Long-term durability of alumina ceramic heads in THA

Background: The optimal type of bearing for hip arthroplasty remains a matter of debate. Ceramic-on-polyethylene (CoP) bearings are frequently used in younger and more active patients to reduce wear and increase biocompatibility compared to Metal-on-Polyethylene (MoP) bearings. However, in comparison to metal heads, the fracture risk of ceramic heads is higher. In addition, ceramic head fractures pose a serious complication which often necessitates major revision surgery. To date, there are no long-term data (>20 years of follow-up) reporting fracture rates of the ceramic femoral heads in CoP bearings. The purpose of this research was to investigate long-term CoP fracture rate. Methods: We evaluated the clinical and radiographic results of 348 cementless THAs treated with 2nd generation Biolox® Al2O3 Ceramic-on-Polyethylene (CoP) bearings consecutively implanted between January 1985 and December 1989. The mean age at implantation was 57 years. The patients were followed for a minimum of 20 years. At the final 111 had died, and 5 were lost to follow-up. The cumulative incidence of ceramic head fractures in the long-term was estimated using a competing risk analysis. Results: The cumulative incidence of ceramic head fracture after 22-years was estimated with a competing risk analysis at 0.29% after 22-years (SE = 2.09%; 95% - CI: 0.03-1.5%). The radiographic analysis revealed no impending failures at final follow-up. Discussion/Conclusion: The fracture rate of second-generation ceramic heads using a CoP articulation remains very low into the third decade after cementless THA

A Cortical Attractor Network with Martinotti Cells Driven by Facilitating Synapses

The population of pyramidal cells significantly outnumbers the inhibitory interneurons in the neocortex, while at the same time the diversity of interneuron types is much more pronounced. One acknowledged key role of inhibition is to control the rate and patterning of pyramidal cell firing via negative feedback, but most likely the diversity of inhibitory pathways is matched by a corresponding diversity of functional roles. An important distinguishing feature of cortical interneurons is the variability of the short-term plasticity properties of synapses received from pyramidal cells. The Martinotti cell type has recently come under scrutiny due to the distinctly facilitating nature of the synapses they receive from pyramidal cells. This distinguishes these neurons from basket cells and other inhibitory interneurons typically targeted by depressing synapses. A key aspect of the work reported here has been to pinpoint the role of this variability. We first set out to reproduce quantitatively based on in vitro data the di-synaptic inhibitory microcircuit connecting two pyramidal cells via one or a few Martinotti cells. In a second step, we embedded this microcircuit in a previously developed attractor memory network model of neocortical layers 2/3. This model network demonstrated that basket cells with their characteristic depressing synapses are the first to discharge when the network enters an attractor state and that Martinotti cells respond with a delay, thereby shifting the excitation-inhibition balance and acting to terminate the attractor state. A parameter sensitivity analysis suggested that Martinotti cells might, in fact, play a dominant role in setting the attractor dwell time and thus cortical speed of processing, with cellular adaptation and synaptic depression having a less prominent role than previously thought

Strategies for head and inlay exchange in revision hip arthroplasty

Due to the increasing number of total hip arthroplasties performed during the last three decades and the limited long-term survival, mainly because of wear, the number of revisions has increased during the last two years. If the implant itself is still considered to be stable, only head and inlay exchange is necessary. This requires comprehensive knowledge of the characteristics of the articulating materials by the surgeon as the wrong choice of wear couple can lead to early failure for a second time. The aim of this paper is to present considerations and strategies for head and inlay exchange in case of failure, either due to wear of the articulation material or of other indications for revision hip arthroplasty

Three-dimensional stress analysis of threaded cups – a finite element analysis

A three-dimensional model of the left acetabulum with inserted threaded cup has been generated, based on the finite element method, to calculate stress patterns in the standing phase during walking. In this study, a hemispherical cup with sharp threads, a parabolic cup with flat threads and a conical cup with sharp threads were analysed and compared. Stress patterns in both implant components and adjacent bony structures were calculated in a directly postoperative situation. The different cups were found to induce different stress patterns, deformations and shifting tendencies. The inlays deform notably and show characteristic rotational movement patterns together with the shell. The inclination angle increases in the hemispherical cup and decreases in the parabolic cup. The conical cup levers outward almost parallel to the bone stock by approximately 0.05 mm. The pole surfaces of the various cups – especially the very convex area next to the threads – induce increased compressive stress in the superior section of the acetabular base. This is increased by a factor of three in the conical cup in comparison to the hemispherical cup and less so in comparison to the parabolic cup. This study illustrates that three-dimensional stress calculations are suitable for procuring additional biomechanical information to augment clinical studies, for evaluating implants and for establishing stability prognoses, especially for newly developed prototypes