epiphysis

**Definition and anatomy** - The epiphysis is the rounded end of a long bone, distal to the physis (growth plate). It is composed mainly of spongy (cancellous) bone with a thin cortical shell and is covered by articular (hyaline) cartilage at the joint surface. - Long bone regions, from end to shaft: epiphysis → physis (growth plate) → metaphysis → diaphysis. - Once growth ends, the physis closes (epiphyseal closure/fusion) and leaves the epiphyseal line. Timing varies by bone and sex, and closure generally occurs in the late teens to early 20s. Girls typically fuse earlier than boys.

**Development** - Long bones form by endochondral ossification. The primary ossification center forms in the diaphysis in utero. - Secondary ossification centers form in the epiphyses, mostly after birth. The distal femoral epiphysis is the classic exception because it is often present at birth, and its presence is used as a marker of fetal maturity. - The physis has zones (from epiphysis toward metaphysis): reserve (resting) → proliferative → hypertrophic → calcification → ossification. Longitudinal growth occurs by chondrocyte proliferation in the proliferative zone. The hypertrophic zone is the weakest, so it is the usual site of physeal fracture and slipping. - Growth regulation: GH/IGF-1, thyroid hormone, and sex steroids promote growth. Estrogen (in both sexes) ultimately causes epiphyseal closure. - **Traction epiphysis (apophysis):** a site of tendon attachment that does not contribute to length, such as the tibial tubercle (Osgood-Schlatter disease) and the calcaneus (Sever disease).

**Clinical correlations** - **Salter-Harris fractures (physeal injuries):** mnemonic SALTR. - I: Slipped (straight across the physis) - II: Above (physis plus metaphysis; the most common type) - III: Lower (physis plus epiphysis, intra-articular) - IV: Through or Two (metaphysis, physis, and epiphysis) - V: Rammed/crushed (worst prognosis, with growth arrest) - Types III–V carry a higher risk of growth disturbance. - **Slipped capital femoral epiphysis (SCFE):** The femoral head (epiphysis) slips posteroinferiorly relative to the neck through the hypertrophic zone. It typically affects obese adolescents (about 10–16 years old, more often boys) and presents with hip, thigh, or knee pain and a limp. Exam shows loss of internal rotation, and the hip externally rotates with flexion (Drehmann sign). Frog-leg lateral X-ray shows the "ice cream falling off the cone" appearance. Management is urgent surgical pinning, and the patient should be non-weight-bearing. Complications include avascular necrosis and chondrolysis. Associations include obesity, hypothyroidism, and renal osteodystrophy. Bilateral disease is possible. Atypical presentations (young age, or a thin patient) should prompt an endocrine workup. - **Legg-Calvé-Perthes disease:** idiopathic avascular necrosis of the femoral head epiphysis in children aged 4–10 years, more often boys. It presents with painless limp and knee or hip pain. Imaging shows a crescent sign or flattening. Compare with SCFE (older, obese). - **Rickets:** defective mineralization of the physis causes widened growth plates, costochondral "rachitic rosary," and bowed legs. - **Achondroplasia:** FGFR3 gain-of-function mutation inhibits chondrocyte proliferation in the physis, causing short limbs with normal trunk length. It is autosomal dominant, and most cases arise from new mutations. - **Osteomyelitis:** In children, hematogenous seeding usually occurs in the metaphysis because of slow, sluggish blood flow in the looped capillaries near the physis. The most common cause is *S. aureus*. *Salmonella* is classic in sickle cell disease. - **Osteochondroma:** a benign cartilage-capped bony outgrowth from the metaphysis near the physis. - **Giant cell tumor:** arises in the epiphysis of long bones in adults aged 20–40 years, classically around the knee. It shows a "soap bubble" appearance and has multinucleated giant cells on histology. - **Chondroblastoma:** a tumor of the epiphysis in adolescents. - **Epiphyseal vs. metaphyseal vs. diaphyseal tumors (high-yield mnemonic):** Epiphysis = giant cell tumor and chondroblastoma; Metaphysis = osteosarcoma and osteochondroma; Diaphysis = Ewing sarcoma. - **Precocious puberty or early estrogen exposure:** early epiphyseal closure results in short adult stature. - **Delayed closure:** hypogonadism and estrogen or aromatase deficiency produce tall stature with unfused epiphyses (eunuchoid proportions). - **Bone age:** a hand/wrist X-ray assesses epiphyseal ossification and closure, and is used to evaluate growth disorders.

**Exam pearls** - Epiphysis = end of bone, secondary ossification center, joint side of the physis. - Physis fractures are the weak point in children, because the physis is weaker than ligaments, so a suspected sprain in a child should be treated as a physeal injury until proven otherwise. - Estrogen closes the growth plates in both sexes.