The growth plate, formally known as the physis or epiphyseal plate, is a specialized disc of cartilage found in the long bones of children and adolescents. It functions to determine the final length of bones like the femur, tibia, and humerus by driving longitudinal growth. This temporary structure is responsible for adding height during childhood. The growth plate represents the weakest point in a growing skeleton, making it particularly susceptible to injury.
Anatomical Structure and Location
The growth plate is composed of hyaline cartilage, a flexible tissue that is softer than mature bone. It is situated at the ends of long bones, positioned between the bone’s main shaft (the metaphysis) and the rounded end (the epiphysis). Most long bones have two growth plates, one near each end, which contribute to the bone’s lengthening.
Because the growth plate is made of cartilage, it does not show up as solid tissue on a standard X-ray. Instead, it appears as a dark, radiolucent line or gap between the denser, calcified bone structures. This visual distinction allows medical professionals to assess skeletal maturity and diagnose injuries in growing patients. The plate is encircled by a ring of bone that provides structural support.
The Process of Bone Elongation
The lengthening of the long bones occurs through a process known as endochondral ossification. This process begins within the growth plate as cells called chondrocytes continually multiply and mature in distinct organizational zones. The resting zone anchors the plate to the epiphysis.
Below this is the proliferative zone, where chondrocytes divide rapidly, stacking into columns and pushing the epiphysis away from the metaphysis. They then enter the hypertrophic zone, where they increase in size and prepare the surrounding matrix for calcification. The final stages involve the death of the enlarged chondrocytes and the subsequent calcification of the cartilage matrix.
Specialized cells and blood vessels then invade this calcified scaffold, removing the dead cartilage and replacing it with new bone tissue. This continuous cycle of cartilage creation, maturation, and replacement by bone drives the growth of the bone shaft.
Why Growth Plates Close
The active process of bone elongation eventually halts when the growth plate undergoes a natural transformation called epiphyseal fusion. This cessation of growth is triggered by hormonal shifts during puberty, specifically the rise in sex hormones like estrogen and testosterone. These hormones signal the chondrocytes to slow their division and accelerate their final maturation steps.
As the body approaches skeletal maturity, the rate of new bone formation begins to outpace the production of new cartilage cells. The cartilage of the growth plate is progressively replaced by solid bone tissue until the entire space is bridged. Once the cartilage is gone, the epiphysis and metaphysis fuse, leaving behind a faint, bony scar called the epiphyseal line.
This fusion marks the permanent end of longitudinal growth. The timing of this closure varies between individuals and sexes. Growth plates in females typically fuse earlier (between 13 and 15), while in males, closure commonly occurs between 15 and 17 years old. The closure sequence also differs between bones.
The Impact of Growth Plate Injuries
The growth plate’s cartilaginous composition makes it structurally weaker than the surrounding ligaments, tendons, and mature bone tissue. Consequently, a force that might cause a sprain in an adult often results in a fracture through the growth plate in a child. These injuries, known as physeal fractures, are common in pediatric orthopedics, frequently affecting the bones of the forearm, fingers, and lower leg.
Damage to the growth plate can disrupt the bone’s future development. If the injury causes a premature closure of the plate or damages the proliferative zone, it can lead to growth arrest. This complication can result in angular deformities or a limb length discrepancy, where one limb is noticeably shorter than the other.
Physicians use the Salter-Harris classification system to categorize these fractures based on the pattern of the break and the specific zones affected. This classification helps predict the likelihood of growth disturbance, as injuries that cross the proliferative and resting zones generally have a higher risk of complications. Prompt and precise treatment is necessary to minimize the long-term effects of a growth plate injury on a child’s skeletal development.