femur. In particular, 3D FE models for an intact femur and a femur implanted with a cementless prosthesis were constructed, and the proximal strains recorded for two loading conditions approximating a one-legged stance: (i) a load applied to the head and an abducting force applied to the greater trochanter; (ii) only a load applied to the head. In modelling the femur, cancellous bone was excluded for simplification.
A press-fitted and a fully bonded bone-prosthesis structure were investigated to identify the stem-bone behaviour for both interface conditions and their implications for proximal bone load transfer. Even if the implemented model included some simplifications, authors found excellent correlation between the experimental test strains and predicted strains in FE model for both loading conditions, so they were able to validate their model.
An important field of application of total hip replacement FE models is the preclinical validation of the implant and the study of its primary stability. Viceconti et al. (2001) implemented a complete FE model of bone-implant complex, with the aim to conduct the pre-clinical validation of a new cemented femoral prosthesis – named cement-locked uncemented prosthesis – by using a synergic combination of experimental and numerical methods. The geometry of the femur was created from reconstruction of CT images; materials properties of stem (titanium alloy), cement mantle and femur were modelled as homogenous, isotropic and linear elastic;
complete model consisted of 6,347 elements. Two load cases were simulated, a heel strike and a pure torsion. The model was validated against in vitro measurements of bone surface strains as well as against primary stability measurements. Authors focussed on the analysis of micro-movements at cement interface but they also observed that under heel strike loading case the cement was more stressed than under the pure torsion load case. As a main result, the FE analysis predicted, for the new implant device tested, a peak stress in the cement mantle not significantly lower than those reported in other studies on usual fully cemented stems. Even if this study represents only one application of preclinical validation procedure, it gives a clear evidence on the importance of it, in particular when innovative devices must be tested before being implanted.
In the same year and with a similar aim, Stolk et al. (2001) used FE analysis to investigate which muscle groups acting around the hip-joint most affect the load distributions in cemented total hip reconstructions, with a bonded and a debonded femoral stem. The final goal was to determine which muscle groups should be included in pre-clinical tests. The geometry of the femur was obtained from CT scans of the proximal part of a human femur; the cement mantle had a minimal thickness of 2 mm; both bonded and debonded stem were simulated; 19 muscle forces were included in the model and a hip joint force was applied to the centre of the prosthetic head. They considered force magnitudes throughout the gait cycle scaled to a body weight of 735 N. The model consisted of 2,226 brick elements.
Results suggest that a loading configuration including the hip-joint contact force and the abductor forces can adequately reproduce in vivo loading of cemented total hip reconstructions in pre-clinical test.
Pancanti et al. (2003) investigated the primary stability of a cementless stem implementing a FE model of a proximal part of human femur with a cementless
stem and without considering muscle forces in the model. Muscle forces were not considered because the aim of the simulation was limited to predict the peak micro- movements over the entire bone-implant surface in four patients performing nine different tasks. The model consisted of 9,349 elements and frictional contact was modelled at the bone–implant interface. Among all patients, the largest value of micro-motion, 107mm, was predicted for a patient performing the stair-climbing task. Interestingly, the distribution of micro-motion seemed not to only depend on the bone-implant geometry but also on how each subject performed the motor task; thus, in the context of pre-clinical validation, the implemented FEA may help to investigate the sensitivity of measurements to the inter-subject variability.
Andreaus et al. (2008), Andreaus and Colloca (2009) conducted a similar study based on a 3D FE model of a physiological and a prosthetic human femur obtained by CT images; a CAD software application was then used to implement an intervention of virtual surgery. The physiological model consisted of 25,637 elements, while the prosthetic model was made of 77,219 elements. Loading was simplified by grouping the action of functionally similar hip muscles. An instant of maximum muscular activity and high joint contact force was then selected for the activities of walking and stair climbing. Numerical simulations – conducted with Comsol 3.3 – showed dramatic variations of maximum and minimum value of stress and strain energy density when comparing walking with stair climbing, even though the spatial locations on the model remained unchanged. Stair climbing was confirmed to be a critical task for primary stability of the prosthetic femur.
Complete models of femur with a prosthesis inside are also used to study the fatigue and fracture of the cement mantle of cemented hip implants in more realistic configurations which take into account for more than only one sample of specific material. Grasa et al. (2005) proposed a new model to predict the reliability of fatigue function of the cement by using a stochastic methodology which allowed to analyse the damage accumulation. The FE model consisted of only proximal part of the femur, made of 11,647 hexahedral bilinear elements; cement-bone interface and cement-stem interface were completely bonded. As for loading conditions, forces developed during walking and stair climbing were taken into account, also including forces acting on the prosthetic head and muscle forces. Analyses were performed with Abaqus v.6.3. After 25 million of loading cycles, a higher failure probability was observed for the specific examined stem with respect to some limited areas located distally in the model; the result was mainly due to the distal cement stress concentration. This interesting methodological study did con- firm – as other works had only predicted – that stair climbing is far more detrimental than walking.
Jeffers et al. (2007) conducted a similar study to investigate the ability of a computational method to predict fatigue cracking in experimental models of a simplified implanted femur structure. Computational models were generated of the same geometry as the experimental specimens (CAD reconstruction), with residual stress and porosity simulated in the cement mantle; steel was chosen for stem and aluminium for the external fixture, the latter having a geometry similar to the proximal part of the human femur. Loading was applied to represent level
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gait for two million cycles. Cement fatigue and creep were simulated using the continuum damage mechanics. Complete cement mantle fractures occurred in the FE models and in the experimental specimens in a similar location. Results of the study suggested that the continuum method is able to predict fatigue locations in a timescale which is comparable with experimental duration; this is an interesting result, notwithstanding simplifications and limitations of the used model.
The scientific peer-reviewed literature also included some studies on the accuracy of total hip replacement surgery (THR) and on the effects of implant positioning on the initial stability. Three of them are briefly described and commented here below.
Lattanzi et al. (2003) investigated the repeatability of orthopaedic surgeons’
decisions in planning THR surgery, and compared planned accuracyvs accuracy achieved by implementing a conventional unassisted surgical procedure. Three surgeons planned the surgery through a CT-based preoperative planning software.
The difference between the implant position achieved at the surgery time and the previously planned position was computed. Results showed that, in most cases, the surgeon positioned the stem more distally and more posteriorly in the pre-operative planning with respect to the stem position during the successive surgery.
In the Reggiani et al.’s study (2008), two FE models of an implanted femur and a titanium alloy stem implant were created, one with the stem positioned according to the pre-surgery planning and the other with the stem fixed in the surgically achieved position (the model had been validated in a previous work by the same author, Reggiani et al. (2007), comparing experimental micro-motion measurements). With a torque of 11.4 Nm applied to the proximal part of hip stem, the planned model predicted a peak Von Mises stress in the bone of 10 MPa; a 12 MPa stress was predicted by the achieved model at the same location. Differences in this case were small because differences between planned and achieved position were quite small, too. However, the study confirms the relevance of prediction of primary stability.
Recently, Bah et al. (2011) used a FE model to describe a statistical investigation into the effects of implant positioning on the initial stability of a cementless total hip replacement. Mesh morphing was combined with design of computer experiments to automatically construct finite element meshes for a range of pre- defined femur-implant configurations and to predict implant micro-motions under joint contact and muscle loading. In particular, a stem of titanium alloy was rotated bya¼[12,8],b¼[2.4,3.5] and g¼[2,0.8] around the Z, Y and X axis of a local coordinate system, to replicate retro-anteversion, varus–valgus and ante-posterior orientations. The FE models were subjected to joint and abductor loads during normal walking mode, applied at a single node at the centre of the top surface of the head of the implant and on the greater trochanter, respectively.
A 4-noded tetrahedral volume mesh was produced using ANSYS11. The percent- age of implant area with micromotion greater than the limit value of 50mm were considered for THR performance assessment. As a result, the highest percentage was found when the implant was rotated towards the anterior part of the femur, i.e. around the medial–lateral axis; implant maximum micromotion was more dependent on antero-posterior (45%) and varus–valgus (43%) than ante-retroversion rotation. The best orientation angles were found to be:11.83 of retroversion,
2.98 of valgus and 1.67 of posterior orientation. Even though the range of orientation angles was quite limited, and configurations were only compared in terms of micromotions, this study gives a good understanding about the effects of different implant positioning.
Based on the above studies, and with the aim to evaluate the effects of the stem positioning in a configuration close to real situation (Viceconti et al.2001), in the present study we designed a second Model, named “Model2”, consisted of: the same femoral stem used for Model1 – i.e. the Model comparable with the experimental test configuration – , fully bonded in a 3 mm cement mantle as in the literature (Lee et al. 1993; Mann et al. 1995; Ramaniraka et al.2000) and the external femur reconstructed dimensionally according to Lattanzi et al. (2003) as shown in Fig.3.
The proximal part of the femur was modeled as in the relevant literature (Mann et al.1995; McNamara et al. 1997; Stolk et al. 2001; Pancanti et al.2003); in particular, it consists of the proximal part of the femur as in Grasa et al. (2005). For Model2, two configurations have been considered, the former with the stem ori- ented according to ISO Standard positioning, the latter according to the critical positioning previously individuated. Boundary and loading conditions were the same as in the previous Model1 – loading applied only at the centre of head prosthesis, interfaces stem-cement and stem-bone completely bounded – . Mesh characteristics of Model2 consisted of 28,150 tetrahedral elements and 123,531 degrees of freedom as shown in detail in Table2. The mechanical properties of the materials used in the simulation are the same as for Model1, but in Model2 the femur was modeled instead of the metallic fixture; values are presented in Table1.
Material properties of cortical bone were chosen according to Andreaus et al.
(2008). Even if, as emerged by literature review, the presence of muscle forces is an important parameter to take into account in order to reproduce in vivo loading (Stolk et al.2001), the present study was conducted in agreement with Pancanti et al.’s study (2003), thus not including muscular forces in the model in order to simplify it at least at this first stage of the study.
Fig. 3 Complete model created to implement finite element analysis: Model2
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To summarise, two main models have been created to conduct the present study: the first one, called “Model1”, to replicate the mechanical test configuration and to simulate standard and critical orientation of the stem; the second one, indicated as “Model2”, which includes the femur with prosthesis inside, to simulate more physiological configurations.