Kyphotic Scapular Posture Increases Posterior Deltoid Demand and Acromial-Spine Strain During Shoulder Motion: A Cadaver Study.

Abstract

Scapulothoracic posture is known to influence shoulder mechanics, with kyphotic (type C) alignment associated with altered scapular orientation and muscle balance. However, the biomechanical consequences of scapular posture on individual muscle loading and bone strain remain poorly defined, limiting the ability to tailor surgical planning and rehabilitation to a patient's baseline alignment. (1) Does a type C posture alter deltoid recruitment and rotator cuff loading patterns compared with a type A upright posture across dynamic shoulder motions? (2) Does type C posture alter acromial and scapular spine strain compared with type A alignment? Six fresh-frozen cadaveric shoulders (mean ± SD age 53 ± 10 years; 5 males, 1 female) were tested using a validated dynamic shoulder simulator capable of independently actuating eight muscle groups (supraspinatus, infraspinatus, teres minor, upper and lower subscapularis, and anterior, middle, and posterior deltoid) and measuring acromial and scapular spine bone strain across Levy zones 1, 2, and 3 (subdivided into 3A and 3B). Each specimen was positioned in either a type A upright (scapulothoracic upward rotation 5°, anterior tilt 13°, internal rotation 30°) or kyphotic type C scapular alignment (upward rotation 6.4°, anterior tilt 23.5°, internal rotation 49.2°) via a 6-DOF Stewart platform. Humerothoracic kinematics were imposed across postures during forward flexion, scapular plane elevation, and weighted (1 kg) internal and external rotation. Continuous muscle forces and acromial-spine strains (Levy zones 1 to 3B) were recorded. Repeated-measures ANOVA was used to evaluate group differences using joint angle (1° increments) as the within-subject factor, with only significant (p < 0.05) spans of ≥ 5° reported because smaller differences (even if detectable in this model) probably would be too small to be perceived clinically. Type C posture shifted deltoid recruitment posteriorly across all tested motions, with posterior deltoid force greater and anterior deltoid force lower; the largest differences during scapular plane elevation were at terminal elevation for the anterior deltoid (type A 96.8 ± 14.5 N versus type C 60.5 ± 19.3 N, mean difference -36.4 [95% confidence interval (CI) -53.7 to -19.1]; p = 0.003) and at mid-elevation for the posterior deltoid (type A 25.6 ± 10.8 N versus type C 60.6 ± 12.2 N, mean difference 35.0 [95% CI 22.4 to 47.7]; p < 0.001). Rotator cuff responses were motion dependent, with supraspinatus and infraspinatus forces varying by task and arc. Type C posture increased scapular spine strain during forward flexion and weighted axial rotation, with the largest increase in zone 3B during forward flexion (type A 261 ± 248 µε versus type C 653 ± 249 µε, mean difference 392 [95% CI 42.2 to 741.5]; p = 0.04), representing a 150% increase in the region of highest clinical fracture risk. Strain did not differ between postures during scapular plane elevation. A type C posture increases posterior deltoid demand and acromial-spine strain while reducing anterior deltoid contribution. These findings suggest posture-dependent mechanical inefficiency rather than joint restriction as a potential driver of reduced shoulder performance in individuals with kyphosis. Preoperative CT-based posture classification may help surgeons anticipate the posterior shift in deltoid demand and elevated acromial strain associated with type C alignment, informing implant positioning decisions to account for the reduced anterior deltoid mechanical advantage in this group. Patients with type C kyphotic scapular alignment experience greater posterior deltoid demand and elevated acromial spine strain during routine shoulder movements compared with those with type A upright alignment. Incorporating scapular posture assessment into preoperative planning may help surgeons identify patients with reduced anterior deltoid mechanical advantage and adjust implant positioning accordingly, while guiding rehabilitation specialists to prioritize anterior deltoid strengthening to reduce fatigue and acromial overload risk in this group.