Anatomy & Physiology

Articular Cartilage

The smooth, white connective tissue covering bone surfaces within joints. It provides a low-friction surface for joint movement and absorbs impact forces.

Articular cartilage is the hyaline cartilage that lines the bone surfaces within all synovial joints. It is firm, smooth, and translucent — visible as the white, glistening surface on the end of chicken or beef bones. In dogs, articular cartilage is found in the hip, elbow, stifle (knee), shoulder, carpus (wrist), and all spinal facet joints.

Structure and Function

Articular cartilage is composed of:

  • Chondrocytes (5–10% of volume): the cells responsible for synthesizing cartilage matrix
  • Extracellular matrix (90–95%): primarily type II collagen and proteoglycans (especially aggrecan), which bind water and provide compressive resistance
  • Water (~65–80% of wet weight): held within the matrix, providing the hydraulic cushioning mechanism

The cartilage has no blood vessels or nerves — it receives nutrients entirely by diffusion from synovial fluid. This avascular nature is both an advantage (no inflammatory nerve signaling when mildly damaged) and a disadvantage (severely limited repair capacity).

Why Articular Cartilage Cannot Regenerate

When articular cartilage is damaged, the chondrocytes have extremely limited ability to replicate and produce new matrix. Full-thickness defects (reaching the underlying bone) can trigger fibrocartilage repair from bone marrow stem cells, but fibrocartilage has inferior mechanical properties and degrades faster than native hyaline cartilage.

This irreversibility is why joint disease is progressive: once damaged, the surface becomes rougher, friction increases, further damage accelerates, and the cycle continues. This is the mechanism underlying osteoarthritis (OA).

Protecting Articular Cartilage

The most evidence-supported interventions for cartilage protection:

  1. Weight management: each additional pound of body weight applies 3–4 times that force to joints during locomotion. Lean body condition is the strongest evidence-based protector.
  2. Low-impact exercise: swimming and leash walking maintain cartilage health through load cycling (cartilage receives nutrients by compression/release) without high peak forces.
  3. Omega-3 fatty acids: reduce prostaglandin-mediated inflammation in cartilage, slowing degradation. Evidence-supported at 75–100 mg/kg/day EPA+DHA.
  4. Glucosamine and chondroitin: provide substrate for proteoglycan synthesis. Evidence quality is moderate; benefit is clearest in dogs with early-stage OA.