What is cartilage and why is it so important?
Cartilage is a flexible connective tissue found in many parts of the body, including the joints, ears, and nose. It is primarily composed of specialized cells called chondrocytes, which produce a large amount of extracellular matrix made of collagen fibers and proteoglycans. This structure gives cartilage its ability to resist compressive forces and provides a smooth surface for joint movement.
There are three main types of cartilage, each with a different role:
- Hyaline cartilage: The most common type, found at the ends of bones in synovial joints. It provides a smooth, low-friction surface for joints to glide effortlessly. This is the type that is most relevant when discussing damaged joint cartilage.
- Fibrocartilage: A tougher, more rigid form found in the intervertebral discs and the menisci of the knee. It contains dense collagen fibers and acts as a shock absorber.
- Elastic cartilage: Contains elastic fibers, making it the most flexible type, found in the external ear and epiglottis.
The unique composition and location of articular (hyaline) cartilage—the type covering the ends of bones—are precisely what limit its natural healing capacity. It's a non-vascular tissue, meaning it has no direct blood supply to deliver nutrients and healing cells to the site of damage, unlike bone, which heals readily.
Why damaged cartilage doesn't heal naturally
The reason damaged cartilage does not grow back is multi-faceted, stemming from its fundamental biological makeup. Several key factors are at play:
- Avascularity: As mentioned, cartilage lacks a blood supply. Nutrients reach chondrocytes via slow diffusion from the synovial fluid in the joint. This slow process is insufficient to support the rapid cell proliferation needed for significant repair.
- Aneural: Cartilage contains no nerves. While this means damaged cartilage tissue itself is not a source of pain, it also means the body receives no warning signal to initiate a strong inflammatory and healing response.
- Limited Chondrocyte Mobility and Reproduction: Mature chondrocytes are fixed in a lacuna within the matrix and have a low mitotic rate (ability to divide). This means they cannot easily migrate to an injury site or produce enough new cells to regenerate lost tissue.
- Formation of Fibrocartilage: When a cartilage injury extends down to the underlying bone (subchondral bone), the bone's blood supply can initiate a repair response. However, the resulting tissue is not hyaline cartilage but inferior fibrocartilage. While fibrocartilage provides some structural support, it is not as durable or smooth as the original tissue, and it often breaks down over time.
Medical procedures for cartilage repair
Although natural regrowth is not possible, modern orthopedic surgery offers several techniques to help repair or replace damaged cartilage. The most suitable option depends on the patient's age, the location and size of the defect, and the overall condition of the joint.
Microfracture surgery
This is one of the most common and longest-standing surgical techniques. It involves arthroscopically creating small holes in the subchondral bone, below the damaged cartilage. This allows blood and bone marrow stem cells to seep out and form a fibrin clot. The clot eventually matures into a type of fibrocartilage. While it offers a short-term solution for smaller lesions, the fibrocartilage is less resilient than hyaline cartilage and may wear away over time.
Autologous Chondrocyte Implantation (ACI) and MACI
Matrix-induced autologous chondrocyte implantation (MACI) is a more advanced, two-stage procedure. First, a surgeon takes a small sample of the patient's own healthy cartilage cells (chondrocytes) from a non-weight-bearing area of the joint. These cells are then sent to a lab to be cultured and expanded over several weeks. In a second surgery, the cultured cells are implanted into the defect, often on a collagen patch, to regrow a new layer of cartilage-like tissue. This technique has shown good long-term results, especially for younger patients with isolated cartilage damage.
Osteochondral autograft transplantation (OATS)
Also known as mosaicplasty, this procedure involves harvesting small plugs of healthy bone and cartilage from a less critical, non-weight-bearing area of the patient's joint. These plugs are then transferred to fill the cartilage defect in a weight-bearing zone, like a mosaic. This method uses the patient's own hyaline cartilage, which is durable, but is typically only used for smaller defects.
Osteochondral allograft transplantation
For larger cartilage and bone defects, a surgeon may use donor tissue (allograft) to resurface the joint. This can be an effective option when autograft plugs are not sufficient to cover the damaged area. A major benefit is that no healthy cartilage needs to be harvested from the patient's own joint.
Comparison of surgical cartilage repair techniques
| Feature | Microfracture | MACI/ACI | OATS | Allograft |
|---|---|---|---|---|
| Technique | Drilling small holes to release stem cells | Two-stage: harvest cells, culture, implant | Transplanting bone/cartilage plugs from patient | Transplanting bone/cartilage plugs from donor |
| Tissue Type | Fibrocartilage (less durable) | Hyaline-like cartilage | Hyaline cartilage | Hyaline cartilage |
| Best For | Small, isolated defects | Isolated defects in younger patients | Small to medium defects | Large or complex defects |
| Invasiveness | Minimally invasive (arthroscopic) | More invasive (two surgeries) | Moderately invasive | Moderately invasive |
| Recovery Time | Shorter (months) | Longer (12-18 months) | Moderate (6 months) | Moderate (6 months) |
| Durability | Limited; may degrade over time | Good; durable repair tissue | Excellent; uses native tissue | Excellent; for large lesions |
Non-surgical management and supplements
For many patients, especially those with generalized arthritis rather than an isolated defect, non-surgical approaches are the primary method of managing symptoms. These strategies do not regrow cartilage but can significantly improve quality of life.
Lifestyle modifications
- Weight management: Reducing excess weight lessens the load on weight-bearing joints, which can slow cartilage wear and decrease pain.
- Low-impact exercise: Activities like swimming, cycling, and walking improve joint lubrication and strengthen surrounding muscles without stressing damaged cartilage.
Injections and other treatments
- Corticosteroid injections: Provide powerful, short-term relief from joint inflammation and pain.
- Viscosupplementation: Injections of hyaluronic acid can help improve joint lubrication and reduce pain. These do not regrow cartilage but act as a temporary synthetic joint fluid.
- Platelet-Rich Plasma (PRP) and stem cell injections: While heavily marketed, many experts caution that there is currently no evidence that these injections can actually regrow cartilage. They may provide anti-inflammatory effects or support healing, but are not a "cure" for cartilage loss.
Supplements
Oral supplements like glucosamine and chondroitin sulfate are widely used but have mixed scientific evidence regarding their effectiveness. The consensus is that they do not regrow cartilage but might help manage symptoms in some individuals. Always consult with a physician before starting any new supplement.
The future of cartilage regeneration research
Scientific research into true cartilage regeneration is advancing. Scientists are studying the limited regenerative capacity that humans possess, similar to that seen in amphibians like salamanders. Promising areas of research include:
- Tissue engineering: Developing scaffolds and hydrogels that can be seeded with cells and growth factors to encourage the growth of new, high-quality cartilage tissue.
- Gene therapy: Techniques to stimulate the patient's own cells to produce cartilage-specific proteins.
- Biologically-active tissue substitutes: Creating living tissue substitutes in a lab to be implanted into the damaged joint.
While this research offers hope for the future, the treatments are not yet available for widespread clinical use.
Conclusion
At present, the definitive answer to the question "can bone cartilage grow back?" is no, not naturally. Mature articular cartilage lacks the biological mechanisms—primarily a blood supply and active stem cells—to repair itself effectively. However, the medical field has developed multiple surgical and non-surgical treatments to address cartilage damage, from implanting a patient's own cultured cells to managing symptoms with exercise and injections. The choice of treatment is a highly individualized decision best made in consultation with an orthopedic specialist.
For those interested in the ongoing medical advances in this field, the Hospital for Special Surgery offers excellent resources on the latest cartilage repair and regeneration techniques.
