Shot peening of orthopaedic implants for tissue adhesion

The portion of an orthopaedic implant to which soft tissue adherence is desired is treated by shot peening using microbead having a diameter in the range of about 10 microns–300 microns. This treatment causes indentations on the surface of the implant of about 10 microns to about 50 microns to provide a fine, shallow texturing of the implant that permits the soft tissue to adhere, but is not rough enough that it will interlock with hard tissue.

TECHNICAL FIELD

This invention relates to orthopaedic implants that have been treated to improve tissue adhesion.

BACKGROUND OF THE INVENTION

Proper adhesion of soft tissue to orthopaedic implants is important but has proven difficult to achieve. For example if the implant surface to which tissue adherence is desired is smooth, tissue cannot easily adhere to the implant and the body forms a tissue capsule around the implant, sealing it off from the rest of the body. This impairs the implant's function. Since the implant constantly moves relative to the tissue, resulting friction causes inflammation and creates a steadily growing capsule of dead tissue. Accordingly, implant surfaces to which tissue adherence is desired have been textured, but too great a degree of surface roughness can permit connective tissue and bone to grow into the fissures. The implant essentially grows into the body and removal of the implant becomes almost impossible, and if possible results in major bone loss.

Shot peening has been used to increase strength and wear resistance of orthopaedic implants, as disclosed in U.S. Pat. No. 5,704,239. Shot peening to increase strength uses shot media of a size and applied at an intensity sufficient to compress the layer just under the surface to thereby increase strength. Shot having a diameter of 0.0011″–0.0023″ would typically be used (identified by industry standard S110–S130shot).

SUMMARY OF THE INVENTION

According to the invention, the portion of an orthopaedic implant to which tissue adherence is desired is treated by shot peening using microbead, that is, shot that is much smaller than shot used to effect strengthening of the implant. Microbead has a diameter in the range of about 10 microns–300 microns and when used at normal intensity causes indentations on the surface of the implant of about 10 microns to about 50 microns. This does not cause compression of the layer just below the surface, but instead provides fine, shallow texturing of the implant that permits the fibroblasts of the connective tissue a surface to which to adhere. However, the implant is not rough enough that it will interlock with hard tissue, such as bone tissue. Furthermore, shot peening is a well known and relatively simple and inexpensive process, which is relatively easily controlled to effect the desired tissue adherence. Other methods of surface treatment are more difficult and expensive, and are less easily controlled to effect the degree of surface roughness that permits soft tissue to adhere, but that is not rough enough that hard tissue will also adhere.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now toFIG. 1, a typical orthopaedic implant is illustrated at10. The implant10as illustrated is a femoral prosthesis used in hip replacement surgery. The implant10includes a stem12, which is implanted within the patient's femur, and a ball14mounted upon the stem12but which projects from the patients femur and engages a compatible prosthesis (not shown) mounted on the patient's pelvis. Since only the stem12is implanted within the femur, and thus it is desired that soft tissue adhere only to the stem12, only the stem12is to be treated as described herein. A conventional shot peening gun used to treat the implant10is illustrated at16and is connected to a source of compressed air (not shown) and to a source of conventional shot or of microbead as will be discussed hereinafter. The shot or microbead is mixed with the air stream from the compressed air source by conventional apparatus assuring that the shot or microbead is mixed with the air stream so that it can be applied to the implant10at the desired intensity. The gun16is moved over the implant10until the desired coverage is achieved. Since it is desired that tissue adhere only to the stem12, only the stem12is to be treated with the microbead.

As discussed above, the conventional shot18, which is shown greatly enlarged inFIG. 2, typically has a diameter of about 0.0011″ to about 0.0023″ (identified in the trade as S-110to S-230shot). The shot18may be steel, ceramic, or any other shot known in the industry. When applied to the outer surface20of the stem12by the gun16, the size of the shot18is sufficient to cause depressions22in the surface20and dimples24between the depressions, thus causing compression of the layer26below the surface20, to thereby effect hardening of the device as is well known to those skilled in the art. As is also well known, the depth and concentration of the depressions is a function of the size of the shot18and the intensity of the shot peening, which may be regulated by known methods. Implants have been hardened by shot peening in the prior art.

The present invention uses shot peening using microbead having a diameter between about 50 microns and about 300 microns to effect texturing of the surface20of the implant stem12to facilitate adhesion thereto by soft tissue, The microbead is illustrated greatly enlarged at28inFIG. 3, but the relative sizes of the micrbead28and the conventional shot18is about as illustrated inFIGS. 2 and 3. The microbead28is applied to the stem12of the implant by the gun16controlling a compressed air stream in which the microbead28is entrained in the same way that the conventional shot18is applied to the stem12. However, the small size of the microbead28does not appreciably compress the layer26below the surface20, and thus does not appreciably affect hardness. As seen inFIG. 6, shot peening using the microbead28results in many small indentations30in the surface20, each having a depth in the range of about 10 microns to about 50 microns thus creating a textured surface compatible with soft tissue adhesion but which indentations are not deep enough to encourage hard tissue growth within the indentations.

Referring toFIG. 7, implants may first be hardened by application of convention shot18, followed by texturing using microbead28. Accordingly, the stem is first shot peened by the conventional shot18to create large depressions22, and is thereafter shot peened using the microbead28, to impose the smaller indentations30on the surface. Accordingly, the implant10is hardened by the conventional shot peening and then textured using microbead.

It is recognized, of course, that most shot is not perfectly spherical. Accordingly, as used herein, the term “diameter” refers to the largest dimension of shot that is not a true sphere.