Abstract
The pedicle screw diameter, composite and design are variables that can affect the threshold of intraoperative electromyographic monitoring. Even though we know that larger diameter objects tend to have less resistance, no study documented the effect that this variable could have on pedicle screw resistance. Using high quality equipment, resistance and resistivity of ten pedicle screws (from four manufacturers) were calculated based on known constant current and measured voltage. Voltage was measured three times for each screw to determine intraobserver measurement variability. Resistance of all screws ranged from 1.4 to 3.9 m ohm (mean = 2.69+/-0.71 m ohm). The screw with largest diameter (7.75 mm) had lower resistance than screws with other diameters. Resistivity of screws ranged from 7.12 to 12.63 micro ohm*m (mean = 9.9+/-1.82 micro ohm*m). Based on the screw design, one manufacturer's pedicle screws (A) had significantly lower resistivity compared to three other manufacturers (p<0.01). Larger diameter screws (7.75 mm in diameter) had lower resistance. Screw design (polyaxial or monoaxial) had no effect on its resistance. Screws of one manufacturer (A) showed lower resistivity compared to those manufactured by other three companies.
Highlights
In the past decade, pedicle screw systems have proven to provide the highest biomechanical stability in spinal instrumentation, which gives a surgeon greater flexibility to accommodate patient’s intrinsic anatomy
Due to a high variability of pedicle geometry ( ), the rates of pedicle cortical perforation have been reported to be between . and ( - ). This is relevant because an incorrect placement of a pedicle screw leads to suboptimal spinal stability and higher incidence of pseudoarthrosis ( ), and may lead to neurological irritation or nerve root injury
Ten titanium alloy (Ti- Al- V) pedicle screws from four different manufacturers (Table ) commonly used in spine surgery were inserted into an aluminum block to provide a connection with the current source
Summary
Pedicle screw systems have proven to provide the highest biomechanical stability in spinal instrumentation, which gives a surgeon greater flexibility to accommodate patient’s intrinsic anatomy. The screw should be placed properly within the pedicle. This is relevant because an incorrect placement of a pedicle screw leads to suboptimal spinal stability and higher incidence of pseudoarthrosis ( ), and may lead to neurological irritation or nerve root injury. An intraoperative electrical testing of pedicle screws is a widely accepted technique of minimizing intraoperative nerve root irritation or an injury during insertion of spinal instrumentation. A properly placed screw can be distinguished from those that perforate a pedicle wall by its minimum level (threshold) of the electrical current needed to elicit a compound muscle action potential (CMAP). The strong likelihood of a pedicle wall defect and a potential screw contact with a nerve root and/or the dura ranged from mA to mA ( - )
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