Trochlear Function

For Medical Students

The Principle

A ball in a deep groove stays put without anything holding it. A ball on a flat surface needs holding.

Shape is therefore a form of stability, and it costs the body nothing to use.

A joint that relies on shape is stable without effort. A joint that has lost its shape has to rely on soft tissue, which can tear.

At the Knee

The lower end of the femur carries a groove between its two condyles, called the trochlea. 2.12.1 covers the anatomy.

The patella sits in that groove once the knee has bent far enough to engage it. 3.12.1 covers engagement.

The outer wall of the groove is normally taller than the inner wall, which is what resists the patella sliding outwards.

In some knees the groove is shallow or flat, which is called trochlear dysplasia. 6.55 covers the condition.

What Changes It

The shape the knee developed with, which is fixed by adulthood. 2.4.2 covers ossification.

The flexion angle, because the groove deepens as it runs down the femur.

Previous injury to the cartilage or bone of the groove.

Nothing about exercise or training, which cannot change the shape of bone.

Clinical Relevance

A shallow groove makes dislocation more likely and makes it more likely to happen again. 6.54 covers recurrent instability.

It is the reason some people dislocate the patella with almost no injury at all.

No amount of strengthening replaces a groove, although muscle can help the soft tissue that is doing the work.

The shape is assessed on imaging rather than on examination. 5.12.1 covers it.

MRCS Level

The Principle

Bony constraint and soft tissue restraint substitute for each other, and the exchange rate is poor. Soft tissue is weaker, and it is the thing that fails.

A joint's stability is therefore best described as how much of the job the shape is doing at a given angle.

In the patellofemoral joint that proportion swings from nearly nothing in extension to nearly everything in flexion. 3.12.1 covers the handover.

At the Knee

Trochlear depth increases from proximal to distal, so constraint increases as the patella descends into the groove with flexion.

The lateral facet of the trochlea is normally higher and more prominent than the medial, which is the principal restraint to lateral displacement.

The sulcus angle describes the groove's opening. A larger angle means a shallower groove.

Dysplasia takes several forms: a shallow groove, a flat groove, a convex floor, and a supratrochlear spur. 5.12.1 covers the imaging classification.

A convex floor is mechanically the worst, because the surface actively pushes the patella out rather than failing to hold it in.

What Changes It

Developmental morphology, which is the dominant factor and is not modifiable. 2.4.2 covers ossification.

The flexion angle at which the patella engages, which decides how much of the groove is available. 3.12.12 covers patellar height.

Cartilage thickness over the trochlear floor, which contributes to the effective depth.

Previous osteochondral injury to the lateral facet.

Femoral rotation, which changes where the groove points relative to the line of pull. 3.12.11 covers it.

How It Is Measured

The sulcus angle is measured on an axial view at a stated flexion angle, and it changes with that angle. 5.12.1 covers the technique.

Trochlear depth and lateral facet inclination are measured on axial magnetic resonance imaging or computed tomography.

The crossing sign and the supratrochlear spur are assessed on a true lateral radiograph, which is why the lateral view still matters.

Cadaveric and robotic testing quantify the force needed to displace the patella at each angle, which is the mechanical measurement. 3.16.2 covers robotic testing.

No single number captures dysplasia, which is why it is classified rather than measured.

Clinical Relevance

Grade the groove before attributing instability to soft tissue failure alone. 3.12.8 covers the mechanics.

Expect a soft tissue reconstruction to work less well where the groove offers less. 7.51 covers the operation.

Order a true lateral radiograph as well as an axial study, because the crossing sign is not visible axially. 5.12.1 covers the views.

Describe dysplasia by its type rather than by a single angle, because the types behave differently.

FRCS Level

At the Knee

Trochlear morphology is the strongest single predictor of recurrent instability, and it is the one factor no soft tissue procedure addresses. 6.54 covers recurrence.

The mechanical reason is that a reconstruction restores a checkrein at low flexion angles and the groove is what holds the patella above them. 3.12.5 covers the ligament.

Displacement force studies show the greatest loss of constraint in the first 30 degrees in dysplastic knees, which is exactly the arc the soft tissue was already covering alone.

The supratrochlear spur deserves separate mention. It converts engagement from a descent into a groove into a climb over a ridge.

Conventional teaching has moved from treating dysplasia as a radiographic curiosity to treating it as the primary lesion in a subgroup. 6.153 covers the controversies.

What Changes It

The type and severity of dysplasia, which decide whether the groove contributes at all.

Patellar height, which interacts with dysplasia by delaying engagement into the shallower proximal part. 3.12.12 covers it.

Tubercle position, which sets the lateral vector the groove has to resist. 3.12.12 covers it.

Femoral anteversion, which can rotate an adequate groove out of line. 3.12.11 covers it.

Previous surgery, which cannot add depth unless it has reshaped the bone. 7.56 covers distal realignment.

How It Is Measured

Classification systems split dysplasia into types from radiographic and axial features, with moderate interobserver agreement at best. 5.12.1 covers them.

Lateral trochlear inclination on axial imaging is the measurement with the most reproducible relationship to instability.

Trochlear depth measured a set distance below the joint line depends on where that distance is taken from.

Robotic displacement testing is the only method that measures constraint rather than shape. 3.16.2 covers it.

The gap between measuring shape and measuring constraint is the reason morphology predicts better in groups than in individuals.

Clinical Relevance

Assess trochlear morphology in every patient with recurrent instability, because it changes the expected result of a soft tissue procedure. 6.54 covers the condition.

Identify a supratrochlear spur specifically, since it changes engagement qualitatively rather than quantitatively.

Weigh morphology, patellar height, tubercle position and alignment together rather than ranking them. 3.12.8 covers the four factors.

Expect shape to predict better across a cohort than for the patient in clinic, and say so.

Take the patient's age, physiology, demands, psychological state and their own ideas, concerns and expectations into the decision, because morphology describes a knee and not a person.