Springer Acta Medica Okayama 0942-2056 28 11 2019 Medial meniscus posterior root tear causes swelling of the medial meniscus and expansion of the extruded meniscus: a comparative analysis between 2D and 3D MRI 3405 3415 EN Yoshiki Okazaki Department of Orthopaedic Surgery, Okayama University Graduate School Takayuki Furumatsu Department of Orthopaedic Surgery, Okayama University Graduate School Takuya Yamaguchi Division of Radiology, Medical Technology Department, Okayama University Hospital Yuya Kodama Department of Orthopaedic Surgery, Iwakuni Medical Center Yusuke Kamatsuki Department of Orthopaedic Surgery, Okayama University Graduate School Shin Masuda Department of Orthopaedic Surgery, Okayama University Graduate School Yuki Okazaki Department of Orthopaedic Surgery, Okayama University Graduate School Takaaki Hiranaka Department of Orthopaedic Surgery, Okayama University Graduate School Ximing Zhang Department of Orthopaedic Surgery, Okayama University Graduate School Toshifumi Ozaki Department of Orthopaedic Surgery, Okayama University Graduate School Purpose</br> This study aimed to clarify the advantages of three-dimensional (3D) magnetic resonance imaging (MRI) over two-dimensional (2D) MRI in measuring the size of the medial meniscus (MM) and to analyse the volumes of MM and the extruded meniscus in patients with MM posterior root tear (MMPRT), at 10�� and 90�� knee flexion.</br> Methods</br> This study included 17 patients with MMPRTs and 15 volunteers with uninjured knees. The MMs were manually segmented for 3D reconstruction; thereafter, the extruded part separated from the tibial edge was determined. The length, width, height, and extrusion of MM were measured by the 2D and 3D methods, and compared. The MM volume, extruded meniscus volume, and their ratio were also calculated using 3D analysis software in the two groups.</br> Results</br> The estimated length and posterior height of MM were larger with 3D MRI than with 2D MRI measurements. The MM volume was significantly greater in MMPRT knees than in normal knees, with increasing MM height. In MMPRT knees, the mean volume of the extruded meniscus and its ratio significantly increased by 304 mm3 (p = 0.02) and 9.1% (p < 0.01), respectively, during knee flexion.</br> Conclusions</br> This study demonstrated that 3D MRI could estimate the precise MM size and that MMPRT caused swelling of the meniscus due to the increased thickness in the posteromedial part. The clinical significance of this study lies in its 3D evaluation of MM volume, which should help the surgeon understand the biomechanical failure of MM function and improve MMPRT repair technique. No potential conflict of interest relevant to this article was reported.