Physis (Growth Plate): Anatomy & Physiology

The physis develops from cartilage-committed mesenchymal cells expressing the SOX family of genes and is the site of endochondral ossification — the orderly conversion of a cartilage template into bone that drives longitudinal growth. Its multilayered structure is built by chondrocyte proliferation and hypertrophy, with synthesis of an extracellular matrix rich in collagens (mainly types II, IX, X and XI) and proteoglycans (aggrecan, decorin). Disturbances of physeal development and physiology produce the skeletal dysplasias.

The hypertrophic zone has the largest cells, the least extracellular matrix per unit volume, and a partially mineralised, transitional structure — mechanically the weakest layer, so shear/avulsion forces propagate through it (the Salter-Harris plane). But the germinal reserve and proliferative zones lie on the epiphyseal side of that plane and keep their epiphyseal blood supply, so most physeal fractures heal without growth disturbance. Growth arrest follows only when the germinal layers or their blood supply are damaged (crush / SH-V, or a bony bar across the plate).

The physes do not contribute equally — which is why some fractures remodel well and why epiphysiodesis timing matters:

• Lower limb — growth is "around the knee": the distal femoral physis is the fastest (about 9 to 10 mm per year, roughly 70% of femoral and about 38% of lower-limb growth) and the proximal tibial physis next (about 6 mm per year, roughly 55 to 60% of tibial / about 27% of lower-limb growth) — together the knee physes give about 65% of lower-limb length.
• Upper limb — growth is "away from the elbow": roughly 80% of humeral growth is at the proximal humerus and about 80% of forearm growth at the distal radius/ulna, so the elbow physes contribute little — which is why elbow fractures remodel poorly and need accurate reduction.
• Maturity & remaining-growth estimation: physes close at roughly age 14 in girls and 16 in boys (better judged by bone age — Greulich-Pyle / Sanders); remaining growth for epiphysiodesis timing is estimated with the Menelaus rule of thumb (about 10 mm/yr distal femur, 6 mm/yr proximal tibia; girls grow to 14, boys to 16), the Green-Anderson growth-remaining charts, the Moseley straight-line graph, or the Paley multiplier.

The developmental signalling framework (PTHrP-Ihh, FGFR3, BMP/Wnt, Sox9/Runx2) is the classic synthesis of Kronenberg 2003 (DOI); the zonal structure, matrix composition and the PTHrP-Ihh regulatory loop are detailed by Burdan et al. 2009 (DOI); and the physeal manifestations of metabolic bone disease (rickets, hypothyroidism) by Chang et al. 2016 (DOI). The classic descriptions of the perichondral ring of LaCroix, the groove of Ranvier and the dual epiphyseal/metaphyseal blood supply are standard physeal anatomy used in these and related works.