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"Diamond" Stress View Radiograph for Pre-Operative Evaluation of Thumb Metacarpophalangeal Joint Hyperextension Laxity
Paul Austin Henkel, DO1; Jeffrey A Marchessault, MD;
1Wellmont Holston Valley Medical Center, Kingsport, TN; 2Lincoln Memorial University-DeBusk College of Osteopathic Medicine, Kingsport, TN

Introduction: Thumb metacarpophalangeal (MCP) joint hyperextension deformity is relatively common in patients with carpometacarpal (CMC) arthritis. It is generally recommended that thumb MCP joint hyperextension greater than 30 degrees be addressed during CMC arthroplasty by MCP arthrodesis or capsulodesis. However, accurate clinical measurement of MCP hyperextension is difficult using standard finger goniometers. The purpose of this study was to compare goniometer measurement and a novel stress radiograph technique of the thumb MCP joint to intra-operative stress radiographs of patients undergoing thumb CMC arthroplasty to validate a pre-operative measurement of MCP joint hyperlaxity.

Materials & Methods: Local IRB approval was obtained. Exclusion criteria included previous thumb surgery. All enrolled patients electing to undergo thumb CMC arthroplasty by a single surgeon were clinically measured for MCP joint hyperlaxity using a standard finger goniometer. The patients were then instructed to form a "diamond" between their hands to stress the MCP joint of both thumbs into hyperextension during the pre-operative radiographic evaluation of the thumb. The radiographic angle of both thumb MCP joints was measured blinded to the other measurements. All thumb MCP joints were then manually stressed in the operating room, under anesthesia, to remove pain as a variable. The intra-operative MCP stress view was considered the gold standard. The differences between the goniometer and intra-operative stress view, and "diamond" view to the intra-operative stress view were compared for statistical significance using Pearson Correlation and paired sample t-test.

Results: Fifty-seven thumb examinations in 30 patients, averaging 61 years of age, were completed. Goniometer, "diamond" view, and intra-operative measurements were averaged for the left thumb and right thumb in each group. Pearson correlations at the .01 level were statistically significant (p<.05) when comparing the goniometer and "diamond" measurements to the intra-operative measurements. Paired sample t-test significance of p>.53 between the "diamond" view and intra-operative measurement was consistent that the two views were inter-changeable. Conversely, paired sample t-test significance of p<0.001 showed the goniometer measurement was statistically different from the intra-operative view.

Conclusions: The thumb hyperextension "diamond" stress radiograph accurately measures MCP joint hyperlaxity, providing objective evidence to patients being counseled on CMC arthroplasty. Goniometer measurement of thumb MCP joint hyperextension does not reliably measure MCP hyperlaxity compared to patients under anesthesia.

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