Digital x-ray analysis for monitoring fracture healing

Abstract

X-ray based evaluation of different stages of fracture healing is a well established clinical standard. However, several studies have shown plain radiography alone to be an unreliable method to assess healing. The advent of digital X-ray systems provides the potential to perform quantitative analysis on X-ray images without disrupting normal clinical practice. Two aspects were explored in this study. The first was the measurement of mechanical fracture stiffness under four point bending and axial loading. The second was the inclusion of an Aluminium step wedge to provide Aluminium-equivalent thickness calibration information. Mechanical sti ness studies involved the development of equipment to perform four point bending on intra-medullary (IM) nailed tibial fractures, equipment to perform axial loading on conservatively treated humeral fractures, and fracture models to ex- amine the developed systems. Computational procedures to automatically measure the angle and offset occurring at the fracture site by comparing loaded and unloaded X-ray images were developed utilising cross-correlation. The apparatus and procedures were tested using the fracture models both in X-ray and using the Zwick materials testing machine. The four point bending system was applied clinically to a series of IM nailed tibial fracture patients and the axial loading system to two conservatively treated humeral fracture patients. Mechanical stiffness results showed that the apparatus worked well in the clinical radiography environment and was unobtrusive to normal practice. The developed X-ray analysis procedure provided reliable measurements. However, in the case of IM nailed tibial fractures, both angular and displacement movements were too small to be accurately assessed or to provide reliable stiffness measurements. This indicated that this patient group was possibly unsuitable for mechanical stiffness measurements or that higher loads needed to be applied to the fracture site. The case studies of conservatively treated humeral fractures showed potential in detecting movement between loaded and unloaded X-rays and using this to provide sti ness information. Further investigation is required to show that this technique has the potential to aid fracture healing monitoring. Investigation into Aluminium step wedge calibration began with the design of different step wedges and X-ray phantoms. Initial image analysis involved studying the automatic processing applied by a digital Computed Radiography (CR) Fuji sys- tem and modelling of the inhomogeneities in X-ray images as well as investigation into the effect of and correction for scatter, overlying soft tissue and bone thickness. Computational procedures were developed to semi-automatically detect the steps of the step wedge, form an exponential Aluminium step thickness to grey level calibration graph, measure soft tissue and bone thickness, and correct for the heel effect and scatter contributions. Tests were carried out on pre-clinical models and results compared to ash weight and peripheral quantitative computed tomography (pQCT). A clinical study of radial fractures was used to investigate the effectiveness of the step wedge calibration system in monitoring fracture healing changes. Results using the step wedge indicated that the calibration technique was e ective in detecting and correcting for aspects in uencing Aluminium-equivalent thickness measures. With careful processing, useful information was obtained from digital X- rays that included the Aluminium step wedge and these correlated well with existing density measures. The use of the wedge in patient images showed that small increases in Aluminium-equivalent thickness of the fracture site could be detected. This was most useful for intra-patient comparisons throughout the course of healing rather than providing quantitative measurements which were comparable to other density measures. In conclusion, this thesis shows the potential for accurate analysis of digital X- rays to aid the monitoring of healing changes in fracture patients, particularly with application of axial loading and the use of step wedge calibration.