Journal of the New Zealand Medical Association, 24-October-2003, Vol 116 No 1184
Epidemiology of slipped capital femoral epiphysis in a population with a high proportion of New Zealand Maori and Pacific children
Susan Stott and Terri Bidwell
Slipped capital femoral epiphysis is one of the most common adolescent hip disorders. It typically occurs within 18 months of growth plate closure with bilateral involvement in 22% to 61% of children.1,2 The aetiology of slipped capital femoral epiphysis is unknown but a number of factors such as femoral retroversion, oblique slope of physis and weight of the child may all contribute to a biomechanically weakened physis at a time of rapid growth.3
The prevalence of slipped capital femoral epiphysis varies between races.2 The lowest prevalence is reported in the Japanese population at 0.7 per 100 000,4 with children of European descent in America having an intermediate prevalence of 3.19 per 100 000.5 Children who are of Polynesian or Maori descent have been reported to have the highest prevalence of slipped capital femoral epiphysis in the world.2 However, this information was based on a study of only 34 children, of whom only 28 were NZ Maori and six Australian Aboriginal.
This study investigates the prevalence of first presentations with slipped capital femoral epiphysis in a population with a high proportion of NZ Maori and Pacific children to more accurately determine the epidemiology of this condition in these ethnic groups.
Admission data from Starship Children’s Hospital were collected for the period 1988 to 2000 and analysed for all diagnoses of slipped capital femoral epiphysis. Readmissions for complications or metalware removal were excluded. Children from outside Auckland were also excluded from the data collection. The hospital database was cross-checked with the handwritten operating-room record logs to ensure that all patients were identified. Radiographs were also audited to ensure that only cases of slipped capital femoral epiphysis were analysed.
The data collected included gender, age at admission, side of slip and the self-identified ethnicity of the patient based on a review of case records and radiographs. The definition of ethnicity used was the same as that used by Statistics New Zealand.6 The relative racial frequencies of slipped capital femoral epiphysis were calculated according to the method described by Loder.2
There were 218 children admitted with slipped capital femoral epiphyses in the time period selected. Of these, 211 had radiographic follow up adequate for analysis.
Analysis by age and gender Table 1 summarises the data collected, including the age at presentation, ethnicity and involved hip/s. Fifty seven children were identified as NZ European, 60 were identified as NZ Maori, and 89 were identified as Pacific. Five came from other ethnic backgrounds, including Indian and Asian.
Table 1. Average age at presentation and bilaterality by ethnic group and gender
The average age at first presentation with a slipped capital femoral epiphysis was 149.5 +/-19.3 months in males (range 99 to 190 months) and 132.6 +/-16.7 months in females (95 to 170 months). These data are shown graphically in Figures 1 and 2. Statistical analysis of the age at first presentation for the different ethnic and gender groupings showed statistically significant differences between the groups at p <0.0001 (one-way ANOVA). A Tukey-Kramer multiple comparisons test confirmed significant differences in age at presentation between NZ European females and males (p <0.01), NZ Maori females and males (p <0.01) and Pacific females and males (p <0.001). NZ European females were an average of 18.1 months younger than NZ European males (95% CI 4.5–31.7 months). NZ Maori females were on average 17.9 months younger than NZ Maori males (95% CI 4.5–31.3 months), and Pacific females were on average 15.8 months younger than Pacific males (95% CI 4.9–26.8 months). However, the age at presentation did not vary significantly between the three ethnic groups, for either males or females.
Figure 1. Age at first presentation with slipped capital femoral epiphysis for males of NZ European, NZ Maori and Pacific ethnicity
Figure 2. Age at first presentation with slipped capital femoral epiphysis for females of NZ European, NZ Maori and Pacific ethnicity
Analysis by site of slip Left-sided slipped capital femoral epiphyses were twice as common as right-sided slipped capital femoral epiphyses. The incidence of left- and right-sided slipped capital femoral epiphyses did not vary significantly between the different ethnic groups or between the genders. Forty children had bilateral slipped capital femoral epiphyses at the time of first presentation and 171 had unilateral slipped capital femoral epiphyses. However, two years after initial presentation, a further 56 children with unilateral slipped capital femoral epiphyses had been readmitted for pinning of the opposite hip, leading to an overall 45.5% bilaterality rate in the short term. A higher percentage of NZ Maori and Pacific children presented with bilateral slipped capital femoral epiphyses at their first presentation than did NZ European children. However, more European children presented with sequential slipped capital femoral epiphyses, leading to a similar bilaterality rate at two years. The rate of presentation with a second slipped capital femoral epiphysis showed an exponential decay (Figure 3), with a t1/2 of 5.75 months (r2 = 0.99). Of all second slipped capital femoral epiphyses, 77% presented within 12 months of the first slipped capital femoral epiphysis.
Figure 3. Time between the diagnosis of the first and second slipped capital femoral epiphyses for the 56 bilateral slipped capital femoral epiphyses with a sequential presentation. The number of hips that became bilateral every six months (mo) after the diagnosis of the first hip follows an exponential decay pattern: number of hips = 62.53.e-0.12 x mo (r2 = 0.99).
Relative racial frequency All but 27 of the 211 children admitted to Starship Children’s Hospital with a slipped capital femoral epiphysis during 1988 to 2000 came from the north, central and west urban zones in Auckland, and were aged between 60 and 179 months. The ethnicity data for these 184 children and the 1991 and 1996 Census data for ethnicity of children of the same age from the same parts of Auckland (Table 2) were used to calculate the relative racial frequency of slipped capital femoral epiphysis.
Table 2. Census figures for 1991 and 1996: children aged 5–14 years in north, central and west urban zones of Auckland
During the study period, the percentage of children of Maori (14%) or Pacific (14.5%) ethnicity remained the same in the population at risk. The relative percentage of NZ European decreased from 64% to 57% largely due to an increase in children from other racial groups, predominantly Asian. Based on these figures, if the presence of slipped capital femoral epiphysis was equally distributed between all ethnicities, we would expect that, over the period 1988 to 2000, approximately 60% of the slipped capital femoral epiphyses would occur in NZ Europeans, 14% in NZ Maori and 14.5% in Pacific children. However, children identified as NZ European made up only 29% of all admissions for slipped capital femoral epiphysis, while 40% of all children admitted for slipped capital femoral epiphysis had Pacific Island ethnicity. Based on the relative population ratios, children of NZ Maori ethnicity were 4.2 times more likely to be admitted to Starship Children’s Hospital with a slipped capital femoral epiphysis than NZ European children in the same age group. The relative racial frequency was higher in Pacific children, who were 5.6 times more likely to be admitted to Starship Children’s Hospital with a slipped capital femoral epiphysis than NZ European children in the same age group (Figure 4).
Figure 4. Relative percentages of NZ European, NZ Maori and Pacific children, aged 60 to 179 months, in north, central and west urban zones of Auckland (based on 1991 and 1996 Census data) compared with relative percentages of children of the same ethnicity, age and residence who were admitted with a slipped capital femoral epiphysis (SCFE) to Starship Children’s Hospital from 1988 to 2000
Relative racial frequencies are a way to estimate the frequency of a condition in a subgroup of the population when the exact denominator, the population numbers, cannot be defined with precision. Our data suggest that NZ Maori and Pacific children have a frequency of slipped capital femoral epiphysis that is 3 to 5 times greater than that of NZ European children. These figures can only be an estimate, due to difficulties in interpretation of self-identified ethnicity and also in determining the ethnic characteristics of the local population. However, the results do support the anecdotal impression of an increased prevalence of slipped capital femoral epiphysis in the NZ Maori and Pacific population.2
The predisposition of NZ Maori and Pacific children to slipped capital femoral epiphysis may be related to the tendency of such children to be larger and physically more developed than their NZ European counterparts at the same age, placing greater stresses on a susceptible growth plate. In New Zealand, the mean body weight and height of Pacific Island children aged 5–12 years is close to the 95 percentile of the National Center for Health Statistics (USA) standards for height and weight.7 Both males and females reach their maximum height one to two years earlier than the USA standard population, a pattern typical of early maturation. As this study is retrospective, we could not determine the body mass index8 or bone age for individual children with a slipped capital femoral epiphysis. Collection of such data prospectively could help determine whether the ethnic variation in prevalence of slipped capital femoral epiphysis found in this study is a reflection of differences in obesity or different patterns of maturation in the three ethnic groups.
The age at first presentation with a slipped capital femoral epiphysis was 11.5 years in females and 12.5 years in males. These figures are one year lower than those reported for the American children of European descent in the study by Loder.2 At Starship Children’s Hospital, we are now seeing slipped capital femoral epiphyses in children as young as eight or nine years of age without any underlying endocrine problem. Body weight has been inversely correlated to the age at presentation with a slipped capital femoral epiphysis2 and may be a contributor to the earlier presentation in our patient population. Obesity in childhood is increasing in Auckland and is a greater problem in NZ Maori and Pacific Island children than NZ European children.8 The relationship between increasing childhood obesity and the incidence of slipped capital femoral epiphysis in New Zealand is not known but must be of concern.
The gender ratio of slipped capital femoral epiphysis in the Auckland population is close to 1:1, with a slight male predominance. The male predominance in slipped capital femoral epiphysis has decreased over the twentieth century. Data from Sweden show that at the turn of the twentieth century, 88% of slipped capital femoral epiphyses were in males.1 However, by the 1970–1980s, the percentage of males had decreased to only 60%.1 In the study by Loder,2 the male predominance persists in Indo-Mediterranean children and similar data have been reported in Japan, where slipped capital femoral epiphysis is very rare.4 However, in other populations, the ratio is closer to 1:1 male to female.5,9,10
The overall short-term incidence of bilateral slipped capital femoral epiphysis in our population was higher than that reported previously and did not vary across the racial groups or with gender. Half of all second slipped capital femoral epiphyses had occurred by six months, with the earliest presentation being within one month of the previous surgery. Thus, there must be a high suspicion of a second slipped capital femoral epiphysis when a child with a unilateral slipped capital femoral epiphysis presents with pain in the opposite hip within a few months of hip pinning. Although children of Maori or Pacific descent in our study had the same incidence of bilaterality as NZ Europeans, they were more likely to present with bilateral slipped capital femoral epiphyses and less likely to develop sequential slipped capital femoral epiphyses. Only a small percentage of NZ Europeans presented with bilateral hip involvement but more had a sequential slipped capital femoral epiphysis. The explanation for this difference is not clear from this retrospective study but may reflect differences in access to healthcare or in parental awareness of the condition.
Author information: N Susan Stott, Associate Professor of Paediatric Orthopaedic Surgery, Discipline of Orthopaedics, Faculty of Medicine and Health Sciences, University of Auckland, Auckland; Terri A Bidwell, Senior Orthopaedic Registrar, Starship Children’s Hospital, Auckland
Acknowledgements: We thank Joy O’Connell (Statistics New Zealand analyst) who provided the census data for the different ethnic groups in the age range 5 to 14 years in north, central and west urban zones of Auckland.
Correspondence: Associate Professor N S Stott, Discipline of Orthopaedics, Faculty of Medical and Health Sciences, University of Auckland, Private Bag 92019, Auckland. Fax: (09) 367 7159; email: email@example.com
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