Patellofemoral Update
Patellofemoral Update
December 31, 2018
2 min read

Novel MRI measurement cited as predictable for patellofemoral instability

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Todd J Lawrence
J. Todd R. Lawrence

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 “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.