Patellar Tracking

For Medical Students

The Principle

A bone sitting inside a tendon changes the direction the tendon pulls, in the way a pulley changes the direction of a rope.

A groove cut into the bone underneath keeps that pulley in line. The deeper the groove, the less work the surrounding soft tissue has to do.

A pulley that is only held by soft tissue is secure in one direction and vulnerable in the other, which is the whole problem of this joint.

At the Knee

The patella sits in the extensor mechanism, between the quadriceps tendon above and the patellar tendon below. 2.4.1 covers the anatomy.

Near full extension it rests above the groove rather than in it, held by soft tissue alone. 2.12.1 covers the trochlea.

It enters the groove as the knee bends, at around 20 to 30 degrees, and becomes progressively more constrained with further flexion.

Through the arc it moves distally and tilts, and its contact with the femur shifts steadily up the back of the patella. 3.12.2 covers contact areas.

What Changes It

Flexion angle, which decides how much of the groove is holding the patella.

Quadriceps pull, and the balance between its medial and lateral parts. 2.12.5 covers the dynamic stabilisers.

Trochlear shape, which is what the groove actually offers. 3.12.4 covers trochlear function.

Patellar height, because a patella sitting high enters the groove later. 3.12.12 covers tubercle position.

Clinical Relevance

Dislocations happen near extension, because that is where the groove is not yet holding the patella.

The soft tissue restraint that matters near extension is the medial patellofemoral ligament. 3.12.5 covers it.

Strengthening the quadriceps changes tracking only where the groove is not already doing the work.

Tracking is assessed by watching the knee bend rather than by a static measurement. 4.15.1 covers the examination.

MRCS Level

The Principle

Constraint in this joint is handed over. Soft tissue holds the patella near extension and bone holds it in flexion, and the handover happens over a narrow arc.

A joint with a handover has a window of vulnerability, and the window is where the clinical problems sit.

Describing the path is therefore less useful than describing when each restraint is in charge.

At the Knee

From full extension the patella translates distally and medially as it engages, then follows the groove laterally and posteriorly through mid flexion.

Tilt and rotation accompany the translation. The patella is not a flat plate sliding on a flat surface. 2.4.1 covers its facets.

By 60 to 90 degrees the patella is deep in the groove and bone is the dominant restraint. Soft tissue contributes little there.

In deep flexion contact moves onto the odd facet and the proximal patella, and the quadriceps tendon itself begins to articulate. 3.12.2 covers that.

What Changes It

The depth and the lateral facet height of the trochlea, which decide when bone takes over. 3.12.4 covers dysplasia.

Patellar height, which shifts the whole handover later in the arc. 3.12.12 covers it.

Tubercle position in the transverse plane, which changes the direction of the distal pull. 3.12.12 covers it.

Femoral and tibial rotation, which move the groove relative to the line of pull. 3.12.11 covers malalignment.

Medial soft tissue integrity, which decides what happens inside the window. 3.12.5 covers the ligament.

How It Is Measured

Axial radiographs at fixed flexion angles give congruence and tilt, and they say nothing about the arc either side. 5.12.3 covers tilt.

Computed tomography and magnetic resonance imaging measure the same indices with better bone and soft tissue detail. 5.12 covers the imaging.

Dynamic imaging through the engagement arc is the measurement that matches the mechanics, and it is not routine.

Clinical tracking assessment is observational and poorly reproducible between observers. 4.15.1 covers it.

Clinical Relevance

Describe tracking by the arc in which it goes wrong rather than as a single path.

Expect instability to present near extension and pain to present in flexion, because the restraints differ there.

Assess the extensor mechanism through movement, since a static axial view misses the engagement arc entirely.

Treat a poorly reproducible clinical sign as a prompt for imaging rather than as a measurement. 4.15.1 covers the tests.

FRCS Level

At the Knee

The engagement arc is the mechanically interesting part of patellar tracking, and almost every index in use measures outside it.

Before engagement the restraint to lateral translation is the medial retinacular complex, of which the medial patellofemoral ligament is the principal band. 2.12.2 covers it.

After engagement the restraining structure is the lateral trochlear facet, and the force needed to displace the patella rises steeply with flexion.

The J sign is this handover failing visibly. The patella sits lateral in extension and snaps medially as it drops into the groove. 4.15.1 covers the sign.

A patella that engages late, from height or from a shallow groove, lengthens the window in which only soft tissue is working. 3.12.12 covers height.

What Changes It

Trochlear morphology, which is the single largest determinant of when the handover happens. 3.12.4 covers it.

Patellar height, which delays engagement without changing the groove. 3.12.12 covers it.

Transverse tubercle position, which sets the lateral vector through the arc.

Femoral anteversion and tibial torsion, which rotate the groove under the extensor mechanism. 3.12.11 covers them.

Quadriceps activation pattern and timing, which matter most in the unloaded part of the arc. 3.12.7 covers dynamic valgus.

How It Is Measured

Static indices measured at 30 degrees or more sit after engagement in most knees and therefore miss the window.

Dynamic computed tomography and magnetic resonance imaging through extension to flexion capture the arc, at a dose or a time cost. 5.12.4 covers subluxation.

Instrumented displacement testing gives a force to displacement curve in cadavers and has no clinical equivalent. 3.16.1 covers cadaveric testing.

No index in routine use reports the flexion angle at which the patella engages, which is the number the mechanics would ask for.

Clinical Relevance

Identify which part of the arc fails before choosing between a soft tissue and a bony procedure. 3.12.8 covers the mechanics.

Read a J sign as late or lateral engagement rather than as a quadriceps problem.

Prefer a dynamic study where the static indices and the history disagree. 5.12 covers the imaging.

State that the flexion angle of engagement is not routinely measured, rather than implying an index captures it.

Expect a soft tissue reconstruction to work inside the window and to do little outside it. 7.51 covers the operation.