Fiber Type Composition of Contiguous Palmaris Longus and Abductor Pollicis Brevis Muscles: Morphological Evidence of a Functional Synergy

Introduction The palmaris longus (PL) tendon can be transferred to the thumb for use in surgical opponensplasty to restore functional hand movements in patients with thenar paralysis. Successful opponensplasty outcomes using the PL tendon have been attributed to an intrinsic synergy of the PL with the abductor pollicis brevis (APB) such that no specific muscular retraining is needed upon tendon transfer. This synergy has been demonstrated electromyographically in which synchronous EMG signals were recorded from the PL and APB during functional thenar movements in healthy young individuals, in vivo . Thus, although an established synergistic relationship between these muscles has been demonstrated, this functional relationship may depend on their spatial relationship and properties of their constituent muscle fibers. The purpose of this study was to compare the proportions of type I and type II muscle fibers in the APB from cadaveric muscle based on its contiguous morphological relationship with the PL tendon for further insight into their functional synergy, contractile capacity and digastric arrangement. Methods Twenty‐four contiguous PL and APB specimens were harvested from the upper limbs (12 right, 12 left) of twelve formalin‐embalmed cadavers (mean age: 74 ± 10 years). The 24 limbs were classified into two groups based on the following morphological criteria: (1) APB with discrete PL tendon connections, or (2) APB with non‐discrete, or rudimentary, PL tendon connections. Cadaveric specimens were obtained from the body bequeathal program at the University of Western Ontario and approved for research use by the Committee for Cadaver Use in Research (REF#21092016). The fiber type composition of the APB and PL muscles was determined by labeling serial cross‐sections with myosin heavy chain type I and type II monoclonal antibodies. Results The APB fascicles from 11 hands (46%) were classified as discrete based on their continuity with the PL tendon. Conversely, the APB fascicles from 13 hands (54%) were classified as non‐discrete due to rudimentary or minimal connections with the PL tendon. The PL consisted of a relatively heterogeneous fiber type composition irrespective of the presence of a discrete (type I: 41 ± 11%, type II: 55 ± 12%, hybrid: 4 ± 3%), or rudimentary (type I: 49 ± 10%,type II: 45 ± 9%, hybrid: 6 ± 4%), tendinous connection with APB musculature. The APB fascicles arranged in a digastric manner with the PL tendon had significantly greater proportions of type II fibers (41 ± 19%) compared to those with rudimentary PL connections (type II: 15 ± 8%). Conclusion The APB fascicles arranged in a digastric relationship with the PL may have the capacity to produce more powerful contractions than those with rudimentary PL tendons based on the known contractile properties of type II muscle fibers. Knowledge of the spatial relationship between the PL and thenar musculature prior to PL autologous tendon transfer may be a useful indicator of the quality of established synergy in vivo. Support or Funding Information Supported by NSERC