Topics In Clinical Chiropractic 2001 (Dec); 8 (1): 1628 ~ FULL TEXT
John J. Triano, DC, PhD
Chiropractic Division, Texas Back Institute,
6300 W. Parker Road, Plano, TX 75093-7916
Purpose: Theoretical foundations for the subluxation model are reviewed and compared with new evidence. Understanding the mechanisms of the lesion will further refine chiropractic theory and empower the elaboration of techniques to improve treatment outcomes and quality of patient care.
Methods: Literature from multiple scientific and clinical disciplines was surveyed with emphasis on evidence related to the concepts of subluxation and manipulation.
Summary: The buckling model builds on clinical observations and supplements them with both direct and indirect biomechanical evidence. This model does not preconceive or proscribe any source of symptoms, but is able to accommodate the multifaceted clinical presentations of patients who respond favorably to manipulation/adjustment. It also can sustain a variety of hypothetical and evidence-based challenges. These findings offer an opportunity to reconceptualize and refine theoretical models of the spinal lesion into a platform for scientific, clinical, and political advancement of the profession.
Key words: biomechanics, orthopaedic manipulation, spine, theoretical models
Spinal manipulation and adjusting have a long history, stretching from ancient Greece through medieval England and the Far East. The chiropractic profession was founded on empirical observations suggesting a role for spinal structure
influencing patients' function and health. A number of explanations on how the mechanics of vertebral subluxation/somatic dysfunction may impact physiology have been offered. [1, 2] Since 1975, with the first multidisciplinary scientific conference on the research status of spinal manipulation, the clinical science of manipulation has made substantial strides. Over 50 clinical trials have been completed, with a preponderance of evidence supporting the use of manipulation for management of acute back pain  and, to a lesser extent,
subacute and chronic back pain, neck pain, and headache.  Internationally, several formal guidelines for the management of spine-related disorders now recommend manipulation/adjustment as a first line of care. In response to increased interest and the need for answers to fundamental questions, the chiropractic profession recently has begun to prioritize the development of theory and the definition of its unique body of knowledge as a discipline. 
While clinical evidence has begun to evolve, basic science data have lagged behind, due in part to a general lack of solid information on the pathophysiologic nature of spinal disorders in general.  Traditional pathophysiologic models, typical of health care, have failed to predict adequately spine injury, prevention, disease, or recovery. Although evidence exists for anatomic and degenerative abnormalities as potential pain generators, their presence alone is neither necessary nor sufficient to produce symptoms or disability. Similarly, normal structure can be present in an individual with disabling nociceptive pain. The focus of much research has been on morphology in addition to the processes of inflammation, pain mediation through peripheral and central mechanisms, and psychological factors influencing the degree to which patients suffer.  Speculation on neurophysiologic, kinetic, biomechanical, trophic, and psychosocial mechanisms of subluxation/spinal dysfunction abound in chiropractic literature as well, but remain primarily deductive. [14, 15] The present work reviews developments in the field of biomechanics that relate directly to the subluxation concept and its resolution through manipulation/adjustment. [16, 17] Understanding the mechanisms of the lesion development and of manipulation will further refine chiropractic theory and empower the elaboration of techniques to improve treatment outcomes and the quality of patient care.
Concepts Underlying Clinical Biomechanics
There are compelling reasons to study the fundamental mechanics of subluxation and spinal manipulation procedures, First, biomechanics is the language by which the spine and joints in general manifests its function. Through studies of successful treatment methods, for example, it may be possible to reconstruct the biomechanical elements of the
underlying lesion or lesions leading to better treatment and prevention, Second, there is need to comprehend the loads that
act on the spine during the administration of these procedures. On the one hand, we must respond to the legitimate concerns
of society for generally safe procedures and warrant, in unequivocal terms, their trust. At the same time, detailed
knowledge of the forces and moments that are induced by our procedures give us the tools by which we can better refine
them and match procedures to patient need on an objective basis. Third, if we are to be more successful and convincing in
communicating our model of health, we must be able to communicate in terms understandable to the broader communities of health care and policy makers. Last, with quantitative knowledge, we are able to develop high clinical standards for
skill and training that are testable,  anchoring our claim for
jurisdictional control of our science. 
Albert Einstein used an elegant refinement of the oft-quoted chiropractic philosophical notion of theory to "keep it
simple." Einstein's version is paraphrased:
everything should be made as simple as possible, but not simpler.
The biomechanics of forces and moments applied to the human body can be both mathematically and physiologically complex. The concepts of lesion development and its correction, however, are relatively straightforward with an understanding of some basic biomechanics. Such information is not simply heuristic but is pragmatically applied. Understanding how the effects of treatment may be affected by age, degeneration, and traumatic changes, combined with manipulation control strategies. [17, 23] provides a rich armamentarium for dealing with a wide variety
of problems, from the simple to the complex.
Biomechanical Characteristics Of Manipulation
Manipulation uses externally applied forces and moments, targeting a joint assembly. [17, 24, 25] The procedures may be characterized by their biomechanics to supply a simple system that
accommodates all of the available techniques. Table 1 summarizes these categories based on current scientific knowledge.
Table 1. Manual procedures classified according to biomechanical characteristics. Loads listed represent published means, where larger sample sizes are available, or typical values for smaller samples.
Procedure type Subgroup Mode Load Speed
Unloaded spinal Continuous passive Mechanical Periodic [<20% BW,
motion motion (CPM) <5% BMI] 0.53 Hz
Flexion-distraction Manual Periodic <0.50 Hz
Mobilization Manual Periodic (~150 N*) <2.00 Hz
procedures Manual Manual Impulse [<560 N, <140 msec
CPM Manual + Periodic + Summed
Impulse hammer Manual + Impulse (<150 N*) <20 msec
* Typical value.
BW = body weight; BMI = body mass index.
The high-velocity, low-amplitude procedure, most often considered the primary treatment used by chiropractors,  produces a brief impulse (100300 msec) load [17, 25, 2730] into the region of an osseous joint. In recent years, Herzog's group in Alberta, Canada, has provided evidence of absolute and relative intervertebral movements resulting from manipulation. [25, 3133] The concept of Sandoz  that synovial joints may be stretched into the "paraphysiologic" joint range is supported. If cavitation (rapid separation of the joint surfaces with formation of an intra-articular gas bubble in the synovial space of the joint) occurs, an audible "pop" can often be heard. 
Brodeur  reviewed literature on joint cavitation, particularly the work of Sandoz  and Mierau et al  that studied the characteristics of cavitation in depth. Mierau et al's work provided the first experimental evidence of increased range of motion after joint cavitation. The boundary separating physiologic and paraphysiologic motion has not been mapped, and its exact relationship to cavitation is not known. Empirical experience suggests that cavitation also may occur before reaching the paraphysiologic space. Regardless of timing, biomechanical behavior of joint structures is known to change following manipulation/adjustment.  The advantage, from a biomechanical standpoint, of altered behavior of the joint is a shift in load-bearing stresses that are transmitted.
Theoretical Mechanics of Spinal Lesion and Dysfunction
Effective manipulation acts on a lesion that is conformable to specific forces and moments in such a way that mechanical
stress concentrations resulting in symptoms are altered. Sustaining beneficial clinical effects depends on the ability for
repair of any tissue damage and for return to the pre-injury function. Many independent hypotheses have been advanced
to account for various aspects of the clinical presentations seen with patients that seem to respond favorably to treatment.
The autonomy of each of these interpretations limits the ability to generalize to the spectrum of patients who seem to
respond favorably when treated. This section reviews the principal features of the main theories and shows how each
may be understood in the context of a unified theory of the lesion, based on biomechanical evidence.
Traditionally, the manipulable disorder (termed "subluxation" in chiropractic, "somatic dysfunction" in osteopathy, and
"fixation" or "functional blockage" in manual medicine) has been characterized as a spinal joint strain/sprain with associated local and referred pain and muscle spasm.  This may be too
narrow a perspective as will be shown in the material that follows. Spinal joint function is assumed to become restricted or
deranged in some fashion, either as an isolated lesion at single or multiple levels, or in combination with other pathology. [16, 17] Several mechanisms for restricted function (fixation, blockage,
hypomobility) have been proposed, but definitive work on their viability remains to be done. Proposed mechanisms for such
hypomobility include the following:
Zygapophyseal joint entrapment of an inclusion or meniscoid (heavily innervated by nociceptors)
Anular fragment entrapment at the intervertebral disc that is innervated by nociceptors
Adhesions and scar formation from earlier injury and/or degenerative change resulting in stiffness and adaptive myofascial shortening
Hyperactive (spastic, hypertonic) deep intrinsic spinal musculature in unilateral, asymmetric patterns
Mechanisms of action from manipulation that have been proposed to affect mechanics include releasing entrapped synovial or disc tissues, reducing pain and restoring mobility, breaking adhesions, and stretching myofascial tissues.  Again, the effects on local tissues within the joint structures require much more research before they can be determined.
Clinically identifying the lesion requires deductive reasoning from the history and examination. The same data are often
useful in selecting the preferred treatment method. Specific procedures are matched to the patient's condition through provocative testing  that evaluates the joint tolerance to a given technique. Modifications in manipulation loads transmitted through the lesioned segment can follow, based on knowledge of the local anatomy, pathology, and response to provocation. Such strategies are aimed at varying the effects of treatment and may include alterations to the patient's initial position in preparation for the procedure, using static or dynamic preload, or altering direction and amplitude of loading. All of these steps are mechanical in nature, and they involve an understanding of the biomechanical actions at the articulation when they are taken.
The core concepts describing the lesions have evolved from multiple sources and may carry different meanings to different
groups, [2, 41, 42] Table 2 relates symptoms by presenting complaint,
pathoanatomic diagnoses, or speculation on the symptom generator. Patients labeled with these descriptors may benefit from
manipulation.  This observation raises several scientific considerations that might include the following:
What is the precise nature of the disorder being treated?
Are there subcategories of lesion that might better be treated by one form of manipulation over the others?
Might some patients respond better to one intervention compared with another?
Are there clinically observable characteristics of spinal dysfunction that allow for an accurate prognosis (based on response to treatment)?
Table 2. Diagnostic terms that relate symptoms by presenting complaint, pathoanatomic diagnosis, or speculation on the symptom generator
Grouping factor Diagnosis
Hypothetical Subluxation/subluxation complex
concept Osteopathic lesion/somatic dysfunction
Pathoanatomy Facet syndrome
Internal disc derangement
Complaint Low back pain/lumbalgia
Modeling Spinal Lesions
Classical chiropractic theory favors the term "subluxation" to characterize the spinal lesion. Leach  and Gatterman  offered reviews of chiropractic reasoning incorporating indirect, sometimes remote, evidence from the literature to correlate or explain clinical observations. Traditional spinal lesion concepts reflect clinical observations. Most early concepts imply an independent
lesion. However, more recent opinions acknowledge common comorbidity between subluxation and common pathoanatomic states. [14, 43, 4547] Descriptive terminology such as subluxation syndrome or subluxation complex has been proposed in an attempt to better convey this concept. [2, 42]
In the fields of manual medicine and osteopathy, Dvorak and Dvorak,  Lewit,  and Greenman  argued similarly to explain clinical observations. Ignoring the syntax of respective schools of thought, [2, 15, 42] the similarities in concepts between the disciplines are striking. Over 106 different terms have been used to define the spinal lesion over the years, with agreement among authors occurring for only nine of them. The motivation for some of the terms seems less to foster scientific and clinical utility than to differentiate professional or philosophical domains. Figure 1 groups terms according to historical consensus regarding the speculative nature of the lesion itself. Over 60% of the proposed definitions reference physical constructs (mechanical/anatomic/structural). Over 20% are ambiguous, as the terms are self-referencing, incorporating the term (or a derivative) being defined.
Based on clinical observations, speculative mechanisms have included mechanical (eg, restricted range of motion),
neurologic (eg, aberrant reflexes, hypertonicity), trophic (eg, inflammatory change), or even psychosocial (eg, stress in the
form of muscle tension)  elements. A summary of the early writings of Palmer outline four required components of subluxation:
(1) articular misalignment,
(2) narrowing of the intervertebral foramen,
(3) nerve pressure, and
(4) interference with nerve function. 
While elements of this cascade may be present in any given case, Boone and Dobson 
correctly point out that there is "insufficient," even contradictory, data for such a narrow and circumscribed model.
Dishman  and Lantz [14, 53] expanded the Palmer model in an effort to account for pathomechanical, biochemical, and physiologic components without proscriptive focus on a given sequence.
Leach  and Mootz  supplement the hypotheses on subluxation seemingly to reconcile explanations shared with other disciplines that use manipulation methods (Table 3). Leach  suggests that subluxation is a form of manipulable lesion. Other modem terms include "somatic" or "segmental" dysfunction. [48, 50] Together they encompass a disturbance of the internal behavior of the functional spinal unit that is reversible. The lesion is an impairment of mobility caused by various forms of mechanical overload and may have remote effects, influencing the vascular, lymphatic, or neural elements. Regardless of origin, there appears to be a cross-discipline consensus that these altered biomechanical behaviors may manifest as local and/or remote clinical signs and symptoms.
Table 3. Historical versus supplemented models of subluxation and the clinical observations associated with them
Historical model Supplemental model Clinical observations
Subluxation Subluxation complex Local symptoms
Vertebral misalignment Kinesiopathology Altered range of motion
Narrowed IVF Myopathology Hypo/hypertonicity
Nerve pressure Neuropathology Pain/paresthesia
Nerve interference Vascular pathology Redness/swelling
Connective tissue Adhesions/degenerative change
IVF = intervertebral foramen.
Biomechanical Concepts Relevant to Modeling the Spinal Lesion
Table 4 reviews biomechanical properties of living tissue that are relevant to modeling the spinal lesion. Posture and
muscle activity, under normal conditions, are considered a response to the tasks being performed. However, a variety of
postural choices may be available, dictating how loading of the spine actually occurs. Differing strategies may be effective in accomplishing the same task but may have different metabolic or mechanical costs. Typically, optimal postures minimize the local stresses by reducing muscular tension, joint compression,  or segmental displacement. 
Table 4. Biomechanical properties of tissue that are relevant to modeling the spinal lesion
Balance point: A small region within the neutral zone where, when a compressive load is applied, there is no movement. This location is uniquely different for each segment but generally near, but posterior to, the geometric center. It may be more right or left of the midline.
Coupled motion: Complex motion involving components in all three cardinal planes (multi-planar).
Creep deformity: A viscoelastic property of tissue that results in a time-dependent increase in strain when under constant load.
Dynamic equilibrium: A condition where forces or moments acting on a structure invoke change in position or velocity that is mathematically offset by the inertial load created by its movement.
Elastic zone: A subcomponent within the range of motion, generally away from its center, where there is a predictable relationship between the amounts of displacement that occur with a given applied load.
Functional spinal unit (FSU): A mechanical linkage between two adjacent vertebrae and the intervening disc, connecting ligaments, and muscles.
Functional spidal region (FSR): A complex mechanical linkage system of multiple functional spinal unit (FSU)s. The boundaries of an FSR vary with the task being performed and the identity of attaching muscles recruited.
Hysteresis: The difference between rate of deformation of a tissue and its recovery in an elastic deformation. The amount of hysteresis reflects the amount of energy absorbed by the tissue.
Inertial load: The effective force or moment generated by the effect of body segment mass in motion.
Motion segment buckling: A local, uncontrolled mechanical response to spine load environment that manifests clinically as a set of spine-related symptoms.
Neutral zone: A region of potential resting positions for the functional spinal unit (FSU). It is a subcomponent within the range of motion, generally near its center, in which very small applied loads result in relatively large displacement.
Normal segmental motion: The boundaries of motion by a healthy functional spinal unit (FSU) are often noted as the normal range of motion. However, the behavior of the functional spinal unit (FSU) within its anatomic limits may vary based on the task, initial starting position, and order of recruitment of attached muscle.
Range of motion: The sum of the neutral zone and elastic zone, on both sides of the center.
Static equilibrium: The balance of forces and moments acting on a structure under static conditions that results in no change in its position, or, if moving, its velocity.
Stress concentration: Localized increase in tissue stress resulting from change in the distribution of load to the tissue, dimension of the contact area between structures, or dimension of the load-bearing structure (eg, from creep deformity).
Recent, Relevant Biomechanical Research
Intersegmental motion is mechanically complex. The path of motion taken by an individual vertebra is determined by the
geometry of the bone-disc-bone junction. Generally, there are common geometric features within functional spinal regions
(FSRs) that guide coupling of multi-planar motions (Table 5). Geometry is not the only guiding factor, however. Movement may be modified, within limits, by muscle recruitment patterns,  the initial posture as a task begins,  tissue condition,  and so forth. That is, the coupled motions change ratio according to spinal region (eg, C2 rotation versus lateral bend at 3:2, decreasing to 1:7.5 at C7)  and whether the functional spinal unit (FSU) begins its motion in flexion or extension. 
Table 5. Intrinsic limiting factors that influence mechanical functioning of vertebra
FSU geometry Vertebral dimensions
Soft tissue dimensions
Disc fiber orientation and thickness
Ligament lengths and thickness
Muscle lines of action and strengths
Tissue properties Stiffnesses
Tasks Moment loads
FSU = Functional spinal unit
Pelvic and sacroiliac motion
Pelvic motion exhibits even greater complexity. [60, 61] Body weight and the working loads from activity are transmitted downward and are balanced by reaction forces and moments from the lower extremity. Motion within the sacroiliac (SI) joints dampens descending loads  much the same as knee
flexion suppresses shock to the spine from heel strike during gait. [62, 63] The direction of motions within the pelvic components, however, is not as straightforward as previously believed. Generally, behavior of the innominate has been held to correspond with ipsilateral movement of the pelvis and the lower extremity as a whole. [60, 64] Information that is more recent shows that this pattern is correct for only a portion of the population. Smidt et al  found considerable symmetry of
pelvic movement during gait. The direction of SI motion, on the other hand, often varies from the expected pattern, falling
into one of three subgroups. In one subgroup, the SI joints move in one direction only, regardless of the stride being
taken. In a second group, the ilium moves in concert with the leading leg, while the remainder shifts that motion to the
contralateral SI joint.
Ligament stretches from coupled motions
Weight bearing and movement result in the loading of the spinal components. An intricate coordination apportions load
sharing between both active and passive tissues. Distribution depends on the task, posture, and muscular fatigue.
Flexion-relaxation phenomenon during trunk flexion is an example. [65, 66]
As the lordosis of the lumbar spine straightens, the muscle activity controlling the movement ceases. The spine then
"hangs" on its ligaments. Complex motions can result in high stresses to the spinal tissues, but only a few studies have
focused on the load-sharing capacity.  Specific ligaments may be primarily responsible for controlling select movements when muscular action is not effective  Pure lumbar flexion produces stretching of the posterior ligaments. Twisting motions principally engage the capsular ligaments. Combined movement stretches the facet capsules and intertransverse and capsular ligaments, with relative sparing of the supraspinous and interspinous structures.
Mechanical failure can be defined as an excessive deformation or loss of continuity (fracture or tearing) of one or more of the tissues.  Degenerative spondylolisthesis might be considered a model of failure by deformation, whereas spondylolitic spondylolisthesis could serve as an example of failure by discontinuity. Under conditions of healthy function, mechanical equilibrium serves to transmit load without physical failure.
The balance of forces acting on a joint (static or dynamic equilibrium) must be maintained both during static postures
and when there is movement.  In the static form, there is little to no motion and the inertial loads from the body segment masses are negligible. For normal activities of daily living, the inertial effects can be significant, and dynamic equilibrium becomes meaningful.
All of this may seem academic. However, it becomes clinically relevant during the normal day, for example, when
a worker becomes fatigued. As trunk muscles tire, there is a redistribution of load to auxiliary muscles and ligaments  along with kinematic changes in spine behavior during lifting tasks. The motions in the primary direction decrease up to 10% while functional spidal region (FSR) coupled motions increase by as much as 50%. With repeated lifting tasks, similar changes are seen in the lower extremities when postural stability is lost.  The use of suboptimal strategies increases the risks for injury. 
Equilibrium of the functional spinal unit (FSU) is made even more interesting by the different behaviors of subregions within the normal intersegmental range of motion. The load-displacement curve (Figure 2), derived by applying measured loads while simultaneously recording motions, can illustrate the roles of particular spinal tissues. Load-displacement tests show that the range of motion is divided into a neutral zone and an elastic zone (see Table 4 for definitions). The ratio may vary from person to person; for example, at C2 the neutral zone may vary by as much as 21°. 
Work like that of Kumaresan et al,  Pintar et al,  Yoganandan and Pintar,  Luttges et al,  and Schultz et al [65, 83, 84] represent examples in which the properties and limiting factors of functional spinal unit (FSU)s and functional spidal region (FSR)s have been tested (Table 5).
Stresses induced by lateral and posterior shear, axial compression, and flexion are absorbed primarily by the disc. The facets play a major role for anterior shear and axial torques with increasing contact forces. [85, 86]
Spinal pain production
The interaction of biomechanics, biochemistry, and neurophysiology may explain complexities encountered in the management of spinal pain. Nociception related to the functional spinal lesion (FSL) is initiated by the mechanical event of
buckling. The pain may be immediate from acute tissue insult or delayed from accumulated trauma or degeneration effects. Typically, treatments directed purely at moderating structural aspects of a patient's problem are frustrating. [11, 12] Local compressive, tensile, or shearing forces are responsible for acute injury. Tissue
failure precipitates a cascade of biochemical and physiologic events and drives chronicity of the case. Secondary effects,
resulting from the cascading biological responses, may become primary pain generators requiring additional modes of treatment. Persistent nociceptive pain may result in increased sensitivity of peripheral nerve endings (peripheral sensitization) and spinal cord neurons (central sensitization). Normal mechanical stimulus and movement then become painful. [87, 88] If the internal biochemical milieu of the disc degrades, the neurotoxicity of breakdown products (eg, phospholipase A2) develops a chemically mediated inflammation or irritation of neural elements. Under these sets of conditions, the patient may benefit most from a multidisciplinary treatment strategy. Chiropractic manipulation/adjustment is used to correct the mechanical element while analgesic/anti-inflammatory medication (oral or local joint/nerve injection) is given to resolve the biochemical component.
Effects of mobility and immobilization
Movement maintains articular and periarticular tissue properties. The viscoelastic properties and ultimate strengths of the
various tissue components are maintained by periodic deformation under different loads and speeds. Sustained or excessive
repetitive load gradually invokes creep deformity, increasing tissue hysteresis and altering its ability to elastically rebound.  A 20minute static bending task exhausts much of the elastic
properties of the spine. After a 2minute rest, it recovers only half of its normal stiffness. After 30 minutes, measurable joint laxity can be demonstrated. Prolonged static postures, even at low intensities, eventually become uncomfortable because of creep with local tissue stress concentration.  In practical terms, this means that heavy exertions should be avoided after prolonged static postures, particularly at the extremes of position. As mentioned earlier, muscle fatigue aggravates the risk by altering recruitment patterns and increasing coupled motions of the functional spidal region (FSR).
Under rapid loads, there is insufficient time for viscoelastic response. Tissue stress values are higher, and failure may
occur at lower amplitudes than with sustained forces.
Repeated short periods of immobilization are as harmful as prolonged immobilization lasting up to 3 months.  A cascade of structural changes (Table 6) begins when the elastic potential of the tissues is not exercised. Without movement, physical shortening of the collagen fibers leads to periarticular fibrosis. Capsular tension increases, and the joint cartilage experiences a chronic increase in compressive stress. Fibrillation and atrophy of the articular cartilage may begin as early as 4 weeks. Videman  found similar cumulative effects from repeated short periods of immobilization in as little as 36 days.
Cascade of tissue changes that may result from relative immobilization of an articulation
Early changes Capsular contracture
Later changes Muscular atrophy
Fibrofatty joint infiltration
Degenerative changes Cartilage changes
Symptoms Joint stiffness
Effects of the neutral zone, hysteresis, and degeneration
The concept of a "normal" rest position for an functional spinal unit (FSU), as commonly represented for static and motion palpation procedures, is technically erroneous. Because of the neutral zone component of range of motion, a vertebra cannot be expected to reliably return to a fixed starting position in relation to its adjacent member. Differences in static resting positions are
likely to occur in normal, healthy subjects. Fortunately, the variation for much of the spine is small. Specific ratios vary
for each segment. In general, the neutral zone in the cervical spine is proportionately larger than in the lumbar spine.
Hysteresis compounds the problem. Prolonged static posture, or repeated activity, results in a temporary shift in the stiffness properties that influence the ability of the vertebra to elastically return to a prescribed initial position. It is no wonder that reliability of static and motion palpation techniques has been so difficult to achieve. Schram  demonstrated this problem as early as 1982 when he attempted to measure pre- and post-treatment changes of location in the upper cervical spine vertebrae. The posttreatment location proved to be unpredictable and random.
All structural properties may be affected by the processes of aging and degenerative change, and they influence the potential to withstand stress and to repair. Yoganandan et al  demonstrated that degenerative disease reduces the capacity for structural loading by one half and the energy absorption by the intervertebral joint to one third. Recovery time frames may depart substantially from experimental evidence from acute injury, confounding long-term prognosis. Injury severity is not clinically quantifiable. Apparently serious injury may recover rapidly with no residual. Minor damage superimposed on degenerative disease may evoke a more severe early symptom picture that then recovers more quickly than expected. It also may evoke persistent symptoms because of the preexisting degenerative changes that prolong recovery. Making matters more complex clinically, psychosocial factors that different individuals display may lead to increased suffering and illness behaviors. 
Aging, like injury, is not a uniform process.  Age-related changes are an interplay of environmental and genetic factors. Aging reduces tissue compliance, strength, and physiologic endurance. [95, 97] Effects accelerate after age 76. Reduction in bone strength with age is well known. Bone density and strength decline in proportion to lean body mass. [98, 99] Static and
dynamic muscular action decrease by 5% after age 45, Muscle endurance falls at a rate of 1% per year. Most muscle capacity
changes result from the sedentary lifestyles that seem to go along with advancing age. 
Aging has serious effects on joint and disc cartilage, proteoglycan concentrations decrease, link proteins fragment, and the size of chondroitin sulfate chains reduce in articular cartilage. The osmotic pressure that retains water content within the matrix drops.  leading to roughening of the joint surface. Similarly, the intervertebral disc undergoes desiccation from loss of the mucopolysaccharides and osmotic pressure, Loss of disc height may or may not occur.  Clinically, changes appear as spondylosis, osteophyte formation, ligamentous and articular capsular hypertrophy, and osteoarthrosis. 
The nervous system also ages. Decreases occur in sensory sensitivity, reflex activity, and innervation of muscle
spindles.  Collectively, these changes affect balance and posture, and how loads are distributed across a joint. [107, 108]
A Unified Theory of Spinal Lesions
The earlier review briefly described the patchwork of alternative hypotheses used to explain the rich set of observations from different patients that respond to manipulation methods. Empirically, structurally undefined abnormalities
appear to respond to manipulation/adjustment. Often described by many terms, they might best be described as FSLs
that alter the biomechanical behavior of the functional spinal unit (FSU). The FSL, arguably, may provide more general understanding of how
such a wide range of signs and symptoms can be explained as a function of the circumstances of injury, [24, 55, 109] The simplest model that fits the various clinical observations is one of
mechanical buckling within the functional spinal unit (FSU) or functional spidal region (FSR).
In simplest form, buckling behavior is a failure by unacceptable deformation for the operating conditions of the joint
and the task being performed. It represents a local, uncontrolled mechanical response to a spinal loading environment
that manifests clinically as a set of symptoms (Tables 3 and 4). Buckling may be characterized as a displacement that is disproportionate to the load imposed by the task. That is, when buckling occurs, the affected area of the spine reaches its maximum range under lower load conditions. It is operating at its extreme, out of phase with the demands of the task. A
functional configuration operating at extremes results, by definition, in altered stress distribution within the functional spinal unit (FSU). But,
does theoretical buckling occur in the real world? Is there independent evidence?
The concept of the FSL is similar to that of compressive buckling injury to the wrist.  The functional spinal unit (FSU) meets the biomechanical criteria for a multimuscular, biarticular chain that
may be subject to "zigzag" collapse [112, 113] or buckling. Buckling in the spine is opposed only by appropriate muscle recruitment for a biomechanical task. The FSL begins with a mechanical overload. It may be either a single traumatic event or aggregate with multiple repetitions. Experiments [63, 114116] have produced these effects in isolated spine segments under conditions where a single functional spinal unit (FSU) is incrementally loaded at the balance point. When a critical load is reached, the force-displacement behavior is interrupted by a disproportionately large displacement. The total distance, however, remains within the normal functional range.
Buckling in isolated functional spidal region (FSR)s occurs with critical loads as low as 20 N [55, 117] to 90 N.  Well-coordinated and timely muscle activation empowers activities of daily living. The intact spine may withstand loads as high as 18,000 N.  Ill-timed or
insufficient muscle recruitment leaves the spine susceptible to sudden, local, disproportionate displacement and strain. Wilder and colleagues,  studying constrained mechanical behavior, first described buckling of isolated functional spinal unit (FSU)s. They demonstrated that functional spinal unit (FSU) buckling is sensitive to the loading site, direction, amplitude, and rate.  Figure 3 illustrates buckling behavior of isolated motion segments during pure flexion and lateral bending tests with the loads applied at the balance point. The displacement that occurs in an functional spinal unit (FSU) under a given load is dependent on the inherent constitutive properties of the tissues as well as the stiffening action of the local muscles.
Essentially, each added increment in load results in a proportionate displacement. Below the buckling value, the displacement per increment is a very small proportion of the total available physiologic range. At the critical buckling load, the
addition of another increment meets with a sudden, large deformation that approaches its maximum normal range of
motion. Removal of the load does not result in elastic recovery to the original equilibrium configuration. Instead, a new
equilibrium, near the extreme position is achieved. Buckling has been observed both in main and coupled motions.  Cholewicki and McGill  captured a buckling event confined to a single functional spinal unit (FSU) in vivo during heavy exertion.
Similar responses have been documented for entire functional spidal region (FSR)s as well as segmental functional spinal unit (FSU)s.  The magnitude of loads necessary to reach the limit of displacement for each functional spinal unit (FSU) is predictably much lower than characteristic of functional spidal region (FSR)s. Buckling response is sensitive to degeneration, which seems to allow it to occur earlier and reach maximum displacement under lower loads. Exposure to vibration, a known risk for the spine, also enhances buckling.
Summary and Conclusion
The language of the spine is biomechanics. Both normal and abnormal functions express themselves in these terms. Recasting theoretical considerations of the subluxation/FSL likewise carries significant advantages. First, it helps to demystify subluxation as a heuristic concept that has neither been seen nor measured. Second, it helps reveal methods by which it may be defined scientifically and monitored clinically. Third, it empowers credible communication, grounded on tangible evidence that can help influence public policy makers and the remaining scientific skeptics. Fourth, it helps lay a foundation to stronger scientific investigation of clinical effects. Finally, a biomechanical understanding of the FSL permits the development of new strategies to refme its prevention, treatment, and resolution.
An advantage of using motion-segment buckling over current hypotheses as a basis for modeling rests on the fact that
this model does not preconceive or proscribe any source of symptoms but is able to accommodate the multifaceted clinical presentations of patients who respond favorably to manipulation/adjustment. It also can sustain a variety of both
hypothetical and evidence-based challenges. For example, injuries (neurogenic and non-neurogenic pain mechanisms)
with potentially local or remote symptoms are dependent entirely on which tissues exceed injury thresholds. Like the
buckling itself, the painful tissue response will be dependent on prior health and the nature, direction, and severity of the
stresses distributed to it.
Thus, the clinical appearances can be many. They may include acute facet capsulitis from a sudden high compressive
or shearing load, ligamentous damage, or discal damage. Discogenic tearing, with neurogenic or non-neurogenic inflammation of the nerve roots or the terminal nerve fibers of the anular material, may result in either dennatomal or sclerotomal radiating pain. Reflex spasm and altered motor control of both the proximal and distal muscle groups may begin. Periarticular tissues also may receive direct damage as with muscular strain/sprain or contact injury to the back or neck. The buckling model builds on clinical observations, supplementing them with both direct and indirect biomechanical evidence. These findings offer an opportunity to reconceptualize and refine theoretical models of the spinal lesion into a platform for scientific, clinical, and political advancement of the profession.
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