J.J. O’Connor

Bone position estimation from skin marker co-ordinates using global optimisation with joint constraints

Widespread use of gait or motion analysis in the diagnosis of patients with locomotor pathology and the subsequent planning and assessment of treatment has been limited because of its reliability, particularly in evaluating frontal and transverse plane components. This is because spatial reconstruction of the musculoskeletal system and calculation of its kinematics and kinetics via a skin marker-based multi-link model are subject to marker skin movement artefacts. Traditional methods treat each body segment separately without imposing joint constraints, resulting in apparent dislocations at joints predominantly because of skin movement artefacts. An optimisation method for the determination of the positions and orientations of multi-link musculoskeletal models from marker co-ordinates is presented. It is based on the minimisation of the weighted sum of squared distances between measured and model-determined marker positions. The model imposes joint constraints. Numerical experiments were performed to show that the new method is capable of eliminating joint dislocations and giving more accurate model position and orientation estimations. It is suggested that, with joint constraints and a global error compensation scheme, the effects of measurement errors on the reconstruction of the musculoskeletal system and subsequent mechanical analyses can be reduced globally. The proposed method minimises errors in axial rotation and ab/adduction at the joints and may extend the applicability of gait analysis to clinical problems.

Kinematic analysis of a multi-segment foot model for research and clinical applications: a repeatability analysis.

An unbiased understanding of foot kinematics has been difficult to achieve due to the complexity of foot structure and motion. We have developed a protocol for evaluation of foot kinematics during barefoot walking based on a multi-segment foot model. Stereophotogrammetry was used to measure retroreflective markers on three segments of the foot plus the tibia. Repeatability was evaluated between-trial, between-day and between-tester using two subjects and two testers. Subtle patterns and ranges of motion between segments of the foot were consistently detected. We found that repeatability between different days or different testers is primarily subject to variability of marker placement more than inter-tester variability or skin movement. Differences between inter-segment angle curves primarily represent a shift in the absolute value of joint angles from one set of trials to another. In the hallux, variability was greater than desired due to vibration of the marker array used. The method permits objective foot measurement in gait analysis using skin-mounted markers. Quantitative and objective characterisation of the kinematics of the foot during activity is an important area of clinical and research evaluation. With this work we hope to have provided a firm basis for a common protocol for in vivo foot study.

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.

Ligaments and articular contact guide passive knee flexion.

The aim of this study was to test the hypothesis that the coupled features of passive knee flexion are guided by articular contact and by the isometric fascicles of the ACL, PCL and MCL. A three-dimensional mathematical model of the knee was developed, in which the articular surfaces in the lateral and medial compartments and the isometric fascicles in the ACL, PCL and MCL were represented as five constraints in a one degree-of-freedom parallel spatial mechanism. Mechanism analysis techniques were used to predict the path of motion of the tibia relative to the femur. Using a set of anatomical parameters obtained from a cadaver specimen, the model predicts coupled internal rotation and ab/adduction with flexion. These predictions correspond well to measurements of the cadaver specimens motion. The model also predicts posterior translation of contact on the tibia with flexion. Although this is a well-known feature of passive knee flexion, the model predicts more translation than has been reported from experiments in the literature. Modelling of uncertainty in the anatomical parameters demonstrated that the discrepancy between theoretical predictions and experimental measurement can be attributed to parameter sensitivity of the model. This study shows that the ligaments and articular surfaces work together to guide passive knee motion. A principal implication of the work is that both articular surface geometry and ligament geometry must be preserved or replicated by surgical reconstruction and replacement procedures to ensure normal knee kinematics and by extension, mechanics.

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.

Validation of a lower limb model with in vivo femoral forces telemetered from two subjects

A mathematical model of the human pelvis-leg system in the sagittal plane, with an anatomical model of the knee, was developed to calculate forces transmitted by the structural elements of the system. The model was used to study the influence of activity of hip flexors and extensors on the forces in the femur during isometric exercises and during level walking. Kinematic and kinetic data together with simultaneous electromyography (EMG) and in vivo axial forces transmitted along the prostheses from two patients implanted with instrumented massive proximal femoral prostheses were obtained. Comparison of the levels of the calculated axial forces in the model femur to the simultaneous telemetered forces showed good agreement for isometric tests. Interaction between the muscles and the bones during isometric tests was examined and bi-articular muscles were shown to play a major role in modulating forces in bones. The study supports the hypothesis that muscles balance the external limb moments, not only at joints but also along the limbs, decreasing the bending moments but increasing the axial compressive forces in bones. It is thus suggested that appropriate simulation of muscle force is necessary in in vitro laboratory experiments and in theoretical studies of load transmission in bones. The sagittal plane model underestimates the value of the maximum axial force in the femur during walking by about 30% but suggests that 70% was due to the action of the extensors or flexors. The results encourage further development of a three-dimensional model with anatomical models of the joints to include coronal and transverse planes for the study of adductors and abductors.

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

Although the anterior drawer test at the ankle joint is commonly used in routine clinical practice, very little is known about the sharing of load between the individual passive structures and the joint response at different flexion angles.A mathematical model of the ankle joint was devised to calculate ligament fibre recruitment and load/displacement curves at different flexion angles. Ligaments were modelled as three-dimensional arrays of fibres, and their orientations at different flexion angles were taken from a previously validated four-bar-linkage model in the sagittal plane. A non-linear stress/strain relationship was assumed for ligament fibres and relevant mechanical parameters were taken from two reports in the literature. Talus and calcaneus were assumed to move as a single rigid body. Antero/distal motion of the talus relative to the tibia was analysed. The ankle joint was found to be stiffer at the two extremes of the flexion range, and the highest laxity was found around the neutral position, confirming previous experimental works. With a first dataset, a 20N anterior force produced 4.3, 5.5, and 4.4mm displacement respectively at 20 degrees plantarflexion, at neutral, and at 20 degrees dorsiflexion. At 10 degrees plantarflexion, for a 6mm displacement, 65% of the external force was supported by the anterior talofibular, 11% by the deep anterior tibiotalar and 5.5% by the tibionavicular ligament. Corresponding results from a second dataset were 1.4, 2.4 and 1.8mm at 40N force, and 80%, 0% and 2% for a 3mm displacement. A component of the contact force supported the remainder.

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

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

Fabrication, Structure and Properties of Tl-Based HTS Thin Films

Thin films of the thallium-based high temperature superconductors are particularly suitable for passive microwave applications. The Tl-2212, Tl-2223 and TM223 phases have critical temperatures in the range 100–120K; thus at the proposed operating temperatures of ~80K, readily achievable with single-stage cryocoolers, their superconducting properties exhibit a slow variation with temperature, thus obviating the need for precise temperature control in maintaining device performance [1]. Device quality films of the above phases have been deposited on a variety of substrates, with and without buffer layers, by a number of groups and a range of practical microwave components have been successfully demonstrated [2]. Critical current densities at 77K and in zero field in excess of GAm-2, and surface resistance values, normalised to 80K and 10GHz, <0.5mΩ have been achieved reproducibly in these films.