MPFL Function

For Medical Students

The Principle

A checkrein is a strap that stops something going too far in one direction and does nothing in the other.

A checkrein only works at the point where it becomes tight. Before that it is slack and contributes nothing.

A structure that works over a small range is easy to overlook and easy to blame for everything, and both mistakes are common here.

At the Knee

The medial patellofemoral ligament runs from the medial side of the femur to the medial edge of the patella. 2.12.2 covers the anatomy.

It resists the patella being pushed outwards, and it does so mainly near full extension. 3.12.1 covers engagement.

Once the knee has bent far enough for the patella to sit in the groove, the ligament has very little left to do. 3.12.4 covers the groove.

It is the structure that tears when the patella dislocates, which almost always happens near extension. 6.59 covers the injury.

What Changes It

Flexion angle, because the ligament is only tight through part of the arc.

Whether it has been torn before, since a healed ligament is usually longer than the original.

The shape of the groove, which decides how much the ligament is left to do. 3.12.4 covers dysplasia.

Muscle around it, particularly the lower part of the inner quadriceps, which blends with it. 2.12.5 covers the dynamic stabilisers.

Clinical Relevance

A first dislocation tears this ligament, which is why the knee often feels unstable afterwards. 6.53 covers the first dislocation.

It is the structure that is reconstructed when the patella keeps dislocating. 7.51 covers the operation.

A reconstruction restores a checkrein. It does not deepen the groove or move the pull. 3.12.12 covers tubercle position.

Section 4 covers how the ligament is tested.

MRCS Level

The Principle

A passive restraint is defined by the arc over which it is tight and by the force it can take before it fails. Both numbers matter.

The medial patellofemoral ligament scores modestly on the second and narrowly on the first, which is why it is necessary rather than sufficient.

A structure like that fails under a load that a stronger structure would hold, and its failure is therefore common rather than exceptional.

At the Knee

The ligament arises from the femur between the adductor tubercle and the medial epicondyle and inserts on the proximal half of the medial patellar border. 2.12.2 covers the footprint.

Sectioning studies attribute roughly half to sixty per cent of the restraint to lateral patellar translation near extension to this ligament.

Its contribution falls away as flexion increases and the trochlea engages, and it is negligible beyond about 30 degrees.

It blends with the medial quadriceps tendon femoral ligament and the medial retinaculum, so it is a complex rather than an isolated band. 2.12.3 covers the MQTFL.

Its failure load in cadaver testing is low compared with the cruciate ligaments, which matches how readily it tears clinically.

What Changes It

Flexion angle, which determines whether the ligament is tight at all.

Previous rupture and healing, which typically leaves it lengthened rather than absent. 6.59 covers the injury.

Trochlear morphology, which sets how much restraint is required of it. 3.12.4 covers it.

Patellar height, because an alta patella keeps the ligament as the only restraint for longer. 3.12.12 covers it.

Vastus medialis activity, which adds a dynamic component to a passive structure. 2.12.5 covers it.

How It Is Measured

Cadaveric sectioning with measured displacement gives the proportional contribution at each angle. 3.16.1 covers the method.

Robotic testing gives force to displacement curves before and after sectioning. 3.16.2 covers it.

Magnetic resonance imaging shows continuity and the site of rupture rather than function. 5.12.5 covers the appearances.

Clinical glide and apprehension testing estimate restraint and are influenced by guarding. 4.15.4 covers the tests.

There is no clinical measurement of the ligament's strength, only of the laxity that follows its failure.

Clinical Relevance

Expect this ligament to be torn after a true dislocation, and look for it rather than for a strain. 6.53 covers the presentation.

Assess lateral translation near extension, because that is the arc in which the ligament works. 4.15.4 covers the test.

Describe the structure as a complex rather than a single band, since the surgical anatomy follows that. 2.12.6 covers it.

Do not attribute instability in mid flexion to this ligament, because it is not tight there. 3.12.4 covers what is.

FRCS Level

At the Knee

The mechanically important point is that this ligament and the trochlea cover different parts of the arc, and a knee needs both.

A reconstruction therefore restores a specific function over a specific arc. Expecting it to compensate for a dysplastic groove is expecting it to work where it was never tight. 3.12.4 covers the groove.

Graft tension and the femoral attachment point govern the result. Tensioning in flexion overconstrains the medial facet and raises medial pressure. 3.12.3 covers pressure.

The femoral origin is close to the physis in a child, which is why the operation differs in a growing knee. 7.94 covers paediatric stabilisation.

Conventional teaching has settled on anatomical femoral placement and low graft tension, and the tension figure remains without a measured optimum.

What Changes It

The arc over which the graft is tight, set by the femoral attachment point.

Graft tension at fixation, which decides whether the construct restrains or compresses.

Trochlear morphology, which sets the demand placed on the construct. 3.12.4 covers it.

Patellar height and tubercle position, which decide how long the construct works alone. 3.12.12 covers them.

Rotational alignment, which changes the direction the patella is pushed. 3.12.11 covers it.

How It Is Measured

Cadaveric and robotic studies give the arc of restraint and the effect of malpositioning the femoral attachment. 3.16.2 covers the method.

Finite element work predicts the medial facet pressure that follows overtensioning. 3.16.4 covers modelling.

Clinically the result is measured as redislocation, apprehension and patient reported outcome rather than as restraint. 4.15.3 covers the sign.

No intraoperative measurement of graft tension is in routine use, so tension is set by feel.

The gap between a measured optimum in a cadaver and a tactile judgement in theatre is the main source of variation in outcome.

Clinical Relevance

Match the procedure to the arc that fails, and add a bony procedure where the failure is outside this ligament's range. 3.12.8 covers the assessment.

Avoid overtensioning, because a tight medial construct raises medial facet pressure and produces pain rather than stability.

Confirm the femoral attachment radiographically, since malposition changes the arc of restraint. 7.51 covers the operation.

State that no measured optimum for graft tension exists, rather than quoting a figure as though it did.

Weigh the patient's age, physiology, comorbidities, knee demand, psychological state and their own ideas, concerns and expectations alongside the morphology.