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In total hip arthroplasty, micromotions at the implant-bone interface influence the long-term survival of the prosthesis. These micromotions are often measured using sensors that are fixed to the implant and bone at points that are remote from the interface. Given that the implant-bone system is not rigid, errors may be introduced. It is not possible to assess the magnitude of these errors with the currently available experimental methods. However, this problem can be investigated using the finite element method (FEM). The hypothesis that the actual interface micromotions differ from those measured in the experimental manner was tested using a case-specific FE model, validated against deflection experiments. The FE model was used to simulate an 'experimental' method to measure micromotions. This 'experimental' method was performed by mimicking the distance between the measurement points; the implant point was selected at the interface while the bony point was at the outer surface of bone. No correlation was found between the micromotions computed at the interface and when using remote reference points. Moreover, the magnitudes of micromotions computed with the latter method were considerably greater. By reducing the distance between the reference points the error decreased, but the correlation stayed unchanged. Care needs to be taken when interpreting the results of micromotion measurement systems that use bony reference points at a distance from the actual interface. [ABSTRACT FROM AUTHOR]

This study assessed whether the Symax™ implant, a modification of the Omnifit(®) stem (in terms of shape, proximal coating and distal surface treatment), would yield improved bone remodelling in a clinical DEXA study, and if these results could be predicted in a finite element (FE) simulation study. In a randomized clinical trial, 2 year DEXA measurements between the uncemented Symax™ and Omnifit(®) stem (both n=25) showed bone mineral density (BMD) loss in Gruen zone 7 of 14% and 20%, respectively (p<0.05). In contrast, the FE models predicted a 28% (Symax™) and 26% (Omnifit(®)) bone loss. When the distal treatment to the Symax™ was not modelled in the simulation, bone loss of 35% was predicted, suggesting the benefit of this surface treatment for proximal bone maintenance. The theoretical concept for enhanced proximal bone loading by the Symax™, and the predicted remodelling pattern were confirmed by DEXA-results, but there was no quantitative match between clinical and FE findings. This was due to a simulation based on incomplete assumptions concerning the yet unknown biological and mechanical effects of the new coating and surface treatment. Study listed under ClinicalTrials.gov with number NCT01695213., (Copyright © 2013 IPEM. Published by Elsevier Ltd. All rights reserved.)

In cementless total hip arthroplasty, long-term implant stability is achieved by bone ingrowth. The strength of the new bond gradually increases in time, due to bone maturation and progression of ingrowth. In finite element simulations, osseointegration generally is implemented as an instant change in the mechanical behavior of the implant-bone interface, although this is a simplified interpretation of the bone ingrowth process. The aim of the present study was to build on previous bone ingrowth simulations and propose a new methodology to simulate bone ingrowth as a time-dependent process. We developed an algorithm to calculate the strength of the local implant-bone bond based of the magnitude of interface micromotions and gaps in time. Our algorithm was subsequently tested in multiple hip reconstructions in which the bone quality and implant-bone contact area were varied. The results of the simulations showed that in the ideal situation (good bone quality and no interface gaps), 91% of implant area could achieve ingrowth, while in the worst case only 17% of implant area showed ingrowth. The initial contact area had a significant effect on ingrowth, overruling the effect of variations in bone quality. The progression of ingrowth had a stabilizing effect on adjacent regions, especially in the high contact area cases. Further development and validation of the presented algorithm requires more information on the nature of the relation between the ingrowth rate and the magnitude of micromotions and gap., (Copyright © 2012 IPEM. Published by Elsevier Ltd. All rights reserved.)

The finite element (FE) method has become a common tool to evaluate peri-prosthetic micromotions in cementless total hip arthroplasty. Often, only the peak joint load and a selected number of muscle loads are applied to determine micromotions. Furthermore, the applied external constraints are simplified (diaphyseal fixation), resulting in a non-physiological situation. In this study, a scaled musculoskeletal model was used to extract a full set of muscle and hip joint loads occurring during a walking cycle. These loads were applied incrementally to an FE model to analyze micromotions. The relation between micromotions and external loads was investigated, and how micromotions during a full loading cycle compared to those calculated when applying a peak load only. Finally, the effect of external constraints was analyzed (full model vs. diaphyseal fixation and reduced number of muscle loads). Relatively large micromotions were found during the swing phase when the hip joint forces were relatively low. Maximal micromotions, however, did concur with the peak hip joint force. Applying only a peak joint force resulted in peak micromotions similar to those found when full walking cycle loads were applied. The magnitude and direction of the micromotions depended on the applied muscle loads, but not on external constraints., (Copyright © 2012 Orthopaedic Research Society.)

Study Design: Mechanical testing of cadaveric tissues., Objective: To compare the strength of discs and vertebrae from the same spines in order to determine which are more vulnerable to injury, and to determine how their relative vulnerability depends on age and gender., Summary of Background Data: Vertebrae can strengthen and weaken according to mechanical demands, but the avascular intervertebral discs may be unable to "keep up." Little is known about the relative strength of discs and vertebrae., Methods: Forty-seven thoracolumbar motion segments were obtained from 30 cadavers 48 to 91 years of age. Each was compressed until a vertebra fractured, and vertebral yield compressive stress (force per unit area) was calculated. Adjacent undamaged intervertebral discs were removed, and circumferential slices, 2.2 mm thick, were cut from the inner, middle, and outer regions of the anterolateral anulus. Slices were stretched to failure to determine their ultimate tensile stress., Results: Yield compressive stress of male and female vertebrae decreased by 69% and 75%, respectively, in the age range of 48 to 91 years (P < 0.001). In contrast, the ultimate tensile stress of the adjacent anulus did not fall significantly with age, except in the outer region of male discs, where it fell by 66% (P < 0.01). Disc strength was proportional to vertebral strength, but only for the outer anulus, and in male spines (r2= 24%, P = 0.019, n = 22)., Conclusion: The outer anulus can adapt to mechanical demands because it is the most metabolically active region of the disc. Disc and bone properties are better matched in male spines because male vertebrae are less affected by variable hormonal changes. The low adaptive potential of intervertebral discs makes them relatively weak in the strengthening spines of young men but relatively strong in the weakening spines of elderly women.