Monday, March 17, 2014

The rotator cuff becomes stiffer with age - a possible reason for cuff tendon failure

The Deep Layer of the Rotator Cuff Tendon Becomes Stiffer with Age: A Possible Cause of Cuff Tear

These authors hypothesized that changes in the mechanical properties of cuff  tendon property may contribute to rotator cuff failure. They measured the stiffness of the rotator cuff tendons in healthy volunteers of various age with use of ultrasound elastography. The cohort represented 210 shoulders/122 healthy volunteers in their 10s to 70s (78 males and 44 females). The shoulders were examined with use of ultrasound elastography in which the stiffness of the superficial and deep layers of the supraspinatus and infraspinatus tendons was measured with a newly developed coupler with known stiffness as a reference. The strain of the tendon was calculated from the color changes on the longitudinal images using an accompanying software. The relationship between age and the ratio of the tendon strain divided by the coupler strain (strain ratio) was analyzed.

They found that the strain ratios of the deep layer of the supraspinatus and infraspinatus tendons became significantly smaller with age (r=-0.5316, r=-0.5874, respectively), which means the deep layer became stiffer with age, whereas the strain ratios of the superficial layer did not change significantly. The difference in the strain ratio between the deep and superficial layers gradually became greater with age.

These data suggest that the deep layer of the rotator cuff tendons becomes stiffer with age.
These findings are consistent with the results of previous reports that the joint-side layer is more vulnerable to a tensile load than the bursal-side layer. The decrease in the inherent elasticity of the cuff tendons may be related to an onset of articular side tear. Also, an increased difference in tendon property between the superficial and deep layers with age may be related to an occurrence of intratendinous tear. 

Comment: Nature has provided a very sophisticated mechanism for the attachment of tendon to bone. This mechanism is designed to maximize suppleness and the gradual transition from a flexible tendon to rigid bone. When this mechanism is compromised and the tendon becomes stiff, the tendon becomes at risk for failure from fatigue failure - just as bending a stiff paper clip back and forth many times leads to breakage. See the figure below from the book Practical Evaluation and Management of the Shoulder

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