Alternatives to spinal fusion

An underlying tenet of design in the biologic realm is that the optimal design of the prosthetic device reproduces exactly the healthy, intact biologic situation in every case (geometric, kinetic). Because the optimal solution is unobtainable, however, because of the limited resources available, compromises must be made. Every compromise inevitably produces complications because of the interdependent nature of biologic systems. The goal then becomes to minimize the detrimental effects of the implant system through the best possible use of the technologies available. It is the author's experience that whenever this fundamental tenet is forgotten in the design of an implant, either the system is affected adversely, or unforeseen complications arise in an associated system. An example can be seen in the evolution of solutions to the problem of loosening of the femoral component of hip prostheses. This loosening is caused, in part, by improper load transmission to the femur. Until recently, most hip implants transferred stress to the femur distally through the stem of the implant. Because of the lack of physiologic stress levels in the proximal femur bone, resorption was common, which often led to loosening of the implant. Many current designs are able to provide proximal loading of the femur through well-designed, proximal press fits. Although this seems to be a better solution, these prostheses fall short of the biologic situation in that they impose hoop stress to bone cells that naturally experience primarily axial stresses. The subsequent remodeling of bone because of the change in the load direction may not provide adequate stability for the implant in the long-term. In the same way, a best effort must be made in the present design to maintain near-physiologic load transfer and motion. Therefore, considerable effort was invested in the design stage of this project to consider the solutions available today to duplicate the stiffness (flexibility) of the lumbar disc in its principal plane of motion. This philosophy of restoration of function was also applied in specifying the geometry and the constraints to motion imposed by the prosthesis. The long life span required of an artificial spinal disc poses a significant challenge to the design effort. Plastics available today cannot be counted on to last for 40 years and 100 million cycles. The proposition of 40 years of metal-on-metal, however, was somewhat alarming. Therefore, quantitative wear-simulation testing has accompanied this development project since the second year of the project. Likewise, other factors, such as fatigue stress and corrosion resistance, become increasingly important because of the extended life span of the device. Therefore, as part of this project, standard material fatigue experiments (such as rotating bend tests) have been performed on the hot isostatatically pressed cobalt-chromium and titanium couples. The loads in the quadruped lumbar spine are largely in shear, which is in contrast to the more upright human lumbar spine, in which the loads are mainly in compression. The fact that bone ingrowth occurred in these animals under the worst-case scenario (shear loads) is most encouraging. There was no foreign body reaction in regional tissues and lymph nodes other than a mild fibrous tissue later over the implant. No soft tissue ingrowth was seen histologically in the springs. Although small numbers of animals were analyzed, the results were encouraging and would suggest that human implementation is feasible with little or no likelihood of mechanical failure, loosening, or disaster. A categoric list of design criteria relevant to the development of an articulating spinal disc implant has been presented. Each of these criteria and many of the questions posed are relevant in the synthesis of any new implant system. Constraints imposed by an implant have been discussed at length. A method of constraint analysis that simplifies specification of motion constraints has been described. Specifications of the preclinical trial design of an intervertebral disc prosthesis have been listed and evaluated on the basis of some of the pretrial experimental data. This design is the product of the detailed prospective design technique that has been the focus of this article. It is a design that will undergo future revisions based on clinical trial data, yet the implant's limitations and its in vitro performance has been assessed thoroughly during the past years to minimize the number of revisions necessary because of unsatisfactory results in clinical trials.