Effect of screw angulation and multiple insertions on load-to-failure of polyaxial locking system

PLoS One. 2023 Dec 11;18(12):e0295526. doi: 10.1371/journal.pone.0295526. eCollection 2023.

Abstract

Purpose: Polyaxial locking plates rely on the alignment between the thread-to-thread connections of the screw head and the plate hole. These implants have provided substantial support for surgeons. In particular, extended screw positioning have proven to be beneficial in the fixation of challenging fractures. This study aimed to investigate the mechanical properties of ChM 5.0 ChLP polyaxial screws inserted in off-axis trajectories, including multiple insertions and to correlate these parameters with the screw head and the plate hole thread-to-thread engagement.

Methods: Polyaxial locking screws were inserted into the plates at various angles (0°,10°,15°, -15° off-axis). Multiple time inserted screws were placed firstly at 15°, then 0° and finally -15° off-axis in the same plate hole. A microCT scan of the plate-hole and screw-head interface was conducted before destructive tests. Representative screws from each group were also examined by Scanning Electron Microscope.

Results: The standard insertion at 0° sustained the greatest maximum bending strength without relocation in the screw hole. Screws inserted at 10° and 15° (one time) showed a significant reduction in load-to-failure of up to 36% and 55%, (p = 0.001) (p = 0.001) respectively. Screws inserted at -15° after a maximum of three multiple insertions with angle shift, showed a total reduction in force of up to 70% (p = 0.001). A microCT analysis of thread engagement showed significant correlations. However, the results obtained for multiple insertions were highly variable.

Conclusions: ChM 5.0 ChLP polyaxial locking system has valuable properties that foster fracture fixation, providing various surgical options. Nevertheless, the freedom of off-axis placement and multiple insertions of the screws comes at the price of reduced force. When possible surgeons should minimize the angles of insertions.

MeSH terms

  • Biomechanical Phenomena
  • Bone Plates
  • Bone Screws
  • Fracture Fixation
  • Fracture Fixation, Internal / methods
  • Fractures, Bone* / diagnostic imaging
  • Fractures, Bone* / surgery
  • Humans
  • X-Ray Microtomography

Grants and funding

This study was supported by the Poznan University of Medical Sciences and Poznan University of Technology (0614/SBAD/1565, 0614/SBAD/1579 and 0614/SBAD/1547). Lukasz Lapaj received royalties from ChM as a consultant and instructor.(ChM Sp. z o.o., Lewickie, Poland). Other authors declare no competing interests. The funder provided support in the form of salaries for authors [LL], but did not have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. The specific roles of these authors are articulated in the ‘author contributions’ section.