To prevent adjacent disc problems after spinal fusion, a pedicle screw with a mobile junction between the head and threaded shaft was newly developed. The threaded shaft of the screw has 10 degrees mobility in all directions, but its structure is to prevent abnormal translation and tilting. This screw was evaluated as follows: (1) endurance test: 106 times rotational stress was applied; (2) biological reactions: novel screws with a mobile head and conventional screws with a fixed head were inserted into the bilateral pedicles of the L3, L4, and L5 in two mini pigs with combination. Eight months after surgery, vertebral units with the screw rod constructs were collected. After CT scan, the soft and bony tissues around the screws were examined grossly and histologically. As a result, none of the screws broke during the endurance test stressing. The mean amount of abrasion wear was 0.0338 g. In the resected mini pig section, though zygapophyseal joints between fixed-head screws showed bony union, the amount of callus in the zygapophyseal joints connected with mobile-head screws was small, and joint space was confirmed by CT. No metalloses were noted around any of the screws. Novel screws were suggested to be highly durable and histologically safe.
Adjacent disc problems after spinal fusion have been considered unavoidable, and the overall annual incidence and predicted 10-year prevalence of further surgery are 2.5% and 22.2%, respectively [
The new pedicle screw is manufactured, which is made of titanium alloy and has a mobile polyaxial structure at the junction between the head and threaded shaft (Figure
Novel pedicle screw with mobile connection. Junction between the screw head and threaded shaft has a crown gear mechanism and 10° mobility in all directions.
Dimples were made on the contact surface of the thread shaft at the junction of the screw to contain the abrasion wear generated after implantation (Figures
Contact surface between screw head and threaded shaft in novel pedicle screw. Dimples on the contact surface contain the abrasion wear generated after implantation.
The mobile head of the novel pedicle screw was fixed in a servo-type fatigue tester from Shimadzu Corporation (Kyoto, Japan), and rotational stress was applied with a motor at an angle of ±5° relative to the threaded shaft, displacement of ±2.2 mm, and frequency of 5 Hz. The stress was applied 106 times, and the room temperature was adjusted to 25°C. Three screws were similarly tested, whether breakage occurred or not, and the amounts of abrasion powder were evaluated. The amount of abrasion wear was calculated by weighing the screw before and after the fatigue test. After durability testing, the screws were washed with acetone and weighed on an electronic balance AW320 (Simadzu). Three screws with no dimples on the contact surface were similarly tested.
Two 6-month-old domestic minipigs weighing 100 kg were anesthetized and screws were inserted into the bilateral pedicles at L3, L4, and L5 under fluoroscopic guidance. In one minipig, novel screws with a mobile head were inserted at L3, and conventional screws with a fixed head (Globalys; KISCO, Kobe Japan) were inserted at L4 and L5. In the other minipig, conventional screws with a fixed head were inserted at L3 and L4, and novel screws with a mobile head were inserted at L5 (Figure
Combination of mobile and fixed-head pedicle screw fixation.
None of the screws tested broke in the predetermined number of trials. The mean amount of abrasion wear, determined by weighing before and after the fatigue test, was 0.0338 g (Table
(a) Novel pedicle screw with dimples. (b) Novel pedicle screw with no dimples.
Time | Pretest (g) | Posttest (g) | Differences (g) |
---|---|---|---|
First | 9.7427 | 9.7133 | 0.0294 |
Second | 9.6567 | 9.6189 | 0.0378 |
Third | 9.6340 | 9.5999 | 0.0341 |
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Average | 0.0338 |
Time | Pretest (g) | Posttest (g) | Differences (g) |
---|---|---|---|
First | 9.7375 | 9.6967 | 0.0408 |
Second | 9.7518 | 9.7185 | 0.0333 |
Third | 9.7214 | 9.6834 | 0.0380 |
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Average | 0.0374 |
No gross metalloses were noted around any of the screws (Figure
Macroscopic findings. Removed vertebral column unit: circles indicate mobile-head pedicle screws.
CT findings of removed vertebral column unit. (a) 3D CT: coronal view. (b) Coronal section: bilateral L3/L4 zygapophyseal joints were not fused. (c) Right sagittal section: L4/L5 zygapophyseal joint was not seen, but L3/L4 zygapophyseal joint was clear. (d) 3D CT: sagittal view. (e) Left sagittal section: L4/L5 zygapophyseal joint was obscure, but L3/L4 zygapophyseal joint was clear. (f) Axial section: though bilateral, L4/L5 bilateral zygapophyseal joints were obscure and bilateral L3/L4 zygapophyseal joints were clear.
Microscopic findings of tissue around a novel pedicle screw. (a) Bony and granulation tissue near the screw head, (b) bony tissue near junction between screw head and shaft, and (c) bony tissue near screw tip.
To solve the problem of adjacent spinal segment disorder after spinal fixation, surgical procedures to treat spinal instability that preserve intervertebral mobility without fusion of vertebral segments have been developed [
Yasuaki Tokuhashi, MD, took (1) 15 thousand dollars from Funding for a Grant-in-Aid for Scientific Research 2010–2012 for “The Development of Spinal Stabilized Systems Retaining Intervertebral Mobility” and (2) stock ownership of pedicle screws and rods of Globalys from KISCO (Kobe, Japan). Masashi Oshima, MD, took (1) 15 thousand dollars from Funding for a Grant-in-Aid for Scientific Research 2010–2012 for “The Development of Spinal Stabilized Systems Retaining Intervertebral Mobility.” Yasumitsu Ajiro, MD, took (1) 15 thousand dollars from Funding for a Grant-in-Aid for Scientific Research 2010–2012 for “The Development of Spinal Stabilized Systems Retaining Intervertebral Mobility.” Hiroshi Uei, MD, declares that there is no conflict of interests regarding the publication of this paper.
The authors acknowledge funding for a Grant-in-Aid for Scientific Research 2010–2012 for “The Development of Spinal Stabilized Systems Retaining Intervertebral Mobility.” And authors also acknowledge stock ownership of pedicle screws and rods of Globalys from KISCO (Kobe, Japan).