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

Area of Science:

• Orthopedic Surgery
• Comparative Anatomy
• Biomechanics

Background:

• Human elbow hyperextension is typically attributed to soft tissue laxity.
• Olive baboons (Papio anubis) commonly exhibit elbow flexion contracture, contrasting with human joint mobility.
• Understanding the anatomical factors influencing elbow extension is crucial for both clinical and evolutionary perspectives.

Purpose of the Study:

• To compare the anatomical and functional roles of the olecranon-coronoid notch angle in terminal elbow extension between humans and baboons.
• To investigate the influence of the notch angle's anterior opening on elbow extension mechanics.
• To determine the contribution of the olecranon-coronoid notch angle to interspecies differences in elbow extension.

Main Methods:

• Active terminal elbow extension measurement using the neutral zero method in 211 human elbows.
• Passive elbow extension assessment and radiographic analysis of 60 baboon extremities.
• Anatomical dissection of 16 baboon arms to evaluate the role of anterior soft tissues.

Main Results:

• The mean olecranon-coronoid notch angle differed significantly between humans (22.2°) and baboons (11.3°).
• The notch angle magnitude was a significant predictor of terminal elbow extension across species (P < 0.001).
• Causal mediation analysis indicated the notch angle accounted for 18% of the interspecies difference in terminal elbow extension (P < 0.001).

Conclusions:

• The olecranon-coronoid notch angle and its anterior opening significantly influence terminal elbow extension in both humans and baboons.
• This anatomical feature plays a role in the distinct elbow extension capabilities observed between the two species.
• The humeroulnar joint primarily limits further elbow extension after anterior soft tissue release in baboons.