Abstract
Nonfusion technologies in spinal surgery are continuously gaining popularity. New ideas are constantly being developed and turned into new products. Each idea has its own philosophy with the principle goal to maintain the motion in the treated segment. In contrast to total disc implants, nucleus implants are designed to preserve as many existing anatomical structures as possible, which include the annulus, the end plates of the vertebral bodies, and the ligaments. In theory, an optimum nucleus replacement should restore the mobility and reestablish the intact disc height, thereby restoring the nominal stresses and strains of the collagen fibers in the annulus. This seems to be a superior situation when compared with a disc left without augmentation following a discectomy.
The origin of the idea to replace the nucleus goes back to the 1950s. The first idea was to fill the nuclear cavity with polymethyl methacrylate or silicon. The first mechanical nucleus implant was suggested in 1966 by Fernstrom, who implanted a stainless steel ball as a spacer into the disc. Since that time, a variety of solutions to replace the nucleus have been developed. These different ideas can be classified into three categories mechanical nucleus devices, polymer implants, and tissue-engineered nucleus implants.
Like other implants, nucleus implants have to fulfill many mechanical and biomechanical requirements. Therefore, before being put into clinical practice, nucleus implants should be tested mechanically in static and dynamic tests. Aside from the mechanical challenges presented to the implant, the implant must reestablish the physiological biomechanics of a spinal segment. Therefore, biomechanical tests with the devices implanted into cadaveric specimens with respect to the surgical approaches for implantation should also be investigated in functional in vitro flexibility tests.
Despite the evaluation and subsequent use of several of these implants in patients, some of them have not performed successfully enough to survive on the market. Depending on the type of device, expulsion or subsidence of the implant has been identified as a problem.
Therefore, it became clear that aggressive testing is warranted. During in vitro testing, they can be subjected to complex cyclic loading with up to 100,000 load cycles. Some of the identified problems can be provoked with such in vitro tests and strategies developed to address the aforementioned issues. However, the ultimate arbiter of the implant's utility is not the biomechanical data, but rather it is still the clinical outcome.
None declared