Biomechanical analysis of three popular tibial designs for TAR with different implant-bone interfacial conditions and bone qualities: A finite element study

A statistical analysis of human talar shape and bone density distribution

The shape of the talus, its internal structure, and its mechanical properties are important in determining talar behavior during loading, which may be significant for the design of surgical tools and implants. Although recent studies using statistical shape modeling have described quantitative talar external shape variation, no similar quantitative study exists to describe the density distribution of internal talar structure. The goal of this study is to quantify statistical variation in talar shape and density to benefit the design of talar implants. To this end, weight-bearing computed tomography (CT) scans of the ankle were collected in neutral, bilateral standing posture, and three-dimensional models were generated for each talus. Local density derived from the Hounsfield unit of each CT voxel was extracted. A weighted spherical harmonic analysis was performed to quantify the talar external shape. One hundred and seventy-nine volumes of interest were placed in the same relative position within each talus to quantify the talar density. Additionally, a finite element analysis (FEA) was conducted on a talus with both heterogeneous and homogeneous material properties to observe the effect of these properties on the stress and strain response. Significant differences were found in the talar density in sex and age, as well as in the stress and strain response between homogeneous and heterogeneous FEA. These differences show the importance of considering heterogeneity when examining the load response of tarsal bones.

Printable functionally graded tibial implant for TAR: FE study comparing implant materials, FGM properties, and implant designs

Tibial implants with functionally graded material (FGM) for total ankle replacement (TAR) can provide stiffness similar to the host tibia bone. The FGM implants with low stiffness reduce stress shielding but may increase implant-bone micromotion. A trade-off between stress shielding and implant-bone micromotion is required if FGMs are to substitute traditionally used Ti and CoCr metal implants. The FGM properties such as material gradation law and volume fraction index may influence the performance of FGM implants. Along with the FGM properties, the design of FGM implants may also have a role to play. The objective of this study was to examine FGM tibial implants for TAR, by comparing implant materials, FGM properties, and implant designs. For this purpose, finite element analysis (FEA) was conducted on 3D FE models of the intact and the implanted tibia bone. The tibial implants were composed of CoCr and Ti, besides them, the FGM of Ti and HA was developed. The FGM implants were modelled using exponential, power, and sigmoid laws. Additionally, for power and sigmoid laws, different volume fraction indices were taken. The effect of implant design was observed by using keel type and stem type TAR fixation designs. The results indicated that FGM implants are better than traditional metal implants. The power law is most suitable for developing FGM implants because it reduces stress shielding. For both power law and sigmoid law, low values of the volume fraction index are preferrable. Therefore, FGM implant developed using power law with 0.1 vol fraction index is ideal with the lowest stress shielding and marginally increased implant-bone micromotion. FGM implants are more useful for keel type fixation design than stem type design. To conclude, with FGMs the major complication of stress shielding can be solved and the longevity and durability of TAR implants can be enhanced.

Assessment of bone ingrowth around beaded coated tibial implant for total ankle replacement using mechanoregulatory algorithm

The long-term performance of porous coated tibial implants for total ankle replacement (TAR) primarily depends on the extent of bone ingrowth at the bone-implant interface. Although attempts were made for primary fixation for immediate post-operative stability, no investigation was conducted on secondary fixation. The aim of this study is to assess bone ingrowth around the porous beaded coated tibial implant for TAR using a mechanoregulatory algorithm. A realistic macroscale finite element (FE) model of the implanted tibia was developed based on computer tomography (CT) data to assess implant-bone micromotions and coupled with microscale FE models of the implant-bone interface to predict bone ingrowth around tibial implant for TAR. The macroscale FE model was subjected to three near physiological loading conditions to evaluate the site-specific implant-bone micromotion, which were then incorporated into the corresponding microscale model to mimic the near physiological loading conditions. Results of the study demonstrated that the implant experienced tangential micromotion ranged from 0 to 71 μm with a mean of 3.871 μm. Tissue differentiation results revealed that bone ingrowth across the implant ranged from 44 to 96 %, with a mean of around 70 %. The average Young's modulus of the inter-bead tissue layer varied from 1444 to 4180 MPa around the different regions of the implant. The analysis postulates that when peak micromotion touches 30 μm around different regions of the implant, it leads to pronounced fibrous tissues on the implant surface. The highest amount of bone ingrowth was observed in the central regions, and poor bone ingrowth was seen in the anterior parts of the implant, which indicate improper osseointegration around this region. This macro-micro mechanical FE framework can be extended to improve the implant design to enhance the bone ingrowth and in future to develop porous lattice-structured implants to predict and enhance osseointegration around the implant.

A macro-micro FE and ANN framework to assess site-specific bone ingrowth around the porous beaded-coated implant: an example with BOX® tibial implant for total ankle replacement

The use of mechanoregulatory schemes based on finite element (FE) analysis for the evaluation of bone ingrowth around porous surfaces is a viable approach but requires significant computational time and effort. The aim of this study is to develop a combined macro-micro FE and artificial neural network (ANN) framework for rapid and accurate prediction of the site-specific bone ingrowth around the porous beaded-coated tibial implant for total ankle replacement (TAR). A macroscale FE model of the implanted tibia was developed based on CT data. Subsequently, a microscale FE model of the implant-bone interface was created for performing bone ingrowth simulations using mechanoregulatory algorithms. An ANN was trained for rapid and accurate prediction of bone ingrowth. The results predicted by ANN are well comparable to FE-predicted results. Predicted site-specific bone ingrowth using ANN around the implant ranges from 43.04 to 98.24%, with a mean bone ingrowth of around 74.24%. Results suggested that the central region exhibited the highest bone ingrowth, which is also well corroborated with the recent explanted study on BOX®. The proposed methodology has the potential to simulate bone ingrowth rapidly and effectively at any given site over any implant surface.

A numerical investigation for the development of functionally graded Ti/HA tibial implant for total ankle replacement: Influence of material gradation law and volume fraction index

Stress shielding is one of the major concerns for total ankle replacement implants nowadays, because it is responsible for implant-induced bone resorption. The bone resorption contributes to the aseptic loosening and failure of ankle implants in later stages. To reduce the stress shielding, improvements can be made in the implant material by decreasing the elastic mismatch between the implant and the tibia bone. This study proposes a new functionally graded material (FGM) based tibial implant for minimizing the problem of stress shielding. Three-dimensional finite element (FE) models of the intact tibia and the implanted tibiae were created to study the influence of material gradation law and volume fraction index on stress shielding and implant-bone micromotion. Different implant materials were considered that is, cobalt-chromium, titanium (Ti), and FGM with Ti at the bottom and hydroxyapatite (HA) at the top. The FE models of FGM implants were generated by using different volume fractions and the rule of mixtures. The rule of mixtures was used to calculate the FGM properties based on the local volume fraction. The volume fraction was defined by using exponential, power, and sigmoid laws. For the power and sigmoid law varying volume fraction indices (0.1, 0.2, 0.5, 1, 2, and 5) were considered. The geometry resembling STAR® ankle system tibial implant was considered for the present study. The results indicate that FGMs lower stress shielding but also marginally increase implant-bone micromotion; however, the values were within the acceptable limit for bone ingrowth. It is observed that the material gradation law and volume fraction index influence the performance of FGM tibial implants. The tibial implant composed of FGM using power law with a volume fraction index of 0.1 was the preferred option because it showed the least stress shielding.

Influence of sidewall retention and interference fit in total ankle replacement on implant-bone micromotion: A finite element study

The success of uncemented total ankle replacement (TAR) is linked to initial stability because bony ingrowth depends upon limited early micromotion. Tibial implant design fixation features resist micromotion aided by bony sidewall retention and interference fit. Our goal was to investigate factors influencing implant-bone micromotion in TAR. Two TAR tibial components were virtually inserted into CT-derived computer models of two distal tibias from patients with end-stage ankle arthritis. Density-based inhomogeneous material assignment was used to model bone compaction during press-fit. Finite element analysis (FEA) was used to simulate three fixation cases: (1) no sidewalls + line-to-line fit, (2) sidewalls + line-to-line fit, and (3) sidewalls + 50, 100, or 200 µm interference fit. Kinetic profiles from the stance phase of gait were simulated and micromotions computed from FEA output. Without sidewalls or interference fit, micromotions were largest in early and late stance, with largest micromotions (averaging ~150-250 µm) observed near heel strike. Micromotions decreased 39%-62% when sidewalls were retained. When interference fit was also modeled, micromotions decreased another 37%-61% to ~10 µm. Micromotion differences between patients persisted with sidewall retention but largely disappeared with interference fit. This study presents new insights into the effects of TAR fixation features on implant-bone micromotion. Stability appeared to be influenced by surrounding bone quality, but this influence was greatly diminished when interference fit was introduced. More complete understanding of TAR implant features and performance is needed, but our results show the importance of bone quality and interference fit in the stability of uncemented TAR.

A numerical investigation of stress, strain, and bone density changes due to bone remodelling in the talus bone following total ankle arthroplasty

Total ankle arthroplasty is the gold standard surgical treatment for severe ankle arthritis and fracture. However, revision surgeries due to the in vivo failure of the ankle implant are a serious concern. Extreme bone density loss due to bone remodelling is one of the main reasons for in situ implant loosening, with aseptic loosening of the talar component being one of the primary reasons for total ankle arthroplasty revisions. This study is aimed at determining the performance and potential causes of failure of the talar component. Herein, we investigated the stress, strain, and bone density changes that take place in the talus bone during the first 6 months of bone remodelling due to the total ankle arthroplasty procedure. Computed tomography scans were used to generate the 3D geometry used in the finite element (FE) model of the Intact and implanted ankle. The Scandinavian Total Ankle Replacement (STAR™) CAD files were generated, and virtual placement within bone models was done following surgical guidelines. The dorsiflexion physiological loading condition was investigated. The cortical region of the talus bone was found to demonstrate the highest values of stress (5.02 MPa). Next, the adaptive bone remodelling theory was used to predict bone density changes over the initial 6-month post-surgery. A significant change in bone density was observed in the talus bone due to bone remodelling. The observed quantitative changes in talus bone density over 6-month period underscore potential implications for implant stability and fracture susceptibility. These findings emphasise the importance of considering such biomechanical factors in ankle implant design and clinical management.

The role of the depth of resection of the distal tibia on biomechanical performance of the tibial component for TAR: A finite element analysis with three implant designs

The depth of resection of the tibia bone in total ankle replacement (TAR) may influence implant-bone micromotion and stress shielding. High implant-bone micromotion and stress-shielding lead to aseptic loosening of the tibial component for TAR. The aim was to improve the outcomes of the different designs of TAR (STAR, Mobility, and Salto) with the variation of the depth of resection of the distal tibia bone. Finite element (FE) models of the implanted tibia with the depth of resection varying from 6 mm to 16 mm and of the intact tibia was prepared. The value of micromotion increased as the depth of resection increased. The micromotion increased in the proximal anterior-posterior portion of the pegs for STAR, the posterior part of the stem for Mobility, and the proximal lateral portion of the keel for Salto with the increase in the depth of resection. Whereas, the stresses (von Mises) decreased in some regions and increased in some regions depending upon the implant design. But overall stresses decreased in the tibia bone. Furthermore, the mean stress shielding increased in all the designs as the depth of resection increased. This in silico study indicated that the depth of resection should be given more importance during TAR surgery. The ideal depth of resection should be minimum i.e., 6 mm based on this FE study.

Clinically useful finite element models of the natural ankle - A review

BACKGROUND

Biomechanical simulation of the foot and ankle complex is a growing research area but compared to simulation of joints such as hip and knee, it has been under investigated and lacks consistency in research methodology. The methodology is variable, data is heterogenous and there are no clear output criteria. Therefore, it is very difficult to correlate clinically and draw meaningful inferences.

METHODS

The focus of this review is finite element simulation of the native ankle joint and we will explore: the different research questions asked, the model designs used, ways the model rigour has been ensured, the different output parameters of interest and the clinical impact and relevance of these studies.

FINDINGS

The 72 published studies explored in this review demonstrate wide variability in approach. Many studies demonstrated a preference for simplicity when representing different tissues, with the majority using linear isotropic material properties to represent the bone, cartilage and ligaments; this allows the models to be complex in another way such as to include more bones or complex loading. Most studies were validated against experimental or in vivo data, but a large proportion (40%) of studies were not validated at all, which is an area of concern.

INTERPRETATION

Finite element simulation of the ankle shows promise as a clinical tool for improving outcomes. Standardisation of model creation and standardisation of reporting would increase trust, and enable independent validation, through which successful clinical application of the research could be realised.

A combined FE-hybrid MCDM framework for improving the performance of the conical stem tibial design for TAR with the addition of pegs

BACKGROUND AND OBJECTIVES

The conical stemmed design of the tibial component for total ankle replacement (TAR) (example Mobility design) uses a single intramedullary stem for primary fixation. Tibial component loosening is a common mode of failure for TAR. Primary causes of loosening are lack of bone ingrowth due to excessive micromotion at the implant-bone interface and bone resorption due to stress shielding after implantation. The fixation feature of the conical stemmed design can be modified with the addition of small pegs to avoid loosening. The aim of the study is to select the improved design for conical stemmed TAR using a combined Finite Element (FE) hybrid Multi-Criteria Decision-Making (MCDM) framework.

METHODS

The geometry and material properties of the bone for FE modeling were extracted from the CT data. Thirty-two design alternatives with varying pegs in number (one, two, four, eight), location (anterior, posterior, medial, lateral, anterior-posterior, medial-lateral, equally spaced), and height (5 mm, 4 mm, 3 mm, 2 mm) were prepared. All models were analyzed for dorsiflexion, neutral, and plantarflexion loading. The proximal part of the tibia was fixed. The implant-bone interface coefficient of friction was taken as 0.5. The implant-bone micromotion, stress shielding, volume of bone resection, and surgical simplicity were the important criteria considered for evaluating the performance of TAR. The designs were compared using a hybrid MCDM method of WASPAS, TOPSIS, EDAS, and VIKOR. The weight calculations were based on fuzzy AHP and the final ranks on the Degree of Membership method.

RESULTS

The addition of pegs decreased the mean implant-bone micromotions and increased stress shielding. There was a marginal decrease in micromotion and a marginal increase in stress shielding when the peg heights were increased. The results of hybrid MCDM indicated that the most preferable alternative designs were two pegs of 4 mm height in the AP direction to the main stem, two pegs of 4 mm height in the ML direction, and one peg of 3 mm height in the A direction.

CONCLUSIONS

Outcomes of this study suggest that the addition of pegs can reduce the implant-bone micromotions. Modified three designs would be useful by considering implant-bone micromotions, stress shielding, volume of bone resection, and surgical simplicity.

Total Ankle Replacement: Biomechanics of the Designs, Clinical Outcomes, and Remaining Issues

The present review paper aimed at discussing the current major issues in total ankle replacement, both the technical and biomechanical concepts, and the surgical and clinical concerns. Designers shall target at the same time restoration of natural ankle kinematics and congruity of the artificial surfaces throughout the range of motion. Surgeons are recommended to expand biomechanical knowledge on ankle joint replacement, and provide appropriate training and key factors to make arthroplasty a good alternative to arthrodesis. Moreover, adequate selection of patients and careful rehabilitation are critical. In the future, custom-made prosthesis components and patient-specific instrumentation are major developments for more complex cases.

Comparison of joint load, motions and contact stress and bone-implant interface micromotion of three implant designs for total ankle arthroplasty

BACKGROUND AND OBJECTIVE

Loosening and wear are still the main problems for the failure of total ankle arthroplasty, which are closely related to the micromotion at the bone-implant interface and the contact stress and joint motions at the articular surfaces. Implant design is a key factor to influence the ankle force, motions, contact stress, and bone-implant interface micromotion. The purpose of this study is to evaluate the differences in these parameters of INBONE II, INFINITY, and a new anatomic ankle implant under the physiological walking gait of three patients.

METHODS

This was achieved by using an in-silico simulation framework combining patient-specific musculoskeletal multibody dynamics and finite element analysis. Each implant was implanted into the musculoskeletal multibody dynamics model, respectively, which was driven by the gait data to calculate ankle forces and motions. These were then used as the boundary conditions for the finite element model, and the contact stress and the bone-implant interface micromotions were calculated.

RESULTS

The total ankle contact forces were not significantly affected by articular surface geometries of ankle implants. The range of motion of the ankle joint implanted with INFINITY was a little larger than that with INBONE II. The anatomic ankle implant design produced a greater range of motion than INBONE II, especially the internal-external rotation. The fixation design of INFINITY achieved lower bone-implant interface micromotion compared with INBONE II. The anatomic ankle implant design produced smaller contact stress with no evident edge contact and a smaller tibia-implant interface micromotion. In addition, significant differences in the magnitudes and tendencies of total ankle contact forces and motions among different patients were found.

CONCLUSIONS

The articular surface geometry of ankle implants not only affected the ankle motions and contact stress distribution but also affected the bone-implant interface micromotions. The anatomic ankle implant had good performance in recovering ankle joint motion, equalizing contact stress, and reducing bone-implant interface micromotion. INFINITY's fixation design could achieve smaller bone-implant interface micromotion than INBONE II.