Determination of the optimal elbow axis for evaluation of placement of prostheses

Objective. To present a method to determine the position and orientation of the mean optimal flexion axis of the elbow in vivo to be used in clinical research. Design. Registering the movements of the forearm with respect to the upper arm during five cycles of flexion and extension of the elbow using a 6 degr ees-of-freedom electromagnetic tracking device. Background. Loosening of elbow endoprostheses could be caused by not placin g the prostheses in a biomechanically optimal way. To evaluate the placemen t of endoprostheses with regard to loosening, a method to determine the elb ow axis is needed. Methods. The movements of the right forearm with respect to the upper arm d uring flexion and extension were registered with a 6 degrees-of-freedom ele ctromagnetic tracking device. A mean optimal instantaneous helical axis of 10 elbows was calculated in a coordinate system related to the humerus. Results. The average position of the flexion/extension axis was 0.81 cm (SD 0.66 cm) cranially and 1.86 cm (SD 0.72 cm) ventrally of the epicondylus l ateralis. The average angle with the frontal plane was 15.3 degrees (SD 2 d egrees). Conclusions. A useful estimation of the position and orientation of a mean optimal flexion axis can be obtained in vivo.