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Pinch Strength of Patients with CMC Arthritis and its Concomitant Conditions
Carissa C Dock, BBME1, Rebecca Stone McGaver, MS, ATC2 and Clare McCarthy, MD3, (1)University of Minnesota, Minneapolis, MN, (2)Twin Cities Orthopedices, Edina, MN, (3)Twin Cities Orthopedics, Edina, MN

Patients with carpometacarpal (CMC) arthritis may present with concomitant metacarpophalangeal (MP) hyperextension, thenar atrophy, and/or MP arthritis. This study hypothesizes that MP hyperextension >30 degrees and/or moderate-to-severe thenar atrophy will affect pinch strength significantly more than the Eaton Classification of Arthritis at the CMC joint.

Patients with CMC arthritis were evaluated prospectively by the senior author. Age, hand dominance, thenar atrophy, MP hyperextension angle (measured with goniometer aligned with metacarpal shaft and proximal phalanx), history of carpal tunnel symptoms or surgery, X-rays (AP, Lat, and Robert’s view), and three key pinch readings (measured on the Baseline pinch gauge with 30lb capacity) were obtained from each patient. Thenar atrophy was classified as follows: none (zero change to thenar muscles), mild (area of thenar muscle flattening), moderate (flat thenar muscles), and severe (indentation or concavity of thenar muscles). Patients were stratified into two groups for comparison, none-to-mild thenar atrophy and moderate-to-severe thenar atrophy. All patients with history of carpal tunnel symptoms or surgery were excluded from analysis. Statistical analysis includes simple t-tests to determine significant difference with P<.05.

Results: Biomechanical studies demonstrate a 4.4% reduction in key pinch for every 10 of MP hyperextension. This study confirms the reduction in key pinch clinically with MP hyperextension and found that moderate-to-severe thenar atrophy had an even greater effect on reduction in key pinch strength. In all patients with CMC arthritis, patients with MP hyperextension <30 degrees, and patients with MP hyperextension >30 degrees had significantly reduced pinch strength when moderate-to-severe thenar atrophy was present(Figure 1). Patients with MP arthritis also had a significantly decrease pinch strength when compared to those without MP arthritis (Figure 2). All patients with MP arthritis had moderate-to-severe thenar atrophy, and 14/25 (56%) had MP hyperextension >30 degrees, indicating end stage disease due to MP instability. Patients with moderate-to-severe thenar atrophy and MP Hyperextension >30 degrees had the weakest pinch, with 20/25 patients being less than 7 lbs, the threshold identified as necessary to complete activities of daily living by Smaby et al.

The key finding of this study is that in patients with CMC arthritis, MP joint is the key to functional pinch. This supports recent studies that have investigated performing an MP procedure at the time of CMC arthroplasty for patients with MP hyperextension >30 degrees and/or moderate-to-severe thenar atrophy.

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