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How Does a Novel Monoplanar Pedicle Screw Perform Biomechanically Relative to Monoaxial and Polyaxial Designs?

  • Clinical Research
  • Published:
Clinical Orthopaedics and Related Research®

Abstract

Background

Minimally invasive spinal fusions frequently require placement of pedicle screws through small incisions with limited visualization. Polyaxial pedicle screws are favored due to the difficulty of rod insertion with fixed monoaxial screws. Recently, a novel monoplanar screw became available that is mobile in the coronal plane to ease rod insertion but fixed in the sagittal plane to eliminate head slippage during flexion loads; however, the strength of this screw has not been established relative to other available screw designs.

Questions/purposes

We compared the static and dynamic load to failure in polyaxial, monoaxial, and monoplanar pedicle screws.

Methods

Six different manufacturers’ screws (42 total) were tested in three categories (polyaxial, n = 4; monoaxial, n = 1; monopolar, n = 1) utilizing titanium rods. An additional test was performed using cobalt-chromium rods with the monopolar screws only. Screws were embedded into polyethylene blocks and rods were attached using the manufacturers’ specifications. Static and dynamic testing was performed. Dynamic testing began at 80% of static yield strength at 1 Hz for 50,000 cycles.

Results

In static testing, monoaxial and monoplanar screws sustained higher loads than all polyaxial screw designs (range, 37%–425% higher; p < 0.001). The polyaxial screws failed at the head-screw interface, while the monoaxial and monoplanar screws failed by rod breakage in the static test. The dynamic loads to failure were greater with the monoplanar and monoaxial screws than with the polyaxial screws (range, 35%–560% higher; p < 0.001). With dynamic testing, polyaxial screws failed via screw-head slippage between 40% and 95% of static yield strength, while failures in monoaxial and monoplanar screws resulted from either screw shaft or rod breakage.

Conclusions

All polyaxial screws failed at the screw-head interface in static and dynamic testing and at lower values than monoaxial/monoplanar screw designs. Monoplanar and monoaxial screws failed at forces well above expected in vivo values; this was not the case for most polyaxial screws.

Clinical Relevance

Polyaxial screw heads slip on the screw shank at lower values than monoaxial or monoplanar screws, and this results in angular change between the rod and pedicle screw, which could cause loss of segmental lordosis. The novel monoplanar screw used in this study may combine ease of rod placement with sagittal plane strength.

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Acknowledgments

The authors thank Daryl D’Lima for his guidance and the Shiley Center for Orthopaedic Research and Education for providing the facility in which to perform this study.

Author information

Authors and Affiliations

  1. Tabor Orthopedics, Division of MSK Group PC, Memphis, TN, USA

    Samuel R. Schroerlucke MD

  2. Shiley Center for Orthopaedic Research and Education at Scripps Clinic, La Jolla, CA, USA

    Nikolai Steklov BS

  3. San Diego Center for Spinal Disorders, La Jolla, CA, USA

    Gregory M. Mundis Jr MD & Behrooz A. Akbarnia MD

  4. Orthopaedic Surgery (private practice), Santee, CA, USA

    James F. Marino MD

  5. Division of Orthopaedic Surgery, Shiley Center for Orthopaedic Research and Education at Scripps Clinic, 10666 North Torrey Pines Road, MS116, La Jolla, CA, 92037, USA

    Robert K. Eastlack MD

Authors
  1. Samuel R. Schroerlucke MD
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  2. Nikolai Steklov BS
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  3. Gregory M. Mundis Jr MD
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  4. James F. Marino MD
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  5. Behrooz A. Akbarnia MD
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  6. Robert K. Eastlack MD
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Corresponding author

Correspondence to Robert K. Eastlack MD.

Additional information

One of the authors (JFM) certifies that he, or a member of his immediate family, is the founder and CEO and holds the majority interest in Trinity Orthopedics, LLC (San Diego, CA, USA).

The institution of one of the authors (RKE) received funding from the San Diego Spine Foundation, which has received grants from Trinity Orthopedics, LLC (San Diego, CA, USA).

The institution of one of the authors (NS) received funding from the San Diego Spine Foundation.

One of the authors (RKE) certifies that he, or a member of his immediate family, has received or may receive payments or benefits, during the study period, an amount of less than USD 10,000, from Aesculap, Inc (Center Valley, PA, USA); an amount of less than USD 10,000, from Synthes, Inc (West Chester, PA, USA); and an amount of USD 10,000 to USD 100,000, from Pioneer RTI (Marquette, MI, USA).

All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research editors and board members are on file with the publication and can be viewed on request.

This work was performed at Tabor Orthopedics (Memphis, TN, USA), Scripps Clinic (La Jolla, CA, USA), and San Diego Center for Spinal Disorders (La Jolla, CA, USA).

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Schroerlucke, S.R., Steklov, N., Mundis, G.M. et al. How Does a Novel Monoplanar Pedicle Screw Perform Biomechanically Relative to Monoaxial and Polyaxial Designs?. Clin Orthop Relat Res 472, 2826–2832 (2014). https://doi.org/10.1007/s11999-014-3711-x

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  • DOI: https://doi.org/10.1007/s11999-014-3711-x

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