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Fixture and a fixture set

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20120264086 patent thumbnailZoom

Fixture and a fixture set


A fixture for insertion into a bore hole arranged in bone tissue is provided. The fixture has a threaded leading portion provided with at least one cutting edge for creating a female thread in the bone tissue, and a threaded trailing portion following the leading portion in the created female thread. Compared to the geometry of a thread peak of said cutting edge of the leading portion, a thread peak of the trailing portion is displaced or overdimensioned in at least one of the coronal and apical directions of the fixture so that a strain is provided to the bone when said displaced or overdimensioned thread peak engages with the created female thread. A fixture set for creating a strain is also provided, the set comprising a fixture and a separate thread maker.
Related Terms: Apical Coronal

Inventors: Stig HANSSON, Anders HALLDIN
USPTO Applicaton #: #20120264086 - Class: 433174 (USPTO) - 10/18/12 - Class 433 
Dentistry > Prosthodontics >Holding Or Positioning Denture In Mouth >By Fastening To Jawbone >By Screw

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The Patent Description & Claims data below is from USPTO Patent Application 20120264086, Fixture and a fixture set.

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TECHNICAL FIELD

The present invention relates to a fixture, such as a dental fixture, for insertion into a bore hole arranged in bone tissue, the fixture comprising a threaded outer surface for engagement with the bone tissue. The invention also relates to a fixture set comprising a fixture and a thread maker.

BACKGROUND OF THE INVENTION

A frequent way today to restore a damaged limb, such as lost tooth, is to install a fixture in the adjacent bone tissue and replace the damaged parts. In this respect, for a successful result, the fixture should become fully stable and correctly joined to the bone. The term osseointegration is used for this joining effect, the basic meaning of this term being the bone tissue growth into the fixture surface. The two major contributors to this joint are a mechanical joint and an organic joint. The former being generally influenced by the macro geometry of the bore into which the fixture is installed, and by the macro geometry of the fixture, and is a direct effect of how well these two work together. The latter one being a continuously evolving and developing effect, particularly the time immediately after installation, and being generally influenced by how well the micro surface structure of the fixture interacts with the bone tissue.

Due to ingrowth there will be an interlocking effect between the bone and the fixture. Also, the mechanical joint is developed over time since the bone tissue, under ideal conditions, may grow into surface cavities of the fixture, and grow into voids left between the fixture and the bore after installation.

During installation of a fixture into the bone tissue, the bone is subjected to both stress and strain. The relationship between stress and strain is substantially linear up to a yield point (yield strain). Up to the yield point the bone is deformed elastically. However, beyond the yield point the bone will deform plastically. In order to provide for good healing conditions and stability of the fixture in the bone, care is taken to maintain the elasticity of the bone tissue and to avoid exceeding the yield point.

There is a continuous endeavour in the industry to further increase the stability of fixtures implanted in bone tissue and to improve the basic conditions during the healing phase after fixture installation. One example is the provision of the fixture surface with different types of structures, such as micro-roughened or blasted structures for increasing the contact surface between the fixture and the bone.

Nevertheless, there is still room for further development of fixtures as regards their stability in bone tissue.

SUMMARY

OF THE INVENTION

An object of the present invention is to provide a fixture, in particular a dental fixture, which has a high bone-anchoring stability/strength during the healing phase of the fixture. This and other objects, which will become apparent in the following, are accomplished by means of a fixture defined in the accompanying claims.

The present invention is based on the insight that exceeding the yield point of the bone during and after implantation may actually be beneficial to the anchoring strength/stability of the fixture during the healing phase of the bone. In particular, the inventors have found that strains in the axial direction which exceed the ultimate strain of the bone, i.e. when the bone cracks, may also be beneficial to trigger the biological response during the healing phase after fixture installation. Although cracks may be formed near the fixture, there will be present stabilizing surrounding bone tissue.

In this application, when strain is discussed, or when different values of strain are discussed, the discussion may relate to tensile strain and/or compressive strain. All strain-related numbers are presented in absolute values.

The insertion of a fixture with a certain torque means that static strains will be induced in the surrounding bone. The magnitude of these static strains do not only depend on the insertion torque but also depend on the fixture design, the shape of the bone preparation, the bone anatomy, the bone quality and possibly also on the fixture surface topography. Rather than to elaborate on these different parameters, some of which are difficult to estimate, the inventors have ingeniously realized that it is possible to achieve an adequately controlled static strain by fixture design.

When a self-tapping fixture having cutting edges or a separate tapper (pretapping before fixture installation) is screwed into a bore hole in the bone tissue a female bone thread (which is complementary to the male fixture or tapper thread) is created in the bone tissue surrounding a bore hole. In cross-section, the female bone thread will have thread peaks which are axially separated by troughs. Such a thread peak in the bone may also be referred to as a bone plug.

The bone plugs of this female thread will have a certain dimension. By providing a fixture with a specific portion having thread peaks that are axially overdimensioned relative to the receiving troughs of the female thread or partly displaced axially relative to the receiving throughs of the female thread, an axial pressure will be applied to the bone plug when said threaded portion is rotated into the bone via said female bone threads. The axially enlarged or displaced thread peaks on the fixture will thus lead to a condensation of the bone tissue.

This means that by controlling the difference in axial extension between the thread peaks of said threaded fixture portion and the female bone thread with which the threads of said portion will mate, a controlled static strain may be achieved.

For instance, said controlled strain may be achieved by having a threaded leading portion of the fixture with a first width of the thread peaks corresponding to the width of the troughs of the created female bone threads, and a threaded trailing portion of the fixture with a second width of the thread peaks which has a larger extension in the axial direction than said first width.

Thus, the inventive idea may in general terms be described as providing a bore hole in the bone tissue, then providing a female thread in the bone tissue surrounding the bore hole, and then applying a static axial pressure to the bone so that a strain is achieved in the bone.

According to at least a first aspect of the invention, a fixture for insertion into a bore hole arranged in bone tissue is provided. The fixture has a geometrical central axis and comprises

a leading portion provided with at least one cutting edge for creating a female thread in the bone tissue, and

a trailing portion following the leading portion in the created female thread,

wherein both of said portions comprise a respective outer surface being threaded for engagement with bone tissue, wherein thread tops and thread bottoms are provided alternatingly in the axial direction of the fixture, each thread top forming part of a thread peak arranged between two consecutive thread bottoms,

wherein a largest thread peak of said cutting edge of the leading portion forms part of a thread having a lead, wherein compared with an imaginary continuation into the trailing portion of said largest thread peak following said lead: a first area of a thread peak of the trailing portion is axially aligned with said imaginary continuation, and a second area of said thread peak of the trailing portion is axially displaced compared with said imaginary continuation, thereby providing a strain to the bone when said thread peak of the trailing portion engages with the created female thread, the comparison of the thread peaks being made at a same first distance from a respective thread bottom measured perpendicularly to said central axis, and/or at a same second distance from a respective thread top measured perpendicularly to said central axis.

Thus, when the cutting edge or edges of the leading portion make a female bone thread in the bone there will be substantially no strain, since bone is cut away rather than pressed away. When the threaded trailing portion enters the female thread in the bone, due to the increased coronal and/or apical extension (relative to the female bone thread) of one or more thread peaks of the trailing portion, the thread peaks will press the bone in the axial direction creating a static strain in the bone tissue.

The largest thread peak of the cutting edge of the leading portion will form the final geometry of the female bone thread in which the trailing portion will follow. The thread peak forms part of a thread having a lead. This lead defines an endless geometrical spiral path around the fixture axis. If the contour/geometry of said largest thread peak would be guided along the geometrical spiral path into the trailing portion it would not completely accommodate the contour/geometry of the strain-providing thread peak of the trailing portion. Instead, the contour/geometry of said thread peak in the trailing portion will have a first area which would be accommodated within the guided contour/geometry of said largest thread peak of the cutting edge and a second area which would be located outside the guided contour/geometry of said largest thread peak of the cutting edge.

It should be understood that when referring to the “largest thread peak of the cutting edge” it is a part of the cutting edge that forms the specific female bone thread in which the strain-providing thread peak of the trailing portion will follow. Thus, if a fixture would have two parallel thread spirals, but only one of the spirals have a strain-providing thread peak in the trailing portion, “the largest thread peak of the cutting edge” is located on that spiral, even if the cutting edge has large thread peaks on the other spiral (which will not have any strain-providing thread peaks in the trailing portion).

It should be understood that the size of the strain-providing thread peak in the trailing portion may be larger, smaller or equal to the largest thread peak of the cutting edge in the leading portion, as long as it has said second area which is displaced as described above.

Although said thread peak of the trailing portion will provide an axial pressure on the bone, the resulting strain is not necessarily just an axial strain, but may also result in a certain amount of radial strain, e.g. depending on the geometry of the thread peaks.

The leading portion comprises one or more cutting edges. However, the trailing portion is suitably non-cutting, i.e. the trailing portion is void of cutting edges, or at least the thread peak or thread peaks providing said strain are non-cutting. It should be noted that when discussing the largest thread peak of the cutting edge, the cutting edge may include several thread peaks which are the “largest”, normally these would be located at a coronal section of the cutting edge.

According to at least one example embodiment, said largest thread peak of said cutting edge of the leading portion forms part of a thread having a lead defining a first helical path in the leading portion, wherein said thread peak of the trailing portion forms part of a thread having a lead defining a second helical path which is partly axially displaced relative to an imaginary extension into the trailing portion of the first helical path. The first helical path will correspond to the path of the created female bone thread. Thus, by having said first and second helical paths displaced relative to each other, said second area of the strain-providing thread peak of the trailing portion will press the created female bone thread. It should be noted that the displacement may be in either the coronal direction or in the apical direction. For instance, at an axial location between the cutting edge of the leading portion and the strain-providing thread peak in the trailing portion a thread may locally (for instance along one thread revolution) be made narrower, resulting in that the coronally following thread will be located slightly more apically than if the thread would not have been made narrower locally. In other words, there is a smaller pitch locally. If instead a thread bottom is widened locally (for instance along one thread revolution), it would result in that the coronally following thread will be located slightly more coronally than otherwise. In other words, there is a larger pitch locally.

According to at least one example embodiment, compared to the geometry of the largest thread peak of said cutting edge of the leading portion, a thread peak of the trailing portion is overdimensioned in at least one of the coronal and apical directions of the fixture so that a strain is provided to the bone when said overdimensioned thread peak engages with the created female thread, the comparison being made at a same first distance from a respective thread bottom measured perpendicularly to said central axis, and/or at a same second distance from a respective thread top measured perpendicularly to said central axis.

If the overdimensioning of the thread peak in the trailing portion is present in both the coronal and the apical direction, then the axial extension (where said comparison is made) of that thread peak is larger than the axial extension of the largest thread peak of the cutting edge of the leading portion.

However, there are other conceivable dimensionings. For instance, the thread peak of the trailing portion may have almost the same geometry as the thread peak of the cutting edge of the leading portion but with the following two differences: 1) the coronal flank of the thread peak in the trailing portion is made concave or is provided with a notch or groove for bone in-growth, and 2) the apical flank of the thread peak in the trailing portion is made convex or provided with a bulge. In this case the thread peak in the trailing portion will be overdimensioned in the apical direction, and will create a pressure on the bone in the apical direction, resulting in said strain, but at the same time the trailing portion is underdimensioned in the coronal direction due to the groove or concavity. Therefore, the axial extension will not necessarily be larger for the thread peak in the trailing portion compared to the thread peak in the leading portion. Of course, the corresponding principle would also be possible for a notch on the apical flank and a bulge on the coronal flank.

Although the axial extension of the thread peak in the trailing portion may be equal to or even smaller than the axial extension of the thread peak of the leading portion (compared at said same first and/or second distance), it may advantagously be larger, which is reflecting in at least one example embodiment. Thus, according to one example embodiment, in the axial direction of the fixture a thread peak of the trailing portion has a longer extension than the largest thread peak of said cutting edge of the leading portion, the comparison being made at a same first distance from a respective thread bottom (or core of the fixture) measured perpendicularly to said central axis, and/or

in the axial direction of the fixture a thread peak of the trailing portion has a longer extension than the largest thread peak of said cutting edge of the leading portion, the comparison being made at a same second distance from a respective thread top measured perpendicularly to said central axis.

A fixture may have a tapered apical end section of the leading portion to facilitate guiding of the fixture into a bore hole in the bone tissue. The profile of the apical end section of the leading portion may vary. Therefore, in this application, when referring to the axial extension of a thread peak of a cutting edge, what is relevant is the largest profile (at the point or points which will be compared to the corresponding points of the thread peak in the trailing portion), since that is what will form the geometry of the female bone thread. Normally, if a cutting edge extends axially over several thread peaks, at least the most coronal of those thread peaks has said largest profile forming the geometry of the female bone thread. Thus, the axial extension of the thread peak of the trailing portion should be compared with a thread peak(s) of the cutting edge(s) that forms the geometry of the femal bone thread into which the trailing portion of the fixture enters.

The difference in axial extension or displacement may be designed at one or more locations of the thread peaks. For instance, at half the height of the thread peak, i.e. radially halfway between thread bottom and thread top, the axial extension of one or more thread peaks in the trailing portion may be larger than that of the thread peaks of the cutting edge in the leading portion. In such case, it does not matter if the comparison is made at a same distance from a respective thread bottom measured perpendicularly to said central axis, or if the comparison is made at a same distance from a respective thread top measured perpendicularly to said central axis. In such case, the above-mentioned first and second distances are of equal value (assuming that the height of the thread is the same in the leading portion and the trailing portion).

However, if only the thread top of the thread peak of the trailing portion has a larger extension than the thread top of a thread peak of the cutting edge in the leading portion, then the comparison should be made at a same distance measured perpendicularly to the central axis from a thread bottom in the trailing and leading portions, respectively.

Similarly, if in the trailing portion, the thread bottoms separating the thread peaks are partially filled up with more material than the thread bottoms in the leading portion, thus making the lowest section of the thread peak wider in the trailing portion, then the comparison should be made at a same distance measured perpendicularly to the central axis from a thread top in the trailing and leading portions, respectively.

Thus, it should be understood that strain may be achieved by various design modifications of the thread peaks in the trailing portion compared to the thread peaks of the cutting edge(s) in the leading portion. Some embodiments reflecting such possibilities are presented below.

According to at least one example embodiment, the axial extension of a thread top in the trailing portion is longer than the axial extension of a thread top of said largest thread peak of said cutting edge in the leading portion. A thread top is the section of the thread peak which is radially farthest off from the central axis of the fixture. The differentiation in thread top design may be accomplished in different ways, even if the thread tops would be arranged at a constant distance from the central axis of fixture throughout the leading and trailing portions. For instance, a thread top at the cutting edge of the leading portion may form an acute angle, while a thread top at the trailing portion forms an obtuse angle. Another example, would be a thread top forming a substantially straight top of a truncated triangle, the axial extension of which is larger for a thread top in the trailing portion than for a thread top of a cutting edge in the leading portion.

According to at least one example embodiment, a top radius of a thread peak in the trailing portion is larger than a top radius of said largest thread peak of said cutting edge in the leading portion. Thus, while both peaks are rounded or curved at the top, the bending of the curvature at the top of a thread peak in the trailing portion is not as strong as for a top of the cutting edge in the leading portion.

An advantage of widening the thread top or increasing the top radius of a thread peak is that a strain is achievable, without augmenting the stress that is naturally present in the bone around a thread peak of a fixture (compared to if the same strain would be achieved with non-rounded thread tops, e.g. trapezoid-shaped thread tops).

According to at least one example embodiment, the axial extension of a thread bottom between two consecutive thread peaks in the trailing portion is shorter than the axial extension of a thread bottom between two consecutive thread peaks of said cutting edge in the leading portion. In other words, one can picture this as partially filling the profile of the thread bottom at the trailing portion with more fixture material compared to the profile of the thread bottom at the leading portion. Expressed differently, the base of the thread peak in the trailing portion is wider than the base of the thread peak in the cutting edge of the leading portion.

This kind of smaller separation of the thread peaks in the trailing portion compared to the separation of the thread peaks in the leading portion is also reflected in at least one other example embodiment. According to at least one example embodiment, each one of the trailing portion and the cutting edge of the leading portion has a first thread peak and an axially consecutive second thread peak located coronally of the first thread peak, wherein the coronal flank of the first thread peak is axially spaced from the apical flank of the second thread peak, wherein said spacing between the flanks of the first and second thread peaks is smaller in the trailing portion than in the cutting edge of the leading portion. The comparison may be made at a same first distance from a respective thread bottom measured perpendicularly to the central axis of the fixture, or at a same second distance from a respective thread top measured perpendicularly to the central axis of the fixture.

According to at least one example embodiment, a bottom radius between two consecutive thread peaks in the trailing portion is smaller than a bottom radius between two consecutive thread peaks of said cutting edge that form the female thread geometry in the bone. Thus, while the thread bottom between two consecutive peaks are rounded or curved at the top in the trailing portion as well as in the leading portion, the bending of the curvature of the thread bottom in the trailing portion is stronger than for the thread bottom of the cutting edge in the leading portion.

An advantage of narrowing the thread bottom or decreasing the bottom radius between thread peaks is that a strain is achievable, without augmenting the stress that is naturally present in the bone around a thread bottom of a fixture (compared to if the same strain would be achieved with non-rounded thread bottoms, e.g. trapezoid-shaped thread bottoms).

According to at least one example embodiment, said thread peak of said cutting edge of the leading portion and said thread peak of the trailing portion having larger axial extension are provided on a common thread such that said thread peak of the trailing portion will follow in the female thread in the bone formed by the thread peak of said cutting edge in the leading portion. In other words said thread peaks are arranged on a common thread spiral. According to at least some other example embodiments, in addition to said thread spiral, the fixture may be provided with one or more additional thread spirals which are provided only in the leading portion, or only in the trailing portion, or in both the leading and the trailing portion. Thus, in some example embodiments, the fixture may be provided with parallel thread spirals in which at least a first thread spiral has the function of providing a strain by differentiating the design of the thread peaks of said first spiral in the trailing portion relative to the design of the thread peaks of said first spiral in the leading portion. At least a second thread spiral may be void of said strain-providing design. Another conceivable embodiment would be to have both thread spirals providing a respective strain. A further conceivable alternative is to have one thread spiral providing a strain to one bone region, for instance the cancellous bone tissue, and another thread spiral providing a strain to another bone region, for instance the cortical bone tissue.

According to at least one example embodiment, in the trailing portion and the leading portion each thread top of a thread peak interconnects a coronal and an apical flank, wherein at least one of a coronal and an apical flank of a thread peak in the trailing portion is axially displaced relative to a respective mating flank of the female thread cut in the bone tissue by the leading portion, wherein at least one of said coronal and apical flanks of the thread peak in the trailing portion provides a strain to the bone tissue when the fixture is installed in the bone. Thus, it is conceivable to only provide for coronally directed condensation at the trailing portion by having one or more of the coronal flanks in the trailing portion axially overdimensioned relative to the coronal flank(s) of the cutting edge in the leading portion. A counter-acting apically directed condensation will be present in the leading portion. Similarly, it is conceivable to only provide for apically directed condensation at the trailing portion by having one or more of the apical flanks in the trailing portion axially overdimensioned relative to the apical flank(s) of the cutting edge in the leading portion. Furthermore, it is conceivable to provide both coronally and apically directed condensation by having the coronal as well as the apical flanks overdimensioned. Still further, it is conceivable to have, for instance, one thread peak in the trailing portion providing a coronally directed condensation, while having another thread peak in the trailing portion providing an apically directed condensation to the bone. As a further possibility, it is conceivable to have a cyclical alternation of thread peaks providing a coronally and apically directed condensation, respectively. In this manner, it would be possible to condense a bone plug from two sides by two consecutive thread peaks, while the axially subsequent bone plug is left without condensation. Such a design would increase primary stability due to the strain provided to the condensed bone plug, while the non-condensed bone plug may favor healing of the bone.



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stats Patent Info
Application #
US 20120264086 A1
Publish Date
10/18/2012
Document #
13445084
File Date
04/12/2012
USPTO Class
433174
Other USPTO Classes
International Class
61C8/00
Drawings
12


Apical
Coronal


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