From the Guidelines
Imaging for leg length discrepancy should begin with standing full-length radiographs of both lower extremities, as it is the gold standard for assessment and provides precise measurement of the femur and tibia lengths bilaterally. This examination, also called a scanogram or teleoroentgenogram, allows for an accurate determination of the discrepancy's location and magnitude. According to the guidelines on the diagnosis and treatment of active charcot neuro-osteoarthropathy in persons with diabetes mellitus (iwgdf 2023) 1, weight-bearing radiographs are preferred, as they may detect dynamic abnormalities, such as joint mal-alignment, joint subluxation, and/or fracture displacement that may not be apparent on non-weight-bearing radiographs.
When considering advanced imaging modalities, MRI is the recommended first option after plain X-ray, as it provides more information to support or exclude the diagnosis of CNO due to better soft tissue contrast, and probably has better specificity 1. However, if MRI is not available or not possible, other modalities, such as nuclear imaging scan or CT scan, can be used for further assessment. Nuclear imaging combined with CT (SPECT-CT) may provide more utility than either nuclear imaging or CT alone due to improved spatial and contrast resolution.
The choice of imaging should be tailored to the patient's age, suspected etiology, and clinical presentation, with radiographs remaining the most cost-effective and accessible initial approach for quantifying leg length differences. For children with suspected growth abnormalities, bone age radiographs of the wrist may be added to predict remaining growth potential. Ultrasound can be helpful for screening infants and young children but lacks the precision of radiographs for definitive measurements.
Some key points to consider when choosing an imaging modality include:
- The patient's age and suspected etiology of the leg length discrepancy
- The need for precise measurement of the femur and tibia lengths
- The availability and accessibility of advanced imaging modalities, such as MRI and CT scans
- The potential risks and benefits of each imaging modality, including radiation exposure and cost.
It is essential to weigh these factors and choose the most appropriate imaging modality to ensure accurate diagnosis and effective treatment of leg length discrepancy, while minimizing potential harms and optimizing patient outcomes, particularly in terms of morbidity, mortality, and quality of life 1.
From the Research
Leg Length Discrepancy Imaging
- Leg length discrepancy (LLD) can be assessed through various methods, including tape measurements, palpation of bony landmarks, and imaging modalities such as radiography 2.
- Imaging modalities, specifically radiography, are more precise and help identify coexistent deformity 2.
- Full-length standing anteroposterior radiography seems to be the most valid and reliable method for measuring anatomic LLD and may be used as a global reference standard to compare clinical methods and imaging diagnostics 3.
- The block test appears to be the most useful method for the clinical assessment of LLD 3.
Diagnostic Considerations
- LLD can be etiologically classified as functional or structural, with length differences typically less than 10 mm being asymptomatic or easily compensated for by the patient 2.
- Discrepancies greater than 20 mm can alter biomechanics and loading patterns, resulting in functional limitations and musculoskeletal disorders, such as functional scoliosis 2.
- A thorough history and examination are essential for diagnosing LLD, as demonstrated in a case report where a 26-year-old woman with a 1-year history of right knee pain was found to have a leg length discrepancy secondary to a growth arrest of the left knee following postoperative infection in childhood 4.
Treatment Implications
- The decision to treat LLD is always elective, and the extent of the discrepancy is not the sole determining factor for the mode of treatment 5.
- Treatment options for LLD include conservative methods such as shoe inserts, high shoes, or orthoses, as well as surgical methods such as leg lengthening with osteotomy and subsequent distraction of the bone callus 5.
- Changes in leg length exert marked mechanical stress on the soft tissues, and predicted leg-length discrepancies exceeding 5 cm may require initial leg-lengthening treatment during the patient's growing years 5.