Mechanics of Glenohumeral Instability.
Last updated Friday, February 04, 2005
Introduction
The most remarkable feature of the glenohumeral joint is its ability to
precisely stabilize the humeral head in the center of the glenoid on
one hand and to allow a vast range of motion on the other. This balance
of stability and mobility is achieved by a combination of mechanisms
particular to this articulation.About the glenohumeral joint In contrast to the hip joint, the glenohumeral joint does not offer
a deep stabilizing socket. An acetabular-like socket would limit motion
by contact of the anatomic neck of the humerus with its rim. Instead,
the small arc of the glenoid captures relatively little of the humeral
articular surface so that neck-rim contact is avoided for a wide range
of positions (see figure 1). (Das, 1966; Maki and Gruen, 1976; Matsen
et al, 1994; Saha, 1971; Turkel et al., 1981)
In contrast to hinge-like joints with shallow sockets, such as the
knee, interphalangeal joints, elbow, and ankle, the glenohumeral joint
does not offer isometric articular ligaments which provide stability as
the joint is flexed around a defined anatomical axis. Instead, the
glenohumeral ligaments play important stabilizing roles only at the
extremes of motion, being lax and relatively ineffectual in most
functional positions of the joint (see figure 2). (Matsen et al, 1994;
Warner et al, 1992)
In spite of its lack of a deep socket or isometric ligaments, the
normal shoulder precisely constrains the humeral head to the center of
the glenoid cavity throughout most of the arc of movement. (Howell and
Galinat, 1989; Howell et al, 1988; Poppen and Walker, 1976; Poppen and
Walker, 1978). It is remarkable that this seemingly unconstrained joint
is able to provide this precise centering, resist the gravitational
pull on the arm hanging at the side for long periods, remain located
during sleep, allow for the lifting of large loads, permit throwing a
baseball at speeds approaching 100 miles an hour, and maintain
stability during the application of an almost infinite variety of
forces of differing magnitude, direction, duration, and abruptness.
The mechanics of glenohumeral stability can be most easily
understood in terms of the relationship between the net force acting on
the humeral head and the shape of the glenoid fossa. A working
familiarity with the mechanics of glenohumeral stability will greatly
enhance understanding of the workings of the normal joint, laboratory
models of instability, clinical problems of instability, and clinical
strategies for managing glenohumeral instability.
How useful was this page or article?
This article is rated  out of 5 stars (312 ratings).
|
|