Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
2002-06-25
2004-11-30
Snow, Bruce E (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S023270
Reexamination Certificate
active
06824568
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an implant for insertion into and permanent anchorage in human skeletal or bone tissue. More specifically, the invention concerns an implant having a screw-threaded anchoring structure comprising a first cylindrical section of a first diameter and a second cylindrical section of a second diameter, said second diameter being less than said first diameter.
The invention is particularly, although not exclusively, concerned with an implant of this type which takes the form of a femur fixture for a hip-joint pros* thesis.
BACKGROUND OF THE INVENTION
Implants of this type in the form of femur fixtures for a hip-joint prosthesis are known from Applicant's prior International patent application publications WO93/01769, WO93/16663 and WO97/25939 with the first and second cylindrical sections being provided with external screw threads for engaging the bone tissue of the femur. The screw threaded first and second cylindrical sections of the femur fixture-disclosed in WO93/01769 are assembled together in the femur by firstly inserting the second cylindrical section medially into the neck of the femur from beneath the greater trochanter and then inserting the first cylindrical section into the neck laterally through the resected section left after resection of the head of the femur. The screw threaded first and second cylindrical sections of the femur fixture disclosed in WO93/16663 and WO97/25939, on the other hand, are integrally formed or pre-assembled prior to anchorage of the fixture in the femur neck by screwing the fixture into the femur neck laterally through the resected section left after resection of the femur head.
In the femur fixtures disclosed in WO93/01769, WO93/16663 and WO97/25939, the first cylindrical section steps into the second cylindrical section. A drawback of this type of implant is the amount of stress present between the bone and the implant following implantation.
SUMMARY OF THE INVENTION
Thus, the object of the present invention is to provide an implant where the above mentioned drawback is reduced.
This and other objects are achieved according to the present invention by providing an implant having the features defined in the independent claim. Preferred embodiments are defined in the dependent claims.
According to the present invention there is provided an implant for insertion into and permanent anchorage in bone tissue, comprising an intraosseous anchoring structure of a generally circular cross-section, said anchoring structure comprising a first cylindrical section of a first diameter and a second cylindrical section of a second diameter, said second diameter being less than said first diameter, said first and second cylindrical sections each being provided with a screw thread profile. The implant is characterised in that said anchoring structure comprises a tapered connecting section provided between and interconnecting said first and second cylindrical sections.
The implant of the invention may be an orthopaedic implant, for example a femur fixture for a hip-joint prosthesis as in the embodiment hereinafter to be described.
Thus, the present invention is based on the advantageous idea of providing an implant of the above-mentioned type with a tapered connecting section between the first and second screw-threaded, cylindrical sections.
The provision of a tapered connecting section would overcome the drawback mentioned above and provide a number of additional advantages. First, the stresses induced by the sharp, step-wise transition present in the prior art implants between the cylindrical sections of differing diameters is radically reduced using the tapered connecting section of the present invention.
A further advantage resulting from the provision of a tapered connecting section when the implant is used for implantation in a cavity of corresponding shape formed in bone tissue, is that the insertion of the implant is facilitated. This is because the distal end of the implant can be guided by the tapered section of the cavity which is arranged for interacting with the tapered connecting section. Thus, the cavity will have no surface facing directly opposite the insertion direction of the implant, as is the case with the prior art implants (see item 50 of FIG. 5 in WO 97/25939).
A still further advantage is that the provision of a tapered connecting section provides a wedging effect during implantation of the implant into bone tissue. This wedging effect improves the short and long term stability of the implant following said implantation. This is mainly due to the radial force component of the normal contact force between the implant and the cancellous bone tissue that surrounds the connecting section upon implantation.
If the longitudinal force exerted by the surrounding tissue on the implant of the invention and on the above-mentioned prior art implants, following implantation of the implant, is essentially the same, then the transversal force on the implant of the invention will be greater compared to the prior art stepped implants. This is because the longitudinal force is carried by the inclined surface of the tapered connecting section, whereby the resulting transversal force will have a radial force component. This radial force component is not present in the prior art implants since only the radial end portion (e.g. item 10 of FIG. 1 in WO 97/25939) of the proximal cylindrical section (e.g. item 2 of FIG. 1 in WO 97/25939) carries the corresponding longitudinal force.
While the overall transversal force is increased, the actual force per surface unit is not necessarily increased. This is due to the fact that the contact surface carrying the longitudinal contact forces will be considerably greater with the tapered connecting section of the present invention as is the case with said radial end portion of the prior art implants.
Also, the provision of the inventive tapered connecting section improves the integration between the implant and the cancellous bone tissue (termed “osseointegration” in the art). This is mainly due to the shape of the contact surface of the connecting section, but also to the increased overall contact force exerted on the implant by the cancellous bone tissue.
Ordinarily, the first cylindrical section is disposed proximally of the second cylindrical section with the taper of the connecting section inclining inwardly in the distal direction, as in the embodiment hereinafter to be described.
Preferably, the tapered connecting section has a frusto-conical profile, even though other tapering shapes are conceivable without departing from the scope of the present invention.
According to preferred embodiments of the invention, the diameter of the first cylindrical section is adapted to the actual size and shape of the femur of the particular patient for whom the implant is intended. Thus, the diameter of the first cylindrical section can vary considerably. However, the diameter of the second cylindrical portion is preferably dimensioned to be within a short, limited range. Thus, the flank angle of the connecting section may vary in dependence of the actual dimensions of the first and second cylindrical sections. Preferably, the flank angle can be varied in the range of 10°-50°, and more preferably in the range of 200-400.
Furthermore, the longitudinal extension of the connecting section is preferably in the range of 5-15 mm, preferably in the range of 7-11 mm.
Advantageously, the end of the tapered connecting section interfacing the first cylindrical section has essentially the same diameter as the first cylindrical section. Likewise, the end of the tapered connecting section interfacing the second cylindrical section advantageously has essentially the same diameter as the second cylindrical section.
According to preferred embodiments of the invention, the connecting section is at least partly provided with a roughened surface. This would even further promote the osseointegration process at the transition area between the cylindrical sections. The roughened surface co
Albrektsson Tomas
Carlsson Lars
Jacobsson Magnus
Macdonald Warren
Wennberg Stig
Astra Tech AB
Birch & Stewart Kolasch & Birch, LLP
Snow Bruce E
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