What is the role of cartilage in joint health?

The Critical Role of Cartilage in Joint Health

Cartilage serves as the essential protective cushion in joints, providing a smooth, low-friction surface that enables pain-free movement while distributing mechanical forces and preventing bone-on-bone contact. 1

Cartilage Structure and Function

Articular cartilage is a specialized connective tissue that covers the ends of bones within synovial joints. It serves several critical functions:

• Shock absorption: Distributes mechanical forces across the joint surface
• Friction reduction: Provides a smooth, lubricated surface for joint movement
• Load bearing: Supports and transfers weight across the joint
• Joint stability: Contributes to proper joint alignment and function

Cartilage is organized into four distinct biological layers or zones, each with specific properties that contribute to its overall function 2:

1. Superficial zone: Outermost layer with densely packed collagen fibers parallel to the surface
2. Middle zone: Contains obliquely arranged collagen fibers and proteoglycans
3. Deep zone: Features collagen fibers perpendicular to the surface with high proteoglycan content
4. Calcified zone: Transitions between cartilage and subchondral bone

Types of Cartilage in Joints

Different types of cartilage serve specific functions in the body 3:

• Hyaline cartilage: The primary type found in synovial joints (knees, hips, shoulders)
• Fibrocartilage: Found in intervertebral discs and menisci
• Elastic cartilage: Present in the external ear and parts of the respiratory tract

Cartilage Damage and Joint Disease

Cartilage damage is a central feature in joint disorders, particularly osteoarthritis (OA). The relationship between cartilage and joint health is complex:

• Delamination injuries: Cartilage can separate from underlying subchondral bone, creating flaps or loose bodies within the joint 1
• Chondral defects: Full-thickness defects expose subchondral bone and can lead to progressive joint deterioration 1
• Osteoarthritis progression: Cartilage loss in OA occurs as part of a complex process resembling aspects of embryonic bone formation 4

The Outerbridge and Beck classifications help categorize the severity of cartilage damage, ranging from softening to complete loss with exposed subchondral bone 1.

Imaging of Cartilage and Joint Health

Imaging plays a crucial role in evaluating cartilage health:

• Radiography: Weightbearing views are essential for optimal detection of joint space narrowing in knee OA 1
• MRI: Superior for evaluating soft tissue and detecting early cartilage damage 5

However, routine imaging for OA follow-up is not recommended unless there is unexpected rapid progression of symptoms or change in clinical characteristics 1.

Management of Cartilage Damage

Treatment options for cartilage damage depend on lesion size, location, and patient factors:

For Small Lesions (<2 cm²):

• Microfracture: Creating small holes in subchondral bone to stimulate fibrocartilage formation 1
• Mosaicplasty: Using autologous osteochondral grafts to fill defects 1

For Medium Lesions (2-6 cm²):

• Osteochondral allograft transplantation: Using donor tissue to replace damaged cartilage and bone 1

For Large Lesions (>6-8 cm²):

• Total joint arthroplasty may be considered for extensive damage 1

Prevention and Maintenance

To maintain cartilage health:

• Exercise therapy: Core strengthening, hip girdle muscle strengthening, and range of motion exercises 5
• Weight management: Reduces mechanical stress on weight-bearing joints 5
• Activity modification: Avoiding high-impact and torsional loads that may increase risk of cartilage damage 6

Clinical Implications

The relationship between cartilage damage and symptoms is complex. Studies have shown that:

• Joints with radiographic progression have higher odds for tenderness 1
• Changes in bone marrow lesions (which often accompany cartilage damage) correlate with changes in pain status 1

Common Pitfalls in Cartilage Management

• Delayed diagnosis: Due to the aneural nature of cartilage, damage may progress before symptoms appear 3
• Drug delivery challenges: The avascular nature of adult cartilage makes targeted treatment difficult 3
• Overreliance on imaging: Radiographic findings don't always correlate with symptoms; clinical assessment remains essential 1

Understanding the fundamental role of cartilage in joint health is crucial for appropriate management of joint disorders and prevention of progressive joint deterioration.