A hypothesis of chronic back pain: ligament subfailure injuries lead to muscle control dysfunction

Abstract

Clinical reports and research studies have documented the behavior of chronic low back and neck pain patients. A few hypotheses have attempted to explain these varied clinical and research findings. A new hypothesis, based upon the concept that subfailure injuries of ligaments (spinal ligaments, disc annulus and facet capsules) may cause chronic back pain due to muscle control dysfunction, is presented. The hypothesis has the following sequential steps. Single trauma or cumulative microtrauma causes subfailure injuries of the ligaments and embedded mechanoreceptors. The injured mechanoreceptors generate corrupted transducer signals, which lead to corrupted muscle response pattern produced by the neuromuscular control unit. Muscle coordination and individual muscle force characteristics, i.e. onset, magnitude, and shut-off, are disrupted. This results in abnormal stresses and strains in the ligaments, mechanoreceptors and muscles, and excessive loading of the facet joints. Due to inherently poor healing of spinal ligaments, accelerated degeneration of disc and facet joints may occur. The abnormal conditions may persist, and, over time, may lead to chronic back pain via inflammation of neural tissues. The hypothesis explains many of the clinical observations and research findings about the back pain patients. The hypothesis may help in a better understanding of chronic low back and neck pain patients, and in improved clinical management.

Fig. 1

Spinal stabilizing system. It consists of three subsystems: spinal column, spinal muscles, and neuromuscular control unit. The spinal column has two functions: structural—to provide intrinsic mechanical stability, and transducer—to generate signals describing spinal posture, motions, loads etc. via the mechanoreceptors. The neuromuscular control unit generates muscle response pattern to activate and coordinate the spinal muscles to provide muscle mechanical stability. There is feedback from the spinal muscles and mechanoreceptors to the control unit. (Adapted from Panjabi 1992)

Fig. 2

Normal circumstances. The intact mechanoreceptors send transducer signals to the neuromuscular control unit, which evaluates the transducer signals and sends out muscle response pattern to coordinate the activation of individual spinal muscles. There is feedback from the muscle spindles and golgi tendon organs of the muscles and mechanoreceptors of the ligaments to the neuromuscular control unit. Under normal circumstances, there are no adverse consequences

Fig. 3

Subfailure injuries of the ligaments. The injured mechanoreceptors send out corrupted transducer signals to the neuromuscular control unit, which finds spatial and temporal mismatch between the expected and received transducer signals, and, as a result, there is muscle system dysfunction and corrupted muscle response pattern is generated. Consequently, there are adverse consequenses: higher stresses, strains, and even injuries, in the ligaments, mechanoreceptors, and muscles. There may also be muscle fatigue, and excessive facet loads. These abnormal conditions produce neural and ligament inflammation, and over time, chronic back pain