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

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.

Pull out Strength of Dual Outer Diameter Pedicle Screws Compared to Uncemented and Cemented Standard Pedicle Screws: A Biomechanical in vitro Study

“…As for the comparison of the cemented and uncemented standard screws in group A, a distinct and significant increase in pull‐out force was measured for the cemented screws, rejecting our first hypothesis. These results correspond well with many other studies comparing uncemented pedicle screws of various designs to cemented pedicle screws. Only for the expansive screw designs, similar pull‐out forces as for the cemented standard screw have been reported in a few studies.…”

“…Regarding the entire group, a distinct but not significant difference was found. This confirms the findings of Christodoulou et al , who report similar results using exactly the same screw designs. In contrast, a study of Amaritsakul et al , which compared screws with several variations of the inner and outer diameter, report contradictory findings.…”

“…In a biomechanical study on artificial bones, a dual outer diameter screw with a slightly different design only achieved smaller pull‐out forces compared to many other designs with varying inner and outer diameter, including conical screws. Another pull‐out study on human vertebrae (T 10 –L 5 ) compared dual outer diameter screws with standard screws and fenestrated cemented screws, resulting in a distinct and significant increase of the cemented screws compared to all other screw types, but only a much smaller and not significant increase of the dual outer diameter screws compared to the standard screws. This study, however, was limited by a low number of tested screws (10 per group), which also made it impossible to further analyze the results for the vertebra level.…”

Multiple Spinal Revision Surgery in a Patient with Parkinson's Disease

“…As a result, the patient had previously undergone bone cement augmentation cage placement with screw fixation in order to resolve instrument failure. Many studies have been performed for the use of bone cement as an augmentation technique during spinal fusion surgeries of osteoporotic bones561017). However, augmentation alone is not sufficient for preventing construct failures.…”

Evaluation of the internal fixation effect of nano-calcium-deficient hydroxyapatite/poly-amino acid composite screws for intraarticular fractures in rabbits

“…The mechanical properties of screw materials, and the flexural and push-out properties of composite screws can vary with the thread and width of a screw. 7,31,51,52 Therefore, adjusting the physical form of composite screws could be a potential and effective method for improving early fixation strength of such nonmetallic screws.…”