Study of dural sac cross-sectional area in early and late phases after lumbar decompression surgery

Abstract

Background context: Lumbar magnetic resonance imaging (MRI) in the early phase after lumbar decompression surgery sometimes reveals an absence in the expansion of the dural sac, regardless of the presence or absence of clinical symptoms; the reason for such a condition is often difficult to explain. There are some reports that compared the dural sac area between the preoperative and early postoperative phases; however, no report exists that compares the early and late phases after lumbar decompression surgery.

Purpose: The purpose of this study was to compare changes in the dural sac cross-sectional area (CSA) in the early and late phases after lumbar decompression surgery. Factors related to the insufficient increase in the postoperative dural sac CSA were also analyzed.

Study design: The dural sac CSA preoperatively and in the early and late phases after lumbar decompression surgery was analyzed retrospectively.

Patient sample: Of 105 patients who underwent lumbar decompression surgery and MRI within 1 week and again more than 1 month after surgery, 83 patients (38 men, 45 women; mean age 65.6 years) were included in this study.

Outcome measures: Cross-sectional areas of the dural sac.

Methods: The dural sac CSA was measured within 1 week (early phase) and more than 1 month (late phase) after surgery, using T2 axial plane MR images. The preoperative and the early and late postoperative CSAs were measured at the same site. The relationship between the dural sac area and age and presence of dural injury was also analyzed.

Results: The mean area of the dural sac preoperatively and in the early and late postoperative phases was 71.2±4.9, 102.2±5.7, and 164.1±6.9 mm(2), respectively. The mean area increased significantly (p<.001) between the preoperative and postoperative early phases and between the early and late postoperative phases. The dural sac area in the early (p=.16) and late (p=.086) phases did not differ significantly between patients aged 75 years or more and those aged less than 75 years. In the case of lumbar spinal stenosis, patients with a preoperative dural sac area of less than 60 mm(2) showed a significantly (p<.001) smaller dural sac area in the early and late postoperative phases, compared with patients with a preoperative dural sac area of 60 mm(2) or more. No significant increase was observed in the dural sac area with regard to the presence or absence of dural injury.

Conclusions: The dural sac area increased significantly between the early and late postoperative phases. No significant difference in the dural sac CSA between the early and late postoperative phases was observed with regard to age or the presence/absence of dural sac injury. A smaller preoperative dural sac CSA resulted in a smaller dural sac CSA in the early and late postoperative phases.

Keywords: Cross-sectional area; Dural sac; Early and late phases; Lumbar decompression surgery; Lumbar spinal stenosis.