Abstract
A changed kinematic elbow axis can cause early loosening of elbow endoprostheses and can decrease the functional outcome. Therefore, these prostheses and their alignment tools are designed to reconstruct normal joint kinematics. We investigated whether it is possible to reconstruct the pre-operative kinematic axis of the elbow when an iBP™ elbow endoprosthesis (Biomet) has been placed.
The calibrated Flock of Birds® electromagnetic tracking device registered controlled passive elbow flexion of ten embalmed upper extremities. The pre-operative kinematic elbow axes were established using helical axes.
Results were expressed in the humeral coordinate system defined by the glenohumeral joint rotation centre and the lateral and medial epicondyle of the humerus. The glenohumeral joint rotation centre was determined using a regression method. The senior author implanted the iBP™ elbow endoprosthesis using standard instrumentation for humeral component alignment. The post-operative kinematic axes were then calculated. A Student’s t-test was performed to compare the pre- and post-operative axes.
No significant differences were found in the direction of the kinematic elbow axes before and after surgery, indicating no alteration in the valgus/varus angle or change in longitudinal rotation of the ulna with respect to the humerus. However, the axis was located significantly more distal (mean difference 7.0 mm, p = 0.004) after surgery. The ventral-dorsal location of the kinematic axis was not significantly different (p = 0.748) after surgery, but there was some variation in individual axes. The iBP™ Elbow System enables the reconstruction of the direction of the pre-operative kinematic elbow axis. While the exact position of the pre-operative axis could not be reproduced in vitro, the kinematic axis of the elbow is expected to be less distal in vivo as a result of the extensive destruction of the rheumatoid elbow. Individually adjustable alignment tools might enable more precise reconstruction.
The abstracts were prepared by Nico Verdonschot. Correspondence should be addressed to him at Orthopaedic Research Laboratory, University Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands.
