ARTICLE IN PRESS JID: JJBE [m5G;July 7, 2015;13:42] Medical Engineering and Physics 000 (2015) 1–7 Contents lists available at ScienceDirect Medical Engineering and Physics journal homepage: www.elsevier.com/locate/medengphy Numerical evaluation of sequential bone drilling strategies based on thermal damage Bruce L. Tai a,∗ , Andrew C. Palmisano b , Barry Belmont c , Todd A Irwin b , James Holmes b , Albert J. Shih c,d a Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843, United States b Department of Orthopaedic Surgery, University of Michigan, Ann Arbor, MI 48109, United States c Department of Biomedical Engineering, University of Michigan, Ann Arbor, MI 48109, United States d Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI 48109, United States article info Article history: Received 3 March 2015 Revised 19 May 2015 Accepted 8 June 2015 Available online xxx Keywords: Sequential bone drilling Thermal dose Finite element analysis (FEA) abstract Sequentially drilling multiple holes in bone is used clinically for surface preparation to aid in fusion of a joint, typically under non-irrigated conditions. Drilling induces a significant amount of heat and accumulates af- ter multiple passes, which can result in thermal osteonecrosis and various complications. To understand the heat propagation over time, a 3D finite element model was developed to simulate sequential bone drilling. By incorporating proper material properties and a modified bone necrosis criteria, this model can visual- ize the propagation of damaged areas. For this study, comparisons between a 2.0 mm Kirschner wire and 2.0 mm twist drill were conducted with their heat sources determined using an inverse method and exper- imentally measured bone temperatures. Three clinically viable solutions to reduce thermally-induced bone damage were evaluated using finite element analysis, including tool selection, time interval between passes, and different drilling sequences. Results show that the ideal solution would be using twist drills rather than Kirschner wires if the situation allows. A shorter time interval between passes was also found to be benefi- cial as it reduces the total heat exposure time. Lastly, optimizing the drilling sequence reduced the thermal damage of bone, but the effect may be limited. This study demonstrates the feasibility of using the proposed model to study clinical issues and find potential solutions prior to clinical trials. © 2015 IPEM. Published by Elsevier Ltd. All rights reserved. 1. Introduction Bone drilling is common in many orthopaedic procedures, includ- ing predrilling for screw placement, temporary bony fixation, and surface preparation for joint fusion. Significant heat is produced dur- ing drilling due to material removal and frictional resistance between the cortical bone and the drill [1]. This heat dissipated from the drilling site can cause damage to the surrounding bone through ther- mal osteonecrosis, which is the result of the temporary or permanent loss of blood supplied to the bone that consequently leads to osteo- cyte and bone death [2–6]. To suppress the heat, studies have shown that drill size, cutting speed, and irrigation have significant effects on bone temperature [7,8]. In particular, irrigation has been shown to significantly decrease drilling temperatures even under intermittent supply [4,5]. However, irrigation is not appropriate for some clini- cal situations. For example, bone drilling to aid in fusion of a joint would be negatively impacted by irrigation because it washes away ∗ Corresponding author. Tel: +1 979 458 9888. E-mail address: btai@tamu.edu (B.L. Tai). the cells one is trying to access by drilling into the the subchon- dral bone. Furthermore, these cases often require repeated sequen- tial passes within a finite region to encourage increased blood flow to aid in healing. Depending on the operating location or simply prefer- ence, a surgeon can choose either a twist drill or a Kirschner wire (K- wire) for sequential drilling. As K-wires are known to produce more heat than twist drills due to lack of flutes [9,10], the risk of thermal damage under near-dry, sequential drilling using them is potentially dangerously high. Both temperature and exposure time are critical factors in deter- mining bone thermal damage. A thermal dose measurement, defined by a cumulative equivalent exposure time at 43 °C (CEM 43 ), is often adopted to predict the onset of bone necrosis [11,12]. Its ultimate va- lidity as a metric is still debatable since it was initially created for cancer therapy. Experimentally, temperatures above 70 °C have been seen to result in immediate bone death [6,13], whereas irreversible cell death of osteocytes occurs after 30 s at a temperature of 55 °C and after 60 s at 47 °C [7,8]. These three conditions, in fact, produce significantly different CEM 43 . The threshold of 47 °C is typically used as an indicator instead of 43 °C when tissues are on the brink of de- struction. http://dx.doi.org/10.1016/j.medengphy.2015.06.002 1350-4533/© 2015 IPEM. Published by Elsevier Ltd. All rights reserved. Please cite this article as: B.L. Tai et al., Numerical evaluation of sequential bone drilling strategies based on thermal damage, Medical Engi- neering and Physics (2015), http://dx.doi.org/10.1016/j.medengphy.2015.06.002