Effect of subscapularis repair on joint contact forces based on degree of posterior-superior rotator cuff tear severity in reverse shoulder arthroplasty

Abstract

Massive irreparable rotator cuff tears (RCTs) affect the clinical outcomes of reverse shoulder arthroplasty (RSA). However, the effects of subscapularis repair on the outcomes of RSA, based on the degree of posterior-superior RCTs, are unclear. This study aimed to examine the effect of subscapularis repair on three-dimensional joint contact forces (JCFs) based on the degree of posterior-superior RCT severity in RSA. Ten human in vivo experimental data were used as input to the musculoskeletal model. A six-degrees-of-freedom (DOF) anatomical shoulder model was developed and validated against three-dimensional JCFs. The 6-DOF musculoskeletal shoulder model of RSA was then developed by importing the reverse shoulder implant into the validated anatomical shoulder model. Based on the various types of posterior-superior RCT severity, inverse dynamic simulations of subscapularis-torn and subscapularis-repaired models of RSA were performed: from isolated supraspinatus tears to partial or massive tears of the infraspinatus and teres minor. The intact rotator cuff model of RSA was also simulated for comparison with the different types of models. Our results showed that the more posterior-superior RCTs progressed in RSA, the more superior JCFs were observed at 90°, 105°, and 120° abduction in the subscapularis-torn model. However, subscapularis repair decreased the superior JCF at those angles sufficiently. In addition, the teres minor muscle-tendon force increased as infraspinatus bundle tears progressed in both the subscapularis-torn and -repaired models, in order to compensate for the reduced force during abduction. However, the teres minor muscle-tendon force was not as high as that of the infraspinatus muscle-tendon, which could result in muscle force imbalance between repaired subscapularis and teres minor. Therefore, our results suggest that repairing the subscapularis and the repairable infraspinatus during RSA can improve glenohumeral joint stability in the superior-inferior direction by restoring muscle force balance between the anterior cuff (i.e., subscapularis) and posterior cuff (i.e., infraspinatus and teres minor). The findings of this study can help clinician decide whether to repair the rotator cuff during RSA to enhance joint stability.

Keywords: biomechanics; glenohumeral joint stability; musculoskeletal model; posterior-superior rotator cuff tear; reverse shoulder arthroplasty; subscapularis repair.

FIGURE 1

The workflow of the overall musculoskeletal modeling and simulation process using the force-dependent kinematics (FDK) method. $q$ : joint angle; $\dot{q}$ : angular velocity; $\ddot{q}$ : angular acceleration; $F^{\left(FDK\right)}$ : FDK residual force; ${\alpha }^{\left(FDK\right)}$ : FDK translations at the glenohumeral joint.

FIGURE 2

Marker placement of the upper body Plug-in-Gait model. An additional maker was attached to the medial epicondyle of the humerus based on the ISB recommendation. RFHD: right forehead; LFHD: left forehead; RBHD: right back of head; LBHD: left back of head; RSHO: right shoulder; RUPA: right upper arm; RELB: right elbow; EM: the most caudal point on medial epicondyle; RFRM: right forearm; RWRA: right wrist marker A (ulnar styloid); RWRB: right wrist marker B (radial styloid); RFIN: right finger; CLAV: clavicle; STRN: sternum; RASI: right anterior-superior iliac; LASI: left anterior-superior iliac; RPSI: right posterior-superior iliac; LPSI: left posterior-superior iliac; C7: seventh cervical vertebra; T10: 10th thoracic vertebra.

FIGURE 3

The illustrations of musculoskeletal shoulder models with the scapular reference frame: (A) anatomical shoulder model; (B) musculoskeletal model of reverse shoulder arthroplasty (RSA). The scapular reference frame in the musculoskeletal model of RSA was positioned at the rotational center of the glenosphere. X-axis, Y-axis, and Z-axis are medial-lateral, superior-inferior, and anterior-posterior, respectively.

FIGURE 4

Rotator cuff configuration in the musculoskeletal model of reverse shoulder arthroplasty. (A) Type A: torn all six bundles of the supraspinatus (No. 1–6 bundle tears); (B) Type B: torn all six bundles of the supraspinatus and torn two bundles of the infraspinatus (No. 1–8 bundle tears); (C) Type C: torn all six bundles of the supraspinatus and torn four bundles of the infraspinatus (No. 1–10 bundle tears); (D) Type D: torn all six bundles of the supraspinatus and infraspinatus (No. 1–12 bundle tears); (E) Type E: torn all six bundles of the supraspinatus, infraspinatus, and teres minor (No. 1–18 bundle tears).

FIGURE 5

The illustrations of the subscapularis-torn and subscapularis-repaired models: (A) subscapularis-torn (all six bundles are torn) model; (B) subscapularis-repaired (all six bundles are intact) model.

FIGURE 6

Comparison of the joint contact forces in the current model (six-degree-of-freedom (DOF) anatomical shoulder model), the public OrthoLoad experimental data, and previously validated 6-DOF anatomical shoulder model: (A) total joint contact force; (B) medial-lateral joint contact force; (C) superior-inferior joint contact force; (D) anterior-posterior joint contact force, respectively.

FIGURE 7

(A) joint compressive force (medial-lateral joint contact force) and (B) posterior deltoid muscle-tendon force in the Type E condition (torn all six bundles of the supraspinatus, infraspinatus, and teres minor). ^aSignificant differences between intact rotator cuff and subscapularis-torn models (p < .05). ^bSignificant differences between intact rotator cuff and subscapularis-repaired models (p < .05). ^cSignificant differences between subscapularis-torn and -repaired models (p < .05).

FIGURE 8

(A) infraspinatus muscle-tendon force and (B) teres minor muscle-tendon force. Type A: isolated bundle tear of the supraspinatus; Type B: Type A + superior bundle tear of the infraspinatus; Type C: Type B + middle bundle tear of the infraspinatus; Type D: Type C + entire bundle tear of the infraspinatus.