Pedicle screws with high electrical resistance: a potential source of error with stimulus-evoked EMG.
METHODS
Clinically relevant aspects of pedicle screws were subjected to electrical resistance testing.


OBJECTIVE
To catalog commonly used pedicle screws in terms of electrical resistance, and to determine whether polyaxial-type pedicle screws have the potential to create a high-resistance circuit during stimulus-evoked electromyographic testing.


BACKGROUND
Although stimulus-evoked electromyography is commonly used to confirm the accuracy of pedicle screw placement, no studies have documented the electrical resistance of commonly used pedicle screws.


METHODS
Resistance measurements were obtained from eight pedicle screw varieties (5 screws of each type) across the screw shank and between the shank and regions of the screw that would be clinically accessible to stimulus-evoked electromyographic testing with a screw implanted in a pedicle. To determine measurement variability, resistance was measured three times at each site and with the crown of the polyaxial-type screw in three random positions.


RESULTS
Resistance across the screw shank ranged from 0 to 36.4 ohms, whereas resistance across the length of the monoaxial-type screws ranged from 0.1 to 31.8 ohms. Resistance between the hexagonal port and shank of polyaxial-type screws ranged from 0 to 25 ohms. In contrast, resistance between the mobile crown and shank of polyaxial-type screws varied widely, ranging from 0.1 ohms to an open circuit (no electrical conduction). Polyaxial-type screws demonstrated an open circuit in 28 of 75 measurements (37%) and a high-resistance circuit (exceeding 1000 ohms) in 5 of 75 measurements (7%).


CONCLUSIONS
Polyaxial-type pedicle screws have the potential for high electrical resistance between the mobile crown and shank, and therefore may fail to demonstrate an electromyographic response during stimulus-evoked electromyographic testing in the setting of a pedicle breech. To avoid false-negative stimulus-evoked electromyographic testing, the cathode stimulator probe should be applied to the hexagonal port or directly to the screw shank, and not to the mobile crown.
Anderson
D Greg
DG
Department of Orthopaedic Surgery, University of Virginia, School of Medicine, Charlottesville, Virginia 22903, USA. dga3k@virginia.edu
Wierzbowski
Lawrence R
LR
Schwartz
Daniel M
DM
Hilibrand
Alan S
AS
Vaccaro
Alexander R
AR
Albert
Todd J
TJ
D000497:Alloys; D001863:Bone Screws / Q000145:classification* / Q000592:standards*; D017097:Electric Impedance; D004558:Electric Stimulation / Q000379:methods; D004566:Electrodes; D004576:Electromyography / Q000379:methods; D013193:Stainless Steel / Q000592:standards; D014025:Titanium / Q000592:standards
D000497:Alloys; D013193:Stainless Steel; D014025:Titanium