David FitzPatrick

Development of a statistical shape model of the patellofemoral joint for investigating relationships between shape and function

Patellofemoral (PF)-related pathologies, including joint laxity, patellar maltracking, cartilage degradation and anterior knee pain, affect nearly 25% of the population. Researchers have investigated the influence of articular geometry on kinematics and contact mechanics in order to gain insight into the etiology of these conditions. The purpose of the current study was to create a three-dimensional statistical shape model of the PF joint and to characterize relationships between PF shape and function (kinematics and contact mechanics). A statistical shape model of the patellar and femoral articular surfaces and their relative alignment was developed from magnetic resonance images. Using 15 shape parameters, the model characterized 97% of the variation in the training set. The first three shape modes primarily described variation in size, patella alta-baja and depth of the sulcus groove. A previously verified finite element model was used to predict kinematics and contact mechanics for each subject. Combining the shape and joint mechanics data, a statistical shape-function model was developed that established quantitative relations of how changes in the shape of the PF joint influence mechanics. The predictive capability of the shape-function model was evaluated by comparing statistical model and finite element predictions, resulting in kinematic root mean square errors of less than 3° and 2.5 mm. The key results of the study are dually in the implementation of a novel approach linking statistical shape and finite element models and the relationships elucidated between PF articular geometry and mechanics.

Influence of gamma-irradiation sterilization and temperature on the fracture toughness of ultra-high-molecular-weight polyethylene

Surface damage of the tibial plateau components of knee prostheses made from medical grade ultra-high-molecular-weight polyethylene (UHMW-PE) has been attributed to delamination wear caused by a fatigue fracture mechanism. It has been proposed that factors such as component design and method of sterilization contribute to such failure mechanisms. Understanding the fracture behaviour of UHMW-PE is therefore critical in optimizing the in vivo life-span of total joint components. The elastic-plastic fracture toughness parameter J was consequently determined for a commercial UHMW-PE at ambient and body temperatures, before and after gamma-irradiation sterilization in air at a minimum dose of 29 kGy. Both ductile stability theory and experimental data suggest that cracks propagate in a stable manner, although stability is affected by the sterilization process. Sterilization with gamma-irradiation results in a loss in fracture toughness JIc of 50% and a decrease in tearing modulus (Tm) of 30%. This dramatic reduction could result in a 50% decrease in the residual strength of the components, maximum permissible crack size under service loading and service life (assuming flaws such as fusion defects exist). The time required for a crack to grow from its original size to the maximum permissible size could be decreased by 30%, resulting in earlier failure. In terms of the design of joint replacement components the critical factor to envisage is the design stress level, which should be halved to account for the irradiation process. A scanning electron microscope study reveals that the material fails in layers parallel to the fracture surface.

Three-dimensional Motion Analysis of the Lumbar Spine during “Free Squat” Weight Lift Training

Background Heavy weight lifting using a squat bar is a commonly used athletic training exercise. Previous in vivo motion studies have concentrated on lifting of everyday objects and not on the vastly increased loads that athletes subject themselves to when performing this exercise. Hypothesis Athletes significantly alter their lumbar spinal motion when performing squat lifting at heavy weights. Study Design Controlled laboratory study. Methods Forty-eight athletes (28 men, 20 women) performed 6 lifts at 40% maximum, 4 lifts at 60% maximum, and 2 lifts at 80% maximum. The Zebris 3D motion analysis system was used to measure lumbar spine motion. Exercise was performed as a “free” squat and repeated with a weight lifting support belt. Data obtained were analyzed using SAS. Results A significant decrease (P < .05) was seen in flexion in all groups studied when lifting at 40% maximum compared with lifting at 60% and 80% of maximum lift. Flexion from calibrated 0 point ranged from 24.7° (40% group) to 6.8° (80% group). A significant increase (P < .05) was seen in extension when lifting at 40% maximum was compared with lifting at 60% and 80% maximum lift. Extension from calibrated 0 point ranged from —1.5° (40% group) to —20.3° (80% group). No statistically significant difference was found between motion seen when exercise was performed as a free squat or when lifting using a support belt in any of the groups studied. Conclusion Weight lifting using a squat bar causes athletes to significantly hyperextend their lumbar spines at heavier weights. The use of a weight lifting support belt does not significantly alter spinal motion during lifting.

Comparison of gas plasma and gamma irradiation in air sterilization on the delamination wear of the ultra-high molecular weight polyethylene used in knee replacements.

Abstract Early failure of knee replacements is thought to be due to the combination of sterilization by gamma irradiation in air and the high cyclic stresses that they endure during use. Such failures are shown through delamination and permanent deformation of the ultra-high molecular weight polyethylene (UHMWPE) component. This study investigated whether gas plasma sterilization, as an alternative to gamma irradiation in air, would give better performance after ageing in a knee replacement using a metal pin on polymer plate wear test. Fourier transform infrared (FTIR) analysis was performed on the components to assess oxidation levels and a finite element stress analysis model is presented to estimate strain at failure in the UHMWPE. Delamination occurred in the majority of the gamma-irradiated plates but did not occur in any of the gas-plasma-sterilized plates. The FTIR analysis showed that the plates gamma irradiated in air were highly oxidized when compared with the gas-plasma-sterilized plates. Plastic strain at failure was determined for the gamma-irradiated plates and found to be less than 2.4-14 per cent.

Anatomic variation of the clavicle: A novel three‐dimensional study

An understanding of the complex anatomy of the clavicle is helpful in the treatment of clavicular fractures. Using three‐dimensional (3D) statistical shape analysis, the author presents a novel method to assess geometric morphology of the clavicle. Fifteen fresh frozen shoulder specimens were scanned using high‐resolution computerized tomography (CT) but four were excluded from the study. A further 16 high‐resolution CT scans of the clavicle were obtained by searching the hospital database. All 27 scans were reconstructed and subsequently imported into and analyzed using a specifically developed statistical software package. Using statistical shape analysis, geometric parameters were then measured. Both gender as well as side specific geometric morphology were observed. Clavicles in men were longer, wider, and thicker than in women. Right clavicles had a greater medial depth than left clavicles, especially in women. Clavicles in men had a greater lateral depth than in women. The sternal angle in women was larger than in men. Using 3D statistical shape analysis and applying it to the clavicle standardizes the study of its anatomy, rules out any variability, and calculates morphological parameters that are accurate, precise, and reproducible. This unique approach provides information that is useful not only to the clinician but also in the modification of current or design of future clavicle fixation devices. More importantly, from an anatomy standpoint, implementation of this novel approach in anatomical studies would eliminate intra‐ and interobserver variation and allow all studies to be standardized and thus more comparable. Clin. Anat. 23:199–209, 2010.

Purity and the dangers of regenerative medicine : Regulatory innovation of human tissue-engineered technology

Abstract This paper examines the development of innovation in human tissue technologies as a form of regenerative medicine, firstly by applying ‘pollution ideas’ to contemporary trends in its risk regulation and to the processes of regulatory policy formation, and secondly by analysing the classificatory processes deployed in regulatory policy. The analysis draws upon data from fieldwork and documentary materials with a focus on the UK and EU (2002–05) and explores four arenas: governance and regulatory policy; commercialisation and the market; ‘evidentiality’ manifest in evidence-based policy; and publics’ and technology users’ values and ethics. The analysis suggests that there is a trend toward ‘purification’ across these arenas, both material and socio-political. A common process of partitioning is found in stakeholders’ attempts to define a clear terrain, which the field of tissue-engineered technology might occupy. We conclude that pollution ideas and partitioning processes are useful in understanding regulatory ordering and innovation in the emerging technological zone of human tissue engineering.

Culturing Cells, Reproducing and Regulating the Self

The emergence of a new tissue economy raises issues for the governance of risk and concepts of the body and self. This article explores the development of autologous cell therapies as a form of tissue engineering and considers how and why autologous applications are seen as less risky and more socially and politically acceptable. In a careful analysis of contemporary debates around the need for new international policies to regulate these technologies, we critically assess the discursive strategies employed to support ideas of the body as a natural entity. Central to these debates are assumptions that autologous applications do not threaten the moral or corporeal integrity of the individual and that they are ‘an ethics-free zone’. Analysis reveals that concepts such as intercorporeality need to be refined if they are to assist our understanding of these cell-based therapies. We consider the biopolitics of Autologous Chondrocyte Implantation (ACI) in order to show the linkages between the culturing of cells, regulation and the reproduction of the self.

A two-dimensional model of cyclic strain accumulation in ultra-high molecular weight polyethylene knee replacements

Abstract As new methods of sterilization of the ultra-high molecular weight polyethylene (UHMWPE) component in knee replacements are introduced, reported incidents of delamination will decrease. The prediction of plastic strain accumulation and associated failure mechanisms will then become more important in knee replacement design. The finite element analysis reported in this paper aims to advance the modelling of strain accumulation in UHMWPE over repeated gait cycles and seeks to determine the effects of the knee replacement design variables of geometry and kinematics. Material testing was performed under cyclic and creep conditions to generate the elastic, viscoplastic material model that has been used in this time-dependent analysis. Non-conforming geometries were found to accumulate plastic strains at higher rates than conforming geometries. The anatomical motion known as rollback initially produced lower strain rates, but predictions of the long-term response indicated that designs which allow rollback may produce higher strains than static designs after only about a week of loading for a knee replacement patient.

The use of the Zebris motion analysis system for measuring cervical spine movements in vivo.

Abstract The cervical spine exhibits the greatest range of motion among the spinal segments due to the complex interaction of its triplanar components of movement. As a result, measurement of movements of the cervical spine and of the various orthoses used in cervical spine injuries has proved difficult with no one method proving satisfactory. This paper uses the Zebris ultrasonic three-dimensional motion analysis system to measure flexion, extension, range of lateral bending, and range of axial rotation in five similar male and five similar female subjects with no history of neck injuries. The subjects were tested unrestrained and in soft and hard collars, as well as in Philadelphia, Miami J, and Minerva orthoses. Results show that the Minerva is the most stable construct for restriction of movement in all planes in both groups. Looking at these results allows ranking of the measured orthoses in order of their three-dimensional stability. Furthermore, by presenting reproducible data incorporating the composite triplanar movements of the cervical spine, thus allowing comparative analysis of the studied orthoses, they propose the Zebris as a reliable, repeatable, and safe method of measurement of cervical spine motion with low intersubject variability.

An application of principal component analysis to the clavicle and clavicle fixation devices.

BackgroundPrincipal component analysis (PCA) enables the building of statistical shape models of bones and joints. This has been used in conjunction with computer assisted surgery in the past. However, PCA of the clavicle has not been performed. Using PCA, we present a novel method that examines the major modes of size and three-dimensional shape variation in male and female clavicles and suggests a method of grouping the clavicle into size and shape categories.Materials and methodsTwenty-one high-resolution computerized tomography scans of the clavicle were reconstructed and analyzed using a specifically developed statistical software package. After performing statistical shape analysis, PCA was applied to study the factors that account for anatomical variation.ResultsThe first principal component representing size accounted for 70.5 percent of anatomical variation. The addition of a further three principal components accounted for almost 87 percent. Using statistical shape analysis, clavicles in males have a greater lateral depth and are longer, wider and thicker than in females. However, the sternal angle in females is larger than in males. PCA confirmed these differences between genders but also noted that men exhibit greater variance and classified clavicles into five morphological groups.Discussion And ConclusionsThis unique approach is the first that standardizes a clavicular orientation. It provides information that is useful to both, the biomedical engineer and clinician. Other applications include implant design with regard to modifying current or designing future clavicle fixation devices. Our findings support the need for further development of clavicle fixation devices and the questioning of whether gender-specific devices are necessary.