436 THE JOURNAL OF BONE AND JOINT SURGERY

The cerebral palsy hip classification is reliableAN INTER- AND INTRA-OBSERVER RELIABILITY STUDY

M. L. Murnaghan, P. Simpson, J. G. Robin, B. J. Shore, P. Selber, H. K. Graham

From the Royal Children’s Hospital, Parkville, Australia

M. L. Murnaghan, MD, MEd, FRCS (C), Orthopaedic Fellow

B. J. Shore, MD, FRCS (C), Orthopaedic Fellow

P. Selber, MD, FRACS, Orthopaedic Surgeon

H. K. Graham, MD, FRCS(Ed), FRACS, Professor of Orthopaedic SurgeryHugh Williamson Gait Laboratory, Royal Children’s Hospital, Flemington Road, Parkville, Victoria 3052, Australia.

J. G. Robin, MB BS, Orthopaedic RegistrarMurdoch Childrens Research Institute, Flemington Road, Parkville, Victoria 3052, Australia.

P. Simpson, BSc (Hons), StatisticianMonash University, Commercial Road, Melbourne, Victoria 3004, Australia.

Correspondence should be sent to Professor H. Kerr Graham; e-mail: kerr.graham@rch.org.au

©2010 British Editorial Society of Bone and Joint Surgerydoi:10.1302/0301-620X.92B3. 23105 $2.00

J Bone Joint Surg [Br] 2010;92-B:436-41.Received 17 July 2009; Accepted after revision 6 November 2009

We have tested the reliability of a recently reported classification system of hip morphology in adolescents with cerebral palsy in whom the triradiate cartilage was closed. The classification is a six-grade ordinal scale, based on the measurement of the migration percentage and an assessment of Shenton’s arch, deformity of the femoral head, acetabular deformity and pelvic obliquity.

Four paediatric orthopaedic surgeons and four physiotherapists received training in the use of the classification which they applied to the assessment of 42 hip radiographs, read on two separate occasions. The inter- and intra-observer reliability was assessed using the intraclass correlation coefficient and found to be excellent, with it ranging from 0.88 to 0.94. The classification in our study was shown to be valid (based on migration percentage), and reliable. As a result we believe that it can now be used in studies describing the natural history of hip displacement in cerebral palsy, in outcome studies and in communication between clinicians.

Population-based studies have shown that dis-placement of the hip affects approximately one-third of children with cerebral palsy1-3 and hasbeen directly related to gross motor function asdetermined by the Gross Motor FunctionClassification System (GMFCS).4 A wide vari-ety of treatments are used in the management ofthis type of hip displacement ranging from brac-ing and botulinium injection to surgical recon-struction and salvage surgery. Understandingthe natural history and clinical outcome is ham-pered by the lack of a valid and reliable classifi-cation system. In children with developmentaldysplasia of the hip (DDH), the Severin5 classi-fication is widely used to describe the outcomeof treatment. However, the inter- and intra-observer reliability of this classification systemhas been questioned.6

In a recent study, a new system describing hipmorphology in adolescents with cerebral palsywas proposed.7 It uses a six-grade ordinal scalebased on the measurement of hip migrationpercentage8 and assessment of the integrity ofShenton’s arch, deformity of the femoral head,acetabular deformity and pelvic obliquity. Ouraim in this study was to test the inter- and intra-observer reliability of this system.

Patients and MethodsFour orthopaedic surgeons (JGR, BJS, PS, AK)and four physiotherapists (AF, TH-I, JR, PT)

were instructed in the grading of hip radiographsusing the classification system.7 Two of the sur-geons were fellowship trained in paediatricorthopaedic surgery and had been in practice fora mean of six years (4 to 10). The other two sur-geons had completed their orthopaedic residen-cies and were in the final months of theirpaediatric orthopaedic fellowship. The fourphysiotherapists had a mean clinical experienceof 20 years (16 to 26) and were actively involvedin research and clinical work with patients withcerebral palsy. All assessors were introduced tothe classification system by an oral presentationand given with written instructions and guide-lines. They were given printed radiographs aswell as a pen or pencil, ruler and calculator.The hip classification. The following six gradesas originally described by Robin et al7 aredescribed below and illustrated in Figure 1.Grade I: normal hip . A morphologically normalhip at skeletal maturity has a migration per-centage < 10%. Shenton’s arch is intact and thefemoral head is round and is covered by a well-developed acetabulum. This includes aneverted, well-defined lateral acetabular mar-gin, and a normal teardrop and sourcil. Pelvicobliquity is < 10°.Grade II: nearly normal hip. A nearly normalhip has a migration percentage ≥ 10% and≤ 15%. Shenton’s arch is intact and the fem-oral head is round or almost round. The

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acetabulum may be normal or only slightly deformed withthe lateral acetabular margin slightly blunted. The teardropis slightly widened, but the sourcil should be well formed.Pelvic obliquity is < 10°.Grade III: dysplastic hip. A dysplastic hip has a migrationpercentage > 15% and ≤ 30%. Shenton’s arch is intact orbroken by ≤ 5 mm. The femoral head may be round or

slightly flattened. The acetabulum is normal or mildly dys-plastic with blunt lateral acetabular margin or a gothic arch.There is a widened teardrop and a poorly developed sourcil.Pelvic obliquity is < 10°.Grade IV: subluxed hip. A subluxed hip has a migration per-centage > 30% and < 100%. There is always some contact ofthe femoral head with the true acetabulum. Shenton’s arch is

Fig. 1

Radiographs and diagrams showing the hip classification system in cerebral palsy. Reproduced with permissionfrom Robin et al.7

438 M. L. MURNAGHAN, P. SIMPSON, J. G. ROBIN, B. J. SHORE, P. SELBER, H. K. GRAHAM

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broken by more than 5 mm. The femoral head has variabledeformity from none to severe with more than half beingaffected. The acetabulum likewise has variable deformityfrom being normally shaped to having a large gothic arch.Grade V: dislocated hip. A dislocated hip has a migrationpercentage ≥ 100%. There is no remaining contact betweenthe femoral head and the true acetabulum. Shenton’s arch iscompletely disrupted. As in grade IV, there is variabledeformity of the femoral head and acetabulum (relating tothe timing and rate of dislocation) with variable degrees ofpelvic obliquity.Grade VI: salvage surgery - loss of hip. These hips haveundergone salvage surgery because of painful dislocation orsubluxation. Salvage surgery may involve valgus osteotomyalone, excision arthroplasty plus or minus valgus osteot-omy, arthrodesis or replacement arthroplasty.

Radiographs of 42 hips of adolescents with cerebralpalsy aged between 14 and 19 years and in whom the tri-radiate cartilage was closed were drawn from a large, pop-ulation-based series of children with cerebral palsy bornbetween January 1990 and December 1992 from the cere-bral palsy register of the State of Victoria, Australia.9 Theradiographs were selected by the first author (MLM). Hehad previously classified the radiographs based on the pub-lished classification system.7 Drawing from approximately80 possible radiographs, 42 were selected in a non-randomised way to provide a balanced representation of allsix grades (grade I, 3 hips; grade II, 10 hips; grade III,6 hips; grade IV, 8 hips; grade V, 9 hips; grade VI, 6 hips).The radiographs were taken using a standardised position-ing protocol employed in the Radiology Department,which has been shown to improve reliability.10 They wereread on two separate occasions by all eight readers. Eachreader was asked to classify each of the hips into one of sixgrades as defined by the classification system. All identify-ing information was removed from the radiographs whichwere presented in two sessions in a different order and sep-arated by four weeks.Statistical analysis. This was undertaken using the intraclasscorrelation coefficient. This is equivalent to the weightedkappa value in which less weight is assigned to agreementsince categories are further apart.11 The intraclass correla-tion coefficient was interpreted using established conven-

tions for kappa in which < 0 is poor agreement, 0 to 0.2slight, 0.2 to 0.4 fair, 0.4 to 0.6 moderate, 0.6 to 0.8 substan-tial, and > 0.8 almost perfect agreement.12 In addition tointer- and intra-observer reliability, comparison was alsomade with an established standard of consensus gradeagreed upon by the first and senior authors (MLM, HKG)who were blinded to the results of the eight readers. Anasymptotic symmetry test was used to test for bias, in orderto determine if one rating was consistently higher or lowerthan the other. All statistical analysis was performed usingStata 10.0 (StataCorp, College Station, Texas).13

ResultsIntra-observer reliability. The results are summarised inTable I. The intraclass correlation coefficients ranged from0.88 to 0.94 demonstrating excellent agreement. A symme-try test indicated that there was no bias when comparingthe first and second readings of any of the raters.Inter-observer reliability. The results are summarised in TableII. Having established the intra-observer reliability on theirfirst and second reading, the data were analysed for inter-observer reliability. The reliability was excellent for both read-ings, with the mean intraclass correlation coefficient increas-ing slightly between the two readings (0.84 to 0.88 and 0.89 to0.92, respectively). When taken individually, the physiothera-pists showed excellent reliability, with noted improvement(although this did not reach statistical significance in the intra-class correlation coefficent between the two readings (0.84and 0.92)). The orthopaedic surgeons were similarly excellentin terms of reliability (0.88 and 0.89).Agreement with the established standards. The data werealso analysed for agreement with the established standard(Table III). Agreement with the established standard wasexcellent with a mean intraclass correlation coefficient of0.91 (0.74 to 0.96). Of 16 readings, 15 scored an intraclasscorrelation coefficient greater than 0.85, with one readerrecording an intraclass correlation coefficient of 0.74which is the only reading not in the ‘almost perfect’ cate-gory when compared with the established standard.Patterns of disagreement. The data were further analysedto determine if there were individual subjects (radiographs)

Table I. Details of intra-observer reliability

Rater ICC* 95% confidence interval (CI)

1 0.92 0.87 to 0.972 0.88 0.82 to 0.953 0.94 0.90 to 0.974 0.91 0.86 to 0.965 0.94 0.90 to 0.986 0.90 0.85 to 0.967 0.93 0.87 to 0.978 0.88 0.87 to 0.95* ICC, intraclass correlation coefficient

Table II. Details of inter-observer reliability

Rater ICC* 95% CI†

All ratersReading A 0.85 0.79 to 0.91Reading B 0.91 0.87 to 0.95

PhysiotherapistsReading A 0.84 0.77 to 0.91Reading B 0.92 0.88 to 0.96

Orthopaedic surgeonsReading A 0.88 0.83 to 0.94Reading B 0.89 0.84 to 0.94

* ICC, intraclass correlation coefficient† CI, confidence interval

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which consistently demonstrated poorer agreement.Table IV highlights those which led to a difference of morethan two points on the hip scale which was assigned.Radiographs 14 and 16 showed repeated disagreementbetween readers, three and four times, respectively. A simi-lar analysis was carried out on subjects which had beenconsistently rated differently by readers and the establishedstandard. Table V summarises these results. In all but onecase, the assigned grade by the readers was higher than theestablished standard.

DiscussionThe migration percentage as described by Reimers8 is themost widely used measurement of hip displacement in chil-dren with cerebral palsy. In his original report he suggestedthat measurement of the migration percentage was morereliable than the use of the centre-edge angle in spastic sub-luxation of the hip. The centre-edge angle was found to bemore variable depending on the position of the child whenthe radiograph was taken and to be non-linear. He esti-mated an SEM of 10% for the migration percentage basedon an estimate of the error involved in measuring a line tothe nearest millimetre. In addition, two groups have studiedthe reliability of the measurement of the migration percent-age. Using carefully standardised methodology, Parrott etal14 found that an experienced reader would be expected tomeasure the migration percentage on a single radiograph towithin 5.8% of the true value and hence a change in migra-

tion percentage between two radiographs taken at differenttimes to within 8.3% of the true value. More recently,Faraj, Atherton and Stott15 found that a change of morethan 13% in the migration percentage, in repeated mea-surements by one assessor, was required in order to be 95%confident of a ‘true’ change. However, both of these studiesexamined only a single radiograph. In a clinical setting,most management decisions are based on the examinationof a series of radiographs taken at different times.

Advantages of the use of the migration percentage arethat it is easily understood, has good face validity, accept-able reliability and there is an extensive body of literaturewhich has related specific values of this important clinicaloutcomes.1-3,8,10,14-17 For example, a threshold value of40% has been shown to be associated with instability and arapid increase in the migration percentage in most hipsstudied.16 Most definitions of subluxation and dislocationas well as thresholds for intervention and managementdecisions, are commonly based on the migration percent-age. A further advantage is that it can be measured from agood quality anteroposterior radiograph of the hip once theossific nucleus is present.

Measurement of the migration percentage from serial hipradiographs has also been proposed as the most practicalmethod of carrying out hip surveillance programmes inchildren with cerebral palsy.17 Given the frequency of silenthip displacement in children with cerebral palsy, a numberof authors have proposed systematic hip surveillance forthe early detection of hip displacement.10,17 It is thereforewidely used for the detection of early hip displacement, thelongitudinal follow-up of children who have been identifiedwith hip displacement and for monitoring the outcome ofvarious interventions.18-20 There are inherent limitations tothe measurement of the migration percentage from a two-dimensional anteroposterior radiograph of the hip. Dis-location of the hip is usually posterolateral in children withcerebral palsy, but is occasionally anterior.21 Both anteriorand posterior dislocations do not always show a markedincrease in the migration percentage although there is almostalways some degree of lateralisation before the femoral headescapes from the acetabulum. Axial imaging using CT com-

Table III. Details of reader agreement with the establishedstandards

ICC (95% CI)* ICC (95% CI)

Rater Reading A Reading B

1 0.94 (0.90 to 0.97) 0.92 (0.87 to 0.97)2 0.92 (0.87 to 0.97) 0.93 (0.88 to 0.97)3 0.94 (0.90 to 0.98) 0.93 (0.89 to 0.97)4 0.86 (0.78 to 0.94) 0.87 (0.82 to 0.95)5 0.96 (0.94 to 0.98) 0.95 (0.92 to 0.98)6 0.91 (0.86 to 0.96) 0.96 (0.93 to 0.98)7 0.90 (0.85 to 0.96) 0.93 (0.89 to 0.97)8 0.74 (0.60 to 0.88) 0.89 (0.83 to 0.96)

* ICC, intraclass correlation coefficient; CI confidenceinterval)

Table IV. Details of patterns of disagreement (individualsubjects (radiographs) which had a difference in gradegreater than 2 between their ratings). Gradings accordingto the classification of Robin et al7

Subject GradingsNumber of readers

12 II, V 114 II, IV 316 IV, VI 420 II, IV 125 IV, VI 138 II, IV 1

Table V. Details of patterns of disagreement (individualsubjects (radiographs) which had a difference in grade > 2between their rating (either A or B) and the establishedstandard. Gradings according to the classification of Robinet al7

Subject GradeEstablished standard

Number of times

1 III I 312 V II 114 IV or VI II 416 VI IV 420 IV or V II 325 IV or VI II 338 I III 1

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plemented by three-dimensional reconstruction, and MRIoffer more detailed information.22,23 However, although CTmay be useful when planning reconstruction of the hip in anindividual patient, neither CT nor MRI is appropriate for theclassification of hips in population-based studies or inrepeated longitudinal examinations in a single subject.24 CTexamination of the hips exposes children to a significant doseof ionising radiation and MRI usually requires generalanaesthesia.24 Given the widespread use of the migrationpercentage in studies of hip displacement in children withcerebral palsy, we considered that this was the key continu-ous variable to be used in the classification of the hip in suchpatients. However, with the development of severe displace-ment, various combinations of deformity of the femoralhead, acetabular deformity (gothic arch) and pelvic obliquitymake the measurement of the migration percentage increas-ingly difficult. The hip classification system was thereforedesigned to describe the morphology of hips after skeletalmaturity in cerebral palsy using a combination of both quan-titative (migration percentage) and qualitative features. Theclassification, based on the use of a continuous variablemigration percentage was consciously based on the GMFCS4

for children with cerebral palsy and its relationship to thegross motor function measure.25

The study by Ward et al6 of the Severin system for DDHfound unacceptably low intra- and inter-observer reliabil-ity. This re-inforces the importance of establishing thevalidity and reliability of a new classification system beforeits widespread acceptance and application. Previous studies

have discussed the influence of the expertise of the readerson interobserver agreement.26,27 In our study, both physio-therapists and orthopaedic surgeons served as readers.There was a spectrum of clinical experience in both groups.There was a subtle increase in the interobserver reliabilitybetween the first (A) and second (B) reading which could beattributed to improved application of the system secondaryto learning from the experience of the first reading. In orderto minimise the influence of recall bias, the radiographswere presented four weeks apart in addition to beingblinded and presented in a different order.

In our study the classification has been shown to be validand reliable when used by experienced physiotherapistsand paediatric orthopaedic surgeons. As a result it can nowbe used to describe the natural history and outcome of hipdisplacement in cerebral palsy. Basing the classification onthe migration percentage allows comparison with studiesreporting a change in the migration percentage in youngerchildren.19,20 However, these rarely show the morpho-logical features which we found to be part of hip disease inadolescents with cerebral palsy. We believe that the changesin hip morphology accelerate during the adolescent growthspurt. Although the rate of change may slow with the onsetof skeletal maturity, it is known that scoliosis, pelvic obliq-uity and changes in hip status may occur in the third andfourth decades.28

The classification may be useful as a means of communica-tion regarding the clinical status of hip morphology in adoles-cents with cerebral palsy as well as a tool for evaluating the

Fig. 2

Anteroposterior radiograph of the hips of a 15-year-old boy with lefthemiplegia, gross motor function classification system level II and pain-ful, neglected left hip subluxation. The right hip is grade I and the left hipis grade IV. The deformity of the femoral head (left hip) is moderate withlateral depression, the acetabulum is dysplastic with a small gothic archand pelvic obliquity is mild, measuring 15°.

Fig. 3

Anteroposterior radiograph of the hips of a 17-year-old boy with spas-tic quadriplegia, gross motor function classification system (GMFCS)level IV. The hips were managed by adductor releases, bilateral fem-oral osteotomies and removal of the implants. At skeletal maturity hehas no pain and comfortable sitting. The right hip is grade III (migra-tion percentage (MP) 28%) and the left also grade III (MP 20%). Despitereconstructive surgery, there are few normal hips at skeletal maturityin GMFCS levels IV and V.

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outcome of treatment and natural history studies. The com-bined use of the GMFCS and the classification may also beuseful. For example, adolescents at GMFCS level I and II,who are long-term independent community walkers mayrequire a normal or near morphologically normal hip (Fig. 2).Those with lower levels of mobility or who are non-walkersmay be able to function with less normal hip development(Fig. 3). Grade-VI hips have been included in the classifica-tion to allow the documentation of hips which have beenpainful and required excision as opposed to those which aredislocated (grade V), but have not yet required salvage(Fig. 4). The relationship between hip morphology, pain andsitting and standing function deserves further exploration.

Our results show that the hip scale in cerebral palsy is areliable tool for the evaluation of radiological findings ofhip morphology in adolescents with cerebral palsy in whomthe triradiate cartilage is closed. The interobserver reliabil-ity was excellent, showing consistent agreement betweenreadings for individual raters.

Supplementary materialAppendices relating to the classification of deformityof the femoral head, acetabular deformity and pelvic

obliquity are available with the electronic version of thisarticle on our website at www.jbjs.org.uk

We wish to thank A. Fosang, T. Hastings-Ison, J. Rodda and P. Thomason fortheir help in grading the radiographs. We also acknowledge the help of M.Sheedy in the preparation of this manuscript.

No benefits in any form have been received or will be received from a com-mercial party related directly or indirectly to the subject of this article.

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Fig. 4

Anteroposterior radiograph of the hips of a 16-year-old girlwith spastic quadriplegia, gross motor function classificationsystem level V. The right hip (grade VI) has been excised forintractable pain. However, there has been proximal migrationwith contact between the femur and pelvis causing pain. Theleft hip is grade II with a mildly flat femoral head which is com-pletely covered (migration percentage 0%) and there is moder-ate pelvic obliquity of 30°.