CAPSULAR PROPERTIES OF THE SHOULDER

The purpose of this study was to determine the structural properties o f the capsule of the glenohumeral joint. Twelve fresh frozen cadaveric shoulders were studied. Capsular strips were prepared from four diffe rent sites (anterior, posterior, superior, and inferior) of the capsul e. One end of the capsular sections was left attached to the humerus, and the other excised was fixed in a clamp of an Instron universal tes ting machine. Maximum load, strength (maximum stress), and modulus of elasticity of these four capsular portions were measured. The most com mon mode of failure was tear at the midsubstance (68%), followed by te ar at the clamp-capsule junction (23%), and detachment from the humeru s (9%). The posterior capsule (1.0 +/- 0.4 mm) was thinner than the an terior (1.8 +/- 0.3 mm), superior (1.6 +/- 0.4 mm), and inferior capsu le (1.5 +/- 0.3 mm). Among the four portions of the capsule, the poste rior capsule showed the greatest strength (216.6 +/- 58.2 kg/cm(2)) an d modulus of elasticity (683.1 +/- 228.8 kg/cm(2)), whereas the superi or capsule showed the least strength (82.4 +/- 33.5 kg/cm(2)). There w ere no significant differences in maximum load. The greater strength o f the posterior capsule may be one explanation for the low incidence o f posterior shoulder dislocation.