SLR - August 2018 - Audris M. Tien

Not All Polyaxial Locking Screw Technologies Are Created Equal: A Systematic Review of the Literature

Reference: Schoch B, Hast MW, Mehta S, Namdari S. Not All Polyaxial Locking Screw Technologies Are Created Equal: A Systematic Review of the Literature. JBJS Rev. 2018 Jan;6(1):e6.

Scientific Literature Review

Reviewed By: Audris M. Tien, DPM
Residency Program: Chino Valley Medical Center, Chino, CA

Podiatric Relevance: There is a plethora of fixation options for fractures on the market today. Polyaxial locking designs are relatively new to the scene and introduce additional costs compared to the traditional nonlocking or fixed-angle locked screw-plate constructs. Currently, four different variable-angle locking mechanisms are available in the United States; however, strong biomechanical data supporting these constructs remains limited. This systematic review aims to educate the reader on the different mechanisms used to achieve locking in these polyaxial screw designs and to compare biomechanical performance of these constructs.

Methods: This systematic review was reported according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. An electronic database search of PubMed and Scopus was conducted through May 12, 2016 using the keywords “polyaxial locking,” “variable angle locking,” “polyaxial screws” and “variable angle screws.” Inclusion criteria included biomechanical studies written in English that directly compared polyaxial locking screw systems. Exclusion criteria included review papers, case reports, clinical papers and technique papers.

All studies used a cantilever bending system model that tested ultimate strength, fatigue strength and failure modes. Ultimate strength was tested at a neutral insertion angle and at various degrees of angular offset.

Results: Polyaxial screw designs achieve locking fixation through a variety of mechanisms: 1) locking cap 2) point-loading thread-in 3) cut-in 4) expansion bushing and 5) screw head expansion. Four articles were included for analysis to compare biomechanical performance of these designs. Given the variability in study design and outcome measures reported, pooled statistics could not be generated. Locking cap constructs were shown to provide the best strength, regardless of the insertion angle. Both point-loading thread-in and cut-in designs showed decreasing strength with increasing angular offset.

Conclusions: Biomechanical testing of several variable-angle locking constructs that achieve locking fixation by different mechanisms show that locking-cap constructs provide the highest ultimate strength, regardless of insertion angle. However, sparse clinical data supports the superiority of locking-cap constructs. After a review of the available literature, there remains concern that certain variable-angle locking mechanisms may not be sufficiently strong to prevent clinical failures. It is important for the surgeon to be familiar with and to understand the different mechanisms by which variable-angle locking constructs work. 

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