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Assessment of Metacarpophalangeal Joint Extension – A Cadaveric Study
Eitan Melamed, M.D.
1; Varun Arvind, MD, PhD
2; Robert J Strauch, MD
31NYC Health + Hospitals/Elmhurst, Queens, NY; 2Columbia University, New York, NY; 3Columbia University Medical Center, New York, NY
Introduction: The extensor complex of the finger metacarpophalangeal (MCP) joint is composed of the extensor tendon (ET), sagittal band (SB), and deep fibrous expansion (DFE) underlying the extensor tendon. The purpose of this study is to compare the role of the ET, SB, and DFE in MCP extension.
Materials and Methods: Six fresh frozen specimens were used (age: 30-66, 33% right hand, 50% male). A 2kg weight was hung from the extensor tendon and a 1.5kg weight from the flexor tendon. The radiocarpal joints were pinned in neutral. Serial sectioning was performed of the ET, SB, and DFE in various order. Sectioning of the ET was performed distal to the SB. Serial images were captured before and after sequential sectioning of the ET, SB and DFE to measure the MCP extension angle. Thumbs were excluded. MCP extension was calculated, and the angular relative differences (in degrees) from baseline were reported. One-way ANOVA was performed with repeated measures with Holm-Sidak's correction for multiple comparisons. P<0.05 was deemed significant.
Results: Sectioning of the ET first led to an extensor lag of 8.9 degrees on average, with no significant increase in MCP extensor lag following further release of the sagittal band (Fig. 1A). In contrast, sectioning of the sagittal band first led to an average extensor lag of 14.5 degrees, with a significant additional increase in extensor lag of 5.7 degrees following release of the ET (p<0.05) (Fig. 1B). There was no significant difference in extensor lag between sectioning of the SB or ET first (p=0.24). In both scenarios, final sectioning of the DFE caused worsening of the extensor lag by an additional 12.1 degrees, on average. DFE was observed originating from the palmar surface of the extensor mechanism and inserting upon the dorsal joint capsule, which itself then inserted onto the dorsal base of the proximal phalanx.
Conclusions: In the biomechanics of MCP joint extension, both the SB and the ET contribute equally. Additionally, it appears the DFE is a secondary extensor of the MCP joint in the absence of the SB and ET distal to the SB.
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