Proprioceptive Role of the Abductor Pollicis Longus in Thumb Carpometacarpal Osteoarthritis: An Anatomical and Immunohistochemical Study

Thumb carpometacarpal (CMC) osteoarthritis (OA) is multifactorial and may involve anatomical variability, altered biomechanics, joint laxity and impaired proprioception. We hypothesized that variability of the abductor pollicis longus (APL) tendon and proprioceptive innervation of its insertions are associated with structural degeneration in thumb CMC OA, including cartilage degeneration and osteophyte area. In this cross-sectional study, twenty fresh-frozen cadaver hands were dissected (mean age 68 years, range 53-95) between June 2022 and December 2023. Anatomical and immunohistochemical analyses were performed using hematoxylin-eosin and alcian blue staining. Protein gene product 9.5 (PGP 9.5) and S-100 (glial marker) immunostaining were assessed. Macroscopic cartilage degeneration was graded using the International Cartilage Research Society (ICRS) classification, and histological degeneration was evaluated using Mankin and Otte scores. Associations between APL tendon morphology, cartilage degeneration and immunohistochemical findings were explored using non-parametric tests. Accessory APL tendinous slips were present in 18/20 (90%) specimens, with 3-6 slips identified per specimen. APL slip number showed weak inverse correlation with OA severity at the trapezium (r = -0.24 [-0.61, 0.22]), weak positive correlation with OA severity at MC1 (r = 0.14 [-0.31, 0.55]), moderate positive correlation with cartilage cellularity (r = 0.47 [0.04, 0.76]), weak positive correlation with Mankin score (r = 0.27 [-0.20, 0.63]), and weak positive correlation with Otte score (r = 0.36 [-0.10, 0.69]). S-100 status showed a moderate inverse association with OA severity at MC1 (r = -0.52 [-0.78, -0.10]). Greater APL slip number was associated with increased cartilage cellularity, consistent with a more cellular cartilage phenotype in specimens with higher APL anatomical complexity. The inverse relationship between S-100 status and MC1 OA severity supports a potential role of innervation-related factors in structural degeneration. However, all analyses were unadjusted, and confounding by age cannot be excluded.