What causes flexion deformity in osteoarthritis?

Causes of Flexion Deformity in Osteoarthritis

Flexion deformity in osteoarthritis primarily results from joint biomechanical changes, muscle weakness, and altered load distribution that develop as the disease progresses.

Pathophysiological Mechanisms

Joint Structure Changes

• Osteoarthritis causes erosion of articular cartilage, which alters the distribution of loads over the joint surface and leads to malalignment 1
• This malalignment interferes with the ability of ligaments to restrict joint motion and maintain physiological load distribution 1
• Progressive cartilage loss results in bone remodeling, osteophyte formation, and synovial inflammation that collectively contribute to joint deformity 1

Biomechanical Factors

• In knee osteoarthritis, approximately 70-80% of joint load passes through the medial compartment during mid-stance phase of gait 1
• This uneven load distribution can be exacerbated by malalignment, further skewing weight distribution across the joint 1
• The external knee adduction moment (EKAM), which is the torque pulling the knee into varus, increases stress on the affected compartment 1

Muscle Weakness

• Quadriceps weakness is common in patients with knee osteoarthritis and contributes to joint instability 1
• Muscle weakness was previously thought to develop solely because patients avoid loading painful joints, but evidence suggests it may actually be a risk factor for osteoarthritis development 1
• Decreased joint stability and reduced shock-absorbing capacity from muscle weakness contribute to disability and altered joint mechanics 1

Proprioceptive Deficits

• Diminished joint position sense (proprioception) contributes to the development of osteoarthritis and subsequent deformities 1
• Proprioception is significantly diminished in older adults with knee osteoarthritis compared to those without 1
• Poor proprioception affects joint stability and control during movement, potentially leading to abnormal joint loading patterns 1

Progression to Flexion Deformity

Pain-Related Adaptations

• Pain avoidance behaviors lead to altered movement patterns that favor flexed positions which are often less painful 1
• Patients adopt positions of comfort that minimize joint stress but can lead to contractures over time 1
• The cycle of pain, altered movement, and further joint damage perpetuates the development of flexion deformity 1

Ligamentous Changes

• Ligamentous laxity develops as part of the disease process, reducing joint stability 1
• Changes in the ligamentous structures around the joint contribute to abnormal joint mechanics 1
• As the disease progresses, these changes become more pronounced, leading to fixed deformities 1

Compensatory Mechanisms

• Patients develop compensatory movement patterns to minimize pain and maintain function 1
• These adaptations often involve maintaining the joint in a slightly flexed position during weight-bearing activities 1
• Over time, these compensatory positions can become fixed due to soft tissue contractures and muscle shortening 1

Management Implications

Exercise Interventions

• Strengthening exercises targeting muscles around affected joints can help reduce progression of deformities 1
• Exercise programs designed to improve muscle strength and joint proprioception can reduce pain and improve mobility 1
• Maintaining joint range of motion through appropriate exercises helps prevent development of fixed flexion deformities 1

Bracing and Orthoses

• Bracing can help correct malalignment and reduce articular contact stress in the affected compartment 1
• Wedged insoles and realigning knee braces aim to redistribute joint loads more evenly 1
• Even knee sleeves without alignment correction can provide symptomatic relief and enhance joint position sense 1

Early Intervention

• Early management of osteoarthritis may help prevent or minimize the development of flexion deformities 1
• Addressing modifiable risk factors such as obesity, muscle weakness, and poor joint biomechanics can slow disease progression 1
• Patient education about proper joint mechanics and activity modification is essential in preventing deformities 1

Clinical Pearls and Pitfalls

• Pitfall: Focusing only on pain management without addressing biomechanical factors can allow deformities to progress 1
• Pearl: A comprehensive approach addressing pain, muscle strength, and joint mechanics is most effective in preventing flexion deformities 1
• Pitfall: Prolonged immobilization to reduce pain can actually worsen flexion deformities by promoting muscle shortening and joint stiffness 1
• Pearl: Maintaining an appropriate balance between rest and activity is crucial for joint health in osteoarthritis 1