While there are several different methods available to the clinician for the assessment of LLD, we were unable to find an in-depth review of the various clinical and imaging modalities as the subject of a single manuscript. Our goal was to enumerate the various modalities that have been described for assessing leg length discrepancy, including the accuracy and interobserver and intraobserver reliability for each technique, to compare the reported results for various assessment tools for LLD, and discuss the potential advantages and pitfalls that have been described with each method. We identified certain trends that were noted across several studies discussed below, along with the potential advantages and pitfalls of each method.
While using a tape measure is an easy, safe, and noninvasive means of assessing LLD, it is less reliable when compared to radiographic techniques such as a scanogram [7
]. The average of two tape measurements of the distance between the ASIS and medial malleolus appears to have acceptable validity and reliability when used as a screening tool for assessing LLD [14
]. However, there are potential sources of error with tape measurements related to differences in leg circumference, angular deformities, and difficulty in accurately palpating bony prominences as well as joint contractures. While the use of standing blocks under the short leg to level the pelvis is slightly more reliable than tape measurement, such a method may still not be precise enough for serial monitoring of LLD [43
There is general consensus that radiographs are more accurate and reliable than clinical exam for analysis of LLD [8
]. Several authors have reported the results of LLD measurement using a variety of imaging techniques such as orthoroentgenogram [13
], CR-based teleoroentgenogram [38
], slit scanogram [28
], microdose digital radiography [3
], CT scanogram [1
], ultrasound [42
], and MRI scanogram [25
]. One needs to consider several issues such as reliability, accuracy, magnification, radiation dose, cost, need for special equipment, convenience, and opportunity to image the entire extremity when choosing the imaging technique for evaluating patients presenting with LLD (Table ).
Comparison of methods for assessing leg length discrepancy
A scanogram is one of the most commonly used methods for assessing LLD. It has excellent reliability [37
] and minimal, if any, magnification error [38
]. However, as supine radiographs that require the patient to remain still between the three radiographic exposures, an orthoroentogenogram and a scanogram are prone to errors related to the patient moving between the exposures. The radiation exposure with scanogram and orthoroentgenogram is also substantially greater than that associated with a full-length standing radiograph and a CT scanogram [1
]. This may be related to the need for three separate radiographic exposures with the scanogram and orthoroentgenogram compared to the single exposure centered at the knee with the standing radiograph as well as the closer xray tube-to-patient distance utilized while performing a scanogram. While taking a scanogram, the xray tube must be centered precisely over the joint since even a minor deviation of the beam can result in measurement error of several millimeters due to distortion by magnification [26
]. Errors in measurement are often seen in patients with clinically important limb-length inequalities when the individual joints of the two limbs are at substantially different levels and thus not visualized on the same radiograph [26
]. Moreover, a scanogram cannot detect angular deformities of the lower limb and may underestimate the LLD in patients with discrepancies in foot height [26
]. Patients presenting with unequal leg lengths often have associated angular deformities of the lower limb. Since the entire lower extremity is imaged on a single radiograph with the patient in the erect position, a comprehensive analysis of limb deformities can be performed as well, along with the assessment of LLD [26
]. Furthermore, unlike a scanogram, the difference in height of the feet is incorporated in the measurement of LLD when using the full-length standing radiograph. There are certain prerequisites that should be met in order to avoid potential errors in using this measurement for clinical decision making. For the standing full-length radiograph, the patient should be stood erect with the pelvis clinically level and the feet plantigrade by using an appropriate-sized lift under the short limb. This will avoid underestimation of the LLD that can occur with the patient plantarflexing the ankle on the short side and flexing the contralateral knee in an attempt to level the pelvis. Similarly, any lower extremity joint contractures or overlying external fixators can diminish the accuracy of LLD measurement using either of the two imaging techniques [36
]. However, there are potential pitfalls with using this radiograph, including the need for special radiographic equipment such as grids, filters, and processors along with the need for long radiographic cassettes that may not be readily available with recent advances in digital imaging and can be difficult to store.
Computed radiography (CR) does not require these additional tools while at the same time uses standard radiographic equipment. The full-length images obtained using CR are readily available on personal computers for preoperative planning and patient/ family education [26
]. Despite a 5% magnification “error” in the measurement of the entire length of the lower extremity, there is minimal effect on assessment of LLD. Furthermore, by placing magnification markers and a ruler next to the patient, this magnification error can be further reduced (Fig. ). Proper training and supervision of the radiology technicians regarding the correct technique and patient positioning for performing standing radiographs, especially with rapidly changing technology, is also critical to ensure appropriate and reproducible imaging studies.
Fig. 7 A standing full-length computed radiograph (modified teleoroentgenogram) of a 14 year old patient following right sided tibial lengthening for a 6 cm LLD. Note the use of a midline ruler and magnification markers adjacent to the right (more ...)
The cost of microdose digital radiography (MDR) is comparable to other imaging techniques [3
], although special equipment is necessary. Moreover, unlike a CT scan, the digital scan has a field length of 150 cm that is sufficient for imaging the entire lower extremity in a single exposure for most patients [3
]. However, this technique is not readily available and not as convenient as a full-length standing AP radiograph that is obtained using computed radiography.
The benefits of ultrasound are that it is inexpensive, does not involve any radiation exposure, is reliable in the hands of experienced users, and is thus a convenient and useful method of assessing LLD [22
]. However, unlike a full-length standing radiograph, an ultrasound does not allow for a comprehensive analysis of the lower extremity including angular deformities and may be less accurate than radiographic methods. This technique may be a useful screening tool in the hands of experienced users [42
A CT scanogram has the advantages of displaying the entire lengths of the femurs and tibias while minimizing the measurement error. There is no magnification when the structure to be measured is centered in the computerized axial tomographic gantry [19
]. While possibly needing longer setup time, a CT scanogram has similar costs and may be more accurate, with excellent reliability and less gonadal radiation, than some of the plain radiographic techniques [1
]. In order to avoid underestimation of limb length, it may also be useful to perform a lateral CT scanogram in patients with flexion contractures of the hip or knee [1
]. However, periarticular and diaphyseal angular deformities as well as joint subluxation and mechanical axis deviations are not as well ascertained on these supine images compared to a standing radiograph. Moreover, this technique is not readily available and usually requires prior scheduling in the department of radiology or an imaging center.
Although an MRI scanogram does not expose patients to ionizing radiation, the measurements obtained using this technique are slightly less accurate than those obtained with a radiographic scanogram or a CT scanogram [25
]. Furthermore, an MRI scanogram has not been well-studied in the clinical setting as an assessment tool for LLD, is probably more expensive, may require sedation in some children, typically requires a longer time to schedule and to complete the study, and may be contraindicated in patients with certain implantable devices. Thus, at this time a supine MRI scanogram remains an investigational tool that requires clinical validation before it can be recommended for general use. Recently, MRI scanners that allow the patient to weight bear during imaging have been introduced in the U.S. market. Such an emerging technique may be an attractive option to comprehensively assess length and alignment of the lower extremities while avoiding radiation exposure to the patient.
Based on our review of the literature, we found several limitations in the available articles dealing with different assessment tools for LLD. The majority of the studies were retrospective case series with multiple confounding variables that were not clearly stated by the investigators. Factors such as magnitude of LLD, level of training and experience of observers, lack of blinding of observers, undocumented body habitus (such as BMI) of subjects, presence of angular deformities and contractures, use of cadaveric and synthetic bone specimens versus live subjects as well as limited number of patients can affect the validity of the authors’ conclusions. Certainly, there are ethical concerns with subjecting patients to multiple diagnostic modalities, especially those involving radiation. However, future investigators can strengthen their research methodology by employing more robust study design and methodology. Our suggestions would include the following: use well-designed prospective, multicenter studies involving a larger number of subjects, clearly state and discuss the confounding variables, perform appropriate statistical analysis, perform adequate tests for reliability and accuracy amongst blinded observers with different levels of training and study emerging technologies that do not involve radiation hazards, such as standing MRI and ultrasound. Hopefully, such efforts can further aid clinicians in performing safe, reliable and accurate assessment of patients presenting with LLD.
An ideal method for assessing LLD should be readily available, accurate, reliable, and affordable, allow visualization of the entire lower extremity, minimize radiation exposure, and have no magnification error. Although at present there is no single imaging method that can be considered ideal, based on our review of the literature, the standing full-length AP computed radiograph of both lower extremities with the pelvis level, along with use of a magnification marker, should be the primary imaging modality for the initial evaluation of LLD in the majority of the patients. A CR teleoroentgenogram is not only an accurate and reliable imaging tool, but the measurements can be obtained with limited radiation exposure in a cost-effective manner [20
]. However, other techniques such as a lateral scout CT scan may be more useful in cases with severe angular deformities, especially those associated with flexion deformities around the knee. In the upcoming years other imaging modalities such as a standing MRI of the lower extremities may prove a viable alternative, without exposing patients to radiation hazards. On the other hand, despite rapidly advancing technology, it is important to consider that the accuracy and ease of obtaining measurements of the patient using any imaging modality is not a substitute for a thorough clinical assessment of the patient presenting with LLD [18
]. Moreover, clinical evaluation of the patient with long-standing limb shortening, especially with associated muscle weakness, using blocks under the short limb can be used to estimate the amount of correction that feels optimal, as this may be different from the true LLD assessed with an imaging modality. Thus, a judicious use of a comprehensive imaging method combined with an astute clinical assessment is the most optimal means of evaluating a patient presenting with leg-length discrepancy.