The Reliability of Radiographic Evaluation of the Posterior Tibial Slope in Skeletally Immature Patients

OBJECTIVES: An association exists between increased posterior tibial slope and anterior cruciate ligament (ACL) injuries in pediatric patients with open physes. Additionally, an increased posterior tibial slope is also associated with increased odds of a further ACL injury after ACL reconstruction. Reliable radiographic measurement techniques are important for investigating limb alignment prior to and following pediatric ACL reconstruction. There have been multiple methods described to measure tibial slope, however, it is unknown if these are reliable in the pediatric population given the altered and developing proximal tibia anatomy during skeletal maturation. The purpose of this study is to evaluate the intra- and interobserver reliability of previously described posterior tibial slope measurements from lateral radiographs of skeletally immature patients. METHODS: A retrospective chart review was performed including patients age 6-18 years old with available lateral knee radiographs and no prior surgery or musculoskeletal pathology. 130 patients (ten in each age group) were analyzed by three reviewers. Measurements were made using the Centricity Enterprise Web PACS System (Version 3.0; GE Medical Systems, Barrington, Illinois). The posterior tibial slope was measured using three previously described methods: the anterior tibial cortex (ATC), posterior tibial cortex (PTC), and the proximal tibia anatomic axis (TPAA) (Figure 1). The radiographs were graded by each reviewer twice, performed two weeks apart. The intra- and interobserver agreements were determined using the intraclass correlation coefficient (ICC) with the second set of measurements used for interobserver agreement. ICC estimates and their 95% confident intervals were calculated using SAS statistical package (Version 9; SAS Institute, Cary, North Carolina) based on an individual ratings, absolute-agreement, two-way mixed-effects model. As described by Landis and Koch, the interpretation of the ICC was as follows—slight: 0.00 to 0.20; fair: 0.21 to 0.40; moderate: 0.41 to 0.60; substantial: 0.61 to 0.80; almost perfect agreement: 0.81 to 1.00. RESULTS: There were 130 patients included with an average age of 12 years old (range 6-18 years) with 47.7% (n=62) male patients. The mean measurements were ATC: 12.3 degrees, PTC 7.2 degrees, and TPAA: 9.3 degrees. Measures of intra-observer agreement met almost perfect agreement criteria among all three reviewers for all three methods of measuring the posterior tibial slope with a mean of 0.88 (range, 0.86-0.92) for ATC, 0.85 (range, 0.82-0.87) for PTC, and 0.87 (range, 0.82-0.92) for TPAA. (Table 1) Measures of inter-observer agreement was substantial across all three reviewers for all three methods of measuring with an average of 0.72 (range, 0.70-0.83) for ATC, 0.74 (range, 0.68-0.83) for PTC, and 0.74 (range, 0.68-0.84) for TPAA (Table 1). CONCLUSION: In accordance with prior reports, the ATC measurement yields larger values and PTC smaller values when measuring posterior tibial slope. The three different methods of measuring demonstrated almost perfect agreement for intra-rater reliability and substantial agreement for inter-rater reliability. There was no difference in reliability across the three different measurement methods. Thus, despite the transforming anatomy during skeletal maturation, the posterior tibial slope can be reliability measured in the skeletally immature population using plain lateral radiographs and any of the three described methods- ATC, PTC, or TPAA.