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Mechanical factors in the initiation and propagation of tears of the rotator cuff

QUANTIFICATION OF STRAINS OF THE SUPRASPINATUS TENDON IN VITRO

Departments of Mechanical Engineering and Musculoskeletal Surgery

A. L. Wallace, PhD, FRACS, Senior Lecturer and Consultant in Orthopaedic Surgery

Department of Musculoskeletal Surgery, Imperial College London, South Kensington Campus, London SW7 2AZ, UK.

R. J. H. Emery, MS, FRCS, Consultant Orthopaedic Surgeon

Department of Orthopaedic Surgery, St Mary’s Hospital, Praed Street, London W2 1NY, UK.

Correspondence should be sent to Professor A. A. Amis.

Differential strain has been proposed to be a causative factor in failure of the supraspinatus tendon. We quantified the strains on the joint and bursal sides of the supraspinatus tendon with increasing load (20 to 200 N) and during 120° of glenohumeral abduction with a constant tensile load (20 to 100 N).

We tested ten fresh frozen cadaver shoulders on a purpose-built rig. Differential variable reluctance extensometers allowed calculation of the strain.

Static loading to 100 N or more increased strains on the joint side significantly more than on the bursal side. During glenohumeral abduction an increasing and significant difference in strain was measured between the joint and bursal sides of the supraspinatus tendon, which reached a maximum of 10.6% at abduction of 120°. The joint side strain of 7.5% reached values which were previously reported to cause failure.

Differential strain causes shearing between the layers of the supraspinatus tendon, which may contribute to the propagation of intratendinous defects that are initiated by high joint side strains.