Abstract
BACKGROUND: Scapular dyskinesis is frequently observed with various types of shoulder instability, but whether scapular dyskinesis could contribute to shoulder instability is still unclear. The purpose of this study was to determine the effects of scapular orientation on anterior and posterior glenohumeral translation using a cadaveric model of anterior and posterior labral tears. METHODS: Twenty fresh-frozen cadaveric shoulders were divided into 2 groups: the anterior lesion (n = 10) and posterior lesion (n = 10) groups. The humeral head was translated anteriorly or posteriorly with a constant 30 N force in the anterior or posterior tear groups, respectively. Humeral head displacement was measured at neutral scapula orientation for the intact labrum and following anterior or posterior labral tears. Following a labral tear, humeral head displacement was also measured at 6 additional scapular orientations (±10° increments from neutral), including downward rotation, upward rotation, posterior tilt, anterior tilt, internal rotation, and external rotation. The humerus was held at 0° of horizontal abduction and 40° of horizontal abduction (the apprehension test position) or 40° of horizontal adduction (the jerk test position) in the anterior lesion or posterior lesion groups, respectively. RESULTS: The presence of isolated labral tears generally increased anterior and posterior translations on the order of 1-2 mm in the neutral scapular orientation (P ≤ .021). Anterior humeral head translation in 0° humeral abduction further increased by approximately 1 mm in the mean upward scapular rotation orientation (P ≤ .021). In the apprehension test, anterior translation increased from posterior to anterior scapular tilt (1.3 mm, P = .017), and from internal to external scapular rotation (1.8 mm, P ≤ .006). Posterior humeral translation in 0° humeral abduction showed trends increasing from downward to upward scapular rotation (1.2 mm, P ≤ .027) and posterior to anterior scapular tilt (2.8 mm, P ≤ .007), while slightly decreasing from internal to external scapular rotation (0.6 mm, P = .014). Posterior translation in the jerk test increased from downward to upward scapular rotation (0.8 mm, P ≤ .012) and posterior to anterior scapular tilt (0.9 mm, P ≤ .043), but slightly decreased from internal to external scapular rotation (0.6 mm, P = .001). CONCLUSION: Increased scapular upward rotation, anterior tilt, and external rotation were associated with increased anterior translation of the humeral head in shoulders with anterior labral lesions. In shoulders with posterior labral lesions, increased scapular upward rotation, anterior tilt, and internal rotation were associated with increased posterior translation of the humeral head. These findings suggest that scapular dyskinesis could contribute to instability recurrence.