THEINTERVERTEBRAL JOINTS I : THE INTERVERTEBRAL DISC Jeanette Mitchell* Summary The structure of the intervertebral disc is described in detail, some reference being made to clinically relevant aspects of the anatomy. Some biomechanical terms, such as motion and force, creep and hysteresis, are explained and discussed in relation to the intervertebral disc as a clinically important struc­ ture. Opsomming Die struktuur van die intervertebrale diskus word in detail beskryf, met verwysing na toepaslike kliniese aspekte van die anatomie. Party biomeganiese terme, soos beweging en krag, “creep” en “hysteresis", word verduidelik en bespreek m et betrekking tot die intervertebrale diskus as 'n kliniese belangrike struktuur. INTRODUCTION M an, the finest example o f bipedalism, shows rem arkable a d ap ­ tation to the erect posture and movem ents, particularly in the stru c ­ ture and functioning o f the spine o r vertebral column. This partly rigid, partly flexible central axis,1,2 extending from the base o f the skull to the end of the coccyx, is stabilised by strong ligaments and muscles which also limit the wide range o f m ovem ents3 taking place at the intervertebral joints. T hese m ovements occur at both the facet joints and the intervertebral discs. Anatomical structure of the intervertebral discs T hese intervertebral joints are a n te rio r1, unpaired, non-syno- vial cartilaginous joints o f the secondary type2,4. T hat is, they arc fibrocartilaginous discs between the vertebral bodies, their shape corresponding to the articulating surfaces of the adjacent vertebral bodies They are designed for strength3, and u nder norm al circum ­ stances, retain their fibrocartilaginous structure throughout life. Each intervertebral disc is composed of: (a) the a n n u lu s flbrosus which consists o f approxim ately 10 to 12 concentric lamellae of collagenous fibres3 running parallel to each o t h e r " (Figure 1). Between adjacent vertebrae, the fibres of these; lamellae are positioned at an angle of about 70 degrees to the vertical and 30 degrees to the horizontal5,6. This gives the appearance, in side-view^ o f the fibres running obliquely between adjacent vertebral bodies in such a way that som e fibres are at right angles to others (Figure 1). This criss-cross arrangem ent o f fibres limits rotational movem ents in both directions . The lamellae a re attached to the bony margins o f the a rticular surfaces o f the vertebral bodies1’ '6. It is of clinical im portance to note that the lamellae are thinner and fewer in num ber, and thus weaker, posteriorly1,3,5,6. Posteriorly too, the fibres o f the lam ellae run mainly vertically , adding to their inherent w eak­ ness in this a re a 1. Slightly posterolaterally, where there is p o o r support by the narrow p osterior longitudinal ligament, the m ajority o f h e rn i­ ations o r protrusions o f disc m aterial occurs3. (b ) the nucleus pulposus which is derived from the embryonic notochord 2’3’4'^. It is b e tte r developed in the cervical and lum bar regions o f the spine , and consists o f reticular and collagen fibres em bedded in a mucoid or^ g elatinous m aterial,1’2,3,5’6’7’!i, with an 88 per cent w ater c o n te n t1,2,5,6’ . It is in contact with the lwaline cartilage covering the articular surfaces o f the vertebral bodies . T he nucleus pulposus is enclosed within the annulus fibrosus and, as such, obeys the laws o f hydrodynamics1, acting as a “shock a b so rb e r”6 for approxi­ mately 75% o f the axial forces7 through the spine3. It is positioned m ore tow ards the p o ste rio r aspect o f the disc1 because o f th e w ider a n te rio r p art o f the annulus fibrosus (F igure 1). In the cervical and lum bar regions o f the spine, this allows the axis around which m ove­ m ent occurs to pass through the v erteb ral colum n slightly p o ste rio r to th e a n a to m ic a l c e n t r e o f th e i n te r v e r t e b r a l disc, a n d d ire c tly through the nucleus pulposus. In the thoracic region, however, the axis o f m ovement passes a n te rio r to the nucleus pulposus . n u c l e u s p u l p o s u s n u c l e u s p u l p o s u s v e r t e b r a l _ b o d y a n n u l u s f i b r o s u s F i g . 1 T h e i n t e r v e r t e b r a l d i s c Je a n e tte Mitchell, MSc, BSc (Physiotherapy), Lecturer D e p artm en t o f Physiotherapy, M edical School, University o f the W itw atersrand, Johannesburg. Bladsy 34 Fisioterapie, Mei 1991, deel 47 no 2 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. ) All the intervertebral discs together constitute approximately one -q u arter of the length of the spine , the remaining three-quarters of the length being m ade up by the vertebral bodies . As would be expected because of their function to w ithstand stresses, the in terv er­ tebral discs increase in size (height and diam eter) and strength from the second cervical to the first sacral vertebra of the spine . (T here a re no discs between the occiput and the atlas o r the atlas and the axis ’ ). T he cervical and lum bar regions of the vertebral colum n are w edge-shaped, being thicker anteriorly to accom m odate the a n te rio r­ ly convex secondary spinal curves2,4. T he intervertebral discs are connected to adjacent vertebral bodies by the strong a n te rio r longitudinal ligament anteriorly and the thinner, w eaker p osterior longitudinal ligament posteriorly, both of which run the e ntire length of the vertebral colum n2,3,4. T he form er prevents hyperextension of the spine, while the latter prevents hyper­ flexion of the vertebral colum n . E ach intervertebral disc is innervated by branches of the sinu- 910 3 vertebral nerve ’ (also known as the m eningeal nerve o r the recur­ rent spinal nerve2). T hese nerve endings a re found in the o u te r lamellae of the annulus fibrosus only^ , and the norm al nucleus 7 10pulposus is devoid of nerves ’ . Similarly, the peripheries of the intervertebral discs only, obtain their blood supply from branches of adjacent blood vessels, such as the a n te rio r radicular arteries. Tile central p arts of the discs, including the annulus fibrosus and the 2.8nucleus pulposus, are avascular . W ith normal ageing process, the intervertebral discs becom e th in n e r due to dehydration* and degeneration of discal materials, resulting in slight height loss . D esiccation of the nucleus pulposus and weakening of the annulus fibrosus3 renders the ageing discs m ore subject to dam age due to m inor strains. Such traum a may lead to a herniated o r prolapsed disc which involves the protrusion o r h ernia­ tion of nucleus pulposus m aterial between the torn fibres of the annulus fibrosus, usually in a posterolateral direction2,3. This m aterial 2.3may impinge on the adjacent spinal nerve root , causing the typical sym ptoms of nerve pressure. The most commonly occurring disc protrusions are the C5/6 and C6/7 intervertebral discs in the cervical region 2’3. and the L4/5 and L5/S1 intervertebral discs in the lum bar f ■ 2region o f the spine . Clinically relevant biomechanics of the intervertebral disc (a) M otion and Force Norm al functional m ovements of the vertebral column causc forces to be exerted on the intervertebral discs. F o r example, flexion and extension, and side-bending o r lateral flexion involve com press­ ion o f that part of the disc closest to the side to which the spine is moved 6’7. S im ila r^ stretching forces are experienced in the opposite sides of the discs ' . On rotation, the lam ellar fibres of the annulus fibrosus, running between the adjacent vertebrae, are stre tc h ed 6' . due to. the twisting movement of the spine (Figure 2). The normal forces acting through the vertebral column and, therefore, on the intervertebral discs are thus due to compression (approxim ation), stretching (traction o r distraction), shearing (gliding forces), bending (com pression and stretching forces), and twisting ( r o t a ti o n a l fo rc e s ) ’ ’ . T h e i n te r v e r t e b r a l d isc s m u s t be s tro n g enough to withstand such forces so as to avoid injury5,6. The annulus fibrosus, under norm al circum stances, can w ith­ stand the compressional forces due to the body's weight u nder the influence o f gravity. T hese and o th e r forces exerted during normal m ovements of the spinal column, a re transm itted to the cartilaginous end plates of the adjacent vertebrae . From here, they are absorbed into the vertebral bodies and thus dissipated6. The discs themselves are capable of absorbing som e of the forces and, as a result, te m p o ­ rarily deform before transm itting the forces to the vertebral bodies . A counteracting force may be exerted within the lam ellar fibres of the a n n u lu s fib ro s u s p a r t o f th e discs, a s th e y a r e s tr e tc h e d o r d e ­ form ed ,7. This force will resist the force-producing m otion, such as rotation 6,7 for example. However, prolonged, sustained o r excessive force leads to m ore perm anent deform ation o f the discs, followed ultimately by degenerative changes in the discs and adjacent vertebral T r a c t i o n C o m p r e s s i o n F l e x i o n / E x t e n s i o n R o t a t i o n F i g . 2 F o r c e s a c t i n g on t h e i n t e r v e r t e b r a l d i s c bodies. T he nucleus pulposus too is designed to sustain and transm it fo rc e s 6,7 due to norm al w eight-bearing and movements. In the n o r ­ mal, unloaded state, the nucleus pul(X>sus exerts an internal pressure due to its w ater-absorbing capacity ’ . O n applying a load o r force to the disc, this internal pressure will increase as the force is absorbed by the nucleus pulposus, and subsequently decrease as the force is transm itted to the annulus fibrosus and then to the adjacent vertebral bodies as before5,6. If the load on the vertebral bodies is asym metrical, the norm al intervertebral disc exhibits a self-stabilising m echanism . T hat is, the annulus fibrosus and nucleus pulposus n earest the side of the disc which is stressed becom es com pressed, and the opposite side of the T h ea n n u lu s f ib r o s u s a n d th e n u c le u s p u lp o s u s is s tr e tc h e d stretched fibres of the annulus fibrosus lengthen ’ m om entarily and, at the sam e time, the increasing, pressure in the nucleus pulposus exerts a horizontal outw ard force . This la tte r force exerts a pressure on the stretched fibres of the annulus fibrosus, causing them to return”7 to th eir original length . This applies a stabilising force on the adjacent vertebral bodies7. T hus, the intervertebral disc, because of these inherent p ro p e r­ ties, accom m odates the various norm al m ovem ents of the spinal colum n, transm its the various forces to which it is subjected, and adds to its stability. (b) C reep and hysteresis T hese concepts are characteristic o f all visco-elastic tissues, such 5 11 as the intervertebral disc ’ . C reep may be defined as that deform ation which occurs in the tissue w hen, a constant force is applied to it11 (F igure 3). T his d e fo r­ m ation is im m ediate and constant with tim e until it reaches a steady state, provided th e force rem ains c o nstant . T he degree o f deform a- Physiotherapy, May 1991, vol 41 no 2 Page 35 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. ) F i g . 3 C r e e p a n d h y s t e r e s i s tion d e pends on the force applied to and the type o f m aterial o f the tis s u e 5. A n example o f creep may be seen in sustained flexion o f the v e r t e b r a l c o lu m n , w h e n th e p o s t e r i o r lo n g itu d in a l lig a m e n t is stretch ed . T h ere will be an im m ediate change in its longitudinal dim ensions which will continue until th e re a ppears to be no fu rth er change occurring. C reep is said to have taken place in the tissue of the p o ste rio r longitudinal ligament. This phenom enon is evident in c ontinuous traction techniques for the spinal column. However, it m ust be noted that as with all applications o f force to tissues, excessive force will result in traum atisation o f that tissue, which may be said to be d u e to excessive creep occurring. H ysteresis may be defined as the difference in the deform ation which occurs in the tissue during the loading and unloading o f an inte rm itten t force 5,11 (Figure 3). T hat is, when a force is applied to the tissue, th ere is an initial deform ation o f the tissue. O n removal o f this force, th ere is som e recovery o f the tissue as an atte m p t is m ade to regain its original dimensions. This recovery is not com plete, however, as time is required before total recovery is possible. W ith fu rth e r applications o f this in term ittent force, the change in the dim ensions o f the tissue between the loading and the unloading cycles, that is the hysteresis, is concom itantly less until a steady state is reached . This phenom enon is seen in the use o f mobilising tec h ­ niques when a force is interm ittently applied to the neural spine o r over the facet jo int o f a vertebra, fo r example, in o rd e r to stretch the soft tissues between them . Again it m ust be noted that excessive force o r force applied beyond the point o f the steady state may result in injury to th at tissue. In this instance, as in all cases o f traum a, an added period to th at tim e required for recovery from hysteresis m ust be allowed. CONCLUSIONS Both spinal m obility and stability a re related to th e in te rv e rte ­ bral discs th e im portant jo in ts o f th e vertebral bodies. A n te ro -p o ste- rio r m ovem ents in th e sagittal plane, lateral m ovem ents in th e coronal plane, a n d rotational m ovem ents a ro u n d a vertical axis all involve changes occurring in these in tervertebral joints. M oreover, intrinsic stability is afforded to th e spine partly by th e fibrocartilaginous in te r­ v ertebral discs. References 1. M orris JM . Biom echanics o f the spine. A rch Surg 1973; 107:418-423. 2. W illiams PL, W arwick R , Dyson M , B a n n iste r L H (eds). G ray’s A n a to m y. E dinburgh: Churchill Livingstone, (37th ed), 1989:Ch 3,4. 3. M oore KL. Clinically Oriented A natom y. Baltim ore: T h e W illiam W ilkins Co, 1982:Ch S. 4. Tobias PV, A rnold M, A llan JC . M a n ’s A natom y: A study in dissection. Johannesburg: W itw atersrand University Press, 1988; V ol III:C h 3. 5. G ilm ore KL. Biom echanics o f the lu m b a r m otion segm ent. In: G rieve G P, (ed), M o d e m M anual Therapy. E dinburgh: Churchill Livingstone, 1986; C h 9:103-111. 6. Je n se n GM . B iom echanics o f th e lu m b a r in te rv erte b ral disk: A re- viewPhysical Therapy 1980; 60(6):765-773. 7. K apandji IA. The Physiology o f Joints. Edinburgh: Churchill Livingstone, (2nd ed), 1979; Vol 111:26-41. 8. N achem son AL. T h e lum bar spine: A n orth o p ae d ic challenge. Spine 1976; 1(1):59-71. 9. C loward RB. T he clinical significance o f th e sinu-vertebral nerve o f the cervical spine in relation to the cervical disk syndrom e. N eurolN eurosurg Psychiat. 1960; 23:321-326. 10. E d g ar MA, Ghadially JA . Innervation o f the lum bar spine. Clin Orth R el Res. 1976; 115:35-41. 11. Bogduk N, Twomey LT. Clinical A n a to m y o f the L u m b a r Spine. E d in ­ burgh: Churchill Livingstone, 1987; Ch 6:49-57. THE UNIVERSITY OF QUEENSLAND MASTER OF PHYSIOTHERAPY STUDIES (M a n ip u la tiv e P h y sio th erap y) The Department o f Physiotherapy, University o f Q ue en­ sland, Brisbane, Australia, offers a full-time masters degree program m e in manipulative physiotherapy. An ap p lica n t with the following qualifications m ay be eligible to enrol: ® Holds a degree of Bachelor of Physiotherapy with first or second class honours from this or another a p ­ proved University or Institute; or: Has co m p le te d the requirements of the Masters Q ual­ ifying examination; and: *3 Has had a t least 1wo years full-time clinical physiother­ ap y experience in the field o f manipulative physiother­ apy; and: » Is in the opinion of the Dean an d the Head o f Depart­ m ent a suitable c a n d id a te for the degree. Masters Qualifying programmes are offered by the d e p a rt­ m ent e a c h year either on a full-time (one year) or part- tim e (1wo years) basis. The Masters coursework is offered on a full-time basis (one year). APPLICATIONS DUE BY 31 JULY 1991. Please direct enquiries to: Dr Y R Burns Head, Department of Physiotherapy University of Queensland Queensland 4072 Australia Bladsy 36 Fisioterapie, Mei 1991, dee! 47 no 2 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. )