Ex vivo evaluation of a 2nd generation Nucleus Pulposus Prosthesis (NPP II) and its ability to restore axial disc height in canine cadaveric specimens
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The effect of a hydrogel nucleus pulposus prosthesis (NPP), made of N-vinyl-2-pyrrollidinone copolymerized with 2-(4’-iodobenzoyl)-oxo-ethyl methacrylate, on intervertebral disc (IVD) height was measured, as well as the assessment of the visibility on radiography, the extent of swelling and number of prostheses remaining intact and in situ after incubation. Both hydrophilic and hydrophobic NPPs were used, in a lumbar and lumbosacral size. The canine cadaveric specimens were radiographed in native state, after nucleotomy and after implantation and swelling of the NPP. The NPPs were weighted and measured before insertion as a xerogel and allowed to swell overnight at 37°C after insertion. After swelling the NPP was macroscopically evaluated in situ by dissection of the IVDs. Thereafter, the prostheses were weighted and measured again. Disc height was measured on radiography by two independent observers, using three different methods, where every method was measured twice. All methods were evaluated on inter- and intraobserver reliability, practical usefulness and restoration of disc height and compared with each other. A restoration of disc height was found during measuring of the radiographs. Also, radiographs revealed intrinsic radiopacity of both hydrophilic and hydrophobic prostheses. Hydrophilic prostheses have a significant greater swelling ability compared to hydrophobic prostheses. Six out of eight (6/8) lumbar hydrophilic NPPs remained intact and in situ after overnight incubation, where 2 of them showed fragmentation. The fragmented NPPs were all migrated through the annulus fibrosus and no longer in situ. All (4/4) lumbosacral hydrophilic NPPs were extruded. Four out of five (4/5) lumbar hydrophobic NPPs remained intact and in situ after overnight incubation, while 1/1 lumbosacral hydrophobic prosthesis was extruded. No fragmentation of the hydrophobic prostheses was seen. Future research should focus on the physical-mechanical properties of both hydrophilic and hydrophobic prostheses.