Population-Based Quantitative Spatial Mapping of Ulnar Motor Branch Origins in the Forearm: An Integrated Anatomical...

Background/Objectives: The extramuscular branching pattern of the ulnar nerve shows substantial interindividual variability, limiting the value of single-pattern anatomical descriptions. It remains unclear whether the origins of its motor branches are spatially random or retain reproducible relationships with the neural, anthropometric, and musculotendinous organization of the forearm. This study characterized the normalized spatial distribution of the origins of the main ulnar motor branches supplying the flexor carpi ulnaris (FCU) and the ulnar portion of the flexor digitorum profundus (FDP), and examined their relationships with anatomical reference variables measured during dissection. Methods: A cross-sectional cadaveric study was conducted on 16 adult right upper limbs. Fifteen specimens were dissected fresh on the day they were received by the Body Donation Programme; one specimen was frozen at -80 °C, thawed overnight, and dissected the following morning. Standardized anatomical dissection identified 54 main motor branches arising from the ulnar nerve and confirmed their target muscles by direct anatomical continuity (36 FCU and 18 FDP branches). For quantitative mapping, only the origin of each branch on the parent ulnar nerve was recorded using a laser-guided Cartesian coordinate system. Anthropometric dimensions and topographical landmarks were normalized to the corresponding forearm dimensions. Spatial organization was characterized using centroids, 95% data ellipses, Euclidean centroid distances, and a specimen-level cluster bootstrap. Generalized estimating equations (GEE) were used for cluster-aware group comparisons and exploratory multivariable association analysis, while ROC analysis and leave-one-cadaver-out cross-validation quantified internal separation and stability within the cadaveric dataset. Results: FCU and FDP branch origins occupied reproducible but partially overlapping normalized spatial territories. Their centroids differed in location, and specimen-level bootstrap resampling supported the stability of the observed separation. Cluster-aware analyses confirmed differences in the proximodistal branch-origin coordinate and in the tendon-crossing landmark. In the multivariable GEE model, the normalized tendon-crossing landmark showed the strongest adjusted association with group membership, while the proximodistal coordinate also remained associated. Conclusions: Despite marked interindividual variation in ulnar nerve branching, the origins of its main motor branches exhibit reproducible population-level spatial organization. Their distributions are related to the anatomically confirmed target muscle and to normalized neural, anthropometric, and musculotendinous landmarks measured during dissection. This integrated reference framework supports interpretation of peripheral motor innervation as an organized spatial system rather than a collection of isolated variants. The findings provide an anatomical basis for future imaging and image-guided research; however, the exploratory regression and ROC analyses do not constitute a clinically validated diagnostic model.