Defining optimal coronal alignment in Total Knee Replacement (TKR) is a controversial and poorly understood subject. Tibial bone density may affect implant stability and functional outcomes following TKR. Our aim was to compare the bone density profile at the implant-tibia interface following TKR in mechanical versus kinematic alignment. Pre-operative CT scans for 10 patients undergoing medial unicompartmental knee arthroplasty were obtained. Using surgical planning software, tibial cuts were made for TKR with 7 degrees posterior slope and either neutral (mechanical) or 3 degrees varus (kinematic) alignment. Signal intensity, in Hounsfield Units (HU), was measured at 25,600 points throughout an axial slice at the implant-tibia interface and density profiles compared along defined radial axes from the centre of the tibia towards the cortices (Hotelling's t-squared and paired t-test).Background
Methods
The increase in revision joint replacement surgery and fractures of bone around orthopaedic implants may be partly addressed by keeping bone healthy around orthopaedic implants by inserting implants with mechanical properties closer to the patient's bone properties. We do not currently have an accurate way of calculating a patient's bone mechanical properties. We therefore posed a simple question: can data derived from a micro-indenter be used to calculate bone stiffness? We received ethical approval to retrieve femoral heads and necks from patients undergoing hip replacement surgery for research. Cortical bone from the medial calcar region of the femoral neck was cut into 3×3×6mm cuboid specimens using a diamond wafering blade. Micro-indentation testing was performed in the direction of loading of the bone using a MicroMaterials (MicroMaterials, UK) indenter, using the high load micro-indentation stage (see Figure 1). To simulate in vivo testing, the samples were kept hydrated and were not fixed or polished. From the unloading curve after indentation, the elastic modulus was calculated, using the Oliver-Pharr method using the indentation machine software. To assess which microindentation machine settings most precisely calculate the elastic modulus we varied the loading and unloading rates, load and indenter tip shape (diamond Berkovich tip, 1mm diameter Zirconia spherical tip and 1.5mm diameter ruby spherical tip). Following this, for 11 patients' bone, we performed compression testing of the same samples after they were indented with the 1.5mm diameter ruby spherical tip to assess if there was a correlation between indentation values of apparent elastic modulus and apparent modulus values calculated by compression testing (see Figure 2). Platens compression testing was performed using an Instron 5565 (Instron, USA) materials testing machine. Bluehill compliance correction software (Instron, USA) was used to correct for machine compliance. The strain rate was set at 0.03mm/s. The apparent elastic modulus was calculated from the slope of the elastic region of the stress-strain graph. The correlation between values of apparent modulus from compression testing and indentation were analyzed using IBM SPSS Statistics 22.Introduction
Methods
Bisphosphonates (BP) are the first-line therapy for preventing osteoporotic fragility fractures. However, concern regarding their efficacy is growing because bisphosphonate use is associated with over-suppression of remodeling. Animal studies have reported that BP therapy is associated with accumulation of micro-cracks (Fig. 1) and a reduction in bone mechanical properties, but the effect on humans has not been investigated. Therefore, our aim was to quantify the mechanical strength of bone treated with BP, and correlate this with the microarchitecture and density of micro-damage in comparison with untreated osteoporotic hip-fractured and non-fractured elderly controls. Trabecular bone cores from patients treated with BP were compared with patients who had not received any treatment for bone osteoporotic disease. Non-fractured cadaveric femora from individuals with no history of bone metabolic disease were also used as controls. Cores were imaged in high resolution (∼1.3µm) using Synchrotron X-ray tomography (Diamond Light Source Ltd.) The scans were used for structural and material analysis, then the cores were mechanically tested in compression. A novel classification system was devised to characterise features of micro-damage in the Synchrotron images: micro-cracks, diffuse damage and perforations. Synchrotron micro-CT stacks were visualised and analysed using ImageJ, Avizo and VGStudio MAX.Introduction
Methods
Bones are thought to become fragile with advancing age due to a loss of mass and structure. However, there are important aspects of bone fragility and fracture that cannot be explained simply by a loss of bone: 30% of all patients told they have healthy bone based on bone mineral density (BMD) measurements go on to fracture. It has been suggested that increased fracture risk might also be due to ageing at the nanoscale, which might deteriorate the overall mechanical properties of a bone. However, it is not clear how mechanics at the level of the collagen-mineral matrix relate to mechanical properties of the whole bone, or whether nano-mechanics contribute to fracture risk. In order to answer these questions our group is developing state of the art methods for analysing the structure and function of the collagen mineral matrix under loading. To image the collagen mineral matrix we obtained beam time on a synchrotron particle accelerator at the Diamond Light Source (Didcot, UK). Electrons are accelerated to near light speed by powerful electromagnets, then slowed to create high energy monochromatic X-Ray beams. Through a combination of X-Ray computed tomography and X-Ray diffraction we have been able to image the collagen/mineral matrix. Furthermore, using in situ loading experiments it has been possible to visualise collagen fibrillar sliding and mineral crystal structure. Our group is analysing how age related changes in nano-structure affect bone mechanical behaviour. As well as comparing fragility fracture patients with ‘healthy’ age matched controls to investigate whether ageing at the nano-scale could increase fracture risk. We are also assessing the effect of common treatments for bone fragility (e.g. bisphosphonate) on nano-mechanics. Unfortunately the expense and high radiation dose associated with synchrotron imaging prevents the technology from being adapted for patients. Therefore the next step will be to identify and test tools that can be used to indirectly assess bone chemistry and mechanical properties at point of care (e.g. laser spectroscopy and indentation). The data could be used to improve the diagnosis, monitoring and treatment of bone fragility.
