Trochlea

For Medical Students

Structure and Location

The trochlea is the groove on the front of the lower femur in which the patella slides. It is formed by the two femoral condyles and the hollow between them.

The lateral facet of the groove is higher and more prominent than the medial facet. That asymmetry is the main bony restraint to the patella moving laterally.

Conventional teaching gives the normal sulcus angle, measured between the two facets at the deepest point, as about 135 degrees. A larger angle means a shallower groove.

The groove does not extend to the top of the femur. It begins at the front of the distal femur and runs down and backwards to become continuous with the intercondylar notch behind. 2.1.5 covers the trochlea as part of the femur.

The cartilage of the trochlea is continuous with the cartilage of the condyles, so the patellofemoral and tibiofemoral joints share one cartilage surface.

Relations

In front of it: the patella in flexion, and the quadriceps tendon and fat pad in extension.

Behind and below: the intercondylar notch, with the cruciate ligaments in it.

On either side: the medial and lateral femoral condyles, which 2.1.3 and 2.1.4 cover.

Above: the supratrochlear region, where the cartilage ends and the bone becomes the anterior femoral cortex.

Function

The groove guides the patella as the knee flexes. The deeper the groove, the more it guides.

The patella is not in the groove in full extension. It sits above it, held by soft tissue, and enters the groove at about 20 to 30 degrees of flexion.

That is why the first part of flexion is the unstable part and why the soft tissue restraints matter most there. 2.12.2 covers the main one.

Clinical Relevance

A shallow or flat proximal trochlea leaves the patella unguided for longer in early flexion. The imaging of that is 5.12.1 and the conditions are in Section 6.

The lateral facet's height is the bony check on lateral movement, so its shape is what a lateral radiograph is being read for.

Because the patella enters the groove at 20 to 30 degrees, an examination of patellar stability is done in that range rather than in deep flexion.

MRCS Level

Structure and Location

Describe the trochlea in three parts: the proximal or entry region, the middle, and the distal region where it meets the notch. The proximal part is the one that varies and the one that matters.

Conventional teaching gives trochlear depth at about 3 cm above the joint line as of the order of 5 mm, measured from the deepest point of the sulcus to the top of the lateral facet.

The sulcus is not in the midline. It runs slightly laterally as it ascends, so the groove and the long axis of the femur are not aligned.

The cartilage of the trochlea is thickest at the proximal lateral facet, where contact pressure is highest in early flexion.

Relations

The supratrochlear fat pad lies above the cartilage margin, between the trochlea and the quadriceps tendon.

The infrapatellar fat pad lies below and in front, between the patellar tendon and the front of the knee.

The trochlea's distal border is continuous with the notch, so the cruciate ligaments lie directly behind the lower part of the groove.

The medial patellofemoral ligament attaches to the femur just below the adductor tubercle, which is behind and above the medial trochlear facet. 2.12.2 covers it.

Blood Supply and Innervation

The trochlear cartilage is avascular, like all articular cartilage, and the subchondral bone is supplied by the genicular anastomosis.

The superior genicular arteries supply the proximal trochlea and the anterior femoral cortex above it.

Nerve supply to the subchondral bone and the overlying soft tissue is from the anterior articular branches, which is why anterior knee pain localises poorly.

Function

Bony stability rises as the knee flexes and the patella sits deeper in the groove, while soft tissue stability falls. The two swap over at around 30 degrees. 3.12.1 covers tracking.

The lateral facet takes the larger share of contact pressure through most of flexion, which 3.12.3 covers.

Groove depth is what converts a quadriceps pull with a lateral vector into straight line patellar movement.

Clinical Relevance

Measure the sulcus angle on an axial view in flexion, and read the proximal trochlea on a true lateral. 5.12.1 covers how.

Examine patellar stability between 0 and 30 degrees, because the anatomy makes that the range where soft tissue carries the restraint.

Expect the proximal lateral facet to be the cartilage that fails first, since it takes the highest early pressure.

Leave dysplasia and its grading to 5.12.1 and Section 6. This page is the normal groove.

FRCS Level

Relations

The proximal trochlea and the anterior femoral cortex meet at a line that is normally smooth. Where it is not, the shape of that junction is what every description of dysplasia is describing, and 5.12.1 owns it.

The medial patellofemoral ligament's femoral attachment lies behind the medial trochlear margin, so the soft tissue restraint and the bony one are not in the same plane.

The groove's distal continuity with the notch means a trochlear procedure and a cruciate tunnel can share bone.

Function

Treat bony and soft tissue restraint as a handover rather than a sum. The soft tissue carries early flexion, the groove carries the rest, and an abnormality of either shows itself in the range the other does not cover.

Groove geometry sets the direction the patella is guided in; soft tissue sets how far it can deviate before the groove takes over. That distinction is what separates a bony from a soft tissue problem.

Contact area rises with flexion while contact pressure peaks in mid flexion, so the cartilage most at risk is not the cartilage most often examined. 3.12.2 and 3.12.3 carry it.

Clinical Relevance

Assess the proximal trochlea specifically on a true lateral radiograph when instability is being investigated, because that is where the geometry varies.

Consider which part of the range a patient's instability occurs in, since the anatomy assigns early flexion to soft tissue and later flexion to bone.

Plan any trochlear work against the notch behind it and the cartilage margins beside it, and 7.56 carries the technique.

Avoid attributing instability to the groove without evidence from the range it controls. A patella that dislocates in full extension has a soft tissue story.