Medical engineering at Cardiff University. Part 2: Postgraduate programmes of study

The Medical Engineering team within the School of Engineering, Cardiff University, delivers two postgraduate programmes of study. Established over 10 years ago, the part-time MSc programmes in Orthopaedic Engineering and Clinical Engineering offer the opportunity of further study while remaining within full-time employment. Both programmes deliver 120 taught credits over two academic years via a series of residential weekends, with successful completion enabling the student to undertake and then defend a 60-credit research dissertation. Fulfilling a specific role on the career pathway for both student cohorts, the strength of each programme is indicated by the consistent number of applicants.

Medical engineering at Cardiff University. Part 1: Undergraduate programmes of study

Cardiff University has offered a medical engineering undergraduate programme since 2001 and hence delivers one of the longest-running and most established medical engineering programmes within the UK. It currently offers BEng (Hons) and MEng (Hons) programmes that are both accredited by the Institution of Mechanical Engineers and include the option to undertake a year in industrial employment. The admissions policy ensures that the intake consists of a diverse range of students and is typically very successful in attracting female students. The programmes consist of six key academic threads which ensure that the content is both relevant and continuous, with all threads tailored to provide a patient-focused learning environment. Students initially learn core and fundamental principles in years 1 and 2, supported by a range of laboratories and practical experimentation. The latter years then encourage the students to corroborate and apply this knowledge, including involvement in a range of project-based learning exercises. The programme is delivered by a core of experienced academic medical engineers, with support from other engineering colleagues, as well as colleagues from the School of Biosciences, the School of Medicine, and the National Health Service. Thus, the programme delivers a wide range of modules which guarantee that graduating students have a thorough understanding of all possible career options. These two factors are significant in making it possible for students to follow their chosen career path upon graduation.

The effect of preparation height and taper on cement lute stress: a three-dimensional finite element analysis

Three dimensional finite element models of an upper second premolar and molar with full veneer gold crown preparations were developed from extracted samples. The cement lute width was kept constant at 40 microm, but the height and preparation taper were varied. For both models the preparation height was either 1.5 mm (short preparation) or 3 mm (long preparation). The preparation taper was either 10 degree or 30 degree, giving a total of eight models. Each model was loaded with a 10 N horizontal load, a 10 N vertical load or a 10 N load distributed across the occlusal surface. The maximum shear stress and the maximum Von Mises' stress in the cement lute of each model were recorded. For the premolar, the maximum shear stresses ranged from 0.3-5.43 MPa and the maximum Von Mises' stress ranged from 1.44-14.98 MPa. For the molar, the maximum shear stresses ranged from 0.15-5.22 MPa and the maximum Von Mises' stress ranged from 0.3 7-15.02 MPa. The stress fields were consistently higher in the premolar with a 30 degree preparation taper compared to the 10 degree taper. The attainment of a cavity taper of 100 is still important to minimise stress in the cement lute and is particularly important in teeth with a lower preparation surface area such as a premolar

Flow instabilities in a graft anastomosis: a study of the instantaneous velocity fields

The major cause of arterial bypass graft failure is intimal hyperplasia. Fluctuating wall shear stresses in the graft, which are associated with disturbed flow, are believed to be important factors in the development and localization of intimal hyperplasia. This study, based upon water as the working fluid, has investigated the flow structure inside a 30 degree Y-junction with different fillet radii at the intersection between the graft and the host artery at various Reynolds numbers and distal outlet segment (DOS) to proximal outlet segment (POS) flow ratios. The structure of the flow has been investigated experimentally using particle image velocimetry (PIV). The two-dimensional instantaneous velocity fields confirm the existence of a very complex flow, especially in the toe and heel regions for the different fillet radii and clearly identify features such as sinks, sources, vortices and strong time dependency.

The influence of the restoration-tooth interface in light cured composite restorations: a finite element analysis

This study examines the early shrinkage behaviour of dental composite resins, and in particular the interfacial stresses around the margins of a composite restoration. The development of stresses at the restoration-tooth interface can have a detrimental effect on the longevity of a restoration. The influence of this interface on the stress system generated in the tooth was examined using finite element analysis. The restoration-tooth interface was simulated using spring elements of varying spring constants (k = 1, 10(2), 10(4), 10(10) N/mm). Interfacial stresses varied from -0.15 to 0.42 MPa for a spring constant of 1 N/mm, and from -19 to 68 MPa for a spring constant of 10(10) N/mm. Correlations between stiffness at the restoration-tooth interface and higher shrinkage stresses due to restricted shrinkage were found. Interfacial failure at the upper and lower regions of the interface, as well as cuspal movements of the order of 2 microm were predicated for the model of the highest spring constant, 10(10) N/mm. The restoration-tooth interface modelled by the spring elements was seen to have a conclusive effect on the ensuing stress system, as well as the longevity of the restoration.

Polymerization shrinkage of dental composite resins

Aesthetic materials have always been a priority for the restoration of anterior teeth; increasingly, they have also gained prominence in the restoration of posterior teeth. This has been due to their advantages as an alternative to amalgam. Their drawbacks however, can include poor marginal adaptation, postoperative sensitivity and cuspal movement. These are particularly associated with the polymerization shrinkage accompanying the placement of composite resins. Consequently, a variety of methods have been used for determining the polymerization shrinkage. These range from dilatometer methods, specific gravity methods and deflecting disc systems to optical methods. In this work a unique method for the qualitative analysis of polymerization shrinkage was undertaken. This method utilized a miniature transducer and provided details of the shrinkage from within the material. The results indicated movement of material towards the initiating light, followed by a return movement away from it. The study was expanded to incorporate clinical aspects, whereby the composite resin was in direct contact with dental tissue, as in a restoration. Tests were performed with surface-treated cavity moulds, as in restoration placement, and without surface treatment. Results indicated that the shrinkage was highly dependent upon the region under investigation, as well as on the state of the surface.

Failure strengths of suture vs. biodegradable arrow and staple for meniscal repair: an in vitro study

A study was performed to determine the in vitro biomechanical behaviour of two 'all inside' meniscal repair techniques (Meniscal Arrow [Bionx Implants Inc.] and Meniscal Staple [Surgical Dynamics Inc.]) and compare these directly with both a horizontal and vertical suture repair. Using 30 fresh bovine medial menisci, vertical 'bucket handle' tears were created 4 mm from the meniscus periphery. Repairs were subsequently performed, using four techniques, with 15 repairs in each group, a horizontally placed 3-metric Ethibond suture, a vertically placed 3-metric Ethibond suture, a single 13-mm arrow and a single 7-mm staple. A tensile test was performed to determine the force at failure for each technique. The mean force at failure of the horizontal and vertical suture groups was 63.2 and 73.9 N, respectively, 44.3 N for the arrow group and 17.8 N for the staple group. The mean forces at failure were significantly different (P < 0.005). The mean tensile strength of the meniscal staple was significantly lower than that of both suture and arrow groups. The 7-mm staple design may not allow adequate interdigitation between the barbed legs and the semicircular collagen fibres of the meniscus.

The effect of curing light intensity and test temperature on the dynamic mechanical properties of two polymer composites

The dynamic modulus of elasticity of two composite materials cured by three light intensities was investigated using a dynamic thermal and mechanical analyser at 37 degrees C and 60 degrees C. The lowest light intensity, 180 mW/cm2, was found to be unsatisfactory in that specimens were weak and mostly untestable. The higher intensities, 350 and 700 mW/cm2, cured the materials to slightly different extents with the elevated temperatures of testing leading to post-cure maturation of the materials. An interesting finding was that one material had a significantly higher modulus when tested at 37 degrees C in comparison with that found at 60 degrees C. In one material the higher light intensity resulted in significantly higher modulus values and these values were increased by post-curing. In the other material the higher light intensity did not result in higher modulus figures at either test temperature, and lower test temperature resulted in better post-cure maturation. These findings are most likely to be caused by differences in base monomer and activator/initiator concentrations. The results indicate that optimum polymerization may not be achieved in all materials by the same method, in particular, high curing light intensities may not achieve the most desirable results.

Comparative biomechanical analysis of a new circumferential flexor tendon repair and a modified Kessler repair

We present the technical details and the results of a biomechanical analysis of a new type of circumferential flexor tendon repair, designed with the more stringent requirements of zone II injuries in mind. Apart from good initial strength we aimed for a design with little bulk at the repair site and good control of the tendon edges. The new repair is achieved using a single, continuous, inverting and locking suture of the periphery of the tendon. The repair was compared with a Kessler core suture of 4/0 polydioxanone, with Tajima and Strickland modifications, to which has been added a simple running circumferential suture (6/0 polypropylene), the repair currently used in our unit. Fresh human cadaver flexor tendons were divided and repaired by one of the two techniques (n = 12 for each technique), using 5/0 polypropylene for the new circumferential suture. A third group of tendons (n = 8) were divided and repaired with a 5/0 multifilament steel circumferential suture. The repaired tendons were tested at longitudinal stress to failure. The first two groups of tendons were tested at two crosshead speeds. Overall, crosshead speed had no effect on ultimate tensile strength (P = 0.5). The 5/0 polypropylene circumferential repair (median 32.29 N) was significantly stronger than the Kessler repair (median 24.03 N) (P = 0.046). The circumferential repair was significantly stronger with steel (median 56.04 N) than with polypropylene (median 32.29 N) (P = 0.007). The size of the repair site, resistance to gap formation and the patterns of failure were analysed on video recordings.

Stress protection due to external fixation

Bone is sensitive to mechanical influences. The presence of an orthopaedic device will impose constraints on the mechanical environment that may influence subsequent remodelling and repair. An Oxford External Fixator was applied to six intact ovine tibiae. The strains engendered during normal walking were then recorded from strain gauges applied to the mid-shaft of the bone. The fixator configuration was then altered such that the intact fixator bar connecting the pins was replaced with a sectioned version that did not permit load transfer through the fixator, and the strain environment re-recorded. The peak strains recorded with the intact bar were significantly lower than those recorded with the sectioned bar. This showed that the use of a fixator with an intact bar resulted in significant stress protection of the underlying bone. A fixator was then applied to both the right and left tibiae of a further six animals and the resulting strain environment and corresponding remodelling response was observed over 16 weeks. In each case one fixator was configured with an intact bar (the stress protected limb), whilst the other utilised a sectioned bar (the unprotected limb). The results showed that over this period the bone mineral content fell by 9% in the stress protected limb compared to the unprotected limp. Quantitative assessment of the bones showed that this bone loss occurred as a direct consequence of resorption on the periosteal and endosteal surfaces. In addition, strain recordings at week 16 showed that the fixator was still stress protecting the tibia.