Standardized loads acting in knee implants

The loads acting in knee joints must be known for improving joint replacement, surgical procedures, physiotherapy, biomechanical computer simulations, and to advise patients with osteoarthritis or fractures about what activities to avoid. Such data would also allow verification of test standards for knee implants. This work analyzes data from 8 subjects with instrumented knee implants, which allowed measuring the contact forces and moments acting in the joint. The implants were powered inductively and the loads transmitted at radio frequency. The time courses of forces and moments during walking, stair climbing, and 6 more activities were averaged for subjects with I) average body weight and average load levels and II) high body weight and high load levels. During all investigated activities except jogging, the high force levels reached 3,372–4,218N. During slow jogging, they were up to 5,165N. The peak torque around the implant stem during walking was 10.5 Nm, which was higher than during all other activities including jogging. The transverse forces and the moments varied greatly between the subjects, especially during non-cyclic activities. The high load levels measured were mostly above those defined in the wear test ISO 14243. The loads defined in the ISO test standard should be adapted to the levels reported here. The new data will allow realistic investigations and improvements of joint replacement, surgical procedures for tendon repair, treatment of fractures, and others. Computer models of the load conditions in the lower extremities will become more realistic if the new data is used as a gold standard. However, due to the extreme individual variations of some load components, even the reported average load profiles can most likely not explain every failure of an implant or a surgical procedure

Is there a mechanical regulation of bone formation in spinal fusion?

Introduction Lumbar interbody fusion using cages is one of the most reliable treatment options for degenerative spinal diseases. Currently, many cage designs are available in the market; however none of them is completely successful, as reflected by non-union rates ranging from 7 to 30%. Cages are made of very different materials (e.g. metals, polymers) and they present a large range of morphological configurations (e.g. solid, ring), leading to distinct mechanical conditions within the fusion region. Mechanical conditions are known to largely influence bone regeneration [Klein, 2003] in long bones, however their role on spinal fusion remains largely unknown. The aim of this study was to investigate how the local mechanical conditions (strains, stress, fluid flow) created by different cage designs might influence bone tissue formation during the spinal fusion process. Methods We developed an iterative computer model to simulate the time course of tissue formation during spinal fusion. The model included the vertebral bodies, the intervertebral space and a spinal cage (Fig. 1). In each time step, tissue formation was regulated by the local mechanical conditions [Prendergast, 1997] within the regenerating region, determined using finite element techniques. The temporal and spatial evolution of tissue formation was investigated for two different cage designs (a solid and a ring cage) and two different levels of stiffness: 1 (soft) and 100 (stiff) GPa. Results Bone formation and maturation started in the most inner region of the intervertebral space and extended over time to the outer region, forming a defined callus shape (Fig. 1a). Model predictions showed a strong influence of cage stiffness and configuration on the fusion outcome (Fig 1b-e). A softer cage showed a more favourable mechanical stimulation for the regeneration of bone, leading to higher amounts of bone tissue formation. For the stiffer cage, better fusion outcome was predicted with a centered solid cage compared to a ring cage (Fig. 1c & e). Stress shielding was observed in the central hollow region of the ring cage, which was more pronounced for the stiffer cage (Fig. 1c). Discussion Mechanical conditions have an influence on bone regeneration. We investigated the effect of the mechanical environment created by different cage designs on the fusion outcome. We observed that cage design, both morphology and material properties, play a key role in the mechanical conditions within the fusion region and therefore the time course of the fusion process. In the future such understanding may be used to optimize the design of spinal fusion implants to guide and foster bone formation and inter-body fusion. References [1] Klein et al., J Orthop Res, 21: 662–669, 2003. [2] Prendergast et al., J Biomech, 30, 539-548, 1997.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Rheology of phosphonium ionic liquids: a molecular dynamics and experimental study

Comparison between the theoretical and experimental viscosity of an ionic liquid.</p

An advanced 3D multi-body system model for the human lumbar spine

Series : Mechanisms and machine science, ISSN 2211-0984, vol. 24A novel 3D multi-body system model of the human lumbar spine is presented, allowing the dynamic study of the all set but also to access mechanical demands, characteristics and performance under work of the individual intervertebral discs. An advanced FEM analysis was used for the most precise characterization of the disc 6DOF mechanical behavior, in order to build up a tool capable of predicting and assist in the design of disc recovery strategies – namely in the development of replace-ment materials for the degenerated disc nucleus – as well as in the analysis of variations in the me-chanical properties (disorders) at disc level or kinematic structure (e.g. interbody fusion, pedicle fixa-tion, etc.), and its influence in the overall spine dynamics and at motion segments individual level. Preliminary results of the model, at different levels of its development, are presented

A Study of 323 Asymptomatic Volunteers

Background The understanding of the individual shape and mobility of the lumbar spine are key factors for the prevention and treatment of low back pain. The influence of age and sex on the total lumbar lordosis and the range of motion as well as on different lumbar sub-regions (lower, middle and upper lordosis) in asymptomatic subjects still merits discussion, since it is essential for patient-specific treatment and evidence-based distinction between painful degenerative pathologies and asymptomatic aging. Methods and Findings A novel non-invasive measuring system was used to assess the total and local lumbar shape and its mobility of 323 asymptomatic volunteers (age: 20–75 yrs; BMI <26.0 kg/m2; males/females: 139/184). The lumbar lordosis for standing and the range of motion for maximal upper body flexion (RoF) and extension (RoE) were determined. The total lordosis was significantly reduced by approximately 20%, the RoF by 12% and the RoE by 31% in the oldest (>50 yrs) compared to the youngest age cohort (20–29 yrs). Locally, these decreases mostly occurred in the middle part of the lordosis and less towards the lumbo- sacral and thoraco-lumbar transitions. The sex only affected the RoE. Conclusions During aging, the lower lumbar spine retains its lordosis and mobility, whereas the middle part flattens and becomes less mobile. These findings lay the ground for a better understanding of the incidence of level- and age-dependent spinal disorders, and may have important implications for the clinical long-term success of different surgical interventions

Total disc replacement for chronic back pain in the presence of disc degeneration

Scientific Assessment and Innovation in Neurosurgical Treatment Strategie