The Comparison of Pullout Strengths of Various Pedicle Screw Designs on Synthetic Foams and Ovine Vertebrae.

AIM: One of the most common problems with transpedicular screws is screw pullout. This study was conducted to measure the pullout strengths of newly designed transpedicular screws. MATERIAL AND METHODS: The design of the three group screws were conical cored standard pedicle screw (Type A), dual threaded pedicle screw (Type B), dual core and dual threaded pedicle screw (Type C), respectively. Polyurethane (PU) blocks in 25 mm and 50 mm thickness were used to investigate the effect of just the pedicle on pullout strength and both distal (vertebral body) and proximal (pedicle) parts of the screw. The screws were also tested in ovine lumbar vertebrae. RESULTS: Type C screw exhibited 5.9% and 12.9% higher pullout strength than Type A and Type B, and 15.4% and 8.6% higher pullout strength than Type A and Type B, respectively on 25 mm and 50 mm thick PU foam block. Type C also exhibited 74.5% and 22.5% higher pullout strength than Type A and Type B, respectively on the ovine vertebrae. CONCLUSION: Transpedicular screws redesigned with modified helical angles exhibit higher pullout strength compared to the classical transpedicular screws and can be inserted more rapidly with the same number of screwing rounds result with doubled insertion depth.

Postfusion pullout strength comparison of a novel pedicle screw with classical pedicle screws on synthetic foams.

Pullout is a very common failure mode on the use of pedicle screws. Numerous studies were completed to increase the pullout strength of pedicle screws especially for osteoporotic bones. In this study, a previously designed pedicle screw type was tested before and after fusion condition. Synthetic polyurethane foams were used in all tests. Three different grades of foams were used in tests to simulate severely osteoporotic, osteoporotic, and healthy bones. Test blocks were produced and characterized in our clinical biomechanics laboratory. Foaming of polyurethane was accepted as fusion process (bone in growth). Pedicle screw including radial holes (new design) was tested both before and after the fusion. It also exhibited remarkably higher pullout strength after fusion than before fusion and most of other alternatives stated in the literature. In total, 70% higher pullout strength was achieved with new design after fusion. On the other hand, new design did not dominate other alternatives when comparison was carried out on severely osteoporotic and healthy bones. To the knowledge of the authors, this is the first study investigating the postfusion properties on synthetic foams.

Design and biomechanical testing of pedicle screw for osteoporotic incidents.

In this study, geometrical features of pedicle screws have been modified and their performances are compared. Performance analysis has been made in terms of pull-out strength and torsional strength. The parameters investigated are core diameter, holes drilled normal to screw axis, angle between sequential holes and distance between holes. Three different core diameter have been studied, which are 4 mm (normal core diameter), 5 mm (medium core diameter) and 5.5 mm (high core diameter). Distance between sequential holes has been arranged such that there is either one hole per pitch or one hole per two pitches. Angle between sequential holes is either 90 degrees or 120 degrees. According to the test results, the screw, with medium core diameter (5 mm) containing one hole per two pitches with 90 degrees angle between sequential holes, has exhibited the optimum performance considering torsional strength and pull-out strength requirements. Its torsional strength is slightly higher than and, when Grade 40 polyurethane foam was used as bone simulating material, its pull-out strength is as good as, an undrilled normal core diameter screw, which is already being used in surgical operations. The fatigue performance of this best performed screw has also been found satisfactory according to the related standard. Its pull-out strength is also tested on a calf vertebra and a promising result has been obtained.