Introduction

Trochlea dysplasia is an important factor in patella instability. When present and severe, it requires treatment in the form of Trochleoplasty. Multiple qualitative and quantitative measures described in the literature in order to define trochlea dysplasia have shown limited use in clinical practice and diagnosis of trochlea dysplasia continues to be challenging.
The goal of this study is to describe what we have termed as “Trochlear Specific Patterns” (TSP) and their distribution on
the “Trochlear Specific Zones” in both normal and abnormal trochleae.
Our hypothesis is that TSP type 3 is diagnostic of trochlea dysplasia.

Methods

MRI scan of 2 cohort of patients were studied.
Group 1: 71 patients who had no patellofemoral problems.
Group 2: 58 patients who had recurrent patella instability due to trochlea dysplasia.

Two transverse lines were defined on sagittal MRI:
- PLFC line. Passing through proximal portion of the Posterior Lateral Femoral Condyle (PLFC), just above the cartilage.
-FIACL line. Through the most proximal part of Blumensaat Line at the level or just above the Femoral Insertion of the Anterior Cruciate Ligament (FIACL).

Based on these reference lines, we defined 2 Trochlear Specific Zones (TSZs):
-zone 0 above the PLFC line
-zone 1 between the PLFC line and FIACL

Three distinct Trochlear Shape Patterns (TSPs) were identified on axial MRI cuts corresponding to the sagittal references. The axial MRI of all patients were studied to identity the TSP in the TSZ 0, at PFCL and FIACL.

Type 1: has a clearly defined sulcus defining a lateral and a medial trochlear facet
Type 2: the sulcus is shallow (sulcus angle >1650), but the lateral and medial facets are present
Type 3: include flat or convex trochleae, subdivided into 3 subtypes described in the article in detail.

Results

In Group 1, we found either type 1 or 2 TSP at all levels:
- TSZ 0: Type 1 in 53 patients. Type 2 in 18 patients
- PFCL: Type 1 in 61 patients. Type 2 in 9 patients. Type 3a in 1 patient (this patient had mild trochlea dysplasia).
- FIACL: type 1 in 67 patients. Type 2 in 4 patients.

In Group 2, we found the following distribution of TSP:
-TSZ 0: All 58 were type 3
- PFCL: Type 1 in 1, type 2 in 5 and type 3 in 52 patients.
- FIACL: the shapes of trochleae were highly variable.

Discussion And Conclusion

The main finding of this study is that in Zone 0, TSP type 3 is never present in patients with a normal trochlea (they show TSP type 1 and 2) and is present in all patients with abnormal trochlea, therefore is pathognomonic of trochlea dysplasia.

Conclusion

The assessment of trochlear shape pattern in zone 0 is a very simple and reliable method to differentiate between normal
and dysplastic trochleae. Being based on MRI scan, it is more reproducible than plain radiographs with no radiation risk and can be a useful screening test to identify dysplastic trochleae that need specialist referral