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CT and MRI measurements of tibial tubercle-trochlear groove distances are not equivalent in patients with patellar instability . BACKGROUND Tibial tubercle-trochlear groove distance ( TT-TG ) is a commonly used measurement for surgical decision making in patients with patellofemoral malalignment and instability . This measurement has historically been performed utilizing axial computed tomography ( CT ) . More recently , magnetic resonance imaging ( MRI ) has been proposed as an equivalent method , but this has not yet been fully validated . PURPOSE To determine the reliability of TT-TG distance measurements on both MRI and CT and to determine whether the measurements are interchangeable with one another . STUDY DESIGN Cohort study ( diagnosis ) ; Level of evidence , 2 . METHODS All patients with patellar instability who underwent both CT and MRI of the knee from 2003 to 2011 were included ( n = 59 knees in 54 patients ) . Two fellowship-trained musculoskeletal radiologists measured the TT-TG distances for each patient by CT and MRI in a randomized , blinded fashion . Interobserver reliability was calculated between radiologists for both imaging modalities , and intermethod reliability was calculated between the 2 imaging modalities . The results are reported using intraclass correlation coefficients ( ICCs ) and Bland-Altman analysis . RESULTS The 59 knees had a mean TT-TG distance of 16.9 mm ( range , 8.3-25.8 mm ) by CT and 14.7 mm ( range , 1.5-25.1 mm ) by MRI . Interobserver reliability between the radiologists was considered excellent for both CT and MRI ( ICC = 0.777 and 0.843 , respectively ) . When comparing CT to MRI , the ICC was considered only fair for each of the raters ( 0.532 and 0.539 ) . Eleven patients ( 19 % ) had a TT-TG distance of ≥20 mm on CT preoperatively and underwent distal realignment by tibial tubercle osteotomy . In this surgical subgroup , the mean TT-TG distance was 22.5 mm ( range , 19.8-25.8 mm ) by CT and only 18.7 mm ( range , 14.4-22.8 mm ) by MRI for a mean difference of 3.80 mm ( P < .001 ) . CONCLUSION The TT-TG distance can be measured with excellent interrater reliability on both MRI and CT ; however , the values derived from these 2 tests may not be interchangeable . This observation should be taken into consideration when MRI is used for surgical planning because MRI may underestimate the TT-TG distance when compared with CT .
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Intervention_Physical
CT
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Intervention_Physical
MRI
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Intervention_Physical
CT
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Intervention_Physical
MRI
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Intervention_Physical
CT
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Intervention_Physical
MRI
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Outcome_Physical
CT and MRI measurements of tibial tubercle-trochlear groove distances
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Outcome_Physical
Tibial tubercle-trochlear groove distance ( TT-TG )
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Outcome_Physical
TT-TG distance measurements
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Outcome_Physical
TT-TG distances
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Outcome_Physical
Interobserver reliability
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Outcome_Physical
intermethod reliability
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Outcome_Physical
TT-TG distance
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Outcome_Physical
Interobserver reliability
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Outcome_Physical
ICC
60488
Outcome_Physical
TT-TG distance
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Participant_Condition
patellar instability
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Participant_Condition
patellofemoral malalignment and instability
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Participant_Condition
patellar instability
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Participant_Sample-size
54