Advances in machining of hard tissues – From material removal mechanisms to tooling solutions

The machining of hard tissues, such as cortical bone and cancellous bone in human bodies and enamel and dentin in teeth, is an essential process in orthopaedics, neurosurgery and dentistry. Owing to their natural high hardness and strength, as well as anisotropic composite structure and properties, the machining process is difficult and prone to generating large cutting forces and high temperatures, potentially leading to necrosis (i.e., cellular death) and microcracks and, therefore, tissue damage. Necrosis and microcracks affect the stability of implant fixation and post-surgery recovery. In this paper, studies in tissue structure and properties, cutting mechanisms, and thermal and mechanical damages formed in the machined surface and subsurface have been reviewed to facilitate the understanding of machining processes and mechanisms for hard tissue. Traditional (orthogonal cutting, drilling, milling and grinding) and non-traditional (vibration-assisted) cutting processes have been considered to explore the underlying mechanisms that control the material removal process and understand the damages that may be formed in hard tissues. The solution strategies developed for minimising the machining-induced damages to the tissues have also been evaluated to provide the knowledge needed for improving surgical tool design. Finally, the research gaps have been identified in this study and future research prospects in hard tissue machining have been proposed. • The machining of hard tissues, including cortical bone, cancellous bone, enamel and dentin, are addressed. • Machining mechanism, including orthogonal cutting, drilling, milling, grinding, vibration-assisted cutting, is summarised. • Machining-induced thermal, mechanical, and thermo-mechanical damages of hard tissues and strategies for minimising damages are summarised. • The recent advancements in tool design and tooling systems suitable for bone cutting in surgical operations are provided. • Clinical purpose, scientific challenges and future work opportunities in the field of hard tissues are highlighted.