INTRODUCTION AND PURPOSE
The measurement of lower extremity limb lengths is a common feature of the orthopedic examination of children. In normal skeletal growth, bones grow equally and in proportion to one another throughout childhood and adolescence, with most bones fully ossified by 20 years of age.1 Questions may arise when a child's limbs grow unequally, resulting in a measurable limb length discrepancy (LLD). Studies have associated an LLD with an increased risk of injury during running, development of back pain, compensatory scoliosis, and gait abnormalities, and LLD is believed to lead to degenerative changes in the lower extremities or lumbar spine.2,3 Limb length discrepancy is among the most common nontraumatic conditions in children for which an orthopedic referral is sought.4 This condition may be the result of congenital limb deficiencies or hemihypertrophy, infections or fractures that injure the physis, neuromuscular disorders, tumors, or trauma that results in overgrowth and disease processes.5
The prevalence of LLD in the general population of children has not been clearly identified. In a review of the literature conducted by Brady et al,6 a prevalence between 4.0% and 40.0% was observed in various studies with adult populations, including subjects without impairments and subjects with a history of recurrent back pain (7.2%), pain of spinal origin (7.7%), chronic low back pain (18.3%), acute low back pain (21.9%), and in athletes (40.0%). For the population without impairments, the prevalence ranged from 4.0% to 8.0%. It should be noted that the prevalence was dependent upon the level the examiners set as a clinically significant difference, with a range from 0.3 to 2.2 cm found in these prevalence studies. In a study by Nissinen,7 of 1060 children developing typically (average age 10.8 years), who were screened for trunk asymmetry and scoliosis, 4.8% of the subjects were found to have an LLD upon visual examination of pelvic alignment.
The most common ways of measuring limb length include direct (tape measure methods), indirect (pelvic leveling), and radiological methods. The direct method of measuring limb length involves a measurement between 2 defined points, typically the anterior superior iliac spine (ASIS) of the pelvis and the malleoli of the ankle joint. Harris et al8 concluded that physical examination using the direct method or the indirect method (such as a block test) was more clinically relevant than CT scanogram in the assessment of LLDs. Their study showed a positive correlation between the patient's perception of an LLD or limp and the physical examination, but no correlation between CT scanogram and the patient's perception of an LLD or limp.
Although radiographs are an accurate and reliable method of measuring limb lengths,3 access to radiology services, the patient's exposure to radiation, and the cost associated with this diagnostic test can present barriers to its use in clinic. Measuring limb lengths using the direct method of tape measurement is a valid and reliable (intraclass correlation coefficient, 0.78–0.99) screening tool to use in the examination.2,9,10 The standard procedure for taking a measurement using this direct method is commonly taught in medical and rehabilitation educational programs. This direct method includes palpation of the ASIS and the medial malleolus of the ankle joint. The subject is in a supine position, with the lower limbs approximately 15 to 20 cm apart and parallel to one another. The lower extremity is in neutral rotation as determined by observation. A measurement is taken from the ASIS to the medial malleolus on both limbs and the lengths are recorded. Measurement from the ASIS to the medial malleolus is considered a measurement of true limb length, and measurement from the umbilicus to the medial malleolus is considered a measurement of apparent limb length. Although current research has been inconclusive in determining a clear threshold for what is considered a significant LLD when measured using the direct method, several studies agree that an LLD of less than 2.0 cm is clinically insignificant.11–14 According to Palmer and Epler,15 a difference of 1.0 to 1.5 cm between limbs is considered to be within normal limits for the adult population. However, in a review of the literature conducted by Brady et al,6 a difference in limb lengths in the adult population was considered clinically significant in a range from 0.3 to 2.2 cm. Opinions are conflicting as to what is the clinically significant LLD in the pediatric population. Tecklin5 states that intervention is generally not indicated for LLD of less than 2.0 cm. In contrast, Campbell et al1 define an LLD as a 2.5-cm or greater difference in limb length. In a study examining the quality of life of 76 children diagnosed with LLD, Vitale et al14 found that patients with an LLD of 2.0 cm or less generally managed better than those with discrepancies of greater than 2.0 cm, but none of their quality-of-life scores were significantly different when comparing the 2 groups. Researchers describing the correction of LLDs and angular deformities in children reported that minimal LLDs of a few millimeters are relatively common and, if the difference is less than 0.5 cm, there is no need for any type of intervention.14
The goal of this study was to identify the prevalence of limb length discrepancies as determined by the direct method in a convenience sample of children aged 8 to 12 years who were developing typically. The information gained from this study will assist the clinician in understanding the prevalence of this clinical variation when examining young children.
Institutional Review Board approval was received before beginning data collection. Packets with information regarding the study along with parental consent forms were sent home with children from 2 private, regular education schools. The sample pool consisted of children from predominately middle-class white families, who are representative of the local community. Sixty-eight children obtained parental consent and agreed to participate in the study. Thirty-seven children from a previous study of LLD, the prevalence of pelvic asymmetry, and sacroiliac joint dysfunction in children were used to add to our subject population. The study consisted of 43 girls and 62 boys ranging in age from 8 to 12 years. Before collecting data, the procedure was reviewed with the child and written assent was obtained from him or her at that time. Excluded from the study were children who had a history of orthopedic surgery or medical diagnosis of a neurological impairment. For demographic information on participants, please see Table 1.
The examiners consisted of 2 physical therapists and 2 physical therapist students. The 2 physical therapists measured and recorded the limb lengths of the children, whereas the physical therapist students obtained the height and weight and explained the procedure to the children. The first 10 children were used to obtain interrater reliability of the 2 physical therapists. Limb lengths were measured as noted later. A discrepancy of 2 cm or greater was considered positive. Interrater agreement for the presence of an LLD between the 2 examiners was 100%.
The child's height in inches and weight in pounds were measured. Next, the true limb length and the apparent limb length were assessed in the following manner. (1) True limb length: The subject was placed in a supine position and the examiner measured limb length from the inferior aspect of the ASIS to the inferior aspect of the medial malleolus. The inferior aspects of the bony prominences were chosen to provide consistency with placement of the measuring tape. This measure was taken in centimeters and a difference of 2 cm or more was recorded as a discrepancy. (2) Apparent limb length: The subject was placed in a supine position and the examiner measured limb length from the umbilicus to the inferior aspect of both the left and right medial malleoli. This measurement was taken in centimeters and a difference of 2 cm or more was recorded as a discrepancy.
The data were analyzed using SPSS for Windows, version 16.0. Descriptive statistics were computed for the data on LLD obtained by standard tape measure method. A chi-square test was used to compare the gender distribution of the group with a true or apparent LLD and the gender distribution of the group without a true or apparent LLD. Independent sample t tests were used to compare the age, height, and weight of the groups. Finally, a linear regression was performed on all of the continuous data to identify any multiple predictors of limb length discrepancies. An alpha level of 0.05 was set to signify statistical significance for all tests.
The original sample consisted of 117 children. One of the children had to be excluded from the study because of a prior orthopedic surgery and 1 child had to be excluded because of a history of neurological impairment. Ten of the children refused to be examined at the time of the data collection, even though their parental consent forms were signed. Descriptive statistics for the remaining 105 children demonstrated an average age of 9.88 years (SD = 1.2), an average height of 55.7 inches (SD = 4.6), and an average weight of 82.5 pounds (SD = 24.6) (see Table 1). Eight of the children (7.6%) presented with an LLD of 2 cm or greater upon examination. Of the 8 children, 6 (5.7%) demonstrated a true LLD and 2 (1.9%) demonstrated an apparent LLD of 2 cm or greater (see Table 2 for a summary of the results).
The gender distribution of the members of the group with an identified true LLD compared with those without a true LLD was not significantly different (P = .696). The gender distribution of the members of the group with an identified apparent LLD compared with those without an apparent LLD was not significantly different (P = .234). The results for age and height were nonsignificant for true limb length discrepancies (P = .137 and .075, respectively). The results for age and height were nonsignificant for apparent limb length discrepancies (P = .400 and .067, respectively). The sample data for weight did not demonstrate a normal distribution required for this statistical analysis. A linear regression was performed on the data to identify any multiple predictors for limb length discrepancies. None of the variables were found to be predictive of true or apparent limb length discrepancies.
The children in this study were in the age range of 8 to 12 years, which corresponds to an age in which the majority of children in the United States enter puberty.16,17 During the rapid growth spurts in puberty, there is a possibility for children's limbs to grow asymmetrically.1 This could be one possible explanation for the LLDs that were found in our sample. We found that 5.7% of children who are developing typically, between the ages of 8 and 12 years, demonstrated a true LLD, and 1.9% demonstrated an apparent LLD, as noted with the direct method of measurement, using the threshold of 2 cm for this type of measurement. The finding of 5.7% is similar to studies in the adult population, which show a prevalence of LLD ranging from 4.0% to 8.0%.6 Awareness of how frequently this examination finding occurs can add to the clinician's understanding of the natural occurrence of this asymmetry. The presence of an LLD may not automatically lead to a specific intervention and may require only monitoring of the condition unless the asymmetry progressively worsens, causes pain, or impairs the functioning of the child. Understanding how a limb length asymmetry can contribute to other impairments in gait or function would make routine screening for an LLD a reasonable addition to a scoliosis screen, physical or preparticipation examination, commonly performed on school-aged children.
A limitation of this study includes the validity of tape measurement for LLD. Several studies have been published that address this issue in adult subjects.2,3,18,19 The literature questions the validity of using this direct method of measurement when the inequality is less than 2 cm.6 We attempted to address this issue by standardizing the procedure, hooking the thumb underneath the bony prominences of the ASIS and medial malleoli to promote consistency with the placement of the tape measure and setting the threshold at greater than 2 cm. Another option would be the use of the lateral malleoli as a reference point. This method takes into consideration the potential sources of error with differences in limb circumference, angular deformities, and joint contractures, if present.2 Measurements less than 2 cm would require further examination with diagnostic imaging to determine an accurate and true LLD and the possible cause.
Another weakness of our study is the lack of diversity in our sample. Our study consisted of 105 subjects recruited from private, regular education schools. All of these subjects were white and from middle-class families. The lack of ethnic diversity in our sample does not allow us to generalize our results to the general population.
In this study, 6.7% of the children (or 1 of 15) examined demonstrated an LLD when measured with the direct method (5.7% presented with a true LLD of 2 cm or greater, and 1.9% presented with an apparent LLD of 2 cm or greater). Our study found a slightly higher percentage of LLD in the pediatric population than the study by Nissinen (4.8%)7 but within the range found in the study of the adult population by Brady et al6 This study sheds some light on the prevalence of LLD in children who are developing typically. It is possible that the presence of an observable LLD in a child who is asymptomatic would be expected in a certain portion of this population. Furthermore, a study on why this LLD occurs and the long-term effects of this variation would be beneficial in the management of the pediatric patient.
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child; leg length inequality; reference standard