The instantaneous helical axis of the subtalar and talocrural joints: a non-invasive in vivo dynamic study

Assistance force-line of exosuit affects ankle multidimensional motion: a theoretical and experimental study

BACKGROUND

The talocrural joint and the subtalar joint are the two major joints of the ankle-joint complex. The position and direction of the exosuit force line relative to these two joint axes can influence ankle motion. We aimed to understand the effects of different force-lines on ankle multidimensional motion.

METHODS

In this article, three assistance force line schemes for ankle exosuits were proposed: perpendicular to the talocrural joint axis (PT), intersecting with the subtalar joint axis (IS), and parallel to the triceps surae (PTS). A theoretical model was proposed to calculate the exosuit's assistance moment. Seven participants completed four experimental tests of ankle plantarflexion, including three passive motions assisted by the PT, PTS and IS schemes, and one active motion without exosuit assistance (Active).

RESULTS

The simulation results demonstrated that all three exosuits were able to produce significant moments of ankle plantarflexion. Among these, the PT scheme exhibited the highest moments in all dimensions, followed by the PTS and IS schemes. The experimental findings confirmed the effectiveness of all three exosuit schemes in assisting ankle plantarflexion. Additionally, as the assistive force lines approached the subtalar joint, there was a decrease in ankle motion assisted by the exosuits in non-plantarflexion directions, along with a reduction in the average distance of ankle angle curves relative to active ankle motion. Furthermore, the linear correlation coefficients between inversion and plantarflexion, adduction and plantarflexion, and adduction and inversion gradually converged toward active ankle plantarflexion motion.

CONCLUSIONS

Our research indicates that the position of the exosuit force line to the subtalar joint has a significant impact on ankle inversion and adduction. Among all three schemes, the IS, which has the closest distance to the subtalar joint axes, has the greatest kinematic similarity to active ankle plantarflexion and might be a better choice for ankle assistance and rehabilitation.

Development and validation of a novel ankle joint musculoskeletal model

An improved understanding of contact mechanics in the ankle joint is paramount for implant design and ankle disorder treatment. However, existing models generally simplify the ankle joint as a revolute joint that cannot predict contact characteristics. The current study aimed to develop a novel musculoskeletal ankle joint model that can predict contact in the ankle joint, together with muscle and joint reaction forces. We modelled the ankle joint as a multi-axial joint and simulated contact mechanics between the tibia, fibula and talus bones in OpenSim. The developed model was validated with results from experimental studies through passive stiffness and contact. Through this, we found a similar ankle moment-rotation relationship and contact pattern between our study and experimental studies. Next, the musculoskeletal ankle joint model was incorporated into a lower body model to simulate gait. The ankle joint contact characteristics, kinematics, and muscle forces were predicted and compared to the literature. Our results revealed a comparable peak contact force and the same muscle activation patterns in four major muscles. Good agreement was also found in ankle dorsi/plantar-flexion and inversion/eversion. Thus, the developed model was able to accurately model the ankle joint and can be used to predict contact characteristics in gait.

Talocalcaneal Ligament Reconstruction Kinematic Simulation for Progressive Collapsing Foot Deformity

BACKGROUND

In progressive collapsing foot deformity (PCFD), an internal and plantar rotation of the talus relative to the calcaneus may result in painful peritalar subluxation. Medial soft tissue procedures (eg, spring ligament repair) aim to correct the talar position via the navicular bone if bony correction alone is not sufficient. The effect of the medial soft tissue reconstruction on the talar reposition remains unclear. We hypothesized that a subtalar talocalcaneal ligament reconstruction might be favorable in PCFD to correct talar internal malposition directly. This pilot study aims to evaluate the anatomical feasibility and kinematic behavior of a subtalar ligament reconstruction in PCFD.

METHODS

Three-dimensional surface model from 10 healthy ankles were produced. A total of 1089 different potential ligament courses were evaluated in a standardized manner. A motion of inversion/eversion and talar internal/external in relation to the calcaneus were simulated and the ligament strain, expressed as a positive length variation, for each ligament was analyzed. The optimal combination for the ligament reconstruction with increased length in internal rotation of the talus, isometric kinematic behavior in inversion/eversion, and extraarticular insertion on talus and calcaneus was selected.

RESULTS

A laterodistal orientation of the talar insertion point in respect to the subtalar joint axis and laterodistal deviation of the calcaneal insertion point presents the highest ligament lengthening in internal talar rotation (+0.56 mm [3.8% of total length]) and presented a near-isometric performance in inversion/eversion (+0.01 to -0.01 mm [0.1% of total length]).

CONCLUSION

This kinematic model shows that a ligament reconstruction in the subtalar space presents a pattern of length variation that may stabilize the internal talar rotation without impeding the physiological subtalar motion.

CLINICAL RELEVANCE

This study investigates the optimal location, feasibility, and kinematic behavior of a ligament reconstruction that could help stabilize peritalar subluxation in progressive collapsing foot deformity.

[Formula: see text].

Ankle and subtalar joint axes of rotation and center of rotation during walking and running in healthy individuals measured using dynamic biplane radiography

The goal of this study was to determine how foot type and activity level affect ankle and hindfoot motion. Dynamic biplane radiography and a validated volumetric registration process was used to measure ankle and hindfoot motion of 20 healthy adults during walking and running. The helical axes of motion (HAM) during stance were calculated at the tibiotalar and subtalar joints. The intersection of each HAM and the rotation plane of interest defined the tibiotalar and subtalar centers of rotation (COR). Correlations between foot type and hindfoot kinematics were calculated using Pearson's correlations. The effect of activity, phase of gait, and dominant vs. non-dominant limb on HAM and COR were evaluated using linear mixed effects models. Activity and phase of gait influenced the superior location of the tibiotalar (p < 0.041) and subtalar (p < 0.044) CORs. Activity and gait phase affected tibiotalar (p < 0.049) and subtalar (p < 0.044) HAM direction during gait. Both HAM orientation and COR location changed with activity and phase of gait. These ankle and hindfoot kinematics have implications for total ankle replacement design and musculoskeletal models that estimate force and moment generating capabilities of muscles.

Hindfoot motion through helical axis image-based on dynamic CT scan using an original simulated weightbearing device

BACKGROUND

Determining the treatment of subtalar joint (STJ) instability requires a better understanding of the biomechanical principles underlying the condition and, a proper diagnosis. This study aimed to analyze "in vivo" the range of motion of the subtalar joint (STJ) measured on two (2D) and three dimensions (3D) image-based on CT Scan using an original device that maintains a simulated weightbearing. The secondary goal was to correlate the 2D and 3D measurement.

METHODS

An observational study was conducted, using an original Dynamic Simulated Weightbearing Device. Asymptomatic ankles were included. Each subject underwent a CT scan under mechanical stress and simulated weightbearing conditions, maintaining maximum eversion and inversion hindfoot positions. The images were obtained, combining both inversion and eversion positions in a single model, which allows for to calculation of the motion vector as well as the helical axis. The helical axis (rotation angle and translation distance), subtalar tilt, anterior drawer, and, subtalar and calcaneocuboid uncoverage were the determinations.

RESULTS

Forty asymptomatic ankles were included. The average range of motion of the STJ amounts to 31.5° ± 9.1° of rotation and 1.56 ± 0.8 mm of translation distance. The anterior drawer and subtalar uncoverage variables were statistically significantly related to each other (r = 0.57; P = 0.00001). However, these 2-D measured variables were not related to kinematic measures of rotation through the helical axis (3D) (p = 0.14; p = 0.19) CONCLUSIONS: The average range of motion of the STJ amounts to 31.5° ± 9.1° of rotation and 1.56 ± 0.8 mm of translation distance. We found no significant correlation between 2D and 3D measurements. In our opinion, the rotation angle and translation distance should be considered the most accurate measurements and should be calculated on every STJ instability for comparison with the asymptomatic population LEVEL OF EVIDENCE: Observational study.

LEVEL OF EVIDENCE III

In vivo evaluation of ankle kinematics and tibiotalar joint contact strains using digital volume correlation and 3 T clinical MRI

BACKGROUND

In vivo evaluation of ankle joint biomechanics is key to investigating the effect of injuries on the mechanics of the joint and evaluating the effectiveness of treatments. The objectives of this study were to 1) investigate the kinematics and contact strains of the ankle joint and 2) to investigate the correlation between the tibiotalar joint contact strains and the prevalence of osteochondral lesions of the talus distribution.

METHODS

Eight healthy human ankle joints were subjected to compressive load and 3 T MRIs were obtained before and after applying load. The MR images in combination with digital volume correlation enabled non-invasive measurement of ankle joint kinematics and tibiotalar joint contact strains in three dimensions.

FINDINGS

The total translation of the calcaneus was smaller (0.48 ± 0.15 mm, p < 0.05) than the distal tibia (0.93 ± 0.16 mm) and the talus (1.03 ± 0.26 mm). These movements can produce compressive and shear joint contact strains (approaching 9%), which can cause development of lesions on joints. 87.5% of peak tensile, compressive, and shear strains in the tibiotalar joint took place in the medial and lateral zones.

INTERPRETATION

The findings suggested that ankle bones translate independently from each other, and in some cases in opposite directions. These findings help explain the distribution of osteochondral lesions of the talus which have previously been observed to be in medial and lateral regions of the talar dome in 90% of cases. They also provide a reason for the central region of talar dome being less susceptible to developing osteochondral lesions.

In Vivo Evaluation of the Position and Orientation of the Geometric Axis of the Tibiotalar Joint

Background

Fitting the surface morphology of the talar trochlea is one of the common methods to define the geometric axis of the tibiotalar joint (GATJ). However, the in vivo motion of such axis during gait has not been fully investigated.

Methods

The ankle kinematic data of fifteen volunteers were collected by a dual fluoroscopic imaging system with a model-image registration method. The GATJ was defined by sphere-fitting the medial or lateral part of the trochlear surface of the talus. The position and orientation of this axis during gait were measured. To verify this axis, the distances of the feature points of the talus to the GATJ during gait were also measured.

Results

There was no statistically significant difference in the distances of feature points of the talus to the GATJ among the seven key poses of the gait cycle. And the position and orientation of the GATJ during gait also showed no statistically significant difference.

Conclusion

The GATJ is the axis about which the talus rotated. And it is one fixed axis during gait. The current finding may help the design of the talar component for total ankle replacement based on the surface morphology of the talar trochlea.

An optimal method for calculating an average screw axis for a joint, with improved sensitivity to noise and providing an analysis of the dispersion of the instantaneous axes

The instantaneous (ISA) and average (ASA) screw axes are techniques commonly adopted in motion analysis to functionally locate the rotation axis and center of rotation of a joint. Several approaches for calculating such axes were proposed in literature and the main limitations were identified as the need for using a threshold on angular displacements or velocities for handling the cases where the ISA is ill-defined and the need for a method for reliably estimating the center or rotation in limit cases, such as a purely rotational motion in the three-dimensional space. Furthermore, in many applications, such as in biomechanics, it is useful to statistically estimate the dispersion or variation of the ISA with respect to the ASA. In this paper we propose a novel method for estimating an ASA. Our method represents an improvement over previous methods as it: (i) exploits an optimization procedure based on the instantaneous differential kinematics (screw twist); (ii) removes the need for a threshold by introducing a weighting based on the norm of angular velocity; (iii) handles the singular cases where the position of the ASA is ill-defined by proposing a regularization term in the optimization. In addition, we proposed a method for estimating the uncertainty in the ASA calculation. The same quantities serve as a measure of the dispersion of the ISAs with respect to the ASA. The method was tested on real data and surrogate data: (i) a human gait analysis trial representing the motion of a knee, (ii) the experimental recording of the free swing motion of a mechanical hinge and (iii) synthetically generated motion data of a purely rotational (cylindrical) motion. The results showed that the new method had a lower sensitivity to noise, was able to reasonably handle the singular cases and provide a detailed analysis of ISA dispersion.

Human ankle joint movements during walking are probably not determined by talar morphology

Knowledge about the orientation of a representative ankle joint axis is limited to studies of tarsal morphology and of quasistatic movements. The aim of our study was therefore to determine the development of the axis orientation during walking. Intracortical bone pins were used to monitor the kinematics of the talus and tibia of five healthy volunteers. The finite helical axis was determined for moving windows of 10% stance phase and its orientation reported if the rotation about the axis was more than 2°. A representative axis for ankle dorsi- and plantarflexion was also estimated based on tarsal morphology. As reported by literature, the morphology-based axis was inclined more medially upwards for dorsiflexion than for plantarflexion. However, when a mean of the finite helical axis orientations was calculated for each walking trial for dorsiflexion (stance phase 15–25%) and for plantarflexion (stance phase 85–95%), the inclination was less medially upwards in dorsiflexion than in plantarflexion in four out of five participants. Thus, it appears that the inclination of a representative ankle joint axis for dynamic loading situations cannot be estimated from either morphology or quasi-static experiments. Future studies assessing muscle activity, ligament behaviour and articulating surfaces may help to identify the determining factors for the orientation of a representative ankle joint axis.

Sensitivity of Internal Tibial Forces and Moments to Static Optimization Moment Constraints At the Subtalar and Ankle Joints

We examined the sensitivity of internal tibial forces and moments during running to different subtalar/ankle moment constraints in a static optimization routine. Seventeen participants ran at 2.20, 3.33, and 4.17 ms-1 while force and motion data were collected. Ankle joint contact force was estimated using inverse-dynamics-based static optimization. Three sets of joint moment constraints were tested. All sets included the flexion-extension and abduction-adduction moments at the hip and the flexion-extension moment at the knee, but differed in the constraints used at the subtalar/ankle: 1) flexion-extension at the ankle (Sag), 2) flexion-extension and inversion-eversion at ankle (Sag+Front), and 3) flexion-extension at the ankle and supination-pronation at the subtalar (Sag+SubT). Internal tibial forces and moments were quantified at the distal one-third of the tibia, by ensuring static equilibrium with applied forces and moments. No interaction was observed between running speed and constraint for internal tibial forces or moments. Sag+SubT resulted in larger internal mediolateral force (+41%), frontal (+79%), and transverse (+29%) plane moments, compared to Sag and Sag+Front. Internal axial force was greatest in Sag+Front, compared to Sag and Sag+SubT (+37%). Faster running speeds resulted in greater internal tibial forces and moments in all directions (=+6%). Internal tibial forces and moments at the distal one-third of the tibia were sensitive to the subtalar and ankle joint moment constraints used in the static optimization routine, independent of running speed.

Tibial torsion correlates with talar morphology

BACKGROUND

There is limited literature on axial rotation of the ankle or variations in anatomy of the talus. We aim to evaluate the rotational profile of the distal tibia and its relationship to talus morphology, radiographic foot-type, and tibiotalar tilt in arthritic ankles.

METHODS

Preoperative imaging was reviewed in 173 consecutive patients with ankle arthritis. CT measurements were used to calculate tibial torsion and the talar neck-body angle (TNBA). Tibiotalar tilt and foot-type were measured on weightbearing plain radiographs.

RESULTS

Measurements indicated mean external tibial torsion of 29.2±9.1˚ and TNBA of 35.2±7.5˚ medial. Tibiotalar tilt ranged from 48˚ varus to 23.5˚ valgus. A moderate association between increasing external tibial torsion and decreasing TNBA was found (ρ=-0.576, p<.0001). Weak relationships were found between external tibial torsion and varus tibiotalar tilt (ρ=-0.239, p=.014) and plantarflexion of the talo-first metatarsal angle (ρ=-0.218, p<.025).

CONCLUSION

We observed a statistically significant correlation between tibial torsion and morphology of the talus, tibiotalar tilt, and first ray plantarflexion. This previously unreported association may provide information regarding the development of foot and ankle deformity and pathology.

LEVEL OF EVIDENCE

Level III.

Subtalar axis determined by combining digital twins and artificial intelligence: influence of the orientation of this axis for hindfoot compensation of varus and valgus knees

PURPOSE

Previous studies evaluating hindfoot and knee alignment have suggested compensation between the knee and the hindfoot deformities. However, these studies did not investigate the influence of the orientation of the subtalar axis on the results.

MATERIAL AND METHODS

Using computed tomography data of patients without osteoarthritis, digital twins, and artificial intelligence, we identified the orientation of the axis of the subtalar joint. Compensation was evaluated in the subtalar joint according to angular knee deformity and subtalar axis direction.

RESULTS

With the inclination angle defined as the angle between the axis and the XY plane (horizontal) and the deviation angle defined as the angle between the projection of axis on the XZ plane, the inclination angle of the subtalar helical axis showed an average angle of 35.3° (range 5° to 48°). The mean deviation angle for the helical axis was 6.4° (range - 4° to + 12°). Our findings indicated that an increase of the inclination angle of the subtalar axis tends to limit adjustment in the hindfoot alignment toward re-balance of the whole lower limb toward a neutral weight-bearing axis when malalignment of the knee occurs.

CONCLUSION

Malalignment of the knee and different compensations in the hindfoot contribute to various combined deformities in the population: associated valgus or varus deformities and inverse associations of varus/valgus deformities.

Comparison between Helical Axis and SARA Approaches for the Estimation of Functional Joint Axes on Multi-Body Modeling Data

Functional methods usually allow for a flexible and accurate representation of joint kinematics and are increasingly implemented both for clinical and biomechanics research purposes. This paper presents a quantitative comparison between two widely adopted methods for functional axis estimation, that is, the helical axis theory and the symmetrical axis of rotation approach (SARA). To this purpose, a multi-body model was developed to simulate the lower limb of a subject. This model was designed to reproduce different motion patterns, that is, by selecting the active degrees of freedom of the simulated ankle joint. Thanks to virtual markers attached to each segment, the multi-body model was used to generate simulated motion capture data that were then analyzed by instantaneous helical axes and SARA algorithms. To achieve a synthetic representation of joint kinematics, a mean helical axis and an average SARA functional axis were estimated, along with dispersion parameters and rms distance data that were used to quantitatively assess the performance of each method. The sensitivity of each algorithm to different combinations of range and speed of motion, scattering of marker clusters, sampling rate, and additive noise on markers’ trajectories, was finally evaluated.

Establishing a New Patient-Specific Implantation Technique for Total Ankle Replacement: An In Vitro Study

BACKGROUND

Revision rates after total ankle replacements (TARs) are higher compared with other total joint replacements. The present study aimed to establish a new patient-specific implantation (PSI) technique for TAR.

MATERIAL AND METHODS

A total of 10 complete Caucasian cadaver legs had whole leg computed tomography scans. The individual geometrical ankle joint axis was determined, and based on this axis, the position of the prosthesis was planned. We assessed prosthesis placement, guiding block position, and preoperative and postoperative ankle rotational axes.

RESULTS

The guiding block position interobserver reliability was 0.37 mm 0.45 (mean ± SD) for the tibial guiding block. The value for the first talar guiding block was 1.72 ± 1.3 mm and for the second talar guiding block, 0.61 ± 0.39 mm. The tibial slope as well as the frontal angles of the anatomical tibial axis compared to the tibial and talar articular surfaces showed no statistically relevant differences with numbers available. The deviation of the assessed preoperative joint axis to the postoperative joint axis was 14.6° ± 7.8.

CONCLUSION

The present study describes the results of an establishing process of a new PSI technique for TAR. The reliability of guiding block positioning and, thereby, prosthesis placement is sufficient.

LEVEL OF EVIDENCE

Biomechanical study.

Validity and Reliability of a Novel Instrument for the Measurement of Subtalar Joint Axis of Rotation

Inclination of the subtalar joint (STJ) in the sagittal and transverse planes may be highly associated with ankle pathology. However, the validity and reliability of measuring the inclination of the STJ axis of rotation (AoR) is not well established. This study aimed to develop a custom-made STJ locator (STJL) and evaluate its reliability and validity. To establish the reliability and validity of the measurement device for STJ AoR, 38 healthy male participants were recruited. For the reliability analysis, test–retest was used, and for validity analysis, Pearson’s correlation and Bland–Altman plot analyses were performed. In the reliability analysis of the STJL, a higher correlation was observed with the sagittal plane (0.930) and transverse plane (0.748) (standard error of measurement: 0.56–0.78; minimal detectable difference: 1.57–2.16). In the validity analysis between radiography and STJL, a significantly higher value of 0.798 was obtained with radiography (42.5) and STJL (43.5) with the sagittal plane. The custom-made STJL may be used in the clinical setting as its validity and intraclass correlation coefficient were high, indicating consistent measurements. Further studies including motion analysis are necessary to provide more information regarding the relationship between STJ AoR inclinations and STJ movements.

Talar neck angle correlates with tibial torsion-Guidance for 3D and 2D measurements in total ankle replacement

Axial plane alignment of the talar component in total ankle arthroplasty is poorly understood and remains a major issue, especially since malpositioning results in increased peak pressure and rotational torque. Further profound knowledge regarding individual anatomy of the talus and its relation to proximal and distal osseous structures is therefore needed. Therefore, three-dimensional (3D) surface models of 50 lower extremities were generated using computed tomography data of patients without ankle osteoarthritis. The talus neck torsion was measured using a novel 3D measurement method. Then, tibial torsion and subtalar joint axis orientation were measured and correlated to the talus neck torsion. Moreover, a 2D measurement method of the talus neck torsion was developed. A statistically significant correlation was found between external tibia torsion and medial talus neck torsion, as well as talus neck axis and subtalar joint axis in the transversal and frontal plane. The novel defined 3D measurement methods indicated excellent inter-rater and intra-rater reliability. The 2D measurement method of the talus neck torsion was in good agreement with the 3D method. The results showed that the rotational profiles of the tibia, talus, and adjacent joints are interconnected, which should be considered in total ankle replacement (TAR). Clinical relevance: This study improves the overall understanding of the talar anatomy, as well as its relationship to adjacent osseous structures. The novel 2D measurement method of the talus neck torsion might improve talar component positioning in the axial plane corresponding to the patient's individual anatomy, and therefore improve the survival rate of TAR.

A methodology for the customization of hinged ankle-foot orthoses based on in vivo helical axis calculation with 3D printed rigid shells

This study aims to develop techniques for ankle joint kinematics analysis using motion capture based on stereophotogrammetry. The scope is to design marker attachments on the skin for a most reliable identification of the instantaneous helical axis, to be targeted for the fabrication of customized hinged ankle-foot orthoses. These attachments should limit the effects of the experimental artifacts, in particular the soft-tissue motion artifact, which affect largely the accuracy of any in vivo ankle kinematics analysis. Motion analyses were carried out on two healthy subjects wearing customized rigid shells that were designed through 3D scans of the subjects' lower limbs and fabricated by additive manufacturing. Starting from stereophotogrammetry data collected during walking and dorsi-plantarflexion motor tasks, the instantaneous and mean helical axes of ankle joint were calculated. The customized shells matched accurately the anatomy of the subjects and allowed for the definition of rigid marker clusters that improved the accuracy of in vivo kinematic analyses. The proposed methodology was able to differentiate between subjects and between the motor tasks analyzed. The observed position and dispersion of the axes were consistent with those reported in the literature. This methodology represents an effective tool for supporting the customization of hinged ankle-foot orthoses or other devices interacting with human joints functionality.

Considerations on the human Achilles tendon moment arm for in vivo triceps surae muscle-tendon unit force estimates

Moment arm-angle functions (MA-a-functions) are commonly used to estimate in vivo muscle forces in humans. However, different MA-a-functions might not only influence the magnitude of the estimated muscle forces but also change the shape of the muscle’s estimated force-angle relationship (F-a-r). Therefore, we investigated the influence of different literature based Achilles tendon MA-a-functions on the triceps surae muscle–tendon unit F-a-r. The individual in vivo triceps torque–angle relationship was determined in 14 participants performing maximum voluntary fixed-end plantarflexion contractions from 18.3° ± 3.2° plantarflexion to 24.2° ± 5.1° dorsiflexion on a dynamometer. The resulting F-a-r were calculated using 15 literature-based in vivo Achilles tendon MA-a-functions. MA-a-functions affected the F-a-r shape and magnitude of estimated peak active triceps muscle–tendon unit force. Depending on the MA-a-function used, the triceps was solely operating on the ascending limb (n = 2), on the ascending limb and plateau region (n = 12), or on the ascending limb, plateau region and descending limb of the F-a-r (n = 1). According to our findings, the estimated triceps muscle–tendon unit forces and the shape of the F-a-r are highly dependent on the MA-a-function used. As these functions are affected by many variables, we recommend using individual Achilles tendon MA-a-functions, ideally accounting for contraction intensity-related changes in moment arm magnitude.

An automated method for defining anatomic coordinate systems in the hindfoot

The absence of a standardized method for defining hindfoot bone coordinate systems makes it difficult to compare kinematics results from different research studies. The purpose of this study was to develop a reliable and robust procedure for defining anatomical coordinate systems for the talus and calcaneus. Four methods were evaluated based upon their anatomic consistency across subjects, repeatability, and their correspondence to functional axes of rotation. The four systems consisted of: 1) interactively identified bony landmarks, 2) a principal component analysis, 3) automatically identified bony landmarks, and 4) translating the tibial coordinate system to the hindfoot bones. The four systems were evaluated on 40 tali and 40 calcanei. The functional axes of rotation were determined using dynamic biplane radiography to image the hindfoot during gait. Systems 2 and 3 were the most repeatable and consistent due to the lack of operator intervention when defining coordinate systems. None of the coordinate systems corresponded well to functional axes of rotation during gait. System 3 is recommended over System 2 because it more closely mimics established bone angles measured clinically, especially for the calcaneus. This study presents an automated method for defining anatomic coordinate systems in the talus and calcaneus that does not rely on manual placement of markers or fitting of spheres to the bone surfaces which are less reliable due to operator-dependent measurements. Using this automated method will make it easier to compare hindfoot kinematics results across research studies.

Centre of Rotation of the Human Subtalar Joint Using Weight-Bearing Clinical Computed Tomography

Accurate in vivo quantification of subtalar joint kinematics can provide important information for the clinical evaluation of subtalar joint function; the analysis of outcome of surgical procedures of the hindfoot; and the design of a replacement subtalar joint prosthesis. The objective of the current study was to explore the potential of full weight-bearing clinical computed tomography (CT) to evaluate the helical axis and centre of rotation of the subtalar joint during inversion and eversion motion. A subject specific methodology was proposed for the definition of the subtalar joint motion combining three-dimensional (3D) weight-bearing imaging at different joint positions with digital volume correlation (DVC). The computed subtalar joint helical axis parameters showed consistency across all healthy subjects and in line with previous data under simulated loads. A sphere fitting approach was introduced for the computation of subtalar joint centre of rotation, which allows to demonstrate that this centre of rotation is located in the middle facet of the subtalar joint. Some translation along the helical axis was also observed, reflecting the elasticity of the soft-tissue restraints. This study showed a novel technique for non-invasive quantitative analysis of bone-to-bone motion under full weight-bearing of the hindfoot. Identifying different joint kinematics in patients with ligamentous laxity and instability, or in the presence of stiffness and arthritis, could help clinicians to define optimal patient-specific treatments.

Estimating the Instantaneous Screw Axis and the Screw Axis Invariant Descriptor of Motion by Means of Inertial Sensors: An Experimental Study with a Mechanical Hinge Joint and Comparison to the Optoelectronic System

The motion of a rigid body can be represented by the instantaneous screw axis (ISA, also known as the helical axis). Recently, an invariant representation of motion based on the ISA, namely, the screw axis invariant descriptor (SAID), was proposed in the literature. The SAID consists of six scalar features that are independent from the coordinate system chosen to represent the motion. This method proved its usefulness in robotics; however, a high sensitivity to noise was observed. This paper aims to explore the performance of inertial sensors for the estimation of the ISA and the SAID for a simple experimental setup based on a hinge joint. The free swing motion of the mechanical hinge was concurrently recorded by a marker-based optoelectronic system (OS) and two magnetic inertial measurement units (MIMUs). The ISA estimated by the MIMU was more precise, while the OS was more accurate. The mean angular error was ≈2.2° for the OS and was ≈4.4° for the MIMU, while the mean standard deviation was ≈2.3° for the OS and was ≈0.2° for the MIMU. The SAID features based on angular velocity were better estimated by the MIMU, while the features based on translational velocity were better estimated by the OS. Therefore, a combination of both measurements systems is recommended to accurately estimate the complete SAID.

The geometrical axis of the talocrural joint-Suggestions for a new measurement of the talocrural joint axis

BACKGROUND

Despite intensive research there is no consensus about the talocrural joint axis. The aim of the present study is a new method to determinate the geometric rotational axis of the talocrural joint.

METHODS

We analyzed 98 CT-scans of full cadaver Caucasian legs. We generated three-dimensional reconstruction models of the talus. A best fitting cone was orientated to the talar articular surface. The geometric rotational axis was defined to be the axis of this cone.

RESULTS

The geometric rotational axis of the talocrural joint is orientated from lateral-distal to medial-proximal (85.6°±10 compared to anatomical tibial axis in torsional plane), from posterior-distal to anterior-proximal (81.43°±44.35 compared to anatomical tibial axis in sagittal plane) and from posterior-medial to anterior-lateral (169.2°±5.91 compared to intermalleolar axis in axial plane).

CONCLUSIONS

The consideration of our results might be helpful for better understanding of ankle biomechanics.

The computed tomographybased anatomy of the ossa cuneiformia

There is a lack of basic anatomic information regarding the ossa cuneiformia. The aim of the present descriptive study was the detailed evaluation of the anatomy of the ossa cuneiformia. We analyzed 100 computer tomography scans of feet without deformities or previous trauma. The length, height and width of each cuneiforme and their articular surfaces were assessed. We itemized the data to gender differences and to foot length. The medial cuneiforme os had a length of 24.0 mm ± 2.4 (mean ± standard deviation), a width of 17.3 mm ± 2.8 and a height of 28.0 mm ± 3.4. The respective values for the intermediate cuneiforme were 18.2 mm ± 2.1, 15.8 mm ± 2.1 and 22.5 ± 2.2 and for the lateral cuneiforme 26.4 mm ± 2.7, 17.2 mm ± 2.9 and 22.8 mm ± 2.9. We found statistical relevant differences regarding gender and foot length subgroups whereas not for all parameters. The present study illustrates basic anatomic data regarding the ossa cuneiformia. This information might be helpful for implant design and placement during midfoot surgery.

Function of ankle ligaments for subtalar and talocrural joint stability during an inversion movement - an in vitro study

Background

The lateral ankle ligament complex consisting of the anterior talofibular ligament (ATFL), the calcaneofibular ligament (CFL) and the posterior talofibular ligament (PTFL) is known to provide stability against ankle joint inversion. As injuries of the ankle joint have been reported at a wide range of plantarflexion/dorsiflexion angles, the aim of the present study was to evaluate the stabilizing function of these ligaments depending on the sagittal plane positioning of the ankle joint.

Methods

Eight fresh-frozen specimens were tested on a custom-built ankle deflection tester allowing the application of inversion torques in various plantarflexion/dorsiflexion positions. A motion capture system recorded kinematic data from the talus, calcaneus and fibula with bone-pin markers during inversion movements at 10° of dorsiflexion, at neutral position and at plantarflexion 10°. ATFL, CFL and PTFL were separately but sequentially sectioned in order to assess the contribution of the individual ligament with regard to ankle joint stability.

Results

Joint- and position-specific modulations could be observed when the ligaments were cut. Cutting the ATFL did not lead to any observable alterations in ankle inversion angle at a given torque. But subsequently cutting the CFL increased the inversion angle of the talocrural joint in the 10° plantarflexed position, and significantly increased the inversion angle of the subtalar joint in the 10° dorsiflexed position. Sectioning of the PTFL led to minor increases of inversion angles in both joints.

Conclusions

The CFL is the primary ligamentous stabilizer of the ankle joint against a forced inversion. Its functioning depends greatly on the plantar−/dorsiflexion position of the ankle joint complex, as it provides the stability of the talocrural joint primarily during plantarflexion and the stability of the subtalar joint primarily during dorsiflexion.

Electronic supplementary material

The online version of this article (10.1186/s13047-019-0330-5) contains supplementary material, which is available to authorized users.

Relative movement on the articular surfaces of the tibiotalar and subtalar joints during walking

Objectives

The objective of this study was to quantify the relative movement between the articular surfaces in the tibiotalar and subtalar joints during normal walking in asymptomatic individuals.

Methods

3D movement data of the ankle joint complex were acquired from 18 subjects using a biplanar fluoroscopic system and 3D-to-2D registration of bone models obtained from CT images. Surface relative velocity vectors (SRVVs) of the articular surfaces of the tibiotalar and subtalar joints were calculated. The relative movement of the articulating surfaces was quantified as the mean relative speed (RS) and synchronization index (SI_ENT) of the SRVVs.

Results

SI_ENT and mean RS data showed that the tibiotalar joint exhibited translational movement throughout the stance, with a mean SI_ENT of 0.54 (sd 0.21). The mean RS of the tibiotalar joint during the 0% to 20% post heel-strike phase was 36.0 mm/s (sd 14.2), which was higher than for the rest of the stance period. The subtalar joint had a mean SI_ENT value of 0.43 (sd 0.21) during the stance phase and exhibited a greater degree of rotational movement than the tibiotalar joint. The mean relative speeds of the subtalar joint in early (0% to 10%) and late (80% to 90%) stance were 23.9 mm/s (sd 11.3) and 25.1 mm/s (sd 9.5), respectively, which were significantly higher than the mean RS during mid-stance (10% to 80%).

Conclusion

The tibiotalar and subtalar joints exhibited significant translational and rotational movement in the initial stance, whereas only the subtalar joint exhibited significant rotational movement during the late stance. The relative movement on the articular surfaces provided deeper insight into the interactions between articular surfaces, which are unobtainable using the joint coordinate system.

Cite this article: C-B. Phan, D-P. Nguyen, K. M. Lee, S. Koo. Relative movement on the articular surfaces of the tibiotalar and subtalar joints during walking. Bone Joint Res 2018;7:501–507. DOI: 10.1302/2046-3758.78.BJR-2018-0014.R1.

Achilles tendon moment arm in humans is not affected by inversion/eversion of the foot: a short report

The triceps surae primarily acts as plantarflexor of the ankle joint. However, the group also causes inversion and eversion at the subtalar joint. Despite this, the Achilles tendon moment arm is generally measured without considering the potential influence of inversion/eversion of the foot during plantarflexion. This study investigated the effect of foot inversion and eversion on the plantarflexion Achilles tendon moment arm. Achilles tendon moment arms were determined using the centre-of-rotation method in magnetic resonance images of the left ankle of 11 participants. The foot was positioned at 15° dorsiflexion, 0° or 15° plantarflexion using a Styrofoam wedge. In each of these positions, the foot was either 10° inverted, neutral or 10° everted using an additional Styrofoam wedge. Achilles tendon moment arm in neutral foot position was 47.93 ± 4.54 mm and did not differ significantly when the foot was positioned in 10° inversion and 10° eversion. Hence, inversion/eversion position of the foot may not considerably affect the length of the Achilles tendon moment arm. This information could be useful in musculoskeletal models of the human lower leg and foot and when estimating Achilles tendon forces during plantarflexion with the foot positioned in inversion or eversion.

The subtalar joint: A complex mechanism

Subtalar joint anatomy is complex and can vary significantly between individuals.

Movement is affected by several adjacent joints, ligaments and periarticular tendons.

The subtalar joint has gained interest from foot and ankle surgeons in recent years, but its importance in hindfoot disorders is still under debate.

The purpose of this article is to give a general overview of the anatomy, biomechanics and radiographic assessment of the subtalar joint.

The influence of the subtalar joint on the evolution of ankle joint osteoarthritis is additionally discussed.

Cite this article: EFORT Open Rev 2017;2:309-316. DOI: 10.1302/2058-5241.2.160050

Model-Based Estimation of Ankle Joint Stiffness

We address the estimation of biomechanical parameters with wearable measurement technologies. In particular, we focus on the estimation of sagittal plane ankle joint stiffness in dorsiflexion/plantar flexion. For this estimation, a novel nonlinear biomechanical model of the lower leg was formulated that is driven by electromyographic signals. The model incorporates a two-dimensional kinematic description in the sagittal plane for the calculation of muscle lever arms and torques. To reduce estimation errors due to model uncertainties, a filtering algorithm is necessary that employs segmental orientation sensor measurements. Because of the model's inherent nonlinearities and nonsmooth dynamics, a square-root cubature Kalman filter was developed. The performance of the novel estimation approach was evaluated in silico and in an experimental procedure. The experimental study was conducted with body-worn sensors and a test-bench that was specifically designed to obtain reference angle and torque measurements for a single joint. Results show that the filter is able to reconstruct joint angle positions, velocities and torque, as well as, joint stiffness during experimental test bench movements.

Biomechanical Evaluation of Strength and Stiffness of Subtalar Joint Arthrodesis Screw Constructs

BACKGROUND

Subtalar arthrodesis is a common treatment for end-stage subtalar joint arthritis as well as many other clinical problems. The best method of subtalar arthrodesis fixation is unknown. The purpose of this study was to compare the strength of subtalar arthrodesis fixation methods including a single posterior screw (SP), 2 posterior minimally divergent screws (MD) and a 2 screw highly divergent screw (HD) construct for subtalar arthrodesis.

METHODS

A biomechanical study was performed including the three different screw configurations (SP, MD, HD). These surrogate bone specimens were subjected to applied inversion and eversion torques about the subtalar joint axis on a servo-hydraulic load frame. Torsional stiffness of the construct and the maximum torque for each configuration were measured. Additionally, a cadaver study was performed using 5 fresh-frozen cadaver specimens. The perpendicular distance from the divergent screw guide-wire placement was measured from anatomic structures.

RESULTS

The HD screw configuration was found to have the highest torsional stiffness in both inversion and eversion, followed by the MD construct and then the SP construct. Similarly, the HD construct had the highest maximum torque versus the MD and SP constructs. All between-group differences were statistically significant (P < .05). The mean distance from key structures to the divergent screw included the sural nerve (13 mm), peroneus brevis tendon (18 mm), tibialis anterior tendon (8 mm), and tibialis posterior tendon (21 mm).

CONCLUSION

This biomechanical and cadaver study supports the use of 2 screws for fixation of subtalar arthrodesis over a single posterior screw. Additionally, we describe a biomechanically superior and potentially safe, alternative 2-screw divergent construct.

CLINICAL RELEVANCE

This study gives biomechanical support for 2 screw, divergent fixation of subtalar arthrodesis or a single over a single screw or two screw minimally divergent construct.

Plantar-flexion of the ankle joint complex in terminal stance is initiated by subtalar plantar-flexion: A bi-planar fluoroscopy study

Gross motion of the ankle joint complex (AJC) is a summation of the ankle and subtalar joints. Although AJC kinematics have been widely used to evaluate the function of the AJC, the coordinated movements of the ankle and subtalar joints are not well understood. The purpose of this study was to accurately quantify the individual kinematics of the ankle and subtalar joints in the intact foot during ground walking by using a bi-planar fluoroscopic system. Bi-planar fluoroscopic images of the foot and ankle during walking and standing were acquired from 10 healthy subjects. The three-dimensional movements of the tibia, talus, and calcaneus were calculated with a three-dimensional/two-dimensional registration method. The skeletal kinematics were quantified from 9% to 86% of the full stance phase because of the limited camera speed of the X-ray system. At the beginning of terminal stance, plantar-flexion of the AJC was initiated in the subtalar joint on average at 75% ranging from 62% to 76% of the stance phase, and plantar-flexion of the ankle joint did not start until 86% of the stance phase. The earlier change to plantar-flexion in the AJC than the ankle joint due to the early plantar-flexion in the subtalar joint was observed in 8 of the 10 subjects. This phenomenon could be explained by the absence of direct muscle insertion on the talus. Preceding subtalar plantar-flexion could contribute to efficient and stable ankle plantar-flexion by locking the midtarsal joint, but this explanation needs further investigation.

The subtalar joint: biomechanics and functional representations in the literature

The subtalar joint is important for gait and function of the foot and ankle. With few external landmarks, the joint is difficult to conceptualize and study in vivo. There have been several functional representations put forth in the literature which can be combined to give a broader understanding of the overall function and mechanics of the subtalar joint. This understanding is clinically important when considering the impact that disease has on the subtalar joint as well as how treatment of the subtalar joint impacts on the surrounding structures.

Analysis of movement axes of the ankle and subtalar joints: Relationship with the articular surfaces of the talus

Bone shape varies among human populations, and in the case of articular joints, the shape of the articulating surfaces strongly influences joint movement. In this study, we aimed to analyse the axes of the talocrural and subtalar joints, using the talar articular surface method, and compare these results with those of the previous studies. We used three-dimensional reconstruction models of tali among Korean individuals. The axes of the talocrural and subtalar joints were calculated by using the least-square sphere fitting method, and the principal axis was used for reference coordination. The shape of the talus was classified based on the shape of its anterior, middle, and posterior calcaneal facets, and the talar shape was compared with the movement axes. The movement axes did not differ according to the type of calcaneal facets of the talus. Interestingly, we also noted that the axes of the talocrural and subtalar joints in this study differed significantly from those reported in the previous studies; these differences may be due to variations in the shape of the bones.

How much does the human medial gastrocnemius muscle contribute to ankle torques outside the sagittal plane?

Ankle movements in the frontal plane are less prominent though not less relevant than movements in the plantar or dorsal flexion direction. Walking on uneven terrains and standing on narrow stances are examples of circumstances likely imposing marked demands on the ankle medio-lateral stabilization. Following our previous evidence associating lateral bodily sways in quiet standing to activation of the medial gastrocnemius (MG) muscle, in this study we ask: how large is the MG contribution to ankle torque in the frontal plane? By arranging stimulation electrodes in a selective configuration, current pulses were applied primarily to the MG nerve branch of ten subjects. The contribution of populations of MG motor units of progressively smaller recruitment threshold to ankle torque was evaluated by increasing the stimulation amplitude by fixed amounts. From smallest intensities (12–32 mA) leading to the firstly observable MG twitches in force-plate recordings, current pulses reached intensities (56–90 mA) below which twitches in other muscles could not be observed from the skin. Key results showed a substantial MG torque contribution tending to rotate upward the foot medial aspect (ankle inversion). Nerve stimulation further revealed a linear relationship between the peak torque of ankle plantar flexion and inversion, across participants (Pearson R > .81, p < .01). Specifically, regardless of the current intensity applied, the peak torque of ankle inversion amounted to about 13% of plantar flexion peak torque. Physiologically, these results provide experimental evidence that MG activation may contribute to stabilize the body in the frontal plane, especially under situations of challenged stability.

Sensitivity, reliability and accuracy of the instant center of rotation calculation in the cervical spine during in vivo dynamic flexion-extension

The instant center of rotation (ICR) has been proposed as an alternative to range of motion (ROM) for evaluating the quality, rather than the quantity, of cervical spine movement. The purpose of the present study was to assess the sensitivity, reliability and accuracy of cervical spine ICR path calculations obtained during dynamic in vivo movement. The reliability and sensitivity of in vivo cervical spine ICR calculations were assessed by evaluating the effects of movement direction (flexion versus extension), rotation step size, filter frequency, and motion tracking error. The accuracy of the ICR path calculations was assessed through a simulation experiment that replicated in vivo movement of cervical vertebrae. The in vivo assessment included 20 asymptomatic subjects who performed continuous head flexion-extension movements while biplane radiographs were collected at 30 frames per second. In vivo motion of C2 through C7 cervical vertebrae was tracked with sub-millimeter accuracy using a volumetric model-based tracking technique. The finite helical axis method was used to determine ICRs between each pair of adjacent vertebra. The in vivo results indicate ICR path is not different during the flexion movement and the extension movement. In vivo, the path of the ICR can reliably be characterized within 0.5mm in the SI and 1.0mm in the AP direction. The inter-subject variability in ICR location averaged ±1.2mm in the SI direction and ±2.2mm in the AP direction. The computational experiment estimated the in vivo accuracy in ICR location was between 1.1mm and 3.1mm.

Importance and challenges of measuring intrinsic foot muscle strength

BACKGROUND

Intrinsic foot muscle weakness has been implicated in a range of foot deformities and disorders. However, to establish a relationship between intrinsic muscle weakness and foot pathology, an objective measure of intrinsic muscle strength is needed. The aim of this review was to provide an overview of the anatomy and role of intrinsic foot muscles, implications of intrinsic weakness and evaluate the different methods used to measure intrinsic foot muscle strength.

METHOD

Literature was sourced from database searches of MEDLINE, PubMed, SCOPUS, Cochrane Library, PEDro and CINAHL up to June 2012.

RESULTS

There is no widely accepted method of measuring intrinsic foot muscle strength. Methods to estimate toe flexor muscle strength include the paper grip test, plantar pressure, toe dynamometry, and the intrinsic positive test. Hand-held dynamometry has excellent interrater and intrarater reliability and limits toe curling, which is an action hypothesised to activate extrinsic toe flexor muscles. However, it is unclear whether any method can actually isolate intrinsic muscle strength. Also most methods measure only toe flexor strength and other actions such as toe extension and abduction have not been adequately assessed. Indirect methods to investigate intrinsic muscle structure and performance include CT, ultrasonography, MRI, EMG, and muscle biopsy. Indirect methods often discriminate between intrinsic and extrinsic muscles, but lack the ability to measure muscle force.

CONCLUSIONS

There are many challenges to accurately measure intrinsic muscle strength in isolation. Most studies have measured toe flexor strength as a surrogate measure of intrinsic muscle strength. Hand-held dynamometry appears to be a promising method of estimating intrinsic muscle strength. However, the contribution of extrinsic muscles cannot be excluded from toe flexor strength measurement. Future research should clarify the relative contribution of intrinsic and extrinsic muscles during intrinsic foot muscle strength testing.

Kinematics of the three components of a total ankle replacement: in vivo fluoroscopic analysis

BACKGROUND

Careful kinematic analysis of ankle joints with newly developed prostheses should be carried out to assess the actual performance in vivo. This study analyzed the pattern of motion of the three components of a ligament-compatible ankle replacement, developed to replicate normal joint kinematics.

MATERIALS AND METHODS

Twelve patients treated with this design were analyzed at 6, 12, and 24 months followup. A series of images were acquired by videofluoroscopy at extremes of the range of motion, and during flexion/extension against gravity and stair-climbing/descending. Three-dimensional positions and orientations of the tibial and talar metal components and of the polyethylene mobile-bearing were obtained from the images by a standard shape-matching procedure. Motion between the three components was calculated and descriptively analyzed.

RESULTS

Large tibiotalar joint mobility of the replaced ankle was observed in all three anatomical planes, particularly in the sagittal. In flexion/extension against gravity, the mean range of flexion was 17.6, 17.7, and 16.2 degrees, respectively, over the three followups. The inclination angle of the mean axis of joint rotation was 3.7 degrees down and lateral in the frontal plane and 4.7 degrees posterior and lateral in the transverse plane, similar to those in the normal ankle. The corresponding antero-posterior translation of the meniscal-bearing with respect to the tibia was 3.3, 3.3, and 3.2 mm, with statistically significant correlation with joint flexion.

CONCLUSION

Physiological motion can be achieved in ligament-compatible ankle joint replacements. The considerable antero-posterior bearing-to-tibial motion and its coupling with flexion support the main original claims of this design.

Multi-rigid image segmentation and registration for the analysis of joint motion from three-dimensional magnetic resonance imaging

We report an image segmentation and registration method for studying joint morphology and kinematics from in vivo magnetic resonance imaging (MRI) scans and its application to the analysis of foot and ankle joint motion. Using an MRI-compatible positioning device, a foot was scanned in a single neutral and seven other positions ranging from maximum plantar flexion, inversion, and internal rotation to maximum dorsiflexion, eversion, and external rotation. A segmentation method combining graph cuts and level set was developed. In the subsequent registration step, a separate rigid body transformation for each bone was obtained by registering the neutral position dataset to each of the other ones, which produced an accurate description of the motion between them. The segmentation algorithm allowed a user to interactively delineate 14 foot bones in the neutral position volume in less than 30 min total (user and computer processing unit [CPU]) time. Registration to the seven other positions took approximately 10 additional minutes of user time and 5.25 h of CPU time. For validation, our results were compared with those obtained from 3DViewnix, a semiautomatic segmentation program. We achieved excellent agreement, with volume overlap ratios greater than 88% for all bones excluding the intermediate cuneiform and the lesser metatarsals. For the registration of the neutral scan to the seven other positions, the average overlap ratio is 94.25%, while the minimum overlap ratio is 89.49% for the tibia between the neutral position and position 1, which might be due to different fields of view (FOV). To process a single foot in eight positions, our tool requires only minimal user interaction time (less than 30 min total), a level of improvement that has the potential to make joint motion analysis from MRI practical in research and clinical applications.

Evaluating foot kinematics using magnetic resonance imaging: from maximum plantar flexion, inversion, and internal rotation to maximum dorsiflexion, eversion, and external rotation

The foot consists of many small bones with complicated joints that guide and limit motion. A variety of invasive and noninvasive means [mechanical, X-ray stereophotogrammetry, electromagnetic sensors, retro-reflective motion analysis, computer tomography (CT), and magnetic resonance imaging (MRI)] have been used to quantify foot bone motion. In the current study we used a foot plate with an electromagnetic sensor to determine an individual subject's foot end range of motion (ROM) from maximum plantar flexion, internal rotation, and inversion to maximum plantar flexion, inversion, and internal rotation to maximum dorsiflexion, eversion, and external rotation. We then used a custom built MRI-compatible device to hold each subject's foot during scanning in eight unique positions determined from the end ROM data. The scan data were processed using software that allowed the bones to be segmented with the foot in the neutral position and the bones in the other seven positions to be registered to their base positions with minimal user intervention. Bone to bone motion was quantified using finite helical axes (FHA). FHA for the talocrural, talocalcaneal, and talonavicular joints compared well to published studies, which used a variety of technologies and input motions. This study describes a method for quantifying foot bone motion from maximum plantar flexion, inversion, and internal rotation to maximum dorsiflexion, eversion, and external rotation with relatively little user processing time.

The 3D in vivo Achilles' tendon moment arm, quantified during active muscle control and compared across sexes

The Achilles' tendon moment arm (ATma) is a critical quantity in that it defines the triceps surae's ability to generate a moment on the calcaneus, which is then transferred to the foot. This measure has been primarily acquired two-dimensionally in small male populations. Thus, the primary purpose of this study was to establish the first in vivo three-dimensional measures of the ATma, measured non-invasively during dynamic activity in a large normative population, inclusive of both males and female subjects (n=20). Subjects were each placed supine in a 1.5 T MRI and asked to repeat a simulated toe-raise while a full sagittal-cine-phase contrast (dynamic) MRI dataset was acquired. From these data, the 3D and 2D ATma was calculated. The ATma was scaled by the distal tibial width, based on a correlation analysis. The 2D ATma overestimated its 3D counterpart by 3.1 mm, on average. The scaled ATma was no different between the male and female cohorts, but the scaled Achilles' tendon area was smaller in the male cohort. The magnitudes of the ATma were most similar to previously reported values when variations in ankle angle were taken into account. The results of this study have important implications for the applicability of ATma data to both clinical questions and modeling. Any future studies should adapt the ATma based on subject size and/or sex, ensure compatibility between the manner in which the ankle angle is defined and the data being used, and account for the influence that muscle force has on the 3D ATma.