DEVELOPMENT OF AN INSTRUMENTED ULNAR HEAD PROSTHESIS TO QUANTIFY LOAD TRANSFER IN-VITRO

Abstract

Quantitative measurements of load transfer through the distal radioulnar joint (DRUJ) are limited. An instrumented ulnar head prosthesis was developed to measure bending and torsion moments about the three anatomic axes of the ulna. This device has shown repeatable loading data following insertion in a cadaveric specimen during active forearm rotation trials conducted in an in-vitro upper extremity joint simulator. The data acquired from this device will have important implications for upper extremity modeling, implant fixation and design, and optimizing surgical procedures related to DRUJ arthroplasty.

To develop a system to quantify in-vitro load transfer through the distal radioulnar joint (DRUJ) following ulnar head arthroplasty during simulated active forearm rotation. Also, the effect of an eccentric ulnar head implant design was investigated.

A load-measuring system was developed that was easily surgically inserted, and produced repeatable loading data.

The instrumented implant developed in this study will contribute to the optimization of surgical procedures and implant design parameters related to distal ulnar arthroplasty.

Four pairs of strain gauges were applied to the stem of an ulnar head prosthesis to measure bending and torsion moments about the three anatomic axes of the ulna. Three ulnar heads were machined with varying eccentricities (axisymmetric, 1.5 mm offset and 3.0 mm offset). The implant was inserted in one unpreserved cadaveric upper extremity and active forearm rotation induced using a computer controlled joint simulator. Repeatability (assessed using the maximum standard deviation over 5 trials of pronation and supination) was less than 9% of the output range for all loads. Bending and torsion moments between −0.4 and 0.5 Nm, correlating to joint loads between 0 and 50 N, were measured. The measured loads followed a consistent pattern with forearm position. Higher loads were noted for the eccentric implant heads compared to the axisymmetric head, especially at the extreme ranges of rotation. Clinical interpretation of these findings is difficult since the optimal loading scenario for distal ulnar implant longevity remains unknown.

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