BioMed Research International
Volume 2018 (2018), Article ID 8639648, 13 pages
https://doi.org/10.1155/2018/8639648

Research Article

Ti-24Nb-4Zr-8Sn Alloy Pedicle Screw Improves Internal Vertebral Fixation by Reducing Stress-Shielding Effects in a Porcine Model

Yang Qu, Shuang Zheng, Rongpeng Dong, Mingyang Kang, Haohan Zhou, Dezhi Zhao, and Jianwu Zhao

Department of Orthopedics, The Second Hospital of Jilin University, Changchun 130041, China

Correspondence should be addressed to Jianwu Zhao; zhao20130922@163.com

Received 4 October 2017; Revised 9 January 2018; Accepted 11 January 2018; Published 8 February 2018

Academic Editor: Weijie Fu

Copyright © 2018 Yang Qu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

To ensure the biomechanical properties of Ti-24Nb-4Zr-8Sn, stress-shielding effects were compared between Ti-24Nb-4Zr-8Sn and Ti-6Al-4V fixation by using a porcine model. Twelve thoracolumbar spines (T12–L5) of 12-month-old male pigs were randomly divided into two groups: Ti-24Nb-4Zr-8Sn (EG, ) and Ti-6Al-4V (RG, ) fixation. Pedicle screw was fixed at the outer edge of L4-5 vertebral holes. Fourteen measuring points were selected on the front of transverse process and middle and posterior of L4-5 vertebra. Electronic universal testing machine was used to measure the strain resistance of measuring points after forward and backward flexion loading of 150 N. Meanwhile, stress resistance was compared between both groups. The strain and stress resistance of measurement points 1, 2, 5, 6, 9, and 10–14 in Ti-24Nb-4Zr-8Sn fixation was lower than that of Ti-6Al-4V fixation after forward and backward flexion loading . The strain and stress resistance of measurement points 3, 4, 7, and 8 was higher in Ti-24Nb-4Zr-8Sn fixation than that of Ti-6Al-4V fixation . Stress-shielding effects of Ti-24Nb-4Zr-8Sn internal fixation were less than that of Ti-6Al-4V internal fixation. These results suggest that Ti-24Nb-4Zr-8Sn elastic fixation has more biomechanical goals than conventional Ti-6Al-4V internal fixation by reducing stress-shielding effects.

1. Introduction

With global population aging, the incidence of degenerative lumbar degeneration has increased [1]. Vertebral fusion has become a gold standard for the treatment of lumbar degenerative diseases in spine surgeons [2, 3]. Lumbar fusion of clinical application has been significantly improved in past decades, and rigid internal fixation technology has increased lumbar fusion rates [4, 5].

Low back pain induced by degenerative diseases is often experienced by the individuals beyond middle age. The standard treatment is decompression and interbody fusion, which is followed by a pedicle screw-rod system to stabilize spine [6, 7]. Conventional pedicle screw-rod system is made from the titanium alloy Ti-6A1-4V, which is biocompatible, corrosion resistant, bendable, and stiff enough to provide sufficient stability for spine [8]. However, the elastic modulus of Ti-6A1-4V (110 GPa) is much higher than that of cortical bone (18 GPa), and thus it changes spinal kinetics. After the implantation of Ti-6A1-4V, the motion of adjacent segments increases to compensate the reduced motion of implanted segments, which accelerate adjacent disc degeneration [9]. A stress-shielding effect, in which some stresses on vertebral bodies are shifted to pedicle-rod systems, decelerates intervertebral fusion and increases the looseness of pedicle screw. To eliminate these complications, some dynamic fixation systems have been developed. The most used one is the Dynesys Dynamic Stabilization System [10, 11], in which titanium alloy rods are replaced with polymolecular cords and spacers. However, the long-term clinical outcomes are not much better than expected [12].

Furthermore, due to strong stress-shielding effects caused by internal fixation, bone stress stimulation was significantly reduced, leading to bone reconstruction and absorption [13]. Biomechanical measurement of fusion techniques shows that all fusion will increase the stress of adjacent vertebrae [14]. Animal experiments also confirmed that strong internal fixation could cause stress occlusion and osteoporosis [15]. The increase of the stress near the segment after strong internal fixation accelerates the degeneration of adjacent segment disc [16, 17], which has become an urgent problem that needs to be solved after lumbar fusion.

In order to solve the problem caused by strong internal fixation, the researchers induce the concept of elastic fixation of spine and the premise of skeletal stability [18]. To reduce the load of the internal fixation and increase sharing load of spine, it is necessary to reduce the stress block and focus. At present, most researchers have applied intramedullary nailing or bone plate, joint replacement prosthesis implantation, and postoperative fixation of spine after internal fixation [19, 20]. However, the degree of occlusion remains unclear [21, 22]. The data show that there is no significant difference between the elastic internal and rigid internal fixation system in the therapy of lumbar degenerative lesions [23]. Low modulus beta-titanium alloys are hoped to provide good biocompatibility and alleviate stress-shielding effects. Titanium alloy Ti-6Al-4V is widely used for implants, which are characterized by high elastic modulus that may induce undesirable stress-shielding effects and toxicity [24]. Ti-24Nb-4Zr-8Sn alloy, as a low modulus (49 GPa), has been approved to a potential bone load-bearing implant material without toxicity [25].

However, the effects of Ti-24Nb-4Zr-8Sn and Ti-6Al-4V fixation on shielding stress were seldom compared. To ensure biomechanical properties of Ti-24Nb-4Zr-8Sn, this experiment is performed to compare the effects of Ti-24Nb-4Zr-8Sn and Ti-6Al-4V fixation on shielding stress. Before and after internal fixation, the strain and stress resistance values of the two kinds of internal fixation on vertebra was compared under the same flexion load.

2. Materials and Methods

2.1. Materials

Twelve thoracolumbar spines (T12–L5) were obtained from 12-month male pigs. Before obtaining the specimens, all spines had no damage and deformity after being scanned by X-ray. All vertebra were randomly assigned into elastic Ti-24Nb-4Zr-8Sn (EG, ) and rigid Ti-6Al-4V (RG, ) fixation groups. Scalpel, posterior longitudinal ligament, anterior longitudinal ligament, and facet joint were removed by scalpel and spatula (Figure 1(a)). The specimens were numbered and basic dimensions were measured (Figures 1(b)–1(d)). Lumbar spine specimens were embedded and fixed with denture base resin solution. Figure 1(e) showed the embedded species. Strain measurement was performed in both groups. The resistance of strain gauge was 121.4 ± 0.1 and gauge sensitivity coefficient was 2.14. Two kinds of pedicle screws Ti-24Nb-4Zr-8Sn and Ti-6Al-4V were used to fix vertebra as Figures 1(f) and 1(g) showed.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Figure 1: Specimen preparation. (a) Pig spine specimens with removal of muscle and other soft tissues. (b) Measurement of vertebral height. (c) Measurement of vertebral width. (d) Measurement of pedicle screw channel length. (e) Embedded specimen after fixation. (f) Ti-24Nb-4Zr-8Sn pedicle screw. (g) Ti-6A1-4V pedicle screw.

2.2. The Establishment of Measuring Points

With a finite element (FE) model and lumbar kinematic analysis according to earlier reports [26–28], biomechanical parameters with flexion loading were predicted. The geometry used in FE model of L4-5 was reconstructed across a porcine thoracolumbar spine. Before and after fixation at L4-5, the maximum strain and stress points were predicted after forward and backward flexion loading.

A total of 14 measuring points with possible high strain and stress resistance were predicted with a finite element (FE) model and lumbar kinematic analysis. Measuring points numbers 1 and 2 were found at left faces of L4-5 vertebra (Figure 2(a)). Measuring points numbers 3, 4, and 7–10 were found at middle faces of L4-5 vertebra (Figure 2(b)). Measuring points 5 and 6 were found at right faces of L4-5 vertebra (Figure 2(c)). Measuring points 11–14 were found at the posterior of L4-5 vertebra (Figure 2(d)). Figure 2(e) showed the measuring points used in Ti-24Nb-4Zr-8Sn group and Figure 2(f) showed measuring points used in Ti-6A1-4V group.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 2: Measuring points predicted with finite element (FE) model and lumbar kinematic analysis. (a) Left L4-5 vertebra (measuring points numbers 1 and 2). (b) Middle L4-5 vertebra (measuring points numbers 3, 4, and 7–10). (c) Right L4-5 vertebra (measuring points 5 and 6). (d) Posterior L4-5 vertebra (measuring points 11–14). (e) Measuring points used in Ti-24Nb-4Zr-8Sn pedicle screw group. (f) Measuring points used in Ti-6A1-4V pedicle screw group. Number 1 point for left pedicle screw fixed at outside the hole of the lumbar vertebrae; number 2 point for left pedicle screw fixed at outside the hole of the lumbar vertebrae; number 3 points for pedicle screw fixed at left side of the transverse of waist L4 vertebral body. Number 5 point for pedicle screw fixed at outside the hole of vertebral vertebrae; number 6 points for right pedicle screw fixed below the ground of the lumbar vertebrae; number 7 points for right pedicle screw fixed at transverse sites of lumbar vertebrae; number 8 points for right pedicle screw fixed at transverse process; number 9 point for right pedicle screw fixed at front of waist L4 vertebral body; number 10 point for right pedicle screw fixed on the 10th vertebrae of the lumbar vertebrae. Number 11 point was at the junction of left fixation rod and L4 pedicle screw. Number 12 point was at the junction of left fixation rod and L5 pedicle screw. Number 13 point was at the junction of right fixation rod and L4 pedicle screw. Number 14 point was at the junction of right fixation rod and L5 pedicle screw.

2.3. Measurement of Strain Resistance after Internal Fixation and Flexion Load

The load was transmitted by the load cell with 300 N ranges, and the displacement was transmitted by photoelectric encoder. First of all, each specimen was installed in the test machine. Each specimen was loaded and unloaded 20 times for preconditioning processing. The strain gauge wire of each measuring point of the specimen was connected with the bridge arm of the dynamic resistance strain gauge junction box and bridged in a half way. The temperature compensation was adapted to the external condition. A load of 150 N was applied to the specimen at a load with increasing rate of 2 mm/min (the load applied in this experiment was within the physiological load range). The strain resistance of each measuring point was measured by dynamic resistance strain gauge. After forward and backward flexion load, strain resistance was measured before pedicle screw fixation (Figures 3(a) and 3(b)). After forward flexion load, strain resistance was measured in Ti-24Nb-4Zr-8Sn group (Figure 3(c)) and Ti-6A1-4V group (Figure 3(d)) after pedicle screw fixation. Strain resistance was also measured in. After backward flexion load, strain resistance was measured in Ti-24Nb-4Zr-8Sn group (Figure 3(e)) and Ti-6A1-4V group (Figure 3(f)) after pedicle screw fixation.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 3: Strain resistance measurements. (a) After forward flexion load, strain resistance was measured before pedicle screw fixation. (b) After backward flexion load, strain resistance was measured before pedicle screw fixation. (c) After forward flexion load, strain resistance was measured in Ti-24Nb-4Zr-8Sn group after pedicle screw fixation. (d) After forward flexion load, strain resistance was measured in Ti-6A1-4V group after pedicle screw fixation. (e) After backward flexion load, strain resistance was measured in Ti-24Nb-4Zr-8Sn group after pedicle screw fixation. (f) After backward flexion load, strain resistance was measured in Ti-6A1-4V group after pedicle screw fixation.

2.4. Each Group of Specimens of Pedicle Screw Fixation after Internal Fixation

As Figure 3 showed, pedicle screw was fixed before application of flexion load. Two kinds of pedicle screw were manufactured by medical instrument company (Chunlizhengda Co., Beijing, China) by using titanium alloy Ti-24Nb-4Zr-8Sn (49 GPa) or Ti-6A1-4V (110 GPa). The lumbar posterior (L4, L5) was fixed with Ti-24Nb-4Zr-8Sn fixator or Ti-6A1-4V fixator. Ten points for pedicle screw fixation were determined. In order to measure the internal rod fixation and the junction of the strain, four additional points were selected for the internal fixation and pedicle screw at the junction (11–14 points). The measurement points were shown in Figure 2. The measuring points were the same between the EG and the RG groups (Figures 2(e) and 2(f)).

For the measurement of the strain gauge after pedicle screw fixation: the same model, batch, sensitivity coefficient, and resistance of strain gauge were used. The stress measurement was performed to measure the strain of the rats under the flexion load before the internal fixation of pedicle screw under flexion load. Ti-24Nb-4Zr-8Sn and Ti-6A1-4V fixation were performed under 150-N flexion load shown in Figures 2(c)–2(f).

2.5. Stress Calculation

The stress formula is provided as follows:

where σ is the stress, is the strain, and is the elastic modulus. In this experiment, the modulus of rigid internal Ti-6A1-4V fixation system is 110 GPa, and the modulus of the Ti-24Nb-4Zr-8Sn elastic internal fixation system is about 70 GPa [29], and the elastic modulus of the lumbar vertebrae is 19.2 GPa [29].

2.6. Statistical Analysis Methods

SPSS software package 16.0 SPSS (Chicago, IL, USA) was used for data analysis. The difference between the two types of variable data of variance was analyzed by using Scheffe method and paired t-test. for the statistical difference was significant.

3. Results

3.1. The Strain Resistance Values of Measuring Points between Ti-24Nb-4Zr-8Sn Internal Fixations Were Similar with Ti-6A1-4V before Internal Fixation

The strain resistance of Ti-24Nb-4Zr-8Sn fixation and Ti-6A1-4V fixation group was compared after flexion loading. Before alloy fixation, there was no significantly statistical difference for strain resistance after forward flexion loading (Table 1) or backward flexion loading (Table 2) between the EG and the RG groups . The results suggested that the specimens were similar with the same strain resistance values of measuring points between the EG and the RG groups.

Table 1: The strain resistance of Ti-24Nb-4Zr-8Sn and Ti-6A1-4V fixation groups under forward flexion loading before fixation (με, ×10⁻⁶).

Measuring points
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Sample 6
Mean ± SD

1

    EG
130
131
133
127
124
129
129 ± 3

    RG
138
145
133
140
137
143
139 ± 4

2

    EG
126
125
129
130
134
127
129 ± 3

    RG
127
134
139
136
129
135
133 ± 5

3

    EG
155
148
159
161
155
150
155 ± 5

    RG
157
153
156
161
159
164
158 ± 4

4

    EG
165
156
160
163
152
157
159 ± 5

    RG
165
159
163
157
160
159
161 ± 3

5

    EG
128
123
127
131
130
129
128 ± 3

    RG
126
135
133
130
128
129
130 ± 3

6

    EG
125
127
129
132
128
123
127 ± 3

    RG
129
126
134
133
125
127
129 ± 4

7

    EG
161
168
164
158
167
159
163 ± 4

    RG
159
164
162
159
169
168
164 ± 4

8

    EG
172
169
159
177
175
171
171 ± 6

    RG
170
165
170
157
165
172
167 ± 5

9

    EG
188
195
193
189
191
192
191 ± 3

    RG
184
188
197
189
196
190
191 ± 5

10

    EG
191
186
187
192
196
191
191 ± 4

    RG
187
190
186
195
188
193
190 ± 4

11

    EG
192
189
202
193
188
197
194 ± 5

    RG
189
191
200
195
192
201
195 ± 5

12

    EG
200
197
195
199
192
189
195 ± 4

    RG
196
188
190
200
196
196
194 ± 4

13

    EG
188
194
192
197
201
196
195 ± 4

    RG
192
198
188
192
205
200
196 ± 6

14

    EG
194
199
189
193
194
187
193 ± 4

    RG
189
195
192
183
187
195
190 ± 5

Table 2: The strain resistance of Ti-24Nb-4Zr-8Sn and Ti-6A1-4V fixation groups under backward flexion loading before fixation (με, ×10⁻⁶).

Measuring points
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Sample 6
Mean ± SD

1

    EG
−200
−189
−201
−205
−192
−197
−197 ± 6

    RG
−203
−187
−197
−199
−201
−197
−197 ± 6

2

    EG
−205
−196
−198
−192
−193
−201
−198 ± 5

    RG
−204
−196
−193
−201
−195
−203
−199 ± 5

3

    EG
−167
−174
−168
−172
−173
−169
−171 ± 3

    RG
−177
−176
−168
−175
−172
−169
−173 ± 4

4

    EG
−178
−169
−177
−169
−174
−170
−173 ± 4

    RG
−173
−169
−175
−174
−178
−165
−172 ± 5

5

    EG
−198
−202
−199
−195
−201
−197
−199 ± 3

    RG
−197
−194
−192
−196
−202
−200
−197 ± 4

6

    EG
−207
−209
−196
−203
−208
−195
−203 ± 6

    RG
−204
−202
−197
−193
−196
−199
−199 ± 4

7

    EG
−150
−162
−155
−157
−159
−161
−157 ± 4

    RG
−155
−161
−157
−154
−159
−163
−158 ± 3

8

    EG
−167
−178
−168
−172
−170
−169
−171 ± 4

    RG
−173
−175
−166
−161
−167
−169
−169 ± 5

9

    EG
−128
−136
−130
−132
−129
−135
−132 ± 3

    RG
−124
−133
−131
−127
−130
−128
−129 ± 3

10

    EG
−134
−137
−133
−137
−135
−130
−134 ± 3

    RG
−132
−130
−137
−135
−129
−131
−132 ± 3

11

    EG
−219
−216
−233
−229
−230
−227
−226 ± 7

    RG
−214
−213
−225
−231
−219
−232
−222 ± 8

12

    EG
−231
−227
−218
−231
−237
−226
−228 ± 6

    RG
−228
−231
−221
−242
−234
−220
−229 ± 8

13

    EG
−217
−241
−229
−236
−224
−238
−231 ± 9

    RG
−220
−233
−231
−232
−231
−229
−229 ± 5

14

    EG
−224
−223
−236
−240
−229
−237
−232 ± 7

    RG
−233
−220
−231
−236
−238
−241
−233 ± 7

3.2. The Strain Resistance of Ti-24Nb-4Zr-8Sn Was Higher Than That of Ti-6A1-4V after Internal Fixation

Strain measurement showed that the strain resistance of the Ti-24Nb-4Zr-8Sn fixation group at 1, 2, 5, 6, 9, and 10–14 measuring was lower than that of the rigid internal fixation under forward flexion loading after fixation (Table 3, ) and backward flexion loading (Table 4, ). In contrast, strain resistance of Ti-24Nb-4Zr-8Sn elastic internal fixation at 3, 4, 7, and 8 measuring points was higher than that of Ti-6A1-4V fixation under forward flexion loading after fixation (Table 3, ) and backward flexion loading (Table 4, ).

Table 3: The strain resistance of -24Nb-4Zr-8Sn and Ti-6A1-4V fixation groups under forward flexion loading after fixation (με, ×10⁻⁶).

Measuring points
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Sample 6
Mean ± SD

1

    EG
149
152
147
152
155
161
152.67 ± 4.93

    RG
188
179
187
193
196
189
188.66 ± 5.82

2

    EG
155
160
153
145
162
154
154.83 ± 5.98

    RG
193
196
201
186
193
182
191.83 ± 6.85

3

    EG
134
131
129
133
126
130
130.50 ± 2.88

    RG
105
97
99
102
106
98
101.17 ± 3.76

4

    EG
137
126
124
135
129
135
131.00 ± 5.40

    RG
107
103
104
99
103
101
102.83 ± 2.71

5

    EG
156
147
149
153
160
156
153.50 ± 4.85

    RG
203
192
183
179
197
185
189.83 ± 9.13

6

    EG
152
154
148
157
155
163
154.83 ± 5.04

    RG
189
198
195
184
204
197
194.50 ± 7.06

7

    EG
132
125
129
136
127
133
130.33 ± 4.08

    RG
96
103
105
102
98
96
100.00 ± 3.85

8

    EG
134
129
124
131
135
137
131.67 ± 4.72

    RG
99
97
108
107
106
103
103.33 ± 4.50

9

    EG
129
135
127
132
131
135
131.50 ± 3.21

    RG
159
165
161
157
163
162
161.17 ± 2.86

10

    EG
132
130
128
135
134
129
131.33 ± 2.80

    RG
164
157
163
160
159
166
161.50 ± 3.39

11

    EG
119
122
125
117
127
133
123.83 ± 5.81

    RG
152
149
162
153
148
157
153.50 ± 5.24

12

    EG
126
122
132
124
131
118
125.50 ± 5.36

    RG
160
157
155
159
152
149
155.33 ± 4.23

13

    EG
125
124
127
123
118
124
123.50 ± 302

    RG
148
154
152
157
161
156
154.67 ± 4.48

14

    EG
129
116
119
129
125
118
122.67 ± 5.75

    RG
154
159
149
153
154
147
152.67 ± 4.23

Note.  versus an RG group. Strain values are positive (tensile) due to elongation caused by forward flexion load.

Table 4: The strain resistance of Ti-24Nb-4Zr-8Sn and Ti-6A1-4V fixation groups under backward flexion loading after fixation (με, ×10⁻⁶).

Measuring points
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Sample 6
Mean ± SD

1

    EG
−245
−254
−249
−257
−261
−258
−254 ± 6

    RG
−310
−305
−319
−304
−318
−301
−310 ± 8

2

    EG
−251
−260
−257
−249
−255
−253
−254 ± 4

    RG
−321
−317
−306
−320
−309
−305
−313 ± 7

3

    EG
−109
−127
−130
−121
−129
−123
−123 ± 8

    RG
−92
−96
−89
−90
−88
−91
−91 ± 3

4

    EG
−121
−123
−107
−114
−125
−110
−117 ± 7

    RG
−87
−98
−85
−92
−96
−93
−92 ± 5

5

    EG
−260
−254
−257
−249
−262
−255
−256 ±5

    RG
−314
−321
−317
−305
−321
−318
−316 ± 6

6

    EG
−257
−248
−263
−246
−250
−259
−254 ± 7

    RG
−309
−307
−312
−318
−317
−311
−312 ± 4

7

    EG
−124
−114
−131
−109
−123
−122
−121 ± 8

    RG
−87
−84
−95
−97
−88
−93
−91 ± 5

8

    EG
−127
−109
−128
−117
−125
−120
−121 ± 7

    RG
−94
−89
−90
−93
−97
−85
−91 ± 4

9

    EG
−87
−83
−91
−82
−88
−90
−87 ± 4

    RG
−98
−117
−104
−117
−103
−119
−110 ± 9

10

    EG
−82
−90
−87
−84
−81
−89
−86 ± 4

    RG
−105
−110
−109
−121
−105
−114
−111 ± 6

11

    EG
−145
−151
−155
−150
−163
−158
−154 ± 6

    RG
−199
−196
−213
−209
−210
−207
−206 ± 7

12

    EG
−156
−148
−161
−157
−153
−148
−154 ± 5

    RG
−211
−207
−198
−211
−217
−206
−208 ± 6

13

    EG
−153
−147
−162
−147
−165
−154
−155 ± 8

    RG
−197
−221
−209
−216
−204
−218
−211 ± 9

14

    EG
−160
−155
−153
−156
−161
−146
−155 ± 5

    RG
−204
−203
−216
−220
−209
−217
−212 ± 7

Note.  versus an RG group. Strain values are negative due to compressive pressure caused by backward flexion load.

3.3. Stress Resistance of Ti-24Nb-4Zr-8Sn Was Lower Than Ti-6A1-4V after Internal Fixation

The stress resistance of Ti-24Nb-4Zr-8Sn and Ti-6A1-4V internal fixation after flexion load was compared. The results showed that stress resistance of Ti-24Nb-4Zr-8Sn fixation was lower than that of Ti-6A1-4V fixation after forward flexion loading (Table 5, ) and backward flexion loading (Table 6, ) at measuring points 1, 2, 5, 6, 9, and 10–14. In contrast, stress resistance of Ti-24Nb-4Zr-8Sn fixation was higher than that of Ti-6A1-4V fixation after forward flexion loading (Table 5, ) and backward flexion loading (Table 6, ) at measuring points 3, 4, 7, and 8. All the results suggest that strain and stress resistance showed more stress-shielding effects in the RG group than in the EG group.

Table 5: The stress resistance of Ti-24Nb-4Zr-8Sn and Ti-6A1-4V fixation groups under forward flexion loading after fixation (MPa).

Measuring points
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Sample 6
Mean ± S.D

1

    EG
2.86
2.92
2.82
2.92
2.98
3.09
2.93 ± 0.09

    RG
3.61
3.44
3.59
3.71
3.76
3.63
3.62 ± 0.11

2

    EG
2.98
3.07
2.94
2.78
3.11
2.96
2.97 ± 0.12

    RG
3.71
3.76
3.86
3.57
3.71
3.49
3.68 ± 0.13

3

    EG
2.57
2.52
2.48
2.55
2.42
2.5
2.51 ± 0.05

    RG
2.02
1.86
1.9
1.96
2.04
1.88
1.94 ± 0.08

4

    EG
2.63
2.42
2.38
2.59
2.48
2.59
2.52 ± 0.10

    RG    
2.05
1.98
2
1.9
1.98
1.94
1.98 ± 0.05

5

    EG
3
2.82
2.86
2.94
3.07
3
2.95 ± 0.09

    RG
3.9
3.69
3.51
3.44
3.78
3.55
3.65 ± 0.18

6

    EG
2.92
2.96
2.84
3.01
2.98
3.13
2.97 ± 0.10

    RG
3.63
3.8
3.74
3.53
3.92
3.78
3.73 ± 0.14

7

    EG
2.53
2.4
2.48
2.61
2.44
2.55
2.50 ± 0.08

    RG
1.84
1.98
2.02
1.96
1.88
1.84
1.92 ± 0.08

8

    EG
2.57
2.48
2.38
2.52
2.59
2.63
2.53 ± 0.09

    RG
1.9
1.86
2.07
2.05
2.04
1.98
1.98 ± 0.09

9

    EG
2.48
2.59
2.44
2.53
2.52
2.59
2.53 ± 0.06

    RG
3.05
3.17
3.09
3.01
3.13
3.11
3.09 ± 0.05

10

    EG
2.53
2.5
2.46
2.59
2.57
2.48
2.52 ± 0.05

    RG
3.15
3.01
3.13
3.07
3.05
3.19
3.01 ± 0.06

11

    EG
8.97
8.79
9.56
9.03
8.73
9.26
9.06 ± 0.31

    RG
13.09
13.42
13.75
12.87
13.97
14.63
13.62 ± 0.64

12

    EG
9.44
9.26
9.15
9.38
8.97
8.79
9.17 ± 0.25

    RG
13.86
13.42
14.52
13.64
14.41
12.98
13.80 ± 0.59

13

    EG
8.73
9.09
8.97
9.26
9.5
9.2
9.13 ± 0.26

    RG
13.75
13.64
13.97
13.53
12.98
13.64
13.58 ± 0.33

14

    EG
9.09
9.38
8.79
9.03
9.09
8.67
9.01 ± 0.25

    RG
14.19
12.76
13.09
14.19
13.75
12.98
13.49 ± 0.63

Note.  versus an RG group. Stress values are positive (tensile) due to elongation caused by forward flexion load.

Table 6: The stress resistance of Ti-24Nb-4Zr-8Sn and Ti-6A1-4V fixation groups under backward flexion loading after fixation (MPa).

Measuring points
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Sample 6
Mean ± SD

1

    EG
−4.7
−4.88
−4.78
−4.93
−5.01
−4.95
−4.88 ± 0.15

    RG
−5.95
−5.86
−6.12
−5.84
−6.11
−5.78
−5.94 ± 0.13

2

    EG
−4.82
−4.99
−4.93
−4.78
−4.9
−4.86
−4.88 ± 0.07

    RG
−6.16
−6.09
−5.88
−6.14
−5.93
−5.86
−6.01 ± 0.14

3

    EG
−2.09
−2.44
−2.5
−2.32
−2.48
−2.36
−2.37 ± 0.15

    RG
−1.77
−1.84
−1.71
−1.73
−1.69
−1.75
−1.75 ± 0.05

4

    EG
−2.32
−2.36
−2.05
−2.19
−2.4
−2.11
−2.24 ± 0.14

    RG
−1.67
−1.88
−1.63
−1.77
−1.84
−1.79
−1.76 ± 0.10

5

    EG
−4.99
−4.88
−4.93
−4.78
−5.03
−4.9
−4.92 ± 0.09

    RG
−6.03
−6.16
−6.09
−5.86
−6.16
−6.11
−6.07 ± 0.11

6

    EG
−4.93
−4.76
−5.05
−4.72
−4.8
−4.97
−4.87 ± 0.13

    RG
−5.93
−5.89
−5.99
−6.11
−6.09
−5.97
−6.00 ± 0.09

7

    EG
−2.38
−2.19
−2.52
−2.09
−2.36
−2.34
−2.31 ± 0.15

    RG
−1.67
−1.61
−1.82
−1.86
−1.69
−1.79
−1.74 ± 0.10

8

    EG
−2.44
−2.09
−2.46
−2.25
−2.4
−2.3
−2.32 ± 0.14

    RG
−1.8
−1.71
−1.73
−1.79
−1.86
−1.63
−1.75 ± 0.08

9

    EG
−1.67
−1.59
−1.75
−1.57
−1.69
−1.73
−1.67 ± 0.07

    RG
−1.88
−2.25
−2
−2.25
−1.98
−2.28
−2.11 ± 0.17

10

    EG
−1.57
−1.73
−1.67
−1.61
−1.56
−1.71
−1.64 ± 0.07

    RG
−2.02
−2.11
−2.09
−2.32
−2.02
−2.19
−2.13 ± 0.11

11

    EG
−11.74
−11.56
−12.57
−12.33
−12.39
−12.21
−12.13 ± 0.40

    RG
−15.95
−16.61
−17.05
−16.5
−17.93
−17.38
−16.90 ± 0.70

12

    EG
−12.45
−12.21
−11.68
−12.45
−12.8
−12.15
−12.29 ± 0.38

    RG
−17.16
−16.28
−17.71
−17.27
−16.83
−16.28
−16.92 ± 0.57

13

    EG
−11.62
−13.04
−12.33
−12.74
−12.04
−12.86
−12.44 ± 0.54

    RG
−16.83
−16.17
−17.82
−16.17
−18.15
−16.94
−17.01 ± 0.83

14

    EG
−12.04
−11.98
−12.74
−12.98
−12.33
−12.8
−12.48 ± 0.42

    RG
−17.6
−17.05
−16.83
−17.16
−17.71
−16.06
−17.07 ± 0.60

Note.  versus an RG group. Stress values are negative due to compressive pressure caused by backward flexion load.

4. Discussion

The strain and stress resistance of the Ti-24Nb-4Zr-8Sn and Ti-6A1-4V fixation increased after flexion load, but the increase in Ti-6A1-4V group was greater than that of Ti-24Nb-4Zr-8Sn group. Strain resistance of 10 measuring points in Ti-6A1-4V group was higher than that of Ti-24Nb-4Zr-8Sn group (Tables 3 and 4, ). Similarly, stress resistance of Ti-24Nb-4Zr-8Sn was also lower than that of Ti-6A1-4V group (Tables 5 and 6, ). Normally, bone material with low modulus elasticity is less resistant to outside stress, while the material with high-modulus elasticity is more resistant to outside stress. The results showed that the stress-shielding effects of Ti-24Nb-4Zr-8Sn fixation were small.

Stress conduction can be caused by lumbar degeneration [16], whereas back pain and other symptoms can be reduced or even disappear if lumbar dynamic internal fixation system precisely limits the activities of the vertebral body [30]. Theoretically, lumbar dynamic stability system reduces adjacent segment degeneration and the stress on the intervertebral bone graft and promotes bone healing and spine recovery. However, high-level stress will cause spinal cord compression and induce nerve tissue damage [31]. Lumbar Ti-24Nb-4Zr-8Sn fixation system is a dynamic stability system made of titanium alloy and polyether ether ketone (PEEK) materials [32]. The materials are safe for patient health and have high strength, excellent mechanical properties, good self-lubricating, corrosion resistance, wear, and other characteristics [33]. Its ultralow elastic modulus in a simple bar-shaped design can achieve a satisfactory dynamic fixation effect. Ti-24Nb-4Zr-8Sn fixation system can reduce the stress resistance of titanium pedicle screw when the stress resistance acts on the elastic connecting rod with certain microaction because polyethylene fiber elastic connecting rod instead of rigid connecting rod with titanium alloys is used. The system reduces vertebral osteoporosis and pedicle screw loosening, fracture, and other risks and maintains intervertebral space and active function.

The load of the segment slows down the degeneration of adjacent segment discs. After the internal fixation of the spine, biomechanical properties of the column are restored, the load sharing of the internal fixation system is reduced, and the reduction of internal fixation load is important to avoid fixation failure [34]. Selective stress occlusion is another way to address stress-induced osteoporosis [35]. Vertebral compression stress stimulates bone growth and strengthens. Selective stress occlusion makes the internal fixation mainly against lateral bending, rotation and shear stress, reducing the compressive stress on internal fixation, thereby increasing the compressive stress of the vertebral body and compressive stress of vertebra. All these results can improve mechanical performance. Ti-24Nb-4Zr-8Sn fixation improved patient's postoperative symptoms significantly when compared with fusion surgery with Ti-6A1-4V [36].

Lumbar Ti-24Nb-4Zr-8Sn internal fixation system has the following advantages: retaining motion function of the surgical segment while reducing the stress resistance of adjacent segments, delaying the occurrence of degeneration and even less damage to the disc, providing sufficient stability while retaining a certain degree of activity of lumbar spine, and delaying the occurrence of adjacent segment degeneration. This experiment is done by establishing a biomechanical model with Ti-24Nb-4Zr-8Sn fixation. The results showed that the effects of stress occlusion caused by Ti-24Nb-4Zr-8Sn fixation were less than those caused by Ti-6A1-4V fixation.

Limitations of the Present Work. There were some limitations of the present study: Ti-24Nb-4Zr-8Sn fixation lacks long-term randomized controlled outcome, and long-term clinical stability, screws loosening, fatigue of screw, and connecting rods remain unclear; the specimens used for the lumbar spine may cause different results when compared with other specimens; only Ti-24Nb-4Zr-8Sn and Ti-6A1-4V fixation were used to compare stress-shielding effects. In the posterior extension, compression, left and right bending, and stress need to be further analyzed in the future.

5. Conclusion

After fixation, strain and stress resistance of measurement points 1, 2, 5, 6, 9, and 10–14 in Ti-24Nb-4Zr-8Sn fixation was lower than that of Ti-6Al-4V fixation after forward and backward flexion loading. In contrast, the strain and stress resistance of measurement points 3, 4, 7, and 8 was higher in Ti-24Nb-4Zr-8Sn fixation than that of Ti-6Al-4V fixation. Stress-shielding effects of Ti-24Nb-4Zr-8Sn fixation were less than those of Ti-6Al-4V fixation. Ti-24Nb-4Zr-8Sn fixation has more biomechanical properties than conventional Ti-6Al-4V fixation by reducing stress-shielding effects.

Conflicts of Interest

The authors have no conflicts of interest to disclose.

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