Patent Application
20080027559
The intramedullary implant stem component of the present invention provides intramedullary fixation when assembled with appropriate total joint replacement implants including knee, hip, and shoulder prosthesis. In other embodiments, the intramedullary implant stem component may be used in the replacement of segments of the long bones of the upper or lower extremity. The intramedullary implant stem component of the present invention has a variable stiffness approaching the stem terminus, which variable stiffness addresses potential problems such as stem tip pain and/or stress shielding associated with high stiffness stems or an abrupt change in stem stiffness.
In accordance with the present invention, the stem component comprises a shaft with a proximal end, a distal end, and a longitudinal length therebetween, wherein the shaft is of an approximately constant diameter along the longitudinal length. In one embodiment, the shaft terminates at the proximal end with a taper or threaded portion. The taper or threaded portion is a connection means that allows for modular assembly to total joint implants designed for the proximal tibia, distal femur, proximal femur, or proximal humerus. In another embodiment, the shaft terminates at the proximal end by integrally meeting another element of the implant without mechanical connection means. To provide the variable stiffness, the stem has a unique geometry of flutes that are distal to the proximal end. The flutes extend along a portion of the shaft and the flute dimensions widen and/or deepen in a continuous fashion as they progress towards the distal end. The flutes may deepen to the extent that they produce a split distal stem with at least three split or discrete end portions. This geometry produces a continuous decrease in bending stiffness of the stem with no abrupt or discontinuous changes in bending stiffness. Further, the flute geometry maintains a substantial percentage of stem material at the nominal stem geometry, such that primary fixation of the stem (e.g. torsional stability) is not compromised.
The terms “distal” and “proximal” by definition refer to a location further from or nearer to, respectively, a reference point. The reference point may vary in different fields, e.g. medical and mechanical. For example, in the medical field, the reference point may be the body midline, or mesial plane. The reference point could also refer to a point of attachment whether the attachment is mechanical or non-mechanical. Herein, to avoid a change of meaning of these terms based on the location or orientation of the implant in the body, proximal shall refer to being next to or nearest the point of attachment, or the point at which the shaft integrally meets another element of the implant device. Specifically, the reference point is the taper or threaded connection in a modular assembly-type implant, or in a one-piece implant, the integral meeting point of the shaft with the remainder of the implant. Similarly, distal shall refer to being situated away from or furthest from the reference point.
The invention will now be explained with reference to the figures wherein like reference numerals are used to refer to like parts throughout the several views. As shown in
The shaft 12 has a proximal end 16 where the shaft 12 adjoins the upper stem portion 14 and an opposing distal end 18. The distal end 18 is adapted to be inserted into a patient's intramedullary canal to secure the stem in place and it may be flat, rounded, bullet-nosed, or any other useful configuration. In an exemplary embodiment, the cross section of the shaft 12 may be substantially circular. The shaft 12 may be substantially straight or curved, for example, such that it matches the curvature of the anatomy, for example the femur. A distal portion of the length of shaft 12 is shown having a series of flutes 20. In accordance with the present invention, there are at least three flutes 20. In one embodiment, the flutes 20 are substantially equidistant from one another and are substantially parallel to the longitudinal axis of the shaft 12. The series of flutes 20 is provided, among other things, for variable bending stiffness. Furthermore, the multiple flutes 20 of the present invention provide a substantially axisymmetrical reduction in bending stiffness of the distal stem. An axisymmetrical bending stiffness does not require a surgeon to orient the stem based on possible directions of force application or bending. Therefore, flute geometries that create an axisymmetrical bending stiffness are disclosed.
The flutes 20 are disposed in a portion of the length of the shaft 12 beginning from a location intermediate the proximal end 16 and the distal end 18 and extending toward the distal end 18. In certain embodiments, flutes 20 having the described configurations may extend the substantial length of the shaft 12 or any part of the shaft 12. In one embodiment, the flutes 20 begin at approximately the middle of the shaft 12.
The unfluted portion of the shaft 12 defines an outer profile of the stem shaft, the diameter of which is referred to as the nominal shaft diameter. The diameter in the fluted portion of shaft 12 is substantially equal to the nominal shaft diameter, such that the diameter of shaft 12 is substantially constant along the length of the shaft 12, in both the fluted and unfluted portions. By substantially maintaining the diameter throughout the length of the shaft 12, maximal contact between the stem 10 and the bone is maintained, thus resulting in maximal stability of the implant. In one embodiment, the outer profile is circular or substantially circular and approximates the cross section of the site of implantation following preparation of the bone, for example following reaming of the bone's intramedullary cavity. However, in other embodiments the outer profile may include other geometries, with the outer profile dimension being maintained substantially constant throughout the length of the shaft.
The stem component 10 may be made from any biocompatible material that has sufficient strength to withstand the patient's weight, examples of which include titanium, titanium alloys, stainless steel, stainless steel alloys, cobalt alloys, and other surgical grade material.
The surface of stem component 10, and in particular the surface of shaft 12, may be provided with a bone in-growth or on-growth feature. This may be a surface of raised splines, a roughened surface, metallic beads, a grit blasted surface, a porous surface, a hydroxyapatite coating, or any other bone growth-promoting substance coating, or combinations of any of these features. This allows the bone into which the stem is implanted to integrate with the stem 10 or otherwise grow into the flutes 20 for increased strength and stability.
In use, the variable stiffness stem helps provide stability to a bone or joint replacing prosthesis component. A surgical method for using the variable stiffness stem of the present invention includes preparing the implantation site for the prosthesis by surgically exposing the site for prosthesis implantation, followed by resection of the bone or joint segment to be replaced. The adjacent portions of bone are then prepared to receive an intramedullary prosthesis stem. This bone preparation can include the reaming of the bone's intramedullary cavity, measuring or sizing the cavity, and/or the removal of bone cavity contents. The bone preparation may be assisted by the use of radiographic imaging or alignment instruments. The variable stiffness stem of desired cross-sectional size and length can then be inserted into the prepared bone cavity. Prior to insertion into the bone, the stem may be attached to other prosthesis elements or it can be attached after insertion. Alternatively, the stem may be an integral part of a one-piece implant device.
The following example is provided to illustrate the invention and is not intended to limit the same.
Three dimensional, finite element stress analysis models of the proximal tibia with total knee replacement (TKR) implants subjected to joint loading associated with a constrained implant in mid-stance gait (
The first principal stress within the cortical bone at the stem tip and the maximum pressure between the prosthetic stem tip and surrounding bone were predicted to vary greatly with implant design. As shown in
The distribution of stress within the tibia was greatly effected by prosthetic component design.
The use of rigid intramedullary stem prosthesis in many joint reconstruction situations can be associated with end of stem pain that limits patient function. This pain may be the result of the localized load transfer, high bone stress development, and high implant-to-bone pressure that occurs at the stem tip. The above analysis demonstrates that implant design features that alter stem terminus bending stiffness can significantly alter the periprosthetic bone loading condition to an extent that stress induced periprothetic pain and function limitation may be reduced. The multiple flute stem geometry of the present invention has a nearly axisymmetric bending stiffness and can provide for smooth, continuous, and substantial bending stiffness reduction that results in less localized load transfer and stress concentration at the stem tip.
Other variations or embodiments of the invention will also be apparent to one of ordinary skill in the art from the above description and example. Thus, the forgoing embodiments are not to be construed as limiting the scope of the claims.
