TT-TG Measurement

For Medical Students

What It Shows

The measurement is the distance between the tibial tubercle and the trochlear groove, taken along a line across the joint. 2.2.5 covers the tibial tubercle.

It is made from axial images, by superimposing a slice through the groove on a slice through the tubercle. 2.1.5 covers the trochlea.

It matters because it is the number used to decide whether the tubercle is transferred, which makes it the most consequential measurement in patellofemoral practice. 5.12.4 covers patellar subluxation.

How It Is Acquired

Identify the deepest point of the trochlear groove on an axial slice. 5.12.1 covers trochlear dysplasia.

Identify the centre of the patellar tendon insertion on the tubercle. 2.2.5 covers the tibial tubercle.

Draw the reference line across the posterior femoral condyles and measure both points against it. 2.1.2 covers the distal femur.

State the knee position, because the measurement changes with flexion. 3.12.1 covers patellar tracking.

State the modality, since computed tomography and magnetic resonance imaging give systematically different values. 5.5.1 covers the standard protocol.

What Normal Looks Like

Both landmarks identifiable on their own slices. 2.1.5 covers the trochlea.

A reference line drawn on a slice where both posterior condyles are visible. 2.1.2 covers the distal femur.

The knee near extension with the quadriceps relaxed, and the position recorded. 3.12.1 covers patellar tracking.

A value reported with the modality, the position and the reference line that produced it. 5.20.1 covers structured reporting.

What Abnormal Looks Like

A tubercle positioned laterally relative to the groove. 2.2.5 covers the tibial tubercle.

A groove too shallow for a deepest point to be found, which makes the measurement unreliable rather than high. 5.12.1 covers trochlear dysplasia.

A value quoted with no modality, no position and no reference line. 5.20.8 covers common reporting errors.

A value compared with a threshold derived by a different method. 5.21.4 covers the patellofemoral imaging literature.

MRCS Level

How It Is Acquired

In a viva the expected answer gives the method before the number.

The expected points are the two landmarks, the reference line, the knee position, the modality, and the dysplastic groove.

Say that the measurement is made on superimposed slices rather than on one. 5.2.22 covers the CT acquisition.

Say that a value without its method is not a measurement. 5.20.1 covers structured reporting.

Say that it is one factor among several rather than a decision by itself. 5.12.1 covers trochlear dysplasia.

What Abnormal Looks Like

The reasoning expected is that the measurement stands in for the lateral force on the patella, which is why it predicts instability and why it is sensitive to everything that alters that geometry. 3.12.6 covers the Q angle.

Flexion engages the patella and rotates the tibia, so the same knee measures differently at different angles. 3.3.5 covers the screw home mechanism.

Computed tomography is acquired with the limb positioned by the table and magnetic resonance imaging with it positioned by the coil, which is part of why the two differ. 5.5.1 covers the standard protocol.

A dysplastic trochlea has no identifiable deepest point, so the measurement inherits that uncertainty rather than reporting it. 5.12.1 covers trochlear dysplasia.

How Good It Is

The measurement is reproducible within one method and is not comparable between methods.

Its best contribution is identifying the patient in whom a tubercle transfer enters the discussion. 5.16.5 covers osteotomy planning.

It is limited by modality dependence, by flexion dependence, by the choice of reference line, and by thresholds derived in cohorts measured differently. 5.21.4 covers the patellofemoral imaging literature.

No figures appear for accuracy on this page, under decision 028, and no distance values or surgical thresholds appear either: those are published quantities tied to particular methods and this environment cannot check them. Pages across Section 5 cite this one as the place that statement is made.

FRCS Level

How Good It Is

At decision level the value contributes to a decision about the tubercle and does not make it. 5.16.5 covers osteotomy planning.

The groove, the patellar height and the rotation are read from the same study, because they interact. 5.12.2 covers patella alta.

A value near a threshold is a reason to weigh the other factors rather than to operate. 5.12.1 covers trochlear dysplasia.

A rotational profile can explain a value that the local anatomy does not. 5.16.3 covers the rotational profiles.

The examination and the history carry the instability itself, which no measurement records. 4.15.3 covers the patellar apprehension test.

Where It Misleads

The commonest error is quoting a value with no method attached. 5.20.8 covers common reporting errors.

A threshold from a computed tomography cohort is applied to a magnetic resonance value. 5.21.4 covers the patellofemoral imaging literature.

A measurement from a flexed knee is compared with one taken near extension. 3.12.1 covers patellar tracking.

A dysplastic groove yields a confident value from an uncertain landmark. 5.12.1 covers trochlear dysplasia.

The value is treated as the indication rather than as one input, which is the error with the largest operative consequence. 5.12.4 covers patellar subluxation.

Fellowship Level

Where It Misleads

At fellowship level the value arrives in a letter, with no modality, no position and no reference line, and a plan already built on it. 5.20.1 covers structured reporting.

Remeasuring is the only way to know what it means, and that needs the images. 5.20.8 covers common reporting errors.

Alternative measurements of the same geometry exist and give different answers, so a second opinion that remeasures differently will disagree legitimately. 5.21.4 covers the patellofemoral imaging literature.

After a tubercle transfer the measurement describes the operation, so it cannot be used to judge the operation's result. 5.12.8 covers the post-operative imaging.

Automated measurement fixes the landmark rules and so fixes one convention, which removes variability without settling which convention is right. 5.18.8 covers artificial intelligence in imaging.

The useful report gives the modality, the knee position, the reference line, both landmark slices, and the value last. 5.20.1 covers structured reporting.