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Nucleus Replacement Device Failure: A Case Report and Biomechanical Study

Study Design. Case report and biomechanical study. Objective. The objectives of this study were to report on a single case of a failed nucleus replacement device and to test the biomechanical properties of the failed device. Summary of Background Data. The use of spine arthroplasty techniques in the treatment of degenerative disc disease is becoming a popular alternative to spinal fusion and discectomy. Nucleus replacement is an emerging surgical treatment that is in the early stages of development. Methods. A 36-year-old woman presented to our institution with excruciating low back pain 15 months after receiving a prosthetic disc nucleus (PDN; Raymedica, Inc.) at L5–S1 as part of an IDE clinical trial. A computed tomography scan showed subsidence of the PDN into the endplates and asymmetric collapse of the L5–S1 disc space. The patient underwent surgery for removal of the device and fusion of L5–S1. After removal, the nucleus replacement device underwent micro-computed tomography imaging and was tested in unconfined and confined compression. Results. The density of the inner core of the PDN was estimated to be 105 g/cm3. Compression testing revealed that the stiffness of the PDN was grossly elevated in comparison to previously published values for human lumbar nuclei and other candidate nucleus replacement hydrogels. The linear-region modulus values were 0.94 MPa for unconfined compression and 32.4 MPa for confined compression. Conclusion. The PDN device excised from this patient failed to reproduce the function of a healthy nucleus. Because preoperative mechanical values were not available for this device, it is difficult to know if the PDN was abnormally stiff at implantation or if it became increasingly stiff after implantation. Whether this was a result of manufacturing, the patient's biologic response to the PDN, or some yet unknown contraindication to PDN placement in this specific patient is unclear.