Imaging for Leg Length Discrepancies
Order a standing full-length anteroposterior (AP) radiograph from hip to ankle as the initial and primary imaging study for evaluating leg length discrepancy. 1, 2, 3
Primary Imaging Protocol
A standing (weight-bearing) full-length hip-to-ankle radiograph is the gold standard because it provides accurate measurement of limb length while simultaneously assessing mechanical axis alignment and any angular deformities in a single study 1, 2, 3
This single radiograph eliminates the need for additional imaging studies (like scanograms), reducing both radiation exposure and cost while providing comprehensive lower extremity evaluation 2, 3
The standing position is critical—weight-bearing films reveal functional alignment under physiologic load that non-weight-bearing studies cannot demonstrate 1, 4
Measurement Technique
Measure from the femoral head center to the ankle joint center on the standing full-length AP radiograph 2, 3
Expect approximately 4-6% magnification (roughly 3-4 cm) on full-length standing radiographs compared to scanograms, but this does not affect the accuracy of leg length discrepancy measurement 2
The correlation between standing AP radiograph and scanogram measurements is excellent (r = 0.96), with mean difference of only 0.5 cm 2
Intraobserver and interobserver reliability is excellent for standing full-length radiographs, with measurements reproducible within 5 mm over 90% of the time 3
When Standing Films May Be Less Accurate
If mechanical axis deviation exceeds 2 cm (significant varus or valgus deformity), the standing radiograph may differ from scanogram by more than 0.5 cm 2
In such cases, the standing film still provides superior clinical information because it shows the true functional deformity, which is what requires treatment 2
Bilateral Comparison
- Consider obtaining bilateral comparison views, particularly in pediatric patients or those with suspected metabolic bone disease, to differentiate true leg length discrepancy from other pathology 4
Common Pitfalls to Avoid
Do not order a scanogram as the initial study—it requires the patient to be supine, provides no information about alignment or deformity, and necessitates additional standing films if angular deformity is present 2, 3
Do not rely on AP pelvic radiographs alone (measuring inter-teardrop distance)—these correlate poorly with true functional leg length discrepancy and miss 33% of clinically significant discrepancies greater than 10 mm 5
Do not accept non-weight-bearing films unless the patient absolutely cannot stand—supine radiographs fail to capture functional alignment abnormalities 1, 6
Do not order separate "alignment films" after obtaining a scanogram—this doubles radiation exposure unnecessarily when a single standing full-length film provides both measurements 2, 3
Clinical Context
Physical examination measurements of leg length discrepancy correlate weakly with radiographic measurements and should not be relied upon for surgical planning 6
A leg length discrepancy of 10-12 mm is generally considered the threshold for clinical significance, though this varies by patient activity level and symptoms 6
The standing full-length radiograph allows simultaneous assessment of hip pathology, knee alignment (varus/valgus), tibial torsion, and ankle joint orientation—all relevant to surgical planning 2, 3