Cross-Sectional Area of the Rotator Cuff Muscles in MRI – Is there Evidence for a Biomechanical Balanced Shoulder?

Abstract

ObjectiveTo provide in-vivo evidence for the common biomechanical concept of transverse and craniocaudal force couples in the shoulder that are yielded by both the rotator cuff muscles (RCM) and the deltoid and to quantitatively evaluate and correlate the cross-sectional areas (CSA) of the corresponding RCM as a surrogate marker for muscle strength using MRI.Materials and MethodsFifty patients (mean age, 36 years; age range, 18–57 years; 41 male, 9 female) without rotator cuff tears were included in this retrospective study. Data were assessed by two readers. The CSA (mm2) of all rotator cuff muscles was measured on parasagittal T1-weighted FSE sequence at two different positions (at the established “y-position” and at a more medial slice in the presumably maximal CSA for each muscle, i.e., the “set position”). The CSA of the deltoid was measured on axial intermediate-weighted FSE sequences at three positions. CSA measurements were obtained using 1.5 Tesla MR-arthrographic shoulder. Pearson’s correlation for the corresponding CSA of the force couple as well as was the intraclass correlation coefficient for the inter- and intra-reader agreement was calculated.ResultsThe mean CSA was 770 mm2 (±167) and 841 mm2 (±191) for the supraspinatus (in the y- and set-positions, respectively) and 984 mm2 (±241) and 1568 mm2 (±338) for the infraspinatus. The mean CSA was 446 mm2 (±129) and 438 mm2 (±128) for the teres minor (in the y- and set-positions, respectively) and 1953 mm2 (±553) and 2343 mm2 (±587) for the subscapularis. The three measurements of the deltoid revealed a CSA of 3063 mm2 (±839) for the upper edge, 3829 mm2 (±836) for the lower edge and 4069 mm2 (±937) for the middle of the glenoid. At the set position Pearson’s correlation of the transverse force couple (subscapularis/infraspinatus) showed a moderate positive correlation of r = 0.583 (p<0.0001) and a strong correlation when the CSA of the teres minor was added to the infraspinatus CSA (r = 0.665, p = 0.0008) and a strong positive correlation of the craniocaudal force couple (supraspinatus/deltoid) that ranged from r = 0.565–0.698 (p<0.0001). Inter-reader agreement (ranged from 0.841 to 0.997, p = 0.0007) and intra-reader agreement were excellent (ranged from 0.863 to 0.999, p = 0.0006).ConclusionThe significant correlation of the CSA of the RCM that form the transverse (subscapularis/infraspinatus-teres minor) and craniocaudal (supraspinatus/deltoid) force couple measured by MR-arthrography supports the biomechanical concept of a dynamically balanced shoulder in patients with an intact rotator cuff.

Comparative analysis of a locking plate with an all-suture anchor versus hook plate fixation of Neer IIb distal clavicle fractures

“…The cross-sectional area of the deltoid muscle was measured on axial CT at three positions: the upper and lower edges, and the middle of the glenoid (Figure 5). 17 The mean value of the three positions was calculated for deltoid muscle volume evaluation, and the ratio of the deltoid muscle volume was calculated using the following formula:…”

“…The cross-sectional area of the deltoid muscle was measured on axial CT at three positions: the upper and lower edges, and the middle of the glenoid (Figure 5). 17 The mean value of the three positions was calculated for deltoid muscle volume evaluation, and the ratio of the deltoid muscle volume was calculated using the following formula: Ratio of deltoid volume ¼ 100 Â 1 year after deltoid volume= preoperative deltoid volume All radiologic measurements were analyzed by two independent examiners (KYK and JSY). The individual value was measured, after which the individual mean value was calculated.…”

The contribution of the supraspinatus muscle at sub-maximal contractions

“…Our result is also consistent with the cross-sectional area of the two muscles. The deltoid is substantially larger than the supraspinatus and capable of producing more force (Aluisio et al, 2003; Bouaicha et al, 2016).…”

“…Based on the shoulder mechanics and muscle anatomy, it would be expected that the deltoid contributes more than the supraspinatus to shoulder elevation, particularly at higher elevation angles. This is because the cross-sectional area of the deltoid is much greater than the supraspinatus (Aluisio et al, 2003; Bouaicha et al, 2016) and the deltoid moment arm increases with humeral elevation, while the supraspinatus moment arm decreases (Ackland et al, 2008).…”

Superior Capsule Reconstruction: Review of a Novel Operative Technique for Management of Irreparable Rotator Cuff Tears

“…Furthermore, SS prevents abnormal inferior-superior translation of the humeral head, particularly during active arm elevation, by compressing the head into the glenoid fossa. The balancing forces between SSC anteriorly and IS and TM posteriorly provide stability in the sagittal plane, and the upward force of the deltoid is balanced by that of IS, TM, and SSC in the coronal plane [11,12].…”