Tibial Tubercle Position

For Medical Students

The Principle

A rope's effect depends on where it is tied as well as how hard it is pulled.

Move the attachment sideways and the direction of pull changes. Move it further from the joint and the lever arm changes.

So one attachment point controls two separate things, and a change to it affects both.

At the Knee

The patellar tendon runs from the bottom of the patella to the tibial tubercle, a bump on the front of the shin. 2.2.5 covers the tubercle.

If the tubercle sits further outwards, the tendon pulls the patella outwards harder. 3.12.6 covers the Q angle.

If the tendon is long, or the tubercle sits low, the patella rides high and enters the groove later. 3.12.1 covers engagement.

A high patella is called patella alta, and it is one of the four factors in instability. 3.12.8 covers them.

What Changes It

The position the tubercle grew in, which is fixed in adults.

Tendon length, which sets how high the patella sits.

Rotation of the shin bone, which moves the tubercle round the front of the leg. 3.12.11 covers it.

Surgery that moves the tubercle deliberately. 7.53 covers the operation.

Clinical Relevance

This is the one thing in the joint that an operation can move precisely, which is why it attracts attention.

Moving it inwards reduces the outward pull. Moving it downwards lowers a high patella. 7.53 covers the operation.

Moving it in the wrong direction or too far causes problems of its own. 3.12.10 covers the wear that follows.

5.12.2 covers how patellar height is measured.

MRCS Level

The Principle

The tubercle sets two independent quantities: the transverse offset of the distal pull, and the height at which the patella sits.

They are independent mechanically and are often moved together surgically, which is a common source of overcorrection.

Describing a knee therefore needs both a transverse index and a height ratio, not one number.

At the Knee

Transverse offset is measured as the tubercle to groove distance, with quoted abnormal values above about 20 millimetres and a soft threshold. 5.12.4 covers it.

Patellar height is measured by ratios of tendon length to patellar length or to tibial landmarks, and the ratios do not agree with each other. 5.12.2 covers them.

A high patella delays engagement, which lengthens the arc in which only the medial soft tissue restrains. 3.12.5 covers the ligament.

A lateral tubercle raises the lateral vector throughout the arc rather than only near extension.

The two therefore produce different clinical pictures: alta gives instability near extension, lateralisation gives it more broadly. 3.12.8 covers the factors.

What Changes It

Developmental tubercle position and tendon length, which are fixed in the adult.

Tibial torsion, which rotates the tubercle laterally without changing the bone's own position. 3.12.11 covers it.

Femoral version, which raises the same index from the other end of the joint.

Knee flexion during imaging, which reduces the measured tubercle to groove distance.

Previous tubercle surgery, which changes both quantities at once. 7.53 covers it.

How It Is Measured

Tubercle to groove distance on computed tomography or magnetic resonance imaging, with the knee position affecting the result. 5.12.4 covers the technique.

Patellar tendon to trochlear groove distance, which excludes tubercle rotation from the measurement.

Patellar height ratios on a lateral radiograph or on sagittal imaging, which disagree systematically. 5.12.2 covers them.

Cadaveric transfer studies, which measure the pressure and displacement effect of moving the tubercle a set distance. 3.16.1 covers them.

No index reports the transverse offset and the height together, so both must be stated.

Clinical Relevance

Report the tubercle to groove distance and a named height ratio together, because they describe different problems.

Record the knee position at imaging, since the transverse index depends on it.

Expect medialisation to reduce the lateral vector and distalisation to advance engagement, and keep the two decisions separate. 7.53 covers the operation.

Avoid treating a single borderline index as an indication, because both thresholds are soft. 3.12.8 covers the pattern.

FRCS Level

At the Knee

Tubercle transfer is the most mechanically direct operation in this joint, which is why its limits are instructive.

Medialisation reduces the lateral vector and raises medial facet pressure. The second effect is why overcorrection produces medial pain and medial wear. 3.12.10 covers the wear.

Anteriorisation reduces contact pressure by changing the lever arm and the resultant, and its effect is largest in the flexion range where the joint is most loaded. 3.12.3 covers pressure.

Distalisation advances engagement and shortens the soft tissue only window, which is a different mechanism from medialisation. 3.12.1 covers engagement.

Cadaveric work supports each effect in isolation. The clinical results are more modest, which is what happens when one term in a multifactorial problem is corrected precisely. 6.153 covers the controversies.

What Changes It

The direction and the magnitude of transfer, which are separate decisions and are often taken as one.

Whether the underlying abnormality is tubercle position or femoral version, because only the first is addressed here. 3.12.11 covers version.

Trochlear morphology, which limits what a vector correction can achieve. 3.12.4 covers it.

Cartilage status, because transfer moves load onto whatever surface receives it. 3.12.10 covers the patterns.

Skeletal maturity, which rules out tubercle osteotomy in a growing knee. 7.94 covers paediatric practice.

How It Is Measured

Preoperative tubercle to groove distance and a named height ratio, acquired at a recorded knee position. 5.12.4 covers the technique.

Rotational profile imaging where the index is raised, to establish the source before correcting the result. 3.12.11 covers it.

Intraoperative measurement of transfer distance, which is the only quantity controlled directly.

Postoperative imaging, which confirms the geometric change and not the pressure change.

No clinical measurement confirms that the intended pressure redistribution occurred, which is a gap between the rationale and the verification.

Clinical Relevance

Establish whether a raised index comes from the tubercle or from femoral version before transferring the tubercle. 3.12.11 covers the distinction.

Decide direction and magnitude separately, since medialisation and distalisation correct different mechanisms.

Limit medialisation, because medial facet overload is the characteristic complication. 3.12.10 covers it.

State that the pressure redistribution is inferred rather than measured after surgery.

Take the patient's age, physiology, comorbidities, knee demand, psychological state and their own ideas, concerns and expectations into the decision, because a precise correction of one term does not by itself make the operation right for this patient.