Residency · Residency · Orthopedic Surgery

Articular Cartilage Structure and Repair Mechanisms

Cartilage Biology

Basic Properties

Hyaline articular cartilage is a remarkable tissue defined by three conspicuous absences: it has no nerves, no blood vessels, and no lymphatic drainage. All nutrition arrives by diffusion from the synovial fluid, a process that depends on the cyclical loading and unloading of the joint (essentially squeezing fluid in and out of the matrix). Chondrocytes occupy only about 1-5% of the tissue volume, making cartilage one of the most sparsely cellular tissues in the body. This low cellularity, combined with avascularity, explains cartilage's extremely limited capacity for intrinsic repair. Despite these limitations, healthy cartilage achieves a coefficient of friction lower than ice sliding on ice (approximately 0.001-0.01), making it one of the most efficient bearing surfaces known.

Zonal Architecture

Articular cartilage is organized into distinct layers, each with specialized structural and functional properties.

Superficial (Tangential) Zone (10-20%)

The superficial zone contains flattened, elongated chondrocytes with collagen fibers oriented parallel to the articular surface. This zone has the highest collagen content and lowest proteoglycan content of any layer. It produces lubricin (PRG4), a glycoprotein critical for boundary lubrication of the joint surface. The parallel fiber arrangement gives this zone the highest tensile strength, enabling it to resist shear forces generated during joint motion.

Transitional (Middle) Zone (40-60%)

The transitional zone features rounded chondrocytes dispersed within randomly (obliquely) oriented collagen fibers. It serves as the first line of resistance to compressive forces and has a higher proteoglycan content than the superficial zone, contributing to the tissue's ability to resist compression through osmotic swelling pressure.

Deep (Radial) Zone (30%)

In the deep zone, chondrocytes are arranged in columns perpendicular to the articular surface, mirroring the perpendicular orientation of the collagen fibers. This zone has the highest proteoglycan content and lowest water content, providing the greatest resistance to compressive forces.

Tidemark

The tidemark is a basophilic line visible on histology that separates the non-calcified cartilage above from the calcified cartilage below. It represents the active mineralization front and serves as an important barrier for nutrient diffusion into the deeper layers.

Calcified Cartilage Zone

The deepest layer contains hypertrophic, sparse chondrocytes surrounded by Type X collagen rather than the Type II collagen found elsewhere. This zone anchors the overlying cartilage to the subchondral bone, with the cement line marking the cartilage-bone interface.

Extracellular Matrix Composition

The extracellular matrix is what gives cartilage its mechanical properties. Water constitutes 65-80% of wet weight, with the highest concentration in the superficial zone. Type II collagen makes up roughly 60% of the dry weight and provides tensile strength through its cross-linked triple helix structure; Types IX and XI collagen contribute to network stability. Proteoglycans, primarily aggrecan, are the other major structural component. Aggrecan monomers bind to a hyaluronic acid backbone via link protein, forming massive aggregates. The negatively charged glycosaminoglycan side chains (chondroitin sulfate and keratan sulfate) attract water molecules, generating an osmotic swelling pressure that resists compressive loads. Non-collagenous proteins including COMP, fibronectin, and lubricin perform various regulatory and lubricating functions.

Chondrocyte Metabolism

Chondrocytes operate under anaerobic conditions in a low-oxygen environment (approximately 1-7% O2) and maintain a low metabolic rate with limited proliferative capacity in mature cartilage. They are mechanoresponsive: moderate cyclic loading promotes matrix synthesis and maintains cartilage health, while excessive loading or prolonged immobilization triggers matrix degradation. The principal catabolic enzymes responsible for matrix breakdown are matrix metalloproteinases (particularly MMP-13, which degrades Type II collagen) and ADAMTS enzymes (aggrecanases that cleave proteoglycans).

Why Cartilage Has Limited Healing

The inability of cartilage to heal itself stems from several interconnected factors. Avascularity prevents the delivery of inflammatory cells and mesenchymal stem cells that would normally initiate a repair response. Resident chondrocytes have limited capacity to migrate to the site of injury or proliferate to fill defects. Damage confined to the superficial zones never reaches the subchondral bone or its blood supply, so no reparative response is triggered. When a defect does penetrate through to the subchondral bone (a full-thickness defect), marrow elements can access the site and produce a repair tissue, but this tissue is fibrocartilage composed predominantly of Type I collagen. Fibrocartilage lacks the organized zonal architecture of native hyaline cartilage and is mechanically inferior, ultimately failing under repetitive joint loading.

Classification of Cartilage Lesions

Outerbridge Classification

The Outerbridge system grades cartilage damage arthroscopically: Grade 0 is normal cartilage; Grade I shows softening and swelling; Grade II has fragmentation or fissuring less than 1.5 cm in diameter; Grade III shows fragmentation or fissuring greater than 1.5 cm extending down to subchondral bone; and Grade IV indicates exposed subchondral bone.

ICRS (International Cartilage Repair Society) Classification

The ICRS system provides a more granular assessment: Grade 0 is normal; Grade 1 is nearly normal with superficial lesions only; Grade 2 involves lesions extending to less than 50% of cartilage depth; Grade 3 describes severely abnormal lesions extending greater than 50% of depth but not through subchondral bone; and Grade 4 indicates lesions penetrating through the subchondral bone plate.

Cartilage Repair and Restoration Techniques

Marrow Stimulation Techniques

Microfracture (Steadman Technique)

Microfracture is indicated for small, contained chondral defects (less than 2-4 cm2) in young patients. The technique involves debriding the damaged cartilage to create a stable rim, removing the calcified cartilage layer, and then creating perforations 3-4 mm deep and spaced 3-4 mm apart using an arthroscopic awl. These perforations allow marrow elements including MSCs and growth factors to fill the defect and form a "super clot" that matures into fibrocartilage. Short-term results are generally good with meaningful symptom relief, but outcomes tend to deteriorate at five or more years because the fibrocartilage repair tissue is mechanically inferior to native hyaline cartilage. The advantages of microfracture are that it is a single-stage, low-cost, arthroscopic procedure.

Enhanced Marrow Stimulation

Newer techniques attempt to improve upon standard microfracture. Autologous matrix-induced chondrogenesis (AMIC) combines microfracture with a collagen membrane overlay to protect the clot. BST-CarGel uses a chitosan-based scaffold applied over the microfracture bed to stabilize the repair tissue during maturation.

Cell-Based Techniques

Autologous Chondrocyte Implantation (ACI)

ACI has evolved through three generations since its introduction by Brittberg in 1994. The first generation involved harvesting chondrocytes arthroscopically, expanding them in culture, and reimplanting them under a periosteal patch. The second generation replaced the periosteal patch with a collagen membrane to reduce the problem of patch hypertrophy. The third generation (MACI) seeds the cultured chondrocytes directly onto a collagen scaffold membrane. ACI is indicated for larger defects (2-10 cm2) in young patients, particularly those who have failed microfracture. It is a two-stage procedure: an initial arthroscopic biopsy for chondrocyte harvest, followed by 4-6 weeks of cell culture, and then a definitive implantation procedure. ACI produces hyaline-like repair tissue in many cases and has demonstrated superior durability compared to microfracture at long-term follow-up, as shown in the STAR trial.

Particulated Juvenile Cartilage Allograft (DeNovo NT)

This single-stage procedure uses minced juvenile articular cartilage allograft fixed with fibrin glue. Juvenile chondrocytes retain greater proliferative capacity than adult cells, potentially producing better repair tissue. However, long-term outcome data remain limited.

Osteochondral Transfer Techniques

Osteochondral Autograft Transfer (OATS/Mosaicplasty)

OATS involves harvesting cylindrical osteochondral plugs from a non-weight-bearing area (typically the periphery of the femoral trochlea) and press-fitting them into the prepared defect site. This provides mature hyaline cartilage with intact subchondral bone architecture in a single-stage procedure. It works best for small-to-medium defects (1-4 cm2). Limitations include donor site morbidity, difficulty matching the articular surface contour, and limited available graft volume.

Osteochondral Allograft Transplantation (OCA)

Fresh osteochondral allograft is suitable for large defects (greater than 3 cm2), particularly those involving subchondral bone loss. The graft must be implanted within 28 days of harvest to maintain chondrocyte viability. OCA eliminates donor site morbidity and can address larger defects than autograft techniques. Concerns include the extremely low but nonzero risk of disease transmission, declining chondrocyte viability with prolonged storage, and issues of cost and availability.

Scaffold-Based Approaches

A variety of synthetic and biologic scaffolds are under active investigation. The goal is to provide a three-dimensional environment that supports cell attachment, proliferation, and matrix production. Materials being studied include collagen, hyaluronic acid, polyglycolic acid, and various composite scaffolds. Some approaches combine scaffolds with growth factors or mesenchymal stem cells. Many of these technologies remain in clinical trials.

Emerging Technologies

The frontier of cartilage repair includes stem cell therapies using bone marrow aspirate concentrate (BMAC) or adipose-derived MSCs, gene therapy approaches for sustained local delivery of growth factors, 3D bioprinting of cartilage constructs with organized zonal architecture, and the use of induced pluripotent stem cells (iPSCs) to generate patient-specific chondrocytes.

<image>A detailed cross-sectional illustration of articular cartilage showing its four distinct zones: superficial zone with flattened chondrocytes and parallel collagen fibers, transitional zone with rounded chondrocytes and oblique collagen, deep zone with columnar chondrocytes and perpendicular collagen, and calcified cartilage zone with the tidemark boundary. Include the subchondral bone plate beneath. Label water and proteoglycan content gradients across zones.</image>

<image>A step-by-step surgical illustration of the microfracture technique. Panel 1: arthroscopic view of a focal chondral defect on the medial femoral condyle. Panel 2: debridement of damaged cartilage to a stable rim with vertical edges. Panel 3: removal of the calcified cartilage layer. Panel 4: creation of microfracture holes with an awl, spaced 3-4 mm apart. Panel 5: the resulting "super clot" filling the defect from marrow bleeding.</image>

<image>A comparative illustration showing three cartilage repair techniques side by side. Left panel: microfracture with marrow clot filling a small defect. Center panel: autologous chondrocyte implantation (ACI/MACI) with cultured chondrocytes on a collagen scaffold placed into a larger defect. Right panel: osteochondral autograft transfer (OATS) with cylindrical plugs of intact cartilage and bone transferred from a donor site. Label the tissue type produced by each (fibrocartilage vs. hyaline-like vs. native hyaline).</image>

Clinical Pearls

Cartilage lesions do not heal spontaneously because the tissue is avascular and chondrocytes have minimal reparative capacity; patients need to understand this when discussing prognosis. Microfracture produces fibrocartilage (Type I collagen), not true hyaline cartilage (Type II collagen), which has implications for the longevity of the repair. Defect size is a key determinant of technique selection: microfracture works best for defects under 2 cm2, while ACI/MACI or osteochondral allograft is preferred for larger defects. A common technical error during microfracture is incomplete removal of the calcified cartilage layer, which prevents marrow elements from adhering to the defect bed. Containment of the defect by intact surrounding cartilage shoulders is important for any repair technique; uncontained defects have consistently worse outcomes. A BMI greater than 30 is a relative contraindication for cartilage restoration procedures because of the increased joint reactive forces. Any concomitant pathology such as malalignment, ligament insufficiency, or meniscal deficiency must be addressed simultaneously, or the cartilage repair will fail regardless of technique. The STAR trial (2020) provided Level I evidence demonstrating the superiority of ACI over microfracture at five years for defects larger than 3 cm2.

References

• Brittberg M, et al. Treatment of deep cartilage defects in the knee with autologous chondrocyte transplantation. N Engl J Med. 1994;331(14):889-895.
• Steadman JR, et al. Microfracture: surgical technique and rehabilitation to treat chondral defects. Clin Orthop Relat Res. 2001;(391 Suppl):S362-369.
• Saris DB, et al. Treatment of symptomatic cartilage defects of the knee: characterized chondrocyte implantation results in better clinical outcome at 36 months in a randomized trial compared to microfracture. Am J Sports Med. 2009;37 Suppl 1:10S-19S.
• Pareek A, et al. Osteochondral autograft transfer versus microfracture in the knee: a meta-analysis of prospective comparative studies at minimum 5-year follow-up. Arthroscopy. 2016;32(10):2118-2130.
• STAR Trial: Randomized Clinical Trial Comparing ACI vs. Microfracture. Am J Sports Med. 2020.

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