Are shoulders with a reverse shoulder prosthesis strong enough? A pilot study.

Area of Science:

• Orthopedic surgery
• Biomechanics
• Rehabilitation medicine

Background:

• Limited active range of motion (ROM) in reverse shoulder prostheses is often attributed to muscle weakness.
• Postoperative strength in these patients has not been previously quantified.

Purpose of the Study:

• To measure joint torques in patients with reverse shoulder prostheses.
• To correlate measured torques with established functional outcome scores.

Main Methods:

• 33 patients (72 ± 8 years) with reverse shoulder prostheses (37 shoulders) underwent isokinetic testing for abduction/adduction and external/internal rotation.
• Patients completed Constant-Murley, DASH, and Simple Shoulder Test ([D]SST) functional scores.
• Follow-up averaged 23 months (range, 4-63 months).

Main Results:

• Only 65% of patients could perform the strength testing protocol.
• Abduction and adduction torques were reduced compared to normal shoulders (15 Nm ± 7 Nm and 16 Nm ± 10 Nm, respectively).
• External and internal rotation torques were markedly lower (9 Nm ± 4 Nm and 8 Nm ± 3 Nm, respectively), with moderate correlations found between strength and functional scores.

Conclusions:

• Patients with reverse shoulder prostheses exhibit significantly reduced muscle strength compared to normative data.
• The greatest strength deficit was observed in external rotation, potentially explaining observed clinical outcomes.
• Limited postoperative strength appears to be a primary factor contributing to reduced range of motion in these patients.