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Peer-reviewedCC BY-NCActa Orthopaedica et Traumatologica Turcica

Length of the fractured level screw has no significant effect on biomechanical stability in short-segment thoracolumbar fixation—in vitro biomechanical study in a sheep model

  • Selahaddin Aydemir1
  • Anıl Murat Öztürk2
  • Ahmet Karakaşlı3
  • Reşit Buğra Hüsemoğlu4
  • Mustafa Çeltik5
  • Onur Süer6
  • Ömer Akçalı3
6 affiliations
  1. 1Department of Orthopedics and Traumatology, İzmir American Hospital, İzmir, Türkiye
  2. 2Department of Orthopaedics and Traumatology, Ege University School of Medicine, İzmir, Türkiye
  3. 3Department of Orthopaedics and Traumatology, Dokuz Eylül University School of Medicine, İzmir, Türkiye
  4. 4Department of Biomechanics, Dokuz Eylül University, İzmir, Türkiye
  5. 5Department of Orthopaedics and Traumatology, Ankara Oncology Research and Training Hospital, Ankara, Türkiye
  6. 6Department of Orthopaedics and Traumatology, İzmir City Hospital, İzmir, Türkiye
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Abstract

Objective: While pedicle screw placement at the fracture level improves stability in short-segment posterior instrumentation for thoracolumbar fractures, the optimal screw length remains controversial. The biomechanical effect of screw length at the fracture level has not been clearly defined under controlled conditions. This study compared short (≤50% vertebral body length) and long (≥70%) screws in a sheep burst fracture model.

Methods: Twenty-one fresh sheep thoracolumbar spines with experimentally induced L1 burst fractures (AO Type A4) were randomly divided into 3 groups: Group 1 (no fracture-level screw), Group 2 (short screws: ≤ 50% of the vertebral body length [20 mm]), and Group 3 (long screws: ≥ 70% of the body length [35 mm]). All specimens were instrumented with pedicle screw constructs. Stability was assessed under axial compression, flexion–extension, lateral bending, and axial rotation using a Shimadzu AG-X testing machine (Kyoto, Japan). Axial displacement was recorded to derive range of motion (ROM) as a primary indicator of construct stability. Post-hoc power analysis revealed 58% power to detect a 0.5 mm displacement difference; therefore, results are interpreted as “no statistically significant difference” rather than equivalence.

Results: Incorporating a fracture-level screw significantly improved stability vs. Group 1 (P < .01). No significant differences were observed between short and long screws in axial compression ROM (0.91 ± 0.19 mm vs. 0.89 ± 0.26 mm, P = .87), flexion/extension ROM (1.41 ± 0.20 mm [3.2° ± 0.5°] vs. 1.17 ± 0.24 mm [2.7° ± 0.6°], P = .12), lateral bending ROM (1.27 ± 0.31 mm [2.9° ± 0.7°] vs. 0.88 ± 0.11 mm [2.0° ± 0.3°], P = .09), or axial rotation failure displacement (80.44 ± 41.31 mm vs. 71.44 ± 29.51 mm, P = .23). Fragment displacement was significantly greater in the long-screw group (3.51 ± 1.25 mm) than in the short-screw group (0.00 ± 0.00 mm, P < .001).

Conclusion: Within the limitations of this sheep model and with only 58% power to detect a 0.5 mm difference, short screws (≤50% vertebral body length) demonstrated no statistically significant difference in biomechanical stability compared to long screws (≥70%) in short segment posterior fixation under the tested conditions. However, long screws were associated with significant anterior fragment displacement, a potentially clinically relevant finding requiring further computed tomography (CT)-based validation. These findings suggest a potential advantage of short screws in avoiding anterior fragment displacement without compromising biomechanical stability. However, given the study’s power limitations and the absence of CT-based fragment displacement validation, these results are preliminary. Clinical recommendations require confirmation in adequately powered human studies with formal equivalence testing and post-instrumentation CT imaging.

Cite this article as: Aydemir S, Ozturk AM, Karakasli A, et al. Length of the fractured level screw has no significant effect on biomechanical stability in shortsegment thoracolumbar fixation—in vitro biomechanical study in a sheep model. Acta Orthop Traumatol Turc., 2026; 60(5), 01021, doi: 10.5152/j.aott.2026.261021.

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Cite

BibTeXRIS
BibTeX
@article{aydemir2026length,
  title = {{Length of the fractured level screw has no significant effect on biomechanical stability in short-segment thoracolumbar fixation—in vitro biomechanical study in a sheep model}},
  author = {Selahaddin Aydemir and Anıl Murat Öztürk and Ahmet Karakaşlı and Reşit Buğra Hüsemoğlu and Mustafa Çeltik and Onur Süer and Ömer Akçalı},
  journal = {Acta Orthopaedica et Traumatologica Turcica},
  year = {2026},
  month = oct,
  doi = {10.5152/j.aott.2026.261021},
  url = {https://doi.org/10.5152/j.aott.2026.261021},
  note = {CC BY-NC},
}
RIS
TY  - JOUR
TI  - Length of the fractured level screw has no significant effect on biomechanical stability in short-segment thoracolumbar fixation—in vitro biomechanical study in a sheep model
AU  - Selahaddin Aydemir
AU  - Anıl Murat Öztürk
AU  - Ahmet Karakaşlı
AU  - Reşit Buğra Hüsemoğlu
AU  - Mustafa Çeltik
AU  - Onur Süer
AU  - Ömer Akçalı
PY  - 2026
DA  - 2026/10/23
JO  - Acta Orthopaedica et Traumatologica Turcica
DO  - 10.5152/j.aott.2026.261021
UR  - https://doi.org/10.5152/j.aott.2026.261021
AB  - Objective: While pedicle screw placement at the fracture level improves stability in short-segment posterior instrumentation for thoracolumbar fractures, the optimal screw length remains controversial. The biomechanical effect of screw length at the fracture level has not been clearly defined under controlled conditions. This study compared short (≤50% vertebral body length) and long (≥70%) screws in a sheep burst fracture model.  Methods: Twenty-one fresh sheep thoracolumbar spines with experimentally induced L1 burst fractures (AO Type A4) were randomly divided into 3 groups: Group 1 (no fracture-level screw), Group 2 (short screws: ≤ 50% of the vertebral body length [20 mm]), and Group 3 (long screws: ≥ 70% of the body length [35 mm]). All specimens were instrumented with pedicle screw constructs. Stability was assessed under axial compression, flexion–extension, lateral bending, and axial rotation using a Shimadzu AG-X testing machine (Kyoto, Japan). Axial displacement was recorded to derive range of motion (ROM) as a primary indicator of construct stability. Post-hoc power analysis revealed 58% power to detect a 0.5 mm displacement difference; therefore, results are interpreted as “no statistically significant difference” rather than equivalence.  Results: Incorporating a fracture-level screw significantly improved stability vs. Group 1 (P < .01). No significant differences were observed between short and long screws in axial compression ROM (0.91 ± 0.19 mm vs. 0.89 ± 0.26 mm, P = .87), flexion/extension ROM (1.41 ± 0.20 mm [3.2° ± 0.5°] vs. 1.17 ± 0.24 mm [2.7° ± 0.6°], P = .12), lateral bending ROM (1.27 ± 0.31 mm [2.9° ± 0.7°] vs. 0.88 ± 0.11 mm [2.0° ± 0.3°], P = .09), or axial rotation failure displacement (80.44 ± 41.31 mm vs. 71.44 ± 29.51 mm, P = .23). Fragment displacement was significantly greater in the long-screw group (3.51 ± 1.25 mm) than in the short-screw group (0.00 ± 0.00 mm, P < .001).  Conclusion: Within the limitations of this sheep model and with only 58% power to detect a 0.5 mm difference, short screws (≤50% vertebral body length) demonstrated no statistically significant difference in biomechanical stability compared to long screws (≥70%) in short segment posterior fixation under the tested conditions. However, long screws were associated with significant anterior fragment displacement, a potentially clinically relevant finding requiring further computed tomography (CT)-based validation. These findings suggest a potential advantage of short screws in avoiding anterior fragment displacement without compromising biomechanical stability. However, given the study’s power limitations and the absence of CT-based fragment displacement validation, these results are preliminary. Clinical recommendations require confirmation in adequately powered human studies with formal equivalence testing and post-instrumentation CT imaging.  Cite this article as: Aydemir S, Ozturk AM, Karakasli A, et al. Length of the fractured level screw has no significant effect on biomechanical stability in shortsegment thoracolumbar fixation—in vitro biomechanical study in a sheep model. Acta Orthop Traumatol Turc., 2026; 60(5), 01021, doi: 10.5152/j.aott.2026.261021.
SN  - 1017-995X
SN  - 2589-1294
ER  -