Axial pullout strength comparison of different screw designs: fenestrated screw, dual outer diameter screw and standard pedicle screw.

Evangelos Christodoulou,Suresh Chinthakunta,Divya Reddy,Saif Khalil,Konstantinos Kafchitsas,Thomas Apostolou,Philipp Drees

doi:10.1186/s13013-015-0039-6

Abstract

BackgroundThe pullout strength of pedicle screws is influenced by many factors, including diameter of the screws, implant design, and augmentation with bone cement such as PMMA. In the present study, the pullout strength of an innovative fenestrated screw augmented with PMMA was investigated and was compared to unaugmented fenestrated, standard and dual outer diameter screw.MethodsTwenty four thoracolumbar vertebrae (T10-L5, age 60 to 70 years) from three cadavers were implanted with the four different pedicle screws. Twelve screws of each type were instrumented into either left or right pedicle with standard screw paired with unaugmented and dual outer diameter screw paired with augmented fenestrated screw in any given vertebra. Axial pullout testing was conducted at a rate of 5 mm/min. Force to failure (Newtons) for each pedicle screw was recorded.ResultsThe augmented fenestrated screws had the highest pullout strength, which represented an average increase of 149%, 141%, and 78% in comparison to unaugmented, standard, and dual outer diameter screws, respectively. Pullout strength of unaugmented screws was comparable to that of standard screws, however it was significantly lower than dual outer diameter screws.ConclusionsFenestrated screws augmented with PMMA improve the fixation strength and result in significantly higher pullout strength compared to dual outer diameter, standard and unaugmented fenestrated screws. Screws with dual outer diameter provided enhanced bone-screw purchase and may be considered as an alternative technique to increase the bone-screw interface in cases where augmentation using bone cement is not feasible. Unaugmented screws can be left in the pedicle even without cement and provide similar pullout strength to standard screws.

Highlights

The pullout strength of pedicle screws is influenced by many factors, including diameter of the screws, implant design, and augmentation with bone cement such as PMMA
For the fenestrated screws augmented with PMMA, entire pedicle was pulled off of the posterior aspect in a few of the specimens, while others failed at the cementbone interface, where a void was created inside the vertebral body after the removal of the screw/cement section
Overall the mean pullout strengths for each screw type for the 3 specimens tested were: standard screw 610 N (±264), unaugmented fenestrated screw 591 N (±238), dual outer diameter screw 827 N (±274), and fenestrated screws with PMMA 1470 N (±218). These values showed that augmentation with PMMA significantly improved the pullout strength to 149%, 141%, and 78% in compare to the unaugmented fenestrated screw (p = 0,00000000835), standard screw (p = 0,0000000394) and dual outer diameter screw (p = 0,0000328), respectively

Summary

Introduction

The pullout strength of pedicle screws is influenced by many factors, including diameter of the screws, implant design, and augmentation with bone cement such as PMMA. The pullout strength of an innovative fenestrated screw augmented with PMMA was investigated and was compared to unaugmented fenestrated, standard and dual outer diameter screw. Previous studies investigating pullout strength have either used different screw designs [1,3,4,5,11] compared different bone cements and parameters for augmentation [2,5,12] or have compared different insertion techniques [9,10,11,13] and have not compared fenestrated screws to dual outer diameter screws in a cadaveric model

Methods

Results

Conclusion