Cartilage is a resilient, flexible connective tissue that covers the ends of bones in joints, acting as a smooth, shock-absorbing surface. While it cannot break with a clean line like a brittle bone, cartilage can be severely damaged, leading to debilitating joint problems. This damage typically involves tearing, rupturing, or wearing away of the tissue, which ultimately compromises the joint’s function.
Understanding the Difference Between Bone and Cartilage Injury
Bone is a hard, mineralized tissue with a rich blood supply and nerve innervation, allowing it to heal effectively and send pain signals when injured. A fracture is a break in this rigid, mineralized matrix, which triggers a robust healing response involving blood clot formation and bone remodeling. Cartilage, particularly the articular cartilage found in joints, is structurally different because it is avascular (lacking a direct blood supply) and aneural (having no nerves).
This lack of blood vessels means that when cartilage is damaged, the necessary healing factors, such as inflammatory cells and stem cells from the bloodstream, cannot easily reach the injury site. Cartilage receives its nourishment slowly through diffusion from the surrounding synovial fluid. Consequently, cartilage does not “fracture” but rather suffers defects, lesions, or tears due to its flexible, non-mineralized nature.
The absence of nerves explains why some cartilage damage can progress without immediate, sharp pain. Pain associated with cartilage loss is often due to the irritation of surrounding joint structures, such as the underlying bone or the joint lining, particularly in conditions like osteoarthritis. Because the specialized cells within cartilage, called chondrocytes, have minimal ability to divide in adults, the tissue has a very limited capacity for self-repair.
Specific Types of Cartilage Damage
Cartilage damage is broadly categorized into acute, traumatic injuries and chronic, degenerative conditions. Acute trauma, often seen in sports injuries, can result in a sudden tear or a localized defect. These defects are classified as chondral lesions if the damage is confined to the cartilage layer.
A more severe injury is an osteochondral lesion, where the damage extends completely through the cartilage and into the underlying subchondral bone. This type of injury is significant because the penetration into the bone can sometimes trigger a limited healing response from the bone marrow. Damage is graded based on severity, ranging from simple softening of the surface (Grade 1) to full-thickness loss that exposes the bone beneath (Grade 4).
Chronic damage is characterized by the gradual, progressive breakdown of the smooth articular surface, most commonly recognized as osteoarthritis. This wear-and-tear phenomenon causes the cartilage to thin and shred over time, losing its ability to cushion the joint. As the disease advances, the cartilage may completely wear away, causing painful bone-on-bone contact and chronic joint inflammation.
Recognizing and Diagnosing Cartilage Problems
Recognizing a cartilage problem often begins with noticing specific symptoms, such as persistent joint pain that worsens with activity. Swelling, stiffness, and a decreased range of motion are common indicators of joint surface irregularity. A distinctive symptom of a cartilage tear or flap is a “catching” or “locking” sensation in the joint, which occurs when a loose fragment of tissue interferes with the joint’s smooth movement.
Diagnosis typically starts with a physical examination but requires advanced imaging because of cartilage’s soft-tissue nature. Standard X-rays are generally insufficient for viewing cartilage directly, as they primarily visualize bone structure. They are useful for identifying bone spurs or signs of joint space narrowing that suggest advanced cartilage loss.
Magnetic Resonance Imaging (MRI) is the preferred non-invasive method for visualizing cartilage, as it can reveal the extent of the defect and the condition of the surrounding soft tissues. For the most definitive diagnosis, an arthroscopy may be performed, a minimally invasive procedure where a surgeon inserts a small camera into the joint. Arthroscopy provides a direct, magnified view of the cartilage surface, allowing for precise grading of the damage and often immediate surgical repair.
Repair and Regeneration Options for Cartilage Injuries
Because cartilage has such a poor natural healing capacity, treatment options range from conservative management to complex surgical procedures aimed at restoration. Non-surgical approaches include physical therapy to strengthen supporting muscles and the use of non-steroidal anti-inflammatory drugs (NSAIDs) to manage pain and swelling. These methods help manage symptoms but do not repair the damaged tissue itself.
Surgical intervention is often necessary for significant defects, utilizing techniques designed to either stimulate new growth or replace the damaged area.
Microfracture
Microfracture is a common arthroscopic procedure where small holes are created in the subchondral bone beneath the defect. This allows bone marrow containing stem cells and blood to access the area, forming a blood clot that eventually matures into repair tissue called fibrocartilage. While fibrocartilage is durable, it is biomechanically inferior to the native hyaline cartilage found in joints.
Cartilage Replacement Techniques
For larger or more active patients, surgeons use replacement techniques:
- Osteochondral Autograft Transfer System (OATS): This involves transplanting plugs of healthy bone and hyaline cartilage from a less weight-bearing area of the joint to the damaged site.
- Autologous Chondrocyte Implantation (ACI): This two-step procedure involves harvesting a patient’s own cartilage cells, growing them in a lab, and then reimplanting them into the defect to promote the growth of hyaline-like tissue.