CONGENITAL DISLOCATION OF THE HIP AND COMPUTERISED

Transcription

1 CONGENITAL DISLOCATION OF THE HIP AND COMPUTERISED AXIAL TOMOGRAPHY J. G. EDELSON, M. HIRSCH, H. WEINBERG, D. ATTAR, E. BARMEIR From the Soroka Medical Center, Beer-Sheba CT scans of 18 hips with typical congenital dislocation have been studied in 16 children. These show that the common position of dislocation is lateral, superior and slightly anterior, and that a false acetabulum can be distinguished even in young children. A defect in the posterior ischium causing distortion of the acetabulum was also present in most cases. The cartilage and the acetabular contents were well shown. Positions of reduction and the anteversion of the acetabulum and the frmoral neck were studied. Hypotheses are presented on the mode of dislocation and on the cause of the pathological changes. Until recently there have been few reports of computerised axial tomography (CT scan) of the hip in young children and not much success in demonstrating the predominantly cartilaginous structures (Padovani et al ; Peterson et al ; Browning, Rosenkrantz and Tarquinio 1982; Visser, Jonkers and Hillen 1982). However, a new generation ofscanners provides sufficient resolution to permit fresh insights into the anatomy of congenital dislocation of the hip (CDH). These findings shed light on this area of orthopaedic practice, in which multiple and sometimes contradictory methods are in use. MATERIAL AND METHODS Eighteen congenitally dislocated hips from 16 children aged between three months and two-and-a-half years were included in this study. The criteria for diagnosing CDH were a minimum of two of the following four features : a positive Ortolani or Barlow test (Smaill 1968); telescoping of the involved limb (Stanisavljevic 1964); radiographic displacement with reference to Perkins and Hilgenreiner s lines (Schottstaedt 1965); and an acetabular index of over 40#{176} (Coleman 1965). There were 14 girls and 2 boys in the study. The left hip was involved in 12 cases and the right in 6 ; two cases were bilateral. No children with arthrogryposis, a J. 0. Edelson, MD, FAAOS, Lecturer in Orthopaedic Surgery M. Hirsch, MD, Director, Department of Diagnostic Radiology D. Attar, MD, StaffOrthopaedic Surgeon E. Barmeir, MD, Senior Staff Radiologist DepartmentsofOrthopaedic Surgeryand Diagnostic Radiology, Soroka Medical Center, University of the Negev, Beer-Sheba, Israel, P.O.B H. Weinberg, MD, Professor oforthopaedic Surgery Hadassah University Hospital, Mount Scopus, Jerusalem, Israel. Requests for reprints should be sent to Dr J. G. Edelson British Editorial Society of Bone and Joint Surgery X/84/4l 14 $2.00 neuromuscular disorder or a genetic malformation were included. In unilateral cases the opposite hip served as the control, though it was appreciated that some abnormality may exist on the normal side (Blockey 1982; Bolton-Maggs and Crabtree 1983). All CT scans were performed by an Elscint 2002 Scanner with 3 mm slice width, 8 s scan time, and 512 x 512 matrix. Care was taken to minimise the exposure to radiation. The cuts were through the superior acetabular margin, the triradiate cartilage and the inferior portion of the acetabulum. Anteversion of the femoral neck and ofthe acetabulum was calculated according to the method described by Visser et a!. (1982). The children were sedated so that they slept through the procedure. They were supine and the hips were allowed to assume the position ofchoice. In a few patients with easily reducible hips these were additionally held gently in flexion, extension, abduction, or rotation to assess the effects of these positions. RESULTS The hips of the sedated children assumed asymmetrical positions, with the involved hip in flexion, abduction and lateral rotation, an attitude reminiscent of the classical picture of a traumatic anterior dislocation (Fig. 1). Displacement ofthe femoral head. CT scan in this position showed that the most striking displacement of the head in congenital dislocation was laterally (Fig. 2). Superior displacement of varying degree also was present in all cases ; this was shown by the consistent appearance of the head of the involved femur before that of the control side as the scan cuts moved caudally from above the hip. Despite the common misconception (McCarroll 1965), posterior displacement is not usual in this age group. As a rule, the head is in the same coronal plane as the opposite head or even anterior to it (Figs 3 and 4). Frank posterior displacement was found in only one of our 472 THE JOURNAL OF BONE AND JOINT SURGERY

2 CONGENITAL DISLOCATION OF THE HIP AND COMPUTERISED AXIAL TOMOGRAPHY 473 Fig. I Figure 1-Photograph of a sedated child to show the resting posture in right sided CDH. Figure 2-CT scan of a child with a right CDH to show lateral displacement of the femoral head. P. pubis ; i, ischium;. femoral head with epiphysial ossification centre:, greater trochanter. patients, an eight-month-old girl who had previously been treated by closed reduction, then held in flexion and abduction in a plaster cast ; this hip re-dislocated on removal of the cast and the patient was then referred to us (Fig. 5). Bony acetabular defects. In 16 of the 18 hips a defect was seen in the bony acetabulum adjacent to the dislocated head, which was displaced upwards and forwards. This defect was a concavity just posterior to the anterior inferior iliac spine ; it contrasted with the normal convexity at this site seen on the control side (Fig. 6). This false acetabulum was more marked in older children (Fig. 7), but was present even in our youngest patient ; it was often difficult to detect on standard anteroposterior radiographs. A similar malformation of the posterior ischial aspect of the acetabulum was found in 15 of the 18 hips. There was blunting and failure of development of the bony acetabulum, leading to distortion and foreshortening in an anteroposterior direction, and making the cup noticeably less capacious than on the control side (Figs 8, 10 and 12). In contrast to the superior false acetabular defect the posterior ischial defect did not appear to increase with age. Soft tissues. The soft tissues were well seen in most cases. The outline ofthe cartilaginous head also was well shown, but subtle distortions of its roundness may have been hidden by adjacent muscle shadows, especially that of the iliopsoas (Fig. 9). The tissues within the acetabulum had the appearance of normal pulvinar fat, except in the two oldest children (21 and 30 months) in whom more dense soft-tissue interposition was present (Fig. 10), and Figure 3-CT scan of four-month-old girl with left sided CDH showing lateral and slight anterior displacement of the left femoral head., metaphysial ossification centre for the femoral neck ; the marked angles are of acetabular anteversion which is increased on the right. Figure 4-Photograph of hemipelvis to show the position of displacement in the early stage of CDH : the effect of extension on the position of the hip is also shown E, epiphysial ossification centre for the femoral head, anterior inferior iliac spine. The uppercurved arrow shows superior and anterior movement of the head when the shaft is placed in extension, as indicated by the lower curved arrow. Fig.4 VOL. 66-B, No. 4, AUGUST 1984

3 474 J. 0. EDELSON, M. HIRSCH, H. WEINBERG, D. ATFAR, E. BARMEIR CT scans of children with CDH. Figure 5-Right sided lesion in an 8- month-old girl to show the lateral and posterior position of the femoral head., distorted posterior lip of the acetabulum. Figure 6-Left sided CDH in a 4fmonth-old girl to show the false acetabulum. anterior inferior iliac spine. Figure 7-CT scan of a 2-year-old child showing a more obvious false acetabulum on the left., anterior inferior iliac spine. Figure 8-Left sided CDH in a 19-month-old girl. Note the blunting of the ischium and diminished size of the cup of the acetabulum. Figure 9-Left sided lesion in a 3fmonth-old girl to show the soft-tissue images. 5, sartorius muscle; T, tensor fascia lata; R, rectus femorus ; I, iliopsoas., posterior lip of acetabulum at the level ofthe triradiate cartilage. Figure 10-left sided CDH in a 2fyearold child. -, soft-tissue interposition;, pulvinar fatty fibrous tissue. At operation the hypertrophied ligamentum teres was causing obstruction and was removed. Figure 1I-Bilateral CDH in a 7-monthold girl. The right side is more severely displaced. The angle of anteversion of the head has been marked, using the elliptical shape of the ossification centre of the femoral head. 11 THE JOURNAL OF BONE AND JOINT SURGERY

4 CONGENITAL DISLOCATION OF THE HIP AND COMPUTERISED AXIAL TOMOGRAPHY 475 influenced the decision in favour of open reduction. In both these older children a hypertrophic and elongated ligamentum teres was seen at operation to block reduction, and the joint capsules were thickened and constricted especially inferomedially. Much fatty fibrous tissue filled the acetabula. In both, the posterior part of the acetabular labrum was enlarged and flattened, but this proved no obstruction to deep and stable reduction ofthe head. In one other case, the child with recurrent dislocation after cast treatment (Fig. 5), the usual smooth meniscoid appearance of the posterior cartilage and labrum was truncated and distorted, but no frank soft-tissue interposition was seen. Anteversion of the femoral neck and of the acetabulum. It is widely held that there is increased anteversion of the femoral neck in CDH (Haupt 1963; Tachdjian 1972). We were unable to confirm a consistent increase in femoral neck anteversion on the involved as opposed to the control side. As already described, the dislocated limb lay on the table in increased lateral rotation which was seen on the CT scan as an apparent increase of anteversion. However, when correction was made for posture by rotating the knees to the neutral position, no consistent increase in anteversion of the femoral neck could be demonstrated. that of the neck which is of the greatest clinical interest. Increased anteversion of the acetabulum may also be significant in CDH (Lloyd-Roberts, Harris and Chrispin 1978). However, such an increase was not found consistently on the involved side, being shown unequivocally in only two cases (Fig. 3). In other cases apparent anteversion of the acetabulum was an artefact resulting from the asymmetrical position. The appearance of anteversion of the normal acetabulum increases with more caudal CT sections and can easily be overestimated. Effects of positioning. Flexion of the hip was shown to move the head progressively backwards against the area of the ischial acetabular defect already described. In one patient with an obvious defect, flexion to 80#{176} caused posterior subluxation (Fig. 12). Extension of the hip tended to exaggerate lateral and anterior displacement of the femoral head. Medial rotation counteracted anteversion, moving the hip posteriorly and deep into the acetabulum. Lateral rotation had the opposite effect. Medial rotation did not move the hip posteriorly as much as flexion, medial rotation to the physiological limit being required to obtain a posterior position roughly comparable to that produced by 80#{176} of flexion. Abduction moved the head anteriorly and deep into Left sided CDH in a 9-month-old girl. Figure 12- The hip is held in 80#{176} of flexion producing posterior subluxation of the femoral head., femoral head ;, greater trochanter. Figure 13-Abduction, as seen in the control hip, moves the femoral head anteriorly. Compare this with the posterior subluxation produced by flexion with minimal abduction shown in Figure 12. It was simpler and undoubtedly more relevant to record the anteversion of the femoral head rather than that of the neck, as is done by traditional methods (Peterson et al ; Visser et a!. 1982). In the young child, the ossification centre of the femoral head is not round but elliptical. Its greatest axis on the CT scan is roughly perpendicular to the long axis of the femoral neck, but not exactly ; the slight difference in axis results from the normal slightly posterior placement of the head on the neck (Harty 1982; Ogden 1982). This gives an angle of anteversion for the head which is slightly less than that for the neck measured in the usual way (Fig. 1 1). It is, however, the position of the head and not the acetabulum, tending to counteract the posterior displacement produced in the flexed or medially rotated positions (Fig. 13). DISCUSSION There are two broad types of CDH (Ogden and Moss 1978). The rare form is teratological and occurs early in the interuterine period ; these children are born with dislocation and malformation of the hips and often have a variety of other birth defects. The more common type, like those in this present study, are subluxated or dislocatable at birth, but are rarely dislocated. The femoral head and the acetabulum are grossly normal in VOL. 66-B, No. 4, AUGUST 1984

5 476 J. G. EDELSON, M. HIRSCH, H. WEINBERG, D. ATFAR, E. BARMEIR size and shape at birth (Somerville 1967; Stanisavljevic 1982), the main abnormality being laxity of ligaments and of the capsule (Wilkinson 1963; McKibbin 1970). These hips become more displaced during the early weeks and months of life. Secondary changes in joint shape and contour may follow and later the hips may come to resemble those of the teratologic variety of dislocation (Ogden and Moss 1978). Mechanics of dislocation. The CT scans show that the laxity of soft tissues may allow enough lateral displacement of the femoral head to free it from the usual constraints of the acetabulum. Excessive lateral rotation of the femur is then possible under the influence of the iliopsoas (McKibbin 1968) and of the lateral rotator muscles, which are well developed in infancy. This produces the asymmetrical lateral rotation seen with the child relaxed and supine (Fig. 1). In this position the femoral head is displaced anteriorly, and this displacement is increased by the considerable amount of anteversion which is normal in the femoral neck of the infant hip. From its lateral and anterior position the head of the femur is free to move superiorly into frank dislocation (Fig. 4). swaddled by certain ethnic groups (Salter 1966); these include the American Indians and the Arab villagers of Hebron both of whom show a multifold increase in the incidence of CDH over comparable population groups. In untreated cases, the head moves back and up even more as the false acetabulum is moulded (Fig. 14) in response to crawling, to standing, and to additional muscle forces. The posterior defect we have shown in the acetabulum may play a role in this, as may the natural posterior slope of the ilium itself. During this same period, the gradual tilting ofthe pelvis by the development of a lumbar lordosis may also help direct the head posteriorly-a possibility we are now looking into. Bony changes in the acetabulum. The demonstration by CT scan of a false acetabulum in very young children was unexpected and would appear to be caused by pressure from the displaced femoral head. The dramatic changes in the posterior ischium, resulting in reduced capacity of the acetabulum, have also not been widely appreciated or visualised by other techniques ; they may be produced by pressure from the displaced and subjacent greater trochanter. However, a more likely explanation is that the Fig. 14 Fig. 15 Figure 14- Photograph ofhemipelvis showing a well developed false acetabulum as seen from the posterolateral aspect. The large arrow indicates the moulding of the cavity of the false acetabulum. FA, false acetabulum; TA, true acetabulum. (Modified from Acetabular dysplasia : skeletaldysplasias in children, edited by Weil, 1978.) Figure 15-Bones of the hip showing the reduced position gained by hip flexion. Flexion of the shaft (small black arrow) produces a posterior and inferior movement of the femoral head (large black arrow)., anterior inferior iliac spine. The mechanics of this type of dislocation after birth has been much discussed and forced extension of the hip during delivery has sometimes been blamed (Tachdjian 1972). We consider that extension may be responsible, but that the child itself may produce this movement. During the first critical weeks of life a baby spends most of its waking hours kicking into extension incessantly and energetically. This tends to move the head upwards and forward (Fig. 4) into the position of dislocation. Extension is also the position in which the hips are posterior ischial defect occurs before and not after birth, and is due to abnormal pressure from the hyperfiexed femoral head in utero. Hyperfiexion, the characteristic posture of the prenatal hip (Wilkinson 1963), places the femoral head (Figs 12 and 15) in a position to press against the area where the posterior defect will later appear. If normal pressures are increased by additional factors such as breech presentation with intact hamstrings (Tonnis 1982), or oligohydramnios (Somerville 1982), acetabular development may be retarded and the THE JOURNALOF BONE AND JOINT SURGERY

6 CONGENITAL DISLOCATION OF THE HIP AND COMPUTERISED AXIAL TOMOGRAPHY 477 defect becomes established. This posterior defect, unlike that producing a false acetabulum, does not appear to grow worse after birth, probably because of the absence of hyperfiexion. These hypotheses seem to conflict with anatomical studies which show only soft-tissue laxity at the time of birth, but the paucity of available specimens of the nonteratological type of CDH (Ogden and Moss 1978) may have prevented observers from noticing what may be, at best, a subtle structural alteration on gross dissection (Tonnis 1982). Soft-tissue interposition. The CT can be useful in showing the acetabular contents, which may be important when operation is contemplated. Further work is needed on the natural history of these tissues in relation to later subluxation and to the long-term result. Anteversion of the femoral neck and the acetabulum. The widely held belief that there is increased femoral anteversion on the involved side in CDH is not confirmed by our studies, nor is there a consistent increase in acetabular anteversion. Our findings are, however, in agreement with those of others working with CT scans. Both femoral neck and acetabular anteversion may be increased in some children with CDH but this affects both the involved and the uninvolved sides (Visser et al. 1982). The observation that the orientation of the epiphysial ossification centre of the femoral head is useful in determining the angle of anteversion is new, and may prove helpful to other investigators. Positioning studies. Displacements in the superoinferior direction during gentle manipulation of the hip cannot easily be appreciated on a CT scan, but the study of models, and of anatomical dissections (McKibbin 1970), as well as the clinical findings (Somerville 1982; Iwasaki 1983), and operative observations(salter 1980)all provide additional evidence. Flexion of the hip is seen to move the head not only posteriorly but inferiorly as well, thus effecting reduction (Fig. 15). Extension has the opposite effect, moving the head superiorly as well as anteriorly into the position of dislocation (Fig. 4). Abduction displaces the hip anteriorly (Fig. 1 3) but also inferiorly, screwing it home deep into the acetabulum by using a soft-tissue fulcrum (McKibbin 1970). The positioning studies show that there is common ground between the two major and superficially dissimilar modes ofholding reduction; The Lorenz position (Ponseti 1966) and its variations such as the human position, and the position in the Pavlik harness, depend on fiexion to move the head down and back. The second mode, the Lange position (Ponseti 1966) and its variations, depends on medial rotation to accomplish this. Both modes then use abduction to move the head down and forward into the depths of the socket. The classical Lange position does include a third element, namely extension, but this has fallen into disrepute (Ponseti 1966), possibly for the reasons we have suggested. Both modes can obtain and hold reduction, and it is important to use the appropriate combination of manoeuvres in each case. Figure 12 shows that too much flexion may unwittingly subluxate the hip posteriorly; this cannot be shown on an ordinary radiograph. The possibility of such subluxation must be weighed against the vascular (Chung 1976) and other complications (Salter 1966) which may be associated with the medial rotation and abduction needed to hold reduction in less flexion. In difficult cases a CT scan can usefully supplement clinical judgment. The knowledge gained from scanning must of course be carefully weighed against the risk of radiation (Peterson et al. 1981). REFERENCES Blockey, NJ. Congenitaldislocation ofthe hip. J BoneJoint Surg[Br] 1982:64-B: Bolton-Maggs fig, Crabtree SD. The opposite hip in congenital dislocation of the hip. J Bone Joint Surg [Br] 1983 :65-B : Browning WH, Rosenkrantz H, Tarquinio T. Computed tomography in congenital hip dislocation : the role of acetabular anteversion. J Bone Joint Surg[Am] l982:64-a: Chung SMK. The arterial supply ofthe developing proximal end ofthe human femur. J Bone Joint Surg [Am] l976;58-a : Coleman 55. Treatment ofcongenitaldislocation ofthe hip in the infant. 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