The Ligamento‐Muscular Stabilizing System of the Spine

Study Design. Electrical and mechanical stimulation of the lumbar supraspinous ligament of three patients with L4‐L5 spinal deficits and of the feline model, respectively, was applied while recording electromyography on the multifidus muscles. Objectives. To determine if mechanoreceptors in the human spine can reflexively recruit muscle force to stabilize the lumbar spine, and to demonstrate, in the feline model, that such ligamento‐muscular synergy is elicited by mechanical deformation of the lumbar supraspinous ligament (and possibly of other spinal ligaments), the facet joint capsule, and the disc. Summary of Background Data. The literature repeatedly confirms that ligaments have only a minor mechanical role in maintaining spine stability, and that muscular co‐contraction of anterior and posterior muscles is the major stabilizing mechanism of the spine. The literature also points out that various sensory receptors are present in spinal ligaments, and that the ligaments are innervated by spinal and autonomic nerves. Data that describe how ligaments and muscles interact to provide stability to the spine were not found. Methods. The supraspinous ligament at L2‐L3 and L3‐L4 was electrically stimulated in three patients undergoing surgery to correct deficits at L4‐L5. Electro‐myography was performed from the multifidus muscles at L2‐L3 and L3‐L4, bilaterally. In 12 cats, the supraspinous ligaments from L1‐L2 to L6‐L7 were mechanically deformed, sequentially, while electromyography was performed from the multifidus muscles of the six levels. Loading of the ligament was applied before and after each of the two vertebrae were externally fixed to prevent motion. Results. Electromyograms were recorded from the multifidus muscles, bilaterally, in the two of the three patients, demonstrating a direct relationship to receptors in the supraspinous ligament. Electromyograms were recorded from the feline multifidus muscle with mechanical loading of the supraspinal ligament at each of the L1‐L2 to L6‐L7 motion segments. In the free‐spine condition the largest electromyographic discharge was present in the level of ligament deformation, and lower electromyographic discharge was recorded in two rostral and caudal segments. After immobilizing any two vertebrae, loading of the ligment resulted in electromyographic discharge in the muscles of the same level and at least one level above and/or below. Conclusions. Deformation or stress in the supraspinous ligament, and possibly in other spinal ligaments, recruits multifidus muscle force to stiffen one to three lumbar motion segments and prevent instability. Strong muscular activity is seen when loads that can cause permanent damage to the ligament are applied, indicating that spastic muscle activity and possibly pain can be caused by ligament overloading.