Experimental evaluation of current and novel approximations of articular surfaces of the ankle joint

Kinematics and flexibility properties of both natural and replaced ankle joints are affected by the geometry of the articulating surfaces. Recent studies proposed an original saddle-shaped, skewed, truncated cone with laterally oriented apex, as tibiotalar contact surfaces for ankle prosthesis. The goal of this study was to compare in vitro this novel design with traditional cylindrical or medially centered conic geometries in terms of their ability to replicate the natural ankle joint mechanics. Ten lower limb cadaver specimens underwent a validated process of custom design for the replacement of the natural ankle joint. The process included medical imaging, 3D modeling and printing of implantable sets of artificial articular surfaces based on these three geometries. Kinematics and flexibility of the overall ankle complex, along with the separate ankle and subtalar joints, were measured under cyclic loading. In the neutral and in maximum plantarflexion positions, the range of motion under torques in the three anatomical planes of the three custom artificial surfaces was not significantly different from that of the natural surfaces. In maximum dorsiflexion the difference was significant for all three artificial surfaces at the ankle complex, and only for the cylindrical and medially centered conic geometries at the tibiotalar joint. Natural joint flexibility was restored by the artificial surfaces nearly in all positions. The present study provides experimental support for designing articular surfaces matching the specific morphology of the ankle to be replace, and lays the foundations of the overall process for designing and manufacturing patient-specific total ankle replacements.     

Biomechanical consequences of subtalar joint arthroereisis in treating posterior tibial tendon dysfunction: a theoretical analysis using finite element analysis

Subtalar joint arthroereisis (SJA) has been introduced to control the hyperpronation in cases of flatfoot. The objective of this study is to evaluate the biomechanical consequence of SJA to restore the internal stress and load transfer to the intact state from the attenuated biomechanical condition induced by posterior tibial tendon dysfunction (PTTD). A three-dimensional finite element model of the foot and ankle complex was constructed based on clinical images of a healthy female (age 28 years, height 165 cm, body mass 54 kg). The boundary and loading condition during walking was acquired from the gait experiment of the model subject. Five sets of simulations (conditions) were completed: intact condition, mild PTTD, severe PTTD, mild PTTD with SJA, severe PTTD with SJA. The maximum von Mises stress of the metatarsal shafts and the load transfer along the midfoot during stance were analyzed. Generally, SJA deteriorated the joint force of the medial cuneonavicular and calcaneocuboid joints during late stance, while that of the metatarsocuneiform joints during early stance were over-corrected. Only the calcaneocuboid joint force at 45% stance demonstrated a trend of improvement. Besides, SJA exaggerated the increased stress of the metatarsals compared to the PTTD conditions, except that of the first metatarsal. Our study did not support the hypothesis that SJA can restore the internal load transfer and midfoot stress. SJA cannot compensate the salvage of midfoot stability attributed by PTTD and could be biomechanically insufficient to restore the biomechanical environment. Additional procedures such as orthotic intervention may be necessary.     

Effect of simulated joint instability and bracing on ankle and subtalar joint flexibility

It is clinically challenging to distinguish between ankle and subtalar joints instability in vivo. Understanding the changes in load-displacement at the ankle and subtalar joints after ligament injuries may detect specific changes in joint characteristics that cannot be detected by investigating changes in range of motion alone. The effect of restricting joints end range of motion with ankle braces was already established, but little is known about the effect of an ankle brace on the flexibility of the injured ankle and subtalar joints. Therefore, the purposes of this study were to (1) understand how flexibility is affected at the ankle and subtalar joints after sectioning lateral and intrinsic ligaments during combined sagittal foot position and inversion and during internal rotation and (2) investigate the effect of a semi-rigid ankle brace on the ankle and subtalar joint flexibility. Kinematics and kinetics were collected from nine cadaver feet during inversion through the range of ankle flexion and during internal rotation. Motion was applied with and without a brace on an intact foot and after sequentially sectioning the calcaneofibular ligament (CFL) and the intrinsic ligaments. Segmental flexibility was defined as the slope of the angle-moment curve for each 1 Nm interval. Early flexibility significantly increased at the ankle and subtalar joint after CFL sectioning during inversion. The semi-rigid ankle brace significantly decreased early flexibility at the subtalar joint during inversion and internal rotation for all ligament conditions and at the ankle joint after all ligaments were cut.     

An image-based kinematic model of the tibiotalar and subtalar joints and its application to gait analysis in children with Juvenile Idiopathic Arthritis

In vivo estimates of tibiotalar and the subtalar joint kinematics can unveil unique information about gait biomechanics, especially in the presence of musculoskeletal disorders affecting the foot and ankle complex. Previous literature investigated the ankle kinematics on ex vivo data sets, but little has been reported for natural walking, and even less for pathological and juvenile populations. This paper proposes an MRI-based morphological fitting methodology for the personalised definition of the tibiotalar and the subtalar joint axes during gait, and investigated its application to characterise the ankle kinematics in twenty patients affected by Juvenile Idiopathic Arthritis (JIA). The estimated joint axes were in line with in vivo and ex vivo literature data and joint kinematics variation subsequent to inter-operator variability was in the order of 1°. The model allowed to investigate, for the first time in patients with JIA, the functional response to joint impairment. The joint kinematics highlighted changes over time that were consistent with changes in the patient’s clinical pattern and notably varied from patient to patient. The heterogeneous and patient-specific nature of the effects of JIA was confirmed by the absence of a correlation between a semi-quantitative MRI-based impairment score and a variety of investigated joint kinematics indexes. In conclusion, this study showed the feasibility of using MRI and morphological fitting to identify the tibiotalar and subtalar joint axes in a non-invasive patient-specific manner. The proposed methodology represents an innovative and reliable approach to the analysis of the ankle joint kinematics in pathological juvenile populations.     

Influence of kinematic analysis methods on detecting ankle and subtalar joint instability

Patients with subtalar joint instability may be misdiagnosed with ankle instability, which may lead to chronic instability at the subtalar joint. Therefore, it is important to understand the difference in kinematics after ligament sectioning and differentiate the changes in kinematics between ankle and subtalar instability. Three methods may be used to determine the joint kinematics; the Euler angles, the Joint Coordinate System (JCS) and the helical axis (HA). The purpose of this study was to investigate the influence of using either method to detect subtalar and ankle joints instability. 3D kinematics at the ankle and subtalar joint were analyzed on 8 cadaveric specimens while the foot was intact and after sequentially sectioning the anterior talofibular ligament (ATFL), the calcaneofibular ligament (CFL), the cervical ligament and the interosseous talocalcaneal ligament (ITCL). Comparison in kinematics calculated from sensor and anatomical landmarks was conducted as well as the influence of Euler angles and JCS rotation sequence (between ISB recommendation and previous research) on the subtalar joint. All data showed a significant increase in inversion when the ITCL was sectioned. There were differences in the data calculated using sensors coordinate systems vs. anatomic coordinate systems. Anatomic coordinate systems were recommended for these calculations. The Euler angle and JCS gave similar results. Differences in Euler angles and JCS sequence lead to the same conclusion in detecting instability at the ankle and subtalar joint. As expected, the HA detected instability in plantarflexion at the ankle joint and in inversion at the subtalar joint.     

Contact pressure in the facet joint during sagittal bending of the cadaveric cervical spine

The facet joint contributes to the normal biomechanical function of the spine by transmitting loads and limiting motions via articular contact. However, little is known about the contact pressure response for this joint. Such information can provide a quantitative measure of the facet joint's local environment. The objective of this study was to measure facet pressure during physiologic bending in the cervical spine, using a joint capsule-sparing technique. Flexion and extension bending moments were applied to six human cadaveric cervical spines. Global motions (C2-T1) were defined using infra-red cameras to track markers on each vertebra. Contact pressure in the C5-C6 facet was also measured using a tip-mounted pressure transducer inserted into the joint space through a hole in the postero-inferior region of the C5 lateral mass. Facet contact pressure increased by 67.6 ± 26.9 kPa under a 2.4 Nm extension moment and decreased by 10.3 ± 9.7 kPa under a 2.7 Nm flexion moment. The mean rotation of the overall cervical specimen motion segments was 9.6 ± 0.8° and was 1.6 ± 0.7° for the C5-C6 joint, respectively, for extension. The change in pressure during extension was linearly related to both the change in moment (51.4 ± 42.6 kPa/Nm) and the change in C5-C6 angle (18.0 ± 108.9 kPa/deg). Contact pressure in the inferior region of the cervical facet joint increases during extension as the articular surfaces come in contact, and decreases in flexion as the joint opens, similar to reports in the lumbar spine despite the difference in facet orientation in those spinal regions. Joint contact pressure is linearly related to both sagittal moment and spinal rotation. Cartilage degeneration and the presence of meniscoids may account for the variation in the pressure profiles measured during physiologic sagittal bending. This study shows that cervical facet contact pressure can be directly measured with minimal disruption to the joint and is the first to provide local pressure values for the cervical joint in a cadaveric model.     

Motion control of the ankle joint with a multiple contact nerve cuff electrode: a simulation study

The flat interface nerve electrode (FINE) has demonstrated significant capability for fascicular and subfascicular stimulation selectivity. However, due to the inherent complexity of the neuromuscular skeletal systems and nerve–electrode interface, a trajectory tracking motion control algorithm of musculoskeletal systems for functional electrical stimulation using a multiple contact nerve cuff electrode such as FINE has not yet been developed. In our previous study, a control system was developed for multiple-input multiple-output (MIMO) musculoskeletal systems with little prior knowledge of the system. In this study, more realistic computational ankle/subtalar joint model including a finite element model of the sciatic nerve was developed. The control system was tested to control the motion of ankle/subtalar joint angles by modulating the pulse amplitude of each contact of a FINE placed on the sciatic nerve. The simulation results showed that the control strategy based on the separation of steady state and dynamic properties of the system resulted in small output tracking errors for different reference trajectories such as sinusoidal and filtered random signals. The proposed control method also demonstrated robustness against external disturbances and system parameter variations such as muscle fatigue. These simulation results under various circumstances indicate that it is possible to take advantage of multiple contact nerve electrodes with spatial selectivity for the control of limb motion by peripheral nerve stimulation even with limited individual muscle selectivity. This technology could be useful to restore neural function in patients with paralysis.     

Physical validation of a patient-specific contact finite element model of the ankle

A validation study was conducted to determine the extent to which computational ankle contact finite element (FE) results agreed with experimentally measured tibio-talar contact stress. Two cadaver ankles were loaded in separate test sessions, during which ankle contact stresses were measured with a high-resolution (Tekscan) pressure sensor. Corresponding contact FE analyses were subsequently performed for comparison. The agreement was good between FE-computed and experimentally measured mean (3.2% discrepancy for one ankle, 19.3% for the other) and maximum (1.5% and 6.2%) contact stress, as well as for contact area (1.7% and 14.9%). There was also excellent agreement between histograms of fractional areas of cartilage experiencing specific ranges of contact stress. Finally, point-by-point comparisons between the computed and measured contact stress distributions over the articular surface showed substantial agreement, with correlation coefficients of 90% for one ankle and 86% for the other. In the past, general qualitative, but little direct quantitative agreement has been demonstrated with articular joint contact FE models. The methods used for this validation enable formal comparison of computational and experimental results, and open the way for objective statistical measures of regional correlation between FE-computed contact stress distributions from comparison articular joint surfaces (e.g., those from an intact versus those with residual intra-articular fracture incongruity).     

In vitro assessment of a motion-based optimization method for locating the talocrural and subtalar joint axes

The locations of the joint axes of the ankle complex vary considerably between subjects, yet no noninvasive method with demonstrated accuracy exists for locating these axes. The moments of muscle and ground reaction forces about the joint axes are dependent on axis locations, making knowledge of these locations critical to accurate musculoskeletal modeling of the foot and ankle. The accuracy of a computational optimization method that fits a two-revolute model to measured motion was assessed using computer-generated data, a two-revolute mechanical linkage, and three lower-leg cadaver specimens. Motions were applied to cadaver specimens under axial load while bone-mounted markers attached to the tibia, talus, and calcaneus were tracked using a video-based motion analysis system. Estimates of the talocrural and subtalar axis locations were computed from motions of the calcaneus relative to the tibia using the optimization method. These axes were compared to mean helical axes computed directly from tibia, talus, and calcaneus motions. The optimization method performed well when the motions were computer-generated or measured in the mechanical linkage, with angular differences between optimization and mean helical axes ranging from 1 deg to 5 deg. In the cadaver specimens, however, these differences exceeded 20 deg. Optimization methods that locate the anatomical joint axes of the ankle complex by fitting two revolute joints to measured tibia-calcaneus motions may be limited because of problems arising from non-revolute behavior.     

Kinematics of the human ankle complex in passive flexion; a single degree of freedom system

The restoration of original range and pattern of motion is the primary goal of joint replacement and ligament reconstruction. The objective of the present work is to investigate whether or not a preferred path of joint motion at the intact human ankle complex is exhibited during passive flexion. A rig was built to move the ankle complex through its range of flexion while applying only the minimum necessary load to drive ankle flexion. Joint motion was constrained only by the articular surfaces and the ligaments. The movements of the calcaneus, talus and fibula relative to the stationary tibia in seven cadaveric specimens were tracked with a stereophotogrammetric system. It was shown that the calcaneus follows a unique path of unresisted coupled motion relative to the tibia and that most of the motion occurred at the ankle, with little motion at the subtalar level. The calcaneofibular and the tibiocalcaneal ligaments showed near-isometric pattern of rotations. All specimens showed motion of the axis of rotation relative to the bones. Deviations from the unique path due to the application of load involved mostly subtalar motion and were resisted. The ankle complex exhibits one degree of unresisted freedom, the ankle behaving as a single degree of freedom mechanism and the subtalar as a flexible structure. We deduced that the calcaneofibular and tibiocalcaneal ligaments together with the articular surfaces guide ankle passive motion, other ligaments limit but do not guide motion.     

Mobility of the subtalar joint in the intact ankle complex

A previous study by these authors showed that the calcaneus follows a unique path of unresisted coupled motion relative to the tibia during passive flexion and that most of this motion occurred at the ankle level. Subtalar motion in the intact ankle complex was observed only when perturbations from this path were induced by the application of force to the calcaneus. Relative motion of the bones of the ankle complex was tracked by stereophotogrammetry in seven specimens. Anatomical landmarks, reference frames and joint angles were defined by standard techniques. Sequential moments were applied to the calcaneus about the long axis of the tibia. Measured movements at subtalar level demonstrated plantarflexion coupled to supination and internal rotation (inversion) and dorsiflexion coupled to pronation and external rotation (eversion). These movements were resisted and were fully recovered when the external load was removed. Subtalar motion diminished as the ankle approached maximal dorsi- and plantarflexion. Two clearly distinguished mean axes of rotation were observed for inversion and eversion runs. The axes of inversion and eversion of the subtalar complex changed orientation along a preferred and repeatable path. The subtalar joint complex occupied only a single stable position in the unloaded state and with no range of unresisted motion. It is inferred that mobility was possible only by the stretching and lengthening of the ligaments and the indentation of the articular surfaces, requiring the application of loads. The subtalar joint complex behaves like a flexible structure.     

Effects of footwear on three-dimensional tibiotalar and subtalar joint motion during running

Running is a popular form of recreation, but injuries are common and may be associated with abnormal joint motion. The objective of this study was to determine the effect of three footwear conditions – barefoot (BF), an ultraflexible training shoe (FREE), and a motion control shoe (MC) – on 3D foot and ankle motion. Dynamic, biplane radiographic images were acquired from 12 runners during overground running. 3D rotations of the tibiotalar and subtalar joints were quantified in terms of plantarflexion/dorsiflexion (PF/DF), inversion/eversion (IN/EV) and internal/external rotation (IR/ER). Across the early stance phase (defined as footstrike to heel-off), BF running demonstrated greater tibiotalar joint range of motion for PF/DF (28.2±8.3°) and IR/ER (7.0±1.4°) than the shod conditions (FREE: PF/DF=15.1±5.9°, IR/ER=4.8±2.1°; MC: PF/DF=15.0±6.2°, IR/ER=4.3±0.7°). Also at the tibiotalar joint, BF running resulted in a position significantly more plantarflexed (BF: 2.0±12.5°, FREE: 15.7±12.2°, MC: 16.5±9.3°) and internally rotated (BF: 12.9±4.5°, FREE: 10.7±4.3°, MC: 10.6±3.9°) at footstrike compared to both shod conditions. No differences were detected between the shod conditions at any point in the early stance phase at the tibiotalar joint. The MC condition demonstrated significant differences compared to FREE at several points throughout the early stance phase at the subtalar joint, with the greatest differences seen at 30% in PF/DF (MC −1.4±8.8°: FREE: −0.5±9.0°), IN/EV (MC −8.1±5.7°: FREE −6.3±5.5°) and IR/ER (MC −9.5±5.3°: FREE: −8.7±5.2°). These findings indicate that footwear has subtle effects on joint motion mainly between BF and shod conditions at the tibiotalar joint and between shod conditions at the subtalar joint.     

Articular contact at the tibiotalar joint in passive flexion

The knowledge of the contact areas at the tibiotalar articulating surfaces during passive flexion is fundamental for the understanding of ankle joint mobility. Traditional contact area reports are limited by the invasive measuring techniques used and by the complicated loading conditions applied. In the present study, passive flexion tests were performed on three anatomical preparations from lower leg amputation. Roentgen Stereophotogrammetric Analysis was used to accurately reconstruct the position of the tibia and the talus at a number of unconstrained flexion positions. A large number of points was collected on the surface of the tibial mortise and on the trochlea tali by a 3-D digitiser. Articular surfaces were modelled by thin plate splines approximating these points. Relative positions of these surfaces in all the flexion positions were obtained from corresponding bone position data. A distance threshold was chosen to define contact areas. A consistent pattern of contact was found on the articulating surfaces. The area moved anteriorly on both articular surfaces with dorsiflexion. The average position of the contact area centroid along the tibial mortise at maximum plantarflexion and at maximum dorsiflexion was respectively 58% posterior and 40% anterior of the entire antero-posterior length. For increasing dorsiflexion, the contact area moved from medial to lateral in all the specimens.     

Analysis of surface-to-surface distance mapping during three-dimensional motion at the ankle and subtalar joints

Joint surface interaction and ligament constraints determine the kinematic characteristics of the ankle and subtalar joints. Joint surface interaction is characterized by joint contact mechanics and by relative joint surface position potentially characterized by distance mapping. While ankle contact mechanics was investigated, limited information is available on joint distance mapping and its changes during motion. The purpose of this study was to use image-based distance mapping to quantify this interaction at the ankle and subtalar joints during tri-planar rotations of the ankle complex. Five cadaveric legs were scanned using Computed Tomography and the images were processed to produce 3D bone models of the tibia, fibula, talus and calcaneus. Each leg was tested on a special linkage through which the ankle complex was loaded in dorsiflexion/plantarflexion, inversion/eversion, and internal/external rotation and the resulting bone movements were recorded. Fiduciary bone markers data and 3D bone models were combined to generate color-coded distance maps for the ankle and subtalar joints. The results were processed focusing on the changes in surface-to-surface distance maps between the extremes of the range of motion and neutral. The results provided detailed insight into the three-dimensional highly coupled nature of these joints showing significant and unique changes in distance mapping from neutral to extremes of the range of motion. The non-invasive nature of the image-based distance mapping technique could result, after proper modifications, in an effective diagnostic and clinical evaluation technique for application such as ligament injuries and quantifying the effect of arthrodesis or total ankle replacement surgery.     

In vivo cartilage contact strains in patients with lateral ankle instability

Damage to the anterior talofibular ligament (ATFL) and cacaneofibular ligament (CFL) during an ankle sprain may be linked to the development of osteoarthritis. Although altered tibiotalar kinematics have been demonstrated, the effects of lateral ankle instability (LAI) on in vivo cartilage strains have not been described. We hypothesized that peak cartilage strains increase, and the location is shifted in patients with ATFL injuries. We used 3-D MRI models and biplanar fluoroscopy to evaluate in vivo cartilage contact strains in seven patients with unilateral LAI. Subjects had chronic unilateral ATFL injury or combined ATFL and CFL injury, and were evaluated with increasing load while stepping onto a force plate. Peak cartilage strain and the location of the peak strain were measured using the contralateral normal ankle as a control. Ankles with LAI demonstrated significantly increased peak strain when compared with ATFL-intact controls. For example, at 100% body weight, peak strain was 29±8% on the injured side compared to 21±5% on the intact side. The position of peak strain on the injured ankle also showed significant anterior translation and medial translation. At 100% body weight, the location of peak strain in the injured ankle translated anteriorly by 15.5±7.1 mm and medially by 12.9±4.3 mm relative to the intact ankle. These changes correspond to the region of clinically observed osteoarthritis. Chronic LAI, therefore, may contribute to the development of tibiotalar cartilage degeneration due to altered cartilage strains.     

The three-dimensional kinematics and flexibility characteristics of the human ankle and subtalar joints--Part I: Kinematics

The in-vitro, three dimensional kinematic characteristics of the human ankle and subtalar joint were investigated in this study. The main goals of this investigation were: 1) To determine the range of motion of the foot-shank complex and the associated range of motion of the ankle and subtalar joints; 2) To determine the kinematic coupling characteristics of the foot-shank complex, and 3) To identify the relationship between movements at the ankle and subtalar joints and the resulting motion produced between the foot and the shank. The tests were conducted on fifteen fresh amputated lower limbs and consisted of incrementally displacing the foot with respect to the shank while the motion of the articulating bones was measured through a three dimensional position data acquisition system. The kinematic analysis was based on the helical axis parameters describing the incremental displacements between any two of the three articulating bones and on a joint coordinate system used to describe the relative position between the bones. From the results of this investigation it was concluded that: 1) The range of motion of the foot-shank complex in any direction (dorsiflexion/plantarflexion, inversion/eversion and internal rotation/external rotation) is larger than that of either the ankle joint or the subtalar joint.; 2) Large kinematic coupling values are present at the foot-shank complex in inversion/eversion and in internal rotation/external rotation. However, only a slight amount of coupling was observed to occur in dorsiflexion/plantarflexion.; 3) Neither the ankle joint nor the subtalar joint are acting as ideal hinge joints with a fixed axis of rotation.; 4) Motion of the foot-shank complex in any direction is the result of rotations at both the ankle and the subtalar joints. However, the contribution of the ankle joint to dorsiflexion/plantarflexion of the foot-shank complex is larger than that of the subtalar joint and the contribution of the subtalar joint to inversion/eversion is larger than that of the ankle joint.; 5) The ankle and the subtalar joints have an approximately equal contribution to internal rotation/external rotation movements of the foot-shank complex.     

A robotic cadaveric flatfoot analysis of stance phase

The symptomatic flatfoot deformity (pes planus with peri-talar subluxation) can be a debilitating condition. Cadaveric flatfoot models have been employed to study the etiology of the deformity, as well as invasive and noninvasive surgical treatment strategies, by evaluating bone positions. Prior cadaveric flatfoot simulators, however, have not leveraged industrial robotic technologies, which provide several advantages as compared with the previously developed custom fabricated devices. Utilizing a robotic device allows the researcher to experimentally evaluate the flatfoot model at many static instants in the gait cycle, compared with most studies, which model only one to a maximum of three instances. Furthermore, the cadaveric tibia can be statically positioned with more degrees of freedom and with a greater accuracy, and then a custom device typically allows. We created a six degree of freedom robotic cadaveric simulator and used it with a flatfoot model to quantify static bone positions at ten discrete instants over the stance phase of gait. In vivo tibial gait kinematics and ground reaction forces were averaged from ten flatfoot subjects. A fresh frozen cadaveric lower limb was dissected and mounted in the robotic gait simulator (RGS). Biomechanically realistic extrinsic tendon forces, tibial kinematics, and vertical ground reaction forces were applied to the limb. In vitro bone angular position of the tibia, calcaneus, talus, navicular, medial cuneiform, and first metatarsal were recorded between 0% and 90% of stance phase at discrete 10% increments using a retroreflective six-camera motion analysis system. The foot was conditioned flat through ligament attenuation and axial cyclic loading. Post-flat testing was repeated to study the pes planus deformity. Comparison was then made between the pre-flat and post-flat conditions. The RGS was able to recreate ten gait positions of the in vivo pes planus subjects in static increments. The in vitro vertical ground reaction force was within ± 1 standard deviation (SD) of the in vivo data. The in vitro sagittal, coronal, and transverse plane tibial kinematics were almost entirely within ± 1 SD of the in vivo data. The model showed changes consistent with the flexible flatfoot pathology including the collapse of the medial arch and abduction of the forefoot, despite unexpected hindfoot inversion. Unlike previous static flatfoot models that use simplified tibial degrees of freedom to characterize only the midpoint of the stance phase or at most three gait positions, our simulator represented the stance phase of gait with ten discrete positions and with six tibial degrees of freedom. This system has the potential to replicate foot function to permit both noninvasive and surgical treatment evaluations throughout the stance phase of gait, perhaps eliciting unknown advantages or disadvantages of these treatments at other points in the gait cycle.     

In vivo kinematics and articular surface congruency of total ankle arthroplasty during gait

Relatively high rates of loosening and implant failure have been reported after total ankle arthroplasty. Abnormal kinematics and incongruency of the articular surface may cause increased contact pressure and rotational torque applied to the implant, leading to loosening and implant failure. We measured in vivo kinematics of two-component total ankle arthroplasty (TNK ankle), and assessed congruency of the articular surface during the stance phase of gait. Eighteen ankles of 15 patients with a mean age of 75±6 years (mean±standard deviation) and follow-up of 44±38 months were enrolled. Lateral fluoroscopic images were taken during the stance phase of gait. 3D-2D model-image registration was performed using the fluoroscopic image and the implant models, and three-dimensional kinematics of the implant and incongruency of the articular surface were determined. The mean ranges of motion were 11.1±4.6°, 0.8±0.4°, and 2.6±1.5° for dorsi-/plantarflexion, inversion/eversion, and internal/external rotation, respectively. At least one type of incongruency of the articular surface occurred in eight of 18 ankles, including anterior hinging in one ankle, medial or lateral lift off in four ankles, and excessive axial rotation in five ankles. Among the four ankles in which lift off occurred during gait, only one ankle showed lift off in the static weightbearing radiograph. Our observations will provide useful data against which kinematics of other implant designs, such as three-component total ankle arthroplasty, can be compared. Our results also showed that evaluation of lift off in the standard weightbearing radiograph may not predict its occurrence during gait.     

Changes in talocrural joint contact stress characteristics after simulated rotationplasty

In order to assess the changes in talocrural joint contact stress after rotationplasty, 10 lower-leg cadaver specimens were axially loaded with 600 N and investigated in two loading situations: (1) Normal loading with a plantigrade foot; (2) in an equinus position of a simulated rotationplasty. Joint contact stress in the talar facet of the talocrural joint was determined with Fuji Prescale film cut to size and analyzed with digital image analysis for joint contact area, mean and peak pressure, contact force, and location of the load application on the trochlea tali. The results demonstrate a significant transfer on the loading zone to the posterior part of the talus (p = 0.005), a significant reduction of the contact area (p = 0.005) and force (p = 0.005), and a significant increase of the mean (p = 0.022) and maximum pressures (p = 0.013). These results indicate that the rotationplasty causes pronounced changes in joint loading characteristics.