J.A. Steiner

The different contributions of cortical and trabecular bone to implant anchorage in a human vertebra.

The quality of the peri-implant bone and the strength of the bone-implant interface are important factors for implant anchorage. With regard to peri-implant bone, cortical and trabecular compartments both contribute to the load transfer from the implant to the surrounding bone but their relative roles have yet to be investigated in detail. However, this knowledge is crucial for the better understanding of implant failure and for the development of new implants. This is especially true for osteoporotic bone, which is characterized by a deterioration of the trabecular architecture and a thinning of the cortical shell, leading to a higher probability of implant loosening. The aim of this study was to investigate the relative biomechanical roles of cortical and trabecular bone on implant pull-out stiffness in human vertebrae. The starting point of our investigation was a micro-computed tomography scan of an adult human vertebra. The cortical shell was identified and an implant was digitally inserted into the vertebral body. Pull-out tests were simulated with micro-finite element analysis and the apparent stiffness of the system with various degrees of shell thickness and bone volume fraction was computed. Our computational models demonstrated that cortical bone, although being very thin, plays a major role in the mechanical competence of the bone-implant construct.

Computational analysis of primary implant stability in trabecular bone

Secure fixation of fractured osteoporotic bone is a serious clinical challenge mainly because the reduced mechanical competence of low-density bone hampers proper implant fixation. Recent experimental findings have shown strong evidence for a rather complex bone-implant interface contact behavior, with frictional and non-linear mechanical properties. Furthermore, the bone microarchitecture is highly diverse even within the same anatomical site of a specific individual. Due to this intrinsic variability experimental studies that could analyze in detail the contributions of screw designs and thread geometry would require a very large amount of bone specimens; this hampers finding potential improvements for implant fixation. As a complementary approach, computational methods may overcome this limitation, since the same specimen can be tested repeatedly in numerous configurations and under various loading conditions. Recent advances in imaging techniques combined with parallel computing methods have enabled the creation of high-resolution finite-element models that are able to represent bone-implant systems in great detail. Yet, the predictive power of the mechanical competence of bone-implant systems is still limited, both on the apparent level and on the local microstructural level. The current strategy in high-resolution FE models to model the bone-implant interface, employing fully bonded cube-like elements, needs to be reconsidered, refined and validated, such that it mimics more closely the actual non-linear mechanical behavior as observed in vitro in order to exploit the full potential of numeric models as an effective, complementary research method to physical in vitro models.

Screw insertion in trabecular bone causes peri-implant bone damage

Secure fracture fixation is still a major challenge in orthopedic surgery, especially in osteoporotic bone. While numerous studies have investigated the effect of implant loading on the peri-implant bone after screw insertion, less focus has been put on bone damage that may occur due to the screw insertion process itself. Therefore, the aim of this study was to localize and quantify peri-implant bone damage caused by screw insertion. We used non-invasive three-dimensional micro-computed tomography to scan twenty human femoral bone cores before and after screw insertion. After image registration of the pre- and post-insertion scans, changes in the bone micro-architecture were identified and quantified. This procedure was performed for screws with a small thread size of 0.3mm (STS, N=10) and large thread size of 0.6mm (LTS, N=10). Most bone damage occurred within a 0.3mm radial distance of the screws. Further bone damage was observed up to 0.6mm and 0.9 mm radial distance from the screw, for the STS and LTS groups, respectively. While a similar amount of bone damage was found within a 0.3mm radial distance for the two screw groups, there was significantly more bone damage for the LTS group than the STS group in volumes of interest between 0.3-0.6mm and 0.6-0.9 mm. In conclusion, this is the first study to localize and quantify peri-implant bone damage caused by screw insertion based on a non-invasive, three-dimensional, micro-CT imaging technique. We demonstrated that peri-implant bone damage already occurs during screw insertion. This should be taken into consideration to further improve primary implant stability, especially in low quality osteoporotic bone. We believe that this technique could be a promising method to assess more systematically the effect of peri-implant bone damage on primary implant stability. Furthermore, including peri-implant bone damage due to screw insertion into patient-specific in silico models of implant-bone systems could improve the accuracy of these models.

A novel in silico method to quantify primary stability of screws in trabecular bone

Insufficient primary stability of screws in bone leads to screw loosening and failure. Unlike conventional continuum finite‐element models, micro‐CT based finite‐element analysis (micro‐FE) is capable of capturing the patient‐specific bone micro‐architecture, providing accurate estimates of bone stiffness. However, such in silico models for screws in bone highly overestimate the apparent stiffness. We hypothesized that a more accurate prediction of primary implant stability of screws in bone is possible by considering insertion‐related bone damage. We assessed two different screw types and loading scenarios in 20 trabecular bone specimens extracted from 12 cadaveric human femoral heads (N = 5 for each case). In the micro‐FE model, we predicted specimen‐specific Youngs moduli of the peri‐implant bone damage region based on morphometric parameters such that the apparent stiffness of each in silico model matched the experimentally measured stiffness of the corresponding in vitro specimen as closely as possible. The standard micro‐FE models assuming perfectly intact peri‐implant bone overestimated the stiffness by over 330%. The consideration of insertion related damaged peri‐implant bone corrected the mean absolute percentage error down to 11.4% for both loading scenarios and screw types. Cross‐validation revealed a mean absolute percentage error of 14.2%. We present the validation of a novel micro‐FE modeling technique to quantify the apparent stiffness of screws in trabecular bone. While the standard micro‐FE model overestimated the bone‐implant stiffness, the consideration of insertion‐related bone damage was crucial for an accurate stiffness prediction. This approach provides an important step toward more accurate specimen‐specific micro‐FE models.

Patient-specific in silico models can quantify primary implant stability in elderly human bone

Secure implant fixation is challenging in osteoporotic bone. Due to the high variability in inter‐ and intra‐patient bone quality, ex vivo mechanical testing of implants in bone is very material‐ and time‐consuming. Alternatively, in silico models could substantially reduce costs and speed up the design of novel implants if they had the capability to capture the intricate bone microstructure. Therefore, the aim of this study was to validate a micro‐finite element model of a multi‐screw fracture fixation system. Eight human cadaveric humerii were scanned using micro‐CT and mechanically tested to quantify bone stiffness. Osteotomy and fracture fixation were performed, followed by mechanical testing to quantify displacements at 12 different locations on the instrumented bone. For each experimental case, a micro‐finite element model was created. From the micro‐finite element analyses of the intact model, the patient‐specific bone tissue modulus was determined such that the simulated apparent stiffness matched the measured stiffness of the intact bone. Similarly, the tissue modulus of a small damage region around each screw was determined for the instrumented bone. For validation, all in silico models were rerun using averaged material properties, resulting in an average coefficient of determination of 0.89 ± 0.04 with a slope of 0.93 ± 0.19 and a mean absolute error of 43 ± 10 μm when correlating in silico marker displacements with the ex vivo test. In conclusion, we validated a patient‐specific computer model of an entire organ bone‐implant system at the tissue‐level at high resolution with excellent overall accuracy.