A novel approach to investigating the tendinous and capsular layers of the rotator cuff complex : A biomechanical study

Abstract

Rotator cuff (RC) muscle insertion was previously thought to consist of singular, individual tendons inserting onto predefined areas on the greater and lesser tuberosities. However, more recent publications describe the RC muscle tendons as forming a singular insertion across the tuberosities, consisting of both tendinous and capsular portions. Orthopaedic surgeons are now considering these two layers in their surgical approach and treatment plans; therefore this study aimed to test and compare the elastic modulus and maximum load to failure for both tendinous and capsular layers taken from supraspinatus (SS), infraspinatus (IS) and subscapularis (SC). Fourteen (n = 14) fresh/frozen arms were used in this study. Each RC muscle was reverse dissected and trimmed to a 2 x 2cm strip, which was separated into its two layers, still attached to the humerus. An Instron 1342 with a 1kN load cell was used to place the samples under tensile testing till failure (Newtons/N). Accompanying Integrated Design Tools (IDT) NX8-S2 cameras captured images for full-field strain measurements with the Image Systems TEMA software package through digital image correlation (DIC). SS, IS, and SC tendinous layers yielded higher average elastic moduli readings (72.34 MPa, 67.04 MPa, and 59.61 MPa respectively) compared to their capsular components (27.38 MPa, 32.45 MPa, and 41.49 MPa respectively). Likewise, the tendinous layers for SS, IS and SC all showed higher average loads to failure (252.74 N, 356.27 N and 385.94 N, respectively) when compared to the capsular layers (211.21 N, 168.54 N and 281.74 N, respectively). These biomechanical differences need to be taken into account during surgical repair owing to the fact that, should these layers be repaired as one singular structure, it may place the weaker less elastic, capsular layer under more strain, possibly leading to either re-tear complications or reduced postoperative healing and functionality. Thus, based on the results, it is recommended that surgeons consider and repair each layer independently for better postoperative biomechanical integrity.