• Philippe E Paré, James L Chappuis, Raja Rampersaud, Amit O Agarwala, Joseph H Perra, Serkan Erkan, and Chunhui Wu.
• Medtronic, Spinal and Biologics, Memphis, TN, USA. philippe.pare@medtronic.com
• Spine. 2011 Aug 15;36(18):E1210-4.

Study DesignComparative biomechanical study was conducted in osteoporotic human cadaveric spines.ObjectiveDetermine the influence of the volume of polymethyl methacrylate injected through a fenestrated pedicle screw on the pullout strength and on the ability to safely remove the implant.Summary Of Background DataPedicle screw fixation in the osteoporotic spine can be improved by the addition of bone cement. Various injection techniques have been used. While improvement has been shown for the pullout strength, the optimal volume of cement to inject has not been previously studied.MethodsSeven osteoporotic spines were instrumented with a standard and a fenestrated pedicle screw augmented with polymethyl methacrylate at each level (T7-L5). Three volumes of bone cement were randomly injected and stratified to the thoracic (0.5 cc, 1.0 cc, and 1.5 cc) and lumbar spine (1.5 cc, 2.0 cc, and 2.5 cc). Axial pullout strength and removal torque of the pedicle screws were quantified.ResultsThe pullout strength of the fenestrated screw was normalized with respect to its contralateral control. Student paired t tests were conducted and a statistically significant increase was noted for 1.0 cc (186 ± 45%) and 1.5 cc (158 ± 46%) in the thoracic spine and for 1.5 cc (264 ± 193%), 2.0 cc (221 ± 93%), and 2.5 cc (198 ± 42%) in the lumbar spine. There was no significant difference with higher volumes of cement. The median removal torque was 0.34 Nm for the standard and 1.83 Nm for the augmented screws. When the augmented implants were removed, the bone cement sheared completely off at the fenestrations in 15 of the 17 cases.ConclusionSignificant increases in pullout strength can be accomplished by injecting a limited quantity of bone cement through a fenestrated screw while minimizing the risks associated with higher volume. The majority of implants were removed without damaging the vertebra as the bone cement sheared off at the fenestrations.

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