Manohar M. Panjabi

The stabilizing system of the spine. Part I. Function, dysfunction, adaptation, and enhancement.

Presented here is the conceptual basis for the assertion that the spinal stabilizing system consists of three subsystems. The vertebrae, discs, and ligaments constitute the passive subsystem. All muscles and tendons surrounding the spinal column that can apply forces to the spinal column constitute the active subsystem. The nerves and central nervous system comprise the neural subsystem, which determines the requirements for spinal stability by monitoring the various transducer signals, and directs the active subsystem to provide the needed stability. A dysfunction of a component of any one of the subsystems may lead to one or more of the following three possibilities: (a) an immediate response from other subsystems to successfully compensate, (b) a long-term adaptation response of one or more subsystems, and (c) an injury to one or more components of any subsystem. It is conceptualized that the first response results in normal function, the second results in normal function but with an altered spinal stabilizing system, and the third leads to overall system dysfunction, producing, for example, low back pain. In situations where additional loads or complex postures are anticipated, the neural control unit may alter the muscle recruitment strategy, with the temporary goal of enhancing the spine stability beyond the normal requirements.

The stabilizing system of the spine. Part II. Neutral zone and instability hypothesis.

The neutral zone is a region of intervertebral motion around the neutral posture where little resistance is offered by the passive spinal column. Several studies--in vitro cadaveric, in vivo animal, and mathematical simulations--have shown that the neutral zone is a parameter that correlates well with other parameters indicative of instability of the spinal system. It has been found to increase with injury, and possibly with degeneration, to decrease with muscle force increase across the spanned level, and also to decrease with instrumented spinal fixation. In most of these studies, the change in the neutral zone was found to be more sensitive than the change in the corresponding range of motion. The neutral zone appears to be a clinically important measure of spinal stability function. It may increase with injury to the spinal column or with weakness of the muscles, which in turn may result in spinal instability or a low-back problem. It may decrease, and may be brought within the physiological limits, by osteophyte formation, surgical fixation/fusion, and muscle strengthening. The spinal stabilizing system adjusts so that the neutral zone remains within certain physiological thresholds to avoid clinical instability.

Clinical spinal instability and low back pain.

Clinical instability is an important cause of low back pain. Although there is some controversy concerning its definition, it is most widely believed that the loss of normal pattern of spinal motion causes pain and/or neurologic dysfunction. The stabilizing system of the spine may be divided into three subsystems: (1) the spinal column; (2) the spinal muscles; and (3) the neural control unit. A large number of biomechanical studies of the spinal column have provided insight into the role of the various components of the spinal column in providing spinal stability. The neutral zone was found to be a more sensitive parameter than the range of motion in documenting the effects of mechanical destabilization of the spine caused by injury and restabilization of the spine by osteophyle formation, fusion or muscle stabilization. Clinical studies indicate that the application of an external fixator to the painful segment of the spine can significantly reduce the pain. Results of an in vitro simulation of the study found that it was most probably the decrease in the neutral zone, which was responsible for pain reduction. A hypothesis relating the neutral zone to pain has been presented. The spinal muscles provide significant stability to the spine as shown by both in vitro experiments and mathematical models. Concerning the role of neuromuscular control system, increased body sway has been found in patients with low back pain, indicating a less efficient muscle control system with decreased ability to provide the needed spinal stability.

Stabilizing function of trunk flexor-extensor muscles around a neutral spine posture

Study Design. This study examined the coactivation of trunk flexor and extensor muscles in healthy individuals. The experimental electromyographic data and the theoretical calculations were analyzed in the context of mechanical stability of the lumbar spine. Objectives. To test a set of hypotheses pertaining to healthy individuals: 1) that the trunk flexor‐extensor muscle coactivation is present around a neutral spine posture, 2) that the coactivation is increased when the subject carries a load; and 3) that the coactivation provides the needed mechanical stability to the lumbar spine. Summary of Background Data. Theoretically, antagonistic trunk muscle coactivation is necessary to provide mechanical stability to the human lumbar spine around its neutral posture. No experimental evidence exists, however, to support this hypothesis. Methods. Ten individuals executed slow trunk flexion‐extension tasks, while six muscles on the right side were monitored with surface electromyography: external oblique, internal oblique, rectus abdominis, multifidus, lumbar erector spinae, and thoracic erector spinae. Simple, but realistic, calculations of spine stability also were performed and compared with experimental results. Results. Average antagonistic flexor‐extensor muscle coactivation levels around the neutral spine posture as detected with electromyography were 1.7 ± 0.8% of maximum voluntary contraction for no external load trials and 2.9 ± 1.4% of maximum voluntary contraction for the trials with added 32‐kg mass to the torso. The inverted pendulum model based on static moment equilibrium criteria predicted no antagonistic coactivation. The same model based on the mechanical stability criteria predicted 1.0% of maximum voluntary contraction coactivation of flexors and extensors with zero load and 3.1% of maximum voluntary contraction with a 32‐kg mass. The stability model also was run with zero passive spine stiffness to simulate an injury. Under such conditions, the model predicted 3.4% and 5.5% of maximum voluntary contraction of antagonistic muscle coactivation for no extra load and the added 32 kg, respectively. Conclusions. This study demonstrated that antagonistic trunk flexor‐extensor muscle coactivation was present around the neutral spine posture in healthy individuals. This coactivation increased with added mass to the torso. Using a biomechanical model, the coactivation was explained entirely on the basis of the need for the neuromuscular system to provide the mechanical stability to the lumbar spine.

Muscle response pattern to sudden trunk loading in healthy individuals and in patients with chronic low back pain.

Study Design. A quick-release method in four directions of isometric trunk exertions was used to study the muscle response patterns in 17 patients with chronic low back pain and 17 matched control subjects. Objectives. It was hypothesized that patients with low back pain would react to sudden load release with a delayed muscle response and would exhibit altered muscle recruitment patterns. Summary of Background Data. A delay in erector spinae reaction time after sudden loading has been observed in patients with low back pain. Muscle recruitment and timing pattern play an important role in maintaining lumbar spine stability. Methods. Subjects were placed in a semiseated position in an apparatus that provided stable fixation of the pelvis. They exerted isometric contractions in trunk flexion, extension, and lateral bending. Each subject performed three trials at two constant force levels. The resisted force was suddenly released with an electromagnet and electromyogram signals from 12 trunk muscles were recorded. The time delay between the magnet release and the shut-off or switch-on of muscle activity (reaction time) was compared between two groups of subjects using two-factor analysis of variance. Results. The number of reacting muscles and reaction times averaged over all trials and directions showed the following results: For healthy control subjects a shut-off of agonistic muscles (with a reaction time of 53 msec) occurred before the switch-on of antagonistic muscles (with a reaction time of 70 msec). Patients exhibited a pattern of co-contraction, with agonists remaining active (3.4 out of 6 muscles switched off) while antagonists switched on (5.3 out of 6 muscles). Patients also had longer muscle reaction times for muscles shutting off (70 msec) and switching on (83 msec) and furthermore, their individual muscle reaction times showed greater variability. Conclusions. Patients with low back pain, in contrast to healthy control subjects, demonstrated a significantly different muscle response pattern in response to sudden load release. These differences may either constitute a predisposing factor to low back injuries or a compensation mechanism to stabilize the lumbar spine.

Mechanical behavior of the human lumbar and lumbosacral spine as shown by three-dimensional load-displacement curves.

The lumbar region is a frequent site of spinal disorders, including low-back pain, and of spinal trauma. Clinical studies have established that abnormal intervertebral motions occur in some patients who have low-back pain. A knowledge of normal spinal movements, with all of the inherent complexities, is needed as a baseline. The present study documents the complete three-dimensional elastic physical properties of each lumbar intervertebral level from the level between the first and second lumbar vertebrae through the level between the fifth lumbar and first sacral vertebrae. Nine whole fresh-frozen human cadaveric lumbar-spine specimens were used. Pure moments of flexion-extension, bilateral axial torque, and bilateral lateral bending were applied, and three-dimensional intervertebral motions were determined with use of stereophotogrammetry. The motions were presented in the form of a set of six load-displacement curves, quantitating intervertebral rotations and translations. The curves were found to be non-linear, and the motions were coupled. The ranges of motion were found to compare favorably with reported values from in vivo studies.

Biomechanical evaluation of spinal fixation devices: I. A conceptual framework.

In the field of spinal fixation devices, there is a profusion of new instrumentations. Often, the biomechanical evaluation is done in a nonstandardized manner, which makes it difficult to compare the results of one researcher with those of another, for the same device or for different devices. There is a need for a conceptual framework under which guidelines may be suggested for the evaluation of these devices in some uniform and comprehensive manner. There are three basic biomechanical tests: strength, fatigue, and stability. The strength test evaluates the failure load of the device, determines its weak points, and is helpful in the initial development of the device. The fatigue test provides a measure of longevity of the device, either alone or as part of the spinal construct, by testing the device to failure using cyclically varying loads. In contrast, the stability test measures the capability of the device to provide multi-directional stability to the injured spine. There is no failure of the device, and the results of this test are clinically important, as they characterize the potential for early fracture healing and early fusion. A conceptual framework for the evaluation of multi-direction stability of spinal fixation devices and guidelines for designing the necessary experiments are described.

Biomechanical analysis of clinical stability in the cervical spine.

This study was undertaken because there is a dearth of objective information in the literature on the clinical instability of the cervical spine below C2. To our knowledge, it is the first biomechanical investigation designed to analyze clinical stability. We have carried out a quantitative analysis of the behavior of the spine as a function of the systematic destruction of various anatomic elements. Under controlled conditions designed to maintain the biological integrity of the specimens, 17 motion segments from 8 cervical spines were analyzed. The spines were studied with either flexion or extension simulated using physiologic loads. Some of the more important findings are: (1) In sectioning the ligaments, one observes small increments of change followed without warning by sudden, complete disruption of the spine; (2) Removal of the facets alters the motion segment such that in flexion, there is less angular displacement and more horizontal displacement; (3) The anterior ligaments contribute more to stability in extension than the posterior ligaments and in flexion, the converse is true; (4) The adult cervical spine is unstable, or on the brink of instability, when any of the following conditions are present: a) All the anterior or all the posterior elements are destroyed or unable to function. b) More than 3.5 mm horizontal displacement of one vertebra in relation to an adjacent vertebra measured on lateral roentgenograms (resting or flexion-extension). c) More than 11 degrees of rotation difference to that of either adjacent vertebra measured on a resting lateral or flexion-extension roentgenogram. These findings can be aptly applied to clinical situations and when instability as determined by the above criteria is present, surgical fusion or some other method to achieve stability should be seriously considered. Work is continuing on this problem as we do not consider this to be altogether complete or definitive. Hopefully, this initial study will stimulate further scientific and clinical investigations.

Biomechanical evaluation of lumbar spinal stability after graded facetectomies.

In an in vitro experiment using fresh human lumbar functional spinal units, the effects of the division of the posterior ligaments (consisting of the supraspinous/ interspinous ligaments) and graded facetectomies were investigated. The graded facetectomies consisted of unilateral and bilateral medial facetectomies, and unilateral and bilateral total facetectomies. Six kinds of moments were applied and ranges of motion (ROM) and neutral zones (NZ) were determined three-dimensionally by stereophotogrammetric methods. Range of motion was not affected by the division of the supraspinous/interspinous ligaments for all load modes. In flexion, ROM increased slightly after unilateral medial facetectomy. In right axial rotation, ROM increased after left unilateral total facetectomy. Range of motion was not affected, even by bilateral total facetectomies, in extension and lateral bendings. This study suggested that medial facetectomy does not affect lumbar spinal stability, and conversely, total facetectomy, even created unilaterally, makes the lumbar spine unstable.

Three-dimensional Movements of the Whole Lumbar Spine and Lumbosacral Joint

Knowledge of the normal movements of whole lumbar spine and lumbosacral joint is important for evaluating clinical pathologic conditions that may potentially produce unstable situations in these regions. At present there are few studies that report systemic three-dimensional movement analysis of these regions. The purpose of this in vitro study was to quantitatively determine three-dimensional movements of the whole lumbar spine and lumbosacral joint. Ten fresh human cadaveric spine specimens including from L1 to sacrum (six specimens) and ilium (four specimens) were studied. Pure moments of a maximum of 10 N-m were applied incrementally. Parameters of neutral zone, elastic zone, and range of motion for rotations as well as for translations were measured. Neutral zones for flexion-extension, right/left axial torque, and right-left lateral bending were, respectively: 1.6°, 0.9°, and 1.4° (L1-2); 1.0°, 0.8°, and 2.0° (L2-3); 1.4°, 0.7°, and 1.4° (L3-4); 1.8°, 0.4°, and 1.6° (L4-5); 3.0°, 0.4°, and 1.8° (L5-S1). Ranges of motion for flexion, extension, axial torque (one side), and lateral bending (one side) were, respectively: 5.8°, 4.3°, 2.3°, and 4.9° (L1-2); 6.5°, 4.3°, 2.6°, and 7.0° (L2-3); 7.5°, 3.7°, 2.6°, and 5.7° (L3-4); 8.9°, 5.8°, 2.2°, and 5.7° (L4-5); 10.0°, 7.8°, 1.4°, and 5.5° (L5-S1). Neutral zone values were small except for flexion at L5-S1. In flexion and extension, more motion took place at lower levels (L4-5, L5-S1) than at upper levels. In axial rotation of the whole lumbar spine, least motion took place at L5-S1. In lateral bending, least motion took place at L1-2 and biggest motion took place at L2-3, while similar magnitudes of motion were seen at L3-4, L4-5, and L5-S1.