5 Results and Discussion
5.4 Computerized simulations of bone remodeling processes in the canine and
I. BEHRENS, B.A., I. NOLTE, P. WEFSTAEDT, C. STUKENBORG-COLSMAN and A.
BOUGUECHA (2009a):
Numerical investigations on the strain-adaptive bone remodeling in the periprosthetic femur: influence of the boundary conditions.
Biomed Eng Online. 8, 7
II. BEHRENS, B.A., A. BOUGUECHA, C. STUKENBORG-COLSMAN, P. WEFSTAEDT
Results and Discussion
37 and I. NOLTE (2009b):
Numerische Untersuchungen zum beanspruchungsadaptiven Knochenumbau im periprosthetischen caninen Femur
Berl Munch Tierarztl Wochenschr. 122, 391-397
BEHRENS et al. (2009a, b) carried out FEA of bone remodeling processes around a cemented prosthetic stem in dogs (Bioméchanique intégrée bioimplant, Bretigny sur Orge, France) as well as in an uncemented femoral prosthesis in humans (BiCONTACT® N (AESCULAP AG, Tuttlingen, Germany). BEHRENS et al. (2009a) found out that for a realistic FE simulation of bone remodeling processes in the human periprosthetic femur it is necessary to consider the whole loading situation within the gait cycle. In contrast, other authors (WEINANS et al. 1992; HUISKES and VAN RIETBERGEN 1995; KUIPER and HUISKES 1997; ENGH and AMIS 1999;
FERNANDES et al. 2002; TAI et al. 2003; GOETZEN et al. 2005) considered only two loading cases of the gait cycle or from stair-climbing for the simulation. The studies of BEHRENS et al. (2009a, b) confirmed the results of other numerical studies, that the changed load distribution in the femur after hip arthroplasty results in biomechanically induced bone remodeling processes in the periprosthetic canine femur (WEINANS et al, 1993; VAN RIETBERGEN et al 1993; WEINANS and SUMNER, 1997). In contrast to these studies BEHRENS et al. (2009a, b) used a loading situation representing the in vivo situation in more detail. Furthermore, the entire femur and not only the proximal part of the femur, was considered in many other studies (WEINANS et al. 1992; WEINANS et al. 1993; VAN RIETBERGEN et al. 1993; HUISKES and VAN RIETBERGEN 1995;
Results and Discussion
38
KUIPER and HUISKES 1997; WEINANS and SUMNER 1997; ENGH and AMIS 1999;
FERNANDES et al. 2002; BITSAKOS et al. 2005) was taken into account for the model setup in the studies of BEHRENS et al. (2009a, b). According to Duda et al. (1998) and Polgar et al. (2003) considering the whole femur for the simulation of the load transfer from the prosthetic stem to the femur represents more realistically the occurring load situation.
In case of the FEA bone remodeling processes of the canine periprosthetic femur, the bone was divided in three regions of analysis (BEHRENS et al. 2009b). BEHRENS et al.
(2009b) could show that there are evident changes in the bone density in each of the analysed areas. In particular the proximal and diaphyseal region of the periprosthetic femur showed a significant loss of bone mass. Thus, statements about a possible reduced secondary stability of the examined canine femoral component Biomechanique® in these regions are possible by means of the finite element analysis.
For the verification of the obtained results comparative analyses between FE calculations and X-rays from implanted dogs of the Small animal hospital were carried out. A good qualitative agreement of bone remodeling processes in the analyzed areas could be shown between the methods. Furthermore, the results from the finite element analysis are in agreement with the results of GERVERS (1998) and GERVERS et al.
(2002). A direct comparison to the results of MOSTAFA et al. (2011) was not possible, as a different type of implant was used in this study.
In case of the FEA of bone remodeling processes in the human prosthetic stem Bicontact® BEHRENS et al. (2009a) used a reduced muscle system according to Heller et al. (2005) and GOETZEN et al. (2005) due to the fact that a correct consideration of
Results and Discussion
39
muscle forces is highly relevant for the calculation of the load distribution as well as resulting bone remodeling processes in the periprosthetic femur. For the bone adaptation model, the model of HUISKES and VAN RIETBERGEN (1995) was modified by consideration of an upper bound for the bone formation rate and an area of bone lysis as this modified model reflects the physiological situation in more detail. In case of the bone remodeling processes around the human prosthetic stem BEHRENS et al.
(2009a) found out that within the investigated loading regime bone mass loss is highest in the proximal region of the femur and much less in the diaphyseal region. One explanation for these findings is due to force transmission from the proximal coated part of the prosthesis to the femur. These findings correspond to the clinical findings described in other studies using the same type of prosthesis (FRITZ et al. 2001, STUKENBORG-COLSMAN et al. 2012). However, BEHRENS et al. (2009a) used only three analysed regions limiting the comparability with results of DEXA investigations as carried out e.g. by STUKENBORG-COLSMAN et al. (2012) for the same type of prosthesis. Future FE studies will have to use identical analyses regions for the FEA to make a validation by the clinical DEXA investigations possible.
With the studies of BEHRENS et al. (2009a, b) it could be shown that the FEM is suitable for the calculation of stress shielding related bone remodeling processes in the periprosthetic femur of both humans and dogs. Furthermore BEHRENS et al. (2009a) found out that considering the loading situation during a whole gait cycle results in a high variation between bone formation and bone loss in contrast to simulations in which only a static loading case was used. In this context it has to be kept in mind that the conclusions made from the computations and models have to be validated by clinical
Results and Discussion
40
examinations of the long term outcome using the same types of prosthesis (LENGSFELD et al. 2002). However, the FEM can be a valuable in silico method for the evaluation of the secondary stability of prostheses both in humans and dogs.
5.5 Establishment of a multibody simulation system for the determination of the loading situation in the canine hip joint
III. HELMS, G., B.A. BEHRENS, M. STOLORZ, P. WEFSTAEDT and I. NOLTE (2009):
Multi-body simulation of a canine hind limb: model development, experimental validation and calculation of ground reaction forces.
Biomed Eng Online. 8, 36
To investigate different loading conditions of the hip joint, computerized simulations are highly desirable due to the fact that these simulations can be carried out without the necessity of animal experiments. In contrast to investigations using dogs with instrumented hip joint implants (BERGMANN 1997), computerized models such as MBS-models allow for the investigation of different loading scenarios and implant positions. At present several studies exist investigating ground reaction forces during gait of dogs (BUDSBERG et al. 1987; ALLEN et al. 1994; BUDSBERG et al. 1996; LEE et al. 2004). However, a direct measurement of hip joint forces is only possible by means of the mentioned instrumented hip joint implants (BERGMANN 1997). Therefore HELMS et al. (2009) established an MBS model of the hind limb capable for the simulation of forces and moments in the hip joint. The model was comprised of an
Results and Discussion
41
anatomic muscle model obtained from CT and MRI data of a 28 kg dog. To calculate the occurring forces during a walking gait, kinematic analyses of a dog with similar height and size were used to animate the MBS model. The model was validated by comparison of the simulated force data with measured ground reaction forces of the same dog walking on an instrumented treadmill. The established multi-body simulation model of the canine hind limb allows the simulation of vertical ground reaction forces during a walking gait showing a similar curve characteristic to the treadmill measurements.
Furthermore measured as well as simulated values are in good accordance to measured values of other working groups.
In contrast, MBS of forces in x- and y-direction showed only a poor similarity to measured ground reaction force data. This finding is most likely to be associated with the modelling of the pad ground contact, which was considered as a simple ellipsoid. To improve simulations also of GRF in x- and y-direction, the model of the pad ground contact has to be enhanced further. The established MBS-model described by HELMS et al. (2009) can serve as a valuable method for future investigations of the detailed dynamic loading situation in the canine hip joint after THR. Furthermore, the obtained values from the dynamic loading situation can be combined with FEA of bone remodeling processes parameters or for the determination of areas of high loadings within the artificial tribological pairing (SHAHAR et al. 2003). Thus, the developed simulation models of BEHRENS et al. (2009a, b) and HELMS et al. (2009) can help to develop optimization strategies for the different components of artificial hip joint prostheses in order to reduce the stress shielding phenomenon and an aseptic loosening of THR.
Summary
42
6 Summary
Total hip replacement (THR) is a routine surgical treatment for severe hip dysplasia in dogs as well as in humans. Aseptic loosening of prostheses components is one key factor influencing the long term outcome of total hip replacements. Current collaborative research between engineers and veterinary as well as medical physicians aim at the improvement of prosthetic materials, prostheses geometries and prosthetic stem alignment within the femoral canal to reduce stress shielding processes in the periprosthetic bone and implant loosening thereof. In this context, computerized modelling methods like the finite element method and multibody simulations can help to provide knowledge about bone remodeling processes in the periprosthetic bone as well as of the loading situation in the artificial joint. To set up these models accurate motion analyses in combination with measured ground reaction forces of the patients are needed. Gait analysis measurements can furthermore be used to quantify the lameness improvement in dogs after orthopaedic surgical interventions such as THRs. To validate the established computerized models and to gain a deep insight into the morphologic processes in the periprosthetic bone modern imaging analyses are necessary.
Within the current work at first the establishment of a gait analysis laboratory at the Small Animal Hospital of the University of Veterinary Medicine Hannover is described.
The laboratory was validated by comparative kinematic and kinetic gait analysis of the hind limb function in dogs walking on a treadmill and force plate. As one result a lower weight bearing behaviour could be demonstrated during walk on the treadmill in comparison to the force plate measurements. In the following, gait analyses were carried
Summary
43
out for the comparison of the functional outcome of dogs undergoing different surgical treatments of common orthopaedic hind limb diseases. Severe hip dysplasia was treated with either cemented or uncemented THR. As one main result, both groups showed similar weight bearing characteristics during a time course of four months after surgery.
Gait analysis data was further used to setup and validate an MBS model for the calculation of joint forces and moments in the canine hind limb. The results of this study show that measured and simulated vertical ground reaction forces are in good accordance to each other. For that reason it can be concluded that the established MBS can be used for the computing of the loading situation of the hip joint during different movements. In this context also combined simulations between MBS and FEM are wanted to simulate the influence of different loading conditions on bone remodeling processes in the periprosthetic femur. Within the here introduced work two studies are presented dealing with the numerical simulation of periprosthetic bone remodeling processes in the canine and human femur after THR. Simulation results suggest that bone remodeling processes mainly occur in the proximal analysis regions of the femur.
In addition to the results from the FEA in case of the dog quantitative measurements of the periprosthetic bone density (grayscale value) by means of postoperative radiographs were carried out to prove whether this technique is capable to allow insights into periprosthetic bone remodeling processes or not. As the results show changes in periprosthetic bone density can be sufficiently analysed for cemented as well as for uncemented prostheses by means of the established technique. Significant bone loss occurred mainly in the region of the greater trochanter of femurs implanted with the uncemented prosthetic stem. For morphologic analysis of periprosthetic bone
Summary
44
remodeling processes in the human periprosthetic femur, DEXA analyses were carried out at different time points before and after implantation of a widely used uncemented THR system. By means of the carried out analyses the hypothesis of a proximal load transfer from the prosthesis to the periprosthetic bone with initial bone loss in the calcar region and the region of the trochanter major could be confirmed. To improve the knowledge and understanding of morphological changes in the joints and the periprosthetic bone after THR, future work will have to combine simulative and morphological analyses of the bone implant interface with functional analyses of the surgical outcome. In this context, the investigations reported here can serve as a basis for the future establishment of optimized THR systems with long term stability.
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