Quantitative determination of articular pressure in the human shoulder joint

The objective of this study was to determine experimentally the effect of individual variations in articular surface geometry on the shoulder joint pressure distribution in functionally important arm positions. The location and size of the load-bearing areas and maximal pressures of 10 cadaver shoulder joints were examined at 30 degrees, 90 degrees, and 150 degrees of abduction (+/-90 degrees external rotation) and 120 degrees flexion with pressure-sensitive film. Only parts of the glenoid cavity were involved in the load transfer, some specimens showing central and other bicentric (superior-inferior) pressure maxima during abduction and flexion. The load-bearing areas were more central at external rotation, suggesting that the humeral head is not a perfect sphere. The maximal pressure was recorded at 90 degrees of abduction with 90 degrees external rotation, with 5.1 MPa for the elevation for single- and >10 MPa for double-arm weight. The study demonstrates that shoulder joint pressure cannot be calculated analytically but depends on subtle variations of joint incongruity.