Establishing reference values for the pubofemoral distance in normal infant medial hips by ultrasound

Abstract

Background

Measuring the pubofemoral distance (PFD) is important for evaluating the effectiveness of hip reduction in the treatment of developmental dysplasia of the hip (DDH). However, reference PFD values have not been established in normal infants.

Purpose

To investigate variations in PFD values measured in normal infant medial hips stratified by age, laterality, and gender.

Material and Methods

A total of 240 infants diagnosed with Graf type Ia and/or Graf type Ib hips by ultrasonography were stratified into eight age groups: 0–1 month; 1–2 months; 2–3 months; 3–4 months; 4–5 months; 5–6 months; 6–7 months; and 7–12 months. The bilateral medial hips were scanned with transinguinal ultrasound. The PFD was defined as the distance between the lateral edge of the superior ramus of pubic bone and the medial edge of the femoral head. Inter-observer reproducibility was assessed.

Results

Among the 240 infants, there were 371 Graf type Ia hips and 109 Graf type Ib hips. Mean ± SD bilateral PFD values of eight groups were measured separately. There were no significant differences in mean PFD values for left or right hips (t = 0.946, P = 0.345) or mean bilateral PFD values in male and female infants (t = 1.445, P = 0.149). Mean PFD values increased linearly with age (left: r = 0.680, P < 0.0001; right: r = 0.682, P < 0.0001). Inter-observer reproducibility was excellent.

Conclusion

This study established reference PFD values from the medial hip in infants aged 0–12 months. PFD values increased with age, but were not significantly influenced by laterality or gender. These data provide detailed information that can support follow-up of infants treated for DDH.

Keywords

Ultrasonography pubofemoral distance medial hip developmental variation infant

Introduction

Developmental dysplasia of the hip (DDH) is a common condition that encompasses the range of structural abnormalities that involve the growing hip. Early diagnosis and treatment are required to optimize functional outcomes in patients presenting with DDH (1,2).

Treatment for persistent dislocated DDH involves closed reduction and immobilization using a pavlik harness, brace, or a spica plaster. During follow-up, accurate imaging should be used to confirm successful reduction of the hip. Inadequate reduction may allow the femur to impinge on and erode the acetabulum causing pseudoarthrosis of the hip and residual dysplasia.

Evidence suggests that transinguinal ultrasound is efficacious for determining the position of the femoral head in a spica cast after closed or open reduction in children with DDH, and that hip reduction can be monitored by measuring the pubofemoral distance (PFD) from the medial hip, defined as the distance between the femoral head and the pubic bone (3,4). Previous reports have evaluated the utility of the PFD in the coronal and axial planes as a screening test to diagnose DDH and as a tool for evaluating the success of DDH treatment in newborns after treatment with a pavlik harness or a Denis Browne hip brace (5 –8).

Currently, literature describing PFD values in normal infant hips is lacking. These normal values are needed to set limits for determining the success of hip reduction during treatment for DDH and informing clinical decision-making. If reduction of the hip joint is unsuccessful three weeks after open or closed reduction and immobilization, other treatment modalities should be considered (9).

The aim of the present study was to establish a reference range of PFD values in the normal medial hip of infants aged 0–12 months stratified by age, laterality, and gender. Findings should expand the utility of the PFD for monitoring the position of the femoral head during the DDH treatment process.

Material and Methods

This was a retrospective study, which was approved by the institutional review board; therefore, informed consent is not applicable.

Sample

A total of 240 infants diagnosed with Graf type Ia and/or Graf type Ib hips by ultrasonography were eligible for this study. Infants with multiple joint contracture, trauma, severe developmental deformity, birth weight <2 kg, gestational age <36 weeks, or ultrasonography showing that the patient’s hip status was not bilateral Ia and/or Ib according to the Graf classification were excluded.

Included infants were stratified into one-month age groups up to seven months: 0–1 month; 1–2 months; 2–3 months; 3–4 months; 4–5 months; 5–6 months; and 6–7 months. Infants aged >7 months seldom undergo ultrasound examination; therefore, infants aged 7–12 months were included in a single subgroup to avoid bias incurred by a small sample size.

Ultrasonic examination

Ultrasonography was performed by two experienced ultrasound physicians and a pediatric radiologist using a Toshiba Aplio 500 system (Toshiba Medical Systems Corporation, Tokyo, Japan) with a high-frequency linear transducer (L 5–12). First, infants were diagnosed with Graf type Ia and/or Graf type Ib hips. They were then placed in the supine position, both hips were flexed and abducted, and the bilateral medial hips were scanned with transinguinal ultrasound. The ultrasonic transducer was placed horizontally on the bilateral inguinal area, and the inner end of the transducer was moved slightly upward, so that the superior ramus of the pubic bone, the anterior margin of the femoral head, and the femoral neck were in the same horizontal plane. The bilateral superior ramus of the pubic bone, femoral heads and femoral necks were clearly visualized by adjusting parameters such as 2D gain, image depth, and focus position.

Ultrasonic measurement

For each hip, a vertical line was made downward through the lateral aspect of the superior ramus of the pubic bone. The PFD was defined by measuring the shortest distance between this tangent and the medial aspect of the femoral head (Fig. 1.).

Fig. 1.

Measuring pubofemoral distance values from bilateral normal infant medial hips on transinguinal ultrasound (white arrow). D, distance; FH, femoral head; FN, femoral neck; L, left; PS, pubic superioris; R, right.

Statistical analysis

Statistical analyses were conducted using SPSS 13.0 (SPSS Inc., Chicago, IL, USA). Measurements are expressed as mean ± SD and corresponding 95% confidence intervals (CI). The associations between PFD and gender, laterality, and age were evaluated using the independent sample t-test, paired sample t-test, or one-way ANOVA, respectively. Inter-observer reproducibility was assessed by comparing a subset of the measurements made by two of the observers on 30 randomly selected cases in a blinded fashion using Kendall’s W test. A P value < 0.05 was considered statistically significant.

Results

The present study included a total of 240 infants diagnosed with Graf type Ia and/or Graf type Ib hips by ultrasonography, including 112 (46.7%) male infants and 128 (53.3%) female infants aged 1 day–12 months. There were 190 firstborn children, 45 second-born children, and five third-born children. Mothers underwent natural childbirth in 138 cases and Cesarean delivery in 102 cases. Gestational age was in the range of 36–42 weeks. Infant birth weight was in the range of 2–3.9 kg. Physical examination showed that hip abduction was limited in eight infants, slightly limited in 74 infants, and unrestricted in 158 infants. Dermatoglyphic symmetry was found in 222 infants and dermatoglyphic asymmetry was found in 18 infants. The infants had been referred for a variety of reasons, including physical examination after birth (n = 204), dermatoglyphic asymmetry (n = 14), abduction deformity or lower limb length discrepancy (n = 6), congenital muscular torticollis (n = 3), varus or valgus foot (n = 4), malposition of the fetus (n = 4), or family history (n = 5).

Among the 240 infants, there were 371 Graf type Ia hips and 109 Graf type Ib hips. Mean ± SD bilateral PFD values were 0.185 ± 0.018 cm (95% CI = 0.180–0.190) in infants aged 0–1 month, 0.198 ± 0.015 cm (95% CI = 0.194–0.202) in infants aged 1–2 months, 0.219 ± 0.016 cm (95% CI = 0.215–0.224) in infants aged 2–3 months, 0.227 ± 0.012 cm (95% CI = 0.224–0.230) in infants aged 3–4 months, 0.224 ± 0.014 cm (95% CI = 0.221–0.228) in infants aged 4–5 months, 0.237 ± 0.017 cm (95% CI = 0.233–0.242) in infants aged 5–6 months, 0.232 ± 0.013 cm (95% CI = 0.229–0.236) in infants aged 6–7 months, and 0.242 ± 0.017 cm (95% CI = 0.237–0.246) in infants aged 7–12 months.

Mean ± SD (95% CI) PFD values across all ages for the left and right hips were 0.221 ± 0.024 cm and 0.220 ± 0.025 cm, respectively, with no significant differences between sides (t = 0.946, p = 0.345) (Table 1).

Table 1.

PFD values from bilateral normal infant medial hips stratified by age.

Age group (months)		PFD (cm)
Age group (months)	Cases	Right	Left
0–1	30	0.185 ± 0.020 (0.180–0.192)	0.185 ± 0.017 (0.179–0.192)
1–2	30	0.195 ± 0.013 (0.190–0.200)	0.200 ± 0.017 (0.194–0.206)
2–3	30	0.220 ± 0.018 (0.214–0.226)	0.219 ± 0.015 (0.213–0.225)
3–4	30	0.226 ± 0.013 (0.222–0.231)	0.228 ± 0.012 (0.224–0.232)
4–5	30	0.223 ± 0.015 (0.218–0.229)	0.225 ± 0.014 (0.220–0.231)
5–6	30	0.238 ± 0.018 (0.232–0.244)	0.237 ± 0.017 (0.232–0.243)
6–7	30	0.233 ± 0.015 (0.227–0.238)	0.232 ± 0.012 (0.228–0.236)
7–12	30	0.242 ± 0.015 (0.237–0.247)	0.242 ± 0.019 (0.235–0.249)

Values are given as mean ± SD (95% CI).

CI, confidence interval; PFD, pubofemoral distance.

Mean ± SD (95% CI) bilateral PFD across all ages in male and female infants were 0.222 ± 0.024 cm and 0.219 ± 0.024 cm, respectively, with no significant differences between genders (t = 1.445, P = 0.149) (Table 2).

Table 2.

PFD values from bilateral normal infant medial hips stratified by age and gender.

			PFD (cm)
Age group (months)	Cases		Mean ± SD (95% CI)
Age group (months)	Male	Female	Male	Female
0–1	9	21	0.182 ± 0.020 (0.173–0.192)	0.186 ± 0.018 (0.181–0.191)
1–2	15	15	0.200 ± 0.015 (0.193–0.203)	0.198 ± 0.016 (0.192–0.203)
2–3	15	15	0.216 ± 0.019 (0.210–0.223)	0.223 ± 0.013 (0.218–0.227)
3–4	15	15	0.228 ± 0.014 (0.223–0.232)	0.227 ± 0.010 (0.223–0.230)
4–5	12	18	0.229 ± 0.016 (0.223–0.235)	0.221 ± 0.012 (0.217–0.225)
5–6	19	11	0.238 ± 0.018 (0.233–0.244)	0.237 ± 0.016 (0.231–0.245)
6–7	19	11	0.234 ± 0.014 (0.230–0.239)	0.229 ± 0.011 (0.224–0.233)
7–12	8	22	0.240 ± 0.016 (0.233–0.248)	0.243 ± 0.017 (0.238–0.248)

Values are given as mean ± SD (95% CI).

CI, confidence interval; PFD, pubofemoral distance.

Mean ± SD (95% CI) PFD increased linearly with age (left: r = 0.680, P < 0.0001; right: r = 0.682, P < 0.0001) (Figs. 2 and 3).

Fig. 2.

Mean pubofemoral distance stratified by infant age.

Fig. 3.

Pubofemoral distance from the right (a) and left (b) hips of each infant stratified by age.

Inter-observer reproducibility was excellent (Kendall’s coefficient of concordance = 0.875, P = 0.008).

Discussion

Diagnosis of DDH in the newborn is usually made by physical examination (10 –12). Ultrasonography of the hip is used to confirm diagnosis and assess the position of the femoral head once treatment is initiated (4,13 –16). Infants with persistent dislocation of the hip who undergo closed or open reduction require immobilization in pelvic external fixators such as a plaster or a brace. During the treatment process, the position of the femoral head should be routinely monitored with imaging methods. Removing the pelvic external fixator to visualize the lateral hip is time consuming and increases the risk for re-dislocation (3). Ultrasound of the medial hip can directly visualize the position of the femoral head without removal of the plaster or brace. Consequently, transinguinal ultrasound of the medial hip can be considered the optimal approach for determining the femoral head position in a pelvic external fixator after closed reduction for DDH (3).

Traditionally, pelvic radiography was used to evaluate the position of the femoral head during treatment for DDH. However, the utility of pelvic radiography for monitoring femoral head reduction is limited. The casting material of the pelvic external fixator device impairs the clarity of the radiographic image, only the ossified portion of the femoral head can be visualized, and the image is provided in a two-dimensional plane; therefore, the exact position of the femoral head relative to the acetabular fossa cannot be defined. In particular, pelvic radiography can provide false-negative examination results in hips that are dislocated in the posterior direction. Conversely, transinguinal ultrasonography of the medial hip is a more reliable method for evaluating femoral head reduction after a closed or open procedure for DDH (3). We recommend the use of both pelvic radiography and transinguinal ultrasonography of the medial hip as the most reliable approach for determining the position of the femoral head in a pelvic external fixator and informing clinical decision-making in infants treated for DDH.

Previous reports show that the PFD has potential as a screening test to diagnose DDH and as a tool for monitoring DDH treatment in newborns after open or closed reduction and immobilization with a pelvic external fixator (5 –8). However, data describing PFD values in normal infant hips (Graf type Ia and/or Graf type Ib) are sparse. Establishing PFD reference values according to infant age, laterality, and gender is essential for understanding the pathology of DDH and monitoring response to treatment. In the present study, PFD values in normal infant medial hips (Graf type Ia and/or Graf type Ib) were measured on transinguinal ultrasonography. Findings showed that the PFD increases with infant age but is relatively constant with laterality and gender. These data imply that comparing the PFD value measured in an infant undergoing treatment for DDH during follow-up to the normal value may be used to assess the efficacy of treatment. If the difference between the diseased treated hip and the normal value approaches zero, treatment could be considered effective. If the difference increases, femoral head reduction was ineffective and the current treatment protocol could be adjusted to avoid further damage to the femoral head and acetabulum.

The present study has some limitations. First, hip ultrasonography requires technical expertise. Risk for measurement error is increased if operators have not undergone appropriate training. Second, if ultrasound is the preferred imaging modality compared to radiography, DDH will be missed or misdiagnosed in patients with co-morbidities who cannot maintain the correct position. Ultrasound and radiograph examinations have their own advantages in the diagnosis of DDH. Clinically, the combination of the two examination methods could be used to increase diagnostic efficacy in DDH.

In conclusion, the present study established reference PFD values from the medial hip in infants aged 0–12 months. PFD values increased with age but were not significantly influenced by laterality or gender. Although the relationship between PFD values and other factors such as birth weight and gestational age were not investigated, the data from the present study provide detailed information that can support follow-up of infants treated for DDH.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conﬂicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received the following financial support for the research, authorship, and/or publication of this article: This study was funded by Shandong Provincial Medical and Health Science and Technology Development program (Nos. 2018WS226, 2016WS0531, 2013WS0086), National Natural Science Foundation of China (No. 81201144), and Postdoctoral Science Foundation of China (No. 2015M582098).

ORCID iD

Yongguang Ban

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