Studies have shown that the rotator cuff (RC) complex is not simply comprised out of four separate tendons inserting onto the tubercles of the humerus, but rather a far more complex, integrated and interconnected system. This system is comprised of multiple layers, each with a unique set of properties. Current surgical practice however, treats the RC tendons as one layer, which only focuses on the bursal/tendinous layer (superficial tendinous layer) during repair. The capsular layer, which lies deep to the bursal/tendinous layer is often overlooked and most often treated together with the bursal/tendinous layer during repair. This has led to multiple postoperative complications including high re-tear rates, tears in new places, limited movement and poor recovery. Therefore, the aim of this study was to investigate the biomechanical properties of healthy RC tendons (both capsular and tendinous layers) in a South African population.
The total sample studied was comprised of 17 fresh/frozen shoulder specimens and 5 cadaveric shoulders. Of the 17 fresh shoulders 7 right and 6 left were harvested from 9 white males and 2 right and 2 left from 2 white females. The mean age of the fresh sample was 64.6 years. The cadaveric shoulders, 1 was male (right = 1; left = 1) and 2 were female (right = 2; left = 1) with a mean age of 64.7 years. Tensile tests were performed on the selected RC tendons using a benchtop MTS Criterion Model 41 tensile testing machine for composite materials, fitted with a 1kN load cell. The tendons were secured with rubber soft tissue clamps and reinforced with sand paper. Each strip was tested to failure at a constant rate of 0.5 mm/s, recording results using the MTS Test suit TWE 126.96.36.1996. Measurements included: peak load of each layer, in Newton force (N) and the modulus of elasticity, in megapascals (MPa).
The results for the peak load (N) value indicated significant differences between the cadaveric Supraspinatus SS layer and the bursal SS layer (p=0.002) and between the cadaveric Infraspinatus IS and bursal IS layer (p=0.0003). For the modulus (MPa) value, there were significant differences between the bursal IS and bursal Subscapular SC (p=0.012), as well as the bursal SS and bursal SC (p=0.020). With regard to the anisotropic nature of the tendons, the graphs gave a better indication of the layer’s differences. The bursal/tendinous layer presented with higher flexibility than the capsular layer. The graphs illustrated that the bursal/tendinous layer’s fibres parted separately during tensile testing, while the capsular layer broke more as a complete section. This corresponds with the more fibrous and cartilaginous properties in the capsular layer.
Therefore, looking at the results, it is evidence that the bursal/tendinous and capsular layer have different biomechanical properties, and that not only statistical values should be used, but a closer look should be taken at how these layers react physically as they experience load or strain. These results may have clinical implications in that surgeons should start to treat the two layers separately during surgical repair procedures.