Trochlea

For Medical Students

Structure and Location

The trochlea is the articular surface on the front of the distal femur. It is a shallow groove running from the front of the intercondylar notch upwards and slightly laterally, with a facet on each side of it.

The lateral facet is larger, steeper and stands further forward than the medial. That asymmetry is the normal arrangement, not a variant. The angle between the two facets, the sulcus angle, is conventionally taught as about 138 degrees in a normal knee.

The groove is deepest in its middle and distal parts and becomes shallow proximally, where it flattens out onto the front of the femur. The patella therefore sits on a shallow surface near extension and in a deeper one as the knee flexes.

Relations

The patella articulates with the trochlea through its own two facets, with the lateral patellar facet against the lateral trochlear facet.

Above the trochlea the synovial cavity continues as the suprapatellar pouch, separated from the quadriceps by fat.

The medial and lateral retinacula pass over the sides of the trochlea without attaching to its articular surface, and the medial patellofemoral ligament runs on the medial side at the level of the proximal trochlea.

Below, the trochlea becomes continuous with the articular surface of both condyles at the front of the notch.

Function

The trochlea does two things. It carries patellofemoral load, and it keeps the patella centred.

Centring is a bony job in flexion and a soft tissue job near extension. Once the patella is engaged in the deeper part of the groove the shape holds it there. In the last part of extension the patella sits proximally, on the shallow part, and the retinacula and the vastus medialis do the work instead.

This is why patellar instability is a problem of the first part of flexion rather than of deep flexion.

Clinical Relevance

A shallow trochlea offers less bony restraint in early flexion, which places more demand on the soft tissues. The range of normal depth is wide.

Classifying trochlear shape, and deciding when a shallow trochlea counts as dysplasia, is an imaging and pathology question owned by 5.12.1 and 6.55. What belongs here is that the shape varies and that the variation is not in itself a diagnosis.

MRCS Level

Structure and Location

Describe the trochlea as an asymmetrical groove with an oblique axis. The groove runs proximally and laterally, so the patella does not travel in a straight line in the coronal plane as the knee extends.

The articular cartilage of the trochlea is the thickest in the knee at the centre of the groove, reaching of the order of 5 mm, and the subchondral bone beneath the lateral facet is denser than beneath the medial, reflecting where load passes.

Proximally the groove flattens and the cartilage thins at the margin where the articular surface meets the anterior femoral cortex. That junction is a normal transition and is where a prominence appears if the trochlea is shallow.

The most distal part of the trochlea blends into the notch, and the central ridge between the condyles at that point is continuous with the floor of the groove.

Relations

Anteriorly: the patella, the prefemoral fat pad above it, and the suprapatellar pouch.

Medially: the medial retinaculum in layers, with the medial patellofemoral ligament the deepest and strongest, running from the femur just distal to the adductor tubercle to the superomedial patella. 2.12.3 and 2.12.4 own the detail.

Laterally: the lateral retinaculum, with its deep transverse fibres attaching to the patella and its superficial oblique fibres continuous with the iliotibial band.

The superior genicular vessels run on either side above the trochlea, deep to the retinacula.

Blood Supply and Innervation

The trochlear subchondral bone is supplied from the genicular anastomosis, mainly the superior medial and superior lateral genicular arteries, with the descending genicular contributing medially.

The articular cartilage itself has no blood supply and is nourished by diffusion from synovial fluid, which is the reason patellofemoral cartilage repairs behave differently from bone healing.

Innervation of the anterior capsule and retinacula is from articular branches of the femoral nerve through the nerves to the vasti, with a contribution from the recurrent branch of the common peroneal nerve laterally.

Function

The trochlea converts the pull of the quadriceps into a force that extends the knee, and the patella sitting in it holds the tendon away from the joint so that the muscle acts at a mechanical advantage.

The depth of the groove and the height of the lateral facet together determine how much lateral force the bone can resist before the soft tissues are loaded.

The oblique axis of the groove means the quadriceps pull and the patellar tendon are not in line, and the trochlea absorbs the difference. 3.12 and 3.13 own the mechanics.

Clinical Relevance

The proximal trochlea is where shape variation shows itself, because that is where the groove is shallowest and where the patella engages.

The transition from articular surface to anterior cortex is the landmark used to judge whether the trochlea is flat or prominent, and it is a normal structure being assessed rather than a lesion being found.

Cartilage is thickest at the centre of the groove, so a defect there removes more tissue before reaching bone than the same defect elsewhere.

In arthroplasty the trochlea is resurfaced by the anterior flange of the femoral component, and the native groove's obliquity is one of the things a symmetrical component does not reproduce.

FRCS Level

Relations

The structures that matter around the trochlea are the ones that restrain the patella when the bone cannot: the medial patellofemoral ligament proximally and medially, the lateral retinaculum laterally, and the vastus medialis obliquus acting through the medial structures.

Their attachments are all close to the proximal trochlea, which is also where the groove is shallowest. The region with the least bony restraint is therefore the region with the most soft tissue attachment, and that coincidence is the anatomy behind patellofemoral surgery.

Function

Trochlear geometry sets the relationship between flexion angle and patellar stability. A deep groove makes stability a function of engagement; a flat one makes it a function of soft tissue tension throughout.

It also sets contact area. A flatter surface concentrates load over a smaller area at any given angle, which is a geometric consequence rather than a pathological one.

Neither statement needs a measurement to be useful at the decision stage, and both would need one to be used quantitatively. 3.12 owns the quantitative account.

Clinical Relevance

Trochlear shape is one of the factors weighed when deciding between soft tissue and bony procedures for patellar instability, and it is the factor that soft tissue surgery does not change.

Where the trochlea is reshaped, the surfaces being altered are the ones described on this page: the groove floor, the facet heights and the junction with the anterior cortex. The procedures themselves are owned by Section 7.

In arthroplasty, a trochlea that was shallow before surgery is replaced by a component groove that is not, and the retinacular tension that had adapted to the native shape has to accommodate the new one.

Classification of trochlear shape is deliberately absent from this page. It is an imaging construct, it has more than one competing system, and 5.12.1 and 6.55 own both the systems and the argument between them.