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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.
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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.
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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.