Archives of Disease in Childhood, 1985, 60, 508-511

Aetiology of idiopathic spinal deformities

There are two types of idiopathic spinal deformity-idiopathic scoliosis, and idiopathic kyphosis(Scheuermann's disease). Both conditions affectotherwise entirely normal, healthy children andwhile idiopathic scoliosis has two peaks of incidenceduring infancy and early adolescence, idiopathickyphosis is mainly prevalent in late adolescence.Idiopathic scoliosis and kyphosis share many fea-tures, being modal at the same site with a commonapex at the eighth and ninth thoracic vertebralbodies and having a similar familial trend andcommunity prevalence rate. As will be seen, theyalso share a common 'pathological process'.

Idiopathic scoliosis

Because many disorders of nerve or muscle areassociated with a scoliosis most workers attemptingto discover the cause of idiopathic scoliosis havefocussed on neuromuscular factors. No such factors,or any other structural or functional abnormalities,have been found which are not secondary to thepresence of a spinal deformity. Moreover, with two A°per cent of teenagers showing an idiopathic scoliosisof 100 or more, a hitherto unknown neuromusculardisorder of epidemic proportions has to be incrimi- -~~nated. What has not received sufficient attention is 7 >the nature of the idiopathic scoliotic deformity. ,,..;

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Three dimensional deformity. A postero-anterior ;radiograph of the spine of a patient with idiopathicthoracic scoliosis (Fig. 1) shows the three compo- -Anents of the deformity, one of which is aetiologically fcrucial. The lateral curvature of the spine withrotation of the vertebrae within the curve are thetwo most obvious, most described, but least impor-tant features. Wherever the idiopathic scolioticdeformity occurs in the spine, for example thoracic,thoraco-lumbar, or lumbar, the direction of rotationis the same, such that the posterior elements rotateinto the curve concavity while the vertebral bodiesrotate anteriorly into the curve convexity. If a line,imaginary or true, is now drawn down the tips of thespinous processes throughout the curve and a similarline is drawn through the centre of the vertebralbodies, it will be seen that the line joining the Fig. 1 Postero-anterior radiograph ospinous processes is the shorter. Therefore, the back idiopathic thoracic scoliosis. There is Gof the spine is shorter than the front and the entire the spine with rotation such that the spideformity must be lordotic.2 are directed towards the curve concavii

fa patient witha lateral curvature of,inous processesity.

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This elementary but important geometrical pointseems to have received little emphasis, and, addi-tionally, spurious impressions of the three dimen-sional nature of the idiopathic scoliotic deformityhave been perpetuated by the taking of inappropri-ate radiographic views. Because of the rotationalcomponent a posterior-anterior radiograph of thepatient must, by definition, be an oblique view ofthe deformity and so, of course, is a lateral view ofthe patient. In order to take true planar views eitherbeam or patient must be rotated according to theamount of rotation of that particular vertebra, andwhen a true lateral projection of the curve apexis obtained (Fig. 2). the lordosis is clearly visual-ised.3 It is attributable solely to sagittal spine shape,the anterior height of the vertebral bodies beinggreater than the posterior and any Schmorl node

Aetiology of idiopathic spinal deformities 509

formation or end plate irregularity more posteriorly.Disc height remains symmetrical and does notcontribute to the lordosis.

Normal sagittal spine shape. The presence of alordosis in the thoracic region implies a radicalalteration in sagittal spine shape. There is normallya kyphosis here, although the spine above and belowis naturally lordotic. The axis of spinal rotation runsthrough these sagittal spine curves so that thenormal thoracic kyphosis, lying behind the axis ofrotation and therefore being under tension and notcompression, is protected from rotation. In thepresence of a thoracic lordosis the axis of spinalrotation will run behind the front of the vertebralbodies, which, particularly on forward flexion,undergo compression and must rotate to the side to

I

Fig. 2 (left) True lateral radiograph of the apical region ofan idiopathic thoracic scoliosis. There is a lordosisat bony level.

(above) Lateral radiograph of the thoracic spine of apatient with Scheuermann's disease. There is akyphosis at bony level.

510 Dickson

be accommodated (the rotational prominence inidiopathic scoliosis is much more noticeable onforward flexion).

In the cervical and lumbar regions lordoses arenormal but they are protected from rotationalinstability by having a large amount of segmentalmovement available, by the presence of powerfulmuscles and fascial systems behind them, and bysquat vertebrae with a broad front which tend toresist rotation. Moreover, these are at the extremeends of the spine which naturally tend to remainfacing the front. Idiopathic scoliosis may occur,however, if the lumbar lordosis is increased or ifstiffness is superimposed. Thus, while the term'kypho-scoliosis' is what most practitioners recall ofspinal deformities, it is a combination which cannotand does not exist.2

Idiopathic kyphosis (Scheuermann's disease)

The lateral radiograph of the thoracic spine inScheuermann's disease (Fig. 2) illustrates the samevertebral changes as in idiopathic scoliosis, but inthe opposite direction with much reduced anteriorvertebral height compared with posterior, andSchmorl node formation and end plate irregularitybeing situated anteriorly.4 Again these are other-wise entirely healthy normal children. If the processof vertebral wedging is asymmetric (for example theright side of the vertebra being more affected thanthe left) then a mild scoliosis can exist at the samesite as the kyphosis but with the spinous processesrotated towards the curve convexity, the oppositedirection to that occurring in idiopathic scoliosis, aswould be entirely expected with an asymmetrickyphosis. The anteriorly situated axis of spinalrotation ensures, however, that the kyphotic regionitself does not rotate. Where a severe scoliosis canoccur in the spine of a patient with Scheuermann'sdisease, is below the area of kyphosis, where there isa compensatory lordosis, which as in idiopathicscoliosis is vulnerable to rotation. Therefore in morethan 50% of patients with Scheuermann's diseasethere is a true idiopathic scoliotic deformity apicalsome five vertebrae below the apex of the kyphosis.Thus in these patients the same otherwise normalchild has the deformities of both Scheuermann'sdisease and idiopathic scoliosis but not at the samesite. This lends strong support to a common patho-logical process.

Normal lateral spinal profile

The normal thoracic kyphosis extends from the thirdto the tenth thoraclc vertebrae, those above belong-

ing to the cervical lordosis and those below belong-ing to the lumbar lordosis. During late childhoodand early adolescence, between the ages of 8 and 12years, the normal thoracic kyphosis reduces in sizeand this occurs at the same time in both boys andgirls.5 Thus, during this period, those with anexcessive amount of flattening become truly lordoticwith obvious rotational consequences, and in girlsthis process is occurring during the phase of in-creased growth velocity of early adolescence. Thisexplains why girls are particularly vulnerable to thedeformity of idiopathic scoliosis. By the time boysgo through their peak adolescent growth velocity thethoracic kyphosis has become fully re-established:they are relatively protected from idiopathic sco-liosis, therefore, but are particularly prone to theopposite deformity of Scheuermann's disease, whichis not prevalent until later adolescence. Lateralprofile is governed, like other aspects of body shape,genetically, and this explains the familial trendobserved in both idiopathic scoliosis and Scheuer-mann's disease.6 The scoliotic patient, however,with a flattened lateral profile seems marginallytaller than a peer, without there being any othergrowth abnormality, because of the uncoiling effect.

Treatment considerations

The kyphotic spine clearly requires extension so thatthe anterior aspects of the growth plates can beunloaded, and if a brace or cast which maintainsspinal extension is applied then there is truephysiological correction of sagittal spinal shape.Scheuermann's disease, therefore, is eminentlytreatable conservatively. The thoracic lordosis ofidiopathic scoliosis would, however, require flexionand this is precisely when the deformity is rota-tionally unstable. Accordingly, the deformity ofidiopathic scoliosis is not treatable conservatively2and, while it can be envisaged that the wearing of abrace 23 hours a day will at least prevent the patientfrom the unfavourable consequences of leaningforward, there is no clear evidence that it alters thenatural history of the condition.7 The deformity ofidiopathic scoliosis can, therefore, only be correctedsurgically. Harrington instrumentation, which isattached above and below the area of rotationaldeformity, cannot be expected to alter the rotationalprominence with which every teenager presents andthis is borne out by studies using computedtomography.8 Only by altering the shape of thespine in the sagittal plane can the normal thoracickyphosis be restored, the spine derotated, all threeplanes of the deformity thereby corrected, and thepatient's presenting complaint effectively dealtwith.4

Aetiology of idiopathic spinal deformities 511

References

Zetterberg C, Aniansson A, Grimby G. Morphology of theparavertebral muscles in adolescent idiopathic scoliosis. Spine1983;8:457-62.

2 Dickson RA, Lawton JO, Archer IA, Butt WP. The patho-genesis of idiopathic scoliosis. J Bone Joint Surg 1984;66B:8-15.

3 Deacon P, Flood BM, Dickson RA. Idiopathic scoliosis in threedimensions. A radiographic and morphometric analysis. J BoneJoint Surg 1984;66B:509-12.

4 Dickson RA, Lawton JO, Butt WP. The pathogenesis ofidiopathic scoliosis. In: Dickson RA, Bradford DS, eds.Management of spinal deformities. London: Butterworths,1984:1-37.

5 Willner S. Spinal pantograph-a non-invasive technique fordescribing kyphosis and lordosis in the thoraco-lumbar spine.Acta Orthop Scand 1981;52:525-9.

6 Wynne-Davies R. Familial (idiopathic) scoliosis. A familysurvey. J Bone Joint Surg 1968;50B:24-30.

7 Kaiser RP, Shufflebarger HL. The Milwaukee brace inidiopathic scoliosis. Evaluation of 123 completed cases. ClinOrthop 1976;118:19-24.

8 Aaro S, Dahlborn M. The effect of Harrington instrumentationon the longitudinal axis rotation of the apical vertebra and onthe spinal and rib-cage deformity in idiopathic scoliosis studiedby computer tomography. Spine 1982;7:457-62.

R A DICKSONDepartment of Orthopaedic Surgery,

St James's University Hospital,Leeds LS9 7TF

Associate Editor

In recent years neonatology has been the scene of referees in connection with neonatal articles and willsome of the most vigorous change and active represent those interests to the Editors. Dr Chiswickresearch within paediatrics. We receive a very large is a consultant paediatrician at the North Westnumber of good, original articles relating to the Regional Perinatal Centre, St Mary's Hospital,newborn and in order to deal with these more Manchester and has been a member of the Nationaleffectively and to strengthen their presentation we Maternity Services Advisory Committee producingare appointing an Associate Editor for fetal and the recent report on neonatal care. He has writtenneonatal medicine. Dr Malcolm Chiswick, a mem- many papers and books relating to perinatal medi-ber of the Editorial Committee, is taking on this task cine; we welcome the expertise and help that he willand will correspond directly with authors and provide.