J Korean Neurosurg Soc.  2022 Nov;65(6):779-789. 10.3340/jkns.2021.0315.

An Experimental Study on the Biomechanical Effectiveness of Bone Cement-Augmented Pedicle Screw Fixation with Various Types of Fenestrations

Affiliations
  • 1Department of Neurosurgery, Armed Forces Capital Hospital, Seongnam, Korea
  • 2Research Center, Sociotech Co, Ltd., Seongnam, Korea
  • 3Research Laboratory, Department of Neurosurgery, Seoul National University Bundang Hospital, Seongnam, Korea
  • 4Department of Neurosurgery and Medical Device Development, Seoul National University College of Medicine, Seoul, Korea
  • 5Department of Neurosurgery, SMG-SNU Boramae Medical Center, Seoul, Korea
  • 6Department of Neurosurgery, Seoul National University College of Medicine, Seoul, Korea
  • 7Department of Neurosurgery, Seoul National University Bundang Hospital, Seongnam, Korea

Abstract


Objective
: To analyze the effects of the number and shape of fenestrations on the mechanical strength of pedicle screws and the effects of bone cement augmentation (BCA) on the pull-out strength (POS) of screws used in conventional BCA.
Methods
: For the control group, a conventional screw was defined as C1, a screw with cannulated end-holes was defined as C2, a C2 screw with six pinholes was defined as C3, and the control group type was set. Among the experimental screws, T1 was designed using symmetrically placed thru-hole type fenestrations with an elliptical shape, while T2 was designed with half-moon (HM)-shaped asymmetrical fenestrations. T3 and T4 were designed with single HM-shaped fenestrations covering three pitches and five pitches, respectively. T5 and T6 were designed with 0.6-mm and 1-mm wider fenestrations than T3. BCA was performed by injecting 3 mL of commercial bone cement in the screw, and mechanical strength and POS tests were performed according to ASTM F1717 and ASTM F543 standards. Synthetic bone (model #1522-505) made of polyurethane foam was used as a model of osteoporotic bone, and radiographic examinations were performed using computed tomography and fluoroscopy.
Results
: In the fatigue test, at 75% ultimate load, fractures occurred 7781 and 9189 times; at 50%, they occurred 36122 and 82067 times; and at 25%, no fractures occurred. The mean ultimate load for each screw type was 219.1±52.39 N for T1, 234.74±15.9 N for T2, 220.70±59.23 N for T3, 216.45±32.4 N for T4, 181.55±54.78 N for T5, and 216.47±29.25 N for T6. In comparison with C1, T1, T2, T3, T4, and T6 showed significantly different ultimate load values (p<0.05). However, when the values for C2 and the fenestrated screws were evaluated with an unpaired t test, the ultimate load value of C2 significantly differed only from that of T2 (p=0.025). The ultimate load value of C3 differed significantly from those of T1 and T2 (C3 vs. T1 : p=0.048; C3 vs. T2 : p<0.001). Linear correlation analysis revealed a significant correlation between the fenestration area and the volume of bone cement (Pearson’s correlation coefficient r=0.288, p=0.036). The bone cement volume and ultimate load significantly correlated with each other in linear correlation analysis (r=0.403, p=0.003).
Conclusion
: Fenestration yielded a superior ultimate load in comparison with standard BCA using a conventional screw. In T2 screws with asymmetrical two-way fenestrations showed the maximal increase in ultimate load. The fenestrated screws can be expected to show a stable position for the formation of the cement mass.

Keyword

Fenestration; Pedicle screw; Bone cement; Ultimate load; Pull-out strength

Figure

  • Fig. 1. Cross-sectional and gross photographs showing the characteristics of each type of screw used in the experiment, and external photos showing the characteristics of each screw group. A : A conventional screw (C1 group). B : A screw with a 2-mm-diameter cannulation and an end hole at the tip of the screw (C2 group). C : A screw with six pin holes with a diameter of 2 mm (C3 group). D : T1 with an elliptical-shaped fenestration and a two-way symmetric thru-hole. E : T2 with a two-way asymmetric half-moon (HM)-shaped fenestration. F : T3 with a one-way HM-shaped fenestration located more distally and occupying three pitches. G : T4 with a one-way HM-shaped fenestration occupying five pitches. H : T5 with a one-way HM-shaped fenestration that was 0.6 mm wider than that in T3. I : T6 with a one-way HM-shaped fenestration that 1.0 mm wider than that in T3. Arrowheads indicate the location and shape of the fenestration.

  • Fig. 2. Pull-out strength test using the MTS system (Bionix 858; MTS systems Corp., Minneapolis, MN, USA). The specimen was mounted on the test jig of a universal material testing machine.

  • Fig. 3. Test results according to ASTM F1717 test standard to analyze the mechanical properties and stability of fenestrated screws (T1). Every test was performed with a T1 screw six times, and the results were displayed on the graph in different colors. In the fenestration compression bending test, the average ultimate load value was 533.97±25.48 N, and the maximum displacement value was 16.58±3.52 mm. At this time, the yield load was 477.75 ±25.05 N, and the yield displacement was 11.00±0.96 mm. Stiffness was measured to be 50.66±4.10 N/mm. A : The tensile test results showed that the yield load was 439.65±36.87 N, and the ultimate load was 505.91±42.87 N. The yield displacement was 16.74±1.3 mm, and the maximum displacement was 23.13±2.66 mm. The stiffness was 28.43±0.58 N/mm. B : The torsional test was performed with an offset of 1.95°. The results of the torsional test were as follows : yield angle, 21.69°±1.23°; yield torque, 37.73±1.36 N·mm; ultimate torque, 46.17±0.87 N·mm; and stiffness, 1.9 ± 0.14 N/mm (C).

  • Fig. 4. Comparison of ultimate load values between the conventional augmented screw group and the fenestrated screw group. A : Comparison of independent paired t-test results between the C2 (cannulated type) and fenestrated screw groups. B : Comparison of C3 pinhole type screws with fenestrated screw groups.

  • Fig. 5. Maximum displacement (mm) (A), and the bone cement volume injected into each screw (B).

  • Fig. 6. Characteristics of the fenestrated screws in radiographic images obtained using fluoroscopy. C1 : A conventional screw without bone cement augmentation. C2 : Bone cement distribution at the tip of screws around the end hole. C3 : Bone cement distribution at the shaft of the screws near the pedicle level. T1-T6 : No flowed-out feature of bone cement at the position of the pedicle.


Reference

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