A new study published in Spinal Deformity examines whether the standard approach for inserting pedicle screws is the best approach.
The study authors conducted two experiments using up to three densities of reference-grade polyurethane foam: 0.16, 0.24 and 0.32 g/cm3. The screws were all provided from K2M and inserted at a 25mm depth. There were a series of pilot holes made as well from 1.5 mm to 6mm in diameter. The researchers also inserted a 6.5 x 45 mm pedicle screw and axially pulled from the material.
A 3 mm pilot hole was also dripped with no tap, 3.5-, 4.5-, 5.5- and 6.5mm-taps and a 6.5 x 45 mm pedicle screw as inserted and axially pulled away from the material, according to the study abstract. The researchers found:
1. The probe /pilot hole size reported a nonlinear, parabolic effect on the pullout strength, which suggests an optimum-sized probe hole for a given pedicle screw.
2. The probe holes that are two big or small could cause rapid falloff pullout strength.
3. Not tapping or undertapping by two or three sizes didn't significantly affect the pullout strength.
4. There was exponential falloff of the pullout strength when the tap size increased to the screw diameter.
5. The researchers concluded the "an ideal pilot hole size half the diameter of the screw is a starting point," and if tapping is necessary they recommended using a tap "two sizes smaller than the screw being implanted."
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