Accessory oblique distal head of the pronator quadratus and its biomechanical implications on the distal radioulnar joint

Introduction Parallel fibers of the pronator quadratus (PQ) bridge the distal shafts of the radius and ulna just proximal to the volar rim and distal radioulnar joint (DRUJ); thus, the PQ facilitates pronation and stabilizes the DRUJ. Morphological variations of the PQ have been described, but biomechanical investigations of distal PQ variations are scarce despite their inherent implications for the DRUJ. The objective of this study is to perform biomechanical analyses on an accessory oblique distal head of the PQ (PQ AODH ) to determine its impact on the DRUJ. Methods A bilateral case of PQ with PQ AODH was discovered during routine human cadaver dissection. The muscles were photographed in situ with scale, and their masses, mean fascicle lengths, and mean sarcomere lengths were measured. A normalized maximal isometric force ( F max ) was calculated for each muscle, which was then trigonometrically evaluated for F max vectors across the DRUJ. Intermuscular architectural comparability indices ( δ 2,1 ) were calculated between paired muscles. Results The left and right PQ AODH muscles were structurally ( δ 2,1 ≤ 0.30) and functionally ( F max = 8.59 N and 8.60 N) comparable. Each PQ AODH originated from the PQ’s typical distal-most attachment on the ulna, coursed obliquely over the volar rim and DRUJ, and inserted about the radial styloid process and to the radiocarpal joint capsule. The PQ AODH muscles produced a mean force of 7.54 N across the DRUJ. The PQ muscles produced similar F max (43.37 N and 48.65 N) but were structurally indifferent (0.30 < δ 2,1 < 0.80). The right PQ displayed significantly less obliquity than the left PQ (2.9° vs. 31.6°), distributing more of its F max across the DRUJ. Conclusions Variant muscles like the PQ AODH may create atypical forces on the DRUJ, and asymmetrical PQ muscles may compound biomechanical concerns. The presence of a PQ AODH may otherwise be advantageous for preserving functional capacity of the DRUJ after PQ injury and/or covering the distal edge of volar plates during surgical fixation of distal radius fractures.