Diagnosis: Bilateral Genu Varum (Bow-Legs) with Abnormal Mechanical Axis Alignment
Based on the radiographic measurements provided, this patient has bilateral genu varum deformity with abnormal mechanical angles, particularly on the right side where the mMPTA of 79° indicates significant tibial varus deformity. 1
Interpretation of Mechanical Angle Measurements
The key to understanding this diagnosis lies in the abnormal mechanical angles:
Right mMPTA of 79°: This is severely abnormal. The normal mechanical medial proximal tibial angle (mMPTA) should be approximately 87° (±3°). A measurement of 79° represents an 8° deviation into varus, indicating significant tibial-sided deformity. 1
Right mLDFA of 99°: This is at the upper limit of normal or mildly abnormal. The normal mechanical lateral distal femoral angle (mLDFA) should be approximately 87-90°. A measurement of 99° suggests mild femoral valgus positioning, though this may represent compensatory alignment. 1
Left mMPTA of 89°: This is within normal range but on the lower end, suggesting mild tibial varus tendency. 1
Left mLDFA of 93°: This is mildly elevated, suggesting slight femoral valgus positioning. 1
Clinical Significance Despite Equal Limb Lengths
The absence of leg length discrepancy (both limbs measure 33 cm) does not exclude significant pathology—the critical issue here is angular deformity affecting the mechanical axis, not length. 1
The mechanical axis deviation is assessed by full-length standing radiographs from hip to ankle, which evaluate how the weight-bearing line passes through the knee joint. 1 Normal alignment has the mechanical axis passing through or near the center of the knee. With the abnormal angles measured (particularly the right mMPTA of 79°), the mechanical axis likely passes medially through the knee, creating increased medial compartment loading. 2
Differential Diagnosis Considerations
Given the bilateral nature with asymmetric severity, consider:
Blount disease (tibia vara): Particularly if this is a child or adolescent, given the severe tibial varus deformity. 3
Metabolic bone disease: X-linked hypophosphatemia (XLH) or other forms of rickets can present with bilateral genu varum and abnormal mechanical angles. 2 Look for other radiographic signs such as metaphyseal fraying, widened growth plates, or osteopenia.
Post-traumatic deformity: Though typically unilateral unless there's bilateral trauma history. 3
Physiologic bowing: Less likely given the severity of angular deformity, especially the 79° mMPTA. 2
Essential Clinical Correlation Required
You must correlate these radiographic findings with the patient's age, clinical examination findings, and assess for associated features: 2
Age is critical: Physiologic bowing is normal in children under 2 years but should resolve by age 3-4. Persistent or progressive varus deformity beyond this age is pathologic. 2
Examine for intercondylar distance: Measure the distance between the knees when standing with ankles together—this quantifies the severity of genu varum clinically. 2
Assess gait pattern: Look for lateral thrust during walking, which indicates dynamic instability and increased medial compartment stress. 2
Check for asymmetric findings: Unilateral or asymmetric deformity suggests acquired pathology rather than developmental. 2
Recommended Additional Imaging
Full-length standing hip-to-ankle radiographs are essential to complete the assessment and should be obtained if not already done. 1 These provide:
- Accurate mechanical axis deviation measurement (distance the weight-bearing line passes from knee center) 1
- Assessment of joint line orientation angles 1
- Evaluation for compensatory deformities at hip or ankle 1
The standing AP view should be obtained with 10° internal rotation to optimize assessment of varus/valgus alignment. 1
Common Pitfalls to Avoid
Do not dismiss abnormal mechanical angles simply because limb lengths are equal—angular deformity and length discrepancy are separate pathologies with different clinical implications. 1, 3
Do not rely on standard AP knee radiographs alone—they cannot accurately measure the mechanical axis, which requires visualization from hip to ankle. 1
Do not assume bilateral symmetry means normal—bilateral pathologic processes (metabolic disease, genetic conditions) can affect both limbs. 2
Do not delay metabolic workup if rickets is suspected—check serum phosphate, alkaline phosphatase, calcium, PTH, and 25-OH vitamin D levels. 2
Clinical Impact and Prognosis
Untreated genu varum with abnormal mechanical axis leads to progressive medial compartment osteoarthritis, pain, and functional limitation. 2 The severity of malalignment (particularly >5° deviation) is associated with accelerated cartilage loss and functional decline. 2
Intervention timing depends on etiology, age, and severity—surgical correction may be indicated for progressive deformity, particularly if the mechanical axis deviation exceeds 2 cm from knee center or if angular deformity causes functional impairment. 3