title: Lumbar spinal canal dimensions measured intraoperatively after decompression are not properly rendered on early postoperative MRI.
abstract: BACKGROUND
  In cases of lumbar spinal stenosis (LSS) treated with surgical decompression, a postoperative magnetic resonance imaging (MRI) is sometimes required. In the experience of the investigators of this study, the obtained decompression observed on early postoperative MRI tends to be disappointing compared to the decompression achieved intraoperatively. This raises the question of whether the early postoperative MRI, performed after lumbar decompression, is a fair representation of the 'real' decompression. This study investigated the correlation between intraoperative and postoperative measurements of the lumbar spinal canal.
  
  
  METHODS
  Twenty patients with LSS underwent surgical decompression on a single level. The orthopaedic surgeon performed direct intraoperative measurements of width, length and height of the spinal canal. Preoperative supine MR images and postoperative prone and supine MR images were acquired. Two radiologists (R.B. and M.G.) measured width, length and height of the spinal canal on the preoperative and postoperative MRIs. Intraoperative measurements were compared to measurements on postoperative MRI in prone position (thus reproducing the intraoperative situation) to avoid positioning bias. Preoperative and postoperative measurements on MR images were also compared. In addition to this, postoperative measurements on supine and prone MR images were also compared.
  
  
  RESULTS
  Interobserver reliability for MRI measurements by both radiologists was generally excellent (intraclass correlation coefficients ≥0.71). The postoperative spinal canal dimensions improved on both prone and supine MRI compared to the preoperative imaging (P < 0.05). Intraoperatively measured dimensions demonstrated a significantly greater height (difference 2.8 ± 3.3 [R.B.] and 1.9 ± 3.7 [M.G.]) and greater width (difference 2.1 ± 3.2 [R.B.] and 2.5 ± 2.7 [M.G.]) compared to postoperative MRI in the prone position (P < 0.05). Postoperative dural sac height was greater on the supine MRI compared to the prone MRI (P < 0.05).
  
  
  CONCLUSIONS
  Surgical decompression of the spinal canal effectively decreases the compression of the dural sac. However, early postoperative MRI after lumbar decompression does not adequately represent the decompression achieved intraoperatively.
authors.affiliation: Department of Neurosurgery, Leiden University Medical Centre, Albinusdreef 2, 2333 ZA, Leiden, The Netherlands. catharina.schenck@gmail.com.
authors.forename: Catharina
authors.initials: C
authors.lastname: Schenck
authors.affiliation: Department of Orthopaedics, Rijnstate Hospital, Arnhem, The Netherlands.
authors.forename: Job
authors.initials: J
authors.lastname: van Susante
authors.affiliation: Department of Radiology, Rijnstate Hospital, Arnhem, The Netherlands.
authors.forename: Maarten
authors.initials: M
authors.lastname: van Gorp
authors.affiliation: Department of Radiology, Rijnstate Hospital, Arnhem, The Netherlands.
authors.forename: Ruben
authors.initials: R
authors.lastname: Belder
authors.affiliation: Department of Neurosurgery, Leiden University Medical Centre, Albinusdreef 2, 2333 ZA, Leiden, The Netherlands.
authors.forename: Carmen
authors.initials: C
authors.lastname: Vleggeert-Lankamp
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D019299:Decompression, Surgical; D005260:Female; D006801:Humans; D008159:Lumbar Vertebrae / Q000473:pathology* / Q000601:surgery*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D009192:Myelography; D015588:Observer Variation; D019637:Orthopedic Procedures; D011184:Postoperative Period; D057234:Preoperative Period; D016684:Prone Position; D015203:Reproducibility of Results; D013115:Spinal Canal / Q000473:pathology* / Q000601:surgery*; D013130:Spinal Stenosis / Q000473:pathology* / Q000601:surgery*; D016896:Treatment Outcome
keywords: Direct intraoperative measurement of spinal canal; Intraoperative versus postoperative dimensions of the spinal canal; Lumbar spinal stenosis; Prone and supine MRI; Surgical decompression; Treatment outcome