Chapter 3: Materials and methods
3.1 Coflex and Coflex-F in non-fusion surgery
3.1.4 FE model of bilateral pedicle screw fixation into the L3-L4 segment (Pedicle
This model was a defect model implanted with pedicle screw fixationat the L3-L4 segment. The difference between the pedicle screw fixation model and the above implantation models was that the pedicle screw fixation model preserved the supraspinous ligaments and interspinous ligaments (Figure 3.5 C). The pedicle screw fixation consisted of two rods (diameter, 4.5 mm) and four pedicle screws (diameter, 6 mm). The pedicle screws were inserted through the pedicles of the L3 and L4 vertebrae bilaterally. The pedicle screws were simplified as cylinders. The screw-bone interfaces were assigned to be fully constrained. The material used for the pedicle screws was Ti-6Al-4V. The Young’s modulus and Poisson’s ratio were assigned to be 113 GPa and 0.3, respectively.
Fig For the pre y of long lu ecting the w
everal meth In this stud el through pressive fol
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42
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is approach ame overall n about this Hybrid test
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addition, the patient’s main aim following surgery is to go back to normal daily life. Thus, the surgically treated spine should be able to go through the same ROM as in a normal person.
Therefore, they suggested that the spinal construct should be tested under the same ROM and the hybrid test method should be more clinically relevant.
In this study, by applying bending moments from the hybrid test method, a 400 N follower load was first applied on the superior surface of the L1 vertebra, and then a moment of 30 N-m was applied incrementally by 1 Nm in 30 loading steps. Therefore, the resultant total ROMs of the implantation models (L1 to L5) under different moments would match the total ROMs of the intact lumbar model which was subjected to 10 N-m loadings according to the in vitro study of Yamamoto et al.[105]. The detailed total lumbar ROMs of the intact model under the hybrid test method are 16.37°in flexion, 10.75° in extension, 15.27° in right lateral bending, and 8.44°in right axial rotation. These ROMs are a baseline with which to match the total lumbar motion among the intact and implantation models under the hybrid test method (Table 3.2). The resulting deviation of ROMs among the three FE models were controlled to within 0.64° in flexion, 0.14° in extension, 0.63° in right lateral bending, and 0.22° in right axial rotation.
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44
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Table 3.2 Intervertebral range of motion and applied moment among the intact, defect, and implantation models under the hybrid test method.
Model ROM (degree) Total lumbar ROM(degree)
(L1-L5) Moment(Nm) L1-L2 L2-L3 L3-L4 L4-L5
Flexion
Intact 3.66 3.78 3.82 5.11 16.37 10 Defect 3.62 3.75 4.32 5.05 16.74 10 Coflex 3.49 3.63 4.14 4.84 16.10 10 Coflex-F 4.33 4.47 1.87 6.01 16.68 12 Pedicle screw fixation 4.51 4.67 1.23 6.31 16.72 13
Extension
Intact 2.70 2.47 2.30 3.27 10.74 10 Defect 2.37 2.05 3.75 2.61 10.78 8 Coflex 3.36 3.06 0.68 3.89 10.99 14 Coflex-F 3.36 3.08 0.54 3.92 10.90 14 Pedicle screw fixation 3.24 2.93 0.22 4.11 10.50 13
Lateral bending
Intact 3.69 3.59 3.67 4.32 15.27 10 Defect 3.69 3.62 3.69 4.33 15.33 10 Coflex 3.72 3.65 3.39 4.34 15.10 10 Coflex-F 3.78 3.70 3.01 4.43 14.92 10 Pedicle screw fixation 4.41 4.17 1.74 5.23 15.55 13
Axial rotation
Intact 1.81 1.90 2.23 2.50 8.44 10 Defect 1.83 1.92 2.26 2.52 8.53 10 Coflex 1.80 1.86 2.13 2.53 8.32 10 Coflex-F 1.80 1.86 2.12 2.53 8.31 10 Pedicle screw fixation 2.17 2.08 1.33 2.79 8.37 13
46
3.2 Coflex-F in fusion surgery
The second subject of following sections includes FE modeling and simulation technique of this study. The first model of second subject was the intact lumbar spine same first subject (3.1.1) model. The other five models were the TLIF or ALIF combined with Coflex-F and TLIF or ALIF combined with bilateral pedicle screw fixation.
3.2.1 FE model of TLIF combined with Coflex-F (Coflex-F + TLIF model)
The intact model was modified to a TLIF model by implanting an AVS-TL cage (30 mm width x 11 mm depth x 21 mm height; Polyetheretherketon (PEEK); Stryker Orthopaedics) (Figure 3.9 E) between the L3 and L4 vertebrae. To simulate the standard TLIF procedure, unilateral total facetectomy and partial discectomy were performed at the L3-L4 segment. The left facet joint, ligamentum flavum, and partial disc were removed, but the posterior elements, contralateral facet joint, supraspinous ligaments, and interspinous ligaments were preserved.
The cage-bone interface was modeled by surface-to-surface contact elements to simulate the early postoperative stage after spinal implantation. These contact elements were able to transmit compression, but not tension. The coefficient of friction at the cage-bone interface was set at 0.8 to mimic the effect that the cage’s small teeth have on contact surfaces. The higher coefficient of friction (0.8) was used in the contact interface to prevent device slip motion [86]. The Young’s modulus and Poisson’s ratio of AVS-TL cage were assigned to be 3.5 GPa and 0.3, respectively.
The TLIF model was again modified to implant the Coflex-F device between the L3 and L4 vertebrae to complete the Coflex-F combined with TLIF model, requiring the removal of supraspinous ligaments and interspinous ligaments (Figure 3.9 A). The Coflex-F is available in five sizes from 8-16 mm in 2 mm increments. In this study, the optimal height for the FE model was 14 mm. Part of the L3-L4 interspinous process was removed to provide sufficient space for implanting the Coflex-F between the interspinous processes. The surface between
the spinous processes and the wings of the Coflex-F was modeled as a surface-to-surface contact. The effect of the teeth on the wings of the Coflex-Fwas simplified by assigning a higher coefficient of friction (0.8) to the wing contact area (Figure 3.9 G, yellow region), and the coefficient of friction for the rest of the contact regions was set at 0.1 (Figure 3.9 G, red region). The rivets were modeled as cylinders (diameter = 2.8 mm) and were constrained to both holes on the wings of the Coflex-Fand the spinous processes in all degrees of freedom (The degrees of freedom of rivet nodes are interpolated with the corresponding degrees of freedom of the nodes on the Coflex and spinous processes during the execution of ANSYS program). The Coflex-F was constructed using Ti-6Al-4V alloy. The Young’s modulus and Poisson’s ratio were assigned to be 113 GPa and 0.3, respectively.
3.2.2 FE model of ALIF combined with Coflex-F (Coflex-F+ ALIF model)
The intact model was modified to an ALIF model by implanting a SynCage-Open cage (Figure 3.9 F) (30 mm width x 24 mm depth x 21 mm height; Titanium alloy; Synthes spine, Inc.) between the L3 and L4 vertebrae. To simulate the standard ALIF procedure, the L3-L4 segment of the intact model underwent partial discectomy and total nuclectomy by the anterior approach, which included removal of the anterior longitudinal ligament, anterior portions of the annulus, and the entire nucleus pulposus. All the other ligaments were preserved. The ALIF cage-bone has the same interface conditions as those of the TLIF cage-bone in section 2.2. The SynCage-Open cage was constructed out of Ti-6Al-7Nb alloy.
The Young’s modulus and Poisson’s ratio were assigned to be 110 GPa and 0.28, respectively.
In addition, the ALIF model was modified for implanting the Coflex-F between the L3 and L4 vertebrae to complete the Coflex-F combined with ALIF model. The ALIF model and the TLIF model implant the Coflex-F under the same conditions (Figure 3.9 B).
48
3.2.3 FE model of TLIF combined with bilateral pedicle screw fixation (Pedicle screw + TLIF model)
The previous TLIF model was combined with bilateral pedicle screws to form the pedicle screw fixation model (Figure 3.9 C). The difference between the pedicle screw fixation model and the Coflex-F model is that the pedicle screw fixation model preserves the supraspinous ligaments and interspinous ligaments. The pedicle screws were inserted bilaterally through the pedicles of the L3 and L4 vertebrae. The pedicle screw fixation in this study consisted of two rods (diameter = 4.5 mm) and four pedicle screws (diameter = 6 mm).
The pedicle screws were modeled as cylinders. The screw-bone interfaces were designed to be fully constrained (The degrees of freedom of screw nodes are interpolated with the corresponding degrees of freedom of the nodes on the Coflex and spinous processes during the execution of ANSYS program). The pedicle screws were made of Ti-6Al-4V alloy. The Young’s modulus and Poisson’s ratio were assigned to be 113 GPa and 0.3, respectively.
3.2.4 FE model of ALIF combined with bilateral pedicle screw fixation (Pedicle screw + ALIF model)
The previous ALIF model was combined with bilateral pedicle screws (Figure 3.9 D).
This model preserved the supraspinous ligaments and interspinous ligaments. Both this model and the previous TLIF model (combined with bilateral pedicle screws) used the same conditions and materials for pedicle screws.
Figur Cofle the T cage porti
re 3.9: Fini ex-F device TLIF mode
in the mid ion of the ve
ite element e combined l; (D) Pedi ddle portion ertebral mod
models: (A d with the A
cle screw f n of the ve del; (G) Co
A) Coflex-F ALIF model
fixation com ertebral mo oflex-F devi
device com l; (C) Pedic mbined wit
del; (F) Sy ce model.
mbined with cle screw fi
h the ALIF ynCage-Ope
h the TLIF ixation com F model; (E en cage in
model; (B) mbined with E) AVS-TL the middle ) h L e
50
3.2.5 Boundary and loading conditions
In this second subject, the boundary and loading conditions are all same first subject (3.1.5). The 400 N compressive follower load was applied to each motion segment through induced contraction in these link elements by decreasing the temperature. The link elements were attached near the centers of each vertebral body such that each element spanned the mid-plane of the discs. These arrangements directed the construction of a nearly ideal follower load, which remains tangent to the spine curve, loading each spinal segment in nearly pure compression.
A 10 Nm moment was applied to the intact model to mimic physiological motion [105].
These motions subject the multilevel lumbar spine to a maximal possible load without causing spinal injury. The other implanted models under comparison were subjected to specific moments that produced overall motions that were equal to those of the intact model, using a hybrid test method. The detailed total lumbar ROMs of the intact model under the hybrid test method are 16.36° in flexion, 10.31° in extension, 15.25° in lateral bending to both sides, and 8.43° in axial rotation to both sides. These ROMs are a baseline to match the total lumbar motion among the intact and implantation models under the hybrid test method (Table 3.3).
The resulting deviation of ROMs among the three FE models were controlled to within 0.33°
in flexion, 0.56° in extension, 0.22° in right lateral bending, 0.24° in left lateral bending, 0.21°
in right axial rotation, and 0.21° in left axial rotation.
Table 3.3 Intervertebral range of motion and applied moment among various surgical models under the hybrid test method.
Model ROM (deg) Total lumbar ROM(deg)
(L1-L5) Moment(Nm) L1-L2 L2-L3 L3-L4 L4-L5
Flexion
Intact 3.66 3.78 3.82 5.10 16.36 10 Coflex-F+TLIF 4.37 4.54 0.96 6.38 16.25 12 Coflex-F+ALIF 4.46 4.60 0.92 6.44 16.42 12 Pedicle screw + TLIF 4.40 4.59 0.62 6.48 16.09 12 Pedicle screw + ALIF 4.49 4.66 0.61 6.47 16.23 12
Extension
Intact 2.27 2.47 2.30 3.27 10.31 10 Coflex-F+TLIF 3.26 3.17 0.43 4.01 10.87 11 Coflex-F+ALIF 3.33 3.01 0.55 3.96 10.85 12 Pedicle screw + TLIF 3.26 3.16 0.27 4.16 10.85 11 Pedicle screw + ALIF 3.32 2.98 0.22 4.09 10.61 12
Right lateral bending
Intact 3.69 3.59 3.67 4.30 15.25 10 Coflex-F+TLIF 4.08 3.97 2.40 5.00 15.45 11 Coflex-F+ALIF 4.46 4.37 1.26 5.15 15.24 12 Pedicle screw + TLIF 4.47 4.32 1.17 5.50 15.46 12 Pedicle screw + ALIF 4.48 4.35 0.92 5.54 15.29 12
Left lateral bending
Intact 3.69 3.59 3.67 4.30 15.25 10 Coflex-F+TLIF 4.11 4.03 1.95 5.07 15.16 11 Coflex-F+ALIF 4.46 4.37 1.26 5.15 15.24 12 Pedicle screw + TLIF 4.47 4.38 1.04 5.51 15.40 12 Pedicle screw + ALIF 4.48 4.35 0.92 5.54 15.29 12
Right axial rotation
Intact 1.81 1.90 2.23 2.49 8.43 10 Coflex-F+TLIF 2.00 2.09 1.52 2.80 8.41 11 Coflex-F+ALIF 2.12 2.19 1.06 2.94 8.31 12 Pedicle screw + TLIF 2.29 2.22 0.81 2.99 8.31 13 Pedicle screw + ALIF 2.38 2.31 0.70 3.13 8.52 14
Left axial rotation
Intact 1.81 1.90 2.23 2.49 8.43 10 Coflex-F+TLIF 2.01 2.09 1.44 2.81 8.35 11 Coflex-F+ALIF 2.12 2.19 1.06 2.94 8.31 12 Pedicle screw + TLIF 2.29 2.22 0.81 2.99 8.31 13 Pedicle screw + ALIF 2.38 2.31 0.70 3.13 8.52 14
52
3.3 Coflex-F in minimally invasive fusion surgery
The third subject of following sections includes FE modeling and simulation technique of this study. The first model of third subject was the intact lumbar spine same first subject (3.1.1) model. The other three models were the TLIF combined with Coflex-F, with unilateral pedicle screw fixation and translaminar facet screw fixation model, and with bilateral pedicle screw fixation.
3.3.1 FE model of TLIF combined with Coflex-F (Coflex-F model)
The intact model was modified to TLIF model and implant Coflex-F between L3 and L4 (Figure 10 A). The model of third subject was the Coflex-F same second subject (3.2.1) model. Both this model and the previous TLIF combined with Coflex-F model used the same conditions and materials.
3.3.2 FE model of TLIF combined with unilateral pedicle screw fixation with translaminar facet screw fixation (UPSF+TFSF model)
The previous TLIF model was combined with pedicle screws between L3 and L4. The pedicle screw fixation consisted of one rods (diameter = 4.5 mm) and two pedicle screws (diameter= 6 mm). The pedicle screws were inserted respectively through the pedicles of L3 and L4 vertebrae unilaterally. The UPSF model was combined with translaminar facet screw fixation (diameter= 4 mm) (Figure 10 B). The unilateral pedicle screw and translaminar facet screw were modeled as cylinders and screw-bone interfaces were designed to be fully constrained. The material used for the pedicle screws and fact screw were Ti-6Al-4V alloy.
The Young’s modulus and Poisson’s ratio were assigned to be 113 GPa and 0.3.
3.3.3 FE model of TLIF combined with bilateral pedicle screw fixation (BPSF model) The intact model was modified to TLIF combined with bilateral pedicle screw fixation between L3 and L4 (Figure 10 C). The model of third subject was the TLIF combined with bilateral pedicle screw fixation same second subject (3.2.3) model. Both this model and the previous TLIF combined with bilateral pedicle screw fixation model used the same conditions and materials.
3.3.4 Boundary and loading conditions
In this third subject, the boundary and loading conditions is all same 3.1.5 and 3.2.5 sections. A 400 N follower load and a 10 N-m moment were applied to the intact model to obtain physiological motions as comparison baseline. The implanted models were subjected to 400 N follower load and specific moments in accordance with the hybrid test method.
These ROMs are a baseline to match the total lumbar motion among the intact and implantation models under the hybrid test method (Table 3.4). The resulting deviation of ROMs among the three FE models were controlled to within 0.35° in flexion, 0.46° in extension, 0.29° in right lateral bending, 0.16° in left lateral bending, 0.11° in right axial rotation, and 0.11° in left axial rotation.
Figur mode (C) T
re 3.10: Fi el combined TLIF model
nite elemen d with unila l combined
nt models:
ateral pedicl with pedicl
54
(A) TLIF le screw fix le screw fix
combined xation with
ation.
with Coflex translamina
x-F device ar facet scre
; (B) TLIF ew fixation;
F
;
Table 3.4 Intervertebral range of motion and applied moment among various surgical models under the hybrid test method.
Model ROM (deg) Total lumbar ROM(deg)
(L1-L5) Moment(Nm) L1-L2 L2-L3 L3-L4 L4-L5
Flexion
Intact 3.66 3.78 3.82 5.10 16.36 10 Coflex-F 4.37 4.54 0.96 6.38 16.25 12 UPSF + TFSF 4.38 4.56 0.61 6.43 16.01 12 BPSF 4.40 4.59 0.62 6.48 16.09 12 Extension
Intact 2.27 2.47 2.30 3.27 10.31 10 Coflex-F 3.26 3.17 0.43 4.01 10.87 11 UPSF + TFSF 3.26 3.17 0.23 4.17 10.83 11 BPSF 3.26 3.16 0.27 4.16 10.86 11 Right lateral bending
Intact 3.69 3.59 3.67 4.30 15.25 10 Coflex-F 4.08 3.97 2.40 5.00 15.45 11 UPSF + TFSF 4.46 4.35 1.23 5.48 15.54 12 BPSF 4.40 4.32 1.17 5.50 15.40 12 Left lateral bending
Intact 3.69 3.59 3.67 4.30 15.25 10 Coflex-F 4.11 4.03 1.95 5.07 15.16 11 UPSF + TFSF 4.48 4.36 0.99 5.55 15.40 12
BPSF 4.47 4.38 1.04 5.51 15.41 12 Right axial rotation
Intact 1.81 1.90 2.23 2.49 8.43 10 Coflex-F 2.00 2.09 1.52 2.80 8.41 11 UPSF + TFSF 2.28 2.24 0.77 3.01 8.32 13 BPSF 2.29 2.22 0.81 2.99 8.32 13 Left axial rotation
Intact 1.81 1.90 2.23 2.49 8.43 10 Coflex-F 2.01 2.09 1.44 2.81 8.35 11 UPSF + TFSF 2.28 2.33 0.77 3.02 8.42 13 BPSF 2.29 2.22 0.81 2.99 8.32 13
56
Chapter 4 Results
4.1 Coflex and Coflex-F in non-fusion surgery
Biomechanical behaviors of the lumbar spine with the Coflex model, the Coflex-F model, and the pedicle screw fixation model were compared with those of the intact model. Data were normalized with respect to the intact model as percentage values under each loading condition.
4.1.1 Range of motion (ROM)
In extension, the ROM increased 64 % in the defect model at the surgical segment (Figure 4.1). After implantation, the ROM effectively decreased 70 % in the Coflex model, 76
% in the Coflex-F model, and 90 % in the pedicle screw fixation model as compared with the intact model. In addition, the ROM increased 24 % in the Coflex and Coflex-F models at the adjacent L1-L3 segments and increased 20 % at the adjacent L4-L5 segment. The ROM increased 19 % in the pedicle screw fixation model at the adjacent L1-L3 segments and increased 25 % at the adjacent L4-L5 segment.
In flexion, the ROM increased 13 % in the defect model and 8 % in the Coflex model at the surgical segment (Figure 4.2). In contrast to the above two models, the ROM decreased 52
% in the Coflex-F and 68 % in the pedicle screw fixation models at the surgical segment. On the other hand, the ROMs of the defect model and the Coflex model were similar to that of the intact model at both adjacent L1-L3 (deviation within 4 %) and L4-L5 segments (deviation within 4 %). However, the ROM increased 17 % to 18 % in the Coflex-F model and 23 % to 24 % in the pedicle screw fixation model at both adjacent L1-L3 and L4-L5 segments.
In lateral bending, the ROM decreased 8 % in the Coflex model, decreased 20 % in the Coflex-F model, and decreased 51 % in the pedicle screw fixation model at the surgical
In lateral bending, the ROM decreased 8 % in the Coflex model, decreased 20 % in the Coflex-F model, and decreased 51 % in the pedicle screw fixation model at the surgical