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Impact of Metacarpal and Proximal Phalanx Morphology on Pyrocarbon Metacarpophalangeal Arthroplasty Outcomes
Audrey Bankes, BS
1, Adam Schluttenhofer, BS
1, Zoe Postal, BS
1, Andy Tom, BS
1; Marco Rizzo, MD
2(1)Mayo Clinic, Rochester, MN, (2)Division of Hand Surgery, Mayo Clinic, Rochester, MN
Introduction: Pyrocarbon arthroplasty is an established surgical option for advanced metacarpophalangeal (MCP) arthritis, but high rates of complications and revisions long-term remain a concern. Little is known about how native joint morphology affects implant survival. This study investigates the impact of preoperative metacarpal and proximal phalanx morphology on revision rates following primary MCP arthroplasty.
Methods & Methods: We retrospectively identified all primary MCP arthroplasties for rheumatoid arthritis (RA) and osteoarthritis (OA) from 2004 to 2022. We used preoperative posteroanterior radiographs to measure metacarpal length, mid-diaphyseal diameter, and neck width, along with proximal phalanx length and neck width. Cluster-robust Cox proportional hazard models were used to evaluate the effect of each measurement on revision risk, controlling for digit (Bonferroni corrected p<0.01 considered significant). Features with significant effects were analyzed in multivariate models controlling for diagnosis and digit. We also tested primary diagnosis as an interaction term and correlated significant features to preoperative grip strength, a known correlate of hand size that affects joint biomechanical forces.
Results: There were 177 included joints (90 OA, 87 RA) in 89 patients with a mean 8.3 years follow-up. There were 32 total revisions (18%), most commonly for ulnar deviation/subluxation (18 of 32). In initial analysis, metacarpal length (p=0.003) and proximal phalanx length (p=0.001) were significant protective factors, and no other feature had a significant effect. In multivariate analysis, increased metacarpal length (HR 0.88, p=0.01) and proximal phalanx length (HR 0.85, p=0.003) were protective factors against revision (Figures 1 & 2). There was no significant interaction between diagnosis and metacarpal length (p=0.46) or phalanx length (p=0.37). Preoperative grip strength was moderately correlated to metacarpal length (r = 0.52, p <0.001) and proximal phalanx length (r = 0.36, p<0.001).
Conclusions: Increased metacarpal and proximal phalanx lengths are significant protective factors against revision following pyrocarbon MCP arthroplasty in both OA and RA. Increased MCP lever arm lengths and higher correlated grip strengths may confer favorable biomechanics and load-sharing. Alternatively, larger joints may allow for easier intraoperative soft tissue balancing and implant sizing. Preoperative assessment of MCP morphology may help risk-stratify patients, though more work is needed to elucidate the mechanisms of conferred protective effects.
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