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| Journal of the New Zealand Medical Association,
24-October-2003, Vol 116 No 1184
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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.
MethodsAdmission 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
ResultsThere 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
 DiscussionRelative 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: s.stott@auckland.ac.nz
References:
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