Contribution of the passive properties of the rotator cuff to glenohumeralstability during anterior-posterior loading

The passive properties of the rotator cuff have been shown to provide some stability during anterior-posterior (AP) translation. However, the relative importance of the rotator cuff to joint stability remains unclear. The pur pose of this study was to quantify the force contributions of the rotator c uff and of capsuloligamentous structures at the glenohumeral joint during A P loading. We hypothesized that the rotator cuff acts as a significant pass ive stabilizer of the glenohumeral joint and that its contribution to joint stability is comparable to the contribution made by the components of the glenohumeral capsule. A robotic/universal force-moment sensor testing syste m was used to determine both the multiple "degrees of freedom" joint motion and the in situ force carried by each soft tissue structure during applica tion of an 89N AP load at 4 abduction angles. The percent contribution of t he rotator cuff to the resisting force of the intact joint during AP loadin g was significantly greater during posterior loading (35% +/- 26%) than dur ing anterior loading at 60 degrees of abduction (P <.05). The contribution of the rotator cuff (ie, 29% +/- 16% at 30 degrees of abduction) was found to be significantly greater than the contributions of the capsule component s during posterior loading at 30 degrees 60 degrees and 90 degrees of abduc tion (P <.05). However, no differences could be Found between the respectiv e contributions of the rotator cuff and the capsule components during anter ior loading. The results support our hypothesis and suggest that passive te nsion in the rotator cuff plays a more significant role than other soft tis sue structures in resisting posterior loads at the glenohumeral joint. The important role of the rotator cuff during posterior loading may be a result of the thin posterior joint capsule compared with the anterior capsule, wh ich has several thickenings. This information increases our understanding o f posterior stability at the glenohumeral joint during clinical laxity test s.