Biomechanical Properties Of Two Different Trajectory Screws Fixation In Osteoporotic Lumbar Vertebrae And Correlation Study Between The Bone Mineral Density Of The Trajectories And Bone Mineral Density Of Vertebrae:cortical Bone Trajectory And Traditional

The development of traditional trajectory(TT)screw instrumentation has grown so rapidly that it is now applied to treat various spinal disorders.The efficacy of TT screw fixation depends on the purchase in the trabecular and subcortical bones of a pedicle rather than a cortical bone.A sufficient screw purchase is difficult to achieve in osteoporosis,and an insufficient purchase can lead to fixation loss.Moreover,inadequate fixation strength of the screw may lead to non-union or sagittal collapse of the construct and kyphosis,and these issues often require a revision procedure.Many researchers have conducted a number of studies and suggested various methods.However,these various strategies have not substantially improved the screw fixation.These techniques demonstrate various complications.Cortical bone trajectory(CBT)screws follow a caudocephalad path sagittally and a lateral path transversally,thereby engaging only the cortical bone in a pedicle.These screws were recently developed as an alternative to TT screw fixation in the lumbar spine.According to biomechanical studies,CBT screws are equal to or better than TT screws in terms of pullout strength and toggle testing.These screws also exhibit marked increases in pullout load compared with those of TT screws in the osteoporotic lumbar spine.However,some studies found the standard pedicle screw had a better fatigue performance compared to the CBT screw.Therefore,biomechanical fixation properties of CBT screw in osteoporotic spines were still in dispute.In addition,researchers investigated the biomechanical properties of TT and CBT screws when each of the screws was used to rescue the other screw during revision.But the possibility of one screw path having the ability to rescue the failure of the other screw in osteoporotic spines has yet to be examined.Therefore,we used osteoporotic human cadaveric vertebrae to assess the behavior of the CBT screw through insertional torque measurement,fatigue testing and pullout testing.After that we assessed whether a CBT screw can salvage a compromised TT screw and whether a TT screw can rescue a failed CBT screw without extension or augmentation of the fixation in either case through the same biomechanical testing.The screw fixation strength is clearly related to the bone mineral density(BMD)of vertebrae,especially the BMD of the screw trajectory.Currently,the dual-energy X-ray absorptiometry(DEXA)and the quantitative computed tomography(QCT)are used to measure vertebral bone density.However,DEXA and QCT can only measure the BMD of vertebrae,which cannot represent the trajectory.The Hounsfield unit(HU)measurements from computed tomography(CT)images are recommended by many studies for BMD assessment and bone strength estimation.A positive correlation between the CT HU value and the DEXA BMD has been confirmed.HUs have also been used to approximate regional oral BMD and have been shown to correlate strongly with insertion torque and the stability of metal implants in vitro and in vivo.Therefore,it is feasible that the HU value of the screw trajectory measured from CT represents the BMD of the screw trajectory.But the correlation between the BMD of vertebrae and the HU values of CBT and TT is yet to be examined.Moreover,lumbar CT is a routine preoperative examination for patients,who need surgery for lumbar degenerative diseases.HU values can make the best use of CT images at no extra cost,and the screw trajectory can be chosen to measure the HU values in the trajectory.This association will be advantageous in applying the HU values of trajectory to estimate bone strength and predict pedicle screw stability,especially in medical institutions without QCT,and may avoid DEXA evaluation for some patients.Therefore,human cadaveric vertebrae from a radiological standpoint was used to assess whether the HU values of CBT and TT were correlated with the BMD of vertebrae,which was measured using DEXA and QCT scans.And then the healthy patients were evaluated such findings by comparing the average bone CT number(HU values)within the area wherein pedicle screws are normally placed for both trajectories and determine the optimum ages and lumbar segments.Objective: The first study: This study was to evaluate the biomechanical efficacy of CBT screws in the osteoporotic lumbar spine and then to investigate whether a CBT screw can salvage a compromised TT screw in osteoporotic lumbar spines and vice versa.The second study: This study used radiological methods to assess the correlation between the bone mineral density(BMD)of vertebrae and Hounsfield unit(HU)values of CBT and traditional trajectory(TT).The third study: This study was designed to compare the bone mineral density(BMD)between the CBT and traditional trajectory(TT)by using Hounsfield unit(HU)values and identify the ideal decades of patients and the suitable lumbar segments using this CBT technology from a radiological standpoint.Methods: The first study:A total of 42 vertebrae from 17 cadaveric lumbar spines were obtained.Bone mineral density was measured,and a CBT screw was randomly inserted into one side of each vertebra.A TT screw was then inserted into the contralateral side.Biomechanical properties were tested to determine insertional torque,pullout strength,and fatigue performance.After checking the screws for the failure of each specimen,the failed screw track was salvaged with a screw of the opposite trajectory.The specimen was then subjected to the same mechanical tests,and results were recorded once more.Repeat pullout test was also performed on TT and CBT screws.The second study: A total of 240 thoracic and lumbar(T9–L5)vertebrae from 40 cadaveric spines were obtained.The specimens were measured using computed tomography(CT).The axial CT images of TT were sliced in a plane horizontal to the pedicle,whereas those of CBT were sliced in a caudocranial plane.In addition,the regions of interest of TT and CBT were selected to calculate an average HU value within the area,wherein the screws were inserted,and fixed at 6.0 mm × 40 mm and 4.0 mm × 30 mm,respectively.The BMD of vertebrae was measured by dual-energy X-ray absorptiometry(DEXA)and quantitative CT(QCT).The third study: Patients were selected randomly from an institutional database based on age(evenly distributed by a decade of life)and gender.A total of 240 healthy patients had a computed tomography(CT)scan of the chest,abdomen,and pelvis.For each patient,axial slices of every vertebra were cut in two planes: one horizontal to the pedicle representing the plane wherein pedicle screws were inserted using the TT and the other in a caudocranial plane representing the plane wherein pedicle screws were inserted using the CBT.For each trajectory,a region of interest(ROI)was selected within the area wherein the screws were inserted.A CT number(HU values)was then calculated within each ROI to represent bone density.Results: The first study:1.The CBT screw had a higher maximum insertional torque(0.35 Nm vs.0.21 Nm,t=3.64,p=0.002)and higher axial pullout strength than the TT screw(396N vs.244 N,t=7.84,p=0.000).Increased BMD was not significantly associated with a higher pullout load.Compared with the TT screw,the CBT screw showed better resistance to fatigue testing and required more cycles to exceed 5 mm(6161 cycles vs.3639 cycles,t=5.62,p=0.000).The failure load for displacing the screws was also significantly greater for the CBT screw than for the TT screw(443N vs.317 N,t=5.75,p=0.000).2.When CBT screws were used to rescue failed TT screws,a 50% increase in the original torque,retention of 81% of the original pullout strength,fatigue performance equal to that of the original screws were found,which were much higher than the strength of the loose TT(TT repeat screws/TT screws were 33%).When TT screws were used to salvage compromised CBT screws,the TT screws retained 51% of the original torque and 54% of the original pullout strength,50% of the original fatigue performance which were still higher than the strength of the loose CBT screws(CBT repeat screws/CBT screws were 12%).While greater decreases in the ratio of pullout strength and fatigue performance were observed between the CBT rescue screws and original CBT screws,only slight changes in the ratio of pullout strength and fatigue performance were found between the TT rescue screws and original TT screws.The second study: The HU value of CBT(286.74 ± 120.80)was almost twice higher than that of TT(165.61 ± 92.38).The average BMD of 240 vertebrae determined using DEXA was 0.540 ± 0.193 g/cm2,and the average cortical and cancellous BMD of 240 vertebrae determined using QCT were 245.63 ± 80.09 and 88.24 ± 61.78 mg/cm3,respectively.The BMD determined using DEXA and QCT was significantly and positively associated with the HU values of CBT and TT.The ratio of the HU values of CBT and TT was significantly and negatively associated with the BMD determined using DEXA and the cancellous BMD determined using QCT and significantly and positively associated with segments(T9-L5)but not with the cortical BMD determined using QCT.The third study: HU values measured at the ROI of CBT were significantly greater than those of the traditional pedicle screw in all age groups,and the specific value between CBT and TT was 1.92.A significant difference was observed between male and female.The HU values of CBT and TT of males were generally higher than those of females(Males: CBT/TT: 1.89±0.45;Females: CBT/TT: 1.95±0.47).The specific value in HU values significantly increased with increasing age(p=0.000)and cauda lumbar level(p=0.000)in males and females.Conclusions: The first study: 1.The CBT screw had a better biomechanical properties in the osteoporotic lumbar spine compared with the traditional pedicle screw.2.CBT and TT screws can be applied as a revision technique to salvage each other in osteoporotic lumbar spines.Besides,CBT and TT screws each retain adequate insertional torque,pullout strength,and fatigue performance when used for revision in osteoporotic lumbar spines.3.The original trajectory of TT screws may remarkably influence the subsequent trajectory of CBT rescue screws;by contrast,the original trajectory of CBT screws may slightly affect the subsequent trajectory of TT rescue screws.As such,the original trajectory of TT screws overlapping with the subsequent CBT rescue screws should be considered prudently.The second study: The HU values of CBT and TT significantly decreased with declining BMD.However,the CBT HU values significantly decreased less than the TT HU values,especially in low BMD vertebrae and cauda lumbar segments.The third study: BMD,as measured by HU values for the ROI of the CBT screw,was significantly greater than that of the traditional pedicle screw,especially in old patients and cauda lumbar segments.