Tibial tubercle to trochlear groove distance and index in children with one-time versus recurrent patellar dislocation: a magnetic resonance imaging study.

PURPOSE To compare the tibial tubercle to trochlear groove (TT-TG) distance and index using magnetic resonance imaging (MRI) between children with onetime and recurrent patellar dislocation. METHODS Records of first-time acute patellar dislocations from 2007 to 2012 were reviewed. 20 males and 23 females aged 10 to 17 years at presentation were included for measurements of the bony and cartilaginous-tendon TT-TG distance and index. Recurrent patellar dislocation was defined as having more than one episode of dislocation within 2 years of the index injury. RESULTS The recurrent dislocation rate was 30.2% (13 out of 43). Patients with recurrent patellar dislocation had a higher mean bony TT-TG distance (17.4 vs. 14.2 mm, p=0.026), cartilaginous-tendon TT-TG distance (18.8 vs. 15.3 mm, p=0.029), and TT-TG index (0.41 vs. 0.33, p=0.008), compared with one-timers. Males had a larger patellofemoral joint than females (49.0 vs. 44.8 mm, p=0.01). The risk of recurrent patellar dislocation was higher in patients with a bony TT-TG distance >14 mm (relative risk [RR]=6.4), a cartilaginoustendon TT-TG distance >13.5 mm (RR=8.8), and a TTTG index >0.26 (RR=6.7). CONCLUSION Children with recurrent patellar dislocation have higher TT-TG distance and index when compared with one-timers.