Abstract
Patello-femoral instability (PFI) affects 40 individuals per 100,000 population and causes significant morbidity. The causes of patello-femoral instability are multi-factorial, and an isolated anatomical abnormality does not necessarily indicate instability. Patello-femoral subluxation ranges from 0% (stable patella tracking) to 100% (dislocation) and there is an established relationship between the amount of subluxation and anterior knee pain. Traditionally, magnetic resonance (MR) imaging and standard radiographs are used to guide the clinician towards a suitable corrective procedure for PFI. The multi-factorial nature of patello-femoral instability is not addressed with current imaging techniques. This study aims to address which anatomical variables assessed on MR images are most relevant to patello-femoral subluxation. This information will aid surgical decision making, particularly in selecting the most appropriate reconstructive surgery.
A retrospective analysis of MR studies of 60 patients with suspected patello-femoral instability was performed. All patients were graded for degree of subluxation using a dynamic MR scan.
The patient scans were assessed for the presence of a specific range of anatomical variables:
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patella alta, (modified Insall-Salvatti)
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patella type (Wiberg classification)
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trochlea sulcus angles for bone and cartilage surfaces
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the distance of the vastus medialis obliquis (VMO) muscle from the patella
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trochlea and patella cartilage thickness
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the horizontal distance between the tibial tubercle and the midpoint of the femoral trochlea (TTD)
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patella engagement – the percentage of the patella height that is captured in the trochlea groove in full extension.
The Wilk’s Lambda test for multi-variate analysis was used to establish whether any relationship was present between the degree of patello-femoral instability and bony or soft tissue anatomical variables. Non-parametric statistical tests were applied across the groups and within the groups to assess their relative significance.
The following variables showed a significant relationship with patellofemoral subluxation; distance of the VMO from the patella (< 0.001), TTD (< 0.001), patella engagement (0.001), sulcus angles (0.004) and patella alta (0.005).
This study agrees with previous work showing a significant correlation between subluxation and trochlea sulcus angle and TTD.
This is the first study to establish a significant correlation between patella engagement and radiological instability. The lower the percentage engagement of the patella in the trochlea, the greater the degree of patello-femoral instability. Patella engagement showed a more significant relationship with subluxation than patella alta.
We report a new method of predicting patello-femoral instability by measuring the overlap of the patella in the trochlea groove.
Correspondence should be addressed to: EFORT Central Office, Technoparkstrasse 1, CH – 8005 Zürich, Switzerland. Email: office@efort.org