Krzysztof Balawender

Background The subscapularis (SSC) is the largest rotator cuff muscle and is involved in the medial rotation, abduction, adduction, and anterior stabilization of the shoulder. It is anatomically variable, as is the morphology and prevalence of the accessory SSC (aSSC), a rare muscle slip attached to the SSC. There is no current review investigating the prevalence and morphometrics of the SSC and aSSC. Purpose To investigate the prevalence of the morphological variants of the SSC and aSSC via meta‐analysis and review the relevant literature involving cadaveric, magnetic resonance imaging, and ultrasonographic studies. Study Design Meta‐analysis. Materials and Methods Literature data reporting the prevalence rates and morphometrics of the SSC and aSSC and their variants were pooled. Literature searches and data analyses were performed in accordance with Preferred Reporting Items for Systematic Reviews and Meta‐Analyses and Anatomical Quality Assurance guidelines. Results Forty‐six studies, totaling 2166 shoulders, were assessed. The SSC showed an overall length, thickness, cross‐sectional area, and volume of 152.2 mm (95% confidence interval, CI, 103.8–200.5 mm), 5.6 mm (95% CI, 4.6–6.6 mm), 18.1 cm 2 (95% CI, 14.2–22.0 cm 2 ), and 126.9 cm 3 (95% CI, 87.2–166.5 cm 3 ), respectively. The SSC displayed substantial variations in its origin and insertion points and in the composition of its tendon. The aSSC had an overall pooled prevalence of 24.6% (95% CI, 0.0%–76.9%). Conclusions The SSC and aSSC have been implicated in multiple shoulder pathologies, including muscle and tendon ruptures and neurovascular compromise. A better understanding of SSC and aSSC variants when diagnosing and treating their related pathologies will reduce patient morbidity and improve treatment regimens.