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A biomechanical investigation of different screw head designs for vertebral derotation in scoliosis surgery
Accepted Manuscript Title: A biomechanical investigation of different screw head designs for vertebral derotation in scoliosis surgery Author: Po-Yi Liu, Po-Liang Lai, Chun-Li Lin PII: DOI: Reference:
S1529-9430(17)30144-4 http://dx.doi.org/doi: 10.1016/j.spinee.2017.04.010 SPINEE 57287
To appear in:
The Spine Journal
Received date: Revised date: Accepted date:
29-11-2016 15-3-2017 10-4-2017
Please cite this article as: Po-Yi Liu, Po-Liang Lai, Chun-Li Lin, A biomechanical investigation of different screw head designs for vertebral derotation in scoliosis surgery, The Spine Journal (2017), http://dx.doi.org/doi: 10.1016/j.spinee.2017.04.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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A Biomechanical Investigation of Different Screw Head Designs for Vertebral Derotation in
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Scoliosis Surgery
3
Authors
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Po-Yi Liu1,2, , Po-Liang Lai2, *, Chun-Li Lin1,*
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Affiliation
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1
Department of Biomedical Engineering, National Yang-Ming University, Taipei, Taiwan
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2
Bone and Joint Research Center, Department of Orthopedic Surgery, Chang Gung Memorial Hospital at
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Linkou, College of Medicine, Chang Gung University, Taoyuan, Taiwan
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PYL [email protected]
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PLL [email protected] CLL [email protected]
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* CLL is corresponding author * PLL is co-corresponding author
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* Corresponding author
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Chun-Li Lin
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Department of Biomedical Engineering, National Yang-Ming University, Taipei, Taiwan, No. 155, Sec. 2,
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Linong Street, Taipei, Taiwan; Phone: 886-2-2826-7000; Fax: 886-2-2821-0847; Email: [email protected]
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* Co-corresponding author
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Po-Liang Lai
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Department of Orthopedic Surgery, Chang Gung Memorial Hospital at Linkou, College of Medicine, Chang
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Gung University, Taoyuan, Taiwan, No. 5 Fushing St. Kweishan, Taoyuan, Taiwan; Phone: 886-3-328-1200;
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Email: [email protected]
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Page 1 of 23
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Abstract
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Background Context: The posterior pedicle screw-rod system, which is widely used to correct spinal
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deformities, achieves a good correction rate in the frontal and coronal planes but not in the axial plane.
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Direct vertebral derotation (DVD) was developed to correct axial plane deformities. However, the design of
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screw head and body connection, in terms of monoaxial, polyaxial, and uniplanar screw, may influence the
7
efficiency of DVD.
8
Purpose: This study compared the efficiency of a newly designed uniplanar screw to that of monoaxial and
9
polyaxial screws in the DVD maneuver.
10
Study design: A porcine spine model and monoaxial, polyaxial and uniplanar screws were used to examine
11
the biomechanics of the DVD maneuver.
12
Patient sample: N/A
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Outcome measures: N/A
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Methods: Six T7-T13 porcine thoracic spine segments were used as test specimens in this study. Pedicle
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screws were inserted in the left pedicles of the T9-T11 spinal segments and then connected with a rod. Three
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types of pedicle screws with different screw head designs (monoaxial, polyaxial and uniplanar) were
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employed in this study. The material testing system (MTS) machine generated a rotational moment through
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the derotational tube on the T10 (apical body) pedicle screw, which simulated the motion applied during the
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surgical vertebral derotational procedure. The pedicle strain and the kinematics of the vertebral body and
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derotational tube were recorded to evaluate the derotational efficiency of different pedicle screw head
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designs. 2
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Result: For the monoaxial, polyaxial and uniplanar screws, the variances of the derotation for the monoaxial,
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polyaxial and uniplanar screws were 2.22° ± 1.43°, 32.23° ± 2.26°, and 4.75° ± 1.60°, respectively; the
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derotation efficiency was 0.65, 0.51, and 0.12, respectively, when the torques of the spinal constructs
4
reached 3 Nm. The rotational variance of the polyaxial screw was statistically greater than that of the
5
monoaxial and uniplanar screws (p < 0.05). The maximum micro strains of the pedicles for the monoaxial,
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polyaxial and uniplanar screws were 1067.45 ± 550.35, 747.68 ± 393.56, and 663.55 ± 271.04, respectively,
7
with no statistically significant differences (p > 0.05).
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Conclusions: The screw head design played an important role in the efficiency and variance of the
9
derotation during the DVD maneuver. The derotational efficiency of the newly designed uniplanar screw was
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closer to that of the monoaxial screw group than to that of the polyaxial screw group. The polyaxial screw
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was inferior for DVD due to a derotational variance between the derotational tube and the apical body that
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was correlated with the range of motion of the screw head. In the present study, the pedicle strain was similar
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in all groups. However, the pedicle strain of the uniplanar screw group was lower than that of the monoaxial
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screw group and was similar to that of the polyaxial screw group when the angle of rotation of the apical
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body increased.
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Keywords: pedicle screw, pedicle strain, scoliosis, uniplanar screw, vertebral derotation, vertebral
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translation.
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Introduction:
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Adolescent idiopathic scoliosis (AIS) is a complex three-dimensional deformity of the spine involving
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deviations in the coronal and sagittal planes and rotations in the axial plane [1–4]. Surgical intervention is
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indicated for scoliosis with a Cobb angle larger than 40–45°. Pedicle screw-based posterior spinal fusion
5
surgery has become the gold standard for the treatment of progressive scoliosis because the pedicle screws
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can transmit force through the pedicle to the vertebral bodies to directly obtain a better correction [3,5].
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Intraoperatively, direct vertebral derotation (DVD) and rod derotation are two major maneuvers used to
8
correct spinal deformities. While deformities in the coronal and sagittal plane are corrected by rod derotation,
9
deformity in the axial plane is corrected by the DVD maneuver [6]. This technique, which was developed to
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improve the correction rate of rotational scoliotic deformities in the axial plane in spinal surgery, involves
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the attachment of a derotational device to pedicle screws. In clinical practice, the apical vertebral rotation
12
(AVR) correction is an index to access the efficiency of DVD [7]. However, the correction of AVR in the
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axial plane is unexpected.
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Varied pedicle screw instrumentation systems are used for the treatment of severe spinal deformities in
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clinical application [1]. An increasing number of studies have indicated that the postoperative AVR is
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significantly different when different screw head designs are used [5,8,9]. The pedicle screws can be
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classified as monoaxial and polyaxial constructs according to the screw head design. Generally, polyaxial
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pedicle screws are designed to provide more degrees of freedom (DOF) on the screw-to-rod connecting
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interface to facilitate easier rod seating [5,10]. Therefore, polyaxial screws are commonly used in clinical
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applications. However, this design may lead to inadequate apical vertebral reduction when performing the
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DVD maneuver. Clinical results show that AVR does not significantly decrease with the use of polyaxial 4
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screws. In contrast, monoaxial screws are immobile at the screw head and are thus superior for performing
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the DVD maneuver to reduce AVR. However, monoaxial screws have a potential disadvantage regarding the
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difficulty of seating the rod into the screw head. Moreover, high stress occurs on the bone-screw interface,
4
which can result in vertebral fracture. Recently, uniplanar screws, which allow for some movement in the
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sagittal plane and axial rotation by itself, have been designed for spinal deformation surgery. This design is
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expected to promote DVD efficiency and decrease vertebral stress.
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The aim of this study was to design an innovative uniplanar pedicle screw to increase the correction rate
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and decrease the amount of stress on the bone-screw interface. The efficiency of this uniplanar screw to
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derotate the vertebrae under an applied torsional moment and the effects of the lateral pedicle strain in
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relation to the torque applied to the spinal specimen were investigated in a porcine spine model. This study
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hypothesized that uniplanar and monoaxial screw-to-rod constructs are more efficient than polyaxial screws
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at achieving derotation and that these constructs subject the spinal pedicle to lower amounts of stress.
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Materials and Methods
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Pedicle Screw Design and Manufacture
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Two traditional monoxial and polyaxial pedicle screws (Stryker Spine, Allendale, NJ) and an innovative
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uniplanar pedicle screw, each measuring 4.5 mm in diameter and 35 mm in length, were used in this study.
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The kinematics of the monoaxial, polyaxial, and uniplanar screw constructs are shown in Figure 1a. The
6
monoaxial screw head was immobile relative to the screw body, thereby preventing any range of motion
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(ROM). The polyaxial screws were designed with an adjustable screw head to allow for several degrees of
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freedom at the screw-rod interface and to allow a maximum 50° ROM at all orientations. The innovative
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uniplanar screw was designed with a ball and socket joint connecting the screw head to the screw body. The
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screw head had a straight, longitudinal slot that allowed the screw body to move and a 60° ROM in a plane
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parallel to the screw cup (Figure 1b). The innovative uniplanar pedicle screws were made of Ti6Al4V by a
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manufacturer with good manufacturing practices and ISO 13485 quality management systems (Huang-Liang
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Co., Ltd., Kaohsiung, Taiwan).
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Specimen Preparation and Biomechanical testing
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Previous studies have indicated that porcine spines are comparable in size to those of human
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adolescents. In this study, the biomechanical tests were conducted on six porcine spines harvested from
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skeletally immature pigs (3–4 months of age, approximately 100 kg) from a slaughterhouse. The test
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specimens were frozen and stored at -20°C until testing. Each test specimen consisted of the T7-T13 thoracic
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vertebrae. After the spines were harvested, the ribs, muscles, and adipose tissues were removed from each
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specimen; the ligamentous structures and intact joint capsules were retained. For fixation, the superior half
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of the T7 (cephalad side) and the inferior half of the T13 (caudal side) vertebral bodies were embedded on an 6
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aluminum disc with epoxy resin (Truetime Industrial Co., Tainan, Taiwan) to provide a stable base
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throughout the test procedure and fixed on the proximal and distal fixtures (Figure 2).
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Each test specimen was thawed at 4°C for 6 hours and then at room temperature for 2 hours; the test
4
specimens were kept moist throughout this period. Each vertebral body (T8- T12) was installed with 3
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makers. The bottom two marker points (proximal and distal marker points) were collinear with the vertebral
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body center. The distance between the vertebral body center and the nearest marker was 30 mm. The line
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that connected the two markers was perpendicular to the long axis of the specimen and parallel to the sagittal
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plane. A total of fifteen marker points were placed on each specimen to track the body trajectory. Three
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pedicle screws of the same design were implanted into the left pedicles of T9 to T11. The three pedicle
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screws were connected with a 6-mm titanium rod to finish the assembly. Screw position was confirmed by
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both anterioposterior (AP) and lateral X-ray photographs. The testing protocol was performed on a materials
12
test system (MTS) (HT-2402EC, Hung Ta Co., Ltd., Taipei, Taiwan). The specimen torque was detected with
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a torque sensor attached to the bottom of the distal fixture. The test specimen was placed and fixed on the
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MTS workspace. The loading device was aligned with the long axis of the apical vertebra (T10) pedicle
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screw. The head of the screw inserted into the apical vertebra (T10) was connected to a derotational tube,
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which was secured by a holding stick. A hinge joint that allowed a 90° rotation was set on the middle
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segment of the holding stick. The other side of the holding stick was fixed on a slider and attached to the
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load cell of the MTS. The test setup is shown in Figure 2.
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In the present study, the T10 vertebral body is considered the apical body, the T9 and T11 vertebral
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bodies are considered the fixed end bodies, and the free mobile spinal segments (T8 and T12) are maintained
21
on either side of the instrumented vertebrae. The two free mobile segments are considered the adjacent 7
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bodies.
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The test performed in this study aimed to reproduce the motion applied to an apical body during the
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DVD maneuver. In this test, the derotational tube transferred an independent load to the pedicle screw of the
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apical vertebra (T10) in the axial plane to generate a rotational force on the body. The hinge joint transferred
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the upper movement of the MTS to a rotational moment. The test was commenced by the displacement of
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the holding stick upward at a rate of 10 mm/min. The spinal torque was monitored, and the sampling rate
7
was set to 10 Hz.
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Two mini-DV video cameras (HDR-XR260V, Sony, Japan; HDR-SR12, Sony, Japan) were placed in
9
front of the MTS workspace. Two-dimensional image coordinates were transformed into 3D object-space
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coordinates using the 3D direct linear transformation algorithm to calculate the T8-T12 vertebral rotation
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[11]. A goniometer (Pasco PS-2138, US) was placed on the hinge joint of the holding stick to measure the
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rotation of the derotational tube at a sampling rate of 10 Hz. The kinematic data, including the vertebral
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rotation, translation, and derotational tube rotation, were collected for analysis.
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The surface of the lateral wall of the left pedicle of the T10 vertebra was polished and glued to a 2-mm
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strain gauge (KFG02-120-C1-16, Kyowa, Electronic Instruments Co., Ltd., Japan) to measure the strain of
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the apical pedicle. The signals were calibrated and amplified by eStrain P4-X signal conditioners at a
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sampling rate of 10 Hz. A data translational board (National Instruments, Austin, TX, USA) and FleXense
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(Chief Si Co., Ltd., Taiwan) software were used to record the strain signals. All data were collected until the
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torsional moment reached 3 Nm. Each specimen was subjected to each of the three tests (i.e., monoaxial,
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polyaxial and uniplanar screws) randomly for a total of 18 tests. The minimum time interval between two
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tests is thirty minutes. The efficiency of derotation was defined as the ratio of the tube rotation to the apical 8
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body rotation. The difference in the rotation of the derotational tube and the apical body represented the
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variance of the derotation.
3 4
Statistical Analysis
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The curve of the derotational tube rotation versus the torque and the apical body rotation versus
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derotational tube rotation on the specimen were analyzed. The derotation efficiency, derotation variance,
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apical vertebra translation, and maximum pedicle strain in each group were determined and then averaged.
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The correlational curve of the correlational curve of the pedicle strain versus the apical body rotation was
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analyzed for each screw type using linear regression models, which were fitted to the linear portion of the
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curve used to compute the mean compliance of each screw design. A one-way analysis of variance (ANOVA)
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and post hoc Tukey’s tests were used to detect a significant difference in the derotation efficiency, derotation
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variance, apical body translation, and maximum pedicle strain in each of the three groups. Similarly,
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ANOVA and post hoc Scheffe’s tests were used to detect a significant difference in the rotation of vertebral
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bodies (apical body, fixed end body, and adjacent body).
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Results
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Kinematic and Dynamic Data
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The rotations of the derotational tubes versus the specimen torques of the monoaxial, polyaxial, and
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uniplanar screw groups are shown in Figure 3. Under incremental rotation, the monoaxial screw group
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showed an approximately linear increase in torque with a rotated derotational tube. The polyaxial screw and
6
uniplanar screw groups showed 28.38° ± 2.50° and 1.2° ± 0.8° delays in the initial stage, respectively. Then,
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similarly to the monoaxial screw group, a trend of approximately linearly increasing torque was observed in
8
the polyaxial screw and uniplanar screw groups.
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The rotations of the derotational tubes versus the rotations of apical bodies of the monoaxial, polyaxial,
10
and uniplanar screw groups are shown in Figure 4. The trend of curve was similar with the Figure 3. The
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polyaxial screw and uniplanar screw groups showed delays in the initial stage. The ratio of the tube rotation
12
to the apical body rotation represent the derotation efficiency. For the monoaxial, polyaxial and uniplanar
13
screw groups, the derotation efficiencies were 0.65, 0.51 and 0.12, respectively.
14
The difference in the rotation of the derotational tube and apical body represented the derotation
15
variance. The derotation variances of the monoaxial, polyaxial and uniplanar screw groups were 2.22° ±
16
1.43°, 32.23° ± 2.26°, and 4.75° ± 1.60°, respectively. The rotational variance of the polyaxial screw was
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statistically larger than that of the monoaxial and uniplanar screws (p < 0.05). The translations of the T10
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apical bodies of the monoaxial, polyaxial and uniplanar screw groups were 3.80 mm ± 1.36 mm, 4.61 mm ±
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1.55 mm and 4.04 mm ± 0.83 mm, respectively, when the torques of the spinal constructs reached 3 Nm.
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Rotation of Vertebral Bodies
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The rotations were calculated based on the marker locations that were tracked using the motion capture 10
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system. Figure 5 summarizes the rotations during the DVD maneuver for the apical, fixed end and adjacent
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vertebral bodies. The rotations of the fixed end bodies were slightly lower than those of the apical body in all
3
groups; no group showed statistical significance (p > 0.05). The rotations of adjacent levels were lower than
4
that of the apical body in all groups, with statistical significance in every group (p < 0.05).
5
Pedicle Strain
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To assess the correlation between vertebral body rotation and the pedicle strain of each screw head
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design group, a linear regression model was applied to fit the data points (Figure 6). The monoaxial,
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polyaxial and uniplanar screw groups exhibited high R-square values of R2 = 0.99, 0.95 and 0.97,
9
respectively. The trend indicated that the pedicle strain of the monoaxial screw group would be higher than
10
that of the other groups when the vertebral body rotation increased. The maximum strains for the monoaxial,
11
polyaxial and uniplanar screw groups were 1067.45 ± 550.35, 747.68 ± 393.56, and 663.55 ± 271.04, with
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no statistically significant differences in any group (p > 0.05).
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Discussion:
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Pedicle screw-based posterior spinal fusion with a rod derotation maneuver has enabled powerful
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coronal and sagittal plane correction in scoliosis surgery. However, the ability to achieve correction in the
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axial plane using this method remains elusive. Lee et al. [3] developed the DVD maneuver, a surgical
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technique that gained popularity due to its better control of the spine in the axial plane and the resulting
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improved corrections of thoracic and lumbar curves. The surgical technique of the DVD maneuver is
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conducted as follows: 1) the pedicle screws are inserted posteriorly from the pedicle and traverse to the
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anterior vertebral body; 2) a pre-contoured rod is inserted into the segmental screws; and 3) correction of the
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vertebral rotation is achieved by applying a force in the opposite direction to that of the deformity. This
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approach enables the transmission of the rotational force to the entire vertebral body, thus allowing the
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correction of rotational deformities in the axial plane. In addition, this technique provides better control of
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the thorax in some cases, eliminating the need for a thoracoplasty to manage the rib hump deformity
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[6,12–14]. However, the clinical outcomes of using the DVD maneuver to treat the vertebral body deformity
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in the axial plane have been disputed. Some studies have indicated no significant improvements even with
15
the additional use of segmental pedicle screw instrumentation in DVD aimed at correcting AIS [12,15,16].
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Kuklo et al. [5] compared the abilities of monoaxial and polyaxial pedicle screws to effectively achieve
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curve correction in a matched cohort of 35 patients with AIS. Monoaxial screws provided superior derotation
18
and restoration of thoracic symmetry compared with polyaxial screws, showing that the choice of screws is
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an important factor in the DVD maneuver. However, few studies have investigated the optimal design of
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screws for use in DVD. In the present study, three pedicle screws with different screw head designs were
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used to compare the derotational efficiency during DVD. The use of monoaxial or uniplanar screws resulted 12
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in a rotation of the apical vertebra that was proportional to the tube rotation of the derotational tube.
2
Therefore, the use of monoaxial or uniplanar screws achieved better efficiency than that of polyaxial screws
3
during the DVD maneuver. On the contrary, in the polyaxial screw group, the surgical segment of the spine
4
rotated only 4.44° ± 1.00° when the derotational tube rotated by 36.67° ± 2.46° under 3 Nm. Even though
5
the monoaxial screw group was more efficient. However, some biomechanical problems can arise after
6
spinal surgery when a monoaxial screw is used to treat a spinal deformity.
7
Monoaxial screws have been shown to be advantageous in correcting spinal deformities in all planes, as
8
these screws greatly improve the stability of the instrumented spine and allow for the application of higher
9
corrective forces to translate and derotate the deformed vertebrae [9,17]. The fixed screw head design of the
10
monoaxial screw offers a mechanical advantage over the pedicle screws with a multi-angle head design in
11
terms of rotational or translated forces. These screws more directly transmitted the force into the vertebral
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body to gain better derotational efficiency. In the present study, the monoaxial group showed a higher
13
derotation efficiency (0.65) and a lower derotation variance (2.22° ± 1.43°) than the other groups (polyaxial,
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0.51 and 32.23° ± 2.26°; uniplanar, 0.12 and 4.75° ± 1.60°). These results were consistent with the opinions
15
that monoaxial screws improve the effects of derotation significantly more than the other designs [1,5,6,9].
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The rotational variance of the monoaxial screw was significant difference with uniplanar screws (p = 0.029).
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The phenomenon maybe explained from the design of uniplanar screws. In order to allow the spherical head
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of screw body to set into the socket of screw cup, the axial diameter of screw head slot was slight larger than
19
the screw diameter of screw body. Therefore, the uniplanar pedicle screw allowed a slight ROM and had a
20
slightly higher rotational variance than the monoaxial pedicle screw. Although there were significant
21
differences among monoaxial, uniplanar and polyaxial screws, the rotational values of monoaxial and 13
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uniplanar screws were much small than polyaxial screws. However, as the screw head of a monoaxial screw
2
is immobile relative to the screw body, the rod can be difficult to seat into the screw head, as the long axis of
3
the screw must be perpendicular to the rod to reach the ideal fixed situation. Typically, each pedicle screw
4
cannot be placed in an identical position to that of the adjacent screws, and the minor screw misalignment is
5
transformed into vertebral misalignment and adverse bone-screw stresses at the time of the rod-screw
6
construct assembly[17,18]. These adverse stresses have no benefit to the deformity reduction but inevitably
7
increase the probability of bone-screw interface damage. This mode of failure has been seen clinically in
8
anterior and posterior methods of deformity corrections and has been demonstrated biomechanically [19,20].
9
Additionally, in clinical practice, monoaxial screws may pose challenges during the rod-screw engagement
10
sequence and failure to match angle preservation between screw-rod interface if they are not very closely
11
aligned in a contiguous arc [21]. Thus, polyaxial screws have often been used in clinical practice to
12
surgically correct scoliosis. Polyaxial screw heads were designed to facilitate the capture of the rod in the
13
screw head when the spine deformity is particularly extreme. However, this design can result in the failure of
14
the apical body to rotate in the DVD maneuver. Furthermore, the correction rate in the axial plane is less
15
than that achieved using the monoaxial screw [1,5,18]. This phenomenon was observed in the present study,
16
in which the polyaxial screws allowed 25° ROM at the joint of the screw head and body. The rotation of the
17
apical body was not measurable until the rotation of the derotational tube achieved 28.38° ± 2.50° (nearly
18
25°). When the inner wall of the screw head touched the screw body, the torque was conducted from the
19
derotational tube to the screw body. The present study revealed that the derotation variance was significantly
20
different among the three groups, which raised concerns that in real clinical situations, the surgeon may not
21
know the real rotation of the apical body during the DVD maneuver. Additionally, soft tissues such as skin 14
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and muscles and hard tissues such as bones around the surgical space may limit the activity of the
2
derotational tube. Since the head design of the polyaxial screw allowed more DOF, the screw head and body
3
could not consistently align after the rod was seated in the screw head. When the screw was implanted more
4
laterally, the screw head would turn medially to compensate for the direction and vice versa. The rotational
5
arc of the derotational tube would be larger than the ROM of polyaxial screws before the screw body
6
generated derotational torque on the apical body. In general, the rotational arc of the derotational tube would
7
be smaller than the ROM of a medially implanted polyaxial screw before the screw body generates a
8
derotational torque on the apical body. Compared with the monoaxial screw, the uniplanar screws, which
9
have a pivoting connection between the screw head and the threaded body, allow for variation in the
10
orientation of screw insertion in the sagittal plane and therefore prevent adverse bending moments in the
11
sagittal plane. Dalal et al. [1] compared the residual postoperative apical vertebral rotations of the uniplanar
12
and polyaxial bilateral pedicle screw constructs in thoracic AIS. Their results showed that the uniplanar
13
screw group had a better apical vertebral derotation (less residual apical vertebral rotation) than the polyaxial
14
screw group. In this present study, uniplanar screws obtained a better efficiency of derotation in the axial
15
plane that was closer to that of the monoaxial screw group than that of the polyaxial screw group; moreover,
16
the uniplanar screws allowed for easy fitting of the rod into the screw head and thereby prevented extra
17
stress on the screw-rod interface. Furthermore, compared with the monoaxial pedicle screw group, the
18
uniplanar screw group had lower pedicle strain during the DVD maneuver.
19
Martino et al. [22] indicated that during the DVD maneuver, the apical body center is translated
20
medially, thus improving the correction of both the main thoracic Cobb angle and the apical translation. In
21
the present study, this phenomenon was also observed during the DVD maneuver. The vertebral body was 15
Page 15 of 23
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not only rotated but also translated medially (Figure 7). In the present study, though the translation of apical
2
body were similar in three groups, when the torques of the spinal constructs reached 3 Nm. However, the
3
translation was difficult to controlled in polyaxial screw group, because the derotational efficiency was lower
4
than other groups. In orienting the DVD maneuver, Parent et al. [23] investigated the failure torque in axial
5
plane DVD maneuvers in both the medial and lateral directions. Their results showed no significant
6
difference in the failure torque between the medial and lateral directions. However, medial screw failure
7
jeopardizes the spinal cord, whereas lateral failure may compromise proximal pulmonary, vascular, or other
8
visceral structures during DVD. This technique is based mostly on the surgeon’s tactile abilities, and the
9
magnitude of force necessary to achieve a desired correction in clinical applications is unknown. Therefore,
10
in this present study, DVD was performed by pulling the derotational tube toward laterally which generating
11
a force pushing the screw toward medially, pivoting around the rod.
12
Previous studies have evaluated the maximum torque and applied power required to achieve complete
13
correction of spinal deformities and vertebral failure [23,24]. Notably, no study has investigated the pedicle
14
strain during the DVD maneuver. In the present study, the strain of the lateral pedicle wall was evaluated. In
15
the linear regression model of the pedicle strain versus the apical body rotation, the monoaxial pedicle screw
16
group had a greater slope than the other groups. The maximum strain was not significantly different when
17
the vertebral bodies underwent 3 Nm torque. However, the difference would be increasingly significant
18
when the torque was increased. These results were consistent with opinions that a higher structural stiffness
19
of the monoaxial pedicle screw would lead to higher stress on the vertebral pedicle, which would increase
20
the probability of bone-screw interface damage. The present study, as with many biomechanical studies, has
21
several limitations. In the absence of a surrogate scoliosis model, we have chosen to use a porcine spine 16
Page 16 of 23
1
model; although this model is similar to a normal human spine in terms of anatomy and vertebral kinematics,
2
it cannot simulate the rotational deformation and altered geometry and mechanics of the vertebrae and
3
intervertebral discs that are typical of a scoliotic spine. The aim of this study was to evaluate the efficiency
4
of different design pedicle screw in direct vertebral derotation. The porcine spines in this study were not
5
scoliotic, because scoliotic porcine spines were not easy to obtain. Also, the testing protocol followed
6
previous studies that all used non-scoliotic cadavers or animal spines to investigate the situation of vertebral
7
body derotation [23,25]. The results illustrated the differences of kinematics of vertebral derotation, as well
8
as the derotational variance, rotation and medial shift of vertebral body among the monoaxial, uniplnar and
9
polyaxial screws. Even though the biomechanical concept and trend between the porcine spine and the
10
human spine are similar, the current study did not necessarily represent the real force or torque in clinical
11
situation. In this study, similar porcine vertebrae were scanned by DEXA (Horizon DXA system,
12
Marlborough, USA) to measure the AP and lateral BMD (bone mineral density). The data ranged between
13
0.8 and 1.1 g/cm2. Boot et al. [26] presented that the BMD of human adolescents (age in 12 - 18 years) was
14
between 0.8~1.2 g/cm2. Therefore, the BMD of test specimens in this study was similar to human
15
adolescents. Screw position was confirmed by both anterioposterior (AP) and lateral X-ray photographs.
16
Nonetheless, numerous studies have used porcine spines for similar biomechanical experiments [27].
17
Although applying the torque to the apical vertebral body is sufficient to perform the vertebral derotation
18
maneuver in a flexible spine, a greater number of derotation levels may be necessary to improve deformity
19
corrections for patients with stiffer and larger spinal curves and vertebral rotations. In clinical, the real torque
20
value used in the process of vertebral body rotation is difficult to measure during the surgery because the
21
deformed level of each patient was difference and amounts of the torque values were not the same in each 17
Page 17 of 23
1
case. The aim of this study was to compare the efficiency of different design of pedicle screws and not at all
2
rotated to failure. The torque of 3Nm slight below failure critical value was set according to the study result
3
of Parent et al. in 2008 [23]. This study indicated that the strength of vertebral body derotated to failure test
4
by using the cadaver spine and showed that minimum torque was close 4 Nm.
18
Page 18 of 23
1
Conclusion:
2
In conclusion, this study investigated the efficiency of three different screw head designs in the direct
3
vertebral derotation maneuver performed on multisegmental porcine spines. Based on the results of this
4
study, the screw head design plays important roles in both derotation efficiency and variance during the
5
direct vertebral derotation maneuver. The derotational efficiency of the newly designed uniplanar screw was
6
closer to that of the monoaxial screw than that of the polyaxial screw. The polyaxial screw was inferior for
7
direct vertebral derotation because there was a derotational variance between the derotational tube and the
8
apical body that was correlated with the range of motion of the screw head. In the present study, the pedicle
9
strain was similar in all groups. However, the pedicle strain of the uniplanar screw group was lower than that
10
of the monoaxial screw group and was similar to that of the polyaxial screw group when the angle of rotation
11
of the apical body increased.
12 13 14 15 16 17 18
Acknowledgements We thank Chih-Yi Chen for her help in preparing the porcine spine. This work was kindly supported by grant CRRPG3E0141 & CRRPG3E0142 from Chang Gung Memorial Hospital, Taiwan.
19 20
19
Page 19 of 23
1
References
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Dalal A, Upasani VV, Bastrom TP, et al. Apical vertebral rotation in adolescent idiopathic scoliosis: comparison of uniplanar and polyaxial pedicle screws. J Spinal Disord Tech 2011;24:251–7.
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Badve SA, Ordway NR, Albanese SA, et al. Toward a better understanding of direct vertebral rotation
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for AIS surgery: Development of a multisegmental biomechanical model and factors affecting
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correction. Spine J 2015;15:1034–40.
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Lee SM, Suk SI, Chung ER. Direct vertebral rotation: a new technique of three-dimensional deformity
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correction with segmental pedicle screw fixation in adolescent idiopathic scoliosis. Spine (Phila Pa
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idiopathic scoliosis. J Korean Neurosurg Soc 2015;58:534–8. [5]
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DiSilvestre M, Lolli F, Bakaloudis G, et al. Apical vertebral derotation in the posterior treatment of adolescent idiopathic scoliosis: Myth or reality? Eur Spine J 2013;22:313–23.
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Pankowski R, Roclawski M, Ceynowa M, et al. Direct Vertebral Rotation Versus Single Concave Rod Rotation. Spine (Phila Pa 1976) 2016;41:864–71.
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Kuklo TR, Potter BK, Polly DW, et al. Monaxial versus multiaxial thoracic pedicle screws in the correction of adolescent idiopathic scoliosis. Spine (Phila Pa 1976) 2005;30:2113–20.
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Kim JY, Song K, Kim KH, et al. Usefulness of simple rod rotation to correct curve of adolescent
Wang X, Aubin CE, Crandall D, et al. Biomechanical Analysis of 4 Types of Pedicle Screws for Scoliotic Spine Instrumentation. Spine (Phila Pa 1976) 2012;37:E823–35.
[9]
Lonner BS, Auerbach JD, Boachie-Adjei O, et al. Treatment of thoracic scoliosis: are monoaxial thoracic pedicle screws the best form of fixation for correction? Spine (Phila Pa 1976) 20
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2009;34:845–51. [10]
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rigidity. J Spinal Disord Tech 2002;15:233–6. [11]
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Hwang SW, Dubaz OM, Ames R, et al. The impact of direct vertebral body derotation on the lumbar prominence in Lenke Type 5C curves. J Neurosurg Spine 2012;17:308–13.
[13]
Hwang SW, Samdani AF, Lonner B, et al. Impact of Direct Vertebral Body Derotation on Rib Prominence Are Preoperative Factors Predictive of Changes in Rib Prominence? Spine (Phila Pa 1976)
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Elipot M, Hellard P, Taïar R, et al. Analysis of swimmers’ velocity during the underwater gliding motion following grab start. J Biomech 2009;42:1367–70.
7 8
Shepard MF, Davies MR, Abayan A, et al. Effects of polyaxial pedicle screws on lumbar construct
2012;37:86–9. [14]
Rushton PR, Grevitt MP. Do vertebral derotation techniques offer better outcomes compared to
12
traditional methods in the surgical treatment of adolescent idiopathic scoliosis? Eur Spine J
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2014;23:1166–76.
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[15]
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Chang DG, Kim JH, Kim SS, et al. How to Improve Shoulder Balance in the Surgical Correction of Double Thoracic Adolescent Idiopathic Scoliosis. Spine (Phila Pa 1976) 2014;39:E1359–67.
[16]
Tang X, Zhao J, Zhang Y. Radiographic, clinical, and patients’ assessment of segmental direct
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vertebral body derotation versus simple rod derotation in main thoracic adolescent idiopathic scoliosis:
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a prospective, comparative cohort study. Eur Spine J 2014;24:298–305.
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[17]
Wang X, Aubin CE, Crandall D, et al. Biomechanical comparison of force levels in spinal
20
instrumentation using monoaxial versus multi degree of freedom postloading pedicle screws. 2011;36;
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E95–104 21
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screws for the treatment of adolescent idiopathic scoliosis. Eur Spine J 2012;21:1082–90. [19]
4 5
Wang X, Aubin CE, Robitaille I, et al. Biomechanical comparison of alternative densities of pedicle
Mahar AT, Brown DS, Oka RS, et al. Biomechanics of cantilever “plow” during anterior thoracic scoliosis correction. Spine J 2006;6:572–6.
[20]
Mohamad F, Oka R, Mahar A, et al. Biomechanical comparison of the screw-bone interface:
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optimization of 1 and 2 screw constructs by varying screw diameter. Spine (Phila Pa 1976)
7
2006;31:E535-9.
8
[21]
set-screw loosening in long spinal constructs: a static and fatigue biomechanical investigation. J
9 10 11
Serhan H, Hammerberg K, O’Neil M, et al. Intraoperative techniques to reduce the potential of
Spinal Disord Tech 2010;23:E31-6. [22]
Martino J, Aubin CE, Labelle H, et al. Biomechanical Analysis of Vertebral Derotation Techniques
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for the Surgical Correction of Thoracic Scoliosis A Numerical Study Through Case Simulations and a
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Sensitivity Analysis. Spine (Phila Pa 1976) 2012;38:E73–83.
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[23]
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of direct transverse plane vertebral derotation. Spine (Phila Pa 1976) 2008;35:1966–9. [24]
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Cheng I, Hay D, Iezza A, et al. Biomechanical analysis of derotation of the thoracic spine using pedicle screws. Spine (Phila Pa 1976) 2010;35:1039–43.
[25]
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Stefan P, Tim O, Richard O et al. Does the direction of pedicle screw rotation affect the biomechanics
Lam FC, Groff MW, Alkalay RN. The effect of screw head design on rod derotation in the correction of thoracolumbar spinal deformity: laboratory investigation. J Neurosurg Spine 2013;19:351–9.
[26]
Boot AM, de Ridder MAJ, Pols AP, et al. Bone mineral density in children and adolescents: relation to puberty, calcium intake, and physical activity. J Clin Endocrinol Metab 1997;82:57–62. 22
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[27]
Wilke HJ, Geppert J, Kienle A. Biomechanical in vitro evaluation of the complete porcine spine in comparison with data of the human spine. Eur Spine J 2011;20:1859–68.
3
Figure 1. (a) Kinematics of monoaxial, polyaxial, and uniplanar pedicle screw constructs; (b) The diameter
4
of minor axis of the slot was the same as the ball of the screw body to limit screw movement on
5
transverse plane. The length of long axis of opening allow a 60-degree movement (cephalad 30
6
degrees, caudally 30 degrees) on sagittal plane.
7
Figure 2. (a) Test setup of simulated DVD maneuver. The test specimen consisted of T7-T13 of porcine
8
spine with the proximal (T7) and this distal (T13) vertebrae fixed. The T8 and T12 vertebrae were
9
free to rotate. The pedicle screws were inserted into left pedicle of T9-T11 and rod was seated into
10 11 12 13 14 15 16 17 18 19
all screw head; (b) Test specimen motion during test processing. Figure 3. The rotations of derotational tube versus the specimen torques of the monoaxial, polyaxial, and uniplanar screws during DVD testing. Figure 4. Rotation of apical vertebra ( T10 ) versus rotation of derotational tube of the monoaxial, polyaxial, and uniplanar pedicle screw during vertebral derotation testing. Figure 5. The rotation of apical vertebra ( T10 ), end instrumented vertebrae ( T9, T11 ) and adjacent vertebrae ( T8, T12 ) of porcine spine during vertebral derotation testing in axial plane. Figure 6. The strain of apical body versus rotation of apical body of the monoaxial, polyaxial, and uniplanar pedicle screw during vertebral derotational testing. Figure 7. The trajectory of apical vertebra during derotational maneuver processing.
20
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Page 23 of 23
S1529-9430(17)30144-4 http://dx.doi.org/doi: 10.1016/j.spinee.2017.04.010 SPINEE 57287
To appear in:
The Spine Journal
Received date: Revised date: Accepted date:
29-11-2016 15-3-2017 10-4-2017
Please cite this article as: Po-Yi Liu, Po-Liang Lai, Chun-Li Lin, A biomechanical investigation of different screw head designs for vertebral derotation in scoliosis surgery, The Spine Journal (2017), http://dx.doi.org/doi: 10.1016/j.spinee.2017.04.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
A Biomechanical Investigation of Different Screw Head Designs for Vertebral Derotation in
2
Scoliosis Surgery
3
Authors
4
Po-Yi Liu1,2, , Po-Liang Lai2, *, Chun-Li Lin1,*
5
Affiliation
6
1
Department of Biomedical Engineering, National Yang-Ming University, Taipei, Taiwan
7
2
Bone and Joint Research Center, Department of Orthopedic Surgery, Chang Gung Memorial Hospital at
8
Linkou, College of Medicine, Chang Gung University, Taoyuan, Taiwan
9 10
PYL [email protected]
11 12
PLL [email protected] CLL [email protected]
13 14 15
* CLL is corresponding author * PLL is co-corresponding author
16 17
* Corresponding author
18
Chun-Li Lin
19
Department of Biomedical Engineering, National Yang-Ming University, Taipei, Taiwan, No. 155, Sec. 2,
20
Linong Street, Taipei, Taiwan; Phone: 886-2-2826-7000; Fax: 886-2-2821-0847; Email: [email protected]
21
* Co-corresponding author
22
Po-Liang Lai
23
Department of Orthopedic Surgery, Chang Gung Memorial Hospital at Linkou, College of Medicine, Chang
24
Gung University, Taoyuan, Taiwan, No. 5 Fushing St. Kweishan, Taoyuan, Taiwan; Phone: 886-3-328-1200;
25
Email: [email protected]
1
Page 1 of 23
1 2
Abstract
3
Background Context: The posterior pedicle screw-rod system, which is widely used to correct spinal
4
deformities, achieves a good correction rate in the frontal and coronal planes but not in the axial plane.
5
Direct vertebral derotation (DVD) was developed to correct axial plane deformities. However, the design of
6
screw head and body connection, in terms of monoaxial, polyaxial, and uniplanar screw, may influence the
7
efficiency of DVD.
8
Purpose: This study compared the efficiency of a newly designed uniplanar screw to that of monoaxial and
9
polyaxial screws in the DVD maneuver.
10
Study design: A porcine spine model and monoaxial, polyaxial and uniplanar screws were used to examine
11
the biomechanics of the DVD maneuver.
12
Patient sample: N/A
13
Outcome measures: N/A
14
Methods: Six T7-T13 porcine thoracic spine segments were used as test specimens in this study. Pedicle
15
screws were inserted in the left pedicles of the T9-T11 spinal segments and then connected with a rod. Three
16
types of pedicle screws with different screw head designs (monoaxial, polyaxial and uniplanar) were
17
employed in this study. The material testing system (MTS) machine generated a rotational moment through
18
the derotational tube on the T10 (apical body) pedicle screw, which simulated the motion applied during the
19
surgical vertebral derotational procedure. The pedicle strain and the kinematics of the vertebral body and
20
derotational tube were recorded to evaluate the derotational efficiency of different pedicle screw head
21
designs. 2
Page 2 of 23
1
Result: For the monoaxial, polyaxial and uniplanar screws, the variances of the derotation for the monoaxial,
2
polyaxial and uniplanar screws were 2.22° ± 1.43°, 32.23° ± 2.26°, and 4.75° ± 1.60°, respectively; the
3
derotation efficiency was 0.65, 0.51, and 0.12, respectively, when the torques of the spinal constructs
4
reached 3 Nm. The rotational variance of the polyaxial screw was statistically greater than that of the
5
monoaxial and uniplanar screws (p < 0.05). The maximum micro strains of the pedicles for the monoaxial,
6
polyaxial and uniplanar screws were 1067.45 ± 550.35, 747.68 ± 393.56, and 663.55 ± 271.04, respectively,
7
with no statistically significant differences (p > 0.05).
8
Conclusions: The screw head design played an important role in the efficiency and variance of the
9
derotation during the DVD maneuver. The derotational efficiency of the newly designed uniplanar screw was
10
closer to that of the monoaxial screw group than to that of the polyaxial screw group. The polyaxial screw
11
was inferior for DVD due to a derotational variance between the derotational tube and the apical body that
12
was correlated with the range of motion of the screw head. In the present study, the pedicle strain was similar
13
in all groups. However, the pedicle strain of the uniplanar screw group was lower than that of the monoaxial
14
screw group and was similar to that of the polyaxial screw group when the angle of rotation of the apical
15
body increased.
16
Keywords: pedicle screw, pedicle strain, scoliosis, uniplanar screw, vertebral derotation, vertebral
17
translation.
18
3
Page 3 of 23
1
Introduction:
2
Adolescent idiopathic scoliosis (AIS) is a complex three-dimensional deformity of the spine involving
3
deviations in the coronal and sagittal planes and rotations in the axial plane [1–4]. Surgical intervention is
4
indicated for scoliosis with a Cobb angle larger than 40–45°. Pedicle screw-based posterior spinal fusion
5
surgery has become the gold standard for the treatment of progressive scoliosis because the pedicle screws
6
can transmit force through the pedicle to the vertebral bodies to directly obtain a better correction [3,5].
7
Intraoperatively, direct vertebral derotation (DVD) and rod derotation are two major maneuvers used to
8
correct spinal deformities. While deformities in the coronal and sagittal plane are corrected by rod derotation,
9
deformity in the axial plane is corrected by the DVD maneuver [6]. This technique, which was developed to
10
improve the correction rate of rotational scoliotic deformities in the axial plane in spinal surgery, involves
11
the attachment of a derotational device to pedicle screws. In clinical practice, the apical vertebral rotation
12
(AVR) correction is an index to access the efficiency of DVD [7]. However, the correction of AVR in the
13
axial plane is unexpected.
14
Varied pedicle screw instrumentation systems are used for the treatment of severe spinal deformities in
15
clinical application [1]. An increasing number of studies have indicated that the postoperative AVR is
16
significantly different when different screw head designs are used [5,8,9]. The pedicle screws can be
17
classified as monoaxial and polyaxial constructs according to the screw head design. Generally, polyaxial
18
pedicle screws are designed to provide more degrees of freedom (DOF) on the screw-to-rod connecting
19
interface to facilitate easier rod seating [5,10]. Therefore, polyaxial screws are commonly used in clinical
20
applications. However, this design may lead to inadequate apical vertebral reduction when performing the
21
DVD maneuver. Clinical results show that AVR does not significantly decrease with the use of polyaxial 4
Page 4 of 23
1
screws. In contrast, monoaxial screws are immobile at the screw head and are thus superior for performing
2
the DVD maneuver to reduce AVR. However, monoaxial screws have a potential disadvantage regarding the
3
difficulty of seating the rod into the screw head. Moreover, high stress occurs on the bone-screw interface,
4
which can result in vertebral fracture. Recently, uniplanar screws, which allow for some movement in the
5
sagittal plane and axial rotation by itself, have been designed for spinal deformation surgery. This design is
6
expected to promote DVD efficiency and decrease vertebral stress.
7
The aim of this study was to design an innovative uniplanar pedicle screw to increase the correction rate
8
and decrease the amount of stress on the bone-screw interface. The efficiency of this uniplanar screw to
9
derotate the vertebrae under an applied torsional moment and the effects of the lateral pedicle strain in
10
relation to the torque applied to the spinal specimen were investigated in a porcine spine model. This study
11
hypothesized that uniplanar and monoaxial screw-to-rod constructs are more efficient than polyaxial screws
12
at achieving derotation and that these constructs subject the spinal pedicle to lower amounts of stress.
13
5
Page 5 of 23
1
Materials and Methods
2
Pedicle Screw Design and Manufacture
3
Two traditional monoxial and polyaxial pedicle screws (Stryker Spine, Allendale, NJ) and an innovative
4
uniplanar pedicle screw, each measuring 4.5 mm in diameter and 35 mm in length, were used in this study.
5
The kinematics of the monoaxial, polyaxial, and uniplanar screw constructs are shown in Figure 1a. The
6
monoaxial screw head was immobile relative to the screw body, thereby preventing any range of motion
7
(ROM). The polyaxial screws were designed with an adjustable screw head to allow for several degrees of
8
freedom at the screw-rod interface and to allow a maximum 50° ROM at all orientations. The innovative
9
uniplanar screw was designed with a ball and socket joint connecting the screw head to the screw body. The
10
screw head had a straight, longitudinal slot that allowed the screw body to move and a 60° ROM in a plane
11
parallel to the screw cup (Figure 1b). The innovative uniplanar pedicle screws were made of Ti6Al4V by a
12
manufacturer with good manufacturing practices and ISO 13485 quality management systems (Huang-Liang
13
Co., Ltd., Kaohsiung, Taiwan).
14
Specimen Preparation and Biomechanical testing
15
Previous studies have indicated that porcine spines are comparable in size to those of human
16
adolescents. In this study, the biomechanical tests were conducted on six porcine spines harvested from
17
skeletally immature pigs (3–4 months of age, approximately 100 kg) from a slaughterhouse. The test
18
specimens were frozen and stored at -20°C until testing. Each test specimen consisted of the T7-T13 thoracic
19
vertebrae. After the spines were harvested, the ribs, muscles, and adipose tissues were removed from each
20
specimen; the ligamentous structures and intact joint capsules were retained. For fixation, the superior half
21
of the T7 (cephalad side) and the inferior half of the T13 (caudal side) vertebral bodies were embedded on an 6
Page 6 of 23
1
aluminum disc with epoxy resin (Truetime Industrial Co., Tainan, Taiwan) to provide a stable base
2
throughout the test procedure and fixed on the proximal and distal fixtures (Figure 2).
3
Each test specimen was thawed at 4°C for 6 hours and then at room temperature for 2 hours; the test
4
specimens were kept moist throughout this period. Each vertebral body (T8- T12) was installed with 3
5
makers. The bottom two marker points (proximal and distal marker points) were collinear with the vertebral
6
body center. The distance between the vertebral body center and the nearest marker was 30 mm. The line
7
that connected the two markers was perpendicular to the long axis of the specimen and parallel to the sagittal
8
plane. A total of fifteen marker points were placed on each specimen to track the body trajectory. Three
9
pedicle screws of the same design were implanted into the left pedicles of T9 to T11. The three pedicle
10
screws were connected with a 6-mm titanium rod to finish the assembly. Screw position was confirmed by
11
both anterioposterior (AP) and lateral X-ray photographs. The testing protocol was performed on a materials
12
test system (MTS) (HT-2402EC, Hung Ta Co., Ltd., Taipei, Taiwan). The specimen torque was detected with
13
a torque sensor attached to the bottom of the distal fixture. The test specimen was placed and fixed on the
14
MTS workspace. The loading device was aligned with the long axis of the apical vertebra (T10) pedicle
15
screw. The head of the screw inserted into the apical vertebra (T10) was connected to a derotational tube,
16
which was secured by a holding stick. A hinge joint that allowed a 90° rotation was set on the middle
17
segment of the holding stick. The other side of the holding stick was fixed on a slider and attached to the
18
load cell of the MTS. The test setup is shown in Figure 2.
19
In the present study, the T10 vertebral body is considered the apical body, the T9 and T11 vertebral
20
bodies are considered the fixed end bodies, and the free mobile spinal segments (T8 and T12) are maintained
21
on either side of the instrumented vertebrae. The two free mobile segments are considered the adjacent 7
Page 7 of 23
1
bodies.
2
The test performed in this study aimed to reproduce the motion applied to an apical body during the
3
DVD maneuver. In this test, the derotational tube transferred an independent load to the pedicle screw of the
4
apical vertebra (T10) in the axial plane to generate a rotational force on the body. The hinge joint transferred
5
the upper movement of the MTS to a rotational moment. The test was commenced by the displacement of
6
the holding stick upward at a rate of 10 mm/min. The spinal torque was monitored, and the sampling rate
7
was set to 10 Hz.
8
Two mini-DV video cameras (HDR-XR260V, Sony, Japan; HDR-SR12, Sony, Japan) were placed in
9
front of the MTS workspace. Two-dimensional image coordinates were transformed into 3D object-space
10
coordinates using the 3D direct linear transformation algorithm to calculate the T8-T12 vertebral rotation
11
[11]. A goniometer (Pasco PS-2138, US) was placed on the hinge joint of the holding stick to measure the
12
rotation of the derotational tube at a sampling rate of 10 Hz. The kinematic data, including the vertebral
13
rotation, translation, and derotational tube rotation, were collected for analysis.
14
The surface of the lateral wall of the left pedicle of the T10 vertebra was polished and glued to a 2-mm
15
strain gauge (KFG02-120-C1-16, Kyowa, Electronic Instruments Co., Ltd., Japan) to measure the strain of
16
the apical pedicle. The signals were calibrated and amplified by eStrain P4-X signal conditioners at a
17
sampling rate of 10 Hz. A data translational board (National Instruments, Austin, TX, USA) and FleXense
18
(Chief Si Co., Ltd., Taiwan) software were used to record the strain signals. All data were collected until the
19
torsional moment reached 3 Nm. Each specimen was subjected to each of the three tests (i.e., monoaxial,
20
polyaxial and uniplanar screws) randomly for a total of 18 tests. The minimum time interval between two
21
tests is thirty minutes. The efficiency of derotation was defined as the ratio of the tube rotation to the apical 8
Page 8 of 23
1
body rotation. The difference in the rotation of the derotational tube and the apical body represented the
2
variance of the derotation.
3 4
Statistical Analysis
5
The curve of the derotational tube rotation versus the torque and the apical body rotation versus
6
derotational tube rotation on the specimen were analyzed. The derotation efficiency, derotation variance,
7
apical vertebra translation, and maximum pedicle strain in each group were determined and then averaged.
8
The correlational curve of the correlational curve of the pedicle strain versus the apical body rotation was
9
analyzed for each screw type using linear regression models, which were fitted to the linear portion of the
10
curve used to compute the mean compliance of each screw design. A one-way analysis of variance (ANOVA)
11
and post hoc Tukey’s tests were used to detect a significant difference in the derotation efficiency, derotation
12
variance, apical body translation, and maximum pedicle strain in each of the three groups. Similarly,
13
ANOVA and post hoc Scheffe’s tests were used to detect a significant difference in the rotation of vertebral
14
bodies (apical body, fixed end body, and adjacent body).
15
9
Page 9 of 23
1
Results
2
Kinematic and Dynamic Data
3
The rotations of the derotational tubes versus the specimen torques of the monoaxial, polyaxial, and
4
uniplanar screw groups are shown in Figure 3. Under incremental rotation, the monoaxial screw group
5
showed an approximately linear increase in torque with a rotated derotational tube. The polyaxial screw and
6
uniplanar screw groups showed 28.38° ± 2.50° and 1.2° ± 0.8° delays in the initial stage, respectively. Then,
7
similarly to the monoaxial screw group, a trend of approximately linearly increasing torque was observed in
8
the polyaxial screw and uniplanar screw groups.
9
The rotations of the derotational tubes versus the rotations of apical bodies of the monoaxial, polyaxial,
10
and uniplanar screw groups are shown in Figure 4. The trend of curve was similar with the Figure 3. The
11
polyaxial screw and uniplanar screw groups showed delays in the initial stage. The ratio of the tube rotation
12
to the apical body rotation represent the derotation efficiency. For the monoaxial, polyaxial and uniplanar
13
screw groups, the derotation efficiencies were 0.65, 0.51 and 0.12, respectively.
14
The difference in the rotation of the derotational tube and apical body represented the derotation
15
variance. The derotation variances of the monoaxial, polyaxial and uniplanar screw groups were 2.22° ±
16
1.43°, 32.23° ± 2.26°, and 4.75° ± 1.60°, respectively. The rotational variance of the polyaxial screw was
17
statistically larger than that of the monoaxial and uniplanar screws (p < 0.05). The translations of the T10
18
apical bodies of the monoaxial, polyaxial and uniplanar screw groups were 3.80 mm ± 1.36 mm, 4.61 mm ±
19
1.55 mm and 4.04 mm ± 0.83 mm, respectively, when the torques of the spinal constructs reached 3 Nm.
20
Rotation of Vertebral Bodies
21
The rotations were calculated based on the marker locations that were tracked using the motion capture 10
Page 10 of 23
1
system. Figure 5 summarizes the rotations during the DVD maneuver for the apical, fixed end and adjacent
2
vertebral bodies. The rotations of the fixed end bodies were slightly lower than those of the apical body in all
3
groups; no group showed statistical significance (p > 0.05). The rotations of adjacent levels were lower than
4
that of the apical body in all groups, with statistical significance in every group (p < 0.05).
5
Pedicle Strain
6
To assess the correlation between vertebral body rotation and the pedicle strain of each screw head
7
design group, a linear regression model was applied to fit the data points (Figure 6). The monoaxial,
8
polyaxial and uniplanar screw groups exhibited high R-square values of R2 = 0.99, 0.95 and 0.97,
9
respectively. The trend indicated that the pedicle strain of the monoaxial screw group would be higher than
10
that of the other groups when the vertebral body rotation increased. The maximum strains for the monoaxial,
11
polyaxial and uniplanar screw groups were 1067.45 ± 550.35, 747.68 ± 393.56, and 663.55 ± 271.04, with
12
no statistically significant differences in any group (p > 0.05).
11
Page 11 of 23
1
Discussion:
2
Pedicle screw-based posterior spinal fusion with a rod derotation maneuver has enabled powerful
3
coronal and sagittal plane correction in scoliosis surgery. However, the ability to achieve correction in the
4
axial plane using this method remains elusive. Lee et al. [3] developed the DVD maneuver, a surgical
5
technique that gained popularity due to its better control of the spine in the axial plane and the resulting
6
improved corrections of thoracic and lumbar curves. The surgical technique of the DVD maneuver is
7
conducted as follows: 1) the pedicle screws are inserted posteriorly from the pedicle and traverse to the
8
anterior vertebral body; 2) a pre-contoured rod is inserted into the segmental screws; and 3) correction of the
9
vertebral rotation is achieved by applying a force in the opposite direction to that of the deformity. This
10
approach enables the transmission of the rotational force to the entire vertebral body, thus allowing the
11
correction of rotational deformities in the axial plane. In addition, this technique provides better control of
12
the thorax in some cases, eliminating the need for a thoracoplasty to manage the rib hump deformity
13
[6,12–14]. However, the clinical outcomes of using the DVD maneuver to treat the vertebral body deformity
14
in the axial plane have been disputed. Some studies have indicated no significant improvements even with
15
the additional use of segmental pedicle screw instrumentation in DVD aimed at correcting AIS [12,15,16].
16
Kuklo et al. [5] compared the abilities of monoaxial and polyaxial pedicle screws to effectively achieve
17
curve correction in a matched cohort of 35 patients with AIS. Monoaxial screws provided superior derotation
18
and restoration of thoracic symmetry compared with polyaxial screws, showing that the choice of screws is
19
an important factor in the DVD maneuver. However, few studies have investigated the optimal design of
20
screws for use in DVD. In the present study, three pedicle screws with different screw head designs were
21
used to compare the derotational efficiency during DVD. The use of monoaxial or uniplanar screws resulted 12
Page 12 of 23
1
in a rotation of the apical vertebra that was proportional to the tube rotation of the derotational tube.
2
Therefore, the use of monoaxial or uniplanar screws achieved better efficiency than that of polyaxial screws
3
during the DVD maneuver. On the contrary, in the polyaxial screw group, the surgical segment of the spine
4
rotated only 4.44° ± 1.00° when the derotational tube rotated by 36.67° ± 2.46° under 3 Nm. Even though
5
the monoaxial screw group was more efficient. However, some biomechanical problems can arise after
6
spinal surgery when a monoaxial screw is used to treat a spinal deformity.
7
Monoaxial screws have been shown to be advantageous in correcting spinal deformities in all planes, as
8
these screws greatly improve the stability of the instrumented spine and allow for the application of higher
9
corrective forces to translate and derotate the deformed vertebrae [9,17]. The fixed screw head design of the
10
monoaxial screw offers a mechanical advantage over the pedicle screws with a multi-angle head design in
11
terms of rotational or translated forces. These screws more directly transmitted the force into the vertebral
12
body to gain better derotational efficiency. In the present study, the monoaxial group showed a higher
13
derotation efficiency (0.65) and a lower derotation variance (2.22° ± 1.43°) than the other groups (polyaxial,
14
0.51 and 32.23° ± 2.26°; uniplanar, 0.12 and 4.75° ± 1.60°). These results were consistent with the opinions
15
that monoaxial screws improve the effects of derotation significantly more than the other designs [1,5,6,9].
16
The rotational variance of the monoaxial screw was significant difference with uniplanar screws (p = 0.029).
17
The phenomenon maybe explained from the design of uniplanar screws. In order to allow the spherical head
18
of screw body to set into the socket of screw cup, the axial diameter of screw head slot was slight larger than
19
the screw diameter of screw body. Therefore, the uniplanar pedicle screw allowed a slight ROM and had a
20
slightly higher rotational variance than the monoaxial pedicle screw. Although there were significant
21
differences among monoaxial, uniplanar and polyaxial screws, the rotational values of monoaxial and 13
Page 13 of 23
1
uniplanar screws were much small than polyaxial screws. However, as the screw head of a monoaxial screw
2
is immobile relative to the screw body, the rod can be difficult to seat into the screw head, as the long axis of
3
the screw must be perpendicular to the rod to reach the ideal fixed situation. Typically, each pedicle screw
4
cannot be placed in an identical position to that of the adjacent screws, and the minor screw misalignment is
5
transformed into vertebral misalignment and adverse bone-screw stresses at the time of the rod-screw
6
construct assembly[17,18]. These adverse stresses have no benefit to the deformity reduction but inevitably
7
increase the probability of bone-screw interface damage. This mode of failure has been seen clinically in
8
anterior and posterior methods of deformity corrections and has been demonstrated biomechanically [19,20].
9
Additionally, in clinical practice, monoaxial screws may pose challenges during the rod-screw engagement
10
sequence and failure to match angle preservation between screw-rod interface if they are not very closely
11
aligned in a contiguous arc [21]. Thus, polyaxial screws have often been used in clinical practice to
12
surgically correct scoliosis. Polyaxial screw heads were designed to facilitate the capture of the rod in the
13
screw head when the spine deformity is particularly extreme. However, this design can result in the failure of
14
the apical body to rotate in the DVD maneuver. Furthermore, the correction rate in the axial plane is less
15
than that achieved using the monoaxial screw [1,5,18]. This phenomenon was observed in the present study,
16
in which the polyaxial screws allowed 25° ROM at the joint of the screw head and body. The rotation of the
17
apical body was not measurable until the rotation of the derotational tube achieved 28.38° ± 2.50° (nearly
18
25°). When the inner wall of the screw head touched the screw body, the torque was conducted from the
19
derotational tube to the screw body. The present study revealed that the derotation variance was significantly
20
different among the three groups, which raised concerns that in real clinical situations, the surgeon may not
21
know the real rotation of the apical body during the DVD maneuver. Additionally, soft tissues such as skin 14
Page 14 of 23
1
and muscles and hard tissues such as bones around the surgical space may limit the activity of the
2
derotational tube. Since the head design of the polyaxial screw allowed more DOF, the screw head and body
3
could not consistently align after the rod was seated in the screw head. When the screw was implanted more
4
laterally, the screw head would turn medially to compensate for the direction and vice versa. The rotational
5
arc of the derotational tube would be larger than the ROM of polyaxial screws before the screw body
6
generated derotational torque on the apical body. In general, the rotational arc of the derotational tube would
7
be smaller than the ROM of a medially implanted polyaxial screw before the screw body generates a
8
derotational torque on the apical body. Compared with the monoaxial screw, the uniplanar screws, which
9
have a pivoting connection between the screw head and the threaded body, allow for variation in the
10
orientation of screw insertion in the sagittal plane and therefore prevent adverse bending moments in the
11
sagittal plane. Dalal et al. [1] compared the residual postoperative apical vertebral rotations of the uniplanar
12
and polyaxial bilateral pedicle screw constructs in thoracic AIS. Their results showed that the uniplanar
13
screw group had a better apical vertebral derotation (less residual apical vertebral rotation) than the polyaxial
14
screw group. In this present study, uniplanar screws obtained a better efficiency of derotation in the axial
15
plane that was closer to that of the monoaxial screw group than that of the polyaxial screw group; moreover,
16
the uniplanar screws allowed for easy fitting of the rod into the screw head and thereby prevented extra
17
stress on the screw-rod interface. Furthermore, compared with the monoaxial pedicle screw group, the
18
uniplanar screw group had lower pedicle strain during the DVD maneuver.
19
Martino et al. [22] indicated that during the DVD maneuver, the apical body center is translated
20
medially, thus improving the correction of both the main thoracic Cobb angle and the apical translation. In
21
the present study, this phenomenon was also observed during the DVD maneuver. The vertebral body was 15
Page 15 of 23
1
not only rotated but also translated medially (Figure 7). In the present study, though the translation of apical
2
body were similar in three groups, when the torques of the spinal constructs reached 3 Nm. However, the
3
translation was difficult to controlled in polyaxial screw group, because the derotational efficiency was lower
4
than other groups. In orienting the DVD maneuver, Parent et al. [23] investigated the failure torque in axial
5
plane DVD maneuvers in both the medial and lateral directions. Their results showed no significant
6
difference in the failure torque between the medial and lateral directions. However, medial screw failure
7
jeopardizes the spinal cord, whereas lateral failure may compromise proximal pulmonary, vascular, or other
8
visceral structures during DVD. This technique is based mostly on the surgeon’s tactile abilities, and the
9
magnitude of force necessary to achieve a desired correction in clinical applications is unknown. Therefore,
10
in this present study, DVD was performed by pulling the derotational tube toward laterally which generating
11
a force pushing the screw toward medially, pivoting around the rod.
12
Previous studies have evaluated the maximum torque and applied power required to achieve complete
13
correction of spinal deformities and vertebral failure [23,24]. Notably, no study has investigated the pedicle
14
strain during the DVD maneuver. In the present study, the strain of the lateral pedicle wall was evaluated. In
15
the linear regression model of the pedicle strain versus the apical body rotation, the monoaxial pedicle screw
16
group had a greater slope than the other groups. The maximum strain was not significantly different when
17
the vertebral bodies underwent 3 Nm torque. However, the difference would be increasingly significant
18
when the torque was increased. These results were consistent with opinions that a higher structural stiffness
19
of the monoaxial pedicle screw would lead to higher stress on the vertebral pedicle, which would increase
20
the probability of bone-screw interface damage. The present study, as with many biomechanical studies, has
21
several limitations. In the absence of a surrogate scoliosis model, we have chosen to use a porcine spine 16
Page 16 of 23
1
model; although this model is similar to a normal human spine in terms of anatomy and vertebral kinematics,
2
it cannot simulate the rotational deformation and altered geometry and mechanics of the vertebrae and
3
intervertebral discs that are typical of a scoliotic spine. The aim of this study was to evaluate the efficiency
4
of different design pedicle screw in direct vertebral derotation. The porcine spines in this study were not
5
scoliotic, because scoliotic porcine spines were not easy to obtain. Also, the testing protocol followed
6
previous studies that all used non-scoliotic cadavers or animal spines to investigate the situation of vertebral
7
body derotation [23,25]. The results illustrated the differences of kinematics of vertebral derotation, as well
8
as the derotational variance, rotation and medial shift of vertebral body among the monoaxial, uniplnar and
9
polyaxial screws. Even though the biomechanical concept and trend between the porcine spine and the
10
human spine are similar, the current study did not necessarily represent the real force or torque in clinical
11
situation. In this study, similar porcine vertebrae were scanned by DEXA (Horizon DXA system,
12
Marlborough, USA) to measure the AP and lateral BMD (bone mineral density). The data ranged between
13
0.8 and 1.1 g/cm2. Boot et al. [26] presented that the BMD of human adolescents (age in 12 - 18 years) was
14
between 0.8~1.2 g/cm2. Therefore, the BMD of test specimens in this study was similar to human
15
adolescents. Screw position was confirmed by both anterioposterior (AP) and lateral X-ray photographs.
16
Nonetheless, numerous studies have used porcine spines for similar biomechanical experiments [27].
17
Although applying the torque to the apical vertebral body is sufficient to perform the vertebral derotation
18
maneuver in a flexible spine, a greater number of derotation levels may be necessary to improve deformity
19
corrections for patients with stiffer and larger spinal curves and vertebral rotations. In clinical, the real torque
20
value used in the process of vertebral body rotation is difficult to measure during the surgery because the
21
deformed level of each patient was difference and amounts of the torque values were not the same in each 17
Page 17 of 23
1
case. The aim of this study was to compare the efficiency of different design of pedicle screws and not at all
2
rotated to failure. The torque of 3Nm slight below failure critical value was set according to the study result
3
of Parent et al. in 2008 [23]. This study indicated that the strength of vertebral body derotated to failure test
4
by using the cadaver spine and showed that minimum torque was close 4 Nm.
18
Page 18 of 23
1
Conclusion:
2
In conclusion, this study investigated the efficiency of three different screw head designs in the direct
3
vertebral derotation maneuver performed on multisegmental porcine spines. Based on the results of this
4
study, the screw head design plays important roles in both derotation efficiency and variance during the
5
direct vertebral derotation maneuver. The derotational efficiency of the newly designed uniplanar screw was
6
closer to that of the monoaxial screw than that of the polyaxial screw. The polyaxial screw was inferior for
7
direct vertebral derotation because there was a derotational variance between the derotational tube and the
8
apical body that was correlated with the range of motion of the screw head. In the present study, the pedicle
9
strain was similar in all groups. However, the pedicle strain of the uniplanar screw group was lower than that
10
of the monoaxial screw group and was similar to that of the polyaxial screw group when the angle of rotation
11
of the apical body increased.
12 13 14 15 16 17 18
Acknowledgements We thank Chih-Yi Chen for her help in preparing the porcine spine. This work was kindly supported by grant CRRPG3E0141 & CRRPG3E0142 from Chang Gung Memorial Hospital, Taiwan.
19 20
19
Page 19 of 23
1
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Shepard MF, Davies MR, Abayan A, et al. Effects of polyaxial pedicle screws on lumbar construct
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3
Figure 1. (a) Kinematics of monoaxial, polyaxial, and uniplanar pedicle screw constructs; (b) The diameter
4
of minor axis of the slot was the same as the ball of the screw body to limit screw movement on
5
transverse plane. The length of long axis of opening allow a 60-degree movement (cephalad 30
6
degrees, caudally 30 degrees) on sagittal plane.
7
Figure 2. (a) Test setup of simulated DVD maneuver. The test specimen consisted of T7-T13 of porcine
8
spine with the proximal (T7) and this distal (T13) vertebrae fixed. The T8 and T12 vertebrae were
9
free to rotate. The pedicle screws were inserted into left pedicle of T9-T11 and rod was seated into
10 11 12 13 14 15 16 17 18 19
all screw head; (b) Test specimen motion during test processing. Figure 3. The rotations of derotational tube versus the specimen torques of the monoaxial, polyaxial, and uniplanar screws during DVD testing. Figure 4. Rotation of apical vertebra ( T10 ) versus rotation of derotational tube of the monoaxial, polyaxial, and uniplanar pedicle screw during vertebral derotation testing. Figure 5. The rotation of apical vertebra ( T10 ), end instrumented vertebrae ( T9, T11 ) and adjacent vertebrae ( T8, T12 ) of porcine spine during vertebral derotation testing in axial plane. Figure 6. The strain of apical body versus rotation of apical body of the monoaxial, polyaxial, and uniplanar pedicle screw during vertebral derotational testing. Figure 7. The trajectory of apical vertebra during derotational maneuver processing.
20
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