| Since the introduction of the transpedicular screw system by Boucher,the application of this system in the treatment of degenerative disorders and unstable fractures,tumors of the spine have become very popular in the last two decades. Recently,internal fixation with pedicle screw has become the most frequently used method in posterior fixation methods of spine. The bone-screw interface is a major determinant in the stability of spinal instrumentation systems. Loosening and failure of the screws are among the most common complication reported,especially for osteoporosis. Many relative researches have been carried out to prevent postoperative complication above-mentioned , including improving design of pedicle screw and strengthening track of pedicle screw. The former,increasing the diameter and/or length of the pedicle screw,can significantly increase the stability of screw fixation,but it also increases the risk of pedicle fracture and anterior body penetration with possible vascular or visceral injury. It has been proved by numerous studies that strengthening track of pedicle screw,injecting solidification materials into the track before fixation, has conspicuous effect on increasing strength of screw fixation. But in this traditionary entire solidification of track,solidification materials wrap up screw completely and tightly that can result in destruction of bone and difficulty in revision. Solidification materials existing between bone and screw for long can form the second interface which threatens long term stability of screw. Therefore,how to avoid materials wrapping up screw completely and decrease destruction of bone,difficulty in revision and problem of the second interface at the same time of strengthening track and increasing stability of screw,has being an urgent and tough problem in spinal surgery. In order to solve this problem,we propose a novel method of track solidification―partial solidification of track.Objective:To evaluate effect on increasing stability of pedicle screw and interface of track through partial solidification of track in vitro and in vivo.Methods:1)design a novel tap―tap with hollow and lateral apertures. Compared with common tap,the major characteristic of this novel tap are lateral apertures and hollow structure. We chose calcium sulfate cement(CSC)as solidification material which has feature of excellent biocompatibility,complete degradation and absorption,osteogenesis and strong mechanical strength. We performed partial solidification of track by injecting CSC into bone through hollow and lateral apertures during track-making;2)Three different methods were randomly carried out on fresh adult goat lumbar vertebrae,A:there was no treatment of track ( blank group ); B : CSC was injected into track directly―entire solidification of track(control group);C:track was partially solidified using the tap with hollow and lateral apertures with CSC―partial solidification of track(experiment group). Micro-CT and biomechanics experiment were used to evaluate distribution of CSC,interface of track and stability of screw;3)Three different methods were randomly carried out on healthy adult goats. After six months,all goats were sacrificed. We used Micro-CT,histology and biomechanics experiment to evaluate interface of track,degradation of CSC and stability of screw.Results:1)In vitro study,there were complete"screw-bone"interface and complete"screw-CSC-bone"interface respectively in group A and B,but it formed a special whole interface consisting of partial"screw-bone"and partial"screw-CSC-bone"interface in group C. Partial solidification of track could significantly strengthen the stability of pedicle screw fixation,but the effect was little weaker than entire solidification of track;2)In vivo study,quality of bone (quantity and density of bone trabeculae)around screw in both group B and C were much better than that in group A,but there was no significant difference between group B and C in bone histomorphometry. Histologically in group C,CSC around the track was completely degradated and much newly formed bone trabeculae crawled around screw tightly forming bone union between bone and screw. Though there were also much bone tissue around screw,a little CSC undegradated was found in cavitas medullaris in group B. In group A,fewer bone trabeculae were found around screw. Partial solidification of track could significantly strengthen the stability of pedicle screw fixation,and the effect was almost as same as that of entire solidification of track.Conclusions: 1)The special whole interface in method of partial solidification of track , consisting of partial"screw-bone"and partial"screw-CSC-bone"interface,not only increased stability of screw ,but also avoid materials wrapping up screw completely and tightly that provided a upstanding precondition for reducing the destruction of bone tissue around screw during it being taken out;2)With complete degradation and adsorption of CSC in vivo,much new bone tissue wrapping up screw tightly forming total and excellent bone union without the second interface,that ensured long term stability of pedicle screw in vivo. |
