The application of physiological loading using a dynamic, multi-axis spine simulator

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.In-vitro testing protocols used for spine studies should replicate the in-vivo load environment as closely as possible. Unconstrained moments are regularly employed to test spinal specimens in-vitro, but applying such loads dynamically using an active six-axis testing system remains a challenge. The aim of this study was to assess the capability of a custom-developed spine simulator to apply dynamic unconstrained moments with an axial preload.Flexion-extension, lateral bending, and axial rotation were applied to an L5/L6 porcine specimen at 0.1 and 0.3. Hz. Non-principal moments and shear forces were minimized using load control. A 500. N axial load was applied prior to tests, and held stationary during testing to assess the effect of rotational motion on axial load.Non-principal loads were minimized to within the load cell noise-floor at 0.1. Hz, and within two-times the load-cell noise-floor in all but two cases at 0.3. Hz. The adoption of position control in axial compression-extension resulted in axial loads with qualitative similarities to in-vivo data.This study successfully applied dynamic, unconstrained moments with a physiological preload using a six-axis control system. Future studies will investigate the application of dynamic load vectors, multi-segment specimens, and assess the effect of injury and degeneration.The authors would like to thank the Higher Education Investment Fund, The Enid Linder Foundation, and the University of Bath Alumni Fund for their support with this study

Dynamic, six-axis stiffness matrix characteristics of the intact intervertebral disc, and a disc replacement

Thorough pre-testing is critical in assessing the likely in vivo performance of spinal devices prior to clinical use. However, there is a lack of data available concerning the dynamic testing of lumbar (porcine model) total disc replacements in all six axes under preload conditions. The aim of this study was to provide new data comparing porcine lumbar spinal specimen stiffness between the intact state and after the implantation of an unconstrained total disc replacement, in 6 degrees of freedom. The dynamic, stiffness matrix testing of six porcine lumbar isolated disc specimens was completed using triangle waves at a test frequency of 0.1 Hz. An axial preload of 500 N was applied during all testing. Specimens were tested both in the intact condition and after the implantation of the total disc replacement. Sixteen key stiffness terms were identified for the comparison of the intact and total disc replacement specimens, comprising the 6 principal stiffness terms and 10 key off-axis stiffness terms. The total disc replacement specimens were significantly different to the intact specimens in 12 of these key terms including all six principal stiffness terms. The implantation of the total disc replacement resulted in a mean reduction in the principal stiffness terms of 100%, 91%, and 98% in lateral bending, flexion–extension, and axial rotation, respectively. The novel findings of this study have demonstrated that the unconstrained, low-friction total disc replacement does not replicate the stiffness of the intact specimens. It is likely that other low-friction total disc replacements would produce similar results due to stiffness being actively minimised as part of the design of low-friction devices, without the introduction of stiffening elements or mechanisms to more accurately replicate the mechanical properties of the natural intervertebral disc. This study has demonstrated, for the first time, a method for the quantitative comparative mechanical function testing of total disc replacements and provides baseline data for the development of future devices. </jats:p

The influence of tibial component malalignment on bone strain in revision total knee replacement.

Revision total knee replacement is a challenging surgical procedure typically associated with significant loss of bone stock in the proximal tibia. To increase the fixation stability, extended stems are frequently used for the tibial component in revision surgery. The design of the tibial stem influences the load transfer from tibial component to the surrounding bone and is cited as a possible cause for the clinically reported pain in the location of the stem-end. This study aimed to analyse the strain distribution of a fully cemented revision tibial component with a validated finite element model. The model was developed from a scanned composite tibia (Sawbones), with an implanted, fully cemented stemmed tibial component aligned to the mechanical axis of the tibia. Loading was applied to the tibial component with mediolateral compartment load distributions of 60:40 and 80:20. Three strain gauged composite tibias with implanted tibial components of the same design using the same loading distribution were tested to obtain experimental strains at five locations in the proximal tibia. The finite element model developed was validated against strain measurements obtained in the experimental study. The strains displayed similar patterns (R2 = 0.988) and magnitudes with those predicted from the finite element model. The displacement of the stem-end from the natural mechanical axis in the finite element model demonstrated increased strains in the stem-end region with a close proximity of the distal stem with the cortical bone. The simulation of a mediolateral compartment load of 80:20 developed peak cortical strain values on the posterior-medial side beneath the stem. This may possibly be related to the clinically reported pain at the stem-end. Furthermore, stem positioning in close proximity or contact with the posterior cortical bone is a contributory factor for an increase in distal strain. </jats:p

Finite element analysis of the tibial component stem orientation in revision total knee replacement

Background: Finite element (FE) models are frequently used in biomechanics to predict the behaviour of new implant designs.To increase the stability after severe bone loss tibial components with long stems are used in revision total knee replacements(TKR). A clinically reported complication after revision surgery is the occurrence of pain in the stem-end region. The aim ofthis analysis was the development of a validated FE-model of a fully cemented implant and to evaluate the effect of differenttibial stem orientations.Methods: A scanned 4th generation synthetic left tibia (Sawbones) was used to develop the FE-model with a virtually implantedfully cemented tibial component. The 500 N load was applied with medial:lateral compartment distributions of 60:40 and 80:20.Different stem positons were simulated by modifying the resection surface angle posterior to the tibias shaft axis. The resultswere compared with an experimental study which used strain gauges on Sawbones tibias with an implanted tibial TKRcomponent. The locations of the experimental strain gauges were modelled in the FE study.Results: Similar patterns and magnitudes of the predicted and experimentally measured strains were observed which validatedthe FE-model. An increase of strain at the most distal gauge locations were measured with the stem-end in contact to theposterior cortical bone. More uniform strain distributions were observed with the stem aligned to the intramedullary canal axis.The load distribution of 80:20 shifts the strains to tensile laterally and a large increase of compressive strain in the medialdistal tibia.Conclusions: A contributory factor of the clinically reported stem-end pain is possibly the direct effect of contact of the tibialstem-end to the posterior region of the cortical bone. The increased load to the medial tibial compartment is more critical forthe development of pain

Antibiotic additives alter the static and viscoelastic properties of bone cements

Introduction: In arthroplasty antibiotics are added to bone cements to prevent deep infection. The static properties of plain and antibiotic laden cements have been extensively described in the literature [1]. Commercially available cements must perform above the minimum values set by ISO 5833:2002 [2]. However, no upper or lower limits are set for the viscoelastic properties of the cements, despite this being a recognised factor affecting the cement-implant performance [3]. The ability of acrylic bone cement to creep and stress relax in conjunction with forceclosed stems in hip arthroplasty affords protection of the vital bone-cement interface. With this design subsidence of the stem within the cement mantle over time does not lead to clinical failure [4]. Conversely, the clinical performance of shapeclosed stem designs can be negatively affected by cements demonstrating excessive creep. This study investigated the effect of antibiotic additives on the static and viscoelastic properties of PMMA cement. Materials and Methods: The mechanical and viscoelastic properties of Simplex P, Simplex Antibiotic and Simplex Tobramycin (Stryker, Limerick, IE) were investigated. This family of cements was chosen as they are characterised by the same polymeric base, that of Simplex P, the plain formulation, but contain different antibiotic additives. In particular Simplex Antibiotic contains 0.5g Erythromycin and 3 million I.U. Colistin, while in Simplex Tobramycin the only additive is 0.5g of Tobramycin. The static properties of the cements (compressive strength, bending strength and bending modulus) were assessed following ISO 5833:2002 [2], while stress relaxation and creep were assessed under quasi static conditions in a four pointbending configuration. The creep experiments were carried out using a custom made apparatus with the specimens positioned in a distilled water bath at 37 o