Incidence of medial plica in 3,889 knee joints in the Japanese population

Purpose: This study aimed to document the incidence of medial plicae (plica synovialis mediopatellaris) in a large sample of the Japanese population and classify them in a type-dependent manner. Methods: The incidence of medial plicae was investigated during arthroscopic surgery of 3,889 knee joints in 3,563 patients (mean age, 35 years); medial plicae were classified according to Sakakibara's classification (type A, a cord-like elevation in the synovial wall; type B, a shelf-like appearance; type C, a large shelf-like appearance covering the anterior surface of the medial femoral condyle; and type D, double insertions into the medial wall or having a central defect). Of all the knee joints examined, 2,006 knees belonged to male patients and 1,883 to female patients. Both knee joints were examined in 326 patients. From these data, the incidence of medial plicae was calculated. Results: The overall incidence rate of medial plicae was 79.9%. The incidence of Sakakibara type A, B, C, and D medial plicae was 35.2%, 22.4%, 12.3%, and 10.0%, respectively. This pattern was reflected in all age groups in both genders, except in women aged 20 to 39 years. The κ coefficient was moderate, at 0.407, in the patients in whom both knee joints were examined. Conclusions: In a large cohort of Japanese patients undergoing arthroscopic knee surgery, the incidence of medial plica was 79.9%. The same pattern was seen in almost all age groups and in both genders (type A > type B > type C > type D). Level of Evidence: Level IV, prognostic case series.