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Trochlear Dysplasia

Page 5.12.1 · Patellofemoral MRI · imaging page

A shallow or flat groove, which is the strongest anatomical predictor of recurrence and the hardest of these measurements to make.

Pipeline status. Editorial draft. Evidence verification did not run. Not consultant reviewed. Not for clinical use.

The same topic at 4 depths

Each tier deepens the one below it and never repeats it. Choose the level you want.

Trochlear Dysplasia

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For Medical Students

What It Shows

Trochlear dysplasia is a trochlear groove that is shallow, flat or convex rather than concave. 2.12.1 covers the trochlea.

Magnetic resonance imaging shows the cartilaginous trochlea as well as the bony one, which a radiograph and a computed tomography study cannot. 5.5.7 covers the cartilage.

It matters because the groove is what holds the patella as flexion begins, so a shallow one lengthens the part of the range in which the patella is unconstrained. 3.12.4 covers the trochlear function.

How It Is Acquired

Assess on axial images through the proximal trochlea, because the dysplasia is proximal and the distal groove can look normal. 5.5.1 covers the standard protocol.

Assess the cartilaginous surface rather than the bony outline, since the cartilage is what articulates. 5.5.7 covers the cartilage.

Assess the sagittal images for a supratrochlear spur and for the crossing of the trochlear floor with the condylar outlines. 5.3.2 covers the lateral view.

Record which level the measurement was made at, because the groove deepens distally. 5.20.8 covers common reporting errors.

Assess the whole patellofemoral joint alongside, since dysplasia rarely occurs alone. 5.12.2 covers patella alta.

What Normal Looks Like

A concave trochlear groove on the proximal axial images with symmetrical facets. 2.12.1 covers the trochlea.

A cartilaginous surface following the bony contour. 5.5.7 covers the cartilage.

No supratrochlear spur on the sagittal images. 5.3.2 covers the lateral view.

A patella seated centrally in the groove. 5.12.4 covers patellar subluxation.

What Abnormal Looks Like

A shallow, flat or convex proximal trochlea. 2.12.1 covers the trochlea.

Asymmetric facets, with the medial facet short. 2.12.1 covers the trochlea.

A supratrochlear spur projecting anteriorly. 5.3.2 covers the lateral view.

A cartilaginous surface flatter than the bony outline suggests, which is the appearance only this modality shows. 5.5.7 covers the cartilage.

Key Learning Points

  • The groove may be shallow, flat or convex rather than concave.
  • Magnetic resonance imaging shows the cartilaginous trochlea as well as the bony one.
  • The dysplasia is proximal and the distal groove can look normal.
  • The groove is what holds the patella as flexion begins.
  • A shallow groove lengthens the unconstrained part of the range.

MRCS Level

How It Is Acquired

In a viva this is the measurement to name as the strongest anatomical predictor of recurrent instability.

The expected points are the proximal axial images, the cartilaginous surface, the sagittal signs, the recorded level, and the associated features.

Say that the dysplasia is proximal, so the level matters. 5.20.8 covers common reporting errors.

Say that the cartilaginous trochlea is what articulates. 5.5.7 covers the cartilage.

Say that several grading systems exist and are not interchangeable. 5.21.4 covers the patellofemoral imaging literature.

What Normal Looks Like

The reasoning expected is that the trochlea constrains the patella only once it has engaged, so the depth of the proximal groove determines how early in flexion that engagement happens. 3.12.4 covers the trochlear function.

A shallow proximal groove therefore extends the range in which the medial soft tissues are the only restraint, which is the mechanism by which dysplasia causes recurrence. 3.12.5 covers the medial patellofemoral ligament function.

Because the groove deepens distally, a measurement made too far distal underestimates the dysplasia, which makes the level part of the measurement. 2.12.1 covers the trochlea.

The cartilaginous and bony surfaces can differ, so the modality that shows cartilage measures the articulating surface while the others measure the bone under it. 5.5.7 covers the cartilage.

How Good It Is

Magnetic resonance imaging assesses trochlear dysplasia better than any other modality and still measures it inconsistently.

Its best contribution is the cartilaginous surface, which is what articulates and which only this modality shows. 5.5.7 covers the cartilage.

It is limited by the level dependence, by the several competing grading systems, and by the shape being three dimensional. 5.16.8 covers three dimensional reconstructions.

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

Key Learning Points

  • The trochlea constrains the patella only once it has engaged.
  • Proximal depth determines how early engagement happens.
  • A shallow groove extends the range in which soft tissue is the only restraint.
  • A measurement made too distal underestimates the dysplasia.
  • The cartilaginous and bony surfaces can differ.

FRCS Level

How Good It Is

At decision level this is the measurement that most changes an instability operation.

Significant dysplasia is the finding that moves the discussion towards addressing the groove rather than only the soft tissues. 5.17.6 covers imaging after MPFL reconstruction.

Mild dysplasia with other abnormal measurements is a combined picture, which is the common one. 5.12.2 covers patella alta.

A normal trochlea in a patient with recurrent instability shifts the question to the other measurements and to the soft tissues. 5.12.5 covers MPFL injury.

The measurement's level and method need stating, because a grade without them is not comparable with a threshold. 5.20.8 covers common reporting errors.

Where It Misleads

The commonest error is measuring too far distal and understating the dysplasia. 2.12.1 covers the trochlea.

The bony outline is measured where the cartilaginous surface was the question. 5.5.7 covers the cartilage.

A grade is quoted without its system. 5.21.4 covers the patellofemoral imaging literature.

The sagittal signs are not reviewed. 5.3.2 covers the lateral view.

Dysplasia is assessed in isolation from the other instability measurements. 5.12.3 covers patellar tilt.

Key Learning Points

  • Measuring too distal understates the dysplasia.
  • The bony outline is measured where cartilage was the question.
  • A grade is quoted without its system.
  • The sagittal signs go unreviewed.
  • Dysplasia is assessed in isolation.

Fellowship Level

Where It Misleads

At fellowship level trochlear dysplasia is the instability measurement with the strongest evidence behind it and the weakest measurement discipline around it.

Several grading systems are in use, they are not interchangeable, and reader agreement on them is moderate, so the measurement that most predicts recurrence is also one of the least reproducible. 5.21.4 covers the patellofemoral imaging literature.

The shape is three dimensional and every routine measurement takes one axial section through it, so the number describes a slice of a surface. 5.16.8 covers three dimensional reconstructions.

The cartilaginous and bony surfaces differ, and the published thresholds come variously from radiographs, from computed tomography and from magnetic resonance imaging, so a threshold and a measurement often come from different tissues. 5.5.7 covers the cartilage.

Three dimensional morphometric analysis would describe the groove properly and is a research technique, which leaves the strongest predictor measured by the crudest available method. 5.16.8 covers three dimensional reconstructions.

The disciplined report names the system, the level and the surface measured, and gives the appearance alongside the grade. 5.20.5 covers reporting cartilage lesions.

Key Learning Points

  • Several grading systems are in use and are not interchangeable.
  • The strongest predictor is among the least reproducible measurements.
  • A routine measurement takes one section through a three dimensional shape.
  • Thresholds and measurements often come from different tissues.
  • Name the system, the level and the surface measured.

Explore Further

Where this topic came from and where it goes next in the spiral.

References

No reference list is attached to this draft. Evidence verification has not run, because source retrieval was unavailable at generation time. Under the Operations Handbook a missing reference is acceptable and an invented one is a critical failure, so nothing has been cited and no citation marker appears in the text. This page carries no diagnostic accuracy figure, which is decision 028 rather than an omission: the comparative standing of the modalities is standard teaching and a specific sensitivity written from memory is the same class of claim as an invented citation, so ACC-001 refuses the number and the page states the comparison. A reviewer with the literature can add the figures into How Good It Is without restructuring the page. Any measurement here, including a sequence parameter, an angle or a millimetre, is written from standard teaching, has not been checked, and is registered under decision 006 in pipeline/config/figures with a verification state of UNVERIFIED_FROM_MEMORY.

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