Mechanics of Glenohumeral Arthritis.

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Stability

a. Humeral articular surface area

An anatomically oriented and sufficiently extensive humeral articular surface area. The orientation of the humeral articular surface can be described in terms of the humeral head center line; a line passing through the center of the humeral articular cartilage and through the center of the anatomic neck. This line usually makes a valgus angle of about 130 degrees with the humeral shaft. The humeral head center line usually makes a retroversion angle of about 30 degrees with the axis of elbow flexion. (Cofield, 1984; Collins, Harryman, Lippitt, et al., 1991; Figgie, Inglis, Goldberg, et al., 1988; Friedman, Thornhill, Thomas, et al., 1989; Hawkins, Bell and Jallay, 1989; Neer, 1990; Neer and Kirby, 1982; Pearl and Lippitt, 1993; Roper, Paterson and Day, 1990; Weiss, Adams, Moore, et al., 1990) Recent studies (Kronberg, Brostrom and Soderlund, 1990; Pearl and Volk, 1995; Roberts, Foley, Swallow, et al., 1991) point out that mean humeral retroversion varies widely from 7 to 50 degrees. Hernigou et al pointed out the importance of clearly defining the reference system when measuring humeral version. (Hernigou, Duparc and Filali, 1995)

The extent of the humeral articular surface area is another critical determinant of stability. In the arthritic glenohumeral joint, stability can be compromised by a reduced amount of available humeral articular surface. Similarly, a prosthetic surface area that comprises only a small part of the total sphere (see figures 18 and 19) can predispose to instability in the same way as does a Hill-Sach's defect in traumatic instability by offering less contact area for joint surface contact (see figure 20).

b. An anatomically oriented glenoid

c. Glenoid concavity

A glenoid concavity with sufficiently large effective arcs. The arc of the glenoid determines the maximal angles that the net humeral joint reaction force can make with the glenoid center line before dislocation occurs (see figure 29).

In the arthritic joint, the effective glenoid arc can be diminished by wear or inflammation, for example posterior wear is typical of glenohumeral osteoarthritis (see figures 24 and 30) and capsulorrhaphy arthropathy (see figure 31) while central erosion of the glenoid is typical of rheumatoid arthritis (see figure 32). At arthroplasty, the effective glenoid arcs need to be restored (see figure 28).

d. Control of the net humeral joint reaction force

The direction of the net humeral joint reaction force is controlled actively by the elements of the rotator cuff and other shoulder muscles (see figure 33). Neural control of the magnitude of the different muscle forces provides the mechanism by which the direction of the net humeral joint reaction force is modulated. For example, by increasing the force of contraction of a muscle whose force direction is parallel to the glenoid center line, the body can change the direction of the net humeral joint reaction force to an orientation of closer alignment with the glenoid fossa (see figure 34).

In glenohumeral arthritis, control of the net humeral joint reaction force may be compromised by tendon ruptures, tuberosity detachment and by deconditioning (see figure 23). The most striking example is in cuff tear arthropathy where the normally stabilizing cuff muscle forces are compromised (see figures 35-37).

If, following glenohumeral arthroplasty, the net humeral joint reaction force is not centered in the glenoid fossa, eccentric loading may produce rocking horse loosening of the glenoid component (see figure 38). A slight degree of mismatch of the glenoid and humeral diameters of curvature allows for minor amounts of force malalignment before rim contact occurs (see figures 39 and 40). Severt et al (Severt, Thomas, Tsenter, et al., 1993) pointed out that high degrees of conformity between the glenoid and humeral joint surfaces increases the translational forces and frictional torque applied to the glenoid component and on this basis advocated the use of less conforming and less constrained designs.

Severe degrees of mismatch may have adverse effects on the glenohumeral contact area (see figure 41) and peak stresses in the polyethylene (see figure 42).

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