Is There a Nerve-free Zone in Which a Subscapularis Split Can Safely be Performed? An Anatomical Study Using Embalmed Specimens

Abstract

Background The subscapularis muscle, which is part of the rotator cuff, is located anterior to the shoulder. In anterior approaches to the shoulder, its supplying nerves are at risk of iatrogenic injury, which may cause postoperative complications. It is unclear whether there is any nerve-free zone in which a subscapularis split can be performed without risking nerve damage. Questions/purposes In an anatomical study, we asked: (1) With the arm abducted 60°and externally rotated, what are the median and shortest distances between the entrance point of the subscapular nerves into subscapularis muscle and the myotendinous junction of this muscle? (2) What are those distances in other positions of the shoulder? (3) Did those measurements differ between specimen sexes or sides? Methods In 84 shoulders of 66 embalmed anatomic specimens, the distance from the myotendinous junction of the subscapularis muscle to the entrance points of the subscapular nerves into the subscapularis muscle was measured using an inelastic thread and a millimeter gauge with the arm abducted 60°and rotated externally. In 16 of 84 shoulders, which were selected randomly, after taking the measurements with the arm abducted 60°and rotated externally, arm positions were changed and further measurements were taken with the arm abducted 60°and rotated neutrally, abducted 60°and rotated internally, and abducted 90°and rotated externally. The positions of the entrance points were described with statistical parameters and compared between different sides, sexes, and joint positions. Measurements were verified using eight freshfrozen shoulders, showing no difference in distances compared with embalmed specimens and confirming reproducibility of measurements. Absolute distances were used to minimize possible distortion when using correlations and for straightforwardness and clinical applicability. Results The median (range) distance was 43 mm (24 to 64) for the upper subscapular nerve and 38 mm (23 to 59) for the lower subscapular nerve with the arm rotated externally Each author certifies that there are no funding or commercial associations (consultancies, stock ownership, equity interest, patent/licensing arrangements, etc.

CORR Insights®: Is There a Nerve-free Zone in Which a Subscapularis Split Can Safely be Performed? An Anatomical Study Using Embalmed Specimens

“…Likewise, the effect of the intraoperative position of the shoulder on the risk of subscapular nerve injury should be examined in greater detail. Similar to Siwetz et al [9], others [3,7] have noted that abduction and external rotation is the safest position with regard to protecting against subscapular nerve injury. However, the influence of this shoulder position on surgical exposure during open stabilization has not been well established, particularly when considered in combination with other factors such as medial retractor placement and the amount of traction applied.…”

“…The findings of this study [9] suggest that sex differences may exist regarding variability in the muscular entry points of the upper and lower subscapular nerves. For example, should the subscapularis of a male football lineman with a BMI of 35 kg/m 2 be managed differently from that of a female soccer player with a BMI of 23 kg/m 2 ?…”

“…The authors also found that placing the shoulder in an abducted and externally rotated position increases the average distance from the musculotendinous junction of the subscapularis muscle to the entry points of the upper and lower subscapular nerves. The present study [9] represents an important step toward addressing heterogeneity among studies by proposing an evidence-based “safe zone” of at least 20 mm from the musculotendinous junction, within which splitting the subscapularis muscle-tendon unit would be unlikely to cause subscapular nerve injury.…”

“…The findings of this study [9] suggest that sex differences may exist regarding variability in the muscular entry points of the upper and lower Moreover, the clinical applicability of the 20-mm "safe zone" should be investigated in additional practical ways. Although it is certainly useful to understand the region of the subscapularis in which the likelihood of nerve injury can be minimized, it remains unknown whether this amount of tendon split would provide sufficient exposure to perform an open shoulder stabilization procedure in a real-world setting.…”

“…In this issue of Clinical Orthopaedics and Related Research ® , Siwetz et al [9] performed a laboratory study in cadavers to evaluate the anatomic location of the major nerves that innervate the subscapularis muscle. The authors found that approximately 88% of included cadaveric specimens demonstrated the classically described pattern of innervation by the upper and lower subscapular nerves, whereas the remaining 12% of specimens demonstrated aberrant anatomy in the form of accessory subscapular nerves.…”