Mechanics of Glenohumeral Arthritis.

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Motion

The requisites of a normal range of glenohumeral motion include:

a. Normal capsular laxity

Normal capsular laxity, allowing a full range of rotation. The glenohumeral capsule is normally lax through most of the functional range of shoulder motion. (Harryman et al., 1992; Lippitt et al., 1994) As the joint approaches the limit of its range, the tension in the capsule and its ligaments increases sharply, serving to check the range of rotation (see figure 1). In many conditions requiring shoulder arthroplasty, the capsule and ligaments are contracted, limiting the range of rotation. Shoulder arthroplasty tends to further tighten the capsule because a degenerated and collapsed humeral head is replaced by a relatively larger prosthesis, and because a glenoid component is added to the surface of the glenoid bone, which may consume more space than the degenerated cartilage it replaces (see figure 2) and "stuff" the joint. Unless sufficient capsular releases (see figures 3 and 4) have been performed to accommodate this stuffing, the joint is "overstuffed." If the joint is overstuffed, joint motion is limited (see figure 5) and more torque (muscle force) is required to move the arm (see figure 6).

Harryman et al (Harryman et al., 1995) determined that all motions, including flexion, external and internal rotation, and maximal elevation are diminished when a too-large humeral head prosthesis is implanted. Furthermore, this overstuffing causes obligate translation of the head to occur on the glenoid; for example forced posterior translation occurs when external rotation is attempted against a tight anterior capsule (see figures 7 and 8). Thus, if normal capsular laxity is lacking, unwanted translation and eccentric glenoid loading may result.

In arthroplasty surgery, the amount of stuffing can be estimated by adding the thickness of the glenoid component to the difference between the amount of intraarticular humerus replaced and the amount of humerus resected. To be comparable, the measurement of the amount of humeral head resected and the measurement of the amount of intraarticular humeral prosthesis added must both be made from the cut surface of humeral neck to the articular surface. In modular humeral heads, the amount of bone replaced must include the thickness of the collar and the exposed part of the Morse taper stem as well as the head itself see (see figure 2).

The amount of stuffing from the glenoid component is related primarily to its thickness as well as the amount of reaming, the presence or absence of cement between the component and bone, and the use of bone grafts. The thickness of currently available glenoid components varies from 3 to over 15 mm. Thicker glenoid polyethylene may help manage contact stresses and may have superior wear properties. (Friedman, LaBerge, et al., 1992) Metal-backed glenoid components affect load transfer and offer opportunities for screw fixation and tissue ingrowth. (Cofield and Daly, 1992) However, both thicker polyethylene and metal-backing contribute to joint stuffing which becomes particularly problematic in shoulders that remain tight even after soft tissue releases. Overstuffing may also predispose the reconstructed shoulder to instability. (Cofield and Daly, 1992) The amount of stuffing from the humeral component is determined by both the geometry of the component and the position in which it is placed. The size of the intraarticular aspect of the humeral component is related to its design, including its radius of curvature, the percent of the sphere represented by its articular surface, and the distance between the base of its collar and the articular surface of the prosthesis (see figures 2 and 9). The position of the component also has a major effect on the degree to which it stuffs the joint. A component inserted into varus will disproportionately stuff the joint when the arm is at the side; this outcome is more likely when the stem of the prosthesis does not fit the humeral canal snugly. A component inserted excessively high will tighten the capsule as the arm is elevated (similar to a mechanical cam) and limit the range of elevation (see figure 10).

Cadaver studies (Matsen, Lippitt, Sidles, et al., 1994) indicate that less that 10 mm of overstuffing can reduce normal capsular laxity by as much as 50%. The overstuffed shoulder is predisposed to obligate translation.

The variables of component size and capsular release are under surgeon control. As a rules of thumb for judging capsular laxity at the time of surgery,

1. the humeral head should translate approximately 15 mm on the posterior drawer,
2. the abducted arm should allow 70 degrees of internal rotation, and
the arm should allow 40 degrees of external rotation at the side after the anterior structures have been repaired (see figure 11)

b. Humeral articular surface area

c. Glenoid articular surface

d. Absence of blocking osteophytes

e. Unrestricted humeroscapular motion interface

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