Can varum deformity cause knee strain?

Varus Deformity and Knee Strain: Biomechanical Impact and Clinical Implications

Yes, varus deformity definitively causes knee strain by creating abnormal mechanical loading patterns that lead to medial compartment overload, cartilage damage, and progressive joint deterioration.

Biomechanical Effects of Varus Deformity

Varus deformity (bowlegs) creates significant biomechanical alterations in knee loading patterns:

• Medial Compartment Overload: During the mid-stance phase of gait, approximately 70-80% of joint load normally passes through the medial compartment 1. Varus alignment further skews this distribution, increasing medial compartment forces 1, 2.

• Abnormal Force Distribution: Varus alignment causes the mechanical axis to shift medially, resulting in:

  • Increased stress on medial tibial plateau cartilage
  • Lateral soft tissue lengthening
  • Medial soft tissue contractures (deep and superficial MCL, posteromedial capsule) 3

• Progressive Cartilage Damage: Recent finite element analysis demonstrates that as varus angle increases, there is a significant increase in peak Von-Mises stress in the medial compartment 2. When varus angle exceeds 6°, there is a markedly increased risk of cartilage damage 2.

Clinical Evidence of Strain and Damage

The relationship between varus deformity and knee strain is supported by several key findings:

• Cartilage Injury Correlation: Clinical studies show a significant correlation between varus deformity angle and cartilage damage in the knee, particularly in the medial plateau and patellofemoral compartment 2.

• Radiographic Manifestations: Varus deformity leads to joint space narrowing, which is visible radiographically as the deformity progresses 1. This narrowing reflects the cartilage wear resulting from abnormal loading patterns.

• Cycle of Deterioration: The American College of Radiology guidelines note that once loads are inappropriately distributed, erosion of articular cartilage results in malalignment, which further interferes with ligament function, creating a vicious cycle of joint deterioration 1.

Biomechanical Assessment

When evaluating varus deformity:

• Radiographic Assessment: Weight-bearing AP and lateral radiographs are essential for evaluating alignment and wear patterns 1.

• Mechanical Axis Evaluation: Full-length standing radiographs (hip-to-ankle) provide accurate information on the weight-bearing mechanical axis in patients with suspected lower limb malalignment 1.

• Alignment Interpretation: AP views obtained in 10 degrees of internal rotation improve interpretation of varus alignment compared to neutral AP views 1.

Management Implications

For patients with varus deformity causing knee strain:

1. Non-surgical approaches:

   • Bracing to realign the joint and reduce medial compartment loading 1
   • Strengthening of hip and proximal muscles to control biomechanical variables, particularly for patients with excessive hip adduction during gait 1

2. Surgical considerations:

   • Early surgical correction when varus angle exceeds 6° to prevent further deformity and restore knee function 2
   • For severe cases requiring total knee arthroplasty, specific attention to bony alignment and ligament balancing is crucial 4
   • The majority of correction should be made on the tibial cut during TKA, as proximal tibial deformity is the major contributor to osteoarthritic varus knee alignment 5

Pitfalls and Caveats

• Underestimating Progression: Varus deformity is not static; it can progressively worsen as medial compartment wear increases, creating a cycle of increasing deformity and strain.

• Overlooking Multiplanar Issues: Varus deformity may be accompanied by rotational abnormalities that further complicate biomechanics and treatment.

• Surgical Complexity: Correction of severe varus deformity during TKA requires greater technical expertise, longer operative times, and careful attention to soft tissue balancing to avoid residual varus alignment 4, 6.

• Recurrence Risk: Recurrence of deformity is common, especially in young children and patients with poor metabolic control 7.

Varus deformity significantly alters knee biomechanics, creating abnormal loading patterns that lead to medial compartment strain, cartilage damage, and progressive joint deterioration. Early identification and appropriate management are essential to prevent this cycle of joint damage.