Contents lists available at ScienceDirect Clinical Biomechanics journal homepage: www.elsevier.com/locate/clinbiomech Non-setting, injectable biomaterials containing particulate hydroxyapatite can increase primary stability of bone screws in cancellous bone Jorge Solana Muñoz a , Ulrike Kettenberger a , Philip Procter b , Dominique P. Pioletti a, ⁎ a Laboratory of Biomechanical Orthopedics, Institute of Bioengineering, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland b Applied Materials Science, Department of Engineering Sciences, Uppsala University, Sweden ARTICLE INFO Keywords: Weak bone quality Screw augmentation Soft material Hydrogel Primary stability Osteoporosis ABSTRACT Background: Fracture fixation in weak bone is still a clinical challenge. Screw augmentation was shown to successfully increase their primary stability. The currently used calcium phosphate or polymeric bone cements, however, present important drawbacks such as induced toxicity and/or impaired bone neo-formation. A new approach to enhance bone screw primary stability without affecting bone formation is the use of non-setting, calcium phosphate loaded soft materials as the augmentation material. Methods: Two types of biomaterials (non-crosslinked hyaluronic acid as viscous fluid and agar as hydrogel) were loaded with 40 wt/vol% of hydroxyapatite particles and characterized. The screw augmentation effect of all materials was evaluated through pull-out tests in bovine cancellous bone and compared to the non-augmented situation (control). The bone mineral density of each test sample was measured with μCT scans and was used to normalize the pull-out strength. Findings: Both materials loaded with hydroxyapatite increased the normalized pull-out strength of the screws compared to control samples and particle-free materials. This counter-intuitive augmentation effect increased with decreasing bone mineral density and was independent from the type of the soft materials used. Interpretation: We were able to demonstrate that non-setting, injectable biomaterials loaded with ceramic par- ticles can significantly enhance the primary stability of bone screws. This material combination opens the unique possibility to achieve a screw augmentation effect without impairing or even potentially favoring the bone formation in proximity to the screw. This effect would be particularly advantageous for the treatment of os- teoporotic bone fractures requiring a stabilization with bone screws. 1. Introduction Osteoporosis related fractures in the elderly population are an in- creasing burden on healthcare systems (Johnell and Kanis, 2006). Hip fracture is by far the most common reason for hospital admission, it may occurs either as a result of a fragility fracture (Acevedo et al., 2018) or a fall. Most hip fracture patients receive operative treatment (Weycker et al., 2016; Williamson et al., 2017). The ability of the proximal femur to bear the internal load actions resulting from a fall is significantly reduced by osteoporosis and in particular, it has been demonstrated that this is related to reduced areal bone mineral density (Curtis et al., 2016). Each form of treatment following a bone fracture may result in undesired outcomes or complications. For example, in the use of screws to fixate undisplaced femoral neck fractures, the holding power of the screw may be limited by the degree of osteoporosis reduction in bone density in the regions where the surgeon tries to obtain purchase (Konstantinidis et al., 2016). Poor screw purchase may result in screw loosening and/or migration during the healing period and may ne- cessitate a further operative intervention which is undesirable in elderly patients (Do et al., 2016; Rogmark et al., 2009). Differing clinical opi- nions on how to deal with this particular complication has led some surgeons to advocate going directly to an endoprosthesis or total hip replacement, whilst others have considered changing the design of the implant and/or modifying the surgical technique (Gardner et al., 2004). In this latter group, the use of cements to augment the fixation of the screw in the bone has been considered. This strategy has been the subject of many preclinical animal studies in which both poly methyl methacrylate (PMMA) and calcium phosphate (CaP) based injectable cements have been proposed as solutions (Stankewich et al., 1996; Stoffel et al., 2008; von der Linden et al., 2006). The purpose of cement augmentation is to improve the holding power of the screw and reduce https://doi.org/10.1016/j.clinbiomech.2018.09.023 Received 15 February 2018; Accepted 21 September 2018 ⁎ Corresponding author at: EPFL/STI/IBI/LBO, Station 9, 1015 Lausanne, Switzerland. E-mail address: dominique.pioletti@epfl.ch (D.P. Pioletti). Clinical Biomechanics 59 (2018) 174–180 0268-0033/ © 2018 Elsevier Ltd. All rights reserved. T