Growth Plates

For Young Learners

Structure and Location

While you are still growing, your bones are not solid all the way along. Near each end there is a disc of cartilage called a growth plate, or physis. It is where new bone is added, which is how your bones get longer.

The knee has two of the most productive growth plates in the body: one at the bottom of the thigh bone and one at the top of the shin bone. Between them they produce roughly two thirds of the length of your leg.

There is also a growing area at the front of the shin, just below the knee, where the big tendon attaches.

Function

Growth plates make bone longer. They close and turn into solid bone when growth finishes, which happens at different ages for different people, usually in the mid to late teens.

Here is the part that matters. While it is open, the growth plate is softer than the ligaments around it. In an adult, a bad twist tears a ligament. In someone still growing, the same twist is more likely to injure the growth plate instead.

This is why a young athlete and an adult can have the same accident and end up with completely different injuries, and why a child's knee injury is always assessed on its own terms.

For Patients

Structure and Location

The physis, or growth plate, is a layer of specialised cartilage near each end of a long bone. At the knee there is one at the distal femur and one at the proximal tibia. The tibial tubercle at the front has its own growing area, called an apophysis, where the patellar tendon attaches.

They close progressively through adolescence, generally earlier in girls than in boys, with most closure around the mid to late teens.

Function

Growth plates are responsible for the lengthening of bone. The two at the knee are the most productive in the lower limb, contributing roughly sixty five per cent of leg length.

Because they contribute so much, injury or interference here has a greater effect on final leg length than at most other sites.

Clinical Relevance

An open growth plate is mechanically weaker than the surrounding ligaments. A force that would tear an adult's ligament tends instead to injure the growth plate in a child, so the same accident produces a different injury depending on skeletal maturity.

This changes how children are assessed. Tenderness directly over a growth plate after an injury is taken seriously even when the x-ray looks normal, because the cartilage itself does not show up on a plain film.

It also changes surgery. Operations that would be routine in an adult may need modifying in a growing child to avoid crossing a growth plate, since damage there can lead to uneven growth or a limb length difference.

The apophysis at the tibial tubercle is the site of a familiar condition in active adolescents, which is covered in the conditions and children's sections rather than here.

For Medical Students

Structure and Location

The physis is a cartilaginous growth plate between epiphysis and metaphysis, organised into reserve, proliferative, hypertrophic and calcification zones. The hypertrophic zone is the mechanically weakest and is where most physeal separations occur.

The distal femoral physis is undulating and interdigitated, which contributes both to its stability and to its propensity for growth arrest after injury. The proximal tibial physis is adjacent to the popliteal neurovascular structures posteriorly.

The tibial tubercle is a traction apophysis, continuous with the proximal tibial physis, receiving the patellar tendon insertion.

Function

The distal femoral physis contributes approximately seventy per cent of femoral length and nine millimetres per year in late childhood; the proximal tibial physis contributes approximately sixty per cent of tibial length and six millimetres per year. Together they account for roughly sixty five per cent of lower limb length.

Physeal closure is progressive and sex dependent, occurring earlier in girls. Skeletal rather than chronological age governs remaining growth, and is estimated radiographically.

Clinical Relevance

The open physis is weaker in tension and shear than the adjacent ligaments and capsule, so loading patterns that produce ligamentous injury in adults produce physeal separation in the skeletally immature. The same mechanism therefore yields different injuries at different ages.

Physeal cartilage is radiolucent, so plain radiographs may appear normal despite significant injury. Focal physeal tenderness after trauma warrants a high index of suspicion.

Physeal injury around the knee carries a disproportionate risk of growth disturbance because of the contribution these plates make, producing angular deformity or limb length discrepancy. This constrains surgical technique, and physeal sparing approaches exist for this reason.

Classification of physeal injuries and their management, and apophyseal conditions, are covered in the children's and conditions sections.