Influence of pilot hole diameter in cancellous screw fixation in a reduced density animal bone model
Introduction
Cancellous and cortical bone screws are surgically implemented as a means of fracture fixation. Cancellous bone screws are characterised by larger thread and pitch sizes (Thakur, 2007) and are commonly utilised as lag screws (Schatzker, 1991; Thakur, 2007) within the metaphyseal and epiphyseal sites of the femur, tibia and pelvis (Chapman et al., 1996), where there is an abundance of cancellous bone (Chapman et al., 1996; Wang et al., 2016). Human cortical bone has an apparent density of approximately 1.80 g/cm3 (Hernandez, 2016), contrasted with cancellous bone's apparent density typically being between 0.10 g/cm3 and 1.10 g/cm3 (Hernandez, 2016), with a surface area to volume ratio approximately eight times larger than cortical bone (Parfitt, 1983). To mitigate the lower density of cancellous bone, the cancellous screw thread contact area is larger than that of a cortical screw – maximising the pull-out strength (Thakur, 2007). Previous work has shown that when the cortical thickness is greater than 1.5 mm the screw pull-out strength can be attributed entirely to the cortical bone (Seebeck et al., 2005). However, in certain clinical procedures either it is not always possible to engage both cortices of bone such as when inserting peri-articularly, when needing to prevent damage to adjacent neurovascular or tendinous structures, or if the cortical bone quality may have been compromised by the surgical method or degenerative diseases (Brown et al., 2013; Stadelmann et al., 2010). In these cases, a cancellous bone screw may be the only viable option (Brown et al., 2013; Stadelmann et al., 2010).
Osteoporosis describes the deterioration of bone quality through structural thinning and reduced interconnectivity of the trabeculae. The World Health Organisation (WHO) defines the osteoporotic bone threshold as being more than 2.5 standard deviations below the mean bone density of a young healthy female adult (Kanis and Kanis, 1994). There were an estimated 3.2 million people in the UK aged over 50 years having osteoporosis in 2010 (Svedbom et al., 2013); the total cost of treating osteoporotic injuries in the UK being approximately £4.4 billion in 2010 and projected to be £5.5 billion in 2025 (Svedbom et al., 2013). Clinical reports suggest a higher screw fixation failure rate in osteoporotic patients (Seebeck et al., 2005; Procter et al., 2015; Broderick et al., 2013; Fletcher et al., 2020), with a lower estimate of one million screw fixation failures caused by loosening occurring worldwide (Procter et al., 2015). The burden of correcting failed fixation surgery is both a doubling of hospital stay duration and costs (Broderick et al., 2013).
The reduction of the hydroxyapatite mineral and collagen content in osteoporotic bone is responsible for the deteriorated mechanical properties of the diseased bone (Faibish et al., 2006). In an attempt to replicate osteoporotic bone conditions for biomechanical testing, previous studies have immersed animal bone samples in hydrochloric (HCl) acid (Akbay et al., 2008; Figueiredo et al., 2011; Fletcher et al., 2018). The calcium-based hydroxyapatite minerals were successfully dissociated by the HCl acid, forming monocalcium phosphate and calcium chloride by demineralisation (Akbay et al., 2008; Figueiredo et al., 2011; Fletcher et al., 2018). In a recent study, juvenile bovine bone was demineralised with 0.6 M HCl acid, to replicate osteoporosis and achieved a 25% reduction in cortical bone density and a 71% decrease in pull-out strength (Fletcher et al., 2018).
Standard operative practice involves inserting surgical screws into predrilled pilot-holes of diameter equivalent to the screw core diameter (Thakur, 2007). Reducing the size of the bone screw pilot-hole diameter to less than the conventional core diameter should effectively result in increasing the local bone density by compaction of the bone trabeculae. Previous studies have demonstrated increases in pull-out force and insertion torque in animal models, when pilot hole diameters were reduced from 3.2 mm to 2.5 mm for 6.5 mm cancellous screws – 15% increase in pull-out force (Steeves et al., 2005) – and from 2.8 mm to 2.0 mm for 4.0 mm pedicle screws −16% increase in insertion torque and 14% increase in pull-out force (Silva et al., 2013). In synthetic models, pilot hole diameter reductions from 1.4 mm to 1.0 mm for 1.6 mm screws increased insertion torque by 51% and pullout force by 44% (Hung et al., 2012). To the authors' knowledge, no previous study has investigated the use of a smaller pilot-hole diameter to offset the lowered bone screw pull-out strength in a reduced density animal bone model. The aims of the present study were to create a low density porcine model using acid demineralisation and to test the following null hypothesis: for a 4 mm diameter cancellous bone screw fixation within an osteoporotic animal bone model, the use of a smaller, 1.5 mm, rather than the manufacturer recommended, 2.5 mm, pilot-hole diameter (Stryker, 2012) achieves no statistically significant difference in pull-out strength or in peak insertion torque.
Section snippets
Porcine cancellous bone model
Thirty porcine tibiae were procured from a local butcher. All samples came from the same species, raised in the same environment for the food chain and slaughtered at the same age (approximately three to four months), minimising inter-specimen variability. The metaphyseal region was extracted from each tibia. Cuts were made in the transverse plane with a minimum sample thickness of 20 mm to ensure sufficient depth for thread engagement. The samples were wrapped in gauze moistened in a
Results
The apparent density of one pilot-hole in two HCl acid demineralisation samples failed to demonstrate a reduction in density. Additionally, one pilot-hole site in one sample and two pilot holes in another sample yielded disproportionately low values of peak insertion torque and pull-out force, potentially caused by structural damage. These data were not including in the analysis.
Discussion
The mean apparent density of the fresh porcine samples validated the choice of porcine cancellous bone for biomechanical screw testing, being within the range of human cancellous bone (0.10 g/cm3 to 1.10 g/cm3) (ASTM, 2017; Hernandez, 2016; Liebschner, 2004). Biomechanical research needs testing models that enable the greatest translation of research findings to clinical practice, whilst being practical to use both financially and ethically. Porcine tibiae are inexpensive, easy to procure and
Conclusion
Porcine bone can be demineralised to model low density cancellous bone. This novel model showed that pullout force is significantly reduced in lower density screw holes, but that this reduction can be mitigated by reducing pilot hole diameter for cancellous screws. Further work is required to validate these findings clinically.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Declaration of Competing Interest
None.
Acknowledgments
We thank Jack Howell, Daniel Ball, Clare Ball and Bruno Hernandez for their guidance and expertise when using the equipment in the University of Bath Biomechanics Laboratory.
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