Lumbar paraspinal muscle morphometry and its correlations with demographic and radiological factors in adult isthmic spondylolisthesis: a retrospective review of 120 surgically managed cases

OBJECTIVE The objective of this study was to assess the cross-sectional areas (CSAs) of lumbar paraspinal muscles in adults with isthmic spondylolisthesis (IS), to compare them with those in the normative population, and to evaluate their correlations with demographic factors and MRI changes in various spinal elements. METHODS The authors conducted a retrospective study of patients who had undergone posterior lumbar interbody fusion for IS, and 2 of the authors acting as independent observers calculated the CSAs of various lumbar paraspinal muscles (psoas, erector spinae [ES], multifidus [MF]) on preoperative axial T2-weighted MR images from the L-3 to L-5 vertebral levels and computed the CSAs as ratios with respect to the corresponding vertebral body areas. These values were then compared with those in an age- and sex-matched normative population and were analyzed with respect to age, sex, duration of symptoms, grade of listhesis, and various MRI changes at the level of the listhesis (pedicle signal change, disc degeneration, and facetal arthropathy). RESULTS Compared with values in normative controls, the mean CSA value for the ES muscle was significantly higher in the study cohort of 120 patients (p = 0.002), whereas that for the MF muscle was significantly lower (p = 0.009), and more so in the patients with PSC (p = 0.002). Magnetic resonance imaging signal change in the pedicle was seen in half of the patients, all of whom demonstrated a Type 2 change. Of the variables tested in a multivariate analysis, age independently predicted lower area values for all 3 muscles (p ≤ 0.001), whereas female sex predicted a lower mean psoas area value (p < 0.001). None of the other variables significantly predicted any of the muscle area values. A decrease in the mean MF muscle area value alone was associated with a significantly increased likelihood of a PSC (p = 0.039). CONCLUSIONS Compared with normative controls, patients with IS suffer selective atrophy of their MF muscle, whereas their ES muscle undergoes a compensatory hypertrophy. Advancing age has a detrimental effect on the areas of the lumbar PSMs, whereas female sex predisposes to a decreased psoas muscle area. Multifidus muscle atrophy correlates with PSC, indicating the role of this deep stabilizer in the biomechanical stability of spondylolisthetic spines. This may be of clinical significance in targeted physiotherapy programs during the conservative management of IS.