Should we recommend occipital plate fixation using bicortical screws or inverted occipital hooks to optimize occipito-cervical junction fusion? A biomechanical study combining an experimental and analytical approach.

BACKGROUND: Occipito-cervical fusion can be necessary in case of cranio-cervical junction instability. Proximal stabilisation is usually ensured by bi-cortical occipital screws implanted through one median or two lateral occipital plate(s). Bone thickness variability as well as the proximity of vasculo-nervous elements can induce substantial morbidity. The choice of site and implant type remains difficult for surgeons and is often empirically based. Given this challenge, implants with smaller pitch to increase bone interfacing are being developed, as is a surgical technique consisting in inverted occipital hook clamps, a potential alternative to plate/screws association. We present here a biomechanical comparison of the different occipito-cervical fusion devices. METHODS: We have developed a 3D mark tracking technique to measure experimental mechanical data on implants and occipital bone. Biomechanical tests were performed to study the mechanical stiffness of the occipito-cervical instrumentation on human skulls. Four occipital implant systems were analysed: lateral plates+large pitch screws, lateral plates+hooks, lateral plates+small pitch screws and median plate+small pitch screws. Mechanical responses were analysed using 3D displacement field measurements from optical methods and compared with an analytical model. FINDINGS: Paradoxical mechanical responses were observed among the four types of fixations. Lateral plates+small pitch screws appear to show the best accordance of displacement field between bone/implant/system interface providing higher stiffness and an average maximum moment around 50 N.m before fracture. INTERPRETATION: Stability of occipito-cervical fixation depends not only on the site of screws implantation and occipital bone thickness but is also directly influenced by the type of occipital implant.

Phase estimation of a 2D fringe pattern using a monogenic-based multiscale analysis.

In this paper, a multiscale monogenic analysis is applied to 2D interference fringe patterns. The monogenic signal was originally developed as a 2D generalization of the well-known analytic signal in the 1D case. The analytic and monogenic tools are both useful to extract phase information, which can then be directly linked with physical quantities. Previous studies have already shown the interest in the monogenic signal in the field of interferometry. This paper presents theoretical and numerical illustrations of the connection between the physical phase information and the phase estimated with the monogenic tool. More specifically, the ideal case of pure cosine waves is deeply studied, and then the complexity of the fringe patterns is progressively increased. One important weakness of the monogenic transform is its singularity at the null frequency, which makes the phase estimations of low-frequency fringes diverge. Moreover, the monogenic transform is originally designed for narrowband signals, and encounters difficulties when dealing with noised signals. These problems can be bypassed by performing a multiscale analysis based on the monogenic wavelet transform. Moreover, this paper proposes a simple strategy to combine the information extracted at different scales in order to get a better estimation of the phase. The numerical tests (synthetic and real signals) show how this approach provides a finer extraction of the geometrical structure of the fringe patterns.

Biomechanical analysis of the thoracolumbar spine under physiological loadings: Experimental motion data corridors for validation of finite element models.

Biomechanical studies that involve normal, injured or stabilized human spines are sometimes difficult to perform on large samples due to limited access to cadaveric human spines and biological variability. Finite element models alleviate these limitations due to the possibility of reusing the same model, whereas cadaveric spines can be damaged during testing, or have their mechanicals behaviour modified by fatigue, permanent deformation or structural failure. Finite element models need to be validated with experimental data to make sure that they represent the complex mechanical and physiological behaviour of normal, injured and stabilized spinal segments. The purpose of this study is to characterize the mechanical response of thoracolumbar spine segments with an analytical approach drawn from experimental measurements. A total of 24 normal and fresh cadaveric thoracolumbar spine segments (T11-L3), aged between 53 and 91 years, were tested in pure flexion/extension, lateral bending and axial torsion using a specific experimental setup. Measurements of global and intervertebral angle variations were performed using three-dimensional mark tracking methods. Load/angle curves for each loading were fitted by a logarithmic approach with two coefficients. The coefficients for the functions describing the response of the spinal segments are given and constitute predictive models from experimental data. This work provides data corridors of human thoracolumbar spine motion segments subjected to pure bending in the three physiological planes. These data could be very useful to validate finite element models of the human spine.

Phase demodulation method from a single fringe pattern based on correlation with a polynomial form.

The method presented extracts the demodulated phase from only one fringe pattern. Locally, this method approaches the fringe pattern morphology with the help of a mathematical model. The degree of similarity between the mathematical model and the real fringe is estimated by minimizing a correlation function. To use an optimization process, we have chosen a polynomial form such as a mathematical model. However, the use of a polynomial form induces an identification procedure with the purpose of retrieving the demodulated phase. This method, polynomial modulated phase correlation, is tested on several examples. Its performance, in terms of speed and precision, is presented on very noised fringe patterns.