Modeling a rotator cuff tear: Individualized shoulder muscle forces influence glenohumeral joint contact force predictions

Abstract

Background: Rotator cuff tears in older individuals may result in decreased muscle forces and changes to force distribution across the glenohumeral joint. Reduced muscle forces may impact functional task performance, altering glenohumeral joint contact forces, potentially contributing to instability or joint damage risk. Our objective was to evaluate the influence of rotator cuff muscle force distribution on glenohumeral joint contact force during functional pull and axilla wash tasks using individualized computational models.

Methods: Fourteen older individuals (age 63.4 yrs. (SD 1.8)) were studied; 7 with rotator cuff tear, 7 matched controls. Muscle volume measurements were used to scale a nominal upper limb model's muscle forces to develop individualized models and perform dynamic simulations of movement tracking participant-derived kinematics. Peak resultant glenohumeral joint contact force, and direction and magnitude of force components were compared between groups using ANCOVA.

Findings: Results show individualized muscle force distributions for rotator cuff tear participants had reduced peak resultant joint contact force for pull and axilla wash (P ≤ 0.0456), with smaller compressive components of peak resultant force for pull (P = 0.0248). Peak forces for pull were within the glenoid. For axilla wash, peak joint contact was directed near/outside the glenoid rim for three participants; predictions required individualized muscle forces since nominal muscle forces did not affect joint force location.

Interpretation: Older adults with rotator cuff tear had smaller peak resultant and compressive forces, possibly indicating increased instability or secondary joint damage risk. Outcomes suggest predicted joint contact force following rotator cuff tear is sensitive to including individualized muscle forces.

Keywords: Computational model; Glenohumeral; Kinematics; Muscle forces; Older adult; Rotator cuff.

Fig. 1:

(A) Functional tasks performed by participants, including a functional pull (top) and axilla wash (bottom). Start/finish (left) and target (right) positions for each task are shown. For the functional pull, participants were seated at a table (height = 0.775m) and pulled against 2.72kg (6lb) resistance from a pulley system. The functional pull task started with the arm forward flexed a distance of 80% of the subject’s forearm length (length was marked on the table) and handle from a weight machine, pull the handle until the arm is in neutral flexion (target), then finish by returning to the start position. For the axilla wash, which is an unloaded task, the participant remained seated but the table was removed. The task started with the elbow extended and arm in neutral, resting quietly at the side. Participants then reached across the torso to touch the lateral aspect of the contralateral shoulder (target), then finished the task by returning to the starting position, with the arm resting quietly at the side in neutral posture. (B) Anatomical locations of retro-reflective markers used with motion capture (pink spheres). Twelve markers were placed at locations including: C7: 7^th cervical vertebra; SC: ventral aspect of the sternoclavicular joint; XP: xiphoid process; AA: lateral aspect of acromial angle of scapula; UA: upper arm, mid-length; LE: lateral epicondyle of humerus; ME: medial epicondyle of humerus; FA: forearm, mid-length; RS: styloid process of radius; US: styloid process of ulna; 5MP: 5^th metacarpophalangeal joint; 2MP: 2^nd metacarpophalangeal joint. An additional marker (not shown) was affixed to the top of the handle used for the functional pull task.

Fig. 2:

Joint contact force comparisons for the functional pull for rotator cuff tear (gray) and control (black) participants. (A) Mean (SD) peak resultant joint contact force was significantly smaller for the rotator cuff tear group (P=0.0244). The compressive (medial-lateral) component of the peak joint contact force was significantly reduced for the rotator cuff tear group (P=0.0248), which can be seen in the (B) coronal and (C) superior views of the joint. Shaded cones represent 1 SD of the mean position of the resultant vector; dashed lines represent 1 SD of the mean magnitude of the resultant force vector. (D) Anterior-posterior and superior-inferior components of the peak joint contact force are overlayed on an oval representing the glenoid fossa.

Fig. 3:

Joint contact force comparisons for the axilla wash for rotator cuff tear (gray) and control (black) participants. (A) Mean (SD) peak resultant joint contact force was significantly smaller for the rotator cuff tear group (P=0.0456). (B) Coronal and (C) superior views of the joint do not exhibit significant differences between the magnitude or direction of the planar components of the resultant force vector for the two groups. Shaded cones represent 1 SD of the mean position of the resultant vector; dashed lines represent 1 SD of the mean magnitude of the resultant force vector. (D) Anterior-posterior and superior-inferior components of the peak joint contact force are overlayed on an oval representing the glenoid fossa, where the superior component of the peak resultant force for one subject with a rotator cuff tear extended beyond the boundary of the glenoid fossa.

Fig. 4:

(A) Comparison of peak resultant joint contact force location within the glenoid rim for computational models with individualized muscle forces (solid markers) and computational models using the muscle forces from the nominal model (hollow markers). (B) Simulations with the nominal model using subject-specific kinematics have peak joint contact forces located closer to the center of the glenoid rim. (C) The ratio of the superior-inferior and medial-lateral components of the peak joint contact force was calculated for simulation results with individualized models (gray bars). Mean (solid line), SD (dashed line) of cohort ratio values are shown. CM02, RF01, and RM01 had peak joint contact forces located near or outside the glenoid rim (panel A) and also had ratio values exceeding 1SD of the cohort values (hashed bars, panel B), with results from analyses with generic models (black bars, panel B) within 1SD of cohort values.