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Mal-aligning humeral offset may not effect shoulder hemiarthroplasty: a biomechanical study.

BACKGROUND
Component mal-positioning may contribute to a poor clinical outcome after hemiarthroplasty. Eccentric head components, recently introduced, allow posterior offset of the humeral head relative to the shaft so the purpose of this study was to determine effects of mal-aligning humeral head offset.


MATERIAL/METHODS
Five glenohumeral joints were each placed in 60 and 90 degrees of shoulder abduction and in apprehension positions using a custom shoulder-testing device. Joint reaction force and contact areas, pressures and patterns were measured. Humeral head offset was then mal-aligned by rotating the eccentric head component 90 degrees and testing was repeated.


RESULTS
There were no significant differences in the joint reaction forces in comparison of the hemiarthoplasies that mimicked and mal-aligned humeral offset. A significant increase in total contact area was found only at 60 degrees of abduction, increasing from 137.4+/-32.2 millimeters(2) to 243.4+/-27.0 millimeters(2). (p<0.05) No changes were found in contact pressures; only the patterns of contact were meaningfully different after hemiarthroplasty mal-aligned in humeral offset. In the apprehension position, rather than elongated ovals, there were multiple irregular patterns in the posterior joint.


CONCLUSIONS
Meaningful alterations in joint biomechanics did not result from hemiarthroplasty that mal-aligned humeral offset, albeit the mal-alignments studied were small. Glenoid impingement with the humeral metaphysis may occur if portions of the humeral osteotomy surface, not covered by the head component, are not chamfered during the surgical procedure.