What is the primary site of involvement in knee Osteoarthritis (OA)?

From the Guidelines

The primary site of involvement in knee osteoarthritis is the medial tibiofemoral compartment, followed by the patellofemoral compartment and then the lateral tibiofemoral compartment. The medial compartment bears approximately 60-80% of the load across the knee during normal gait, making it particularly susceptible to degenerative changes 1. This weight-bearing distribution explains why most patients with knee osteoarthritis develop pain and cartilage loss on the inner (medial) side of the knee first. The patellofemoral compartment is the second most commonly affected area, particularly in individuals who perform activities involving repetitive knee flexion or those with abnormal patellar tracking. Lateral compartment osteoarthritis is less common and may be associated with valgus knee alignment or previous lateral meniscus injury.

The progression of knee osteoarthritis typically involves cartilage degradation, subchondral bone changes including sclerosis and cyst formation, osteophyte development at joint margins, and varying degrees of synovial inflammation. Understanding the compartmental involvement helps guide both non-surgical management approaches and surgical interventions when necessary. According to recent guidelines, non-surgical management of knee osteoarthritis includes education, structured exercise, and weight loss as core treatments, with topical NSAIDs as first-line treatments and oral NSAIDs and intra-articular injections for persistent pain 1.

Key points to consider in the management of knee osteoarthritis include:

• The importance of weight loss and exercise in reducing symptoms and slowing disease progression
• The use of analgesics, including NSAIDs and intra-articular corticosteroid injections, for pain management
• The potential benefits and limitations of surgical interventions, such as joint replacement surgery, in advanced cases of knee osteoarthritis. It is essential to consider the individual patient's needs and circumstances when developing a treatment plan, taking into account factors such as disease severity, comorbidities, and personal preferences 1.

From the Research

Primary Site of Involvement in Knee Osteoarthritis

The primary site of involvement in knee osteoarthritis can vary, but research suggests that the medial compartment is most commonly affected.

• A study published in 2018 found that medial compartment involvement was present in 91.7% of knees, while lateral compartment and patellofemoral joint involvement were found in 32.1% and 33.9% of knees, respectively 2.
• Another study published in 2007 found that the greatest cartilage volume loss was found in the medial compartment, with risk factors including female gender, joint space width, meniscal lesions, and bone changes at baseline 3.
• The medial compartment was also found to be the most commonly affected in a study published in 2019, which examined the patterns of cartilage degeneration in knees with medial tibiofemoral offset 4.

Patterns of Compartment Involvement

The patterns of compartment involvement in knee osteoarthritis can vary, but research suggests that:

• Medial compartment OA with or without patellofemoral joint involvement is a common pattern, found in 48.2% of knees in one study 2.
• Isolated lateral OA is less common, found in 6.5% of knees in one study 2.
• No patients showed isolated patellofemoral joint OA in one study 2.

Risk Factors for Cartilage Volume Loss

Research has identified several risk factors for cartilage volume loss in knee osteoarthritis, including:

• Female gender 3.
• Joint space width 3.
• Meniscal lesions 3.
• Bone changes at baseline 3.
• High body mass index (BMI) 3.
• Smaller joint space width 3.
• Increases in Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) pain and patient global scores over time 3.
• Greater joint space narrowing 3.