Neuromuscular signatures differentiating functional and impaired cases in full-thickness rotator cuff tears

The relationship between structural damage and functional impairment in patients with full-thickness rotator cuff tears (FT-RCTs) remains inconsistent. While some patients maintain high function despite large tears, others experience profound disability, suggesting that compensatory neuromuscular mechanisms may be more closely associated with functional status than with anatomical integrity. This study aims to identify the clinical, structural, and neuromuscular factors that differentiate high-functioning from poor-functioning individuals with FT-RCTs.

Fifty-three patients with FT-RCTs were evaluated. Assessments included ultrasonographic measurement (tear size, acromiohumeral distance [AHD], humeral head migration, and coracohumeral ligament [CHL] thickness). Isometric strength (shoulder flexor, upper/lower trapezius [LT], serratus anterior, teres major [TM]) was quantified using hand-held dynamometry. Scapular biomechanics, including three-dimensional kinematics and muscle activation, were recorded using an electromagnetic tracking system and electromyography during controlled arm elevation in the scapular plane. Functional levels were determined using the Flexilevel Scale of Shoulder Function score. To identify clear differentiators, comparisons were made between the upper quartile (Good-Function) and lower quartile (Poor-Function) groups.

Structural parameters, including larger tear size (r = 0.317, P = .025), thinner CHL (r = -0.365, P = .009), and narrower AHD (r = -0.340, P = .016), showed a statistically significant moderate correlation with pain intensity but not with functional scores. Conversely, superior functional ability was moderately associated with shoulder flexor strength (r = 0.485, P < .001), LT strength (r = 0.376, P = .007), and reduced superior humeral head migration (r = 0.311, P = .028). Neuromuscularly, better function correlated with increased activation of the serratus anterior (r = -0.338 to -0.335, P = .017) and LT (r = -0.318 to -0.347, P = .014-.024). The Good-Function group demonstrated significantly greater shoulder flexor strength (11.2% ± 3.2% vs. Poor-Function; P = .001) and significantly lower compensatory activation of the TM (8.0%-13.4%; P = .015-.018).

Functional capacity in FT-RCTs appears to be associated with distinct neuromuscular compensatory mechanisms rather than the severity of structural damage. High-functioning individuals exhibit superior strength in the shoulder flexors, while poor function is characterized by excessive TM activation. Although statistically significant, the moderate effect size indicates that additional unmeasured factors likely influence the functional status of patients with FT-RCTs. Rehabilitation for FT-RCTs may benefit from incorporating a neuromuscular-based perspective alongside the traditional structure-centric model. Given the observed associations, clinicians might consider prioritizing the strengthening of shoulder flexors and the LT, while monitoring for excessive TM activity as a potential clinical marker of suboptimal compensation.