INTRAMUSCULAR PRESSURE IS LESS WITH MINIMALLY INVASIVE SPINAL RETRACTORS THAN WITH OPEN RETRACTORS Kee D. Kim, MD 1 ; David Spenciner, P.E., Sc.M 2 ; Marike Zwienenberg-Lee, MD 1 ; James E. Boggan, MD 1 Department of Neurological Surgery University of California Davis 1 RIH OrthopaedicFoundation, Inc. 2 Introduction Spinal muscle retraction increases the intramuscular pressure (IMP) and decreases blood flow to the paraspinal muscles that may adversely affect postoperative function. 1,2 Endoscopic placement of pedicle screws is done with less retraction than an open procedure and may thus cause less ischemic damage. The exact effect of retraction on the muscle has not been demonstrated previously. We have designed a cadaver study in which IMP measurements using a minimally invasive retractor and an open retractor are compared. Methods Two unembalmed cadavers were used to compare FlexPosure, a flexible minimally invasive retractor (Endius, Plainville, MA) and Versa-Trac open lumbar retractor (V. Mueller, McGaw Park, IL). An ultra- miniature pressure transducer catheter (Millar Instruments, Houston TX) was used to measure pressure at specific locations next to the incision during retraction. A 3.5 cm paramedian incision for L4-5 posterolateral fusion was made after a serial dilation and FlexPosure was deployed. The needle pressure transducer was inserted into the paraspinal musculature and IMP was measured at three sites: 1.5 cm cephalad and caudad to the incision and 2.5 cm lateral to the incision. These steps were repeated on the contralateral side. Midline incision followed by open retraction with Versa-Trac retractor necessary for same posterolateral fusion was performed. The IMP measurements were again recorded at three different sites: 2.5cm lateral to cephalad, caudad and center of the incision. For the second cadaver, the same sequence of measurements was performed but only one side. Maximum IMP was measured three times at each site. A Mann-Whitney Rank Sum Test was used to analyze the data. Results The mean IMP measured with the minimally invasive retractor was /- 6.3 mm Hg (n=27) and the mean IMP with the open retractor was / mm Hg (n=18) (P<0.001). The maximum pressure was maintained throughout the time that the open retractor was applied. The maximum pressure with the minimally invasive retractor, in contrast, was noted only briefly with the initial expansion. Planning of IMP measurement sites No of measures (n) Mean IMP (mm Hg) Openprocedure / Endoscopic procedure /- 6.3 Discussion and conclusions This study shows that the peak IMP during spine surgery with a minimally invasive retractor is significantly less than with the open retractor. This may in part explain the diminished post-operative pain and more rapid recovery of endoscopically treated patients. It encourages the use of the endoscopic technique in eligible patients. References 1 Taylor H. et al. The impact of self-retaining retractors on the paraspinal muscles during posterior spinal surgery. Spine 27: , Datta G. et al. Back pain and disability after lumbar laminectomy: Is there a relationship to muscle retraction? Neurosurgery 54: , 2004 Endoscopic retraction Open retraction Acknowledgements: This study was funded by Endius,Inc., Plainville MA. Positioning of the fiberoptic probe