gms | German Medical Science

German Congress of Orthopedic and Trauma Surgery (DKOU 2017)

24.10. - 27.10.2017, Berlin

Influence of Screw design on extraction torque of cement augmented pedicle screws

Meeting Abstract

  • presenting/speaker Nina Renner - Unfallchirurgie Universitätsklinikum Erlangen, Erlangen, Germany
  • Milena Pachowsky - Universitätsklinikum Erlangen, Unfallchirurgische Abteilung, Erlangen, Germany
  • Kolja Gelse - Universitätsklinik Erlangen, Unfallchirurgische Abteilung, Erlangen, Germany
  • Friedrich F. Hennig - Universitätsklinikum Erlangen, Unfallchirurgische Abteilung, Erlangen, Germany
  • Arnd Kleyer - Medizinische Klinik 3 Universität Erlangen, Erlangen, Germany
  • Fabian Stemmler - Medizinische Klinik 3 Universität Erlangen, Erlangen, Germany

Deutscher Kongress für Orthopädie und Unfallchirurgie (DKOU 2017). Berlin, 24.-27.10.2017. Düsseldorf: German Medical Science GMS Publishing House; 2017. DocPO20-1354

doi: 10.3205/17dkou737, urn:nbn:de:0183-17dkou7372

Published: October 23, 2017

© 2017 Renner et al.
This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License. See license information at http://creativecommons.org/licenses/by/4.0/.


Outline

Text

Objectives: The aim of the present study is to investigate the torque required to remove two different shaped fenestrated pedicle screws augmented with PMMA using two different models of osteoporotic spine (cadaveric spine and saw bone model). We further tested the influence of different biomechanical shape designs on the stability and safety of revising augmented screws in osteoporotic vertebrae.

Methods: To address this issue we used to different models for screw and cement application. Saw bone models and eight human osteoporotic cadaveric lumbar vertebrae were instrumented with 2 different types of cement augmented pedicle screws via a dedicated applicator. Extraction torque measurements were carried out manually by using a dedicated Newtonmeter adapted to a corresponding hand-driver. This was performed by an experienced surgeon to reproduce a realistic setting. An angle of 210° was regarded as sufficient and the maximum torque during this phase was recorded as the representative value.

To obtain cement volumina after explantation, models were measured using a high resolution peripheral quantitative computed tomography CT Scanner (XtremeCT; Scanco Medical AG, Brüttisellen, Switzerland) with an isotropic voxel size of 82 micrometer. Models were scanned before and after intervention. After removing the screws remaining cement was contoured and segmented using a threshold based technique using the standard software provided by the manufacturer. Thresholds were set after visual correction, cement was defined by values between 400-700.

Statistical evaluation was performed by using the unpaired t-test. All comparisons were made at a statistical significance level of 0.05.

Results and Conclusion: Mean torque values for the two different screw designs (Brehm vs Synthes) were 1.85 Nm (SD +/-0.52) vs 3.05 Nm (+/- 0.52) with a statistic difference in the saw bone model (p= 0.0103). Regarding the human osteoporotic vertebrae mean torque values were 1.60Nm (SD +/-0.4) vs. 2.48Nm (SD +/-0.6) with a statistic significance (p= 0.0411). Using the HR-pQCT technique we were able to obtain cement volume in mm³ in both models. The mean Volume in the saw bone model was 2047 mm³ (SD+/-81) vs. 1791 mm³ (SD+/-279) and 2399 mm³ (SD+/-148) vs. 2085 mm³ (SD+/-348) in the cadaver vertebrae, both with no statistic significant difference (p= 0.0649 and p=0.1787). The applied cement volume ranged from 1533 mm³ to 2559 mm³.

In this study we applied cement augmented screw construct in two different models. We were able to ex-torque screw devices in human cadaveric vertebrae without noticing a relevant damage in HR-pQCT measurements, whereas looking at the saw bone model we were not able to remove Synthes screw without damage. We did not observe differences in the volume of cement that was left in the models.

The different screw design showed a significant impact on the torque that was necessary to remove the pedicle screw from the cement mantle in the saw bone model and also led to the destruction of the specimen.