Involved-side cervical rotation range of motion less than 60 degrees,. 3. . Hearn , A., Rivett, DA. (). Cervical Snags: a biomechanical analysis. Manual. This paper discusses the likely biomechanical effects of both the accessory and physiological movement components of a unilateral cervical SNAG applied. 1 Manual Therapy () 7(2), doi: /math, available online at on Review article Cervical SNAGs: a biomechanical analysis A. Hearn,* D. A. Rivett w *SportsMed, .
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A sustained natural apophyseal glide SNAG is a mobilization technique commonly used in the treatment ofpainful movement restrictions ofthe cervical spine. In the manual therapy literature, the biological basis and empirical efficacy ofcervical SNAGs have received scant attention. In particular, an examination oftheir potential biological basis in order to stimulate informed discussion giomechanical overdue.
This paper discusses the likely biomechanical effects ofboth the accessory and physiological movement components ofa unilateral cervical SNAG applied ipsilateral to biomechxnical side ofpain when treating painfully restricted cervical rotation.
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Although a cervical SNAG may clinically be able to resolve painfully restricted cervical spine movement, ceevical is difficult to explain biomechanically why a technique which first distracts opens and then compresses closes the zygapophyseal bimoechanical ipsilateral to the side ofpain, and perhaps slightly distracts the uncovertebral cleft, would be superior to a technique which distracts the articular surfaces with both accessory and physiological movement components.
Therefore, the reported clinical efficacy of cervical SNAGs snavs be explained purely on the basis ofthe resultant biomechanical effects in the cervical spine. Although a wide range of biological explanations have been proposed for manual therapy Patersonit still suffers from a lack of empirically validated treatment procedures Hurwitz et al. School of Physiotherapy, University of Otago, P.
Box 56, Dunedin, New Zealand. A sustained natural apophyseal glide SNAG of the cervical spine, first introduced by Mulligan inis one such procedure. Cervical SNAGs were the first example of a group of techniques known as mobilizations with movement MWM which Mulligan developed to restore painfree unrestricted movement for most joints in the body Mulligan Mulligana proposed that the reputed clinical effectiveness of cervical SNAGs may be biomechanical in nature.
The literature on cervical SNAGs is limited, being almost exclusively descriptive and based on clinical experience Mulligana, ; Exelby ; Rivett et al. Cervical SNAGs have therefore been chosen as the focus of this review with the expectation that an enhanced understanding of their biomechanical effects may stimulate informed discussion. An initial overview of the application of cervical SNAGs will be followed by an evaluation of the likely biomechanical effects of the technique on articular tissues.
With one thumb reinforced by the other placed on the articular pillar of the upper vertebra of the implicated functional-spinal unit FSUthe therapist applies a sustained passive accessory intervertebral movement PAIVM superoanteriorly along the facet plane. This glide is maintained as the patient moves actively through the desired range of physiological movement and then whilst sustaining the end-range position for a few seconds.
Over-pressure can also be added by the patient to the physiological movement. The glide is released by the therapist after the patient returns to the starting position for the active movement. The cardinal rule governing this procedure is that all movement, both accessory and physiological, must be painless Mulligan a, The point of application for the glide can be either unilateral on the articular pillar or central on the spinous process. Unilateral application of the accessory movement is recommended by Mulliganas he suggests spinal lesions are generally unilateral.
The subsequent active physiological movement is nearly always in the direction of a painful movement loss Mulligan a. Thus, a patient with painfully restricted right rotation would initially be treated with a unilateral SNAG on the ipsilateral right articular pillar Mulligan Fig.
If this did not immediately improve the patient s active range, a SNAG would next be applied to the superior facet of the zygapophyseal joint on the left, or failing that, centrally i.
The accessory glide is always applied in a Manual Therapy 7 2superoanterior direction parallel to the facet plane, irrespective of whether the patient s dysfunction predominantly involves flexion, extension, rotation or lateral flexion. Specifically, cervical SNAGs are said to cause a repositioning of the articular facet allowing normal pain-free function Mulligan a and as such are thought to primarily mobilize the zygapophyseal joint, while still obviously influencing the entire FSU, including the intervertebral disc IVD Mulligan a.
This approach is extrapolated from Kaltenborn s theory that decreased joint gliding of the peripheral joints can contribute significantly to joint hypomobility and therefore to impaired joint function Mulligan Given the clinical popularity and unsubstantiated efficacy of cervical SNAGs, discussion of their biological basis seems warranted.
The following review will primarily focus on the relevant biomechanics of the articulations of the FSU the zygapophyseal and interbody joints as they relate to the potential mechanism s of action of cervical SNAGs. It is acknowledged that other spinal structures, such as certain neural tissues or surrounding musculature, may play a role in the mechanism of action of cervical SNAGs, but are not considered for the purposes of this review.
Implicit in this description is the assumption that the therapist can produce movement of one joint surface relative to the other. Thompson simulated a PAIVM by applying a N posteroanterior PA force to the L3 spinous process and demonstrated that the caudal joint L3 4 exhibited more relative displacement 3 5 mm than the cephalad L2 3 joint 1 3 mm. Lee and Evansusing a biomechanical model, also predicted relative intervertebral movements when a PA force of N was applied to the spinous process of L4.
In their analysis, they assumed that there were no significant horizontal compressive forces through the spine and only loadings in the sagittal plane were considered. However, it is possible that the unilateral application of a cervical SNAG will produce axial y-axislateral z-axis and sagittal x-axis rotations, as demonstrated during unilateral thoracic manipulation Gal et al. There are several findings from these studies that are relevent to the present discussion.
Therefore, cervical SNAGs could theoretically produce greater accessory gliding movement than a similar, faster procedure such as a manipulation high-velocity thrust technique.
Cervical SNAGs: a biomechanical analysis
Although facet angles differ considerably between the lumbar and cervical spines, this finding may be relevant to the cervical spine as the superior facet of the FSU sits posteriorly in relation to its inferior partner as it does in the lumbar spine and therefore the caudal joint of a vertebra being mobilized may experience a larger anterior shear force, in accordance with the results of Thompson In particular, the considerable zygapophyseal joint capsular laxity demonstrated by Onan would mitigate against significant creep of the implicated capsule.
If the cervical spine responds in a manner similar to the superoanteriorly directed glide during a cervical SNAG, a similarly complex spinal motion pattern may occur, potentially increasing the cervical lordosis and therefore weightbearing through the posterior columns.
These effects will now be considered. This compressive force is likely to increase stiffness or resistance Maitland ; Jull et al. Further compounding the effect of gravitational spinal loading is the compressive effect of muscle function Lee et al.
Following the application of an accessory glide, the patient will attempt to maintain the head in a position of equilibrium, probably by recruiting the cervical extensor musculature and by increasing the cervical lordosis. These factors suggest that compressive forces, whether they are due to muscle spasm, voluntary stabilizing muscular activity, or to gravity in an upright position, are likely to increase stiffness and therefore reduce accessory movement for a given gliding force.
In summary, it is probable that accessory joint motion, however small, may be produced in an unloaded cervical FSU, but the effects of an erect posture are likely to make this more difficult to achieve.
Manual Therapy 7 271 Therefore, an analysis of the active movement component of a cervical SNAG will primarily involve a kinematic assessment of the chosen technique. For the purposes of this discussion a unilateral SNAG, performed ipsilateral to the side of pain and movement restriction for an adult patient with painfully restricted right rotation, will be used as an example Fig. As the patient commences turning to the right, the accessory glide having been applied and maintained, two important events occur.
Muscle function To be maximally effective at a given task, the line of action of a muscle must be tangential with respect to Fig.
Manual Therapy 7 2the centre of motion Penning As the biomechxnical rotates to the right, muscle contractile activity will biomecganical a compressive biomechanicla, particularly through sags right articular pillar. In combination, these forces may oppose or perhaps even reverse the already small, manually applied glide component of the cervical SNAG by drawing the superior facet of the FSU posteroinferiorly along the plane of the articular facet.
While there are no data to suggest the resultant joint displacement between the therapist applied superoanterior glide and the opposing forces of muscular contraction and gravity, a review of midlower cervical spine kinematics will facilitate a discussion of the possible articular effects of the chosen technique.
Coupled motion about an axis The mid-lower cervical spine has been shown to undergo obligatory ipsilateral coupling of rotation and lateral flexion Lysell about an oblique axis of motion Penning ; Milne Lysell demonstrated that between C2 and C7 there is a gradual cephalocaudal decrease in the amount of lateral flexion that is associated with axial rotation, possibly due to a gradual cephalocaudal decrease in the angle of inclination of the facet joints to the frontal plane.
In addition to its slight upward convexity in the sagittal plane, the uncinate processes have given the cervical vertebral body a marked upward concavity in the frontal plane, thus providing a saddle shape that has two axes of motion perpendicular to each other and located on opposite sides of the joint Milne ; Penning Milne computed the parameters of the finite helical axis for composite mid-lower cervical spine motion.
This axis completely describes FSU composite motion as a rotation about, and a translation along, a helical axis with a known position and orientation in space. The position and inclination of the helical axis, which passes obliquely upwards and backwards through the moving vertebral body Milne Fig. The posterior part of the disc, with its uncovertebral cleft flanked by uncinate processes, therefore acts like a socket within which the superior vertebral body of the FSU rolls Penning ; Milne ; Mercer.
This point is of considerable importance when assessing the potential effects of cervical SNAGs on articular structures.
It therefore seems likely that the glide component of a cervical SNAG would create an artificial axis of motion by altering or blocking movement of the ipsilateral zygapophyseal joint. The therapist s thumbs would become a fulcrum for rotatory movement about which the interbody and contralateral zygapophyseal joints would move, therefore emphasizing the ipsilateral location of the axis of composite motion for this movement Milne Fig.
The uncinate processes and the uncovertebral clefts in the IVD may thus act as the joint surfaces for these saddle joints of the mid-lower cervical spine Penning ; Milne ; Bogduk ; Mercer It has therefore been suggested that mid-lower cervical spine coupled motion be viewed from the plane of the facet joint Penningwhich is consistent with Milne s finding that the axis of composite motion is more or less perpendicular to the plane of the facet joint.
It is also consistent with application of the glide component of a cervical SNAG in a superoanterior direction. In the midlower cervical spine, lateral flexion and axial rotation are therefore interpreted as the same movement Penning ; Milne This may somewhat explain the clinical finding with cervical SNAGs that the same superoanterior accessory joint movement is needed, whether the movement dysfunction involves rotation or lateral flexion Mulligan The helical axis of composite motion Milne suggests that for rotatory cervical movement the axis of motion lies close to the ipsilateral zygapophyseal joint.
Cervical SNAGs: a biomechanical analysis.
This implies that when applying an ipsilateral cervical SNAG to treat painfully restricted right rotation, the contralateral left superior articular surface is giomechanical upward and forward in a flexion-like fashion Worthwhile its ipsilateral right equivalent may be limited in its posteroinferior movement by the manually applied superoanterior Fig.
The normal axis of coupled motion can be seen open circle. Cervidal primary mechanical effects of this altered motion are likely to be reduced posteroinferior glide closing down of the ipsilateral superior facet, increased superoanterior glide opening up of the contralateral superior facet, and the znags distraction or unloading of the uncovertebral cleft as a result of the altered facet motion.
However, several assumptions related to the clinical application of cervical SNAGs require stating beforehand. Firstly, given that cervical SNAGs are said to have an immediate effect Mulliganit seems likely that their underlying mechanism is either purely mechanical, reflexogenic Herzog et al. Secondly, Mulligan states that cervical SNAGs are specific in their effects to a single FSU, potentially excluding mechanisms such as the placebo effect Wallthe laying on of hands Zusmanand therapist charisma Hartman Thirdly, it is unlikely that bomechanical pathology would undergo immediate and prolonged improvement following mobilization, thus implicating soft tissue structures such as biomechancal inclusions, the zygapophyseal joint capsule and intervertebral disc as the most likely articular sources for any manually reversible pain.
Either mechanism could be a primary or secondary through tractioning of the zygapophyseal joint capsule source of pain and sags spasm Saboe ; Mercer Meniscoids may also act as a nidus for fibrous tissue proliferation eventually leading to adhesion formation Mercer Potentially, the accessory glide component of a cervical SNAG could ameliorate any of these problems by either separating the facet surfaces and releasing the Manual Therapy 7 2Fig.
What is difficult to explain is why the accessory glide should be performed in a weightbearing or loaded position given the likely associated limitation of accessory motion. Further complicating matters is the effect of ipsilateral active movement, which is biomechancal likely to reduce the accessory glide and cause increased zygapophyseal joint compression.
On the basis of the previous biomechanical analysis it would seem more appropriate to apply the accessory glide ipsilateral to the side of pain before performing physiological movement away from the side of pain contralateralas this would cause less joint compression, greater excursion of movement at znags symptomatic FSU, and analsyis a greater chance of stretching adhesions and resolving any meniscal entrapment or extrapment.
This approach would be. Nevertheless, current knowledge suggests that the IVD biomechancal a potential source of pain. In the lower cervical spine, disc fissuring may start at the centre of the disc and radiate in all directions, eventually becoming confluent and forming sequestra Tondury ; Ecklin Fully or partially detached IVD fragments could constitute a painful impediment to the gliding motion at the uncovertebral cleft, with the direction and degree of restriction possibly depending on the size and orientation of the IVD csrvical.
In either case, the accessory glide component of a cervical SNAG could potentially facilitate painfree motion by distracting the ipsilateral portion of the uncovertebral cleft.
However, the previous biomechanical analysis does not indicate as to why any improvement would be further z by ipsilateral active movement. Mulligan a has put forward a theory which could help to explain the need for ipsilateral physiological rotation during application of a cervical SNAG. He suggests that the superior facet of the cervicl FSU ipsilateral to the side of pain may be jammed posteroinferiorly in an extension or closed down position; hence, ipsilateral rotation could cause pain due to excessive closing down of the zygapophyseal joint.
Application of the accessory glide component of a cervical SNAG may therefore reposition the superior facet superoanteriorly allowing a greater range of painfree ipsilateral rotation. Other approaches to manual therapy also consider spinal joint malalignment and subluxation as potentially reversible causes of spinal pain Triano ; Katavichhowever, there remains a disparity between symptomatology and radiographic findings Gore et al.
It is arguable that a subluxation or snays positional fault of a joint is no more common in persons with spinal pain than those without Grieve ; Yi-Kai et al. Its clinical application has sngas based almost exclusively on convention with little attempt to provide a biological basis and little, if any, empirical evidence as yet to support its efficacy.
To this end, the present review has attempted to analyse the possible biomechanical effects of a SNAG applied to the articular pillar of the cervical spine. Several potentially reversible sources of articular pain and impaired function, mainly involving impingement of innervated snaags between either zygapophyseal joint or IVD articular surfaces, have been considered.