Professor Donal McNally is Head of the Human Factors Research Group.
The major focus of my research is in the field of spinal mechanics, particularly the mechanical function of the intervertebral disc and how this relates to internal structure. To further my research goals, I have developed a wide range of expertise ranging from functional and anatomical imaging, through standard biomechanical testing, to highly novel techniques to quantify the internal mechanical behaviour of intact skeletal structures.
I have considerable experience in the mechanical evaluation of both intact spinal segments and medical devices such as total disc replacements, motion preserving stabilisation systems and fusion devices.
In addition to conventional loading and bending testing, I have developed a number of techniques that allow me to study the internal mechanical behaviour of disc tissues. Internal stresses can be quantified using a needle mounted transducer that can measure the compressive stress within the extracellular matrix in two orthogonal directions. Internal displacements and strains can be quantified using a combination of clinical and high frequency ultrasound techniques. Clinical ultrasound permits internal structures, such as individual lamellae, to be imaged and tracked throughout a loading cycle. Scanning acoustic microscopy permits fibre orientation to be measured as discrete points within the outer annulus.
The ability to quantify both the internal and external mechanical function of intervertebral discs permits me to determine the effects of treatments, such as radio-frequency heating probes, and the effects of surgery on adjacent levels.
To be able to design an implant effectively it is important to know a number of things; what the dysfunction is that needs to be corrected, what the device needs to be able to do, what effect the device will have on adjacent structures, and how the device is likely to fail or degrade.
My considerable experience in the field of spinal mechanics is of great value in identifying the underlying mechanical failure or dysfunction that leads to conditions such as discogenic back pain. In most cases sensitive diagnostic techniques need to be developed before the correct treatment can be selected. My work with transducer systems and functional imaging is directed, in part, to improving diagnosis so that treatment outcomes can be improved.
The motion and load bearing nature of spinal segments is highly complex compared to other joints such as hips and knees. For example the intervertebral disc rotates about 3 axes and translates in 3 directions. Similarly the spatial location of the axes of rotation change with motion and disease. I have considerable expertise in disc mechanics and kinematics.
Structures such as the intervertebral disc or facet joint have an enormous mechanical influence on the surrounding structures. Any implant design that does not replicate these effects, or that introduces new mechanical conditions for the surrounding structures, is likely to fail or create further clinical problems. I am able to evaluate the performance of such devices and determine their effects on surrounding structures.
Clearly, the level of experienced mechanical testing that I can provide is of great importance in determine the failure characteristics of devices, or their interface with the body. Such failures can occur in conditions of simple overload, repetitive fatigue loading, or high strain rate 'impact' type loads.
Mechanical and Physiological Modeling
There is a limit to the understanding of spinal mechanics that can be gained through experiment. Frequently, greater insight can be gained through the use of mathematical models. Clearly, the ability to combine both experiment and theory so that one can develop from the other is of great value.
The mechanics and nutrition of the disc are interdependent. This is largely because the disc is 60-80% water that is free to move, under the influence of mechanical loads, both within the disc and in and out of it. Nutritional and other metabolites are convected on such mechanically induced flows, but also move by diffusion along concentration gradients. Such gradients are created by the metabolic activity of the cells within the disc, but such activity is also dependant on the concentration of the metabolites and therefore the nutritional pathways. Similarly the metabolic activity of the disc cells determines the composition of the extracellular matrix and therefore its mechanical and diffusive properties.
I have considerable experience modelling the disc as a multiphase system, and also in modelling the respirational metabolism of the disc cells. Similarly, I am experienced in modelling the mechanically complex behaviour of the disc and how this affects the tissues of the disc itself, and also adjacent structures such as the vertebral endplate.
Another important aspect of my work is the field of impact biomechanics, where my research focuses on the mechanical behaviour of tissues, such as ligament and bone, and whole structures, such as limbs, under loading conditions comparable to those found in automotive accidents.
Accidents involving pedestrians and other people external to vehicles are one of the largest causes of fatalities and serious injury on the roads. Unlike occupants of vehicles where position and motion can be controlled by restraint systems and airbags, pedestrians are hit directly by the car. Understanding of these impacts requires a detailed knowledge of the behaviour of tissues, such as bone, and parts of the body, such as legs.
Injury Thresholds and Criteria
The unique test facilities at the Institute of Biomechanics permit the mechanisms of injuries, such as ankle fractures in car crashes and brain damage in baby shaking, to be investigated and quantitative injury thresholds and criteria to be determined. This information is useful for 'calibrating' ATDs (crash test dummies) and drafting safety legislation.
High Strain Rate/Impact Testing
My laboratory is equipped with a combination of facilities for evaluating biological tissues and structures at high strain rates that is unique within the United Kingdom. These facilities include; a crash track capable of programmed acceleration of up to 100g, a 2m drop tower, and a pneumatic impact gun capable of propelling 10kg specimens into targets at velocities up to 11ms-1.
My research vision is to solve important healthcare problems using my understanding of and expertise in soft tissue biomechanics.
My primary research focus is back pain, which is estimated to cost the UK £12.3 billion in healthcare and social costs. research prizes, and total disc replacements that have been implanted throughout Europe.
I also have a focus on engineering solutions to reduce the numbers of fatalities (currently around 1800 per year), and serious injuries on British roads. This work has been cited by the Transport Select Committee in the areas of whiplash injuries and bicycle safety.
SHERLOCK KE, TURNER W, ELSAYED S, BAGOURI M, BAHA L, BOSZCZYK BM and MCNALLY D, 2015. The Evaluation of Digital Rectal Examination for Assessment of Anal Tone in Suspected Cauda Equina Syndrome. Spine. 40(15), 1213-8
DIXON, JAMES E., SHAH, DISHEET A., ROGERS, CATHERINE, HALL, STEPHEN, WESTON, NICOLA, PARMENTER, CHRISTOPHER D. J., MCNALLY, DONAL, DENNING, CHRIS and SHAKESHEFF, KEVIN M., 2014. Combined hydrogels that switch human pluripotent stem cells from self-renewal to differentiation PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA. 111(15), 5580-5585