7.P.10 EVALUATION OF STABILITY OF ROTATING HINGE KNEE PROSTHESES: A BIOMECHANICAL ANALYSIS

Abstract

Introduction: Rotating hinge knee prostheses are designed to provide a stable knee reconstruction when the intrinsic soft-tissue stability of the knee had been lost as a result of tumor resection, multiple knee replacements, trauma or surgical reconstruction. Instability is the main risk factor for implant’s dislocation. We performed a biomechanical analysis to establish the association between design of the central rotational stem (length and taper) and the implant’s stability, using a self constructed biomechanical apparatus on a test bench in the laboratory.

Materials and Methods: The lengths and tapers of the central rotational stem of three different rotating hinge knee implants (LPS/M.B.T. (DePuy)-cylindrical, length: 46mm, taper: 0°; S-ROM Noiles (DePuy) -conical, length: 46mm, taper: 5°; GMRS (Howmedica) -cylindrical, length: 47mm, taper: 0°) were measured using a self-constructed biomechanical testing device. The degree of tilting of the central rotational stem within the vertical post-in channel by extending the distraction was measured as well the maximum amount of distraction before the stem’s dislocation.

Results: The GMRS implant design was superior to the LPS/M.B.T. and the S-ROM Noiles implant design concerning stability and maximum amount of distraction before dislocation (38 vs. 27 vs. 26 mm). The GMRS system had a steep rising distraction-angular displacement curve until the dislocation at 38mm occurred while the laxity curve of S-ROM Noiles showed a poor increasement. Besides early high laxity it required only 26mm of distraction to dislocate.

Conclusions: Our conclusion is that rotating hinge prostheses with a short and markedly tapered central rotational stem have the highest instability/angular laxity at any given amount of distraction. Such knee prostheses should be used with caution in patients after excessive soft tissue resection. A long and cylindrical central rotational stem seems to prevent early instability and implant’s dislocation.