Effects of Titanium Surface Modification on Inflammation and Osseous Healing in Diabetes

Abstract

Successful osseous healing is achieved by highly organised and controlled biological processes. Inflammation is an initial and integral part of these biological processes, influencing the fate of overall healing outcomes. Indeed, it has been well established that the skeletal and immune systems closely interact and influence each other in order to regulate bone homeostasis, both in healthy and diseased conditions. Compromised wound healing is one of the major clinical complications of diabetes, often leading to high morbidity and mortality. Pathophysiological changes, such as chronic hyperglycaemia and a resultant pro-inflammatory microenvironment, are considered to be the main aetiological factors in diabetes, contributing to a compromised immune system and poor wound healing potential. Macrophages play a pivotal role in both the initiation and resolution of inflammation by expressing an array of functional phenotypes (e.g. M1 and M2) in response to diverse local microenvironmental chemical cues. A timely transition of inflammatory status from ‘proinflammatory (M1)’ to ‘anti-inflammatory/pro-healing (M2)’, is believed to be a crucial biological step in order to achieve successful wound healing, and there is increasing evidence supporting the notion that macrophages are responsible for this ‘switch’ of microenvironment. This important cellular phenomenon of M1 and M2 phenotypic transition is disrupted in diabetes due to various systemic and local factors, however the underlying cellular mechanisms are yet to be fully elucidated. Recently, numerous in vitro and in vivo studies have investigated the influence of different biomaterial surface characteristics on the resultant immune responses, demonstrating that the expression of macrophage phenotypes can be modulated by surface topography, chemistry and hydrophilicity, thus resulting in enhanced osseous healing. However, there is insufficient evidence in the current literature to support whether such biomaterial enhancements can result in similar outcomes in diabetic conditions, where the osteoimmunological interactions are compromised. Therefore, we hypothesized that a hydrophilic moderately rough titanium surface promotes M2 macrophage phenotype polarization at the early inflammatory phases of osseous healing, creating a microenvironment that promotes osteogenesis, even under the systemically compromised healing conditions encountered in diabetes. In the first part of this thesis (Chapter 2), we established a Type-1 Diabetes like animal model to demonstrate that a hydrophilic moderately rough titanium surface was able to influence macrophage polarization towards an anti-inflammatory M2 phenotype, even in extreme hyperglycaemic and pro-inflammatory experimental conditions. Based on the results obtained from the pilot study (Chapter 2), we explored further whether the promotion of M2 macrophage phenotype polarization via a modified biomaterial surface had a direct impact on osteoblastic activity (Chapter 3). This in vitro study showed for the first time that the upregulation of osteogenic gene expression (TGF‐ß/BMP signalling pathway) in osteoblasts co‐cultures with M2 phenotype macrophages was enhanced if the macrophages were cultured on the modified biomaterial surface compared to a control surface. In the final part of the thesis, we employed a more clinically relevant diabetic model, Goto-Kakizaki rat in order to investigate the immunomodulatory effects of titanium surface modification on macrophage polarization and inflammation in Type-2 Diabetes like conditions (Chapter 4). In a series of in vitro and in vivo studies, we demonstrated that the modified titanium surface can compensate for the compromised diabetic macrophage function by creating an environment that attenuates the pro-inflammatory response during the early stages of osseous wound healing, thus restoring macrophage homeostasis and resulting in enhanced osseous healing in Type 2 diabetic conditions.