Adolescents with Low Body Mass Index Are More Vulnerable to Pediatric Nondisplaced Femoral Neck Fractures

Abstract

Background

Femoral neck fracture is a rare condition among fractures in the pediatric population. However, its potential for grave complications such as avascular necrosis or severe limb length discrepancy warrants prompt diagnosis and management. Much effort has been made to assess fracture risk in young adolescents in relation to obesity, low vitamin D levels, or young age osteoporosis. However, to our knowledge, there has been no literature that highlights a higher incidence of femoral neck fracture in adolescents with low body mass index (BMI).

Methods

At a level I trauma center, 22 early adolescents aged 10 years and older who had femoral neck fractures and underwent surgical treatment were included in the study. At the time of injury, BMI of each patient was stratified into 5 categories (underweight, normal weight, overweight, moderate obesity, and extreme obesity) based on BMI for age. Underweight was defined as a BMI below the 5th percentile, normal weight as between the 5th and 85th percentile, overweight as between the 85th and 95th percentile, and obese as above the 95th percentile. Then the patients were divided into 2 groups according to trauma degree: high-energy trauma and low-energy trauma. Low-energy fractures were defined as those caused by all types of trauma except for accidents involving motor vehicles, bicycles, or ski and all falls from greater than standing height. Independent samples t-tests and Pearson’s chi-square tests were conducted between the 2 trauma groups.

Results

Excluding 2 patients lost to follow-up, 4 of 13 patients (30.77%) in the low-energy fracture group were underweight, whereas none in the high-energy fracture group were underweight. Including valgus impacted femoral neck fractures, 7 femoral neck fractures were nondisplaced, while 13 were complicated with displacement and required closed reduction. Avascular necrosis was observed in 4 cases and limb length discrepancy in 3 cases. The mean BMI percentile differed statistically significantly between the 2 trauma groups (p < 0.05).

Conclusions

Low-energy femoral neck fractures in adolescents appeared to be associated with low BMI. Future studies are required to clarify the relationship between low BMI and fracture risk.

Femoral neck fracture is a rare condition among pediatric fractures. According to Renni et al.,¹⁾ fractures of the proximal femur account for 0.1% of all pediatric fractures, and the average age at the time of injury is 10.3 years. However, its potential for grave complications such as avascular necrosis (AVN) or physeal growth arrest leading to severe limb length discrepancy warrants prompt diagnosis and management.²,³⁾ Multiple studies have assessed fracture risk in pediatric populations and suggested several risk factors including low vitamin D levels, young age osteoporosis, and obesity.⁴,⁵,⁶⁾ Obesity, in particular, has been highly scrutinized and proven to be a risk factor for various pediatric conditions such as Legg-Calvé-Perthes disease, slipped capital femoral epiphysis, Blount’s disease, genu varum deformity, and lower extremity fractures.⁴⁾ However, the fracture risk in underweight children, especially in cases of femoral neck fracture, has received relatively little attention. To our knowledge, investigations into the causes of femoral neck fractures resulting from low-energy trauma are absent in the literature. This study aims to determine whether pediatric femoral neck fractures caused by low-energy trauma are associated with low body mass index (BMI) or underweight status in adolescents.

METHODS

The study was conducted with Institutional Review Board approval (Ajou University Medical Center; IRB No. IRB-DB-2024-192), and the requirement for informed consent was waived given the retrospective design and minimal risk to participants.

This study was conducted at a level I trauma center. All children and adolescents aged 10–19 years⁵⁾ who had a traumatic femur neck fracture and visited the institution between 2011 to 2023 were recruited. Data for patients who received surgical treatment, either by screw fixation or sliding hip screw, were collected (n = 22). Patients who failed to follow up until radiographic union was achieved (n = 2) were excluded, and demographic data including predisposing conditions that might affect bone quality such as osteogenesis imperfecta, mucopolysaccharidosis, or other metabolic bone diseases were confirmed.

At the time of injury when patients were enrolled in the emergency department, data including age, height, and weight were collected. Based on this information, BMI was calculated and categorized according to the Center for Disease Control protocol.⁷⁾ BMI percentile was recorded for each patient, and patients were divided into 4 groups based on their percentile: underweight, normal weight, overweight, and obese. Underweight was defined as a BMI below the 5th percentile, normal weight as between the 5th and 85th percentile, overweight as between the 85th percentile and 95th percentile, and obese as above the 95th percentile.

Charts and radiographs were reviewed to determine demographic information, mechanism of injury, fracture pattern, and complications. All patients were divided into 2 groups: low-energy fractures and high-energy fractures. Low-energy fractures were defined as those caused by all types of trauma except for accidents involving motor vehicles, bicycles, and ski and all falls from more than standing height.⁸,⁹,¹⁰,¹¹,¹²⁾ Fracture patterns were classified according to the Delbet and AO systems. Surgical treatment was conducted by 2 pediatric surgeons at our institution (JHC and THK). The surgical procedure involved either screw fixation or a sliding hip screw (Synthes femur neck system), and the number of screws used for stable fixation was determined by the surgeon for each case (Fig. 1). Surgical complications, such as painful implants, nonunion, malunion, AVN, and physeal growth arrest, were closely monitored and followed.

Fig. 1. A 13-year-old female adolescent presented to the emergency department with right hip pain. (A) Preoperative x-ray of the right proximal femur. (B) Coronal view of a preoperative computed tomography scan, showing no definite femoral neck fracture line or displacement. (C) Preoperative T2-weighted magnetic resonance imaging showing high signal intensity in the femoral neck area. (D) Postoperative x-ray showing fixation with two 7.0-mm cannulated screws.

All statistical analysis was carried out using IBM SPSS Statistics for Windows version 25.0 (IBM Corp.). The Kolmogorov-Smirnov normality test was used to determine whether the patients’ BMI percentile was normally distributed. An independent samples t-test was conducted to compare low-energy fracture and high-energy fracture groups. Pearson’s chi-square test was also performed according to each BMI class (underweight, normal, overweight, obese). A p-value of less than 0.05 was considered statistically significant.

RESULTS

There were 20 patients who met the inclusion criteria, including 9 male patients (45%) and 11 female patients (55%). Mean age at the time of injury was 11.5 years (range, 8–14; standard deviation, 1.73). Common mechanisms of injury were slips (11 / 20, 55%), motor vehicle accidents (4, 20%), and falls from height (2, 10%). In all, 4 (20%) were underweight, 14 (70%) were normal weight, and 2 (10%) were overweight. One patient had a history of mental retardation, but his BMI was within the normal range. No patient had a predisposing condition that might affect bone quality or increase fracture risk. Fracture types were classified using the AO classification system: 4 were 31B1.1 type fractures, 3 were 31B1.2 type fractures, 6 were 31B2.1 type fractures, 2 were 31B2.3 type fractures, and 5 were 31B3 type fractures.

The demographic data of patients and clinical characteristics of fractures are presented in Table 1. Contingency table for each weight class is shown in Table 2. Statistically significant difference between the 2 energy groups in age, sex, union time, BMI, or displacement was not found. Most fractures occurred in the Delbet type II transcervical region, and only 2 cases in the low-energy fracture group had Delbet type III basicervical fractures. Location of the fracture showed no statistical difference. Four patients developed AVN during follow-up and 3 patients experienced limb length discrepancy due to physeal growth arrest. Two cases of AVN occurred after low-energy trauma, and the other 2 cases of AVN followed high-energy trauma. There were 13 patients who suffered displaced fractures, and 7 patients had non-displaced fractures. Valgus impaction type fractures were included in the non-displaced category as minimal displacement occurs with this fracture type. Regarding the 2 main surgical modalities used, 18 patients received screw fixation after closed reduction on a fracture table, while 2 patients received a sliding hip screw.

Table 1. Patients’ Demographic Data and Clinical Characteristics of Fractures in the 2 Groups.

Variable		Low-energy fracture (n = 13)	High-energy fracture (n = 7)	p-value
Mean age (yr)		11.62 ± 0.49	11.29 ± 0.68	0.69
Sex				0.92
	Girl	9 / 13 (69.2)	2 / 7 (28.6)
	Boy	4 / 13 (30.8)	5 / 7 (71.4)
Union time (mo)		4.00 ± 0.48	5.57 ± 0.87	0.10
Delbet classification
	I	0	0	0.27
	II	11 / 13 (84.62)	7 / 7 (100)
	III	2 / 13 (15.38)	0
	IV	0	0
AO classification
	31B1.1	4 / 12 (33.33)	0	0.42
	31B1.2	2 / 12 (16.67)	1 / 8 (12.50)
	31B2.1	3 / 12 (25.00)	3 / 8 (37.50)
	31B2.3	1 / 12 (8.33)	1 / 8 (12.50)
	31B3	2 / 12 (16.67)	3 / 8 (37.50)
Follow-up (mo)		27.62 ± 10.24	30.28 ± 17.84	0.89
BMI classification
	Underweight	4 / 13 (30.77)	0	0.26
	Normal weight	8 / 13 (61.54)	6 / 7 (85.71)
	Overweight	1 / 13 (7.69)	1 / 7 (14.29)
	Obese	0	0
Displacement
	Nondisplaced	9 / 13 (69.23)	3 / 7 (42.86)	0.27
	Displaced	4 / 13 (30.77)	4 / 7 (57.14)

Values are presented as mean ± standard deviation or number (%).

BMI: body mass index.

Table 2. Number of Patients According to BMI Percentile Class in the 2 Groups.

BMI percentile	Low-energy trauma	High-energy trauma
Underweight (< 5th)	4	0
Normal weight (5th–85th)	8	6
Overweight (85th–95th)	1	1
Obese (> 95th)	0	0
Total	13	7

BMI: body mass index.

p = 0.255.

Pediatric populations typically utilize BMI percentile rather than BMI itself, as age and growth must be comprised in the calculation. We performed a normality test (Kolmogorov-Smirnov) and found that our BMI percentile date (n = 20) were normally distributed (p = 0.154). An independent samples t-test was done between BMI percentile of the low-energy and high-energy fracture groups. Average BMI percentile for the low-energy fracture group was 27.08 (± 7.97) while that of the high-energy fracture group was 55.71 (± 9.12). The difference between the 2 groups was statistically significant (p = 0.038). Pearson chi-square test or linear by linear association for different weight classes (underweight, normal weight, overweight, obese) was statistically insignificant.

DISCUSSION

To our knowledge, this is the first study to examine the association between lower BMI percentiles and femoral neck fracture risk in a pediatric population, a contrast to the more commonly discussed role of increased BMI or obesity. While our data suggest that the low-energy fracture group exhibited a lower average BMI percentile than the high-energy fracture group, the Pearson chi-square test did not reach statistical significance. Additionally, the proportion of underweight patients in the low-energy fracture group (4 / 13, 31%) compared to none in the high-energy fracture group reflects a notable trend rather than a definitive conclusion. It is important to interpret these findings cautiously and acknowledge that the small sample size may have limited statistical power. Larger-scale studies are needed to confirm whether a meaningful association between low BMI percentile and increased fracture risk exists.

In 2014, Sabhaney et al.¹³⁾ reported that underweight children exhibited the highest odds ratio for fractures (odds ratio [OR], 1.44; 95% CI, 1.00–2.07) when compared to their normal-weight peers. In addition, Kessler et al.¹⁴⁾ conducted a study investigating the relevance of childhood obesity to lower extremity fracture risk. In the study, they divided the fracture group into 3 categories: foot fractures in group A, ankle, leg, and knee fractures in group B, and hip to the distal femur fractures in group C. The study revealed that increasing BMI was associated with increased odds of foot, ankle, leg, and knee fractures in children (group A/B). However, in group C, fracture risk in underweight patients was 66% higher than in normal-weight patients (OR, 1.66; 95% CI, 1.06–2.59).¹³⁾ These findings align with our study, suggesting that adolescents with low BMI percentiles may have a stronger association with proximal femur fractures caused by low-energy trauma. Although the authors of the study attributed this result to the fact that adolescents tend to participate more in sports, resulting in high-energy trauma, we believe that there may be distinct characteristics that these low BMI adolescents might possess other than increased exposure to sports. Low muscle mass or fat mass to absorb shock during traumatic events, low activity level, low socioeconomic status, malnutrition, low vitamin D level, and low bone mineral density (BMD) can all be possible causes of low-energy trauma femoral neck fracture cases.

As patients were enrolled in our study after traumatic event, it was unfeasible to collect previous vitamin D levels or pre-fracture BMD. This might have contributed as confounding factors to our result regarding the association between low BMI and femoral neck fracture. However, the relationship between BMI and bone health remains multifaceted and contentious. A systematic review by Misra et al.¹⁵⁾ supported the notion that low BMI in anorexia nervosa compromises bone health during adolescence, with key factors including reduced lean mass, hormonal imbalances, and deficient IGF-1. Golden and Abrams¹⁶⁾ also outlined the critical role of nutritional status, including BMI, in maintaining pediatric bone health, emphasizing that low BMI significantly compromises bone strength during key growth phases. On the other hand, Franzoni et al.¹⁷⁾ conducted a prospective study of 79 adolescents with anorexia nervosa, demonstrating that even after a sustained 1-year increase in BMI, there were no improvements observed in lumbar BMD nor BMD Z-score. Assaf et al.¹⁸⁾ conducted a population-based control study which showed correlation between low-energy fracture and low vitamin D level. They showed vitamin D deficiency (< 20 ng/mL) increased low-energy fracture risk by almost 5 times. However, Gou et al.⁵⁾ revealed that femoral neck BMD and nutritional factors in the pediatric age group did not show a significant association. Collectively, these findings indicate that BMI, BMD, and vitamin D status should not be treated as singularly predictive, but rather as inter-related factors requiring further research to unravel their independent and collective roles in pediatric bone fragility.

There are some apparent limitations to this study. First, as femur neck fractures in the pediatric population are very rare, the number of patients is relatively small. A multi-center study with a larger patient cohort would better clarify the relationship between low BMI and femoral neck fracture risk. Second, this is a retrospective analysis of prospectively collected patient data. Due to the nature of the injury, it was not feasible to carry out a prospectively designed study for femoral neck fracture. Third, although low BMI adolescents showed some correlation, confounding factors such as low vitamin D levels or pediatric osteoporosis, which were not included in this study, might be present. As malnutrition, low socioeconomic status, low vitamin D levels, and low BMD scores are all closely interconnected, future studies are required to fully understand the relationship between fracture risk of any type and low BMI adolescents. Furthermore, the study’s analysis assumes that each low energy/high energy population cohort has similar proportions of BMI percentile groups. However, different proportions within each group may exist, and this disproportionate exposure to low-energy trauma among underweight children undermines our emphasis on low BMI percentile as a potential risk factor for proximal femoral fracture in early adolescents.

Femoral neck fracture is a rare condition in the pediatric age group, but it is very important due to its potential for serious complications. Our study shows a statistically significant association between low BMI percentile and femoral neck fractures resulting from low-energy trauma. Therefore, when adolescents with low BMI experience low-energy trauma to the hip region, closer examination for nondisplaced femoral neck fractures is warranted.