Influence of cementless hip stems on femoral cortical strain pattern depending on their extent of porous coating

The extent of porous coating of cementless total hip stems is held responsible for radiological periprosthetic changes, the rate of thigh pain, and even its long-term success. However, there is only sparse knowledge on how the biomechanical loading conditions of the femur are influenced by the extent of porous coating in the early phase after implantation of a cementless hip stem. Aiming to evaluate the effect of surface structuring on the strain pattern of the femur, we implanted three anatomic hip stems with different extents of porous coating (full, two-thirds proximal, and penguin type) in second-generation composite femora coated with a photoelastic layer. A cortical strain mapping was conducted before and after insertion of the implants under standardized loading conditions considering relevant muscle forces. The results of the statistical analysis of three different implantation sequences proved that composite femora are suitable for repeated measurements within the applied experimental setup. Cortical strain changes including stress-shielding effects medially (-60%) and laterally (-50%) were validated with a cadaver femur. The extent of porous coating had no significant influence on the surface strain pattern for an immediate postoperative situation

Bone-to-bone and implant-to-bone impingement : a novel graphical representation for hip replacement planning

Bone-to-bone impingement (BTBI) and implant-to-bone impingement (ITBI) risk assessment is generally performed intra-operatively by surgeons, which is entirely subjective and qualitative, and therefore, lead to sub-optimal results and recurrent dislocation in some cases. Therefore, a method was developed for identifying subject-specific BTBI and ITBI, and subsequently, visualising the impingement area on native bone anatomy to highlight where prominent bone should be resected. Activity definitions and subject-specific bone geometries, with planned implants were used as inputs for the method. The ITBI and BTBI boundary and area were automatically identified using ray intersection and region growing algorithm respectively to retain the same ‘conical clearance angle’ obtained to avoid prosthetic impingement (PI). The ITBI and BTBI area was then presented with different colours to highlight the risk of impingement, and importance of resection. A clinical study with five patients after 2 years of THA was performed to validate the method. The results supported the study hypothesis, in that the predicted highest risk area (red coloured zone) was completely/majorly resected during the surgery. Therefore, this method could potentially be used to examine the effect of different pre-operative plans and hip motions on BTBI, ITBI, and PI, and to guide bony resection during THA surgery

Optimal design of composite hip implants using NASA technology

Using an adaptation of NASA software, we have investigated the use of numerical optimization techniques for the shape and material optimization of fiber composite hip implants. The original NASA inhouse codes, were originally developed for the optimization of aerospace structures. The adapted code, which was called OPORIM, couples numerical optimization algorithms with finite element analysis and composite laminate theory to perform design optimization using both shape and material design variables. The external and internal geometry of the implant and the surrounding bone is described with quintic spline curves. This geometric representation is then used to create an equivalent 2-D finite element model of the structure. Using laminate theory and the 3-D geometric information, equivalent stiffnesses are generated for each element of the 2-D finite element model, so that the 3-D stiffness of the structure can be approximated. The geometric information to construct the model of the femur was obtained from a CT scan. A variety of test cases were examined, incorporating several implant constructions and design variable sets. Typically the code was able to produce optimized shape and/or material parameters which substantially reduced stress concentrations in the bone adjacent of the implant. The results indicate that this technology can provide meaningful insight into the design of fiber composite hip implants

Ion beam sputter etching of orthopedic implanted alloy MP35N and resulting effects on fatigue

The effects of two types of argon ion sputter etched surface structures on the tensile stress fatigue properties of orthopedic implant alloy MP35N were investigated. One surface structure was a natural texture resulting from direct bombardment by 1 keV argon ions. The other structure was a pattern of square holes milled into the surface by a 1 keV argon ion beam through a Ni screen mask. The etched surfaces were subjected to tensile stress only in fatigue tests designed to simulate the cyclic load conditions experienced by the stems of artificial hip joint implants. Both types of sputter etched surface structures were found to reduce the fatigue strength below that of smooth surface MP35N

Performance of the resurfaced hip. Part 1: the influence of the prosthesis size and positioning on the remodelling and fracture of the femoral neck

Hip resurfacing is an established treatment for osteoarthritis in young active patients. Failure modes include femoral neck fracture and prosthesis loosening, which may be associated with medium-term bone adaptation, including femoral neck narrowing and densification around the prosthesis stem.Finite element modelling was used to indicate the effects of prosthesis sizing and positioning on the bone remodelling and fracture strength under a range of normal and traumatic loads, with the aim of understanding these failure modes better.The simulations predicted increased superior femoral neck stress shielding in young patients with small prostheses, which required shortening of the femoral neck to give an acceptable implant–bone interface. However, with a larger prosthesis, natural femoral head centre recreation in the implanted state was possible; therefore stress shielding was restricted to the prosthesis interior, and its extent was less sensitive to prosthesis orientation. With valgus orientation, the implanted neck strength was, at worst, within 3 per cent of its intact strength.The study suggests that femoral neck narrowing may be linked to a reduction in the horizontal femoral offset, occurring if the prosthesis is excessively undersized. As such, hip resurfacing should aim to reproduce the natural femoral head centre, and, for valgus prosthesis orientation, to avoid femoral neck fracture

A fast and robust patient specific Finite Element mesh registration technique: application to 60 clinical cases

Finite Element mesh generation remains an important issue for patient specific biomechanical modeling. While some techniques make automatic mesh generation possible, in most cases, manual mesh generation is preferred for better control over the sub-domain representation, element type, layout and refinement that it provides. Yet, this option is time consuming and not suited for intraoperative situations where model generation and computation time is critical. To overcome this problem we propose a fast and automatic mesh generation technique based on the elastic registration of a generic mesh to the specific target organ in conjunction with element regularity and quality correction. This Mesh-Match-and-Repair (MMRep) approach combines control over the mesh structure along with fast and robust meshing capabilities, even in situations where only partial organ geometry is available. The technique was successfully tested on a database of 5 pre-operatively acquired complete femora CT scans, 5 femoral heads partially digitized at intraoperative stage, and 50 CT volumes of patients' heads. The MMRep algorithm succeeded in all 60 cases, yielding for each patient a hex-dominant, Atlas based, Finite Element mesh with submillimetric surface representation accuracy, directly exploitable within a commercial FE software

The effect of boundary constraints on finite element modelling of the human pelvis

The use of finite element analysis (FEA) to investigate the biomechanics of anatomical systems critically relies on the specification of physiologically representative boundary conditions. The biomechanics of the pelvis has been the specific focus of a number of FEA studies previously, but it is also a key aspect in other investigations of, for example, the hip joint or new design of hip prostheses. In those studies, the pelvis has been modelled in a number of ways with a variety of boundary conditions, ranging from a model of the whole pelvic girdle including soft tissue attachments to a model of an isolated hemi-pelvis. The current study constructed a series of FEA models of the same human pelvis to investigate the sensitivity of the predicted stress distributions to the type of boundary conditions applied, in particular to represent the sacro-iliac joint and pubic symphysis. Varying the method of modelling the sacro-iliac joint did not produce significant variations in the stress distribution, however changes to the modelling of the pubic symphysis were observed to have a greater effect on the results. Over-constraint of the symphysis prevented the bending of the pelvis about the greater sciatic notch, and underestimated high stresses within the ilium. However, permitting medio-lateral translation to mimic widening of the pelvis addressed this problem. These findings underline the importance of applying the appropriate boundary conditions to FEA models, and provide guidance on suitable methods of constraining the pelvis when, for example, scan data has not captured the full pelvic girdle. The results also suggest a valid method for performing hemi-pelvic modelling of cadaveric or archaeological remains which are either damaged or incomplete

Development and Implementation of a Computational Modeling Tool for Evaluation of THA Component Position

The human body is a complicated structure with muscles, ligaments, bones, and joints. Modeling human body with computational tools are becoming a trend [1]. More importantly, using computational tools to evaluate human body is a non-invasive technique that could help surgeons and researchers evaluate implant products [2]. Therefore, the development of a model which can analyze both implant sizing suggestion and kinematics of subject specific data could prove valuable. For total hip arthroplasty, one common complication is in vivo separation and dislocation of the femoral head within the acetabular cup [3] [4]. Developing a successful computational tool to address this issue includes developing a dynamic model of hip joint, implementing implant sizing suggestion algorithms and computing component alignments. Due to advancement in technology, the current focus has been to develop patient-specific solutions, a combined program of both hip model and implant suggestion model has been developed. In this dissertation, the primary objective is to develop a fully functional hip analysis software that not only can suggestion and template the implant sizing and position, but the software can also utilize the patient specific data to run simulation with different activities. The second objective of this dissertation is to conduct hip analysis studies using hip analysis software. Overall, the results in this dissertation discuss the effect of different stem positions and surgeon preferences on the outcome of the Total Hip Arthroplasty

Design and testing of additively manufactured lattice structures for musculoskeletal applications

Additive manufacturing (AM) methods present a new frontier in engineering, allowing the fabrication of porous lattice structures with tailored mechanical properties. AM structures can be made using bio-inert metals, creating controlled stiffness biomaterials. As bone formation is strain dependent, these AM biomaterials can be used in implants to optimise the strain in surrounding trabecular bone for peak bone formation. However, the behaviour of AM lattices varies and is subject to manufacturing constraints. The aim of this PhD was to investigate the mechanical behaviour of AM lattices, and maximise the clinical benefits of AM for musculoskeletal applications. Lattice architecture was shown to affect the anisotropy of an AM lattice biomaterial, increasing the stiffness in directions not often tested in the literature. The mechanical and morphological properties of individual struts within powder bed fusion (PBF) lattices were also shown to vary depending on the orientation of the struts to the build direction. The ultimate tensile strength of titanium alloy (Ti6Al4V) struts more than doubled when built at a low angle versus perpendicular to the build platform, and other properties were substantially lower than for the bulk material. Geometric imperfections were found for struts built at low angles. As such, a low stiffness modified stochastic lattice was designed and tested which avoided the problems found with struts built at low angles. The resulting lattice had improved stiffness isotropy and could be used for musculoskeletal applications, tuned to match the mechanical properties in local trabecular bone and enhancing bone formation.Open Acces