Comparison of Terminal Elbow Extension between Humans and Baboons (Papio anubis)

It is assumed that hyperextension of the elbow joint beyond 0° in humans is due to local ligament and other soft tissue laxity. The common extant old-world olive baboon (Papio anubis), with a quadrupedal gait, commonly has a mild elbow joint flexion contracture. This study compares anatomic and functional roles of the olecranon-coronoid notch angle of the proximal ulna and the anterior direction of its opening on terminal elbow extension in humans and baboons. Active terminal elbow extension was measured in 211 elbows of skeletally mature humans using the neutral zero method. Passive elbow extension and radiographs were performed on 60 extremities of 30 anesthetized baboons. The mean olecranon-coronoid notch angle for humans and baboons was 22.2 ± 6.0° and 11.3 ± 3.2°, respectively (P < 0.001). Mixed effect regression analysis showed that the magnitude of the angle was a significant predictor of terminal elbow extension (P < 0.001) when accounting for species. Also, the causal mediation analysis showed that 18% of the difference in terminal elbow extension between species could be attributed to the olecranon-coronoid notch angle (P < 0.001). Anatomic dissection of 16 baboon arms showed that sectioning of all the anterior soft tissue structures increased mean terminal elbow extension from -18.2 ± 5.9° to -7.1 ± 6.0° and that further elbow extension was blocked primarily by the intact humeroulnar joint. The size of the notch angle and the positioning of its anterior proximal opening played a significant role in deciding terminal elbow extension in both species.