PMMA Teeth-Pedicle Screw Spacers in Revision Spinal Surgery: A Technical Note

Abstract

BACKGROUND Pedicle screw instrumentation was first described by Boucher in 1959 and subsequently popularised by Camille et al. in 1963. It has since become the most prevalent spinal fixation technique with indications for pedicle screw and rod constructs including congenital disease, trauma, infection, and spinal deformity including scoliosis. Initially used in lumbar spine surgery, the use of pedicle screws has since been extended to the thoracic spine where, because of the narrow pedicle corridor and proximity of critical structures, there is less margin for error in screw placement.1 Pedicle screw misplacement has been described in up to 40% of lumbar spine cases and up to 55% in thoracic spine cases.2 Complications associated with pedicle screw placement include damage to spinal cord, neural elements, nerve roots, pedicle fractures, wall cut out, and difficulty fitting instrumentation to the pedicle. Pedicle screw placement techniques can be grossly divided into freehand and image-guided techniques, with freehand techniques including drill assisted and pedicle gearshift (probe) methods while image-guided techniques incorporate fluoroscopic assisted, navigation assisted and robotic-assisted means. Pedicle screw placement into previously fused spines can be challenging with vital anatomic landmarks altered or obscured by the fusion mass.3 PMMA (Polymethyl methacrylate) was first introduced by Charnley in 1960 for cemented total hip arthroplasty. In spine surgery, PMMA has been used in numerous forms including kyphoplasty, disk substitution, and pedicle screw augmentation. In 1972 Bucholz and colleagues began combining PMMA with antibiotics to achieve high local antibiotic concentration in the setting of osteomyelitis; as such PMMA antibiotic loaded spacers have been well described in revision arthroplasty. Staged revision spinal surgery has been advocated by some authors in the setting of deep infection.4 Our senior author has noted that identification of pedicle screw tracts during second stage surgery can be challenging. Rationale We describe a novel surgical technique for the use of PMMA Teeth in the setting of staged revision spinal surgery to maintain the patency of pedicle screw tracts, permitting prompt identification on proceeding surgery stages and achieving local antibiotic delivery when necessary. Surgical Technique A standard posterior approach to the spine is performed identifying the in situ instrumentation. After removal of the in situ spinal rods and pedicle screws, the pedicle screw tracts are examined evaluating the medial, lateral, superior, and inferior walls of the tract. Residual fibrous tissue is debrided. A standard mix of Palacos R cement (Heraeus Medical, Newbury, UK) with the addition of 2 g vancomycin is performed. The PMMA teeth are subsequently fashioned from the Palacos R cement (Fig. 1). Similar to the technique described by Tsung et al.5 in revision hip arthroplasty, the principle for fixation is that of macroscopic fit as opposed to microscopic interlock of the cement; therefore, the PMMA teeth are inserted once the cement has cured.FIGURE 1: PMMA Teeth.PMMA indicates Polymethyl methacrylate.Expected Outcomes The PMMA teeth technique can assist the spinal surgeon in identifying the pedicle screw tract promptly and safely in staged revision spinal surgery with the added benefit of local antibiotic delivery. Complications Potential complications include PMMA Teeth fracture in situ, however, given the PMMA Teeth are inserted once the cement has cured, removal should be uncomplicated. DISCUSSION & CONCLUSION The incidence of revision spinal fusion has a reported range of 8 to 45%, with the revision rate increasing with follow-up; 21% of patients undergoing revision spinal surgery will require further spinal surgery.6 The rate of pedicle screw perforation has been reported to be as high as 28%, and 19% with a perforation greater than 2mm. This poses a significant challenge with considerable potential for pedicle screws to be misplaced in the revision spinal surgery setting.