Biomechanical parameters and intermuscular comparability of asymmetrical pronator quadratus muscle variations with reversed tendon aponeuroses

Background: The pronator quadratus (PQ) is a quadrangular muscle deep in the anterior compartment of the forearm that executes pronation and stabilizes the distal radioulnar joint (DRUJ). A thin, superficial tendon aponeurosis is commonly present on its ulnar side. Reports of non-typical PQ morphologies involving biomechanical parameters and intermuscular comparability are scarce despite the important knowledge they would contribute to understanding PQ variations and their clinical implications. Thus, the purpose of this study is to investigate the biomechanical parameters and intermuscular comparability of asymmetrical PQ variations with reversed tendon aponeuroses. Methods: A case of asymmetrical bilateral PQ variations was discovered during routine human cadaver dissection. The variations were photographed in situ with scale, and anatomical parameters of each muscle were measured. Mean postmortem fixed sarcomere states were evaluated via light microscopy to calculate a normalized maximal isometric force (Fmax) for each muscle. Intermuscular architectural comparability indices (δ2,1) were calculated between each muscle to indicate suitability as flap/autograft tissue. Results: Despite differences in mass and Fmax, the left distal PQ muscle (PQd; 3.81 g; Fmax = 2.22 N), left proximal PQ muscle (PQp; 4.13 g; Fmax = 2.72 N), and right PQd (6.97 g; Fmax = 4.16 N) were architecturally indifferent (0.30 < δ2,1 < 0.80). However, each of these muscles were significantly different in structure (δ2,1 ≥ 0.80), size, and functional capability compared to the right PQp (0.87 g; Fmax = 0.79 N). Notably, each ipsilateral pair of variant PQ muscles exhibited reversed location of their tendon aponeuroses. Conclusions: The PQ is encountered and utilized in many surgical procedures, including volar (modified Henry) or open reduction internal fixation (ORIF) approaches for surgically repairing a distal radius fracture. Chronic refractory myofascial pain in the distal forearm and DRUJ osteoarthritis may warrant differential evaluation for variations of the PQ. This report may provide new and important insights to orthopedists, clinical anatomists, and allied health professionals when diagnosing and treating patients with PQ variations.