Physiological Considerations in Bracing of the Spine1

It is well recognized that braces in current use cannot produce complete immobilization of the spine, especially in the lumbosacral region. In fact, it has been shown that motion at the lumbosacral joint may be increased during trunk flexion when a long spinal brace is worn, since there is compensation for decreased motion of the thoracolumbar region. 1 Clinically, however, despite the obvious incompleteness of immobiliza­ tion, bracing frequently results in symptomatic improvement of low-back disorders. Apparently, either partial immobilization or the support provided, in some manner, to this region brings about the improvement. In this regard it has been observed that in certain cases of low-back pain caused by disc degeneration or so-called mechanical instability, compression of the abdomi­ nal viscera often relieves the pain. This compression may be accomplished by use of a circumferential bandage, a well-lilting corset, a brace with an abdominal pad which can be tightened, or a snug plaster body jacket. In an effort to expand our understanding of the physiological factors in support of the spine and their possible application to orthotics, a study of the mechanics of stability and support of the spine was undertaken at the Bio­ mechanics Laboratory. In particular, the role of the compartments of the trunk (thorax and abdomen) in helping to provide stability of the spine was investigated. The spinal column, which serves as a sustaining rod for the maintenance of the upright position of the body, may be considered lo have both an intrinsic and an extrinsic stability. Intrinsic stability is provided by the alternating rigid and elastic components of the spine which are bound to­ gether by ligaments, while extrinsic stability is provided by the paraspinal and trunk muscles. The trunk muscles, especially those of the abdomen, form a contractile muscular wall about the body compartments which is capable of compressing the viscera. With the contraction of these muscles, the intra­ cavitary pressures are increased, aiding in many bodily functions such as childbirth, respiration, return of venous blood, and. as will be shown, stabili­ zation or support of the spine. The isolated ligamentous spine behaves like a modified elastic rod. 2 When it is fixed at the base, its critical load—i.e., the greatest load it can sustain without buckling -is approximately 4 1/4 pounds, or much less than the body weight alone.2 The stability of the spine in the living is therefore dependent largely on the extrinsic support provided by the trunk muscula-