| Background:Cervical spondylosis is common in middle-aged and elderly people in China.With the country gradually entering an aging society,the incidence of cervical spondylosis in Chinese population is increasing year by year.As one of the most important treatments for cervical spondylosis,internal fixation surgery has been continuously improved and its therapeutic efficacy has been verified in clinical practice for many years.Posterior cervical screw fixation technique is an important component of cervical spine surgery.At present,pedicle screw(PS)technique and lateral mass screw(LMS)technique are the main internal fixation techniques in clinical practice.Abumi first reported cervical PS in 1994,and subsequently reported that it was superior to anterior screw-plate fixation and LMS in the treatment of subaxial cervical spine traumatic fractures,tumors,infections,degenerative lesions and secondary surgery for failed anterior fixation and fusion.At present,PS has obvious advantages over other posterior internal fixation instruments.The pedicle is a very strong structure in the cervical spine,and there is relatively dense cancellous bone at the junction between the pedicle and the lamina.A large number of biomechanical studies have reported that the peak pullout strength of PS is significantly higher than that of LMS,its fatigue resistance and stability are also better than those of LMS,and its advantages were more obvious in patients with osteoporosis.In addition,for patients with severe cervical trauma or tumors,the cervical stability provided by PS can reduce the incidence of secondary anterior cervical fixation.Previous studies have shown that PS has longer path length.However,the anatomical variation of cervical pedicle is high and the structure of pedicle is fine.Thus,for the same cervical segment of different individuals and different cervical segments of the same individual,the height and width,axial projection point,axis angle of pedicle,as well as the starting point and the direction of pedicle screw are markedly different,which are important causes of surgical complications and postoperative adverse outcomes.It has been reported that the pedicle cortex penetration rate of freehand PS placement can be as high as 16.8%.In addition,anatomically,the pedicle of cervical vertebra is closely associated with vertebral artery,nerve root,dural sac and esophagus.During screw insertion,if clear anatomical markers are lacking,catastrophic surgical complications such as vertebral artery rupture(0.6%-1.0%),nerve root injury(0.44%-7.6%),cerebrospinal fluid leakage(<0.1%)and esophageal perforation(rare)often emerge.These factors together determine the high difficulty of PS placement.The accuracy of PS placement can be partially improved by assistive techniques such as three-dimensional computerized tomography(CT)navigation,O-arm navigation and 3D printed drill guide template,but these techniques are limited by the proficiency of the surgeon and are not yet popular.LMS was originally proposed by Roy-Camille in 1972,and up to now,Roy-Camille technique,Magerl technique and Anderson technique for LMS fixation have been developed.LMS has become one of the most commonly used posterior screw fixation techniques for subaxial cervical spine surgery due to its high surgical safety,relatively low screw placement difficulty and satisfactory cervical stability The main complications of LMS are nerve root injury(3.8%-5.6%),vertebral artery injury(1.3%)and articular surface invasion(16.7%).However,for patients requiring rigid fixation,or with lateral mass fracture or severe osteoporosis,especially at C7 segment,LMS cannot provide sufficient strength and stability.In 2014,Aramomi proposed a new technique called paravertebral foramen screw(PVFS)for the fixation and fusion of subaxial cervical spine.The screw starting point was located at 1 mm medial to the cross-point of the midline of the lateral mass and the inferior margin of the inferior articular process of the cranially adjacent vertebra,and a 4.5 mm×12 mm screw was inserted along the direction inclined 20°-25°medially.PVFS was adjusted on the basis of cervical PS,and "thick and short" screws were selected,and this technique allows screws to purchase the relatively compact cancellous bone near the entry zone of the cervical pedicle,which are helpful to enhance the strength and stability of PVFS.Shorter screw can avoid direct injury of vertebral artery by the tip of the screw,which theoretically ensures surgical safety.Biomechanical experiments have shown that PVFS has higher pullout strength than LMS,and PVFS is considered as an effective substitute for failed LMS placement or lateral mass fracture that cannot implant LMS.However,there are still few reports on the study of anatomical parameters and biomechanics of PVFS based on the Chinese population.In this study,the screw path length and insertion angle of PVFS,LMS and PS in Chinese population were measured on radiographic images.Based on the parameters we carried out initial pullout strength test,fatigue test and residual pullout strength test to explore and compare the differences of mechanical properties among PVFS,LMS and PS.Objective:To measure the length and angle parameters of the screw paths,and test and compare the biomechanical strength of PVFS,LMS and PS of subaxial cervical spine in Chinese population,which provided theoretical basis for the surgical design and screw selection of posterior cervical surgery instruments in clinical practice.Method:This study included the cervical computerized tomography(CT)scans of 50 healthy volunteers(25 males and 25 females)in our hospital from January 2018 to June 2018.The average age of the volunteers was 56.00±15.90 years(29-89 years).After three-dimensional reconstruction of CT data,the screw starting points,the length of screw paths,optimal medial angles,maximum medial angles and minimum medial angles of PVFS,LMS(Magerl technique)and PS on C3-C7 segments were designed and measured on the reconstructed 3D model,and the pedicle widths at various segments of cervical vertebrae were measured as well.Every two weeks,the same spine surgeon measured each parameter again,and took the average of the two measurements as a result of the parameter.Thirty cervical spine vertebrae(C3-C6)specimens were harvested from eight fresh-frozen cadavers of donors whose average age was 45.25 ± 11.23 years at death.The cervical spine vertebrae specimens were randomly divided into three groups for the placement of PVFS,LMS and PS.The screws used for PVFS,LMS and PS insertion were 4.5 mm x 12 mm screws,3.5 mm x 14 mm screws and 3.5 mm x 24 mm screws,respectively.One side of each vertebra was randomly chosen for initial pullout strength test,and the other side for fatigue test and residual pullout strength test.Results:In general,the optimum length and medial angle of the PVFS in Chinese population were 10.65 mm and 21.120 at C3;10.12 mm,22.620 at C4;9.82 mm,23.660 at C5;9.19 mm,24.13° at C6;9.10 mm,27.540 at C7.The C3 segment had the longest general optimal length(p<0.05),and the C7 segment had the shortest general optimal length of PVFS(p<0.05).However,there was no significant difference in optimal length of PVFS between C6 and C7 vertebrae(p≥ 0.05).The C7 vertebra had the largest general medial angle(p<0.05),meanwhile the C3 vertebra had the smallest general medial angle(p<0.05).The optimum lengths of screw paths in males at the segments of C4 and C6-C7 vertebrae were longer than those in females and the optimal medial angle at C4 and C6-C7 segments were smaller than those in females(10.44mm vs 9.81mm,21.11° vy 24.12s at C4;9.69mm vs 8.69mm,22.10° vs 26.16° at C6;9.90mm vs 8.30mm,24.95。vs 30.13° at C7,p<0.05).The optimum length and medial angle of PS in Chinese population were 30.94 mm,33.92° at C3;30.50 mm,34.95° at C4;31.92 mm,33.42° at C5;30.50 mm,31.94° at C6;29.87 mm,31.01° at C7.The general pedicle widths were 5.35mm at C3;5.56mm at C4;5.99mm at C5;6.34mm at C6;6.86mm at C7.The optimum length of screw path and pedicle width in males at C3-C7 vertebrae were longer than those in females(32.59mm vs 29.29mm,5.89mm vs 4.81mm at C3;32.06mm vs 28.94mm,6.06mm vs 5.06mm at C4;33.85mm vs 29.98mm,6.40mm vs 5.57mm at C5;32.22mm vs 28.78mms 6.77mm vs 5.90mm at C6;31.56mm vs 28.18mm,7.26mm vs 6.46mm at C7,p<0.05).At C4-C5 and C7 segments,the optimum medial angle in males was smaller than that in females(33.66° vs 36.24° at C4;32.24° vs 34.59° at C5;29.01° vs 33.02° at C7,p<0.05).The optimum lengths of LMS paths in Chinese population were C3,14.84 mm;C4,15.33 mm;C5,15.44 mm;C6,14.74 mm;C7,14.06 mm.At C3-C7 segments,the optimal length of screw path in males was longer than that in females(15.56mm vs 14.12mm at C3;15.95mm vs 14.71mm at C4;16.11mm vs 14.77mm at C5;15.68mm vs 13.80mm at C6;14.82mm vs 13.29mm at C7,p<0.05).The general optimal length of PVFS were shorter than those of LMS and PS at C3-C7 segments,and the general optimal medial angles were smaller than those of PS in C3-C7 segments(p<0.05).The initial pullout strength of PVFS,LMS and PS was 327.10 ± 17.07 N,305.71±11.63 N and 635.67±22.82 N,respectively.The residual pullout strength of PVFS,LMS and PS was 265.62 ±18.19 N,192.80 ± 17.10 N and 494.89± 41.79 N,respectively.The residual pullout strength of PVFS,LMS and PS decreased by 18.8%,36.93%and 22.15%,respectively,compared with their initial pullout strength(p<0.001).The initial pullout strength of PS was higher than that of PVFS and LMS(p<0.001),and the initial pullout strength of PVFS was slightly higher than that of LMS(p=0.004).The residual pullout strength of PS was the highest,followed by PVFS,and LMS was the lowest(p<0.001).For the fatigue test,the load at the first cycle and at the first time when the set position was reached in PVFS were higher than those of LMS and PS(p<0.05),but there was no significant difference between LMS and PS(p=0.550;p=0.067).The load at the last cycle of PVFS and PS was higher than that of LMS(p<0.001;p=0.003),but it was not significantly different between PVFS and PS(p=0.126).Conclusion:The optimal length of PVFS were shorter than those of LMS and PS at C3-C7 segments,and the optimal medial angles were smaller than those of PS at C3-C7 segments.In Chinese population,the length of the screw path of PVFS in males was slightly longer than that of females,and the medial angle was smaller than that of females.The safety angle of insertion of PVFS is large.Because of the length of screw path of PVFS is limited,it does not have the risk of directly injuring the vertebral artery.The direct pullout strength of PVFS was slightly higher than that of LMS,and the residual pullout strength was significantly higher than LMS.The property of fatigue resistance of PVFS was similar to PS and obviously better than LMS.In summary,PVFS can be used as an effective substitute for LMS and PS. |
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