In order to design a good cementless femoral implant many requirements need to be fulfilled. For instance, the range of micromotions at the bone-implant interface should not exceed a certain threshold and a good ratio between implant-bone stiffness that does not cause bone resorption, needs to be ensured. Stiff implants are known to evoke lower interface micromotions but at the same time they may cause extensive resorption of the surrounding bone. Composite stems with reduced stiffness give good remodeling results but implant flexibility is likely to evoke high micromotions proximally. Finding a good balance between these incompatible design goals is very challenging. The current study proposes a finite element methodology that employs subsequent ingrowth and remodeling simulations and can be of assistance when designing new implants. The results of our simulations for the Epoch stem were in a good agreement with the clinical data. The proposed implant design made of porous tantalum with an inner CoCrMo core performed slightly better with respect to the Epoch stem and considerably better with respect to a Ti alloy stem. Our combined ingrowth and remodeling simulation can be a useful tool when designing a new implant that well balances mentioned incompatible design goals.
"This method has been applied to femoral implants in THR (Kerner et al 1999; Turner et al 2005), RHR (Gupta et al 2006; Pal et al 2009; Rothstock et al 2011; Dickinson et al 2012; Perez et al 2014), to acetabular cups (Ghosh et al 2013), and in other joints (van Lenthe et al 1997). Advanced approaches have combined strain adaptive bone remodelling with other associated mechanobiological processes, including cementless implant ingrowth (Tarala et al 2011) and periprosthetic defect healing (Dickinson et al 2012). "
[Show abstract][Hide abstract] ABSTRACT: Bone morphology and density changes are commonly observed following joint replacement, may contribute to the risks of implant loosening and periprosthetic fracture and reduce the available bone stock for revision surgery. This study was presented in the 'Bone and Cartilage Mechanobiology across the scales' WCCM symposium to review the development of remodelling prediction methods and to demonstrate simulation of adaptive bone remodelling around hip replacement femoral components, incorporating intrinsic (prosthesis) and extrinsic (activity and loading) factors. An iterative bone remodelling process was applied to finite element models of a femur implanted with a cementless total hip replacement (THR) and a hip resurfacing implant. Previously developed for a cemented THR implant, this modified process enabled the influence of pre- to post-operative changes in patient activity and joint loading to be evaluated. A control algorithm used identical pre- and post-operative conditions, and the predicted extents and temporal trends of remodelling were measured by generating virtual X-rays and DXA scans. The modified process improved qualitative and quantitative remodelling predictions for both the cementless THR and resurfacing implants, but demonstrated the sensitivity to DXA scan region definition and appropriate implant-bone position and sizing. Predicted remodelling in the intact femur in response to changed activity and loading demonstrated that in this simplified model, although the influence of the extrinsic effects were important, the mechanics of implantation were dominant. This study supports the application of predictive bone remodelling as one element in the range of physical and computational studies, which should be conducted in the preclinical evaluation of new prostheses.
Biomechanics and Modeling in Mechanobiology 07/2015; DOI:10.1007/s10237-015-0678-9 · 3.15 Impact Factor
"We used the strain adaptive remodelling theory to simulate changes in bone mineral density in time (d/dt) . The size of 'dead zone' and computer time unit were determined in our previous remodelling study in which we utilized the same bone model . In that study the FE remodelling prediction around the EPOCH FullCoat stem was fitted to 2 year clinical DEXA data in order to define the adequate 'dead zone' and to determine the time unit in the simulation . "
[Show abstract][Hide abstract] ABSTRACT: 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.
"Micromotion can be estimated by numerical analyses or by a multitude of methods, involving in vitro measurements (Baleani et al., 2000; Buhler et al., 1997; Gortchacow et al., 2011; Gortz et al., 2002; Kassi et al., 2005; Nogler et al., 2004; Tarala et al., 2011). Experimental studies have found that excessive micromotion can compromise or inhibit the biological integration of bone at the implant surface (Engh et al., 1999; Jasty et al., 1997; McKellop et al., 1991; Pilliar et al., 1986; Soballe et al., 1992b), however the exact range of motion that will allow osseointegration is not known. "
[Show abstract][Hide abstract] ABSTRACT: Background
Uncemented implants are dependent upon initial postoperative stability to gain bone ingrowth and secondary stability. The possibility to vary femoral offset and neck angles using modular necks in total hip arthroplasty increases the flexibility in the reconstruction of the geometry of the hip joint. The purpose of this study was to investigate and evaluate initial stability of an uncemented stem coupled to four different modular necks.
A cementless femoral stem was implanted in twelve human cadaver femurs and tested in a hip simulator with patient specific load for each patient corresponding to single leg stance and stair climbing activity. The stems were tested with four different modular necks; long, short, retro and varus. The long neck was used as reference in statistical comparisons. A micromotion jig was used to measure bone-implant movements, at two predefined levels.
A femoral stem coupled to a varus neck had the highest value of micromotion measured for stair climbing at the distal measurement level (60 μm). The micromotions measured with varus and retro necks were significantly larger than motions observed with the reference modular neck, p < 0.001.
The femoral stem evaluated in this study showed acceptable micromotion values for the investigated loading conditions when coupled to modular necks with different length, version and neck-shaft angle.
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