Despite of the high success of TKA, 20% of recipients remain dissatisfied with their surgery. There is an increasing discordance in the literature on what is an optimal goal for component alignment. Furthermore, the unique patient specific anatomical characteristics will also play a role. The dynamic characteristic of a TKR is a product of the complex interaction between a patient's individual anatomical characteristics and the specific alignment of the components in that patient knee joint. These interactions can be better understood with computational models. Our objective was to characterise ligament characteristics by measuring knee joint laxity with functional radiograph and with the aid of a computational model and an optimisation study to estimate the subject specific free length of the ligaments.

Pre-operative CT and functional radiographs, varus and valgus stressed X-rays assessing the collateral ligaments, were captured for 10 patients. CT scan was segmented and 3D–2D pose estimation was performed against the radiographs. Patient specific tibio-femoral joint computational model was created. The model was virtually positioned to the functional radiograph positions to simulate the boundary conditions when the knee is stressed. The model was simulated to achieve static equilibrium. Optimisation was done on ligament free length and a scaling coefficient, flexion factor, to consider the ligaments wrapping behaviour.

Our findings show the generic values for reference strain differ significantly from reference strains calculated from the optimised ligament parameters, up to 35% as percentage strain. There was also a wide variation in the reference strain values between subjects and ligaments, with a range of 37% strain between subjects. Additionally, the knee laxity recorded clinically shows a large variation between patients and it appears to be divorced from coronal alignment measured in CT. This suggests the ligaments characteristics vary widely between subjects and non-functional imaging is insufficient to determine its characteristics. These large variations necessitate a subject-specific approach when creating knee computational models and functional radiographs may be a viable method to characterise patient specific ligaments.

Introduction: Pre-operative coronal curve flexibility assessment is of key importance in the surgical planning process for scoliosis correction. The fulcrum bending radiograph is one flexibility assessment technique which has been shown to be highly predictive of potential curve correction using posterior surgery, however little is known about the extent to which soft tissue structures govern spinal flexibility. The aim of this study was to explore how the mechanical properties of spinal ligaments and intervertebral discs affect coronal curve flexibility in the fulcrum bending test. To this end a biomechanical analysis of a scoliotic thoracolumbar spine and ribcage was carried out using a three dimensional finite element model.

Methods: CT-derived spinal anatomy for a 14 year old female adolescent idiopathic scoliosis patient was used to develop the 3D finite element model. Physiological loading conditions representing the gravitational body weight forces acting on the spine when the patient lies on their side over the fulcrum bolster were simulated. Initial mechanical properties for the spinal soft tissues were derived from existing literature. In six separate analyses, the disc collagen fibre and ligament stiffness values were reduced by 10%, 25% and 40% respectively, and the effects of reduced tissue stiffness on fulcrum flexibility were assessed by comparison with the initial model. Finally, the effect of discectomy on fulcrum flexibility was simulated for thoracic levels T5 to T12.

Results: Reducing disc collagen fibre stiffness resulted in a greater change in segmental rotations in the fulcrum bending test than reducing ligament stiffness. However, reductions of up to 40% in disc collagen fibre stiffness and ligament stiffness produced no clinically measurable increase in fulcrum flexibility (increase of 1.2%). By contrast, following removal of the discs, the simulated fulcrum flexibility increased by more than 80% compared to the initial case.

Discussion: Disc collagen fibre and ligament stiffness both have minimal influence on scoliotic curve flexibility. However, discectomy simulation shows that the intervertebral discs are of critical importance in determining spinal flexibility.

Introduction: Low back pain (LBP) is an ailment affecting a large portion of the population and may result from degeneration of the intervertebral discs. Degeneration of the discs may be characterized by a loss of hydration, a more granular texture in the disc components and the presence of anular lesions which are tears in the anulus fibrosus. Research to date has been lacking in defining a relationship between LBP and anular lesions. In this study a materially and geometrically accurate finite element model (FEM) of an L4/5 intervertebral disc was developed in order to study the effects of anular lesions on the disc mechanics.

Methods: An anatomically accurate transverse profile for the disc FEM was derived from transversely sectioned human cadaveric discs. The anulus fibrosus ground substance was represented as an incompressible material using an Ogden hyperelastic strain energy equation. Material parameters were derived from experimentation on sheep discs. In order to separately assess the effects of degeneration of the nucleus and of the entire disc, four models were analysed. A healthy disc was modelled as reference and the three degenerate models comprised a degenerate nucleus (no hydrostatic nucleus pressure) with either a healthy anulus, or with a radial or rim anular lesion. Loading conditions to simulate the extreme range of physiological motions about the 6 axes of rotation were applied to the models and the peak rotation moments compared.

Results: The reduction in peak moment between the Healthy Disc FEM and the Healthy Anulus FEM ranged from 24% under flexion to 86% under right lateral bending. When the lesions were simulated, the rim and radial lesion resulted in variations in peak moment from the Healthy Anulus FEM of 1–10% and 0–4%, respectively.

Conclusions: The analysis suggested that loss of the nucleus pulposus pressure had a much greater effect on the disc mechanics than the presence of anular lesions. This indicated that the development of anular lesions prior to the degeneration of the nucleus would have minimal effect on the disc mechanics. But the response of an entirely degenerate disc would show significantly different mechanics compared to a healthy disc. With the degeneration of the nucleus, the disc stiffness will reduce and the outer innervated anulus may become overloaded and painful.