Ossification

For Medical Students

Structure and Location

The patella begins as cartilage. A single ossification centre appears within it in early childhood, conventionally taught as between 3 and 5 years of age, and it is usually earlier in girls than boys.

Ossification spreads outwards from that centre and the bone takes its adult shape by adolescence. Until it does, the patella on a radiograph is smaller and more irregular than the cartilage it sits in.

Where more than one centre appears and they fail to fuse, the result is a bipartite patella: two bony fragments within one cartilage envelope.

Relations

The cartilage envelope is always larger than the ossified part in a child, so the articular surface extends beyond what a radiograph shows.

The quadriceps and patellar tendons attach to the cartilage before the bone is formed and remain attached to it throughout, which is why a young patella can fail at the pole through the cartilage rather than the bone.

The lower pole is the last part to ossify and is the site of the apophysis that gives trouble in adolescence.

Function

An unossified or partly ossified patella still functions mechanically, because the cartilage transmits load. The mechanical role does not wait for the bone.

The significance of ossification is diagnostic rather than functional: it determines what is visible and what injuries are possible at a given age.

Clinical Relevance

A bipartite patella is a normal variant, present in about 2 per cent of people, and most often sits at the superolateral corner. It is usually painless and found by chance.

On a radiograph the separation has smooth, rounded, sclerotic edges, unlike the sharp irregular edges of a fracture. It is also frequently bilateral, so the other knee is useful.

Sleeve fractures and apophysitis at the lower pole are paediatric problems arising from this anatomy, and Section 10 owns them.

MRCS Level

Structure and Location

The patella ossifies from a centre that appears in early childhood and may be multiple. Multiple centres normally coalesce; failure to coalesce produces a bipartite or rarely a tripartite patella.

Bipartite patellae are conventionally classified by the position of the accessory fragment, and the superolateral position is by far the commonest. The classification itself is an imaging construct and 5.12 owns it.

The accessory fragment is joined to the main bone by fibrocartilage rather than by bone, and the vastus lateralis inserts on it, which is the mechanical reason it can become symptomatic.

Relations

The fragment's relation to the vastus lateralis insertion is the clinically relevant one. Traction through that muscle loads the fibrocartilaginous junction.

The articular surface is usually continuous across the junction, so a bipartite patella is not a defect in the joint surface.

In a child the whole bone sits within a cartilage envelope attached to the extensor mechanism, so a sleeve fracture separates cartilage and periosteum from the ossified bone rather than breaking bone.

Blood Supply and Innervation

The developing patella is supplied by the same genicular anastomosis as the adult bone, entering anteriorly, and the accessory fragment in a bipartite patella has its own supply through its soft tissue attachments.

That independent supply is why excision of a symptomatic fragment is feasible and why the main bone is not devascularised by it.

Innervation follows the anterior articular branches of the femoral nerve, and pain from a symptomatic junction is localised rather than referred.

Function

Ossification does not change the patella's mechanical role, which the cartilage performs from the start.

A bipartite patella is mechanically normal in most people. It becomes symptomatic when the junction is loaded repeatedly, usually in an adolescent athlete, and the symptom arises from the junction rather than from the joint surface.

Clinical Relevance

Distinguish bipartite patella from fracture on four features: smooth rounded sclerotic margins, the superolateral position, frequent bilaterality, and the absence of a mechanism that fits.

A symptomatic bipartite patella in an adolescent is a traction problem at the fibrocartilaginous junction. Treatment belongs to Sections 6 and 7 and the paediatric aspects to Section 10.

A sleeve fracture in a child is easy to under call on a radiograph, because the separated part is largely cartilage. The clinical finding of a high riding patella and loss of active extension matters more than the film.

In an adult, an incidental bipartite patella found before arthroplasty or before a fracture fixation changes the plan, because the fragment is not a fracture to be reduced.

FRCS Level

Relations

The junction between fragment and main bone is a soft tissue interface carrying a muscle insertion. Operating on it means deciding whether to excise the fragment, release the insertion, or fix the junction, and all three are decisions about soft tissue rather than bone.

In the growing knee the patellar attachment of the medial patellofemoral ligament lies close to a bone that is still ossifying, which constrains fixation.

Function

Excising a superolateral fragment removes part of the vastus lateralis insertion and therefore part of the lateral pull on the patella. That is a change to tracking as well as a treatment for pain.

The effect is small in most people and is not small in a patella that was already marginally stable, which is why tracking is assessed before the fragment is removed.

Clinical Relevance

Before treating a bipartite patella as the cause of pain, exclude the other sources on the same page of the differential. The variant is common and incidental far more often than it is symptomatic.

In fracture fixation, a pre-existing bipartite fragment should be recognised on the preoperative film rather than discovered intraoperatively, where it will be mistaken for comminution.

In the skeletally immature knee every procedure on or near the patella has to account for incomplete ossification, and Section 10 owns the consequences.

The prevalence figure on this page and the age of ossification are both written from standard teaching and registered as unchecked. Both are the kind of number that varies between populations and between studies.