A calcium phosphate implant with controlled macroporosity for anterior L5-S1 interbody fusion

Abstract

In the United States approximately 50 million people suffer from low back pain. In the 20-50 years age group, this is the most important and expensive health care problem. The causes of low back pain are various. A herniated disk or spinal stenosis may trap nerve roots exiting the spinal column. Spondylolisthesis or degenerative disorders affecting the facet joints may cause some spinal units to become unstable. Although the majority of patients suffering back pain respond well to conservative treatments such as medication and physical therapy, a fraction of them will require spinal surgery. Different methods of stabilization exist, such as posterior, posterolateral and interbody fusion. Titanium screws and rods can be used for a posterior or posterolateral stabilization, with or without bone graft. Metallic cages or screws can also be used for a posterior lumbar interbody fusion (PLIF) or an anterior lumbar interbody fusion (ALIF), with or without bone graft inside the implant. A common drawback and risks to all of these implants is that they are made out of metal, with all the potential problems engendered by these materials. The present dissertation work aims at designing an improved anterior lumbar interbody fusion (ALIF) implant for the L5-S1 segment. The global objective of this thesis is to develop an alternative biomaterial that can replace at the same time the metallic cage and the bone graft. It must be biocompatible, bioresorbable, osteoinductive and strong enough to resist the high local stresses. The implant made of this biomaterial should also present an appropriate macroporosity to allow the blood vessels and bone cells to colonize quickly the implant. The implant should then be degraded in a controlled fashion by fragmenting over a defined period of time and releasing non-toxic ions which can be metabolized or excreted by the body. If this could be achieved, then no more graft would be necessary and the synthetic material would disappear with time allowing a complete bone fusion. In order to find an answer to this problem, five studies were defined: An anthropometric study of L5-S1 and the available space for insertion of an implant (Chapter 2). A complete mechanical characterization of two different phosphocalcic cements (Chapter 3). A new method to manufacture hydroxyapatite scaffolds with controlled porosity (Chapter 4). A comparison between an unconstrained and a partially constrained system for in vitro biomechanical testing of the L5-S1 functional spinal unit (Chapter 5). A numerical study using a finite element model of the L5-S1 functional spinal unit, validated with experimental data (Chapter 6). Our study provides the necessary geometrical parameters for the design of an ALIF cage to treat patients with low back pain (Chapter 2). Compression, tension and torsion tests together with the use of a conewise linear elasticity model and a Tsai-Wu failure criterion provide an exhaustive characterization of both elastic and failure properties of a brushite and a hydroxyapatite cement (Chapter 3). A new method to manufacture hydroxyapatite scaffolds with controlled porosity and predictable elastic properties is developed. Since precipitated hydroxyapatite cements are biocompatible, biodegradable and osteoconductive, the manufactured scaffold represents a biomaterial of choice for bone reconstruction in weight bearing areas dominated by compressive stresses (Chapter 4). According to our findings, particular care should be given when quantifying and comparing the kinematics and the flexibility curves of the intact or instrumented spine with experimental set-ups of various degrees of constraints for axial rotation and to an even larger extent for lateral bending (Chapter 5). A three-dimensional finite element model of the L5-S1 FSU is presented. The kinematics of the segment is found to be similar between numerical and experimental results for all major motions (Chapter 6). Following the application of the numerical model to the developed ALIF implant with or without posterior stabilisation, two applications for lumbar fusion are envisaged (Chapter 7). First, this macroporous scaffold can be used as an ALIF implant in conjunction with a posterior stabilization. Main advantages compared to current solutions are that no more bone graft is necessary and that no more metal is inserted anteriorly. Second, this macroporous scaffold can be used as a bone graft material, which can be inserted in a metallic cage of sufficient elasticity.