Magnetic Resonance Imaging and Radiographic Patellar Height Ratio Measurement Reliability Is Not Degraded by Skeletal Maturity or Patellofemoral Instability

PURPOSE: To identify the most reliable patellar height ratios (PHRs) between and within imaging modalities and evaluate whether skeletal maturity or patellofemoral instability (PFI) diagnosis impacts the PHR reliability. METHODS: A radiologic database of lateral knee radiographs and magnetic resonance imaging (MRI) studies from patients treated at a children's hospital was established with a priori power analysis. Seven fellowship-trained orthopaedic surgeons performed PHR measurements (Insall-Salvati Index [ISI], Caton-Deschamps Index [CDI], Blackburne-Peel Ratio, Knee Triangular Ratio, Modified ISI, Koshino Index, and Patellotrochlear Index). Intraclass correlation coefficients were calculated to assess inter- and intrarater reliability on radiographs, MRI, and between the 2 modalities. Comparisons of reliability based on skeletal maturity and PFI diagnosis were performed using Wilcoxon signed-rank testing. RESULTS: Among 77 subjects, 29 (37.7%) were skeletally immature, whereas 37 (48.1%) had a PFI diagnosis. Radiograph-derived ISI inter-rater reliability ranged from 0.69 to 0.82, whereas intrarater reliability ranged from 0.75 to 0.95. For CDI, inter-rater reliability ranged from 0.64 to 0.78 and intrarater reliability ranged from 0.65 to 0.78. Radiographic reliability was greater for patients with PFI for both inter-rater (P < .001) and intrarater reliability (P = .035). Radiographic intrarater reliability was greater among men than women (P = .012). Inter-rater (0.88-0.92) and intrarater (0.89-0.93) reliability on MRI were both excellent for Patellotrochlear Index. Inter-rater reliability of MRI-derived PHRs was greater among PFI patients (P = .021). Both intrarater (P < .001) and inter-rater (P = .007) reliability were greater among men than women. For intermodality reliability, all PHRs except ISI showed questionable or unacceptable reliability, whereas ISI consistently showed very good intermodality reliability (0.65-0.73). Intermodality reliability of PHRs was greater among skeletally immature subjects (P = .005). CONCLUSIONS: ISI and CDI are the most reliable PHRs in radiographic assessment of patellar height. On MRI, Patellotrochlear Index is the most consistently reliable, whereas ISI and CDI show very good to excellent reliability. ISI is the only PHR showing very good intermodality reliability. Neither skeletal immaturity nor a PFI diagnosis degrades the reliability of PHRs. LEVEL OF EVIDENCE: Level III, retrospective diagnostic case series.