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Ulnar Collateral Ligament Strain of the Thumb Metacarpophalangeal Joint: Biomechanical Comparison of Two Postoperative Immobilization Techniques
John R. Lien, MD; Abhishek Julka, MD; Alexander Brunfeldt, MS; Jeffrey N. Lawton, MD; University of Michigan Health System
University of Michigan Health System, Ann Arbor, MI, USA

Hypothesis
This biomechanical study compares postoperative immobilization techniques of the thumb metacarpophalangeal ulnar collateral ligament. During simulated pinch, we hypothesize the ulnar collateral ligament (UCL) will experience significantly lower strain with transarticular pin fixation and splint application when compared to splint application alone.

Methods
Ten cadaveric specimens were used in this study. After isolation of the thumb metacarpophalangeal ulnar collateral ligament, a differential variable reluctance transducer was applied across the proper UCL. After specimen mounting, simulated pinch was achieved through loading the flexor pollicis longus (FPL) tendon. This simulated pinch induced strain in prior biomechanical studies. The metacarpophalangeal joint was held in neutral position with a transarticular Kirschner wire and a plaster forearm based thumb spica splint was applied. The FPL tendon was loaded to an equivalent of 2, 4, and 6 pounds of tip pinch force. Displacement measurements were repeated four times. The Kirschner wire was then removed with the splint in place, and the measurements were repeated. Measurements with a valgus force application to the thumb were performed as well.

Results
Using the induced strain values from 2, 4, and 6 pounds of thumb tip pinch force, average values of -0.005, -0.010, -0.014 with pin and splint immobilization were noted, respectively. With splint immobilization alone, strain values were -0.001, 0.000, and 0.000 at 2, 4 and 6 pounds of pinch force, respectively. Both sets of values are lower than those previously published. The differences between immobilization methods did not reach statistical significance with our sample size. The negative strain values with some pinch forces suggest compression across the UCL. A recognized limitation of this cadaveric study is the lack of recruitment of intrinsic hand musculature. With sole recruitment of the FPL tendon, the observed compression of the UCL could be due to paradoxical thumb metacarpophalangeal extension, with the splint acting as a fulcrum.

Summary Points
- There are no previous biomechanical studies comparing two commonly used postoperative immobilization techniques after UCL repair.
- The thumb metacarpophalangeal UCL may experience negative strain (compression) during simulated pinch with both transarticular pin fixation and splinting and splinting alone.
- After ulnar collateral ligament repair, forearm based thumb spica splinting with or without supplemental transarticular pin fixation effectively protects the ligament from strain.
- The use of temporary metacarpophalangeal pin fixation may be useful in difficult cases, such as those with volar plate rupture or noncompliance.


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