Why does knee articular cartilage degeneration cause joint space narrowing?

Why Knee Articular Cartilage Degeneration Causes Joint Space Narrowing

Knee joint space narrowing occurs because articular cartilage degeneration reduces the cushioning space between the femur and tibia, allowing the bones to move closer together on weight-bearing radiographs. 1, 2

Biomechanical Mechanism of Joint Space Narrowing

• Articular cartilage serves as a cushioning layer between the femur and tibia, and its degeneration directly reduces the physical space between these bones 1, 2
• The medial compartment is most commonly affected in osteoarthritis, carrying approximately 70-80% of joint load during mid-stance phase of gait, making it particularly vulnerable to cartilage loss 1
• As cartilage degrades, the load distribution across the knee becomes altered, potentially accelerating further cartilage degeneration in a progressive cycle 1, 2
• Malalignment of the mechanical axis of the lower limb is both a risk factor for and a consequence of joint space narrowing, contributing to the progression of cartilage loss 1, 3

Radiographic Detection of Joint Space Narrowing

• Joint space narrowing is a cardinal radiographic sign of osteoarthritis, visible on weight-bearing radiographs 3, 1
• Standing anteroposterior (AP) and lateral radiographic views are recommended as standard for assessing degenerative changes and joint space narrowing 3, 2
• Weight-bearing radiographs are essential for accurate assessment as they demonstrate the functional narrowing that occurs under load 3
• Radiographic joint space narrowing appears specific (91%) but not sensitive (23%) for cartilage loss, meaning radiographs will miss a substantial proportion of knees with cartilage loss 4

Limitations of Radiographic Assessment

• Radiographs cannot directly visualize cartilage and may miss early cartilage changes before joint space narrowing becomes apparent 5, 4
• A substantial proportion of knees (42%) show cartilage loss on MRI when no radiographic progression is apparent 4
• In some studies, 33% of patients with radiographic tibiofemoral joint space narrowing had normal articular cartilage at arthroscopy (false-positive findings) 6
• Radiographs cannot distinguish between cartilage loss and other causes of joint space narrowing such as meniscal extrusion or degeneration 6, 7

Advanced Imaging for Cartilage Assessment

• MRI is superior to radiography for detecting early cartilage changes due to its direct multiplanar acquisition and superior soft-tissue contrast 5, 4
• MRI can detect cartilage loss in specific regions that may not be apparent on standard radiographs, particularly in the posterior femoral regions 4
• Standardized-flexion radiographs show significant correlation with cartilage volume and thickness in the medial femorotibial compartment in neutral and varus knees, and in lateral tibial subregions in valgus knees 8
• However, no statistically significant correlation exists between radiographic joint space width and cartilage loss in anterior and posterior knee subregions 8

Clinical Implications

• Joint space narrowing alters biomechanical loading patterns across the knee, potentially accelerating disease progression 3, 1
• Early detection of cartilage degradation before significant joint space narrowing may allow for earlier intervention to potentially slow disease progression 5
• Realignment bracing can increase medial condylar separation by an average of 1.2 mm and change tibiofemoral coronal angle by approximately 2.2° at initial contact, potentially modifying the progression of joint space narrowing 3, 2
• Knee cartilage defects are associated with increased type II collagen breakdown, which contributes to decreased cartilage volume and subsequent joint space narrowing 9

Risk Factors for Progressive Joint Space Narrowing

• Obesity significantly increases joint loading and accelerates progression of joint space narrowing 1
• Muscle weakness, particularly of quadriceps, hamstrings, and gastrocnemius, contributes to joint instability and progression 3, 1
• Previous joint injury, especially to cruciate ligaments or menisci, increases risk of progression by altering joint biomechanics 1, 7
• Bilateral lower limb malalignment of more than 5° is associated with greater physical functional decline and accelerated joint space narrowing 3, 2