Intraoperative determination of the load-displacement behavior of scoliotic spinal motion segments: preliminary clinical results

Introduction: Spinal fusion is a widely and successfully performed strategy for the treatment of spinal deformities and degenerative diseases. The general approach has been to stabilize the spine with implants so that a solid bony fusion between the vertebrae can develop. However, new implant designs have emerged that aim at preservation or restoration of the motion of the spinal segment. In addition to static, load sharing principles, these designs also require a profound knowledge of kinematic and dynamic properties to properly characterise the in vivo performance of the implants. Methods: To address this, an apparatus was developed that enables the intraoperative determination of the load-displacement behavior of spinal motion segments. The apparatus consists of a sensor-equipped distractor to measure the applied force between the transverse processes, and an optoelectronic camera to track the motion of vertebrae and the distractor. In this intraoperative trial, measurements from two patients with adolescent idiopathic scoliosis with right thoracic curves were made at four motion segments each. Results: At a lateral bending moment of 5Nm, the mean flexibility of all eight motion segments was 0.18±0.08°/Nm on the convex side and 0.24±0.11°/Nm on the concave side. Discussion: The results agree with published data obtained from cadaver studies with and without axial preload. Intraoperatively acquired data with this method may serve as an input for mathematical models and contribute to the development of new implants and treatment strategie

Patient Specific Properties of Scoliotic Spinal Motion Segments: Experiments and Parameter Identification

Spinal fusion is a widely and successfully performed strategy for the treatment of spinal deformities and degenerative diseases. The general approach has been to stabilize the spine with implants so that a solid bony fusion between the vertebrae can develop. During the last decades, new implant designs have emerged that aim at preservation or restoration of the motion of spinal segments. In addition to static, load sharing principles, these designs also require a profound knowledge of kinematic and dynamic properties to properly characterise the in-vivo performance of the implants. Most existing approaches to measure spinal stiffness intraoperatively in an in- vivo environment use a distractor. However, these concepts usually assume a planar loading and motion, whereas the spine exhibits complex three-dimensional movements. The aim of this thesis was to measure the in vivo mechanical properties of motion segments and then determine mechanical parameters solving an inverse problem. An apparatus was developed that enables the intraoperative, three-dimensional determination of the load-displacement behaviour of spinal motion segments. The apparatus consists of a sensor-equipped distractor to measure the applied force, and an optoelectronic camera to track the motion of vertebrae and dis- tractor. As the orientation of the applied force and the three dimensional motion is known, also moment-angle relations could be determined. The proposed concept was validated with three cadaveric lumbar ovine spines and compared to measurements on a spinal loading simulator, which was considered to be gold standard. The mean values of the stiffness determined with the pro- posed concept were within a range of ±15% compared to data obtained with the spinal loading simulator under applied loads of less than 5 Nm. An intraoperative pilot study was conducted at two patients with adolescent idiopathic scoliosis with right thoracic curves and load-displacement relations were measured at eight motion segments in total. The scoliotic motion segments showed an asymmetric mechanical behaviour. At a lateral bending moment of 5 Nm, the mean flexibility of all eight motion segments was 0.18 ± 0.08 deg/Nm on the convex side and 0.24 ± 0.11 deg/Nm on the concave side. The intraoperative measurements were then used to solve an inverse problem in order to identify material parameters of the connective tissues, using a finite element model with patient-specific geometry. The ligaments and the annulus fibrosus were modelled as hyperelastic, anisotropic materials with a continuum mechanical approach. The finite element model includes all ligaments, costotransverse, costovertebral and facet joints. In order to achieve good agreement between simulation and experiment, the error squares of the three Euler angles were minimized, considering measurements on the convex and the concave side. Five material constants, which describe the properties of the annulus fibrosus and the ligaments and allow for asymmetric mechanical be- haviour, are included in the parameter vector. A sampling of twenty parameter vectors was used to evaluate the robustness of the optimization. With the chosen approach the parameter identification best fits the experimentally measured motion in lateral bending. This thesis addressed several aspects of the patient-specific characterization of mechanical properties of spinal motion segments. The concept of navigated distractor measurements was developed, validated and tested intraoperatively at patients with adolescent idiopathic scoliosis. The intraoperatively acquired data was used to identify material parameters of ligaments and the annu- lus fibrosus. As a next step, additional load cases, i.e. flexion and axial rotation should be included in the intraoperative measurements. This leads to a better posed optimization problem and thus to a more accurate determination of the material parameters

Intraoperative determination of the load-displacement behavior of scoliotic spinal motion segments: preliminary clinical results

Introduction: Spinal fusion is a widely and successfully performed strategy for the treatment of spinal deformities and degenerative diseases. The general approach has been to stabilize the spine with implants so that a solid bony fusion between the vertebrae can develop. However, new implant designs have emerged that aim at preservation or restoration of the motion of the spinal segment. In addition to static, load sharing principles, these designs also require a profound knowledge of kinematic and dynamic properties to properly characterise the in vivo performance of the implants. Methods: To address this, an apparatus was developed that enables the intraoperative determination of the load–displacement behavior of spinal motion segments. The apparatus consists of a sensor-equipped distractor to measure the applied force between the transverse processes, and an optoelectronic camera to track the motion of vertebrae and the distractor. In this intraoperative trial, measurements from two patients with adolescent idiopathic scoliosis with right thoracic curves were made at four motion segments each. Results: At a lateral bending moment of 5 N m, the mean flexibility of all eight motion segments was 0.18 ± 0.08°/N m on the convex side and 0.24 ± 0.11°/N m on the concave side. Discussion: The results agree with published data obtained from cadaver studies with and without axial preload. Intraoperatively acquired data with this method may serve as an input for mathematical models and contribute to the development of new implants and treatment strategies

Specimen specific parameter identification of ovine lumbar intervertebral discs: On the influence of fibre–matrix and fibre–fibre shear interactions

Numerical models of the intervertebral disc, which address mechanical questions commonly make use of the difference in water content between annulus and nucleus, and thus fluid and solid parts are separated. Despite this simplification, models remain complex due to the anisotropy and nonlinearity of the annulus and regional variations of the collagen fibre density. Additionally, it has been shown that cross-links make a large contribution to the stiffness of the annulus. Because of this complex composite structure, it is difficult to reproduce several sets of experimental data with one single set of material parameters. This study addresses the question to which extent the ultrastructure of the intervertebral disc should be modelled so that its moment-angle behaviour can be adequately described. Therefore, a hyperelastic constitutive law, based on continuum mechanical principles was derived, which does not only consider the anisotropy from the collagen fibres, but also interactions among the fibres and between the fibres and the ground substance. Eight ovine lumbar intervertebral discs were tested on a custom made spinal loading simulator in flexion/extension, lateral bending and axial rotation. Specimen-specific geometrical models were generated using CT images and T2 maps to distinguish between annulus fibrosus and nucleus pulposus. For the identification of the material parameters the annulus fibrosus was described with two scenarios: with and without fibre-matrix and fibre-fibre interactions. Both scenarios showed a similar behaviour on a load displacement level. Comparing model predictions to the experimental data, the mean RMS of all specimens and all load cases was 0.54±0.15° without the interaction and 0.54±0.19° when the fibre-matrix and fibre-fibre interactions were included. However, due to the increased stiffness when cross-links effects were included, this scenario showed more physiological stress-strain relations in uniaxial and biaxial stress states. Thus, the present study suggests that fibre-matrix and fibre-fibre interactions should be considered in the constitutive law when the model addresses questions concerning the stress field of the annulus fibrosus

Comparative digesta retention patterns in ratites

Ratites differ distinctively in the anatomy of their digestive tract. For example, Ostriches (Struthio camelus) have a particularly long, voluminous colon and long paired caeca, Rheas (Rhea spp.) are characterised by a short colon with particularly prominent paired caeca, and Emus (Dromaius novaehollandiae) – have neither very prominent caeca nor a prominent colon. We tested whether digesta excretion patterns corresponded to these differences in anatomy, expecting Ostriches to have the longest and Emus the shortest digesta retention times, and Rheas possibly showing a selective retention of fluids observed in other birds and mammals with prominent caeca. We used 6 Ostriches (97-123kg), 5 Greater Rheas (R. americana, 22-27kg) and 2 Emus (32-34kg) fed a common diet of alfalfa pellets ad libitum in captivity. Intake per unit of metabolic body mass did not differ between Ostriches and Rheas but was significantly higher in Emus, which also displayed higher defecation frequencies and lower fiber digestibility. Mean digesta retention time for small fiber particles (2 mm) differed significantly among species (Ostrich: 30-36h; Rhea: 7-19h; Emu: 1.3-1.8h), but there were no differences between the retention of 2 mm or 8 mm particles or a solute marker within species. The shape of the marker excretion curves corresponded to digesta mixing in the digestive tract of Ostriches and Rheas but not Emus. The calculated dry matter gut fill (% of body mass) was significantly higher in Ostriches (1.6-1.8) than Rheas (0.3-1.0) and Emus (0.2). Ostriches had the highest, and Emus the lowest fecal dry matter concentration. These physiological findings match the differences in digestive anatomy and support the concept that in ratites, herbivory – and hence flightlessness – evolved repeatedly in different ways

Statistical shape modeling of pathological scoliotic vertebrae: A comparative analysis

Statistical shape models (SSMs) have been used widely as a basis for segmenting and interpreting complex anatomical structures. The robustness of these models are sensitive to the registration procedures, i.e., establishment of a dense correspondence across a training data set. In this work, two SSMs based on the same training data set of scoliotic vertebrae, and registration procedures were compared. The first model was constructed based on the original binary masks without applying any image pre- and post-processing, and the second was obtained by means of a feature preserving smoothing method applied to the original training data set, followed by a standard rasterization algorithm. The accuracies of the correspondences were assessed quantitatively by means of the maximum of the mean minimum distance (MMMD) and Hausdorf distance (H(D)). Anatomical validity of the models were quantified by means of three different criteria, i.e., compactness, specificity, and model generalization ability. The objective of this study was to compare quasi-identical models based on standard metrics. Preliminary results suggest that the MMMD distance and eigenvalues are not sensitive metrics for evaluating the performance and robustness of SSMs

Validation of intra-operative measurement apparatus to determine the stiffness properties of spinal motion segments

The load-displacement behavior of spinal motion segments is commonly determined from in-vitro experiments on cadaveric spines. However, clinically, it is often desirable to quantify the patient specific biomechanical properties of the spine in-vivo. Load-displacement measurement requires direct access to the appropriate anatomy, which is typically available in spinal surgeries that aim to correct lumbar spinal instability or scoliosis. We propose an approach to measure the spinal load-displacement behavior for use during these surgeries.</jats:p