The restoration of lumbar intervertebral disc load distribution: a comparison of three nucleus replacement technologies.

STUDY DESIGN: A validated L3-L4 nonlinear finite element model was used to evaluate strain and pressure in the surrounding structures for 4 nucleus replacement technologies. OBJECTIVE: The objective of the current study was to compare subsidence and anular damage potential between 4 current nucleus replacement technologies. It was hypothesized that a fully conforming nucleus replacement would minimize the risk of both subsidence and anular damage. SUMMARY OF BACKGROUND DATA: Nucleus pulposus replacements are emerging as a less invasive alternative to total disc replacement and fusion as a solution to degenerative intervertebral discs. Multiple technologies have been developed and are currently undergoing clinical investigation. METHODS: The testing conditions were applied by excavating the nucleus of the intact model and virtually implanting models representing the various nucleus replacement technologies. The implants consisted of a conforming injectable polyurethane (E = 4 MPa), soft hydrogel (E = 4 MPa), stiff hydrogel (E = 20 MPa), and polyether-etherketone (PEEK) on PEEK articulating designs. The model was exercised in flexion, extension, lateral bending, axial rotation (7.5 Nm with 450 N preload), and compression (1000 N). Vertebral body strain, anular maximum shear strain, endplate contact pressure, anulus-implant contact pressure, and bone remodeling stimulus were reported. RESULTS: The PEEK implant induced strain maxima in the vertebral bodies with associated endplate contact pressure concentrations. For the PEEK and hydrogel implants, areas of nonconformity with the endplate indicated adjacent bone resorption. Lack of conformity between the implant and inner anulus for the PEEK and hydrogel implants resulted in inward anular bulging with associated increased maximum shear strain. The conforming polyurethane implant maintained outward bulging of the inner anular wall and indicated no bone resorption or stress shielding adjacent to the implant. CONCLUSION: A fully conforming nucleus replacement resulted in a decreased propensity for subsidence, anular bulging, and further degeneration of the anulus when compared with nonconforming implants.

Mechanical and biomechanical characterization of a polyurethane nucleus replacement device injected and cured in situ within a balloon.

BACKGROUND: The DASCOR device has recently been introduced as an innovative nucleus replacement alternative for the treatment of low-back pain caused by degenerative intervertebral disc disease. The purpose of this study was to characterize, through a series of preclinical mechanical bench and biomechanical tests, the effectiveness of this device. METHODS: A number of samples were created using similar preparation methods in order to characterize the nucleus replacement device in multiple mechanical bench tests, using ASTM-guided protocols, where appropriate. Mechanical bench testing included static testing to characterize the device's compressive, shear properties, and fatigue testing to determine the device's compressive fatigue strength, wear, and durability. Biomechanical testing, using human cadaveric lumbar spines, was also conducted to determine the ability of the device to restore multidirectional segmental flexibility and to determine its resulting endplate contact stress. RESULTS: The static compressive and shear moduli of the nucleus replacement device were determined to be between 4.2-5.6 MPa and 1.4-1.9 MPa, respectively. Similarly, the ultimate compressive and shear strength were 12,400 N and 6,993 N, respectively. The maximum axial compressive fatigue strength of the tested device that was able to withstand a runout without failure was determined to be approximately 3 MPa. The wear assessment determined that the device is durable and yielded minimal wear rates of 0.29mg/Mc. Finally, the biomechanical testing demonstrated that the device can restore the multidirectional segmental flexibility to a level seen in the intact condition while concurrently producing a uniform endplate contact stress. CONCLUSIONS: The results of the present study provided a mechanical justification supporting the clinical use of the nucleus replacement device and also help explain and support the positive clinical results obtained from two European studies and one US pilot study. CLINICAL RELEVANCE: Nucleus replacement devices are rapidly emerging to address specific conditions of degenerative disc disease. Preclinical testing of such devices is paramount in order to potentially ensure successful clinical outcomes post implantation.