What are the primary causes of knee joint space narrowing?

Primary Causes of Knee Joint Space Narrowing

The primary causes of knee joint space narrowing are osteoarthritis, meniscal tears, and malalignment of the mechanical axis of the lower limb, with osteoarthritis being the most common cause affecting nearly one in two people by age 85. 1

Osteoarthritis as Primary Cause

Osteoarthritis (OA) is the leading cause of knee joint space narrowing and the most common cause of disability in adults in the United States. The disease process involves:

• Loss of hyaline cartilage, subchondral bone attrition and remodeling, meniscal degeneration, ligamentous laxity, fat pad extrusion, and impairments of neuromuscular control 1
• Altered distribution of loads over the articular surface, creating a cycle where aberrant loading leads to further disease progression 1
• Medial compartment involvement is most common, with approximately 70-80% of joint load passing through this compartment during mid-stance phase of gait 1

Radiographic Assessment of Joint Space Narrowing

Joint space narrowing (JSN) on radiographs is considered a surrogate marker for cartilage loss in osteoarthritis:

• Standing AP, lateral, and Merchant radiographic views are standard for assessing degenerative changes and joint space narrowing 1
• The degree of JSN correlates with disease severity, with OARSI JSN grades 1,2, and 3 displaying approximately 5.2%, 18%, and 44% smaller cartilage thickness in weight-bearing medial femorotibial compartments compared to knees without JSN 2
• The central part of the weight-bearing femoral condyle is most strongly affected by JSN-related cartilage loss 2

Meniscal Tears and Joint Space Narrowing

Meniscal tears contribute significantly to joint space narrowing:

• Higher grades of JSN are correlated with a higher frequency of meniscal tears (odds ratio 6.00,95% CI 1.29-27.96 for grade 2 vs grade 1 JSN) 3
• Among patients with JSN who don't have articular cartilage degeneration, meniscus degeneration accounts for 50% of cases 4
• Duration of knee pain is also associated with higher grades of JSN (OR 1.25,95% CI 1.01-1.53 for grade 3 vs grade 1 JSN) 3

Malalignment and Joint Space Narrowing

Malalignment of the mechanical axis of the lower limb contributes to joint space narrowing:

• Malalignment is a risk factor for progression of radiographic joint-space narrowing, cartilage loss, and decline of physical function 1
• Bilateral lower limb malalignment of more than 5° (either varus or valgus) is associated with greater physical functional decline compared to less than 5° of malalignment 1
• Varus alignment increases loading in the medial compartment, while valgus alignment increases loading in the lateral compartment 5

Clinical Relevance of Joint Space Narrowing

Joint space narrowing has important clinical implications:

• A minimum joint space narrowing of 0.7 mm or more over three years is associated with a 5.15 times higher risk (95% CI, 1.70 to 15.60) of requiring osteoarthritis-related surgery during eight years of follow-up 6
• Knees with higher JSN grades display larger subchondral bone areas, suggesting that an increase in subchondral bone area occurs in advanced OA 2
• Fluoroscopic assessment during gait has demonstrated that 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 1

Caution in Interpretation

It's important to note that radiographic JSN doesn't always correlate perfectly with cartilage damage:

• 33% of patients with radiographic tibiofemoral JSN may have grossly normal articular cartilage in both tibiofemoral compartments at arthroscopy (false-positive) 4
• Of patients with greater than 50% medial JSN, 41% may have normal articular cartilage in the medial compartment at arthroscopy 4
• JSN may be associated with patellofemoral compartment cartilage degeneration (22%) rather than tibiofemoral degeneration in some cases 4