594 THE JOURNAL OF BONE AND JOINT SURGERY Mechanical factors in the initiation and propagation of tears of the rotator cuff QUANTIFICATION OF STRAINS OF THE SUPRASPINATUS TENDON IN VITRO P. Reilly, A. A. Amis, A. L. Wallace, R. J. H. Emery From Imperial College of Science, Technology and Medicine and St Mary’s Hospital, London, England P. Reilly, MS, FRCS, Clinical Research Fellow A. A. Amis, DSc, FIMechE, Professor of Orthopaedic Biomechanics Departments of Mechanical Engineering and Musculoskeletal Surgery A. L. Wallace, PhD, FRACS, Senior Lecturer and Consultant in Orthopae- dic 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, Lon- don W2 1NY, UK. Correspondence should be sent to Professor A. A. Amis. ©2003 British Editorial Society of Bone and Joint Surgery doi:10.1302/0301-620X.85B4.12062 $2.00 ifferential 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. J Bone Joint Surg [Br] 2003;85-B:594-9. Received 16 January 2001; Accepted after revision 6 September 2002 In his 1998 Codman Lecture Fukuda 1 drew particular atten- tion to partial-thickness tears of the rotator cuff “because these conditions and their characteristics occupy a most sig- D nificant position in the spectrum of rotator cuff disease”. Many fundamental questions, however, relating to their pathogenesis, progression and symptomatology remain unanswered. Cadaver 2 and radiological 1 studies have suggested that intratendinous tears occur frequently, accounting for up to half of all partial-thickness tears. In 1939 Lindblom and Palmer 3 first considered the role of intratendinous shearing in the pathogenesis of tears of the rotator cuff. Since then a number of studies have suggested that the supraspinatus tendon has a heterogeneous structure and non-linear response to loading. 4-7 The anterior third of the supraspina- tus tendon was found to be thicker, with a higher ultimate load and failure stress than the posterior third. It was con- cluded that the anterior area was the strongest part and per- formed the main functional role of the tendon. 6 It has also been shown that the joint and bursal layers of the tendon have different properties, with the joint side having a higher modulus of elasticity and lower ultimate strain. 4 From engineering practice, it is clear that defects in a material can lead to concentrations of stress which may cause the defects to propagate. We hypothesised that the dif- ferent mechanical properties of the joint and bursal regions of the supraspinatus tendon would lead to different tissue strains under load. If correct, shear stress concentration between the layers could lead to propagation of partial- thickness tears of the supraspinatus tendon. To our knowledge this is the first attempt to quantify the strains which occur in the critical zone of the supraspinatus tendon under different loading conditions in vitro. Materials and Methods Thirteen cadaver specimens were harvested and frozen within 24 hours of death. The mean age of the specimens was 69 years (48 to 80) and seven were from men and six from women. There was no premorbid history available. Each specimen consisted of the lateral 7 cm of the scapula and the proximal 15 cm of the humerus, along with the sur- rounding soft tissues after removal of the deltoid muscle. The specimens were defrosted individually over 24 hours in normal saline-soaked bags. Throughout testing they were moistened with physiological saline. The rotator interval was opened, the long head of biceps was divided in the