Muscle activation variability in relation to functional internal rotation in reverse total shoulder arthroplasty

Area of Science:

• Orthopedic Surgery
• Biomechanics
• Kinesiology

Background:

• Reverse total shoulder arthroplasty (rTSA) use is increasing, yet functional internal rotation (fIR) outcomes vary significantly.
• This variability impacts patients' independence in daily activities.
• Previous research explored patient, kinematic, and surgical factors, but the role of muscle activation remains unclear.

Purpose of the Study:

• To investigate the relationship between muscle activation patterns and functional internal rotation (fIR) variability after rTSA.
• To identify specific muscle activation patterns associated with high and low fIR levels.
• To explore correlations between kinesiophobia, internal rotation strength, and fIR outcomes.

Main Methods:

• Prospective study of 16 rTSA patients (minimum 1 year post-op).
• Electromyography (surface and intramuscular) measured muscle activations during hand-to-back movements, normalized via maximal isometric voluntary contraction (MIVC) trials.
• Kinematics captured with markers; fIR measured by hand dorsum to C7 distance; kinesiophobia assessed with Tampa-17 scale.

Main Results:

• High fIR patients exhibited greater activation in posterior deltoid, upper/lower trapezius, and teres major.
• Negative correlations found between fIR and activations of pectoralis major, middle/lower trapezius, serratus anterior, teres minor, and latissimus dorsi.
• Kinesiophobia weakly correlated (R_s=-0.32, p=0.025) and internal rotation strength showed a slight positive correlation (R_s=0.28, p=0.054) with fIR.

Conclusions:

• Lower fIR levels in rTSA patients are associated with overactivation of specific shoulder girdle muscles.
• Muscle activation patterns of the posterior deltoid, upper/lower trapezius, and teres major are crucial for achieving high fIR.
• Increased kinesiophobia and decreased internal rotator strength are linked to poorer fIR outcomes post-rTSA.