• Mark J Sokolowski, Timothy A Garvey, John Perl, Margaret S Sokolowski, Burak Akesen, Amir A Mehbod, Kevin J Mullaney, Daryll C Dykes, and Ensor E Transfeldt.
• Trinity Orthopaedics, SC, Oak Park, IL, USA.
• Spine. 2008 Jan 1;33(1):114-9.

Study DesignProspective clinical series with comparison to retrospectively collected data.ObjectiveTo compare direct measures of postoperative hematoma volume against a new measure of hematoma effect on the thecal sac: the critical ratio.Summary Of Background DataAsymptomatic epidural hematoma is common after lumbar surgery. Symptomatic patients demonstrate a typical progression from sharp peri-incisional pain to bilateral neurologic deficits. Little is known about what differentiates symptomatic and asymptomatic patients. Magnetic resonance imaging (MRI) measures of hematoma size or mass effect may correlate with postoperative symptoms.MethodsThe study population consisted of 3 patient groups evaluated by MRI 2 to 5 days after lumbar decompression with or without fusion. Fifty-seven consecutive prospectively enrolled patients comprised the asymptomatic group. No patient developed severe postoperative pain or neurologic deficit. Search of our institutional database identified 4978 surgical patients within the last 24 months. Seventeen developed new postoperative symptoms. The painful group included 12 patients with severe peri-incisional pain without neurologic deficit. The cauda equina (CE) group included 5 patients with postoperative CE syndrome. Digital imaging software was used to calculate thecal sac cross sectional area on pre- and postoperative MRI at each level, hematoma volume, volume per level decompressed, and critical ratio for each patient. Critical ratio was defined as the smallest ratio of postoperative to preoperative cross sectional area within the lumbar spine. RESULTS.: The critical ratio was the only measure found to differ significantly (P < 0.05) among all 3 groups. Mean critical ratios were asymptomatic (0.8), painful (0.5), and CE (0.2).ConclusionThe critical ratio correlates more closely with the presence or absence of postoperative symptoms than measures of hematoma volume, and is consistent with the clinical expectation that greater thecal sac compression may result in more severe symptoms. Few guidelines exist for postoperative lumbar MRI interpretation. The critical ratio is an important contribution.

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