The Coronal Plane Alignment of the Knee Classification in Patients With Medial Meniscal Posterior Root Tears

Background: The medial meniscus posterior root tear (MMPRT) is a contributing factor to osteoarthritis (OA). The Coronal Plane Alignment of the Knee (CPAK) classification is a method for characterizing knee alignment phenotypes by incorporating the arithmetic hip–knee–ankle (aHKA) and the joint line obliquity (JLO). Purpose: To evaluate the CPAK type in patients with MMPRT while comparing the radiographic parameters between the MMPRT knees and the unaffected knees. Study Design: Cross-sectional study; Level of evidence, 3. Methods: A total of 74 patients with MMPRT, including 87 knees, were confirmed using magnetic resonance imaging. Radiographic parameters of long-leg radiographs were measured separately by 2 reviewers at 2 different time points to classify the CPAK phenotypes, where the neutral aHKA was determined by 0°± 2° and neutral JLO by 180°± 3°. Results: Patients with MMPRT were predominantly women (72.97%), with a mean age of 55.1 ± 9.3 years and a body mass index of 27.7 kg/m 2 . Among 74 patients, 13 had bilateral MMPRT. According to a mean aHKA of −2°± 3.4° and JLO of 173.9°± 4.2°, a total of 87 MMPRT knees tended to demonstrate varus (41.4%) to neutral (49.4%) alignment, and apex distal JLO (73.4%). The CPAK distribution of MMPRT was type 2 (39.1%), type 1 (28.7%), type 4 (11.5%), type 5 (10.3%), type 3 (5.6%), type 6 (3.5%), and type 7 (1.2%); types 8 and 9 were not found. There was no difference in CPAK distribution between the MMPRT knees and the non-affected knees. The intra- (0.74) and interrater (0.80) reliability of the CPAK classification were in substantial agreement. The intrarater reliability for the lateral distal femoral angle (LDFA) and the medial proximal tibial angle (MPTA) was good (intraclass correlation coefficient [ICC] = 0.89). Interrater reliability was excellent for the LDFA (ICC = 0.91) and MPTA (ICC = 0.93). Conclusion: Among patients with MMPRT, the most common CPAK phenotypes are types 2, 1, and 4, respectively. Most patients had an apex-distal JLO and varus-to-neutral alignment. These findings suggest that apex distal JLO may be a contributing factor for the development of MMPRT.