Spinal Stability

Part of the difficulty with spinal stability is that the word has been used in two quite different senses that are easily confused. The first is mechanical, borrowed from engineering. A structure is mechanically stable if, when it is pushed slightly out of position, it tends to return towards its resting state rather than buckling or running away into ever larger displacement. In this sense stability is a property of a physical system and can, in principle, be calculated. The second sense is clinical. Here instability has traditionally meant something closer to an abnormal, symptom-producing behaviour of a spinal segment: excessive or disordered movement thought to cause pain, deformity, or risk to the nerves. These two meanings do not map neatly onto one another, and a great deal of confusion in the literature has flowed from treating them as if they did. A spine can be mechanically sound yet painful, and a spine can move more than average without ever causing trouble.

The most influential clinical definition came from Alf Nachemson and, above all, from Augustus White and Manohar Panjabi, whose textbook Clinical Biomechanics of the Spine framed clinical instability as the loss of the spine's ability, under physiological loads, to maintain its patterns of movement so that there is neither damage nor irritation of the neural structures, nor the development of deformity or incapacitating pain. It is a careful, deliberately broad definition, and its very breadth reflects an honest problem: the profession has never had a simple, agreed yardstick for instability that applies to the common, painful, degenerate back.

The modern scientific account of how the spine stays stable owes most to the work of Manohar Panjabi, who in 1992 proposed that spinal stability is produced not by any single structure but by three interdependent subsystems working together. The first is the passive subsystem: the vertebrae, the intervertebral discs, the facet joints and, crucially, the ligaments and joint capsules. These provide the bulk of the spine's stability towards the ends of its range of movement, when they are stretched taut, but they contribute little near the upright, neutral posture where they are slack. The second is the active subsystem: the muscles and their tendons, which generate the forces that hold the column upright and resist perturbation. The third is the neural subsystem: the nerves and the central nervous system, which continuously sense the position and loading of the spine through receptors in the muscles, tendons and ligaments, and command the muscles to contract with the right timing and force. In Panjabi's scheme, stability emerges from the coordinated function of all three; a deficit in one may be compensated by the others, but if compensation fails, the system as a whole may become unstable.

An isolated cadaver spine, stripped of its muscles, is a revealing object. Studies by Panjabi and others showed that such a specimen, loaded from above, buckles under a strikingly small weight, far less than the loads the living spine carries every day. It is the muscles, coordinated by the nervous system, that make the difference. Biomechanical experiments by Wilke, Crisco, Cholewicki, McGill and their colleagues went on to quantify how much different muscle groups contribute, and to show mathematically that the spine behaves like a flexible rod held upright by guy-ropes: without sufficient, well-timed muscular tension it is prone to buckling, and with it the same column becomes robust. This reframing, from stability as a fixed property of bones and ligaments to stability as an actively maintained, moment-to-moment achievement of muscle and nerve, is one of the most important conceptual shifts in the whole field.

Within Panjabi's account sits a particularly influential concept: the neutral zone. When a spinal segment is moved from full bending in one direction to full bending in the other, it does not resist evenly throughout. Near the middle of the range, around the neutral upright posture, the segment offers very little resistance and moves easily; this laxity is the neutral zone. Towards the extremes of movement, resistance rises steeply as ligaments and other passive tissues tighten; this is the elastic zone. Panjabi and his colleagues showed experimentally that when a disc is injured, or a segment degenerates, or ligaments are cut, it is chiefly the neutral zone that enlarges: the region of easy, poorly controlled motion grows, even when the total range of movement changes rather little. He argued that the size of the neutral zone is a more sensitive indicator of instability than the total range of motion, and that muscular control is precisely what normally keeps the neutral zone in check. The idea was attractive because it offered a measurable, mechanically grounded target and helped explain why a back could feel unstable and painful without any gross excess of movement on an ordinary examination.

If muscle is central to stability, which muscles? An important refinement came from the Swedish engineer Anders Bergmark, who in 1989 distinguished two functional systems. The global system comprises the large, superficial muscles that span many segments and produce the powerful movements and general bracing of the trunk. The local system comprises the smaller, deeper muscles that attach directly to individual vertebrae and are well placed to control the fine relationships between adjacent segments, tuning stability where the large muscles cannot. This distinction directed attention to two deep muscles in particular. The multifidus, a series of short muscles bridging the small joints at the back of the spine, and the transversus abdominis, the deepest sheet of the abdominal wall, which wraps the trunk like a corset. Both were argued to be important local stabilisers of the lumbar spine.

The work that most captured clinical imagination came from Paul Hodges and Carolyn Richardson in Brisbane. In the mid 1990s they reported that in healthy people the transversus abdominis contracts a fraction of a second before a limb movement, in anticipation of the disturbance the movement will create, whereas in people with low back pain this anticipatory, feedforward contraction is delayed. Around the same time, Julie Hides and colleagues showed that the multifidus muscle, after a first episode of acute low back pain, wastes rapidly and does not simply recover on its own once the pain settles. Taken together, these observations suggested that the deep stabilising muscles behave abnormally in people with back pain, and that this might be both a consequence and a perpetuating cause of the problem, a loop that treatment might be able to break.

The third of Panjabi's subsystems, the neural controller, has its own body of evidence. The ligaments, discs and muscles of the spine are richly supplied with sensory receptors that report on position, tension and load. Work by Solomonow, Holm, Indahl and others demonstrated that stretching or loading spinal ligaments reflexively activates the deep muscles, a sensorimotor loop that appears to protect the segment; and that when these tissues are damaged or fatigued, the reflexes are blunted and muscular protection falters. Panjabi later drew these threads into a broader hypothesis, suggesting that minor, sub-rupture injuries to spinal ligaments might corrupt the sensory signals they send, leading in turn to faulty muscle control, to abnormal loading, and to a self-sustaining cycle of pain and dysfunction. Whether or not that particular hypothesis proves correct, it exemplifies the mature view of stability as a control problem, in which timing and coordination matter as much as raw strength.

While the biomechanists were dissecting the mechanics of the ordinary back, a parallel and rather separate history was unfolding in the management of spinal injuries, and here the concept of stability had immediate, practical bite. When a vertebra is broken, the first question is whether the injury is stable, meaning it can be trusted to heal without displacing and damaging the spinal cord, or unstable, meaning it may collapse or shift and therefore may need to be held. The answer determines whether a patient is treated with a brace and time or with an operation, so a clear rule for judging stability was of the greatest value.

The first widely adopted rule came from the British surgeon Frank Holdsworth, who in an influential paper of 1970 proposed a two-column model of the spine. He divided the vertebral column into an anterior column, comprising the vertebral bodies and discs, and a posterior column, comprising the bony arch and the strong posterior ligaments, which he christened the posterior ligamentous complex. In Holdsworth's scheme, stability depended above all on the integrity of that posterior ligamentous complex: if it was torn, the injury was unstable. This gave surgeons a workable framework for a generation.

The framework was recast in 1983 by the American surgeon Francis Denis, who, on the basis of a large series of injuries studied with the newly available computed tomography, argued that a two-column model was insufficient and proposed instead a three-column spine. Denis divided the anterior part of Holdsworth's model in two, defining an anterior column of the front of the vertebral body and disc, a middle column of the back of the vertebral body and disc together with the posterior longitudinal ligament, and a posterior column of the bony arch and its ligaments. His central claim was that the middle column was the key to stability, and that injury involving two of the three columns, particularly the middle, indicated an unstable fracture. The three-column concept, described in similar terms at around the same time by Raymond Louis in France, dominated the classification and surgical decision-making of spinal trauma for decades, precisely because it linked a mechanical judgement about stability to a concrete choice about treatment.

The column theories were enormously useful, but they were also heavily anatomical and did not always predict outcomes well, nor did observers always agree when applying them. Over the following decades the field moved towards classification systems that combine the morphology of the fracture with the state of the crucial ligaments and, importantly, with the patient's neurological condition. Systems such as the Thoracolumbar Injury Classification and Severity score and the later AOSpine classification retain the Holdsworth insight that the integrity of the posterior ligamentous complex is central to stability, but they fold it into a broader, more reproducible assessment. The direction of travel has been from a purely structural notion of stability towards one that also weighs how the injury behaves and how it affects the nervous system, mirroring the same shift, from bones to whole-system function, seen in the study of the everyday back.

The idea that instability causes pain, and that eliminating movement should therefore relieve it, is the intellectual engine behind spinal fusion, an operation that joins two or more vertebrae into a single block of bone. Fusion has deep roots: Russell Hibbs and Fred Albee independently described operations to fuse the spine around 1911, initially to arrest the deforming, destructive instability of tuberculosis and of scoliosis. When Mixter and Barr showed in 1934 that a ruptured intervertebral disc could compress a nerve and cause sciatica, surgery on the lumbar spine expanded rapidly, and fusion was increasingly added in the belief that the affected segment was unstable and that stabilising it would prevent recurrent trouble. Over the twentieth century, as instrumentation improved, from bone grafts alone to screws, rods and cages, fusion became one of the most commonly performed spinal operations in the world.

Yet the evidence that fusion reliably cures the common degenerate, painful back proved harder to establish than the confident theory suggested. Randomised trials from the turn of the century, notably the Swedish Lumbar Spine Study led by Peter Fritzell, found that fusion could reduce pain and disability more than the rather unstructured non-surgical care of the time, but the advantage was modest and came with the costs and risks of surgery; other trials comparing fusion with structured rehabilitation found little difference. For specific problems with a clear mechanical basis, such as an unstable fracture, a slipping vertebra, deformity, tumour or infection, the logic of fusion remained strong. For non-specific low back pain attributed to vague segmental instability, the results were sobering, and they prompted a re-examination of the underlying assumption.

If fusing a segment solid was an imperfect answer, perhaps the problem was the solidity itself. Fusion abolishes movement at the treated level and, by doing so, may throw extra load onto the neighbouring segments, a phenomenon known as adjacent segment degeneration. This concern, together with the wider enthusiasm for stability as controlled rather than abolished motion, drove a wave of motion-preserving devices from the 1980s onwards. Artificial intervertebral discs sought to replace a worn disc while keeping the segment mobile. Dynamic stabilisation systems, of which the Dynesys device described by Stoll and colleagues in 2002 is the best known, used flexible cords and spacers anchored to screws to restrain and guide movement rather than eliminate it, aiming, in effect, to shrink the enlarged neutral zone while preserving useful motion. The concept was elegant and directly rooted in the biomechanical theory of stability.

In practice, the results of these motion-preserving approaches have been mixed. Total disc replacement performs comparably to fusion for carefully selected patients in the trials that have been done, but has not displaced it. Dynamic stabilisation systems have suffered mechanical problems and have not clearly outperformed fusion, and enthusiasm for them has cooled. The episode is instructive: a plausible mechanical theory generated ingenious devices, but the clinical benefit that the theory predicted did not straightforwardly materialise, a recurring lesson in this field.

The same theory that inspired implants also inspired a revolution in physiotherapy. If back pain reflected faulty control of the deep stabilising muscles, then retraining those muscles, the transversus abdominis and the multifidus in particular, ought to help. From the 1990s, specific stabilisation or motor control exercise, developed by Richardson, Hodges, Hides and others, taught patients to isolate and activate these deep muscles, and the broader idea of core stability escaped the clinic to become a fixture of gyms, Pilates studios and sports training everywhere. For a time it seemed that the mechanism of common back pain had been identified and a rational, targeted treatment devised.

As the evidence accumulated, a more measured picture emerged. Motor control exercise does help people with chronic low back pain, but systematic reviews, including a Cochrane review by Saragiotto and colleagues in 2016, found it to be about as effective as other active forms of exercise, not clearly superior. Careful studies also questioned some of the founding assumptions: the changes in deep-muscle timing seen in people with pain are variable and may be a general response to pain rather than a specific fault to be corrected, and simply strengthening one muscle does not reliably fix pain. Critics such as Eduardo Lederman argued forcefully that the "core stability" concept had outrun its evidence. The consensus that has settled is that keeping active and moving matters, that exercise helps, but that the particular brand of exercise matters less than was once believed, and that the deep muscles are part of a system rather than a magic switch.

The pendulum of thought has, in a sense, swung and then settled nearer the middle. The early, structural view located stability in bones and ligaments; the biomechanical revolution relocated it to a dynamic system of muscle and nerve; the clinical enthusiasm that followed sometimes overreached, attributing too much common pain to measurable instability and promising too much from stabilising surgery and stabilising exercise alike. Voices such as Robert Mulholland questioned whether "lumbar instability" was even the right way to think about the degenerate painful back, suggesting that abnormal loading and the patient's response to it, rather than excess movement as such, might be closer to the truth. Modern back pain science increasingly frames persistent pain as a complex biopsychosocial phenomenon, in which tissue mechanics are only one strand alongside sensitisation of the nervous system, beliefs, fear of movement and general health.

Where does this leave spinal stability? As a mechanical concept it remains indispensable, especially in trauma, deformity and reconstruction, where the question of whether a spine can safely bear load without failing is concrete and consequential. As an explanation for ordinary, non-specific back pain it is now held more lightly, one useful idea among several rather than the master key it once seemed. The three-subsystem model endures as the best framework we have for understanding how the healthy spine holds itself together, and the neutral zone remains a valuable way of thinking about the loss of fine control. But the field has learned humility about translating an elegant mechanical theory directly into treatment. The history of spinal stability is, in the end, a case study in how a beautiful idea can illuminate a subject profoundly while also, if held too tightly, mislead, and in how good science slowly sorts the one from the other.

Few ideas in the study of the back have proved as powerful, or as slippery, as spinal stability. It is invoked to explain how a slender column of small bones can carry the weight of the body and permit graceful movement without collapsing or injuring the delicate cord it protects; it is blamed when the back is painful; and it has been used to justify a great deal of treatment, from strengthening exercises to metal implants. Yet for much of medical history there was no agreed definition of what stability actually meant, still less a way to measure it. This piece traces the concept: what spinal stability is understood to be, the mechanics by which the spine holds itself together and moves, and the long evolution of thought that carried the idea from the fracture clinic of the mid twentieth century, through the biomechanics laboratory, into the physiotherapy gym, and back again to a more cautious modern view. Along the way it shows how each shift in understanding reshaped the treatment of spinal conditions, and where that reasoning stands today. It is purely educational. It describes the science and its history; it offers no advice and makes no recommendations.

• —Panjabi MM. The stabilizing system of the spine. Part I. Function, dysfunction, adaptation, and enhancement. J Spinal Disord. 1992;5(4):383-389; discussion 397. PMID: 1490034.
• —Panjabi MM. The stabilizing system of the spine. Part II. Neutral zone and instability hypothesis. J Spinal Disord. 1992;5(4):390-396; discussion 397. PMID: 1490035.
• —Panjabi MM. Clinical spinal instability and low back pain. J Electromyogr Kinesiol. 2003;13(4):371-379. PMID: 12832167.
• —Panjabi MM. A hypothesis of chronic back pain: ligament subfailure injuries lead to muscle control dysfunction. Eur Spine J. 2006;15(5):668-676. PMID: 16047209.
• —Denis F. The three column spine and its significance in the classification of acute thoracolumbar spinal injuries. Spine (Phila Pa 1976). 1983;8(8):817-831. PMID: 6670016.
• —Denis F. Spinal instability as defined by the three-column spine concept in acute spinal trauma. Clin Orthop Relat Res. 1984;(189):65-76. PMID: 6478705.
• —Holdsworth F. Fractures, dislocations, and fracture-dislocations of the spine. J Bone Joint Surg Am. 1970;52(8):1534-1551. PMID: 5483077.
• —Louis R. Spinal stability as defined by the three-column spine concept. Anat Clin. 1985;7(1):33-42. PMID: 3994851.
• —Hodges PW, Richardson CA. Inefficient muscular stabilization of the lumbar spine associated with low back pain. A motor control evaluation of transversus abdominis. Spine (Phila Pa 1976). 1996;21(22):2640-2650. PMID: 8961451.
• —Hodges PW, Richardson CA. Contraction of the abdominal muscles associated with movement of the lower limb. Phys Ther. 1997;77(2):132-142. PMID: 9037214.
• —Bergmark A. Stability of the lumbar spine. A study in mechanical engineering. Acta Orthop Scand Suppl. 1989;230:1-54. PMID: 2658468.
• —Crisco JJ 3rd, Panjabi MM. The intersegmental and multisegmental muscles of the lumbar spine. A biomechanical model comparing lateral stabilizing potential. Spine (Phila Pa 1976). 1991;16(7):793-799. PMID: 1925756.
• —Wilke HJ, Wolf S, Claes LE, Arand M, Wiesend A. Stability increase of the lumbar spine with different muscle groups. A biomechanical in vitro study. Spine (Phila Pa 1976). 1995;20(2):192-198. PMID: 7716624.
• —Cholewicki J, McGill SM. Mechanical stability of the in vivo lumbar spine: implications for injury and chronic low back pain. Clin Biomech (Bristol, Avon). 1996;11(1):1-15. PMID: 11415593.
• —Panjabi M, Abumi K, Duranceau J, Oxland T. Spinal stability and intersegmental muscle forces. A biomechanical model. Spine (Phila Pa 1976). 1989;14(2):194-200. PMID: 2922640.
• —Hides JA, Richardson CA, Jull GA. Multifidus muscle recovery is not automatic after resolution of acute, first-episode low back pain. Spine (Phila Pa 1976). 1996;21(23):2763-2769. PMID: 8979323.
• —MacDonald DA, Moseley GL, Hodges PW. The lumbar multifidus: does the evidence support clinical beliefs? Man Ther. 2006;11(4):254-263. PMID: 16716640.
• —Hodges PW, Moseley GL. Pain and motor control of the lumbopelvic region: effect and possible mechanisms. J Electromyogr Kinesiol. 2003;13(4):361-370. PMID: 12832166.
• —Holm S, Indahl A, Solomonow M. Sensorimotor control of the spine. J Electromyogr Kinesiol. 2002;12(3):219-234. PMID: 12086817.
• —Adams MA, Dolan P. Spine biomechanics. J Biomech. 2005;38(10):1972-1983. PMID: 15936025.
• —Nachemson A. Lumbar intradiscal pressure. Experimental studies on post-mortem material. Acta Orthop Scand Suppl. 1960;43:1-104. PMID: 14425680.
• —Kirkaldy-Willis WH, Farfan HF. Instability of the lumbar spine. Clin Orthop Relat Res. 1982;(165):110-123. PMID: 6210480.
• —Beazell JR, Mullins M, Grindstaff TL. Lumbar instability: an evolving and challenging concept. J Man Manip Ther. 2010;18(1):9-14. PMID: 21655418.
• —Virk S, Qureshi S, Sandhu H. History of Spinal Fusion: Where We Came from and Where We Are Going. HSS J. 2020;16(2):137-142. PMID: 32523481.
• —Hibbs RA. An operation for progressive spinal deformities: a preliminary report of three cases from the service of the orthopaedic hospital. 1911. Clin Orthop Relat Res. 2007;460:17-20. PMID: 17620807.
• —Mixter WJ, Barr JS. Rupture of the intervertebral disc with involvement of the spinal canal. N Engl J Med. 1934;211(5):210-215.
• —Fritzell P, Hägg O, Wessberg P, Nordwall A; Swedish Lumbar Spine Study Group. 2001 Volvo Award Winner in Clinical Studies: Lumbar fusion versus nonsurgical treatment for chronic low back pain: a multicenter randomized controlled trial from the Swedish Lumbar Spine Study Group. Spine (Phila Pa 1976). 2001;26(23):2521-2532. PMID: 11725230.
• —Weinstein JN, Lurie JD, Tosteson TD, et al. Surgical versus nonsurgical treatment for lumbar degenerative spondylolisthesis. N Engl J Med. 2007;356(22):2257-2270. PMID: 17538085.
• —Stoll TM, Dubois G, Schwarzenbach O. The dynamic neutralization system for the spine: a multi-center study of a novel non-fusion system. Eur Spine J. 2002;11(Suppl 2):S170-S178. PMID: 12384741.
• —Ding F, Jia Z, Zhao Z, et al. Total disc replacement versus fusion for lumbar degenerative disc disease: a systematic review of overlapping meta-analyses. Eur Spine J. 2017;26(3):806-815. PMID: 27448810.
• —Prud'homme M, Barrios C, Rouch P, Charles YP, Steib JP, Skalli W. Clinical Outcomes and Complications After Pedicle-anchored Dynamic or Hybrid Lumbar Spine Stabilization: A Systematic Literature Review. J Spinal Disord Tech. 2015;28(8):E439-E448. PMID: 25093644.
• —Mulholland RC. The myth of lumbar instability: the importance of abnormal loading as a cause of low back pain. Eur Spine J. 2008;17(5):619-625. PMID: 18301932.
• —McGill SM. Low back stability: from formal description to issues for performance and rehabilitation. Exerc Sport Sci Rev. 2001;29(1):26-31. PMID: 11210443.
• —Lederman E. The myth of core stability. J Bodyw Mov Ther. 2010;14(1):84-98. PMID: 20006294.
• —Saragiotto BT, Maher CG, Yamato TP, et al. Motor control exercise for chronic non-specific low-back pain. Cochrane Database Syst Rev. 2016;(1):CD012004. PMID: 26742533.
• —Kjaer P, Bendix T, Sorensen JS, Korsholm L, Leboeuf-Yde C. Are MRI-defined fat infiltrations in the multifidus muscles associated with low back pain? BMC Med. 2007;5:2. PMID: 17254322.
• —Danneels LA, Vanderstraeten GG, Cambier DC, Witvrouw EE, De Cuyper HJ. CT imaging of trunk muscles in chronic low back pain patients and healthy control subjects. Eur Spine J. 2000;9(4):266-272. PMID: 11261613.
• —Abedi A, Mokkink LB, Zadegan SA, et al. Reliability and Validity of the AOSpine Thoracolumbar Injury Classification System: A Systematic Review. Global Spine J. 2019;9(2):231-242. PMID: 30984504.
• —White AA, Panjabi MM. Clinical Biomechanics of the Spine. 2nd ed. Philadelphia: J.B. Lippincott; 1990.

This information is for general patient education and is not a substitute for an individual consultation. Always seek personalised advice from a qualified spinal specialist.