The influence of abductor lever-arm changes after shoulder arthroplasty

Clinically, pain relief is usually satisfactory after shoulder arthroplasty, but the range of motion can be limited. The changes of the lever-arms of deltoid and supraspinatus muscles are considered to be important factors influencing postoperative active motion. After excluding patients with muscle weakness resulting from a neurologic condition, patients with chronic rotator-cuff tears, patients who did not follow rehabilitation guidelines, and patients with bilateral arthroplasties, 22 arthroplasty cases remained and were evaluated. Compared with the unaffected side, four radiologic parameters were assessed: (A) offset of humeral head, (B) lateralization of humeral head, (C) acromiohumeral interval, and (D) superoinferior migration of humeral head. The differences were measured and statistically analyzed. In summary: (1) The active and passive range of motion of abduction in scapular plane were 89.5° ± 26.1° and 109.7° ± 25.0°. (2) Parameters A, B, and C statistically changed (p < 0.05). (3) B value of the operated side was reduced below zero in six (27.3%). (4) There was a tendency to create medial shifting and inferior subluxation with the arthroplasty. This had a moderate adverse correlation to the active range of motion (correlation coefficient 0.498). (5) Before normalization for bone size, only C as a single parameter had moderate correlation with motion; after normalization, this decreased. (6) If combined parameters were assessed, A + C had a moderate correlation (correlation coefficient 0.435) with motion. (7) If an accurate anatomic relationship cannot be completely restored, A seemed more significant than any other parameters (p = 0.002); namely, shoulders with a smaller offset had a more limited active range of motion in abduction.