The pullout strengths of various pedicle screw
designs are compared using tapped
The objective of this study is to compare the pullout strength
of various pedicle screw
designs. The designs are compared using tapped
pilot holes. By using several different screw designs, it is possible to gain an understanding of whether there is a correlation between tapping a pilot hole
and the ultimate pullout strength
Summary of Background Data.
Most bone screws originally developed were intended to be installed in a pretapped pilot hole
. This same technology has been carried over to the development of more modern bone screws for use in spinal fixation
applications. Many pedicle screws in use today are still intended to be installed in a tapped
hole. Preparing the vertebrae and tapping of a pilot hole
involve additional trauma to the patient as well as increased operating time.
Pedicle screws from various manufacturers are installed in tapped
pilot holes and then loaded to failure. A uniform synthetic material was used to provide a consistent test of each screw design by eliminating variability seen in bone.
Tapping pilot holes did not increase the pullout strength
of the screws tested in this study. It was observed during testing that tapping some of the holes degraded the material. This degradation led to pullout strengths that were lower than in the untapped
case, and generally larger standard deviations.
The pullout strength
was not increased by tapping for the screws in this study. Screws placed in untapped
holes generally had higher pullout strengths and lower standard deviations. The results of this study suggest that tapping does not increase pullout strength
in bone with densities near 20 lb/ft3
, which correlates with low density cancellous or osteoporotic bone.