Anatomical explanation of the screw motion of the elbow joint.

Aim: This study aimed to explore the anatomical factors contributing to the screw motion of the elbow joint during a flexion/extension movement of the elbow joint.

Background: Historically, the humero-ulnar joint has been simplified to a symmetrical, mono-axial hinge joint. More recent studies suggest an asymmetrical screw motion or helical movement during the flexion movement. This subtle varus-valgus motion during flexion and extension likely has evolutionary explanations. When the arm is in a dependent position (upper arm next to the body), the valgus angle of the elbow allows the arm to relax comfortably without interference with the pelvis. However, no anatomical studies have been conducted to confirm this biomechanical hypothesis. This study aims to explore the screw motion of the elbow joint anatomically.

Methods: Non-coupled statistical shape models of the humerii and ulnae were separately created using CT scans of 210 humerii (115 right, 95 left) and 156 ulnae (75 right, 81 left). The mean shape and the 95% CI of the variation of the first five shape modes were calculated and analyzed.The deepest points of the osseous trochlear grove were indicated on the five shape modes. This was performed in 3-matic by making sections of the articular humerus in different flexion angles. The mediolateral distance of each point to the sagittal plane was measured. The same procedure was performed for the greater sigmoid notch of the ulna. The geometry of the notch was investigated by determining the mediolateral shift of the notch with respect to the point of origin.

Results: The mean shape model of the humeral osseous groove has a helical anatomy. The ulnar greater sigmoid ridge has a circular anatomy, without any helical anatomy, as there was no mediolateral shift of the notch in the different section planes with respect to the point of origin. The maximal valgus position of the elbow was reached in extension, while the maximal varus position was reached in deep flexion.

Conclusion: This is the first anatomical study clarifying the origin of the vortical movement of the elbow due to the helical path of the distal humeral groove based on a statistical shape model of the humerus. This morphological effect should be taken into account when performing osteosynthesis or in the development of total elbow implants.