Junction for a modular implant

The present invention provides an improved junction for modular implant components.

BACKGROUND

Medical implants to replace or augment various parts of the mammalian body have been successfully used to reduce pain and improve function. For example, orthopaedic implants for replacing portions of bones and joints damaged by disease and/or trauma often eliminate pain and/or increase mobility. Orthopaedic implants for hips, knees, shoulders, ankles, elbows, wrists, the digits of the hands and feet, vertebral bodies, spinal discs, and other bones and joints have been developed. Many medical implants are made more versatile by providing them as separate modular components that can be combined to form an implant suited to a particular patient's condition. Where such modular components are supplied, a means for attaching them to one another is provided.

SUMMARY

The present invention provides a junction for modular implant components.

In one aspect of the invention, a modular joint implant includes a male/female junction between first and second joint components. The first component includes a bore having a longitudinal junction axis and a bore opening. The second component includes a projection engageable with the bore in male/female seating arrangement. A first portion of the bore opening is offset axially relative to a second portion of the bore opening.

In another aspect of the invention, the first portion is offset in a direction of increasing stiffness of the wall surrounding the bore.

In another aspect of the invention, the first portion is offset in a direction of increasing wall thickness.

In another aspect of the invention, the first portion is offset on a side of the implant that is generally in tension when the implant is loaded.

In another aspect of the invention, a modular joint implant includes a male/female junction having a side that is predominately in compression in use and a side that is predominately in tension in use. The implant includes a first component including a bore having a bore opening and an interior surface forming a female side of the male/female junction. The first component further has an exterior surface. The interior and exterior surfaces define a wall between them having a wall thickness. The wall thickness on the tensile side of the implant being greater than the wall thickness on the compressive side of the implant.

In another aspect of the invention, a modular joint implant includes a male/female junction having a side that is predominately in compression in use and a side that is predominately in tension in use. The implant includes a first component including a bore having a bore opening and an interior surface forming a female side of the male/female junction. The bore has a side on the tensile side of the implant that is shifted axially relative to a side of the bore on the compressive side of the implant.

These and other aspects of the invention will be described in reference to the appended drawings.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Embodiments of a junction for a modular implant are applicable to a variety of implants for use throughout the body. A femoral hip stem has been used to illustrate the invention. However, the invention may also be applied to various other implants including orthopaedic implants for hips, knees, shoulders, ankles, elbows, wrists, the digits of the hands and feet, vertebral bodies, spinal discs, and other suitable implants.

FIGS. 1–2depict a modular femoral hip implant10for replacing the proximal head and neck of a femur of a hip joint that has been damaged due to injury or disease. In use, the proximal head and neck are surgically removed and the femoral hip implant10is inserted into the proximal femur. The femoral hip implant10supports a femoral head12that may be a modular and separate component as shown. Optionally, the femoral head12may be integral to the femoral hip implant10. An acetabular component14may be implanted in the acetabulum of the pelvis to articulate with the femoral head12. Optionally, the femoral head12may articulate with the natural acetabulum. The femoral hip implant10has a medial aspect16and a lateral aspect18. When the patient loads the joint, such as by standing, walking, or other activities, forces are transmitted to the femoral hip implant10through the head12. These forces tend to create a bending moment that places the medial aspect16of the femoral hip implant in compression and the lateral aspect18in tension.

The femoral hip implant10may include modular components such as a proximal body20and a stem22. The proximal body20has a top end24and a bottom end26. A neck28extends upwardly and medially from the top end24to support the femoral head12for articulation with the acetabular component14. The proximal body20and stem22include a male/female junction for holding them together. In the illustrative embodiment, the female side of the junction is depicted in the proximal body20and the male side of the junction is depicted on the stem22. It is contemplated that the male/female portions may be reversed and still fall within the scope of the invention. The proximal body20includes a bore30(FIG. 2) having a bore opening32and an interior surface34forming the female side of the male/female junction. The bore30has a longitudinal junction axis36. The proximal body20has an exterior surface38spaced from the interior surface34of the bore30. The exterior38and interior34surfaces define a wall40between them. The wall40has a wall thickness42that may be constant or that may increase from the bore opening32toward the top end24as shown inFIG. 2. An increasing wall thickness may be accomplished by tapering the bore such that it narrows from the bore opening32toward the top end24as in the illustrative embodiment. A tapered bore can be made self-locking as is known in the art.

The stem22includes a bottom end44and a top end46. The bottom end44is configured for insertion into the intramedullary canal of the patient's femur. The top end46includes a projection48having an exterior surface50forming the male side of the male/female junction. The projection48is engageable with the bore30in male/female seating arrangement along the junction axis36. A threaded stud33extends from the projection48and is received by a counter bore35formed in the proximal body20. A nut37threads onto the stud33to secure the male/female junction.

When the femoral hip implant10is loaded, the medial aspects of the proximal body20and stem22are placed in compression and the lateral aspects of the proximal body20and stem22are placed in tension. Due to differences in the bending stiffness of the proximal body20and stem22in the region of the male/female junction, the bore wall40may move relative to the exterior surface50of the projection48. Cyclic loading can lead to fretting between the interior surface34of the bore30and exterior surface50of the projection48. This may be more problematic on the tension side since tensile forces may initiate and propagate fatigue cracks.

The magnitude of the fretting motion is related to the relative stiffness of the male and female parts of the junction. The relative motion at the opening32of the bore30is an accumulation of the relative motion along the entire length of the junction. This accumulated relative motion may be decreased by decreasing the length along which the relative motion accumulates on the tensile side of the junction. However, it is ineffective to simply make the junction shorter, because this will create higher stresses on the smaller diameter male cross section where such a shortened junction would end. However, by shortening only the tensile side, the compressive side is still supported and the tensile stress at the end of the tensile side increases only slightly while the relative fretting motion decreases significantly. In the illustrative embodiment, a lateral portion52of the bore opening32is offset axially upwardly relative to a medial portion54. This offset, or relieved, portion52can be created by stepping up the lateral portion52, sloping up the lateral side such that the bore opening32is transverse to the junction axis36, or by forming the bore opening in some other suitable shape.

Fretting in the male/female junction can also be reduced by better matching the stiffness of the male and female sides of the male/female junction. The present investigators have found that one way to better match the stiffness of the male and female sides is to increase the stiffness of the bore wall40adjacent the bore opening32on the tensile side of the implant10. Increasing the stiffness can be accomplished by increasing the outer diameter of the proximal body20adjacent the bore opening32to move material radially away from the junction axis36such that the bending moment of inertia is increased. Increasing this stiffness can also be accomplished by increasing the wall thickness of the proximal body20at the junction of the proximal body20and projection48. In the illustrative embodiment, the bore30is tapered so that it narrows proximally and the wall thickness42increases proximally. By offsetting a lateral portion52of the bore opening32axially upwardly relative to a medial portion54the wall thickness on the lateral side18of the bore opening32is increased. This offset, or relieved, portion52can be created by sloping the lateral side such that the bore opening32is transverse to the junction axis36as shown, by stepping up the lateral portion52, or by forming the bore opening in some other suitable shape. In the illustrative embodiment, the projection48forming the male side of the junction narrows upwardly. Thus, moving the lateral portion52upwardly also moves the lateral interface to an area of decreased stiffness of the projection48to further match the stiffness of the male and female portions laterally. Finally, by moving the lateral side upwardly while having the medial, anterior, and posterior sides of the junction extend further downwardly, the fatigue properties of the lateral side are improved while the overall interface of the male and female parts is kept relatively large to facilitate secure engagement between them.

Another way increase the stiffness of the bore wall40is to enlarge the bore30on just the tensile side of the junction as shown inFIG. 3such that the bore wall is offset away from the projection48. This has the same effect as moving the tensile side axially upwardly as described above. Other ways of better matching the stiffness of the male and female sides of the junction may also be used and are considered within the scope of this invention.

It will be understood by those skilled in the art that the foregoing has described illustrative embodiments of the present invention and that variations may be made to these embodiments without departing from the spirit and scope of the invention defined by the appended claims.