tissue adjacent the
implant 10 and consequently promote better remodelling of the bone
tissue into apposition with the implant 10. Moreover, as the
microthreads are positioned at the coronal end 5 of the implant 10
the loads transferred thereby helps alleviate the problem of bone
tissue resorption at the coronal surface of the maxilla or mandible
(marginal bone tissue resorption).
[0056] The provision of micro threads in the coronal section 19
also enables a reasonable wall thickness to be retained around the
tapered coronal section 13 of the socket 9 in the implant 10, when
compared to the wall thickness that would result from use of
macrothreads in the coronal section 19 in any event. This helps
preserve the mechanical strength of the implant 10.
[0057] To conclude, the dental implant 10 has a screw threaded
outer surface 19, 21 which (i) makes it straightforward for the
implant 10 to be screwed into a bone tissue structure, and (ii)
promotes the short- and long-term stability of the implant 10 in
the bone tissue structure.
[0058] It will be appreciated that the invention has been
illustrated with reference to an exemplary embodiment and that the
invention can be varied in many different ways within the scope of
the appended claims. As an example, although the illustrated
example is a dental implant the invention has equal application in
other areas, for example, the orthopaedic area.
[0059] Finally, it is to be noted that the inclusion in the
appended claims of reference numerals used in the Figures of
drawings is purely for illustrative purposes and not to be
construed as having a limiting effect on the scope of the
claims.
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