Clinicians use palpatory assessment of individual intervertebral segments prior to the application of a thrust technique. The osteopathic profession has used Fryette's model of the physiological movements of the spine to assist in the diagnosis of somatic dysfunction and the application of treatment techniques. Fryette' outlined his research into the physiological movements of the vertebral column in 1918. He presented a model that indicated that coupled motion occurred in the spine and displayed different coupling characteristics dependent upon spinal segmental level and posture. The 'muscle energy' approach is one system of segmental spinal lesion diagnosis and treatment predicated upon Fryette's laws.2 Practitioners utilizing the muscle energy technique (MET) use these laws of coupled motion, as a predictive model, both to formulate a mechanical diagnosis and to select the precisely controlled position required in the application of both muscle energy and thrust techniques. Current literature challenges the validity of Fryette's laws.
Convention dictates that intervertebral motion is described in relation to motion of the superior vertebra upon the inferior vertebra. Motion is further defined in relation to the anterior surface of the vertebral body; an example of this is the direction of vertebral rotation that is described in relation to the direction in which the anterior surface of the vertebra moves rather than the posterior elements.
In the clinical setting, vertebral motion is described using standard anatomical cardinal planes and axes of the body. Spinal motion can be described as rotation around, and translation along, an axis as the vertebral body moves along one of the cardinal planes. By convention, the vertical axis is labelled the y-axis, the horizontal axis is labelled the x-axis, and the anteroposterior axis is the z-axis (Fig. A.3.1).
In biomechanical terms, flexion is anterior (sagittal) rotation of the superior vertebra around the x-axis while there is accompanying forward (sagittal) translation of the vertebral body along the z-axis. In extension, the opposite occurs and the superior vertebra rotates posteriorly around the x-axis and translates posteriorly along the z-axis. In sidebending there is bone rotation around the anteroposte-rior z-axis, but sidebending is rarely a pure movement and is generally accompanied by vertebral rotation. The combination, and association, of one movement with others is termed coupled motion. The concept of coupled motion is not recent. As early as 1905, Lovett3 published his observations of coupled motion of the spine.
Coupled motion is described by White and Panjabi4 as a 'phenomenon of consistent association of one motion (translation or rotation) about an axis with another motion about a second axis'. Bogduk5 describes coupled movements as 'movements that occur in an unintended or unexpected direction during the execution of a desired motion'. Stokes et al6 simply state coupling to be when 'a primary (or intentional) movement results in a joint also moving in other directions'. Where rotation occurs in a consistent manner as an accompaniment to sidebending, it has been termed conjunct rotation.7-* Therefore, in rotation, the vertebra should rotate around the vertical y-axis, but translation will be complex dependent upon the extent and direction of coupling movements. Coupling will cause shifting axes of motion.
Greenman9 maintains that rotation of the spinal column is always coupled with side-bending, with the exception of the atlantoaxial joint. The coupled rotation can be in the same direction as sidebending (e.g. sidebending right, rotation right) or in opposite directions (e.g. sidebending right, rotation left). The osteopathic profession developed the convention of naming the coupled movements as type 1 and type 2 movements (see Figs A.3.2 and A.3.3).
These concepts of vertebral motion are attributed to Fryette. Fryette acknowledges the contribution made to his understanding of spinal movement by Lovett, who had undertaken research on cadavers in order to understand the structure and aetiology of scoliotic curves.
Fryette acknowledged that Lovett's findings for the thoracic and lumbar spine were correct for the position in which Lovett had placed the spine for his cadaveric experiments but maintained that they would not be true if the lumbar and thoracic spine were placed in different positions of flexion or extension. Fryette performed his own experiments upon a 'spine mounted in soft rubber' and introduced the concept of neutral (facets not engaged) and non-neutral (facets engaged and controlling vertebral motion) positioning. Fryette defined neutral to mean 'the position of any area of the spine in which the facets are
idling, in the position between the beginning of flexion and the beginning of extension'. In the cervical spine below C2, the facets are considered to always be in a non-neutral position and are therefore assumed to control vertebral motion. The thoracic and lumbar regions have the possibility of neutral and non-neutral positioning. Mitchell2 summarizes Fryette's laws as follows:
• Law 1. Neutral sidebending produces rotation to the other side, or in other words, the sidebending group rotates itself toward the convexity of the sidebend, with maximum rotation at the apex.
• Law 2. Non-neutral (vertebra hyperflexed or hyperextended) rotation and sidebending go to the same side, individual joints acting at one time.
• Law 3. Introducing motion to a vertebral joint in one plane automatically reduces its mobility in the other two planes.
Research into coupled movement has been undertaken on cadavers and live subjects. Cadaver research has allowed precise measurements to be taken of coupling behaviour but has the disadvantage of being unable to reflect the activity of muscles or the accurate effects of load on different postures. Plane radiography has been superseded by the more accurate biplanar radiographic studies that allow research to be undertaken imder more normal physiological conditions. Most research has been performed on the lumbar spine.
Brown"111 indicates, after an extensive literature review, the conflicting results of studies into coupled motion. Many authors have demonstrated a coupling relationship between sideflexion and rotation,8-^-19 but there is inconsistent reporting of the direction of coupling. Other authors maintain that sidebending and rotation are purely uniplanar motion occurring independently of each other.2021
Stoddard'2 demonstrated radiologically that sidebending in the cervical spine is always accompanied by rotation to the same side regardless of cervical posture. Stoddard's observations in relation to the cervical spine are consistent with Lovett's findings and Fryette's laws. These findings are further supported by research undertaken using biplanar X-ray analysis.17 In 20 normal male volunteers, when the head was rotated, lateral bending occurred by coupling in the same direction at each segment below the C3 vertebra. Interestingly, coupling was not restricted to lateral bending. At the same time, flexion took place by coupling at each segment below the C5-6 vertebrae and extension above the C4-5 level.
While there is agreement as to the direction of axial rotation and lateral flexion coupling in the cervical spine, i.e. sidebending and rotation occurring to the same side, the patterns for coupling in the lumbar spine are less clear. Stoddard's findings in the lumbar spine were that 'sidebending is accompanied by rotation to the opposite side if the commencing position is an erect one of extension. If, however, the starting position is full flexion, sidebending is then accompanied by rotation to the same side'.12 Other authors do not support these findings and report inconsistent coupling.ai4'I6J8
Plamondon et al,16 using a stereoradiography method to study lumbar intervertebral motion in vivo, demonstrated that axial rotation and lateral bending were coupled motions but reported there was 'no strict pattern that the vertebrae follow in executing a movement'.
Pearcy and Tibrewal,'4 in a three-dimensional radiographic sfridy of normal volunteers, with no history of back pain requiring time off work or medical treatment, found that the relationship between axial rotation and lateral bending is not consistent at different levels of the lumbar spine. Some individuals occasionally demonstrate 'movements in the opposite direction to the voluntary movement at individual intervertebral levels, most commonly at L4-5 and L5-S1. In lateral bending there was a general tendency for L5-S1 to bend in the opposite direction to the voluntary movement'. This unexpected finding is consistent with a study by Weitz.22
Panjabi et al, *8 using fresh human cadaveric lumbar spines from Ll-sacrum, assessed coupled motion under load in different spinal postures using stereophotogrammetry. They concluded that coupling is an inherent property of the lumbar spine as advocated by Lovett, but that in vitro coupling patterns are more complex than generally believed. They demonstrated that the presence of muscles is not a requirement for coupled motion but acknowledged that they may significantly alter coupling behaviour. The specific effect of physiological loading and muscle activity upon coupled motion is presently unknown. In a neutral posture, left axial torque produced right lateral bending at the upper lumbar levels and left lateral bending at the lower two levels with L3-4 being a transition level. They concluded that the 'rotary coupling patterns in the lumbar spine are a function of the intervertebral level and posture'. At the upper lumbar levels, axial torque produced lateral bending to the opposite side, whereas at the lower lumbar levels, axial torque produced lateral bending to the same side. It was also noted that 'the spine does not exhibit mechanical reciprocity'; for example, at L4-5, applied left axial torque produced coupled left lateral bending, but applied left lateral bending produced coupled right axial rotation.
Panjabi et al's's finding that 'at L2-3, coupled lateral bending increased from about 0.5° in the fully extended posture to 1.5° in neutral and to about 2° in flexed postures' conflicts with Fryette's third law which indicates that introduction of motion to a vertebral joint in one plane automatically reduces its mobility in the other two planes. In lumbar flexion, the coupled lateral bending increased by OS from the neutral to the flexed position.
A number of studies have indicated that coupled movement occurs independently of muscular activity.8-']'23 In 1977, Pope et al13 utilized a biplanar radiographic technique to evaluate spinal movements in intact cadaveric and living human subjects. They confirmed that 'vertebral motion occurs as a coupling motion, and that axial rotation uniformly is associated with lateral bend'. Frymoyer et al23 measured spinal mobility using orthogonal radiography on 20 male cadavers and nine male living subjects. They found that complex coupling does occur in the lumbar spine and demonstrated remarkably similar spinal behaviour between the two groups. These studies indicate that coupling occurs independently of muscular activity.
Vicenzino and Twomey8 used four human male postmortem lumbar spines from L1 to the sacrum, with ligaments intact and muscles removed, to assess conjunct rotation of the spine when sidebending was introduced in both a flexed and extended position. They found that in the flexed position, lateral flexion of the lumbar spine was associated with conjunct rotation to the same side. This is consistent with Fryette's laws. However in the extended position, lateral flexion was associated with conjunct rotation to the opposite side, which supports Stoddard's12 radiographic observations of coupled motion in the extended position. These findings are not consistent with Fryette's laws, which predict sidebending and rotation to the same side as the facets are not 'idling' when in the extended position, Vicenzino and Twomey's8 study reveals that the L5-S1 segment is unique in that conjunct rotation was always in the same direction as sideflexion independent of flexion or extension positioning, This finding for the L5-S1 segment was supported by Pearcy and Tibrewal14 who found that during axial rotation at L5-S1, lateral bending always occurred in the same direction as the axial rotation.
Vicenzino and Twomey8 draw the conclusion that as both in vitro and in vivo studies have demonstrated conjunct rotation, the non-contractile components of the lumbar spine may have primary responsibility for the direction of conjunct rotation and that neuromuscu-lar activity may only modify the coupling, The impact of muscular activity on coupled motion in both the normal and the dysfunctional intervertebral joint requires further study.
The presence of apophysial joint tropism might influence spinal motion and confound predictive models of vertebral coupling. The incidence of facet tropism has been reported as
20% at all lumbar levels but may increase to 30% at the L5-S1 segment.5 The incidence of facet tropism is also higher in patient populations attending manual medicine practitioners. It has been estimated that as many as 90% of patients presenting with low back pain and sciatica have articular tropism with pain occurring on the side of the more obliquely oriented facet.5 Cyron and Hutton24 subjected 23 cadaveric lumbar intervertebral joints to a combination of compressive and shear forces. When asymmetric facets were present, the vertebrae that have such facets rotated towards the side of the more oblique facet. They concluded that articular tropism could lead to lumbar instability manifesting itself as joint rotation towards the side of the more oblique facet. This was not a study of coupled motion and no clear comments can therefore be made about the influence of facet tropism on patterns of coupling, but it does suggest that tropism can influence spinal mechanics.
Disc degeneration and spinal pathology presenting with pam and nerve root signs might also influence spinal coupling. In 1985, Pearcy et ar5 undertook a three-dimensional radiographic analysis of lumbar spinal movements. They studied patients with back pain alone and patients with back pam plus nerve tension signs demonstrated by restricted straight leg raise. Coupled movements were increased only in those patients without nerve tension signs, indicating the possibility of asymmetrical muscle action. It was concluded that 'the disturbance from the normal pattern of coupled movements in the group with back pain alone suggests that the ligaments or muscles were mvolved unilaterally, and thus acted asymmetrically when the patient moved'. The fact that coupled movements were increased in the back pain group suggests that muscular activity, while not bemg essential for couplmg, can influence the magnitude of coupled movement. The action of the contractile elements in normal, dysfunctional and pain states requires more study before any definite statements can be made relating to their effect upon coupled motion.
It is evident that many factors, such as facet tropism, vertebral level, intervertebral disc height, back pain and spinal position, might influence the degree and direction of coupling.
While it appears that Fryette's laws are open to question, there are still only two possibilities for the coupling of sidebending and rotation, i.e. to the same or the opposite side. With this in mind, it appears reasonable to classify spinal movement as type 1 and type 2 in relation to coupled sidebending and rotation. What is not clearly established is the influence of flexion and extension in relation to type 1 and type 2 movements.
Conclusions that can be drawn from the literature are limited for a number of reasons. Cadaver studies exclude the effects of muscular activity and normal physiological loading; the studies were also often single segment analysis and generally of small sample size. Plane radiographic studies have inherent measuring difficulties associated with extrapolating three-dimensional movements from two-dimensional films. The use of biplanar radiographic assessment improved the accuracy of measurement and allowed studies to be performed with muscular activity and in more normal physiological conditions; again. however, the groups studied were small. Notwithstanding these observations, there are a number of conclusions that can be drawn:
• Coupled motion occurs in all regions of the spine.
• Coupled motion occurs independently of muscular activity, but muscular activity might influence coupled movement.
• Coupling of sidebending and rotation in the lumbar spine is variable in degree and direction.
• There are many variables that can influence the degree and direction of coupled movement, including pain, vertebral level, posture and facet tropism.
• There does not appear to be any simple and consistent relationship between conjunct rotation and intervertebral motion segment level in the lumbar spine.
There is evidence to support Lovett's initial observations and Fryette's laws in relation to sidebending and rotation coupling in the cervical spine, i.e. sidebending and rotation occur to the same side.1217 However, the evidence in relation to lumbar spine coupling is inconsistent.8'14'1618
While Fryette's laws may be useful for predicting coupling behaviour in the cervical spine, caution should be exercised for the thoracic and lumbar spine, where modification of the model may be necessary.
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