The contribution of the tibial tubercle to patellar instability: analysis of tibial tubercle–trochlear groove (TT-TG) and tibial tubercle–posterior cruciate ligament (TT-PCL) distances

Purpose: The purpose of this study is to assess the reliability of measuring the tibial tubercle to posterior cruciate (TT-PCL) distance compared to the tibial tubercle to trochlear groove (TT-TG) distance on magnetic resonance imaging (MRI), establish baseline TT-PCL values in patellar instability patients, and determine the predictive value of an excessive TT-PCL distance (≥24 mm) for recurrent patellar instability compared to a TT-TG distance ≥20 mm. Methods: TT-TG and TT-PCL distances were calculated on MRI in a randomized and blinded fashion by two reviewers on 54 patients (59 knees) with patellar instability. Interobserver reliability was assessed using interclass correlation coefficients (ICC). TT-PCL distances were also assessed to establish mean values in patellar instability patients. The ability of excessive TT-PCL and TT-TG distances to predict recurrent instability was assessed by comparing odds ratios, sensitivities, and specificities. Results: Interobserver reliability was excellent for both TT-TG (ICC = 0.978) and TT-PCL (ICC = 0.932). The mean TT-PCL in these 59 knees was 21.7 mm (standard deviation 4.1 mm). Twelve (20 %) of 59 knees had a single dislocation, and 47 (80 %) exhibited 2 or more dislocations. The odds ratios, sensitivities, and specificities of a TT-TG distance ≥20 mm for identifying patients with recurrent dislocation were 5.38, 0.213, and 1.0, respectively, while those of a TT-PCL distance ≥24 mm were 1.46, 0.298, and 0.583, respectively. Of the 10 knees with a TT-TG distance ≥20 mm, all 10 (100 %) had recurrent instability, while 14 (73.7 %) of the 19 knees with a TT-PCL ≥24 mm experienced multiple dislocations (n.s.). Conclusion: Both TT-PCL and TT-TG can be measured on MRI with excellent interobserver reliability. In this series, the mean TT-PCL value in patients with patellar instability was 21.8 mm, but the range was broad. A TT-PCL distance ≥24 mm was found to be less predictive of recurrent instability in this series. For patients experiencing multiple episodes of patellar instability in the setting of a normal TT-TG distance, obtaining the TT-PCL measurement may provide a more focused assessment of the tibial contribution to tubercle lateralization. Level of evidence: III.