What imaging modalities are used to diagnose leg length discrepancy (LLD)?

From the Guidelines

Imaging for leg length discrepancy should typically begin with weight-bearing radiographs, including a standing anteroposterior (AP) pelvis and full-length lower extremity radiographs, as they provide accurate measurements of bone lengths and overall limb discrepancy. 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.

For children, a hand-wrist radiograph should be included to assess skeletal maturity and growth potential. CT scans offer more precise measurements and can identify rotational abnormalities, while MRI is useful for evaluating soft tissue abnormalities, growth plate issues, or tumors that might contribute to the discrepancy 1. Ultrasound can be helpful for infants or young children to minimize radiation exposure.

The imaging approach should be tailored to the patient's age, suspected etiology, and clinical presentation. For significant discrepancies (>2 cm) or when surgical intervention is being considered, more advanced imaging may be necessary. These imaging studies help determine whether the discrepancy is structural (anatomic) or functional (due to joint contractures or axial deformities), which guides appropriate treatment planning.

Some key points to consider when choosing an imaging modality include:

• The need for accurate measurements of bone lengths and overall limb discrepancy
• The potential for dynamic abnormalities, such as joint mal-alignment or subluxation
• The presence of soft tissue abnormalities, growth plate issues, or tumors
• The patient's age, suspected etiology, and clinical presentation
• The potential risks and benefits of each imaging modality, including radiation exposure and cost.

It is also important to note that advanced imaging modalities, such as MRI, may not always be available or feasible, and alternative modalities, such as nuclear imaging or CT scans, may be necessary in some cases 1. Ultimately, the choice of imaging modality should be guided by the patient's individual needs and the clinical presentation, with the goal of providing accurate and timely diagnosis and treatment.

From the Research

Leg Length Discrepancy Imaging

• Leg length discrepancy (LLD) can be assessed directly through tape measurements or indirectly through palpation of bony landmarks 2.
• Imaging modalities, specifically radiography, are more precise and help identify coexistent deformity 2.
• A standing anteroposterior radiograph of the lower extremity should be the imaging modality of choice when evaluating patients with limb length inequality who may have angular deformities 3.
• Plain film radiography can confirm the presence of leg length discrepancy, as seen in a case where the right limb measured 30 mm longer than the left 4.
• Radiological measurement of leg length inequality is complicated by a lack of consensus, but a difference of less than 10 mm on AP view plain radiographs is generally considered clinically acceptable 5.

Imaging Techniques

• Scanogram and standing anteroposterior radiograph are two reliable techniques for measuring leg length discrepancy, with excellent intraobserver and interobserver reliability 3.
• Computed radiography can be used to measure LLD with both techniques, and repeated measurements are within 5 mm of the first measurement greater than 90% of the time 3.
• Imaging modalities can help identify coexistent deformities, such as angular deformities, and are essential for evaluating patients with limb length inequality 2, 3.

Clinical Implications

• Leg length discrepancy can cause pain, functional limitations, and musculoskeletal disorders, such as functional scoliosis 2.
• Discrepancies greater than 20 mm can alter biomechanics and loading patterns, resulting in functional limitations and musculoskeletal disorders 2.
• A thorough history and examination are essential for diagnosing leg length discrepancy, and imaging modalities can confirm the presence of LLD 4.