Abstract
Introduction: Successful shoulder arthroplasty is based on restoration of the individual’s proximal humeral morphology with a precise osteotomy of the humeral head at the level of the anatomical neck. The objective of this study was to determine the geometry of the articular portion of the humeral head in contact with the glenoid in the neutral position and compare the orientation to the geometry of the humeral head determined using the cartilage/calcar interface of the anatomical neck.
Methods: An intact rotator cuff and joint capsule were exposed for six cadaveric full arms. Precision perspex reference cubes were attached to the greater tuberosity of the humerus and to the coracoid process of the scapula on each specimen. Each shoulder was mounted in a custom built jig with the arm fixed in the neutral position and a Microscribe 3D-X digitizer used to digitize three faces of each precision cube. The shoulder joint was then disarticulated and both the humerus and scapula re-mounted on the same jig, independently. The cube faces were re-digitized and relevant points, lines and surfaces were identified and digitized on each humerus and scapula. The humeri were then scanned using a high precision surface laser scanner.
The data collected from both digitizing tools were merged into the same coordinate system and graphically represented. Paired Student’s t-tests were used to compare the inclination and retroversion angles for the two techniques.
Results and discussion: The study found a significant difference in inclination (p less than 0.02) and no difference in retroversion (p equal to 0.75) when the glenoid position was used to calculate humeral head orientation (Inclination: Mean 11.5 deg., StD. 11.2 deg.; Retroversion: Mean 20.5 deg., StD. 6.6 deg.) when compared to using the cartilage/calcar interface (Inclination: Mean 134.1 deg., StD. 1.9 deg.; Retroversion Mean 21.7 deg., StD. 13.9 deg.).
Small deviations in the recovery of head orientation in shoulder arthroplasty may impact on the longevity of an implant. The differences in inclination and retroversion noted in this study may alter the load on the glenoid and/or rotator cuff mechanism in joint replacement. Further research is necessary.
Correspondence should be addressed to Mr Carlos Wigderowitz, Senior Lecturer, University Department of Orthopaedic and Trauma Surgery, Ninewells Hospital and Medical School, Dundee DD1 9SY.
