Cho SM, Kim SH, Ha SK, et al. - Researchers tried to find out 1-year postoperative changes in paraspinal muscle volume using a simple formula applicable to magnetic resonance imaging (MRI) or computed tomography (CT) images. This study included a total of 40 individuals with degenerative lumbar spinal stenosis who underwent posterior interbody fusion (PLIF) at the L4/5 level in the period between May 2010 and June 2017. They evaluated radiologic parameters by applying MRI or CT images which were taken before surgery and at 1 year after surgery. The volume of the paraspinal muscles was measured utilizing a simple formula that was obtained from the formula for determining the volume of truncated elliptic cones. The data indicated that the volume of the MF muscles was markedly reduced, and the degree of reduction was apparent in the MRI after the posterior lumbar fusion. At 1 year after surgery, the volume of the ES muscles, which are located relatively laterally, also tended to reduce.