THE INTERVERTEBRAL JOINTS I I : THE FACET JOINTS by J e a n e t t e M it c h e l l M .S c ., B.Sc.(Physiotherapy) Department o f A natom y and Human B i o lo g y , U n iv e r s it y o f th e W itwatersrand M edical School (F o rm e rly : D ep a rtm en t o f P h y sio ­ therapy University of the Witwatersrand; Department of Physiotherapy Education, Avon & Gloucestershire College of Health, Glenside Centre, Bristol, England.) INTRODUCTION The vertebral column is unique in being a rigid yet flexible structure, allowing movement to occur at the intervertebral joints24,36. As physiotherapists appreciate, a knowledge and understanding of spinal anatomy and biomechanics is basic to the successful treatment of patients with dis­ orders of the joints of the vertebral column. The joints of the spine can be classified as the intervertebral discs and the facet joints. The intervertebral discs, described by the author previously22, are the joints of the vertebral bodies23, ,36, while the facet jo in ts are the jo in ts o f the v erteb ral arches23,34,36. This paper gives a detailed account of the structure and functioning of the facet joints of the spine, known also as the zygapophyseal joints, the apophyseal joints or the lateral intervertebral joints. In this description, they will be referred to by the simpler term "facet joints", denoting articulations between opposing facets on the articular processes of adjacent verte­ bral arches. Anatomical structure of the facet joints The facet joints are present between the vertebrae from occiput to sacrum. They are ty p ic a l s y n o v ia l jo in ts o f the p la n e . 7 ,2 3, 24, 26 ,3 3, 34, 36 . u u -ty p e .....................and, as such, are multi- axial as regards planes of movement pos­ sible23. They are the posterior24, paired %ft joints of the vertebral column ' ' . The articular processes which bear the articular facets arise near the junction of the pedicles and laminae of each verte­ bra2 , 3 6 . Each facet joint is enclosed by a fibrous capsule (Fig.l). This is reinforced posteriorly by fibres of the multifidus mus­ cle inserting into the capsule, and adjacent mammillary process in the lumbar verte­ brae, and antero-medially by the elastic 2 23 33ligamentum flavum ' ' . The articular capsule is generally lax, particularly in the cervical spine where a greater range of movement is evident7,23, . This is of par­ ticular relevance in the treatment of condi­ tions in which the manifested signs and symptoms are related to movements of these cervical joints. In the lumbar spine, the facet joint intra- capsular spaces, lined with synovial mem­ brane, have been shown to extend beyond the superior and inferior articular proc- 32 38esses and under the ligamenta flava ' . These polar recesses may be filled with fat pads or have fat-filled synovial folds pro­ jecting from them into the joint spaces1 for two to four millimetres32. The intra-articu- lar fat may become enlarged, extending from the polar recesses into the joint cav- 3 2 38ity ' , and may cause reduced move­ ments at the particular joint involved. This may be associated with increased me­ chanical stresses on the joint and, eventu­ ally, joint degenerative changes5,14,32. These intracapsular folds have been de- 7 28scribed also as meniscoid-like in nature ' , giving the impression of the presence of discs in the facet joints. In the facet joints of the c e r v ic a l s p in e , Y u , S e th e r and Haughton (1987)39 d em on strated four types of menisci, composed of dense con­ nective tissue in children but collagen and fat in adults and diseased or damaged joints. These rudimentary folds of synovial membrane within each facet joint move freely during movement of the joint5,14,32 and can become entrapped between the moving joint surfaces. This may be the /Summary ^ The a n a to m y o f the fa c e t joints, also known as the zygapophyseal, apophyseal or lateral intervertebral joints, is described in detail. Particu­ lar reference is m ade to the ch a n g ­ ing orientation o f the articular fa c ­ ets in the cervical, th o ra c ic and lum bar parts o f the spine, allowing different ranges o f m ovem ent in these regions. Possible causes for back pain are explained as a fu n c­ tion of the biom echanics o f the ver­ tebral colum n. V_________________ :________ J /'O psom m ing ^ Die a n a to m ie van die faset ge- wrigte, ook genoem die "zygapo­ physeal", "apophyseal" o f sy-inter- vertebrale gewrigte, w ord in detail beskryf. Besondere verwysing is ge- m aak na die veranderende orien­ te e r in g v a n d ie a r tik u le r e n d e fasette in die servikale, thorasiese en lum bale dele van die werwelk- olom, w a t verskillende bew eging- som vange in hierdie areas toelaat. M oontlike oorsake van rugpyn, as 'n funksie van die biom eganika van ^ d ie werwelkolom, w ord beskryf. j ca u se o f the " lo c k e d " jo in t seen by physiotherapists, an advocated treatment being manipulation to free the entrapped tissue and hence the joint. These intracap­ sular folds contain blood vessels14 and nerves7,14,15,27. The nerve supply is from the medial branches of the posterior pri­ m a ry ra m i o f the s p in a l nerves8,13,23,27,34,36, and is thought to have a nociceptive function15, relaying the sen­ sation of pain from the particular facet joint/s involved8,27. The orientation of the articular facets of the facet joints differs in the various re­ gions of the vertebral column23,26,36 (Fig.2), and is central to the type of movement/s 23 26 33which can take place at the joints ' ' .A n appreciation of this influences the choice of the m ovem ents used by m anipulative therapists in the treatment of spinal prob­ lems. Considering the superior articular facets, for example, these are generally ori­ entated superiorly and slightly posteriorly in the cerv ical spin e; posteriorly and slightly laterally in the thoracic spine; and medially and posteriorly in the lumbar spine23, 6'33,36. The specific orientation of the articular surfaces of the facet joints (Fig.2) allows the greatest range of motion of rotation and flexion/ extension in the cervical spine; of rotation in the thoracic spine, and of flexion/extension in the lum­ bar spine9,23,36. Despite the fact that there continued on page 12... Physiotherapy, February 1994 Vo! 50 No I Page 11 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) .. continued from page 11 Fig 2: F ace t jo in t o rien ta tio n (su p e rio r and lateral view s). (A fte r W illia m s e t al 36* is very little rotation possible between each vertebra in the lumbar spine, rotation mo­ b ilis a tio n is c o n s id e r e d by m any physiotherapists to be a most effective technique for the treatment of low back pain. This may be more because of the hysteresis effect of such mobilisations on the soft tissues involved rather than the movement of the facet joints themselves. At the junction's between these regions, the orientation of the articular facets of the last vertebra of the region above will closely resemble that of the first vertebra of the region below rather than that which is typical of the particular region ’ .F o rex - ample, the superior articular facets of the twelfth thoracic vertebra face posteriorly, as do those of the vertebrae above. The inferior articular facets, however, are ori­ entated laterally, which is more typical of those of the lumbar vertebrae and which will effectively reduce the degree of rota­ tion and increase the flexion/extension possible at this joint. This changing orien­ tation of the articular facets will influence the stress patterns experienced in these re­ gions of the spine. For example, many pa­ tients present with spinal pain in the thora­ columbar area, where degenerative pat­ terns of these facet joints seen radiologi- cally are typical of the particular stresses experienced here. These joints may be­ come "locked" in flexion/extension, giv­ ing rise to pain and stiffness. These factors must be considered for the effective treat­ ment of thoracolumbar dysfunction. In the cervical spine, the paired inter- vertebral joints between the occiput and the first vertebrae (the atlas), known as the craniovertebral joints, are atypical in that they are not true facet joints2 . These joints are paired synovial joints, but are of the j i i 2 3 , 3 4 ,3 6 t l condylar type . 1 he superior articu­ lar facets of the atlas, which articulate with the articular facets of the occiput, are larger than those of the vertebrae below, kidney­ shaped and face anteriorly and medially. The inferior articular facets of the atlas are orientated inferiorly, medially and slightly posteriorly23'34,36. The equivalent joints between the atlas and axis (the second vertebra) are synovial joints of the plane type. A third joint exists between the atlas and axis where most rotation occurs. This is a synovial joint of the pivot type, between the dens or odon­ toid peg of the axis and the inner surface of the anterior arch of the atlas23'34,36. The third to seventh cervical vertebrae also have a second set of plane-type synovial joints, known as the uncoverte- bral joints23'36. These appear as a lateral lipping (the uncinate processes) along the lateral edges of the superior margins of the vertebral bodies, articulating with the infe­ rior margins of the bodies of the vertebrae above23' 6 (Fig.2). The orientation of these small joints again play a role in determin­ ing the degree and direction of movements occurring in the cervical spine. D ISCO V ER A M E R IC A Five hundred years ago Columbus discovered America. Today, Action Medical is right here in South Africa helping physiotherapists and occupational therapists to make similar discoveries of thetr own. Some are travelling the continent, exploring a wide variety of destinations by working on short assignments. 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R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) Clinically relevant biomechanics of the facet joints Normal functional spinal movements, such as flexion, extension, lateral-flexion or s id e -b e n d in g and a x ia l rota- tion16'17'19'23'25'27̂ 6, involving transla­ tion and rotation, can occur in any of the three fundam ental planes (horizontal, sagittal and coronal), or a combination of 19 2 3these planes ' . For example, flexion and extension occur in the sagittal plane, and lateral-flexion in the coronal plane, both movements involving gliding movements of the facet joints. Axial rotation occurs in a horizontal plane, around a vertical axis. It is restricted by the facet joints, particu­ larly in the lumbar spine. This movement can be increased by a coupling of move­ ments (e.g. lateral-flexion and flexion/ex­ tension) and a combination of the respec- .• , 2 , 1 9 ,2 3 ,2 7 ,3 5tive planes . The range of movements possible be­ tween vertebrae in each of the cervical, thoracic and lum bar spinal regions is small, but the sum of the individual move­ ments gives a considerable range of mo­ tion over the length of the vertebral col- 19 2 3 2 7umn ' ' . In the cervical spine, there is little measurable axial rotation between 19 2 7the occiput and the first vertebra ' , and no lateral flexion between the atlas and axis 5. Lateral-flexion is controlled be­ tween each of the remaining cervical ver- 2 3 2 7tebrae by the uncovertebral joints ' . In the thoracic spine, the ribs limit this move­ ment, while rotation is restricted to one to two degrees between any two vertebrae in 1 9 2 3 2 7the lumbar spine by the facet joints ' ' . The forces which are exerted on the ver­ tebral column during these normal func­ tional movements of the spine are a result of compression (approximation), distrac­ tion (traction), shearing (translation/glid­ in g ), b e n d in g and tw is tin g (ro ta- 1 6 1 9 2 3tion) ' ' . Loading of the spine involves an increase in com pression forces, ab­ sorbed mainly by the intervertebral discs and an increase in shearing forces at the facet joints16'17, which are ideally situated in the individual vertebrae to absorb and support these shear forces10'11. The greater percentage of the compres­ sion forces are borne by the intervertebral discs16,19'23. The facet joints, however, play a major role in the control of these forces. That is, these joints provide stability to the spine, particularly in the cervical and lum- 5 9 2 3 3 lbar regions ' ' ' , by restricting excessive movements of the vertebral column, which w ill d a m a g e the in te r v e r te b r a l discs2'5'9'23'31' , and, in the cervical spine, the vertebral arteries too, ultimately3 . However, as with all synovial joints, the facet joints too can be deformed and dam­ aged when overloaded. In order to mini­ mise this, the forces on the spine are spread throughout the length of the vertebral col­ umn , and the activity of the paraverte­ bral muscles, particularly, play a role in decreasing stress on the in 1 rvertebral joints. This helps to ensure the preserva­ tion of the spine as a stable yet functional unit17. The facet joints, therefore, allow mobility yet give stability to the vertebral column . N evertheless, abnorm ally excessive forces through these joints will result in trauma to both the facet joints and the intervertebral discs, leading to the signs and symptoms typically found by practis­ ing physiotherapists and other clinicians. The lumbar spine facet joints18, for exam- 18 20pie, as. well as the related soft tissues have been im plicated as a sign ifican t source ir. low back pain. Therefore, some knowledge of spinal kinematics is essential in an appreciation of normal movements and in the understanding and manage­ ment of spinal problems3 . Spinal biomechanics are studied in the neutral upright position when the normal spinal curvatures are taken into account. For instance, in erect standing, the majority of the compression forces pass through the intervertebral discs16'19'2 ,25, particularly in the lumbar spine. Such compression forces, acting over a period of hours, cause a narrow ing of the intervertebral disc spaces, an increase in pressure between the articular surfaces of the facet joints, and, if sustained, an increase in the normal lum- 6 19 3 7bar spinal curvature ' ' . These factors, and an increase in extension of the lower spine, lead to an increase in the impinge­ ment or degree of impaction of the inferior articular processes onto the superior ar­ ticular processes and laminae of the verte- 2 6 2 9 3 0brae below ' ' ' . This is because the facet joints cannot sustain vertically applied loads6. This may lead to the "locked" joint with associated pain and decreased move­ ment, in the lower back, for example. Re­ versing the stresses on the joints b y the use of traction and mobilisation of the joints, for instance, helps to relieve these symp­ toms. Further studies of the lumbar facet joints6'29'30,33 have shown that the antero- medial/coronal part (one third) of the facet joints (Fig.2) restrict excessive flexion of the spine by restraining the forward trans­ lational component of flexion; whereas the posterior/sagittal part (two-thirds) of the facet joints restrict excessive axial rotation. The axial rotation which occurs in the lum­ bar spine before impaction of the articular processes is, therefore, small1'4'6. An un­ derstanding of this is of particular rele­ vance when using rotational movements of the lumbar spine in the treatment of low back pain, for example. In this way, although the facet joints resist most of the intervertebral shearing forces in the lumbar spine, they share the compression load in the more lordotic pos­ tures2. In rotation, the facet joints protect the posterior part of the intervertebral disc by the facet surfaces taking up the forces due to the torsion. In flexion, the capsules of the facet joints further serve to protect the intervertebral discs by restricting ex­ cessive movements. However, the facet joints enhance the coupling motion of the vertebral colum n 2'19' 3-27'35. T h e fa c e t joints and intervertebral discs, therefore, have distinct roles in the absorption and transfer of energy due to forces on the spine, both in maintaining postures and in locomotion, involving normal functional movements of the spine17. Sustained postures, particularly at the extremes of any range of motion, in any of the planes of motion, result in strain of the capsules, the ligamenta flava and the mul- tifidus attachments, possibly leading to the sym ptom s of low back pain2'21 experi­ enced by many patients. Furthermore, sus­ tained excessive compression forces, caus­ ing loss of vertebral height of the interver­ tebral discs if continued over time, will eventually lead to subluxation of the facet joints12. This, in turn, results in arthritic changes and pain. Flexibility of the spine also depends, to some degree, on the elasticity in spinal ligaments, which decreases with age, thus making the vertebral column appear to be more stable3. In conclusion, a knowledge of the anat­ om y an d an u n d e r s ta n d in g o f the biomechanics of the facet joints give an appreciation of the different spinal move­ ments possible. However, there is still a great deal to be learnt about the efficient control of the forces acting on the vertebral column, and particularly at the extremes of the ranges of motion. This is evidenced by the on-going problem of spinal pain seen daily by many physiotherapists and other clinicians. Acknowledgements The assistance of colleagues in both Jo­ hannesburg, South Africa and Bristol, Eng­ land in the preparation of this paper is gratefully acknowledged. References 1. A dam s M A , H u tto n W C. T h e relevan ce of torsion to the m echanical d eran g em en t o f the lu m b ar spine. Spine 1981;6(3):241-248. 2. A d am s M A , H u tto n W C. T he m echanical fun ctio n o f the lum bar ap o p hyseal joints. Spin e 1983;8(3):327-330. 3. A dam s M A , H u tton W C , Sto tt JR R . T h e resis­ tance to flexion o f the lu m b ar intervertebral joint. Spine 1980;5(3):245-253. 4. A hm ed A M , D u ncan NA, Burke DL. The effect of facet g eom etry on the axial torque- ro ta tio n re s p o n se of lu m b a r m otio n seg­ m ents. S pin e 1990;15(5):391-401. 5. B ogduk N , E ngel R. T he m enisci of the lum bar R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. )