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Background: The multifidus muscles are vital for spine stability and movement. Changes in muscle morphology are considered to be related to spine problems. The aim of this study was to examine multifidus muscles morphology in patients with sacralization.

Methods: Seventy CT images were examined for the presence of sacralization phenomena, using 3-D and 2-D images on the coronal and sagittal planes. Sacralization was identified as a partial or full fusion between the L5 transverse process and the sacrum. The cross-sectional area (in mm²) and fat infiltration (in Hounsfield units) were measured bilaterally on axial images on the L4 and L5 levels.

Results: Sacralization phenomena were found in 9 subjects (mean age 72 ±10.3 years). In order to age-match the controls, only 29 out of the 61 subjects were included in the analysis (mean age 69.9 ±7.9 years). Smaller cross-sectional areas of the multifidus muscles examined on both levels were observed in the sacralization group compared with the controls (not significant). The size of the muscles on the L4 level was larger than on the L5 level in the sacralization group. A significant difference (p<0.05) in fat infiltration was found between the L4 and L5 levels in both groups without a significant difference between groups.

Conclusion: Amongst the sacralization patients, the multifidus muscles were observed to be more active on the L4 level than the L5, although the total cross-sectional areas of the muscles were smaller compared to the controls. Fat infiltration within the muscles is not associated with the sacralization phenomena.