96 – OUTCOMES AFTER REVISION FOR MECHANICAL FAILURE OF THE KOTZ MODULAR FEMORAL TIBAL REPLACEMENT (KMFTR) PROSTHESIS

Abstract

Purpose: Massive endoprostheses have become the mainstay of treatment for reconstruction after resection of primary bone tumours. The Kotz Modular Femoral Tibial Replacement (KMFTR, Kotz prosthesis, Stryker Inc.) system has been one of the most widely utilized uncemented modular systems. Although this prosthesis has excellent bone ingrowth characteristics and a low aseptic loosening rate, we have identified a significant incidence of mechanical failure and breakage of the prosthesis. The purpose of this investigation is to review the outcomes after prosthetic revision for a broken Kotz prosthesis.

Method: A retrospective review was undertaken of our institutional database from the years 1989, when we first utilized the Kotz prosthesis, until present. We identified all patients who had undergone a revision of the prosthesis for mechanical failure or prosthetic breakage. Periprosthetic fractures and revisions for polyethylene bushing wear were excluded.

Results: 119 distal femoral, 55 proximal tibial and 47 proximal femoral Kotz endoprostheses (221 in total) have been implanted in our center since 1989. There were 21 revisions (9.5% of total prostheses) for mechanical failure. Of these, 16 were in the distal femur, four in the proximal tibia and one in the proximal femur. Mechanical failures occurred at a mean of 77 months (range 24–170). Of the 21 metal failures, 8 stems broke at the junction of the stem and body, 8 fractured through screw holes in the stem, 3 fractured the derotation lug, one fractured the tibial housing and one lateral side-plate failed. Of these failures only three implants had associated definite loosening and two of these three were cemented. Broken stems initially required extraction whilst preserving as much of the longitudinal and transverse bone stock as possible in order to facilitate osseo-mechanical integration of the revision prosthesis. This was accomplished using trephines to core the ingrown broken stem out of the bone. Over the last 20 years, the 16 broken stems have been revised in 5 patients to larger Kotz uncemented stems, 2 to cemented GMRS stems with an adaptor to the KMFTR system, 3 to Restoration uncemented revision hip stems with a custom adaptor to the KMFTR system, 2 to custom GMRS uncemented stems with an adaptor to the KMFTR system, and 4 to total femurs. All except one patient was alive with no evidence of disease. Post-revision, 14 patients had TESS, MSTS87, MSTS93 scores of 80.5, 25.5 and 70 respectively.

Conclusion: Despite very low aseptic loosening rates, mechanical failure of the Kotz prosthesis continues to be a significant clinical problem even several years after implantation. Fatigue failure often leads to the difficult scenario of removing a well-ingrown uncemented stem. Our data illustrates that these prostheses can often be successfully revised by trephining out the broken stem and inserting new uncemented stems. Functional outcome continues to be good and is comparable to pre-revision levels.