This study was the first to focus on the relationship between RCT and the scapular body. The relationship between RCT and acromion and glenoid morphology has already been extensively reported. Bigliani et al. classified the acromion shape using lateral radiographs and reported that RCT is more prevalent in patients with a hooked acromion than in those with a curved or flat acromion . Aoki et al. measured the acromial slope on skeletal preparations in the sagittal plane and found that a low acromial slope is a predisposing factor for subacromial impingement . Moreover, Banas et al. described LAA on oblique coronal magnetic resonance imaging and discovered that lower LAAs are associated with RCT . Nyffeler et al. proposed the acromion index (AI), which is the ratio between the distance from the glenoid to the lateral edge of the acromion and the distance from the glenoid rim to the lateral edge of the humeral head, to quantify the lateral extension of the acromion on true anteroposterior radiographs; they found that a large AI is associated with full-thickness RCT . Torrens et al. also proposed the acromial coverage index, which is a different measurement method, and reported that the lateral extension of the acromion is higher in patients with RCT than in those with an intact rotator cuff . Glenoid inclination has been measured on anteroposterior radiographs or CT using various methods. Some studies showed that glenoid inclination correlates with RCT , whereas other studies came to the opposite conclusion . Moor and Gerber et al. described CSA as a measure of the glenoid inclination and of the lateral extension of the acromion on anteroposterior radiographs; they found that a smaller CSA is associated with osteoarthritis, whereas a larger CSA is associated with RCTs [2, 3]. The anteroposterior acromial coverage is reportedly not related to RCT [11, 12]. In a cadaveric study, individuals with RCT had a greater anterior acromial projection . In the present study, the results of univariate analysis revealed that the RCT group had larger CSA, ACA, AAPA, SSA and IAA and a smaller CSSA than the non-RCT group. However, ACA, CSSA, AAPA and SSA were not significantly different in the presence or absence of RCT in the multivariate analysis but the CSA and IAA were related to RCT. The present study is the first to report not only high CSA but also the inclination of the inferior angle of the scapula and the bending of the scapular body in RCT cases.
The bending of the scapular body in RCT may be explained by the idea that the morphology of the scapular body affects scapular motion. The inferior part of the trapezius generates a scapular posterior tilt and upward rotation. Decreased trapezius activity reportedly causes insufficient elevation of the acromion and scapular anterior tilt, resulting in subacromial impingement [13–15]. The serratus anterior is the primary muscle that stabilises the medial border and inferior angle of the scapula to prevent scapular winging and anterior tilt . Ludewig et al. reported that the scapula shows an anterior tilt when the serratus anterior is dysfunctional, as evaluated using surface electromyography, thus causing subacromial impingement [17, 18]. Cases of impingement syndrome reportedly have lesser upward rotation and external rotation of the scapula . Hébert et al. quantified the contribution of each scapular rotation to the scapular total range of motion in impingement syndrome; the results suggest that patients with impingement syndrome have a lesser posterior tilting of the scapula in the symptomatic shoulder than in the asymptomatic contralateral shoulder . According to Smith et al., the movement of the scapula into a protracted or retracted position results in reduced isometric shoulder elevation strength . Kijima et al. compared the three-dimensional scapular kinematics during scapular-plane abduction of symptomatic RCTs, asymptomatic RCTs and healthy shoulders and suggested that the posterior tilt of the scapular was smaller in the symptomatic RCTs than in the healthy shoulders . These previous studies identified an anterior tilt of the scapula in individuals with shoulder impingement or symptomatic RCTs. The increased anterior tilt of the scapula might result in a decrease in the subacromial space, between the greater tuberosity and the acromion, which may aggravate symptoms. In the present study, which investigated the morphology of the scapula, the angle between the inferior angle and the scapular plane was larger and the scapular body was bent in cases with RCT. The serratus anterior originates from the ribs and inserts on the medial border and inferior angle of the scapula. In other words, the inferior angle of the scapula corresponds to the insertion of the muscle that stabilises the scapula in the thorax. When the inferior angle is inclined and the scapular body is bent, the scapula’s superior part may be more likely to tilt anteriorly, leading to impingement, which may be related to the pathogenesis of RCT and pain.
Another possible reason for the bending of the scapular body may be related to the thorax. Kebaetse et al. reported that a slouched position reduced the posterior tilt of the scapula during scapular plane abduction and was associated with a decreased active shoulder abduction range of motion and muscle force . Culham et al. also reported that the scapula tilts more anteriorly as the slope of the upper thoracic axis increases . However, the relationship between the thoracic shape and the scapular body curvature remains unconfirmed. Therefore, it remains unclear why the scapular body was bent in the cases with RCT.
This study has several limitations. First, the tear size and location varied in the RCT group. Second, the thoracic shape and scapular dynamics were not examined. Further investigation considering the thorax and dynamic analyses are required to clarify why the scapular body is bent in RCT cases. Third, considering that the subacromial spur was excluded from the analysis, whether the subacromial spur existed before or after the RCT remains unclear. Evaluating the subacromial spur would have given different results.