Patellar Tracking

For Medical Students

Structure and Location

In full extension the patella sits above the trochlea, resting on the fat pad and the front of the femur rather than in the groove. It is mobile and can be pushed sideways.

As the knee flexes the patella descends and enters the groove. Conventional teaching puts engagement at around 20 to 30 degrees of flexion. From that point the bone holds it and it can no longer be displaced by hand.

With further flexion the patella travels down the groove and then between the condyles, and the part of its surface in contact moves progressively from the lower pole towards the upper pole.

Relations

The contact sequence is the useful fact. Near extension the lower part of the articular surface is in contact. In mid flexion the middle. In deep flexion the upper part, and at the extreme the odd facet engages the medial condyle.

The restraints change hands as the knee bends. Near extension the retinacula and the vastus medialis hold the patella; once engaged, the trochlea does.

The patella's path is not a straight line in the coronal plane. It moves slightly laterally at first, then medially as it engages, then laterally again in deep flexion.

Function

Normal tracking means the patella is centred in the groove through the range with its contact area moving smoothly up its surface.

The system is designed so that the moment of least bony support coincides with the moment of least load. Near extension the patella is barely constrained, and the patellofemoral force is low. As the force rises with flexion, the constraint rises too.

That coincidence is why the normal knee tolerates a wide range of anatomy without symptoms, and why losing either half of it matters.

Clinical Relevance

Patellar dislocation almost always happens in early flexion, because that is where bony constraint is least while the muscle is already pulling.

Examining the patella therefore means examining it near extension, where it can move, rather than in flexion where it cannot.

Where cartilage change sits on the patella tells you at which flexion angle it is loaded, which tells you which activities will hurt.

MRCS Level

Structure and Location

Describe tracking in three planes. In the sagittal plane the patella descends and tilts as it engages. In the coronal plane it shifts medially on engagement and laterally again in deep flexion. In the axial plane it rotates slightly, following the obliquity of the groove.

Four anatomical variables govern all of it: patellar height, tubercle position relative to the groove, trochlear depth and the balance of the retinacula. Three of the four can be altered surgically.

The groove's axis runs proximally and laterally, so the patella does not travel in a straight line even in a normal knee.

Relations

Vastus medialis obliquus inserts on the superomedial border at an angle to the rest of the quadriceps and is the dynamic medial restraint. It acts through the medial retinacular layers rather than directly on the groove.

The medial patellofemoral ligament runs from the femur between the adductor tubercle and the medial epicondyle to the superomedial patella, and it is close to isometric over the first part of flexion, which is exactly the range where it is needed.

The lateral retinaculum's deep transverse fibres tether the patella laterally, and its superficial oblique fibres are continuous with the iliotibial band, so hip position affects lateral tension.

The fat pad occupies the space the lower pole moves through and limits how far the patella descends.

Blood Supply and Innervation

The anterior soft tissues through which tracking is controlled are richly innervated from the femoral nerve's articular branches and from the recurrent branch of the common peroneal laterally.

That innervation is why maltracking can be painful before any surface change is visible: the painful structure is retinaculum and fat pad, not cartilage.

The patella's blood supply enters anteriorly, so the retinacular releases that alter tracking also alter perfusion. A complete lateral release divides part of the ring.

Function

Through the first part of flexion the medial patellofemoral ligament is the primary passive restraint and the trochlea contributes little. Once the patella is engaged the contributions reverse.

Contact area increases with flexion while contact moves proximally on the patella, so the surface loaded in deep flexion is larger and higher than that loaded near extension.

The odd facet articulates only at the extremes of flexion, which is why change there may be silent in someone who does not kneel or squat.

Clinical Relevance

Instability is an early flexion problem and the restraints that fail are soft tissue, which is why medial patellofemoral ligament reconstruction addresses most of it.

Maltracking with a normal medial patellofemoral ligament points at the bony variables: height, tubercle position or trochlear shape. Soft tissue surgery will not correct those.

Where cartilage change sits identifies the loading angle, so a distal pole lesion and an upper pole lesion are different clinical problems with different symptom patterns.

Measurement of tracking, of patellar height and of tubercle to groove relationship all belong to 5.12 and to 3.12.1. The engagement angle on this page is written from standard teaching and registered as unchecked.

FRCS Level

Relations

The relation to establish before operating is which restraint has failed. A patella that dislocates with a normal trochlea and normal height has a soft tissue problem; one that dislocates with a flat trochlea has a bony problem with a soft tissue consequence.

The two present identically to the patient and differently on imaging, which is why imaging precedes the decision rather than confirming it.

Function

Tracking is the output of four variables, and correcting one does not correct the others. That is the whole of why patellofemoral surgery is staged by variable rather than by procedure.

The range that matters is the first thirty degrees or so of flexion. A reconstruction that is correct at ninety degrees and wrong at twenty has failed, and the reverse is rarely true.

Restoring normal tracking is not the same as achieving stability. A stable patella that tracks laterally still loads the lateral facet, and that distinction separates a successful operation from a satisfied patient.

Clinical Relevance

Assess all four variables before instability surgery. Addressing the one that is easiest to change is the commonest route to a stable, painful knee.

A medial patellofemoral ligament reconstruction tightened in flexion will tether the patella medially through the range, and the error is in the femoral attachment rather than the graft. 2.1.6 covers why.

Trochlear shape is the variable soft tissue surgery cannot touch, which is the anatomical argument for considering trochleoplasty in a selected few. 7.51 onwards owns the decision.

After arthroplasty, tracking is governed by component rotation and position rather than by native anatomy, so the same four variables are replaced by three implant ones.