颈椎前路带锁与非带锁钢板螺钉系统固定强度的比较
作者：陈德玉 沈强 宋滇文
单位：陈德玉(第二军医大学长征医院骨科，上海 200003)；沈强(第二军医大学长征医院骨科，上海 200003)；宋滇文(第二军医大学长征医院骨科，上海 200003)
关键词：颈椎；钢板；螺钉；固定强度
第二军医大学学报000704 [摘要] 目的：比较带锁与非带锁颈椎前路钢板螺钉系统的即刻固定强度和抗疲劳强度。方法：采用CSLP和Orion两种颈椎前路带锁钢板与Acroplate非带锁钢板固定于新鲜颈椎椎节。带锁钢板测定锁定和非锁定状态；非带锁钢板测定单侧椎体皮质骨固定和螺钉穿透椎体前后侧骨皮质两种长度。测定其即刻固定强度、抗疲劳强度和钢板螺钉的抗拉出强度。结果：锁定状态的CSLP和Orion系统的即刻固定强度和抗疲劳强度均显著高于各自非锁定状态及Acroplate钢板单侧皮质螺钉固定。未锁定之带锁钢板和非锁定钢板的抗疲劳强度较各自的即刻固定强度显著减弱。带锁钢板和相同螺钉长度的非带锁钢板的螺钉抗拉出强度无明显差别。结论：锁定机制提高了颈椎前路钢板螺钉的即刻固定强度和抗疲劳强度。
, 百拇医药
[中图分类号] R 687.32 [文献标识码] A
Comparison of fixation strength of the locking and non-locking anterior cervical plate systems
CHEN De-Yu SHEN Qiang SONG Dian-Wen
(Department of Orthopaedics, Changzheng Hospital, Second Military Medical University, Shanghai 200003， China)
[ABSTRACT] Objective: To compare the prompt fixation strength and antifatigue strength of the locking and non-locking anterior cervical plating systems. Methods:AO Cervical Spine Locking Plate (CSLP), Danek Orion plate and AcroMed Acroplate were used on the lamb cervical spines. The CSLP and Orion were tested with screws locked and unlocked, and the Acroplate with unicortical and bicortical purchase. The fixation strength and pull-off strength of the screw-plate constructs were performed initially and after fatigue. Results: Locked CSLP and Orion constructs were more rigid than all unlocked unicortical systems initially and after cyclic loading. After fatigue testing, the strength of all unlocked constructs decreased significantly. There was no significant difference in pull-off strength between the CSLP, the Orion and the unicortical Acroplate. Conclusion: The locking mechanism significantly increases the prompt fixation strength and antifatigue strength of the tested screw-plate systems.
, 百拇医药
[KEY WORDS] cervical vertebrae; plate; screw; fixation strength
[Article Code] 0258-879X(2000)07-0610-05
The anterior plating systems are extensively used to treat fracture and dislocation in the lower cervical spine[1-5].There are different types of system available, they can be divided into the locking and the non-locking anterior cervical plating systems. The examples of the former include the Synthes Cervical Spine Locking Plate (CSLP) and Danek Orion plate,and the latter are Casper plate and AcroMed Acroplate. In the past, most studies on biomechanics of anterior cervical internal fixation have been limited to the non-locking anterior cervical plating systems[6-9].Little experimental work has been performed to determine the effect of locking screws on the stability of anterior cervical plating.In this study，the currently available locking and non-locking anterior cervical plate systems were tested biomechanically. Comparisons of fixation strength and pull-out strength between the different systems were performed.The effects of locking screws on the stability of anterior cervical plating were determined.
, http://www.100md.com
1 MATERIALS AND METHODS
1.1 Specimen preparation Fourteen fresh lamb， aged between 6-8 months,their cervical spines with T1 were collected from a local abattoir.The specimens were frozen in double thickness plastic bags at -20°C.Just prior to biomechanical testing, the specimens were thawed at room temperature.Individual vertebrae(C3-T1) were dissected from the spines，and the surrounding soft tissues were cleaned.The sagittal vertebral diameter was measured by a caliper.A total of 80 vertebrae were used in this study and randomized to different instrumented test conditions.
, 百拇医药
1.2 Testing instrumentation and groups Each system was instrumented using the superior half of plate fixed with 2 screws to the vertebral body according to its technical principle.A torque screwdriver was used to contro1 the peak insertion torque. The CSLP and Orion were tested with screws locked and unlocked，and the Acroplate with unicortical and bicortical screw purchase.There were 8 vertebrae for each testing condition. The initial fixation strength was measured first and then followed by fatigue strength and pull-out strength. In addition, the CSLP and Orion with screws locked，and the Acroplate with unicortical and bicortical screw purchase were tested only for determining the initial pull-out strength.Each group had 8 vertebrae too.
, 百拇医药
1.3 Biomechanical testing Biomechanical testing for fixation strength and pull-out strength of the screw-plate constructs were performed on an Instron (Model l321，Instron Corporation，Cantor，MAS) with the vertebrae held in a custom test fixture.The plate was oriented vertically to the floor for cyclic loading and parallel to the floor for pull-out testing. A steel connecting block was fixed on each test plate.One end of a threaded stainless steel rod，as an adjustable moment arm，was attached to the steel block on the plate, and the other end was connected with a universal ball joint to the actuator of Instron.Cyclic loads (total 2.5 N/m bending moment for both flexion and extension，at a rate of l Hz) were generated under load control to simulate cervical flexion-extension.The fixation strength was determined in a11 1ocked and unlocked constructs, and the unicortical and bicortical Acroplate initially and after 105 cycles, followed by pull-out testing to failure of the screws within the plate.For the pull-out test of the screw-plate constructs，the ball joint was taken off, the sample was turned 90°, and the threaded rod was attached to the actuator directly in line with the long axis of the anchor screws. The pull-out strength after fatigue was determined at a rate of 10 cm/min until failure of the construct occurred. In addition, the initial pull-out strength was measured in the locked CSLP, the locked Orion, and the uni- and bicortical Acroplate constructs.Test data were treated by an IBM compatible computer.
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1.4 Statistical analysis A t-test was performed to compare the fixation strength and pull-out strength of the screw-plate constructs for the different systems.The relationship between the pull-out strength and middle sagittal diameters of vertebral bodies were determined by correlation analysis.
2 RESULTS
2.1 The fixation strength of the different plating systems The fixation strength of each tested construct is shown in Figure l.The locked CSLP and Orion system were significantly more rigid than their corresponding unlocked constructs initially and after cyclic loading.The locked Orion construct showed significantly higher strength than the unicortical Acroplate construct，initially and after fatigue testing，and than the bicortical Acroplate construct after fatigue testing.The locked CSLP showed significantly higher strength than the unicortical Acroplate construct only after fatigue testing.The locked Orion constructs had higher strength than the locked CSLP before and after cyclic loading; however，this was not significant.After 105 cycles，the strength of all unlocked constructs decreased significantly.There was no significant change in fixation strength of the locked CSLP and Orion constructs after cyclic loading.
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Fig 1 The fixation strength of different plate
systems before and after cycling loading
□： Before cycling； ■： After 10 000 cycles
A:CSLP(locked); B:CSLP(unlocked);
C:Acroplate (bicortical)； D:Acroplate (unicortical);
E:Orion (locked); F:Orion (unlocked)
2.2 Pull-out strength of the screws The pull-out strength of the screws in different constructs is shown in Figure 2.The bicortical Acroplate had a significant higher pull-out strength than all unicortical screws-plating constructs initially and after cyclic loading.There was no significant difference in pull-out strength between the CSLP，the Orion，and the unicortical Acroplate. After 105 cycles, pull-out strength of the screws in all tested constructs decreased, insignificantly though.
, http://www.100md.com
Fig 2 Pull-out strength of the screws in different plating
constructs initial and after fatigue testing
□: Initial; ■: Cycled; □: Unlocked and cycled
A: CSLP；B: Acroplate (unicortical);
C: Acroplate (bicortical); D: Orion
2.3 Relationship between sagittal diameter of the vertebral body and pull-out strength The middle sagittal diameter of individual vertebral bodies was varied from 15.5 mm to 19.4 mm with a mean of 17.3 mm. There was no significant difference in diameter between the different tested construct groups. However, a negative correlation was observed between the initial pull-out strength of the screws and the sagittal diameter of the vertebral body in all unicortical screws-plate constructs (Figure 3).
, http://www.100md.com
Fig 3 Relationship between initial pull-out strength
and the middle diameter of verlebral body
r=-0.65,n=24
3 DISCUSSION
There are anatomic differences between the lamb and human cervical spines.The lamb vertebrae are taller in body height and somewhat smaller in transverse and sagittal diameters than the human vertebrae.The middle sagittal diameter of the lamb individual vertebral bodies(C3-T1)vary from 15.5 mm to 19.4 mm with a mean of 17.3 mm and are big enough to be instrumented with typical human unicortical screw-plating (14 mm).In addition，the lamb vertebrae have the advantage of easy availability and greater specimen to specimen continuity.
, 百拇医药
Previous studies on the biomechanics of anterior cervical internal fixation have focused on comparison of non-locking plate techniques.Grubb et al [6] examined the stability potential of the Casper plate secured with unicortical or bicortical screws using a porcine cervical spine. A C5 corpectomy, strut grafting and plating were performed. The immediate stability，failure moment，and mechanism of failure were evaluated. No difference were found in the immediate stabilities of unicortical versus bicortical screw placement.Failure of the unicortical Casper plate group resulted from a combination of screw loosening and pull-out greater than 3.5 mm，whereas failure for the bicortical Casper group was primarily the result of plate deformation，and screw loosening in few cases.Ryken et al[7] evaluated the immediate and post fatigue stabilities of Casper plates secured with unicortical and bicortical screws on fresh human cervical motion segments.The results indicated that there was no difference in the pre-fatigue load displacement characteristics of plated spines with segmental disruption when the unicortical and bicortical groups were compared.However，a significant difference was observed between these groups with 5 000 cycles of fatigue loading.Half of the unicortical group became so unstable that further testing was precluded.Gallagher et al[8] compared unicortical to bicortical Casper screw placement in a single human cadaveric vertebra undergoing 200 cycles of simulated flexion-extension.A marked improvement on stability was observed with bicortical purchase.In their study，however，only a single fixation screw was used to absorb all the load，which did not simulate the case in clinical situation where at least 2 screws are used at each vertebral level.In our study，two locking screw-plate systems and one non-locking system were evaluated to examine the effects of locking screws on the rigidities of anterior plating.A basic unit of anterior cervical internal fixation, which includes a plate and 2 screws secured on single vertebral body, was tested.A bending moment was used to simulate the load on the construct during physiological cervical flexion-extension.Our results indicate that both bicortical and unicortical Acroplate constructs had lower strength than the locked CSLP and Orion constructs.After 105 cycles loading，the unlocked CSLP and Orion，unicortical and bicortical Acroplate became significantly less rigid; the rigidities of the locked CSLP and Orion showed no significant change.Biomechanically，the stability of the non-locking plate system requires pressure of the screw head to ho1d the plate against the bone.If the plate is not flat against the bone，or if there is even minimal looseness of the screw，the stability of the construct will be lost.In the locking plate system, once the locking screw has been inserted into the anchor screw, the screw and plate function as a single unit. This is especially advantageous in the cervical spine. The anterior surface of the vertebral bodies have an undulating contour precluding direct bony contact through the entire length of the construct.Loosing at the screw-plate junction may occur. The CSLP and Orion allowed for securing screw in the plate holes by means of the locking screw.The CSLP uses a locking hub screw which expands the anchor screw head against the plate.The Orion plate uses a large flat-head screw along the centerline of the plate that tightens down over a portion of 2 anchor screws at the same level.However, there was no significant difference in the fixation strength between the locked Orion and CSLP.
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The screw migration has been a main complication of the unlocked screw-plate systems.Published descriptions of the surgical technique for bicortical screws have emphasized the importance of penetration through the posterior vertebral cortex for appropriate stability[2].Maiman et al[9] examined the pull-out behavior of Caspar screws in iso1ated human cadaveric cervical vertebral bodies，to examine the importance of posterior cortical penetration.There were no significant differences between the 2 groups with the posterior penetration and without posterior penetration.The screws of the CSLP and Orion systems are 14 mm in length. The sagittal diameter of the human cervical vertebral body is 17-21 mm, so there is a 3 mm safety factor at least. Without the need for posterior vertebral body cortical purchase, the risk of screw misplacement and spinal cord injury was decreased. In this study, when the tested screws had the same length, a negative correlation between the pull-out strength of the screw and the sagittal diameter of the vertebral body was observed. This is different from the results obtained by Maiman and co-workers. The unicortical screw (14 mm) we tested was shorter than those used by Maiman which were subcortical and very close to the length of bicortical screws.In addition, the cadaveric vertebrae tested by Maiman were relative old and therefore had poorer bony quality. Except for the anatomic differences，the length of the screw is another key fact in determining the pull-off strength under given diameter. Therefor, use of longer unicortical screws may increase fixation strength in patients with larger cervical vertebrae.
, 百拇医药
Biography:CHEN De-Yu,MD,associate professor,tutor of master.
Foundation：This project is supported by Shanghai Leading Subject of Key Medical Science Foundation.NO.1995-Ⅳ-008,1998-Ⅳ-008.
[REFERENCES]
[1] Aebi M, Zuber K, Marchesi D. Treatment of cervical spine injuries with anterior plating: indications, techniques, and results[J]. Spine, 1991,16(3 Suppl):38-45.
, 百拇医药 [2] Caspar W, Barbier DD, Klara PM. Anterior cervical fusion and caspar plate stabilization for cervical trauma[J]. Neurosurgery, 1989,25(4):491-502.
[3] Goffin J, Loon JV, Calenbergh FV, et al. Long-term results after anterior cervical fusion and osteosynthetic stabilization for fracture and /or dislocation of the cervical spine[J]. J Spinal Disord, 1995,8(6):500-508.
[4] 袁 文，贾连顺，戴力扬, 等. AO纯钛带锁钢板在颈椎前路固定的初步报告[J]. 中国脊柱脊髓杂志, 1996, 6(4):161-163.
, 百拇医药
[5] 刘忠军，党耕町，刘晓光，等. AO带锁型钢板螺钉在颈椎外伤治疗中的应用[J]. 中华骨科杂志，1997, 17(1):33-36.
[6] Grubb MR, Currier BL, Shin JS, et al. Biomechanical evaluation of anterior cervical spine[J]. Spine， 1998，23(8):886-892.
[7] Ryken TC, Goel VK, Clausen JD, et al. Assessment of unicortical and bicortical fixation in a quasistantic cadaveric model[J]. Spine， 1995，20(17):1861-1867.
[8] Gallagher MR, Maiman DJ, Reinartz J, et al. Biomechanical evaluation of Caspar cervical screws: comparative stability under cyclical loading[J]. Neurosurgery， 1993，33(6):1045-1050.
[9] Maiman DJ, Pintar FA, Yoganandan N, et al. Pull-off strength of Caspar cervical screws[J]. Neurosurgery， 1992，31(6):1097-1101.
Received 2000-04-20
Accepted 2000-07-10, http://www.100md.com
单位：陈德玉(第二军医大学长征医院骨科，上海 200003)；沈强(第二军医大学长征医院骨科，上海 200003)；宋滇文(第二军医大学长征医院骨科，上海 200003)
关键词：颈椎；钢板；螺钉；固定强度
第二军医大学学报000704 [摘要] 目的：比较带锁与非带锁颈椎前路钢板螺钉系统的即刻固定强度和抗疲劳强度。方法：采用CSLP和Orion两种颈椎前路带锁钢板与Acroplate非带锁钢板固定于新鲜颈椎椎节。带锁钢板测定锁定和非锁定状态；非带锁钢板测定单侧椎体皮质骨固定和螺钉穿透椎体前后侧骨皮质两种长度。测定其即刻固定强度、抗疲劳强度和钢板螺钉的抗拉出强度。结果：锁定状态的CSLP和Orion系统的即刻固定强度和抗疲劳强度均显著高于各自非锁定状态及Acroplate钢板单侧皮质螺钉固定。未锁定之带锁钢板和非锁定钢板的抗疲劳强度较各自的即刻固定强度显著减弱。带锁钢板和相同螺钉长度的非带锁钢板的螺钉抗拉出强度无明显差别。结论：锁定机制提高了颈椎前路钢板螺钉的即刻固定强度和抗疲劳强度。
, 百拇医药
[中图分类号] R 687.32 [文献标识码] A
Comparison of fixation strength of the locking and non-locking anterior cervical plate systems
CHEN De-Yu SHEN Qiang SONG Dian-Wen
(Department of Orthopaedics, Changzheng Hospital, Second Military Medical University, Shanghai 200003， China)
[ABSTRACT] Objective: To compare the prompt fixation strength and antifatigue strength of the locking and non-locking anterior cervical plating systems. Methods:AO Cervical Spine Locking Plate (CSLP), Danek Orion plate and AcroMed Acroplate were used on the lamb cervical spines. The CSLP and Orion were tested with screws locked and unlocked, and the Acroplate with unicortical and bicortical purchase. The fixation strength and pull-off strength of the screw-plate constructs were performed initially and after fatigue. Results: Locked CSLP and Orion constructs were more rigid than all unlocked unicortical systems initially and after cyclic loading. After fatigue testing, the strength of all unlocked constructs decreased significantly. There was no significant difference in pull-off strength between the CSLP, the Orion and the unicortical Acroplate. Conclusion: The locking mechanism significantly increases the prompt fixation strength and antifatigue strength of the tested screw-plate systems.
, 百拇医药
[KEY WORDS] cervical vertebrae; plate; screw; fixation strength
[Article Code] 0258-879X(2000)07-0610-05
The anterior plating systems are extensively used to treat fracture and dislocation in the lower cervical spine[1-5].There are different types of system available, they can be divided into the locking and the non-locking anterior cervical plating systems. The examples of the former include the Synthes Cervical Spine Locking Plate (CSLP) and Danek Orion plate,and the latter are Casper plate and AcroMed Acroplate. In the past, most studies on biomechanics of anterior cervical internal fixation have been limited to the non-locking anterior cervical plating systems[6-9].Little experimental work has been performed to determine the effect of locking screws on the stability of anterior cervical plating.In this study，the currently available locking and non-locking anterior cervical plate systems were tested biomechanically. Comparisons of fixation strength and pull-out strength between the different systems were performed.The effects of locking screws on the stability of anterior cervical plating were determined.
, http://www.100md.com
1 MATERIALS AND METHODS
1.1 Specimen preparation Fourteen fresh lamb， aged between 6-8 months,their cervical spines with T1 were collected from a local abattoir.The specimens were frozen in double thickness plastic bags at -20°C.Just prior to biomechanical testing, the specimens were thawed at room temperature.Individual vertebrae(C3-T1) were dissected from the spines，and the surrounding soft tissues were cleaned.The sagittal vertebral diameter was measured by a caliper.A total of 80 vertebrae were used in this study and randomized to different instrumented test conditions.
, 百拇医药
1.2 Testing instrumentation and groups Each system was instrumented using the superior half of plate fixed with 2 screws to the vertebral body according to its technical principle.A torque screwdriver was used to contro1 the peak insertion torque. The CSLP and Orion were tested with screws locked and unlocked，and the Acroplate with unicortical and bicortical screw purchase.There were 8 vertebrae for each testing condition. The initial fixation strength was measured first and then followed by fatigue strength and pull-out strength. In addition, the CSLP and Orion with screws locked，and the Acroplate with unicortical and bicortical screw purchase were tested only for determining the initial pull-out strength.Each group had 8 vertebrae too.
, 百拇医药
1.3 Biomechanical testing Biomechanical testing for fixation strength and pull-out strength of the screw-plate constructs were performed on an Instron (Model l321，Instron Corporation，Cantor，MAS) with the vertebrae held in a custom test fixture.The plate was oriented vertically to the floor for cyclic loading and parallel to the floor for pull-out testing. A steel connecting block was fixed on each test plate.One end of a threaded stainless steel rod，as an adjustable moment arm，was attached to the steel block on the plate, and the other end was connected with a universal ball joint to the actuator of Instron.Cyclic loads (total 2.5 N/m bending moment for both flexion and extension，at a rate of l Hz) were generated under load control to simulate cervical flexion-extension.The fixation strength was determined in a11 1ocked and unlocked constructs, and the unicortical and bicortical Acroplate initially and after 105 cycles, followed by pull-out testing to failure of the screws within the plate.For the pull-out test of the screw-plate constructs，the ball joint was taken off, the sample was turned 90°, and the threaded rod was attached to the actuator directly in line with the long axis of the anchor screws. The pull-out strength after fatigue was determined at a rate of 10 cm/min until failure of the construct occurred. In addition, the initial pull-out strength was measured in the locked CSLP, the locked Orion, and the uni- and bicortical Acroplate constructs.Test data were treated by an IBM compatible computer.
, http://www.100md.com
1.4 Statistical analysis A t-test was performed to compare the fixation strength and pull-out strength of the screw-plate constructs for the different systems.The relationship between the pull-out strength and middle sagittal diameters of vertebral bodies were determined by correlation analysis.
2 RESULTS
2.1 The fixation strength of the different plating systems The fixation strength of each tested construct is shown in Figure l.The locked CSLP and Orion system were significantly more rigid than their corresponding unlocked constructs initially and after cyclic loading.The locked Orion construct showed significantly higher strength than the unicortical Acroplate construct，initially and after fatigue testing，and than the bicortical Acroplate construct after fatigue testing.The locked CSLP showed significantly higher strength than the unicortical Acroplate construct only after fatigue testing.The locked Orion constructs had higher strength than the locked CSLP before and after cyclic loading; however，this was not significant.After 105 cycles，the strength of all unlocked constructs decreased significantly.There was no significant change in fixation strength of the locked CSLP and Orion constructs after cyclic loading.
, http://www.100md.com
Fig 1 The fixation strength of different plate
systems before and after cycling loading
□： Before cycling； ■： After 10 000 cycles
A:CSLP(locked); B:CSLP(unlocked);
C:Acroplate (bicortical)； D:Acroplate (unicortical);
E:Orion (locked); F:Orion (unlocked)
2.2 Pull-out strength of the screws The pull-out strength of the screws in different constructs is shown in Figure 2.The bicortical Acroplate had a significant higher pull-out strength than all unicortical screws-plating constructs initially and after cyclic loading.There was no significant difference in pull-out strength between the CSLP，the Orion，and the unicortical Acroplate. After 105 cycles, pull-out strength of the screws in all tested constructs decreased, insignificantly though.
, http://www.100md.com
Fig 2 Pull-out strength of the screws in different plating
constructs initial and after fatigue testing
□: Initial; ■: Cycled; □: Unlocked and cycled
A: CSLP；B: Acroplate (unicortical);
C: Acroplate (bicortical); D: Orion
2.3 Relationship between sagittal diameter of the vertebral body and pull-out strength The middle sagittal diameter of individual vertebral bodies was varied from 15.5 mm to 19.4 mm with a mean of 17.3 mm. There was no significant difference in diameter between the different tested construct groups. However, a negative correlation was observed between the initial pull-out strength of the screws and the sagittal diameter of the vertebral body in all unicortical screws-plate constructs (Figure 3).
, http://www.100md.com
Fig 3 Relationship between initial pull-out strength
and the middle diameter of verlebral body
r=-0.65,n=24
3 DISCUSSION
There are anatomic differences between the lamb and human cervical spines.The lamb vertebrae are taller in body height and somewhat smaller in transverse and sagittal diameters than the human vertebrae.The middle sagittal diameter of the lamb individual vertebral bodies(C3-T1)vary from 15.5 mm to 19.4 mm with a mean of 17.3 mm and are big enough to be instrumented with typical human unicortical screw-plating (14 mm).In addition，the lamb vertebrae have the advantage of easy availability and greater specimen to specimen continuity.
, 百拇医药
Previous studies on the biomechanics of anterior cervical internal fixation have focused on comparison of non-locking plate techniques.Grubb et al [6] examined the stability potential of the Casper plate secured with unicortical or bicortical screws using a porcine cervical spine. A C5 corpectomy, strut grafting and plating were performed. The immediate stability，failure moment，and mechanism of failure were evaluated. No difference were found in the immediate stabilities of unicortical versus bicortical screw placement.Failure of the unicortical Casper plate group resulted from a combination of screw loosening and pull-out greater than 3.5 mm，whereas failure for the bicortical Casper group was primarily the result of plate deformation，and screw loosening in few cases.Ryken et al[7] evaluated the immediate and post fatigue stabilities of Casper plates secured with unicortical and bicortical screws on fresh human cervical motion segments.The results indicated that there was no difference in the pre-fatigue load displacement characteristics of plated spines with segmental disruption when the unicortical and bicortical groups were compared.However，a significant difference was observed between these groups with 5 000 cycles of fatigue loading.Half of the unicortical group became so unstable that further testing was precluded.Gallagher et al[8] compared unicortical to bicortical Casper screw placement in a single human cadaveric vertebra undergoing 200 cycles of simulated flexion-extension.A marked improvement on stability was observed with bicortical purchase.In their study，however，only a single fixation screw was used to absorb all the load，which did not simulate the case in clinical situation where at least 2 screws are used at each vertebral level.In our study，two locking screw-plate systems and one non-locking system were evaluated to examine the effects of locking screws on the rigidities of anterior plating.A basic unit of anterior cervical internal fixation, which includes a plate and 2 screws secured on single vertebral body, was tested.A bending moment was used to simulate the load on the construct during physiological cervical flexion-extension.Our results indicate that both bicortical and unicortical Acroplate constructs had lower strength than the locked CSLP and Orion constructs.After 105 cycles loading，the unlocked CSLP and Orion，unicortical and bicortical Acroplate became significantly less rigid; the rigidities of the locked CSLP and Orion showed no significant change.Biomechanically，the stability of the non-locking plate system requires pressure of the screw head to ho1d the plate against the bone.If the plate is not flat against the bone，or if there is even minimal looseness of the screw，the stability of the construct will be lost.In the locking plate system, once the locking screw has been inserted into the anchor screw, the screw and plate function as a single unit. This is especially advantageous in the cervical spine. The anterior surface of the vertebral bodies have an undulating contour precluding direct bony contact through the entire length of the construct.Loosing at the screw-plate junction may occur. The CSLP and Orion allowed for securing screw in the plate holes by means of the locking screw.The CSLP uses a locking hub screw which expands the anchor screw head against the plate.The Orion plate uses a large flat-head screw along the centerline of the plate that tightens down over a portion of 2 anchor screws at the same level.However, there was no significant difference in the fixation strength between the locked Orion and CSLP.
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The screw migration has been a main complication of the unlocked screw-plate systems.Published descriptions of the surgical technique for bicortical screws have emphasized the importance of penetration through the posterior vertebral cortex for appropriate stability[2].Maiman et al[9] examined the pull-out behavior of Caspar screws in iso1ated human cadaveric cervical vertebral bodies，to examine the importance of posterior cortical penetration.There were no significant differences between the 2 groups with the posterior penetration and without posterior penetration.The screws of the CSLP and Orion systems are 14 mm in length. The sagittal diameter of the human cervical vertebral body is 17-21 mm, so there is a 3 mm safety factor at least. Without the need for posterior vertebral body cortical purchase, the risk of screw misplacement and spinal cord injury was decreased. In this study, when the tested screws had the same length, a negative correlation between the pull-out strength of the screw and the sagittal diameter of the vertebral body was observed. This is different from the results obtained by Maiman and co-workers. The unicortical screw (14 mm) we tested was shorter than those used by Maiman which were subcortical and very close to the length of bicortical screws.In addition, the cadaveric vertebrae tested by Maiman were relative old and therefore had poorer bony quality. Except for the anatomic differences，the length of the screw is another key fact in determining the pull-off strength under given diameter. Therefor, use of longer unicortical screws may increase fixation strength in patients with larger cervical vertebrae.
, 百拇医药
Biography:CHEN De-Yu,MD,associate professor,tutor of master.
Foundation：This project is supported by Shanghai Leading Subject of Key Medical Science Foundation.NO.1995-Ⅳ-008,1998-Ⅳ-008.
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, 百拇医药
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Received 2000-04-20
Accepted 2000-07-10, http://www.100md.com