Three-dimensional scapular morphology is associated with rotator cuff tears and alters the abduction moment arm of the supraspinatus

Abstract

Background: Numerous studies have reported an association between rotator cuff injury and two-dimensional measures of scapular morphology. However, the mechanical underpinnings explaining how these shape features affect glenohumeral joint function and lead to injury are poorly understood. We hypothesized that three-dimensional features of scapular morphology differentiate asymptomatic shoulders from those with rotator cuff tears, and that these features would alter the mechanical advantage of the supraspinatus.

Methods: Twenty-four individuals with supraspinatus tears and twenty-seven age-matched controls were recruited. A statistical shape analysis identified scapular features distinguishing symptomatic patients from asymptomatic controls. We examined the effect of injury-associated morphology on mechanics by developing a morphable model driven by six degree-of-freedom biplanar videoradiography data. We used the model to simulate abduction for a range of shapes and computed the supraspinatus moment arm.

Findings: Rotator cuff injury was associated with a cranial orientation of the glenoid and scapular spine (P = .011, d = 0.75) and/or decreased subacromial space (P = .001, d = 0.94). The shape analysis also identified previously undocumented features associated with superior inclination and subacromial narrowing. In our computational model, warping the scapula from a cranial to a lateral orientation increased the supraspinatus moment arm at 20° of abduction and decreased the moment arm at 160° of abduction.

Interpretations: Three-dimensional analysis of scapular morphology indicates a stronger relationship between morphology and cuff tears than two-dimensional measures. Insight into how morphological features affect rotator cuff mechanics may improve patient-specific strategies for prevention and treatment of cuff tears.

Keywords: Biplanar videoradiography; Imaging - computed tomography; Musculoskeletal modeling; Rotator cuff; Shoulder; Statistical shape modeling.

Fig. 1.

(a) Anterior and (b) Posterior view of the musculoskeletal model. Fibres are modelled as a string of 40 points, optimized such that each fibre traversed the shortest distance from origin to insertion without penetrating the bone surface. The origin and insertion points were automatically mapped to all generated shape models and muscle fibres were calculated for each frame of the kinematic data. The supraspinatus (SUP) fibres are labelled.

Fig. 2.

The glenohumeral joint angles for 160 degrees of thoracohumeral abduction. The abduction and internal rotation angles were determined from the linear fit to subject in vivo kinematic data and the elevation plane angle was determined from a quadratic fit to the in vivo data.

Fig. 3.

Overview of the methodology including data acquisition, data processing, and shape and biomechanical analyses.

Fig. 4.

(a) Scatter plot of PC4 vs. PC2 scores for asymptomatic subjects and subjects with full thickness supraspinatus tears. (b) Variation explained by each PC.

Fig. 5.

The first four principal components describe the greatest modes of variation in the dataset. PC2 and PC4 significantly distinguished the asymptomatic and tear subjects, while PC1 and PC3 did not. (a) PC1 altered the aspect ratio of the scapular body. (b) PC2 shows a change from cranial to lateral orientation of the glenoid as the shape moves from tear- to asymptomatic. (c) PC3 shifted the orientation and position of the acromion and coracoid process and was not associated with tears. (d) PC4 shows an increase in the subacromial space and narrowing of the supraspinous fossa as the shape moves from tear- to asymptomatic.

Fig. 6.

Supraspinatus moment arm curves for 160 degrees of thoracohumeral abduction. Moment arm curves for the 21 generated shape models are plotted for each principal component. The arbitrary scale of −10 to +10 spans the range of the PC scores calculated in the dataset, where a negative score is associated with the tear subjects and a positive score is associated with the asymptomatic subjects.

Fig. 7.

Supraspinatus abduction moment arms at 20°, 90°, and 160° of abduction for scapular shape models generated across (a) PC2, representing change from cranial to lateral orientation, and (b) PC4, representing change from small subacromial space to large subacromial space. R² and P values are shown for the results of the statistical analysis.