MRI Features of Psoas Major Muscles in Patients with Paraspinal Muscle Atrophy
Authors: Kita Teruo , Takayama Kazushi , Nakamura Hiroaki , Kanematsu Fumiaki , Yasunami Toshiya , Nishino Kazuya , Yabu Akito , Sakanaka Hideki , Yamano Yoshiki 
Published: May 2015

Introduction Maintaining both lumbar lordosis and paraspinal muscle function is important for prevention of sagittal imbalance. We previously reported that the cross-sectional area of the paraspinal muscles tends to decrease with age. To quantify the depth of the groove between the paraspinal muscles, we have developed our own parameter, the T-back value, which indicates the length of the bulge of muscle on the attachment of the spinous process. The T-back value is strongly correlated with the CSA of the paraspinal muscles. The occurrence of T-back value of 0 or less shows a marked increase in individuals aged 60 years or older. The CSA of the paraspinal and psoas muscles shows a similar decreasing trend with age, but fat infiltration of the paraspinal muscles is far more prominent than that of the psoas muscles in the elderly. The aims of this study are to investigate the CSA and fat infiltration of the psoas muscles in patients with paraspinal muscle atrophy, and to compare the findings with those in patients having a T-back value above 0. Methods Among 704 patients who underwent MRI of the lumbar spine at our hospital during 2010, 141 patients had a T-back value of 0 or less. Of these 141 patients, there were 120 patients, 40 men and 80 women, aged 60 years or older. We defined these as T-back − group. The control group included 120 age- and sex-matched individuals with a T-back value above0. We defined them as T-back + group. We compared their data from the two groups. The body mass index (BMI) of participants was calculated. Lumbar lordosis was measured on sagittal T2-weighted MR images. Axial T2-weighted MR images and a digital image analysis technique were used to measure the cross-sectional area and the percentage of fat infiltration of the paraspinal and psoas muscles at the intervertebral disc levels from L1/2 to L4/5 bilaterally. Then the mean values of both sides were used as CSA and the percentage of fat respectively Results The T-back − group had a significantly lower BMI than the T-back + group. Lumbar lordosis was significantly smaller in the T-back − group than in the T-back + group. In addition, the CSA of the paraspinal muscles was significantly smaller in the T-back − group than the T-back + group. The %Fat of the paraspinal muscles was significantly higher in the T-back − group than the T-back + group. When the CSA of the psoas muscles was compared across all spinal levels, there was no significant difference between the two groups. The %Fat of the psoas muscles also showed no significant difference between the two groups. Conclusion The patients in this study were divided into two groups according to the T-back value. The T-back − group had a smaller BMI, less lumbar lordosis, and the advanced atrophy of the paraspinal muscles than the T-back + group. However, MRI features of the psoas muscles were similar in the two groups. This study demonstrates that the psoas muscles are less susceptible to atrophy than the paraspinal muscles.