Purpose Pronator teres (PT) to extensor carpi radialis brevis (ECRB) or extensor pollicis longus (EPL) tendon transfer surgery is often indicated for patients lacking wrist or thumb extension. The purpose of this investigation was to determine if architectural differences among these muscles contributes to the hand function deficits seen in some patients following this procedure.
Methods Ten fresh-frozen arms were skinned and fixed in 10% buffered formalin for 72 hours. Arms were then rinsed in 1X phosphate-buffered saline and PT, ECRB, and EPL muscles were excised. Measurements of mass were made on each of the muscles. Pennation angle and fascicle length were then measured at four predetermined locations along each muscle. This allowed the organization of fibers to be described and permitted the humeral and ulnar heads of PT to be characterized. Using laser diffraction, sarcomere lengths were measured, allowing muscle fiber lengths to be normalized to 2.7μm. Calculations of physiological cross sectional area (an index of force-generating capacity) and normalized fiber length (an index of muscle excursion) were then made. The architectural features of each muscle were compared using one way ANOVAs and post-hoc Tukey tests. P-values less than 0.05 were considered statistically significant.
Results Interestingly, the ulnar head of PT was present in only six of 10 specimens. The average physiological cross sectional area (PCSA) of PT was similar to ECRB (4.1 ± 0.5 cm2 vs. 3.3 ± 0.3 cm2) but was significantly larger than EPL (4.1 ± 0.5 cm2 vs. 1.1 ± 0.1 cm2). The average fiber length for PT was similar to ECRB (70.1 ± 5.7 mm vs. 61.7 ± 1.9 mm) but was significantly longer than EPL (70.1 ± 5.7 mm vs. 54.4 ± 3.2 mm). However, the fiber lengths in the ulnar head of PT (44.0 ± 5.7 mm) were significantly shorter than ECRB or EPL when present.
Conclusions These data indicate that, based on architecture, PT is an excellent donor choice for transfer to the ECRB for restoration of wrist extension. While PT muscles had significantly larger PCSAs and longer fiber lengths compared to EPL muscles, these differences would provide force production and excursion advantages when compared to the host muscle's native architecture. In some cases, however, the ulnar head of PT may limit the total excursion range of the muscle as its fibers are significantly shorter than those of ECRB and EPL. Future investigations should attempt to identify excursion limitations imposed by the ulnar head of PT in tendon transfer patients with persistent wrist and thumb extension deficits.