The Prognostic Value of a Novel Magnetic Resonance Imaging-Based Classification for Septic Arthritis of the Shoulder

Background: The purposes of the present study were (1) to create a magnetic resonance imaging (MRI)-based classification system for septic shoulder arthritis and to show the results of arthroscopic debridement based on the classification in eradicating the infection, and (2) to determine the distribution of the Gächter stages, which are determined on the basis of arthroscopic findings only, in each grade of the novel classification system. Methods: The present study included 31 patients who underwent arthroscopic debridement for the treatment of acute septic arthritis of the shoulder between January 2001 and December 2015. The mean duration of follow-up was 2.3 years. The included patients were assessed with use of a novel classification system based on preoperative radiographic and MRI findings. MRI findings were classified as Grade I (only synovitis or joint effusion), Grade II (marrow edema in the bare area of the humeral head), Grade III (cartilaginous erosion), Grade IV (osseous erosion), and Grade V (osseous erosion on MRI also observed on radiographs). No changes on radiographs were observed in Grades I to IV. The intraclass correlation coefficients (ICCs) for interobserver and intraobserver reliability were calculated. Results: The overall reinfection rate was 54.8% (17 of 31). Among patients with Grade-I, II, III, IV, and V involvement, the reinfection rates were 0 of 4, 1 of 6, 2 of 4, 9 of 12, and 5 of 5, respectively. A diverse distribution of Gächter stages were found in each grade of the novel classification system, with the exception of Grade V. The ICCs for interobserver and intraobserver reliability were 0.81 and 0.80 for the novel classification system. The ICCs for interobserver and intraobserver reliability were 0.75 and 0.78 for Gächter stages. Conclusions: This novel classification system for the grading of acute shoulder infections on the basis of radiographs and MRI allows preoperative grading of septic arthritis to be performed objectively. Patients who were classified as Grade III or higher in the novel classification system had higher reinfection rates than those who were classified as Grade I or II and required more aggressive treatment to eradicate the infection.