Multi-axis passive and active stiffnesses of the glenohumeral joint

Objective. To investigate passive and active glenohumeral stiffness in the anterior, posterior, superior, and inferior directions at different lateral positions of the humerus. Design. Glenohumeral stiffness along multiple axes was determined in fresh-frozen shoulder specimens under both passive (no simulated muscle contraction) and active (with simulated muscle contraction) conditions. Background. Glenohumeral laxity has been evaluated in various studies with focus on one of the multiple directions. However, glenohumeral stiffness characterizing the force-displacement relationship and stability has not been evaluated in all four directions under passive and active conditions. Methods. The humeral head was translated in the posterior, anterior, inferior and superior directions relative to the glenoid with different lateral positions, and multi-axis glenohumeral stiffness generated by passive and active structures were investigated. Results. Without muscle loading, glenohumeral stiffness in the superior direction (K-sup = 5.83 N/mm) was higher than that in the inferior (K-inf = 4.32), anterior (K-ant = 3.67), and posterior (K-post = 2.89) directions (P < 0.008), and K-inf was higher than K-post (P = 0.011). Stiffness in the different directions were correlated to each other (P < 0.001), and shifting the humerus laterally increased stiffness in all directions (P < 0.05) except for the superior direction. With moderate muscle loads, the glenohumeral joint became significantly stiffer in all four directions (P < 0.05) with less difference among different directions. Conclusions. Glenohumeral stiffnesses are different in the different directions but are correlated to each other and contribute jointly to glenohumeral stability. Muscle contractions can increase glenohumeral stiffness significantly.