J Sign

For Medical Students

What It Tests

The J sign is an abrupt lateral deviation of the patella as the knee reaches full extension, so the patella's path traces a J rather than a straight line.

It indicates that the patella is leaving the trochlear groove and being pulled laterally by the quadriceps once the bone no longer constrains it. 2.12.1 covers the trochlea.

It is the clearest and most specific clinical sign of patellar maltracking, and it is read during active extension rather than passively. 4.15.1 covers tracking.

How It Is Performed

Sit the patient on the edge of the couch with the knees flexed and the feet hanging. 4.3.4 covers the sitting assessment.

Watch the patella from directly in front as the patient extends the knee slowly and fully.

Look specifically at the last part of the arc, since that is where the sign occurs.

Note whether the deviation is abrupt or a gradual drift, because the abruptness is what makes it the J sign rather than a mild tracking abnormality.

Repeat as the patient flexes from full extension, where the patella should reduce abruptly back into the groove.

What a Positive Means

An abrupt lateral deviation in terminal extension is a positive J sign and indicates substantial maltracking.

It usually indicates trochlear dysplasia, patella alta, or a deficient medial restraint, and often more than one of those. 4.15.8 covers dysplasia.

A positive sign in a patient with instability episodes is a structural finding supporting the diagnosis. 4.2.22 covers the dislocation event.

A positive sign in a patient whose complaint is pain rather than instability still indicates maltracking, and whether it explains the pain is a separate question. 4.15.9 covers patellofemoral pain.

A gradual lateral drift without an abrupt component is a milder tracking abnormality and should be recorded as that rather than as a J sign. 4.15.1 covers it.

MRCS Level

How It Is Performed

In a viva the J sign is the answer when asked for a clinical sign of maltracking, and knowing what it implies structurally is the follow up.

The expected points are the sitting position, the view from the front, the terminal part of the arc, the abruptness, and the reduction on flexion.

Say what the sign implies: that the patella is unconstrained in extension, which points at the trochlea, the patellar height or the medial restraint. 4.15.8 covers dysplasia.

Say that you would assess the patellar height, since patella alta keeps the patella out of the groove through more of the range. 4.6.5 covers patellar tenderness.

Say that you would measure the rotational alignment, since malrotation contributes and changes the operation. 4.5.6 covers it.

What a Positive Means

The reasoning expected is that the trochlea's depth constrains the patella through flexion, so in extension, where the patella sits above the groove, only the soft tissues hold it. 2.12.1 covers the trochlea.

A dysplastic trochlea is shallow proximally, so the patella engages later and leaves earlier, which widens the arc in which it is unconstrained. 3.12.4 covers trochlear function.

Patella alta raises the patella so it engages later, with the same effect through a different mechanism. 2.4.1 covers the patellar anatomy.

A deficient medial patellofemoral ligament removes the restraint that resists the quadriceps' lateral pull in that arc. 2.12.2 covers the ligament.

How Good It Is

The J sign is the most specific clinical sign in the patellofemoral examination, because an abrupt reproducible deviation is hard to produce in a normal knee.

It is insensitive. Plenty of knees with significant maltracking and instability do not show it, so a negative sign excludes little.

It is more reproducible than most patellofemoral findings, because the observation is binary rather than graded.

It cannot distinguish its causes, which imaging does, and in this chapter the imaging genuinely leads. 4.1.6 covers that relationship.

No figures appear for any accuracy comparison on this page, under decision 023, and the comparison is the claim being made.

FRCS Level

How Good It Is

At decision level a J sign contributes to the case for bony surgery rather than soft tissue surgery, which is the central patellofemoral decision.

A J sign with trochlear dysplasia and instability is the combination in which a medial patellofemoral ligament reconstruction alone is least likely to be enough. 4.15.7 covers the ligament.

A J sign with patella alta contributes to the case for distalising the tubercle, which is a different operation decided largely on imaging. 4.6.7 covers the tubercle.

A J sign in a patient with pain and no instability is a finding to record and not an indication, and operating on it is a recognised way to make a painful knee worse. 4.15.9 covers patellofemoral pain.

After surgery the sign's persistence is the clearest clinical evidence that the maltracking was not corrected. 4.20.6 covers that assessment.

Where It Misleads

The commonest error is calling a gradual lateral drift a J sign.

The second is assessing it passively or supine.

The third is reading it as an indication in a patient whose complaint is pain.

The fourth is attributing it to the medial restraint alone, when dysplasia and patellar height are the commoner structural contributors.

The fifth is not imaging the trochlea, which is where the operative decision is actually made.

Fellowship Level

Where It Misleads

In referred practice a persisting J sign after patellofemoral surgery is the finding that explains the operation that did not work.

A patient with a medial patellofemoral ligament reconstruction and a persisting J sign had a soft tissue operation for a bony problem, and the revision is a bony one. 4.15.8 covers the assessment that would have shown it.

A patient whose J sign has gone and whose instability persists has a different problem, and that is worth distrusting as a sign that the instability was not patellar at all. 4.2.2 covers instability as a symptom.

A patient with a J sign and generalised hypermobility has a soft tissue contribution that no bony correction fully addresses, and that belongs in the consent. 4.23.3 covers those patients.

In a knee with a previous tubercle osteotomy the sign's persistence may reflect an incomplete correction rather than a failed soft tissue repair, and the pre-operative imaging separates them. 4.6.7 covers the tubercle.

In medicolegal work a recorded J sign is a useful objective observation, and it is one of the few patellofemoral findings specific enough to carry weight. 4.23.7 covers that setting.