epiphyseal plate

**Location and function:** The physis lies between the epiphysis (secondary ossification center) and the metaphysis in children and adolescents. It drives longitudinal bone growth by endochondral ossification. Appositional growth (width) occurs separately through the periosteum by intramembranous ossification. At skeletal maturity the plate ossifies and becomes the epiphyseal line.

**Histologic zones (from epiphysis to metaphysis):** 1. Reserve (resting) zone: small, scattered chondrocytes that anchor the plate and store lipid and glycogen. 2. Proliferative zone: chondrocytes divide and stack into columns. This is the zone of active growth. 3. Hypertrophic zone: cells enlarge and mature. This is the weakest zone, and most physeal fractures pass through it. 4. Zone of provisional calcification (calcified cartilage): matrix calcifies and chondrocytes undergo apoptosis. 5. Ossification zone: osteoblasts lay down woven bone on the cartilage scaffold, and blood vessels invade from the metaphysis.

**Regulation:** - Growth hormone acts through IGF-1 to stimulate chondrocyte proliferation. - Thyroid hormone, sex steroids and vitamin D are also needed. Estrogen causes epiphyseal closure in both sexes, which is why the pubertal growth spurt ends. - Indian hedgehog (Ihh) and PTHrP form a feedback loop that keeps chondrocytes proliferating and delays hypertrophy. - FGF3 acts as a negative regulator. An activating mutation causes achondroplasia, with decreased chondrocyte proliferation in the proliferative zone. This produces short limbs with a normal trunk and normal intelligence. Mutations are usually sporadic and associated with advanced paternal age.

**Clinical correlations:** - **Salter-Harris fractures (SALTR mnemonic):** - Type I: **S**traight across the physis (slipped). - Type II: **A**bove the physis (fracture runs through the metaphysis). This is the most common type. - Type III: **L**ower, below the physis (through the epiphysis into the joint). - Type IV: **T**hrough or traversing the physis (metaphysis, physis and epiphysis). - Type V: **R**ammed or crushed physis. It has the worst prognosis because of growth arrest. - Types III to V carry a higher risk of growth disturbance, limb length discrepancy or angular deformity. - **Slipped capital femoral epiphysis (SCFE):** Typically seen in obese adolescents (age 10-16). The femoral head slips posteroinferiorly through the hypertrophic zone. It presents with hip, thigh or knee pain and a limp, and the leg is externally rotated. Frog-leg lateral radiograph shows the "ice cream scoop falling off the cone" appearance. Treatment is surgical pinning. - **Rickets:** Vitamin D deficiency impairs mineralization of the zone of provisional calcification. The result is a widened, cupped, frayed physis, with rachitic rosary, bowed legs and craniotabes. - **Achondroplasia:** FGFR3 gain-of-function mutation (see above). - **Growth hormone deficiency or excess:** Deficiency causes short stature and excess causes gigantism if it occurs before plate closure. Acromegaly occurs if it happens after closure. - **Precocious puberty:** Early estrogen exposure closes the plates early, so the child is tall initially but has a short final adult height. - **Osgood-Schlatter disease:** Traction apophysitis of the tibial tubercle in adolescent athletes. - **Osteomyelitis:** In children, hematogenous seeding occurs at the metaphysis because of sluggish, looped blood flow near the physis. - **Osteosarcoma:** Commonly arises in the metaphysis of long bones (distal femur and proximal tibia) in adolescents. - **Ewing sarcoma:** Arises in the diaphysis. - **Giant cell tumor:** Arises in the epiphysis after plate closure.

**High-yield points:** Physeal cartilage is the weakest part of the growing skeleton, weaker than the ligaments, so a child with joint trauma and tenderness over the physis but normal radiographs should be treated as having a Salter-Harris I fracture. Closure generally occurs around 14-16 years in girls and 16-18 years in boys, and bone age on a hand and wrist radiograph is used to assess remaining growth.