Supraspinatus tendon load during abduction is dependent on the size of the critical shoulder angle: A biomechanical analysis.
Shoulders with supraspinatus (SSP) tears are associated with significantly larger critical shoulder angles (CSA) compared to disease-free shoulders. We hypothesized that larger CSAs increase the ratio of joint shear to joint compression forces (defined as "instability ratio"), requiring substantially increased compensatory supraspinatus loads. A shoulder simulator with simulated deltoid, supraspinatus, infraspinatus/teres minor, and subscapularis musculotendinous units was constructed. The model was configured to represent either a normal CSA of 33° or a CSA characteristic of shoulders with rotator cuff tears (38°), and the components of the joint forces were measured. The instability ratio increased for the 38° CSA compared with the control CSA (33°) for a range of motion between 6° to 61° of thoracohumeral abduction with the largest differences in instability observed between 33° and 37° of elevation. In this range, SSP force had to be increased by 13-33% (15-23 N) to stabilize the arm in space. Our results support the concept that a high CSA can induce SSP overload particularly at low degrees of active abduction.
Department of Orthopaedics, Balgrist University Hospital, Zurich, Switzerland.
Christian
C
Gerber
Jess G
JG
Snedeker
Daniel
D
Baumgartner
Arnd F
AF
Viehöfer
D001696:Biomechanical Phenomena; D019245:Compressive Strength; D057645:Deltoid Muscle / Q000503:physiopathology; D006801:Humans; D006811:Humerus / Q000503:physiopathology; D008953:Models, Anatomic; D009119:Muscle Contraction; D020959:Polyethylene / Q000737:chemistry; D016059:Range of Motion, Articular; D000070636:Rotator Cuff Injuries; D012540:Scapula / Q000503:physiopathology; D033081:Shear Strength; D012782:Shoulder / Q000503:physiopathology; D000070599:Shoulder Injuries; D013314:Stress, Mechanical; D013710:Tendons / Q000033:anatomy & histology* / Q000503:physiopathology*
D020959:Polyethylene
biomechanics; critical shoulder angle; etiology; rotator cuff tear; tendon