| BackgroundDisc degeneration is highly prevalent, Lumbar disc herniation is a common disease causing the low-back pain and sciatica. It is very important for its radiograph, biomechanics and biology in many spine disorders. It has been most commonly treated with conservative care and surgery (discectomy, spinal fusion). For patients with persistent pain and disabling, despite of non-operative therapy, surgery treatment may be required. No matter discectomy, lumbar fusion and so on, they are not without complications. As our understanding of the structure and function of the human spine increase, the performance expectations of medical therapies also rise. Recently, no-fusion technique of spine and intervertebral disc arthroplasty are highly prevalent. Prosteses of total disc and disc nucleus are now available that may maintain motion and theoretically correct spinal segment dysfunction. They are considered to be used to reconstruct intervertebral disc. It is logical to consider the replacement of the degenerated nucleus with a prosthetic nucleus. The prosthetic disc nucleus (PDN) also functions in the same manner as a healthy natural disc nucleus. Before 2002, PDN replacement was not used in China. At present, short-term clinical results of PDN replacement was satisfied. Unfortunately, the procedures evaluate the long-term follow-up results have not been validated. Several complications have been reported. Subsidence is the erosion of prosthese into the adjacent vertebral body, can lead to progressive spinal deformity, resultant compression of neural elements, decrease of disc height and accelerate the degeneration of motion segment. The biomechanics character of endplate is one of important reasons.To prevent implant subsidence, the interface between the implant and the endplate must have sufficient strength to resist the large in vivo loading. The strength of this interface was dependent on biomechanics of endplate and implate area. Some authors have reported that the inferior, posterior, and lateral regions of vertebral endplate are strongest, whereas others have suggested that the central region below the nucleus is strongest. At present, to our knowledge, no published investigations have addressed this issue of the regional interface strength variation of the chondral endplate and relative biomechanics study of prothestic nucleus.Therefore, the aim of this study was to evaluate the mid-term clinical result of prosthetic disc nucleus replacement, determine if there are regional differences in the exteranl surface stress distribution of chondral endplate, determine the effect of prosthetic disc nucleus replacement on biomechanical property of lumbar structure.Objectives1. To evaluate clinical results of prosthetic disc nucleus (PDN) replacement for the treatment of the lumbar disc herniation2. To determine if there are regional differences in the exteranl surface stress distribution of end-plate according to Modic classification, and whether any differences identified are affected by endplate3. To determine if there are differences in primary biomechanics properties of lumbar functional range in establishing prosthetic disc nucleus replacement models.Materials and methods 1. 65 patients with lumbar disc herniation were implanted with one single PDN device from March 2002 to Novmember 2003. 57 patients come to the clinical, functional, and radiographic mid-term follow-up examinations. Independent analysis was performed by carefully review of the charts (Oswestry and Prolo scales), operative notes, preoperative and postoperative radiographs, magnetic renasonce imaging and follow-up records of all patients.2. 26 fresh lumbar vertebrae were harvested. The specimens were divided into four groups with MRI results according to Modic type. There were 6 Modic typeâ… , 4 typeâ…¡, 4 typeâ…¢and 6 normal. The specimens were mounted in Polymethylmethacrylate (PMMA). Indentation tests were performed at standardized test sites in the middle-regions of superior and inferior end-plate of human vertebrae (L4/5) using a pressure sensor, a material test system (MTS) machine with a load-test to 1300N. The load and stiffness pressure were determined directly with MTS machine with. Three-way analyses of variance were used to analyze the resulting data. Indentation tests were same as part two.3. The stability tests were performed using 3-D laser scanner. 8Nm load of anterior-flexion, posterior-extension and axial torsion respectively were loaded. Gemagic studio software analyzed the data.4. The vertevetebral stress tests were performed using L4/5 finite element model. 500N were loaded on the superior surface of the L4 vertebral body continually to simulate human body physiological load. 4Nm load of anterior, flexion, posterior, extension and axial torsion respectively were loaded. ANSYS software analyzed the data.Results1. After implantation, a significant proportion of patients experienced pain relief. Improvements were noted in pain intensity, walking distance, neurological weakness, Oswestry and Prolo scores, and lumbar mobility. Disc height measurements have been not well maintained. Compared with preoperative height of intervertebral disc. Several potential complications were associated with the implantation of PDN, including transient low-back pain, implant dislocation, malposition of the implant, breakages of the end plates and subsidence of implant.2. The stiffness stresses were varied significantly across the endplate surface, with center regions being stronger than bilateralis regions when the disc was intact. The characteristic distributions were identified in the lumbar superior and lumbar inferior endplate. The stress distributions of superior endplates were found to be stronger than inferior endplate.3. The stiffness stress was varied significantly across the endplate surface, with center regions being stronger than bilateralis regions when the disc was intact. The characteristic distributions were identified in the lumbar superior and lumbar inferior endplate. The stress distributions of superior endplates were found to be stronger than inferior endplate. When the nucleus was resected, the stress changed across the endplate surface with bilateralis regions being stronger than center regions. When prosthetic disc nucleus was replaced, the stress of bilateralis regions were also little stronger than center regions. In Modic typeâ… and Modic typeâ…¡, the variation of stress were almost same as intact models. However, the variation of stress was difference in Modic typeâ…¢. The stress was always varied significantly across the endplate surface, with bilateralis regions being stronger than center regions when the disc was intact, nucleotomy and prosthetic disc nucleus replacement models. The primary height of disc of nucleotomy were decreased than the intact disc at L4/5. The stress of nucleotomy was the strongest. The range of motion (ROM) in nucleotomy specimen increased significantly compared with the intact models. The prosthetic disc nucleus replacement models decreased the ROM of lumbar compared with the nucleotomy. There was no significant difference between the intact and prosthetic disc nucleus replacement models.4. The vertebral body's stress was the smallest after implating PDN, and its stressdistributed equally, but the stress under post-discectomy were more bigger than the normal disc all the motion state, in the other, tile stress distribution state were the changed while tile spine were in different motion state.Conclusions1. The clinical effect of PDN is certainly preferred and can effectively increase the range of lumbar motion in patients with lumbar disc herniation. Breakages of the end plates and subsidence of implant are potential complications However, Long-term reliability, safety and clinical efficacy of PDN implantation need to be intensively studied.2. Highly significant regional strength variations were identified in the lumbar endplates with Modic types. The pressure crossing the endplate surface of bilateralis region was smaller than center regions. The center of the endplate, where nucluse are currently placed, is not strongest region of the lumbar endplate.3. Highly significant regional strength variations were identified in the lumbar endplates with Modic types. When the disc nucleus was resected, the pressure crossing the endplate surface of bilateralis region was stronger than center regions. The center of the endplate, where implants are currently placed, is not strongest region of the lumbar endplate. The effect of prosthetic disc nucleus replacement to recover the primary range of lumbar motion and height of disc were certainly preferred. The vertebral body's pressur was the smaller after implating PDN than discectomy, and its pressure distributed equally.
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