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Novel MRI measurement cited as predictable for patellofemoral instability
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According to recently published study results, the width of the patellar tendon beyond lateral trochlear ridge distance may be a reliable, predictable and discriminative measurement for patellofemoral dislocations.
“We started this project trying to think of reliable ways to describe the relationship between the extensor mechanism and the trochlear morphology that has to contain it during knee range of motion,” J. Todd R. Lawrence, MD, told Healio.com/Orthopedics. “It seemed to us that most other widely used quantitative assessment tools for patellar instability only measured trochlear morphology or extensor mechanism alignment but not the relationship between these two factors. The new measurement described in this paper, termed [width of the patellar tendon beyond lateral trochlear ridge] PT-LTR, quantifies how much of the extensor mechanism is actually outside of the containment area of the trochlea. In doing so it helps to integrate trochlear morphology with overall alignment (and maybe to a certain extent soft tissue restraints as well). In essence we feel that it helps describe how well an individual patient’s trochlea contains their extensor mechanism.”
Investigators analyzed radiology records from 2005 to 2014 for patients aged between 5 years and 18 years and without patellofemoral dislocations and who had knee MRIs. Two blinded reviewers assessed 215 MRI studies. In a case-control design, standard and novel morphology measurements were calculated for each knee. The intraclass correlation coefficient was used to assess the interobserver reliability of each measure. The two-tailed independent-samples Student t tests were used to calculate predictability for patellofemoral dislocation. Receiver operating characteristic analyses and area under the curve were used to calculate the discriminative capacity. Investigators used resultant sensitivity and specificity to calculate an optimal measurement cutoff.
Results showed excellent agreement between raters in standard measurements of tibial tubercle- trochlear groove (TT-TG) distance, tangential axial width of the patella (TAWP), and tangential axial trochlear width (TATW). Investigators noted the lateral condyle length had good agreement and PT-LTR, the novel measurement had excellent agreement. Significant predictors of patellofemoral instability included TT-TG, TAWP, TATW, and PT-LTR.
According to results from the receiver operating characteristic analysis, the AUC for TAWP was 0.65, below the 0.8 threshold and the AUC for TATW was 0.814 and when less than 32.5 mm it was 76% sensitive and 77% specific for dislocations. TT-TG had an AUC of 0.806. When TT-TG was 13.5 mm or more, it was 76% sensitive and 76% specific for dislocations. The PT-LTR had an AUC of 0.876 and when it was 5.55 mm or greater, it was 73% sensitive and 89% specific for patellofemoral dislocation.
“While it is still unclear how useful this measurement will be in predicting the risk of a recurrent dislocation or how it will factor into surgical decision making, we hope that it will help change the discussion on patellar instability from simply considering individual factors to that of containment,” Lawrence said. – by Monica Jaramillo
Disclosures: Mistovich reports he is paid consultant for OrthoPediatrics; receives educational support from Arthrex; and holds stock or stock options in Right Mechanics. Please see the full study for a list of all other authors’ relevant financial disclosures.
Perspective
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Shital N. Parikh, MD, FACS
Patellofemoral instability is multifactorial and there is a complex interplay between the trochlea, tibial tuberosity and extensor mechanism. The authors of this study introduced a novel MRI measurement (width of the patellar tendon beyond lateral trochlear ridge or PT-LTR) to estimate the containment of extensor mechanism in the trochlea. They found a significant difference in PT-LTR values between patellar instability (9 mm) and control group (2 mm). At a cutoff value of 5.55 mm, PT-LTR had 73% sensitivity and 89% specificity to predict patellar instability. It can be used in lieu of TT-TG distance which has similar sensitivity (76%), but less specificity (76%). PT-LTR does not have the other drawbacks of TT-TG, such as occasional inability to identify the trochlear groove and varied values with knee flexion and rotation. However, like TT-TG, the PT-LTR measurement is an absolute value that does not account for different knee sizes and patellar tendon widths, age or gender of the patient and other risk factors, such as trochlear dysplasia or patella alta. It is a bit surprising that measurement of patellar tendon width was unreliable with the lowest correlation between raters, but measurement of patellar tendon width beyond LTR was reliable with excellent correlation. Overall, PT-LTR, if validated in future studies, would be a step forward in dynamic assessment of extensor mechanism.
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