Articular cartilage is a specialized, smooth connective tissue covering the ends of bones in the knee joint, serving to reduce friction and absorb shock during movement. This tissue allows for the smooth gliding motion necessary for activities like walking or running. Damage to this cartilage is a highly prevalent issue, often resulting from acute trauma or the cumulative wear associated with conditions like osteoarthritis. Restoring the structure and function of this tissue is a significant medical challenge, requiring approaches from lifestyle adjustments and injections to advanced surgical repair techniques.
Why Knee Cartilage Does Not Heal Easily
The fundamental challenge in healing damaged knee cartilage stems from its unique biological structure. Unlike most other tissues in the body, articular cartilage is avascular, meaning it lacks a direct blood supply. This absence prevents the delivery of inflammatory cells, growth factors, and stem cells necessary to initiate the typical wound healing cascade.
Nutrients reach the resident cartilage cells, called chondrocytes, only through slow diffusion from the surrounding synovial fluid. Chondrocytes have a limited capacity for proliferation and cannot multiply fast enough to repair significant defects. The dense extracellular matrix surrounding these cells further isolates them, hindering cellular migration into the injury site. This biological isolation explains why injuries larger than a few millimeters rarely heal naturally, often leading to the formation of biomechanically inferior fibrocartilage rather than the original hyaline cartilage.
Conservative Strategies for Joint Health
Initial management of knee cartilage issues often centers on non-invasive conservative strategies aimed at slowing degeneration and alleviating symptoms. One of the most impactful interventions involves active weight management to reduce the mechanical load placed on the joint. Every extra pound of body weight can translate into several additional pounds of pressure on the knees during activities like walking or climbing stairs.
Physical therapy plays a significant role by focusing on strengthening the muscles surrounding the knee, particularly the quadriceps, hamstrings, and glutes. Strengthening these muscle groups provides better support and stability, which helps offload stress from the damaged cartilage and improves overall joint alignment. Therapists often incorporate low-impact activities, such as cycling, swimming, or aquatic therapy, to maintain joint mobility without introducing excessive impact forces.
Activity modification requires patients to substitute high-impact movements, such as running or jumping, with less strenuous alternatives. Assistive devices or bracing may be utilized temporarily to reduce strain and provide external stability during periods of increased pain. Some individuals use over-the-counter dietary supplements like glucosamine and chondroitin, which are natural components of healthy cartilage. Studies on these supplements have yielded mixed results regarding their ability to reduce pain, but they are often considered safe due to their low risk profile.
Injectable and Biologic Therapies
When conservative strategies are insufficient, the next level of treatment often involves intra-articular injections directly into the knee joint. Viscosupplementation involves injecting hyaluronic acid (HA) into the joint space to restore the natural lubricating properties of the synovial fluid. HA, a complex sugar molecule, acts as a shock absorber and lubricant, reducing friction between joint surfaces. The injection is intended to temporarily replenish the HA content, which is typically diminished in arthritic knees, and may also exert anti-inflammatory effects.
Corticosteroid injections are another common treatment, primarily used for their anti-inflammatory effects to rapidly reduce pain and swelling. These injections contain synthetic cortisone, which dampens the localized inflammatory response within the joint. Corticosteroids do not promote cartilage healing, and frequent injections may accelerate cartilage thinning or damage over time. Due to these concerns, physicians typically limit the frequency of these injections in weight-bearing joints.
Regenerative medicine treatments, known as biologics, aim to harness the body’s own healing mechanisms. Platelet-Rich Plasma (PRP) therapy concentrates the patient’s blood platelets, which are rich in growth factors, and injects them into the joint. These growth factors signal the body to initiate repair processes and reduce inflammation. Stem cell therapy involves introducing mesenchymal stem cells, often harvested from the patient’s bone marrow or fat tissue, into the damaged area. While promising, many stem cell treatments remain experimental in the context of definitive cartilage repair.
Surgical Procedures for Cartilage Repair
Surgical intervention is typically reserved for patients with symptomatic, localized defects in the articular cartilage that have not responded to less invasive measures. The microfracture procedure is a technique that aims to stimulate the body’s natural healing response. The surgeon uses a specialized awl to create tiny holes in the subchondral bone. This penetration allows blood and bone marrow, which contain mesenchymal stem cells, to seep out and form a super-clot over the defect. This clot eventually matures into fibrocartilage, which is biomechanically inferior to the native hyaline cartilage but can provide a functional repair surface.
Another option is the Osteochondral Autograft Transfer System (OATS), also known as mosaicplasty, which involves transplanting healthy tissue. This single-stage procedure requires the surgeon to harvest small plugs of bone and hyaline cartilage from a non-weight-bearing area of the patient’s knee. These plugs are then transferred and implanted into the prepared defect site, resembling a mosaic pattern. OATS offers the advantage of immediately filling the defect with mature hyaline cartilage, though the size of the defect that can be treated is limited by the availability of donor tissue.
For larger defects, Autologous Chondrocyte Implantation (ACI), commonly performed as Matrix-Induced ACI (MACI), is a two-step procedure. The first step involves an arthroscopic biopsy to harvest a small sample of the patient’s chondrocytes. These cells are then cultured in a laboratory to increase their numbers over several weeks. In the second procedure, the cultured cells are seeded onto a specialized collagen membrane and implanted into the prepared cartilage defect, secured with fibrin glue. This technique encourages the growth of durable repair tissue that closely resembles the native hyaline cartilage.