Patella Alta

For Medical Students

What It Shows

Patella alta is a patella positioned proximally relative to the joint, so it engages the trochlea later in flexion. 2.4.1 covers the patellar anatomy.

It is measured as a ratio rather than a length, which is what allows the measurement to transfer between studies. 5.2.5 covers magnification.

It matters because a late engaging patella spends longer unconstrained, which is the same mechanism trochlear dysplasia acts through. 3.12.4 covers the trochlear function.

How It Is Acquired

Measure on a sagittal image or a true lateral radiograph, since the measurement is sagittal. 5.3.2 covers the lateral view.

Keep the flexion angle consistent, because patellar height changes through flexion. 5.3.2 covers the lateral view.

Use a ratio, so that magnification and patient size cancel. 5.2.5 covers magnification.

State which ratio was used, because several exist and their thresholds differ. 5.21.4 covers the patellofemoral imaging literature.

Assess the cartilaginous articular surfaces where the ratio depends on them, which magnetic resonance imaging allows and a radiograph does not. 5.5.7 covers the cartilage.

What Normal Looks Like

A patella whose articular surface sits opposite the trochlea at the measured flexion angle. 2.12.1 covers the trochlea.

A ratio within the normal range for the convention used. 5.21.4 covers the patellofemoral imaging literature.

Engagement with the trochlea early in flexion. 3.12.4 covers the trochlear function.

Symmetry with the other knee, which is a useful reference. 5.12.4 covers patellar subluxation.

What Abnormal Looks Like

A patella positioned proximal to the trochlea at the measured angle. 2.12.1 covers the trochlea.

A ratio above the normal range for the convention used. 5.21.4 covers the patellofemoral imaging literature.

Delayed engagement, with the patella entering the groove later than expected. 3.12.4 covers the trochlear function.

An associated shallow trochlea or lateralised tuberosity, since the features cluster. 5.12.1 covers trochlear dysplasia.

MRCS Level

How It Is Acquired

In a viva the expected answer names the ratio used before quoting any value.

The expected points are the sagittal measurement, the consistent flexion, the ratio rather than a length, the named convention, and the cartilaginous surfaces.

Say that a ratio without its convention is uninterpretable. 5.21.4 covers the patellofemoral imaging literature.

Say that a ratio cancels magnification where a length does not. 5.2.5 covers magnification.

Say that the feature clusters with the others. 5.12.1 covers trochlear dysplasia.

What Normal Looks Like

The reasoning expected is that the patella is stabilised by the trochlea once engaged, so its height determines when in flexion that protection begins, and alta delays it. 3.12.4 covers the trochlear function.

A ratio is used because the absolute distance depends on the patient's size and on the projection, and dividing by a nearby dimension removes both. 5.2.5 covers magnification.

Different conventions use different reference dimensions, including the patellar tendon length, the articular surface length and the trochlear position, so their numbers are not comparable. 5.21.4 covers the patellofemoral imaging literature.

The conventions that use the articular surface need cartilage to be visible, which is why a magnetic resonance measurement and a radiographic one can differ legitimately. 5.5.7 covers the cartilage.

How Good It Is

The measurement is reproducible within a convention and not comparable between conventions.

Its best contribution is identifying the patient in whom a distalisation is part of the discussion. 5.16.5 covers osteotomy planning.

It is limited by the competing conventions, by the flexion dependence, and by the thresholds coming from different cohorts measured differently. 5.21.4 covers the patellofemoral imaging literature.

No figures appear for accuracy on this page, under decision 028, and no ratio values or thresholds appear either: those are published quantities with competing conventions and this environment cannot check them.

FRCS Level

How Good It Is

At decision level this measurement contributes to whether a tuberosity procedure is part of the operation.

Significant alta with recurrent instability is the picture in which distalisation is discussed. 5.16.5 covers osteotomy planning.

Alta with a normal trochlea and a normal tuberosity position is a different patient from one with all three abnormal. 5.12.1 covers trochlear dysplasia.

The measurement must be reported with its convention or the surgeon cannot compare it with a threshold. 5.20.8 covers common reporting errors.

Patellar height also changes after a tuberosity procedure, so the pre-operative value is the reference for later studies. 5.12.8 covers patellofemoral post-operative imaging.

Where It Misleads

The commonest error is quoting a ratio against a threshold from a different convention. 5.21.4 covers the patellofemoral imaging literature.

The flexion angle differs between studies and the height appears to change. 5.3.2 covers the lateral view.

A length is measured instead of a ratio. 5.2.5 covers magnification.

An oblique lateral is used and the measurement is invalid. 5.3.2 covers the lateral view.

Alta is treated as the whole instability assessment. 5.12.1 covers trochlear dysplasia.

Fellowship Level

Where It Misleads

At fellowship level patella alta is the clearest example in this chapter of competing conventions producing incompatible numbers for one anatomical fact.

Several ratios are in routine use with different reference dimensions and different normal ranges, so the same patella is normal by one and abnormal by another, and the report usually gives a number without its method. 5.21.4 covers the patellofemoral imaging literature.

Some conventions measure bone and some measure the articular surface, so the modality changes the answer as well as the convention, which compounds the problem. 5.5.7 covers the cartilage.

Flexion dependence means a measurement is only comparable with one made at the same angle, and the angle is not recorded on either study. 5.3.2 covers the lateral view.

The surgical thresholds for distalisation derive from particular conventions in particular cohorts, so applying a threshold to a differently measured value is the routine practice rather than the exception. 5.16.5 covers osteotomy planning.

The disciplined report states the convention, the flexion angle and the surface used, which costs one sentence and makes the number usable. 5.20.8 covers common reporting errors.