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.

2011

dissertation

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