RESUMO
The dynamic orientation of total hip replacement acetabular cups during walking may vary substantially from their assumed position at surgical implantation and may vary between individuals. The scale of this effect is of interest for both pre-clinical device testing and for pre-operative surgical planning. This work aimed to evaluate (1) patient variation in dynamic cup orientation; (2) whether walking speed was a candidate proxy measure for the dynamic cup orientation; and (3) the relationships between dynamic cup orientation angles and planar pelvic angles. Pelvic movement data for patients with fast (20 patients) and slow (19 patients) self-selected walking speeds were used to calculate acetabular cup inclination and version angles through gait. For aim 1, the range and extremes of acetabular cup orientation angles were analysed for all patients. A large patient-to-patient variation was found in the ranges of both inclination angle (1° to 11°) and version angle (4° to 18°). The version angle was typically retroverted in comparison to the implantation position (greatest deviation 27°). This orientation is substantially different to the static, 0° version, simplifying assumptions in pre-clinical 'edge loading' testing. For aim 2, the cup orientation angles were compared between the fast- and slow-walking groups using statistical parametric mapping. The only significant differences observed were for cup version angle, during ~12% of the gait cycle before toe-off (p < 0.05). Therefore, self-selected walking speed, in isolation, is not a sufficient proxy measure for dynamic acetabular orientation. For aim 3, correlations were recorded between the acetabular cup orientation angles and the planar pelvic angles. The cup inclination angle during gait was strongly correlated (Spearman's coefficient -1) with pelvic obliquity alone, indicating that simple planar assessment could be used to anticipate inclination angle range. The cup version angle was correlated with both pelvic rotation and tilt (Spearman's coefficient 0.8-1), indicating that cup version cannot be predicted directly from any single pelvic movement. This complexity, along with the interaction between inclination angle and range of version angle, supports the use of computational tools to aid clinical understanding.
RESUMO
BACKGROUND: Up until 2017, medical devices were placed on the European Union's (EU) single market in accordance with either Medical Device Directive 93/42/EEC for general medical devices or Medical Device Directive 90/385/EEC for active implantable devices. However, some devices that complied with these directives still failed catastrophically. In the orthopaedic device field, these failures were most pronounced in metal-on-metal hip devices causing severe patient morbidity with increased need for revision surgery which had unpredictable outcomes. Subsequently, the newly introduced Medical Device Regulations 2017/745 are aimed at addressing patient safety based on previous experience and thorough device assessment prior to and post-release on the EU single market; to accommodate for this they are substantially different (and more stringent). This poses a greater challenge for manufacturers and regulatory bodies in terms of time and resources. METHODS: A review of the EU directives and published literature was undertaken. This review provides the rationale behind this change and its potential impact on research, industry, and clinical practice. DISCUSSION: The change in legal requirements for the medical devices to be put on the EU single market ultimately leads to increased patient safety, which is supported by clinical professionals. The new requirements for data transparency, post-market surveillance, and implant information availability increase the chance of catastrophic failure prevention. However, the exact method of implementation remains uncertain, and some essential rules on the data requirements for compliance have not yet been published by the EU. These limitations may limit the availability of products on the market including withdrawal of existing devices and a decrease in new medical device innovation. It is speculated that lack of new technologies within the medical device area can dramatically affect patient safety itself by not allowing potentially safer materials and methods on the EU single market, as the focus for the manufacturer becomes existing devices.
RESUMO
The complex motion and geometry of the spine in the cervical region makes it difficult to determine how loads are distributed through adjacent vertebrae or between the zygapophysial (facet) joints and the intervertebral disc. Validated finite element modes can give insight on this distribution. The aim of this contribution was to produce direct validation of subject-specific finite element models of Functional Spinal Units (FSU׳s) of the cervical spine and to evaluate the importance of including fibre directionality in the mechanical description of the annulus fibrosus. Eight specimens of cervical FSU׳s were prepared from five ovine spines and mechanically tested in axial compression monitoring overall load and displacements as well as local facet joints pressure and displacement. Subject-specific finite element models were produced from microCT image data reproducing the experimental setup and measuring global axial force and displacement as well as local facet joints displacement and contact forces. Material models and parameters were taken from the literature, testing isotropic and anisotropic materials for the annulus fibrosus. The validated models showed that adding the direction of the fibres to their non-linear behaviour in the description of the annulus fibrosus improves the predictions at large strain values but not at low strain values. The load transferred through the facet joints was always accurate, irrespective of the annulus material model, while the predicted facet displacement was larger than the measured one but not significantly. This is, to the authors׳ knowledge, the first subject-specific direct validation study on a group of specimens, accounting for inter-subject variability.
