The Epidemiology of Slipped Capital Femoral Epiphysis in American Samoa

Abstract

The Maori of New Zealand have been identified as a high-risk population for slipped capital femoral epiphysis (SCFE). This study assessed whether the burden of disease from SCFE in the American Territory of American Samoa is similar to that identified in the Maori. This was a retrospective review of children from American Samoa treated for SCFE at a tertiary care pediatric hospital between 2005–2014. Demographic, clinical, and radiographic information was collected. All patients were followed for at least one year after surgery and prophylactically pinned hips were not included. Data for determining incidence was obtained from the United States Census Bureau. Between 2005–2014, 55 American Samoan youth were treated for 73 SCFE. The incidence in the “at-risk” population 5–14 years of age was 53.1 per 100,000. Patients had a mean BMI of 29.5 (19.4–46.4) and mean weight of 76.7 kg (45.9–139 kg). Southwick angle was a mean 40.6°+/− 20.4° (6–83°). Overall, 82.2% of hips were treated with in-situ cannulated screw fixation. At a minimum one-year after initial surgery, 22 hips (30.1%) required major surgery including intertrochanteric osteotomy, osteochondroplasty, or total hip replacement. The incidence of SCFE in American Samoa is extremely high, 53.1 per 100,000 of “at-risk” population 5–14 years old. The mean weight and BMI in SCFE patients from American Samoa is substantially higher than previously published reports. Furthermore, morbidity from SCFE in this population is substantial with 30.1% requiring major surgery either at the time of physeal stabilization or in subsequent years.

Introduction

Slipped capital femoral epiphysis (SCFE) is a pediatric hip disorder characterized by displacement through the proximal femoral physis leading to variable degrees of deformity, pain and dysfunction. Untreated SCFE can lead to limitations in motion, early degenerative hip arthritis, and increased need for hip arthroplasty. Risk factors for SCFE are described as multi-factorial. Weight, race, mechanical stress based on osseous anatomy, and endocrine factors are all thought to be contributors.^1–8 Among these factors, increased body mass is the most common finding in children with SCFE and its importance is well established.^2,5,7,8 Other risk factors can often be traced back to increased weight: hormonal changes associated with obesity may weaken the physis;⁹ reduced femoral anteversion, pre-disposing to SCFE, is more prevalent in obese adolescents;¹⁰ and, the simple fact that the mechanical forces across the femoral head with gait are 6.5 times body weight.³

The incidence of SCFE varies between different racial groups, with the Maori from New Zealand demonstrating the highest incidence.^1,11,12 American Samoa is an American Territory consisting of a group of seven islands located 2,600 miles southeast of Hawai‘i and 1,800 miles northeast of New Zealand. The population in 2010 was 55,636.¹³ Greater than 90% of the population of these islands identify as Polynesian in US Census Bureau statistics.¹³ Given the documented high incidence in the Maori, the incidence in a predominantly Samaon population could be expected to be high. Limited access to healthcare and necessary intra-operative imaging in peripheral hospitals has led to all children diagnosed with SCFE in American Samoa being treated at a tertiary referral hospital in Honolulu, Hawai‘i for several decades.

The objective of our study was to determine the incidence of SCFE in American Samoa and compare to previously published data in other populations. Given the long-term disability due to hip dysfunction and arthritis from untreated SCFE, or, SCFE not treated in an expedient fashion, a high incidence of this disorder may warrant increased screening in the susceptible adolescent population.

Materials and Methods

A retrospective chart and radiographic review was conducted of all patients from American Samoa treated for SCFE at a tertiary pediatric orthopaedic hospital in Honolulu, Hawai‘i over a ten-year period between 2005–2014. Due to the fact that resources were not available for treatment of SCFE in American Samoa, diagnosis was made by orthopaedic surgeons in American Samoa and patients were transported to Hawai‘i for surgical treatment. Charts were reviewed one year after the last surgical case included. Institutional review board approval was obtained from the University of Hawai‘i.

In determining incidence and demographic details, if bilateral SCFE were treated asynchronously, only the first side was included. Thus, if a patient presented with a right sided SCFE, but one year later had a left-sided SCFE, only the right side was counted to determine incidence. Parameters assessed in this manner included age, gender, height, weight, body mass index (BMI), medical comorbidities, duration of antecedent symptoms, time between diagnosis and surgery, and family history of SCFE. Radiographic measurements of skeletal maturity were assessed by assessing the Risser status, the revised modified Oxford Bone Score, and status of the triradiate cartilage (open versus closed). Risser status assesses ossification of the iliac apophysis on the pelvis and ranges from 0 to 5. Risser 1 occurs at a mean age of 13.5 years in girls and 15.5 years in boys, at a point when two-thirds of puberty has passed. The revised modified Oxford Bone Score is another measurement of skeletal maturity, based on an AP pelvic x-ray designed to predict the risk of SCFE on the opposite hip when children present with a unilateral SCFE. A score of 0–2 corresponds to lesser skeletal maturity and a high risk of another SCFE on the opposite hip. In bilateral cases, both hips were assessed independently for laterality, stability, Southwick angle (SA),¹⁴ slip severity, type of fixation, and further surgery. Southwick angle is the most common measurement of hip deformity in SCFE. Mild deformity is defined as a SA < 30°, moderate deformity SA is 30–50° and in severe deformity the SA is > 50°. Further surgery was subdivided into minor (including irrigation and debridement, screw revision, screw removal) and major. Prophylactically pinned hips were not included.

Data to calculate incidence was obtained from the United States Census Bureau.¹³ Year-by-year data was available with the population of American Samoa split into 5-year age groups: 0–4 years, 5–9 years, 10–14 years, and 15–19 years. Given the heterogeneity of the calculation of incidence in previous studies, incidence was calculated for the following groups: 5–14 years or the “at-risk” population as defined by Phadnis, et al,¹¹ 5–19 years, and total population to allow comparison.

Statistical analyses included descriptive statistics, and chi-square or independent sample t-tests to assess gender differences for each study parameter as described above using SAS 9.3 (SAS Institute Inc, 2011–2012).

Results

Patient Population

Between 2005–2014, 55 American Samoan patients were treated for 72 SCFE. All patients were Samoan. Demographic details of the study sample are presented in Table 1. Thirty-seven patients were male (67.3%) and 18 were female (32.7%). Mean age at treatment was 12.3 years (+/− 2.2, 7.3–18.1). BMI was a mean of 29.5 (19.4–46.4) and mean weight was 76.7 kg (45.9–139 kg). The left and right hip were equally involved: 37 left, 35 right. Bilateral SCFE, either on presentation, or at a later date, were present in 30.9% of patients (n=17). Four hips were unstable (5.5%) and two patients had medical conditions predisposing to SCFE - panhypopituitarism and a pituitary tumor (data not shown). Seven children had a first degree relative with a history of SCFE requiring treatment. Weight percentiles of female and male patients were based on World Health Organization data and categorized into the following percentiles: greater than 99th (n=25, 45.5%), 97th-99th (n=17, 30.9%), 90th-97th (n=6, 10.9%), 75th-90th (n=4, 7.3%), and less than 75th (n=3, 5.5%) (Figure 1).

Table 1.

Demographic Information of Study Sample

Demographics (n=55)
Male / Female (n, %)	37 (67.3%) / 18 (32.7%)
Mean Age (years), range	12.3 years (+/−2.2, range 7.3–18.1)
Mean Body Mass Index (kg/m2), range	29.5 (19.4–46.4) kg/m2
Mean Weight (kg), range	76.7 (45.9–139) kg
Yes, family history of SCFE	7 (55%)
Slipped capital femoral epiphysis (n=73)
Unilateral / Bilateral	55 (76.4%) / 17 (23.6%)
Right / Left	37 / 35

SCFE, slipped capital femoral epiphysis

Figure 1.

Patients' Weights by Centiles. Percentile weight of American Samoan slipped capital femoral epiphysis using World Health Organization, age-specific, normative values.

Twenty-five patients described a history of acute SCFE (45.5%, defined as antecedent symptoms of three weeks or less) and mean duration of antecedent symptoms, prior to radiographic diagnosis, was 43 days (0–60 days) (data not shown). There were four unstable slips, one of which proceeded to avascular necrosis. The remaining, stable patients were given crutches at the referral hospital and advised to limit weight-bearing until transport on a commercial flight could be arranged for treatment at our facility. Radiographic indicators of skeletal maturity derived from pelvic X-rays at the time of treatment are presented in Table 2. Risser status was 0 in 87.3% (48/55) of patients, and 32.7% of patients had an open triradiate cartilage (18/55). Revised modified Oxford Bone Score was 0–2 in 45.5% (25/55) and 3–10 in the remaining 54.5%.

Table 2.

Radiographic Markers of Skeletal Maturity

(N=55)	Radiographic indicators associated with younger skeletal age	Radiographic indicators associated with greater skeletal age
Risser Status (0 vs 1–5)	Risser 0: 87.3% (48/55)	Risser 1–5: 12.7% (7/55)
Tri-radiate cartilage status (open vs closed)	Triradiate Open: 32.7% (18/55)	Triradiate Closed: 67.3% (37/55)
Revised Modified Oxford Bone (RMOBS) Score (0–2 vs 3–10)	RMOBS 0–2: 45.5% (25/55)	RMOBS 3–10: 54.5% (30/55)

Radiographic Deformity and Treatment

Mean Southwick angle was 40.6° (+/− 20.4°, 6–83°) with the distribution of mild, moderate and severe slips being relatively equal. Twenty-six SCFE were mild (SA < 30°, 36.1%), twenty-four were moderate (SA 31–50°, 33.3%) and twenty-two were severe (SA > 50°, 30.1%) (Table 3). Fifty-nine of seventy-two hips (81.9%) were treated with either a single (n=56) or multiple (n=3) cannulated screw(s). Due to severity of deformity, the initial surgical procedure included in-situ cannulated screw fixation with simultaneous intertrochanteric osteotomy in eleven hips (15.3%), intertrochanteric osteotomy alone in one hip presenting late (1.4%), and modified Dunn sub-capital realignment in one hip (1.4%) (Table 4).

Table 3.

Radiographic Measurement of Deformity in Hips Diagnosed with SCFE

Deformity-Southwick Angle - SA	Number (%)	Mean Southwick Angle (range)
Mild (SA < 30°)	26 (36.1%)	19.9° (6–30°)
Moderate (SA 31–50°)	24 (33.3%)	44.7° (31–50°)
Severe (SA > 50°)	22 (30.6%)	65.9° (52–83°)
Overall	72 (100%)	40.6° (6–83°)

Table 4.

Treatment of Slipped Capital Femoral Epiphysis

Treatment	Number (%)
Single Cannulated Screw	56 (81.9%)
Multiple Cannulated Screws	3 (4.2%)
Cannulated Screw(s) and Intertrochanteric Osteotomy	11 (15.3%)
Intertrochanteric Osteotomy	1 (1.4%)
Modified Dunn Sub-Capital Realignment	1 (1.4%)
72 (100%)

Minor additional future surgery was performed in five patients including screw removal (3 patients), hematoma irrigation, and revision screw fixation for failure of physeal closure. Major future surgery was performed in ten patients including intertrochanteric osteotomy (7), osteochondroplasty (1) and total hip replacement (2). Avascular necrosis occurred in two patients, one stable, one unstable, both of whom underwent total hip replacement. Combined with those who underwent intertochanteric osteotomy primarily at the time of physeal stabilization, 22 of 73 (30.1%) hips required significant surgery.

Incidence

The incidence of SCFE in the “at-risk” population, 5–14 years old, between 2005–2014 was 53.1 per 100,000. Among 5–19 year olds, the incidence was 33.6 per 100,000. Incidence calculated for the total sample was 10.1 per 100,000. Figure 2 demonstrates changes of incidence over the study period. The “at-risk” population, 5–14 years of age demonstrated a trend towards increasing incidence over the study period, but there were outlier years (2008, 2012) in which the annual incidence was substantially lower.

Figure 2.

SCFE Incidence for Age Groups. Annual incidence of slipped capital femoral epiphysis per 100,000 population aged 0 to 19 years old (▲), 5 to 19 years old (■), 5 to 14 years old (◆) (“at risk” group).

Discussion

The incidence of SCFE in American Samoan children aged 5–14 is extremely high, 53.1 per 100,000. Based on self-reported US Census Bureua data, the population of American Samoa is 92.6% Polynesian, 95.2% at least partially Polynesian, with 88.9% of individuals specifically self-identifying as Samoan.¹³ This data adds specific information regarding the incidence of SCFE in Samoan children and also to the limited reports regarding Polynesian populations in general, a group that is growing rapidly in the United States.

Few studies have reported an increased risk of SCFE in Polynesians, previously exclusively in the Maori of New Zealand. Loder coordinated a worldwide, multi-collaborator study, in which he determined the relative racial frequency of SCFE in Polynesians to be 4.5 times that of the Caucasian population.¹ However, in this study of 1615 children in whom race was known, the group characterized as Polynesian/Native Australian consisted of only 28 Maori and 6 Australian aboriginal children, the latter of which is a distinct group from Polynesians. Stott and Bidwell followed up Loder's findings, reporting on the children treated for SCFE at Starship Children's Hospital in New Zealand between 1988–2000.¹² Compared to Caucasian children, children of Maori descent were 4.2 times, and those of other Pacific heritage 5.6 times, more likely to be admitted for SCFE.

Phadnis', et al, described the highest incidence of SCFE yet reported, in a New Zealand health district with a 22% Maori population.¹¹ Incidence in the Maori population among children 5–14 years of age was 81 per 100,000 compared to 11.3 per 100,000 in Caucasian children, or a relative risk of 7.17. In Maori males, incidence was 94.9 per 100,000. This is the only study describing a group with an incidence of SCFE higher than what is described in the present study among American Samoans.

Polynesians, particularly Samoans and Native Hawaiians, are a large and rapidly growing group in the United States. The 2010 US census shows 1.2 million individuals who self-identify as Native Hawaiian/Pacific Islander alone, or, in combination.¹⁵ This group grew by 40% as compared to the 2000 census, a rate four times that of the general US population over the same span.

While risk factors for SCFE are described as multi-factorial, weight is thought to be the most consistent contributor.^2,5.⁷.⁸ In the adult American Samoan population, 74.6% are classified as obese (BMI ≥30),¹⁶ while 84% of Samoan males living in the continental United States are obese¹⁷ as compared to 36% of the US adult male population as a whole.¹⁸ Forty-four percent of American Samoan kindergarten children were identified as overweight/obese (BMI > 25), progressing to 71.3% by the 11th grade.¹⁹ With a mean weight of 76.5 kg and BMI of 29.5, our cohort of SCFE patients is fully 10 kg heavier than previously published SCFE studies, and the BMI is 3.5 kg/m² greater.^1,11,20 More than three quarters of this group was greater than the 97th percentile body weight for age.

While our cohort was substantially heavier than those of previously published reports, our patients were relatively typical in terms of their age, male-to-female ratio, and degree of slip.^{1,2,7,8,11,12,20–24} Overall, 5.5% of slips were unstable, consistent with other series, one of which developed AVN (25%).^25–27 Interestingly, the external rotation deformity created by SCFE is culturally well-accommodated in Samoa by traditional practices of sitting cross-legged on the floor as it makes this sitting position easier.

Despite our hospital being the sole referral center for children with SCFE from American Samoa, our findings likely underrepresent the incidence of SCFE in this region. Though an American Territory, some cases likely go untreated and American Samoa continues to have difficulties with access to care and education surrounding SCFE. Some of these challenges are reflected in the length of time between diagnosis and treatment for our patients. Furthermore, children with parents from the independent nation of Samoa (formerly Western Samoa) may have been unable to easily travel to the United States for treatment due to parental visa restrictions, and may not have had treatment, or, travelled to Samoa for treatment. Delay and difficulty with transportation is reflected in only 82.2% of hips undergoing in-situ cannulated screw fixation, as compared with eleven (15.1%) which underwent a concomitant intertrochanteric osteotomy, and one case in which delay was substantial enough that the physis was closed by the time of arrival. The 30.1% rate of significant surgery, either primarily or subsequently, reflects the morbidity of this condition in this population, and, further emphasizes access to care challenges.

The retrospective nature of our study is a weakness, as is the question of what age groups one uses to calculate incidence. Previous studies' parameters are a reflection both of the age group at risk for SCFE but also the information available to those authors. Loder reported a mean age of 12 +/− 1.5 years for girls and 13.5 +/− 1.7 years for boys¹ and 76.7% of our patients fell within the classic 10–14 years of age. Previous literature variably utilizes 5–14 years¹¹, 9–16 years^20,21 7–16,²² < 25,²³ 7–18²⁴ and other partitions. To facilitate comparison to other studies, we provide incidence utilizing the 5–14 and 5–19 year populations and total population.

Conclusions

Our study demonstrates that the American Samoan population has the second highest incidence of SCFE worldwide, and yet the incidence is likely an underestimate. This rapidly growing demographic group has higher rates of obesity in the continental United States than in American Samoa, and thus great vigilance is recommended when assessing Samoan or any Polynesian youth with lower extremity pain or limp. Current efforts are underway to increase awareness of SCFE in American Samoa and facilitate timely treatment to improve outcomes.

Conflict of Interest

None of the authors identify any conflicts of interest.