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Background: Acute compartment syndrome has been an underreported complication during spine surgery with the patient positioned on the so-called 90/90 kneeling frame (with 90 degrees of both hip and knee flexion), presumably because of elevated intramuscular pressures in the dependent leg compartments. The purpose of the present study was to characterize and quantify certain parameters that affect the risk for acute compartment syndrome experimentally and to make objective comparisons with other spine surgery positions.Methods: Eight healthy volunteers were positioned in three spine surgery positions: the 90/90 kneeling position, the so-called 45/45 suspended position (with the hips and knees both flexed to 45 degrees with the legs suspended on a sling), and the prone position. Intramuscular pressures were measured in all four left leg compartments with slit catheters. Local blood pressure and applied load beneath the leg were also measured.Results: The 90/90 kneeling position was associated with significantly increased intramuscular pressure in the anterior compartment (30.8 +/- 5.7 mm Hg) in comparison with the prone position (13.5 +/- 1.7 mm Hg) and the 45/45 suspended position (13.8 +/- 1.7 mm Hg). In the 90/90 kneeling position, these values correlated with subject weight (r = 0.72, p = 0.045) and the applied body weight load measured beneath the leg (r = 0.74, p = 0.037). The mean differences between intramuscular pressure and ankle blood pressure were more pronounced as the position of the ankle dropped below the level of the heart in the 45/45 suspended and the 90/90 kneeling positions.Conclusions: The 90/90 kneeling position results in elevated intramuscular pressure in the anterior compartment of the leg. This elevated pressure correlates also with subject weight. The 90/90 kneeling position may predispose patients to the development of an acute compartment syndrome during prolonged spine surgery, with heavier patients being at increased risk. In certain instances, the surgeon may consider using the 45/45 suspended position to minimize this risk. [ABSTRACT FROM AUTHOR]