Comparison of Supine and Prone Methods of Leg Length Inequality Assessment
Introduction
Leg length inequality (LLI) assessment is performed by doctors of chiropractic, physical therapists, and doctors of osteopathy for a number of reasons.1 The test involves determining the baseline relative position of the feet in the prone or supine position, which amounts to a y-axis positional asymmetry of the distal lower extremities. There are also leg checking protocols in which LLI is assessed as an evoked response, as when the head is turned or the knees flexed to 90°2 or when the examiner or patient makes contact with a part of the patient’s body.3 A review of the literature on the reliability and validity of measures used in manual therapy to localize the site of spinal manipulation4 found varying levels of reliability for supine and prone LLI assessment procedures.5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18 There was also some support for the validity of measures of supine and prone LLI.19, 20, 21, 22, 23, 24, 25
The clinical interpretation of LLI crucially depends on the distinction between anatomic LLI (LLIa),26, 27, 28 wherein the legs are measurably of different length, and functional LLI (LLIf), in which the legs are de facto equal in length and yet 1 has been drawn cephalad in some manner (Table 1).29, 30 A review article on pelvic torsion includes discussion of various models for the functional short leg.31
Descriptions of leg length assessment procedures do not uniformly take into account that observed LLI (LLIo) may reflect primarily LLIa or LLIf, let alone what diagnostic difference this may make. It has been suggested that a functional short leg may be associated with posterior innominate rotation, whereas an anatomical short leg has been found to predict anterior innominate rotation.29 Considerations such as these suggest that discrimination of functional from anatomic short legs may have an impact on clinical outcomes.32
Practitioners who focus on the upper cervical spine are also entrenched in functional leg checking.33, 34 The assumption for this group is that it may be a surrogate measure of the state of atlas alignment, given that upper cervical radiographs cannot be obtained during every office visit. An upper cervical monograph states: “Not only does the short-leg indicate the presence of nervous imbalance in the CNS [central nervous system], but the amount of shortness can indicate the degree of neurological imbalance.”34 Although upper cervical chiropractors typically assess LLI in the supine position,35 other chiropractic techniques employ LLI assessments the prone position.33
Although supine and prone leg checks are used in practice, there are few published studies as to whether their results agree. There are also few studies evaluating if the information supplied by leg checking influences clinical outcomes. A previous study reported that supine and prone leg length assessments manifested different associations with pelvic obliquity, but no direct comparison was drawn between the leg checking methods.36 A different study comparing the results of leg checking with radiographs included a module assessing the agreement of supine and prone leg checks; however, only part of the data was reported.22
We feel that it would be worth knowing if prone and supine leg checks may be regarded as equivalent procedures, or if they should be regarded as dissimilar procedures that provide different information. Therefore, the primary objective of the current study was to determine the intermethod reliability of supine and prone LLI assessment. The secondary objective was to determine if the degree of examiner confidence had an impact on the degree of intermethod agreement.
Section snippets
Methods
The number of participants to be included in this study was based on the following: for κ agreement coefficients, the required number of participants depends on (1) the relative error r, where it has been suggested that any estimated interrater reliability coefficient should differ from its “true" value by no more than 20%; and (2) the difference pa − pe between the overall agreement probability pa and the chance-agreement probability pe.37 Under the best-case scenario that chance agreement in
Results
A convenience sample of 43 asymptomatic and minimally symptomatic student volunteers were recruited. Five (11.6%) participants reported leg pain ranging from 1 to 6 on an 11-point scale (mean = 3). Three participants (7.0%) reported prior histories of lower extremity injuries and surgeries. The sample was 37% female, and their mean age was 25.5 years (range: 23–42).
Intermethod reliability data are summarized in Table 2. The supine leg checker found 25 of 43 legs (58.1%) short on the left; the
Discussion
To our knowledge, this is the first study to compare LLI prone and supine assessments. According to the often-cited Landis and Koch scale for Kappa Statistic Strength of Agreement, 0.00-0.20 indicates slight; 0.21-0.40 equals fair; 0.41-0.60 indicates moderate; 0.61-0.80 equals substantial; and 0.81-1.00 indicates almost perfect.38 According to this scale, intermethod agreement for the full data set in this study was slight. When both examiners were confident, their agreement was perfectly nil,
Conclusions
The findings of this study suggest that supine and prone leg length assessment procedures appear to measure different phenomena, and their results in this study were not interchangeable. There may be mechanical reasons that the supine body position mitigates against LLIo attributable to pelvic torsion, whereas the prone position may increase its observability. The discrepancy between prone and supine leg checking results may then reflect that placing the patient in the prone position may
Funding Sources and Conflicts of Interest
No funding sources or potential conflicts of interest were reported for this study.
Contributorship Information
Concept development (provided idea for the research): R.C.
Design (planned the methods to generate the results): R.C., M.L.
Supervision (provided oversight, responsible for organization and implementation, writing of the manuscript): R.C.
Data collection/processing (responsible for experiments, patient management, organization, or reporting data): M.L.
Analysis/interpretation (responsible for statistical analysis, evaluation, and presentation of the results): R.C.
Literature search (performed the
Acknowledgments
The authors thank Dr. Terri Payton and Dr. John Lockenour for their help with study design and data acquisition.
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