Compliant Mechanism as a Motion-Preserving Artificial Spinal Disc: A Novel Concept

Investigations of spinal disc replacement options have been conducted for several decades but, as yet, the suggested solutions have not been proven to correctly preserve the natural joint motion. This paper focuses on a new structure of an artificial intervertebral disc joint that closely supports a close-to-natural three-dimensional motion of two adjacent vertebrae. The disc design is based on a passive parallel mechanism, with different stiffnesses for each link. Optimization of the artificial disc dimensions and link stiffnesses enabled convergence of the finite screw axis (FSA) of the artificial disc joint with that of a natural disc. As a result, the natural motion of the spine vertebrae was maintained and the loads on the facet joints minimized. The mechanism design was optimized, built, tested, and proven to be a feasible artificial disc with natural motion preservation characteristics.

BIOMECHANICAL PERFORMANCE OF A MODIFIED DESIGN OF DYNAMIC CERVICAL IMPLANT COMPARED TO CONVENTIONAL BALL AND SOCKET DESIGN OF AN ARTIFICIAL INTERVERTEBRAL DISC IMPLANT: A FINITE ELEMENT STUDY

Most of the implants used for total disc replacement (TDR) surgery are designed as a ball and socket pair aimed at providing a three-dimensional unconstrained motion. However, one of the major concerns with ball and socket design is the wear of the implant which limits its life. In this study the biomechanical performance of two types of implant designs is compared — a conventional ball and socket type (Prodisc-C) and a modified design of dynamic cervical implant (DCI) using FE analysis. A 3-dimensional geometrical model of cervical spine (C1–T1) was developed using CT scan data of a middle-aged healthy male. Subsequently, using FE analysis, the ROM values were validated with the existing literature using a compressive load in combination with different physiological motions of the neck. Furthermore, FE analysis on the two implants, fitted at C5–C6 segment, showed a significant increase in the ROM of implanted segment using Prodisc and decrease in the ROM of inferior segment, but modified-DCI restored the motion of the implanted and adjacent segments. Analysis of average bone strains adjacent to the implant showed a possibility of stress shielding for Prodisc. However, higher stress distribution on the modified-DCI limited its clinical use.

Bone Remodelling Model Incorporating Both Shape and Internal Structure Changes by Three Different Reconstruction Mechanisms. A Lumbar Spine Case

The paper presents a method of analysis of bone remodelling in the vicinity of implants. The authors aimed at building a model and numerical procedures which may be used as a tool in the prosthesis design process. The model proposed by the authors is based on the theory of adaptive elasticity and the lazy zone concept. It takes into consideration not only changes of the internal structure of the tissue (described by apparent density) but also surface remodelling and changes caused by the effects revealing some features of “creep”. Finite element analysis of a lumbar spinal segment with an artificial intervertebral disc was performed by means of the Ansys system with custom APDL code. The algorithms were in two variants: the so-called siteindependent and site-specific. Resultant density distribution and modified shape of the vertebra are compared for both of them. It is shown that this two approaches predict the bone remodelling in different ways. A comparison with available clinical outcomes is also presented and similarities to the numerical results are pointed out.