Ligament fibre recruitment and forces for the anterior drawer test at the human ankle joint

Comparative finite element analysis of contact and stress distribution in tibiotalar articular cartilage: Healthy versus varus ankles

Background

The distribution of forces within the ankle joint plays a crucial role in joint health and longevity. Loading disorders affecting the ankle joint can have significant detrimental effects on daily life and activity levels. This study aimed to enhance our understanding of the mechanical behavior of tibiotalar joint articular cartilages in the presence of varus deformity using finite element analysis (FEA) applied to patient-specific models.

Methods

Two personalized ankle models, one healthy and another with varus deformity, were created based on CT scan images. Four static loading scenarios were simulated at the center of pressure (COP), coupled to the hindfoot complex. The contact area, contact pressure, and von Mises stress were computed for each cartilage.

Results

It was found that the peak contact pressure increased by 54% in the ankle with varus deformity compared to the healthy ankle model. Furthermore, stress concentrations moving medially were observed, particularly beneath the medial malleolus, with an average peak contact pressure of 3.5 MPa and 4.7 MPa at the tibial and talar articular cartilages, respectively.

Conclusion

Varus deformities in the ankle region have been consistently linked to elevated contact pressure, increasing the risk of thinning, degeneration, and eventual onset of osteoarthritis (OA), emphasizing the need for prompt interventions aimed at mitigating complications.

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.

Influence of cancellous bone material and dead zone on stress-strain, bone stimulus and bone remodelling around the tibia for total ankle replacement

Extreme bone resorption due to bone remodelling is one of the reasons for ankle component loosening. Finite element (FE) analysis has been effectively used nowadays for pre-clinical analysis of orthopaedic implants. For FE modelling, the selection of bone material and dead zone play a vital role to understand the bone remodelling. This study deals with the effects of different cancellous elastic modulus-density relationships and dead zone on bone remodelling around the tibia owing to total ankle replacement (TAR), using finite element analysis with physiological loading conditions. This study also investigated the bone stimulus distribution in the tibia to identify the initial indication of bone density changes due to bone remodelling. Additionally, the Hoffman failure criterion was used to investigate the chances of implant-bone interface failure due to different cancellous bone material modelling and bone remodelling. The present bone remodelling study consists of three different dead or lazy zones (±0.75, ±0.60 and ±0.35) to examine the influence of the dead zone on bone remodelling. Differences in stress/strain distribution were observed in the tibia bone due to different cancellous bone material modelling. Despite little variations, bone density changes due to bone remodelling were found to be almost similar for two FE models having different cancellous bone material. Similar to these results, the effect of different dead zone on bone density changes due to bone remodelling was found to be minimal. Bone stimulus distribution in the cancellous bone was found to be almost similar for FE models having different cancellous bone material modelling and different dead zones. To understand the stress/strain and interface related failure of the tibial component, cancellous bone material modelling plays a crucial role. However, cancellous bone material modelling and dead zone have minimal influence on bone remodelling around the tibia cancellous bone due to TAR.

Contact patterns in the ankle joint after lateral ligamentous injury during internal rotation: A computational study

Lateral ankle instability, resulting from the inability of ankle ligaments to heal after injury, is believed to cause a change in the articular contact mechanics that may promote cartilage degeneration. Considering that lateral ligaments' insufficiency has been related to rotational instability of the talus, and that few studies have addressed the contact mechanics under this condition, the aim of this work was to evaluate if a purely rotational ankle instability could cause non-physiological changes in contact pressures in the ankle joint cartilages using the finite element method. A finite element model of a healthy ankle joint, including bones, cartilages and nine ligaments, was developed. Pure internal talus rotations of 3.67°, 9.6° and 13.43°, measured experimentally for three ligamentous configurations, were applied. The ligamentous configurations consisted in a healthy condition, an injured condition in which the anterior talofibular ligament was cut, and an injured condition in which the anterior talofibular and calcaneofibular ligaments were cut. For all simulations, the contact areas and maximum contact pressures were evaluated for each cartilage. The results showed not only an increase of the maximum contact pressures in the ankle cartilages, but also novel contact regions at the anteromedial and posterolateral sections of the talar cartilage with increasing internal rotation. The anteromedial and posterolateral contact regions observed due to pathological internal rotations of the talus are a computational evidence that supports the link between a pure rotational instability and the pattern of pathological cartilaginous load seen in patients with long-term lateral chronic ankle instability.

Estimating the stabilizing function of ankle and subtalar ligaments via a morphology-specific three-dimensional dynamic model

Knowledge of the stabilizing role of the ankle and subtalar ligaments is important for improving clinical techniques such as ligament repair and reconstruction. However, this knowledge is incomplete. The goal of this study was to expand this knowledge by investigating the stabilizing function of the ligaments using multiple morphologically subject-specific computational models. Nine models were created from the lower extremities of nine donors. Each model consisted of the articulating bones, articular cartilage, and ligaments. Simulations were conducted in ADAMS™ - a dynamic simulation program. During simulation, tibia and fibula were fixed while cyclic moments in all three anatomical planes were applied to the calcaneus one-at-a-time. The resulting displacements between the bones and the forces in each ligament were computed. Simulations were conducted with all ligaments intact and after simulated ligament serial sectioning. Each model was validated by comparing the simulation results to experimental data obtained from the specimen used to construct the model. From the results the stabilizing role of each ligament was established and the effect of ligament sectioning on Range of Motion and Overall Laxity was identified. On the lateral side, ATFL provided stabilization in supination, CFL restrained inversion, external rotation and dorsiflexion and PTFL limited dorsiflexion and external rotation. On the medial side, PTTL restrained dorsiflexion and internal rotation, ATTL limited plantarflexion and external rotation, and TCL limited dorsiflexion, eversion and external rotation. At the subtalar joint, ITCL limited plantarflexion and its posterior-lateral bundle restrained subtalar inversion. CL restrained plantarflexion/dorsiflexion, and internal and external rotation. The large inter-model variability observed in the results indicate the importance of using multiple subject-specific models rather than relying on one "representative" model.

Experimental and finite element investigation of total ankle replacement: A review of literature and recommendations

This paper briefly reviews the different methodology, technology, challenges, and outcomes of various studies related to TAR prosthesis based on numerical and experimental techniques. Very less in-vitro experimental studies on TAR have been found than finite element (FE) studies. Due to the invasive nature of the experimental approach, inadequacy and less clinical information, computational modelling has been widely used by the researchers. This paper critically examines the part related to FE modelling and experimental analysis. Some recommendation related to modelling of bones, cartilages, ligaments, muscles, and implant-bone interface condition were discussed for better understanding the results and better clinical significance.

Bone remodelling around the tibia due to total ankle replacement: effects of implant material and implant-bone interfacial conditions

One of the major causes of implant loosening is due to excessive bone resorption surrounding the implant due to bone remodelling. The objective of the study is to investigate the effects of implant material and implant-bone interface conditions on bone remodelling around tibia bone due to total ankle replacement. Finite element models of intact and implanted ankles were developed using CT scan data sets. Bone remodelling algorithm was used in combination with FE analysis to predict the bone density changes around the ankle joint. Dorsiflexion, neutral, and plantar flexion positions were considered, along with muscle force and ligaments. Implant-bone interfacial conditions were assumed as debonded and bonded to represent non-osseointegration and fully osseointegration at the porous coated surface of the implant. To investigate the effect of implant material, three finite element models having different material combinations of the implant were developed. For model 1, tibial and talar components were made of Co-Cr-Mo, and meniscal bearing was made of UHMWPE. For model 2, tibial and talar components were made of ceramic and meniscal bearing was made of UHMWPE. For model 3, tibial and talar components were made of ceramic and meniscal bearing was made of CFR-PEEK. Changes in implant material showed no significant changes in bone density due to bone remodelling. Therefore, ceramic appears to be a viable alternative to metal and CFR-PEEK can be used in place of UHMWPE. This study also indicates that proper bonding between implant and bone is essential for long-term survival of the prosthetic components.

Biomechanical Model for Stress Fracture-related Factors in Athletes and Soldiers

Stress fractures (SF) are one of the most common and potentially serious overuse injuries.

PURPOSE

This study aimed to develop a computational biomechanical model of strain in human tibial bone that will facilitate better understanding of the pathophysiology of SF.

METHODS

The MRI of a healthy, young male was used for full anatomical segmentation of the calf tissues, which considered hard-soft tissues biomechanical interactions. From the undeformed coronal MR images, the geometry of bones, muscles, connecting ligaments, and fat were reconstructed in three dimensions and meshed to a finite element model. A force that simulated walking was applied on the tibial plateaus. The model was then analyzed for strains in the tibia under various conditions: unloaded walking, walking with a load equivalent to 30% of bodyweight, and walking under conditions of muscular fatigue. In addition, the effect of tibia robustness on strain was analyzed.

RESULTS

The model showed that the tibia is mostly loaded by compression, with maximal strains detected in the distal anterior surface: 1241 and 384 microstrain, compressive and tensile, respectively. Load carriage resulted in ~30% increase in maximal effective strains. Muscle fatigue has a complex effect; fatigued calf muscles (soleus) reduced the maximal effective strains up to 9%, but fatigued thigh muscles increased those strains by up to 3%. It had also been shown that a slender tibia is substantially prone to higher maximal effective strains compared with an average (22% higher) or robust tibia (39% higher).

CONCLUSIONS

Thigh muscle fatigue, load carriage, and a slender tibia were detected as factors that may contribute to the development of SF. The methodology presented here is a novel tool for investigating the pathophysiology of SF.

Rotational Instability after Anterior Talofibular and Calcaneofibular Ligament Section: The Experimental Basis for the Ankle Pivot Test

The clinical diagnosis of the anterior talofibular ligament (ATFL) rupture is based on the findings from the medical history and the anterior drawer test, a maneuver that allegedly pushes the talus and rearfoot anteriorly, although with great variability in its sensitivity. We consider that an ATFL rupture is best evaluated by a rotational vector (i.e., a pivot test) owing to the uncompromised medial ligaments that will block any pure anterior translation of the talus underneath the tibia. We idealized a constrained ankle cadaver model that only allows talar movements in the axial plane. Our hypothesis was that progressive sectioning of the lateral ankle ligaments in this model would cause a progressive and significant angular laxity in internal rotation. Our results showed 3.67 degrees ± 1.2 degrees of talus rotational laxity in the intact ankle, 9.6 degrees ± 3.2 degrees after ATFL sectioning, and 13.43 degrees ± 3.2 degrees after ATFL and calcaneofibular ligament sectioning, indicating almost threefold increase in internal talocrural rotation after single ATFL sectioning and an almost fourfold increase after double (ATFL and calcaneofibular ligament) sectioning. We consider this evidence of rotational ankle laxity to be a major step in defining the correct movement to diagnose an ATFL rupture and propose a new term to avoid further inconsistencies and variability, "the pivot test."

Effects of implant orientation and implant material on tibia bone strain, implant–bone micromotion, contact pressure, and wear depth due to total ankle replacement

The aim of this study is to investigate the effects of implant orientation and implant material on tibia bone strain, implant–bone micromotion, maximum contact pressure, and wear depth at the articulating surface due to total ankle replacement. Three-dimensional finite element models of intact and implanted ankle were developed from computed tomography scan data. Four implanted models were developed having varus and valgus orientations of 5° and 10°, respectively. In order to determine the effect of implant material combination on tibia bone strain, micromotion, contact pressure, and wear depth, three other finite element models were developed having a different material combination of the implant. Dorsiflexion, neutral, and plantarflexion positions were considered as applied loading condition, along with muscle force and ligaments. Implant orientation alters the strain distribution in tibia bone. Strain shielding was found to be less in the case of the optimally positioned implant. Apart from the strain, implant orientation also affects implant–bone micromotion, contact pressure, and wear depth. Implant materials have less influence on tibia bone strain and micromotion. However, wear depth was reduced when ceramic and carbon fibre–reinforced polyetheretherketone material combination was used. Proper orientation of the implant is important to reduce the strain shielding. The present result suggested that ceramic can be used as an alternative to metal and carbon fibre–reinforced polyetheretherketone as an alternative to ultra-high molecular weight polyethylene to reduce wear, which would be beneficial for long-term success and fixation of the implant.

Finite element analysis of knee and ankle joint during gait based on motion analysis

Contact pressures in the articular cartilage during gait affect injuries and the degenerative arthritis of knee and ankle joints. However, only contact forces at the knee and ankle joints during gait can be estimated by using a rigid body dynamic model. The contact pressure distribution can be obtained quantitatively for a static posture by using finite element (FE) analysis in most cases. The purpose of this study is to develop a new method to obtain the contact pressure distribution at the knee and ankle joints during gait by integrating FE analysis with rigid body dynamic analysis. In this method, a reference FE model of the lower extremity is constructed first and is then transformed to each stance phase of the gait obtained from dynamic analysis by using homogeneous transformation. The muscle forces and ground reaction force (GRF) during gait obtained from the dynamic analysis were used as loading conditions for FE analysis. Finally, the contact pressure distribution at the tibia plateau cartilage and talus cartilage were estimated at the 1st peak, mid-stance, and the 2nd peak at the same time. The present method can provide the contact pressure distribution at the knee and ankle joints over the entire gait.

Contact stresses, pressure and area in a fixed-bearing total ankle replacement: a finite element analysis

BACKGROUND

Mobile-bearing ankle implants with good clinical results continued to increase the popularity of total ankle arthroplasty to address endstage ankle osteoarthritis preserving joint movement. Alternative solutions used fixed-bearing designs, which increase stability and reduce the risk of bearing dislocation, but with a theoretical increase of contact stresses leading to a higher polyethylene wear. The purpose of this study was to investigate the contact stresses, pressure and area in the polyethylene component of a new total ankle replacement with a fixed-bearing design, using 3D finite element analysis.

METHODS

A three-dimensional finite element model of the Zimmer Trabecular Metal Total Ankle was developed and assembled based on computed tomography images. Three different sizes of the polyethylene insert were modeled, and a finite element analysis was conducted to investigate the contact pressure, the von Mises stresses and the contact area of the polyethylene component during the stance phase of the gait cycle.

RESULTS

The peak value of pressure was found in the anterior region of the articulating surface, where it reached 19.8 MPa at 40% of the gait cycle. The average contact pressure during the stance phase was 6.9 MPa. The maximum von Mises stress of 14.1 MPa was reached at 40% of the gait cycle in the anterior section. In the central section, the maximum von Mises stress of 10.8 MPa was reached at 37% of the gait cycle, whereas in the posterior section the maximum stress of 5.4 MPa was reached at the end of the stance phase.

DISCUSSION

The new fixed-bearing total ankle replacement showed a safe mechanical behavior and many clinical advantages. However, advanced models to quantitatively estimate the wear are need.

CONCLUSION

To the light of the clinical advantages, we conclude that the presented prosthesis is a good alternative to the other products present in the market.

Results at a minimum follow-up of 5 years of a ligaments-compatible total ankle replacement design

BACKGROUND

A new design of 3-part ankle replacement was developed to achieve compatibility with the natural ligaments by allowing certain fibers to remain isometric during passive motion.

METHODS

We evaluate 75 ankle prostheses implanted from July 2003 to December 2008, at a mean follow-up 6.5±1.1years (range 5-9 years). The mean age at surgery was 62±13years (range 29-82).

RESULTS

The mean AOFAS scores achieved at pre-op and at last follow-up were respectively 37±5 (23-45) and 78±8 (64-98). (p<0.001). Clinical range of motion of the ankle measured by goniometer pre op was 1°±2 of dorsiflexion and 12°±4° of plantarflexion; at last follow-up range of motion increased to 6°±5° in dorsiflexion (p<0.01) and 18°±7° in plantarflexion (p<0.05). Radiographs showed no loosening and little signs of radiolucency. Two revisions necessitated component removal, neither for implant loosening. The overall survival rate was 97.3%.

CONCLUSIONS

Function and Range-of-motion showed significant improvements. These results demonstrate that ligaments-compatible shaped talar and tibial components, with a fully conforming interposed meniscal bearing, can provide satisfactory survival rates and clinical outcomes in the middle term.

A numerical study on stress distribution across the ankle joint: Effects of material distribution of bone, muscle force and ligaments

OBJECTIVE

The goal of this study is to develop a realistic three dimensional FE model of intact ankle joint.

METHODS

Three dimensional FE model of the intact ankle joint was developed using computed tomography data sets. The effect of muscle force, ligaments and proper material property distribution of bone on stress distribution across the intact ankle joint was studied separately.

RESULTS

Present study indicates bone material property, ligaments and muscle force have influence on stress distribution across the ankle joint.

CONCLUSION

Proper bone material, ligaments and muscle must be considered in the computational model for pre-clinical analysis of ankle prosthesis.

A Three-Dimensional Ankle Kinetostatic Model to Simulate Loaded and Unloaded Joint Motion

A kinetostatic model able to replicate both the natural unloaded motion of the tibiotalar (or ankle) joint and the joint behavior under external loads is presented. The model is developed as the second step of a sequential procedure, which allows the definition of a kinetostatic model as a generalization of a kinematic model of the joint defined at the first step. Specifically, this kinematic model taken as the starting point of the definition procedure is a parallel spatial mechanism which replicates the ankle unloaded motion. It features two rigid bodies (representing the tibia–fibula and the talus–calcaneus complexes) interconnected by five rigid binary links, that mimic three articular contacts and two nearly isometric fibers (IFs) of the tibiocalcaneal ligament (TiCaL) and calcaneofibular ligament (CaFiL). In the kinetostatic model, the five links are considered as compliant; moreover, further elastic structures are added to represent all the main ankle passive structures of the joint. Thanks to this definition procedure, the kinetostatic model still replicates the ankle unloaded motion with the same accuracy as the kinematic model. In addition, the model can replicate the behavior of the joint when external loads are applied. Finally, the structures that guide these motions are consistent with the anatomical evidence. The parameters of the model are identified for two specimens from both subject-specific and published data. Loads are then applied to the model in order to simulate two common clinical tests. The model-predicted ankle motion shows good agreement with results from the literature.

Biomechanics of the natural, arthritic, and replaced human ankle joint

The human ankle joint complex plays a fundamental role in gait and other activities of daily living. At the same time, it is a very complicated anatomical system but the large literature of experimental and modelling studies has not fully described the coupled joint motion, position and orientation of the joint axis of rotation, stress and strain in the ligaments and their role in guiding and stabilizing joint motion, conformity and congruence of the articular surfaces, patterns of contact at the articular surfaces, patterns of rolling and sliding at the joint surfaces, and muscle lever arm lengths.

The present review article addresses these issues as described in the literature, reporting the most recent relevant findings.

BIOMECHANICS OF THE INTACT AND REPLACED HUMAN ANKLE JOINT

The main objective of the study was to develop advanced biomechanical models of the intact human ankle complex. It was also aimed at designing a total ankle replacement which would better reproduce the physiological function of the joint. Passive flexion was analyzed in a number of lower-leg preparations with stereophotogrammetry and radiostereometry. The articular surfaces and fibres within the calcaneofibular and tibiocalcaneal ligaments were observed to prescribe the changing positions of bones, ligaments and instantaneous axis of rotation. Joint motion included rolling as well as sliding. Computer-based models elucidated this kinematics at the intact joint, and how changing positions of the centre of rotation and muscle lines of action affect lever arm length at different flexion angles. The mechanical response of the joint to anterior drawer and talar tilt tests was explained in terms of fibre recruitment. The experimental evidence and the geometrical models gave the basis for the design of a novel ankle replacement. A three-component, convex-tibia prosthesis was developed with articular surface shapes that are compatible with the geometry of the ligaments. The proposed prosthesis based on ligament/shape compatibility is showing encouraging results in initial implantation.

Early clinical results of the BOX ankle replacement are satisfactory: a multicenter feasibility study of 158 ankles

A new design for a 3-part ankle replacement was developed in an effort to achieve compatibility with the naturally occurring ligaments of the ankle by allowing certain fibers to remain isometric during passive motion. In order to test the design concept clinically, 158 prostheses were implanted in 156 patients within a 9-center trial and were followed up for a mean of 17 (range 6 to 48) months. The mean age at the time of surgery was 60.5 (range 29.7 to 82.5) years. Outcome measures included the American Orthopaedic Foot & Ankle Surgery hindfoot-ankle score and range of motion measured on lateral radiographs of the ankle. The preoperative American Orthopaedic Foot & Ankle Surgery score of 36.3 rose to 74.6, 78.6, 76.4, and 79.0, respectively, at 12, 24, 36, and 48 months. A significant correlation between meniscal bearing movement on the tibial component (mean 3.3 mm; range 2 to 11 mm) and range of flexion at the replaced ankle (mean 26.5°; range 14° to 53°) was observed in radiograms at extreme flexions. Two (1.3%) revisions in the second and third postoperative years necessitated component removal (neither were for implant failure), and 7 (4.4%) further secondary operations were required. The results of this investigation demonstrated that non-anatomic-shaped talar and tibial components, with a fully conforming interposed meniscal bearing, can provide safety and efficacy in the short term, although a longer follow-up period is required to more thoroughly evaluate this ankle implant.

Wear behaviour in total ankle replacement: a comparison between an in vitro simulation and retrieved prostheses

BACKGROUND

To minimise wear of the meniscal component in total ankle replacement, a three-component artificial joint has recently been developed. This new prosthesis has convex spherical tibial and anticlastic talar metal components with non-anatomic but ligament-compatible shapes in the sagittal plane, and a fully conforming ultra-high-molecular-weight-polyethylene meniscal component inserted in between. The in vitro wear of meniscal components can be assessed using a four-station joint simulator. The study was aimed at comparing wear patterns obtained in vitro with those observed in implant retrievals with the same design.

METHODS

The wear tests were run in a joint wear simulator at a frequency of 1.1 Hz for two million cycles. Three bearings within corresponding metal components were subjected to flexion/extension (range 0-58 degrees), anterior-posterior translation (0-5.2 mm), internal-external rotation (-1.9 degrees to +5.7 degrees), and a maximum axial load of 2.6 KN. These conditions were taken from the most recent findings in ankle joint mechanics. Three prostheses of the same type were harvested from patients due to replacement failures not associated with the device, 24, 24 and 9 months, respectively, after implantation. The in vitro worn components and the three retrievals were analysed by using a scanning electron microscope, a Coordinate Measuring Machine, and micro-Raman spectroscopy.

FINDINGS

Visual and microscopic observations, analyses, and Raman crystallinity-based measurements showed similarity between the patterns generated experimentally in the wear simulator and those seen in retrievals with similar wear life.

INTERPRETATION

A joint wear simulator like the one used in this study, once configured properly, appears to be suitable to assess wear rates also in total ankle prostheses.

Orthopaedic sport biomechanics - a new paradigm

This article proposes a new paradigm, "Orthopaedic sport biomechanics", for the understanding of the role of biomechanics in preventing and managing sports injury. Biomechanics has three main roles in this paradigm: (1) injury prevention, (2) immediate evaluation of treatment, and (3) long-term outcome evaluation. Related previous studies showing the approach in preventing and managing anterior cruciate ligament rupture and anterior talofibular ligament tear are highlighted. Orthopaedics and biomechanics specialists are encouraged to understand what they could contribute to the current and future practice of sports medicine.

Robotic cadaver testing of a new total ankle prosthesis model (German Ankle System)

BACKGROUND

An investigation was carried out into possible increased forces, torques, and altered motions during load-bearing ankle motion after implantation of two different total ankle prostheses. We hypothesized that the parameters investigated would not differ in relation to the two implants compared.

METHODS

We included two different ankle prostheses (Hintegra, Newdeal, Vienne, France; German Ankle System, R-Innovation, Coburg, Germany). The prostheses were implanted in seven paired cadaver specimens. The specimens were mounted on an industrial robot that enables complex motion under predefined conditions (RX 90, Stäubli, Bayreuth, Germany). The robot detected the load-bearing (30 kg) motion of the 100(th) cycle of the specimens without prostheses as the baseline for the later testing, and mimicked that exact motion during 100 cycles after the prostheses were implanted. The resulting forces, torques, and bone motions were recorded and the differences between the prostheses compared.

RESULTS

The Hintegra and German Ankle System, significantly increased the forces and torques in relation to the specimen without a prosthesis with one exception (one-sample-t-test, each p < or = 0.01; exception, parameter lateral force measured with the German Ankle System, p = 0.34). The force, torque, and motion differences between the specimens before and after implantation of the prostheses were lower with the German Ankle System than with the Hintegra (unpaired t-test, each p < or = 0.05).

CONCLUSIONS

The German Ankle System prosthesis had less of an effect on resulting forces and torques during partial weightbearing passive ankle motion than the Hintegra prosthesis. This might improve function and minimize loosening during the clinical use.

Mathematical models of passive motion at the human ankle joint by equivalent spatial parallel mechanisms

The paper presents a theoretical model of the ankle joint, i.e. tibio-talar articulation, which shows how the articular surfaces and the ligaments, acting together as a mechanism, can control the passive kinematics of the joint. The authors had previously shown that, in virtually unloaded conditions, the ankle behaves as a single degree-of-freedom system, and that two ligament fibres remain nearly isometric throughout the flexion arc. Two different equivalent spatial parallel mechanisms together with corresponding kinematic models were formulated. These assumed isometricity of fibres within the calcaneal-fibular and tibio-calcaneal ligaments and rigidity of the articulating surfaces, taken as three sphere-plane contacts in one model, and as a single spherical pair in the other. Geometry parameters for the models were obtained from three specimens. Motion predictions compare quite well with the measured motion of the specimens. The differences are accounted for by the simplifications adopted to represent the complex anatomical structures, and might be reduced by future more realistic representations of the natural articular surfaces.

Meniscal wear at a three-component total ankle prosthesis by a knee joint simulator

Despite the fundamental value of wear simulation studies to assess wear resistance of total joint replacements, neither specialised simulators nor established external conditions are available for the human ankle joint. The aim of the present study was to verify the suitability of a knee wear simulator to assess wear rates in ankle prostheses, and to report preliminary this rate for a novel three-component total ankle replacement design. Four intact 'small' size specimens of the Box ankle were analysed in a four-station knee wear simulator. Special component-to-actuator holders were manufactured and starting spatial alignment of the three-components was sought. Consistent load and motion cycles representing conditions at the ankle joint replaced exactly with the prosthesis design under analysis were taken from a corresponding mechanical model of the stance phase of walking. The weight loss for the three specimens, after two million cycles, was 32.68, 14.78, and 62.28mg which correspond to a linear penetration of 0.018, 0.008, and 0.034mm per million-cycle, respectively for the specimens #1, #2, and #3. The knee wear simulator was able to reproduce load-motion patterns typical of a replaced ankle. Motion of the meniscal bearing in between the tibial and talar components was smooth, this component remaining in place and in complete congruence with the metal components throughout the test.

Finite element analysis of a total ankle replacement during the stance phase of gait

Total ankle replacement (TAR) designs have still several important issues to be addressed before the treatment becomes fully acceptable clinically. Very little is known about the performance, in terms of the contact pressures and kinematics of TAR when subjected to daily activities such as level gait. For this purpose, an explicit finite element model of a novel 3-component TAR was developed, which incorporated a previously validated mechanical model of the ankle ligament apparatus. The intermediate mobile polyethylene meniscal bearing was modelled as an elastic-plastic continuum while the articulating surfaces of the tibial and talar metal components as rigid bodies. Overall kinematics, contact pressures and ligament forces were analysed during passive, i.e. virtually unloaded, and active, i.e. stance phase of gait, conditions. Simulation of passive motion predicted similar kinematics as reported previously in an analytical four-bar linkage model. The meniscal bearing was observed to move 5.6 mm posteriorly during the simulated stance and the corresponding antero-posterior displacement of the talar component was 8.3 mm. The predicted pattern and the amount (10.6 degrees ) of internal-external rotation of the ankle complex were found to be in good agreement with corresponding in vivo measurements on normal ankles. A peak contact pressure of 16.8 MPa was observed, with majority of contact pressures below 10 MPa. For most ligaments, reaction forces remain within corresponding physiological ranges. A first realistic representation of the biomechanical behaviour of the human ankle when replaced by prosthetic joints is provided. The applied methodology can potentially be applied to other TAR designs.

Mechanics of the anterior drawer test at the ankle: the effects of ligament viscoelasticity

The anterior drawer test at the human ankle joint is a routine clinical examination. The relationship between the mechanical response of this joint and the flexion angle was elucidated by a recent mathematical model, using purely elastic mechanical characteristics for the ligament fibres. The objective of the present work was to assess the effect of ligament viscoelasticity on the force response of the ankle joint for anterior displacements of the foot relative to the tibia, at different ankle flexion positions. A viscoelastic model of the ligaments from the literature was included in the recently proposed mathematical model. Drawer tests were simulated at several flexion angles and for increasing velocities of the imposed anterior displacement. The stiffness of the model ankle joint increased only modestly with velocity. The response force found for a 6mm displacement at 20 degrees plantarflexion increased by only 13% for a one hundred-fold increase in velocity from 0.1 to 10 mm/s. The flexion angle was confirmed as the most influential parameter in the mechanical response of the ankle to anterior drawer test.

Ligament fibre recruitment at the human ankle joint complex in passive flexion

Knowledge of ligament fibre recruitment at the human ankle joint complex is a fundamental prerequisite for analysing mobility and stability. Previous experimental and modelling studies have shown that ankle motion must be guided by fibres within the calcaneofibular and tibiocalcaneal ligaments, which remain approximately isometric during passive flexion. The purpose of this study was to identify these fibres. Three below-knee amputated specimens were analysed during passive flexion with combined radiostereometry for bone pose estimation and 3D digitisation for ligament attachment area identification. A procedure based on singular value decomposition enabled matching bone pose with digitised data and therefore reconstructing position in space of ligament attachment areas in each joint position. Eleven ordered fibres, connecting corresponding points on origin and insertion curves, were modelled for each of the following ligaments: posterior talofibular, calcaneofibular, anterior talofibular, posterior tibiotalar, tibiocalcaneal, and anterior tibiotalar. The measured changes in length for the ligament fibres revealed patterns of tightening and slackening. The most anterior fibre of the calcaneofibular and the medio-anterior fibre of the tibiocalcaneal ligament exhibited the most isometric behaviour, as well as the most posterior fibre of the anterior talofibular ligament. Fibres within the calcaneofibular ligament remain parallel in the transverse plane, while those within the tibiocalcaneal ligament become almost parallel in joint neutral position. For both these ligaments, fibres maintain their relative inclination in the sagittal plane throughout the passive flexion range. The observed significant change in both shape and orientation of the ankle ligaments suggest that this knowledge is fundamental for future mechanical analysis of their response to external forces.

Mobility of the human ankle and the design of total ankle replacement

Our prior research has shown that currently available total ankle implants fail to restore physiologic ankle mobility. Most of the modern mobile-bearing designs that feature a flat tibial component and a talar component with anatomic curvature in the sagittal plane function nonphysiologically with the natural ligament apparatus. To establish a more natural relationship between the implanted components and the retained ankle ligaments, we have developed a new design. According to our prior research, we suggest that physiologic ankle mobility is reproduced best with a design featuring a spherical convex tibial component, a talar component with radius of curvature in the sagittal plane longer than that of the natural talus, and a fully conforming meniscal component. Our preliminary observations in trial implantation and in a few patients suggest that while reproducing physiologic ankle mobility, the new design is capable of maintaining complete congruence at the two articulating surfaces of the meniscal bearing over the entire motion arc, with the prospect of minimizing wear of this component.