Biomechanical comparison of three different compression screws for treatment of odontoid fractures evaluation of a new screw design

https://doi.org/10.1016/j.clinbiomech.2020.105049Get rights and content

Highlights

  • The quotient of maximum and plateau torque can be improved by optimal predrilling.

  • The application torque can be indicative of the stripping torque.

  • Compression force and torque are correlated for compression and sleeve-nut screws.

  • Double-threaded and sleeve-nut screws show a higher tolerance to screw tearing.

  • Sleeve-nut screw is superior in compression.

Abstract

Background

Lag screw osteosynthesis in odontoid fractures shows a high rate of pseudarthrosis. Biomechanical properties may play a role with insufficient fragment compression or unnoticed screw stripping. A biomechanical comparison of different constructed lag-screws was carried out and the biomechanical properties determined.

Methods

Two identical compression screws with different pilot holes (1.25 and 2.5 mm), a double-threaded screw and one sleeve-nut-screw were tested on artificial bone (Sawbone, densities 10-30pcf). Fragment compression and torque were continuously measured using thin-film force sensors (Flexiforce A201, Tekscan) and torque sensors (PCE-TM 80, PCE GmbH).

Findings

The lowest compression reached the double-threaded screw. Compression and sleeve-nut-screw achieved 214–298% and 325–546%, respectively, of the compression force of double-threaded-screw, depending on the test material. The pilot hole optimization led to a significant improvement in compression only in the densest test material. Screw stripping took place significantly later with increasing density of the test material on all screws. In compression screws this was done at a screw rotation of 180–270°, in sleeve nut screw at 270–720° and in double-threaded screws at 300–600° after reaching the maximum compression.

Interpretation

Double-threaded screw is robust against screw stripping, but achieves only low fragment compression. The classic compression screws achieve better compression, but are sensitive to screw stripping. Sleeve-nut screw is superior in compression and as robust as double-threaded screw against screw stripping. Whether the better biomechanical properties lead to a reduction in pseudarthrosis must be proven in clinical trials.

Section snippets

Background

Anterior screw osteosynthesis in patients with Anderson's and D'Alonzo's odontoid type II fracture (Anderson and D'Alonzo, 1974) is the only operative procedure with movement preservation for this type of injury. At the same time, surgical treatment improves survival at a comparable complication rate compared to conservative treatment such as halo-fixation or orthosis immobilization. However, there is a non-union rate of 27% (Robinson et al., 2014). Mechanical factors such as tissue

Investigated compression screws

Three different lag screws for the treatment of the odontoid fracture with different compression principles were selected for testing. These are a classic cannulated spongiosa screw with smooth shaft and screw head (CS, DePuy Synthes, Raynham, US), a non-cannulated double threaded or Herbert screw (Ulrich Medical, Ulm, Germany) and a cannulated spongiosa screw with shaft and sleeve nut (Signus Medizintechnik GmbH, Alzenau Germany) (Figure 1).

The technical details are shown in Table 1. The

Density of the test material, compression force, and torque

All screws show a significant increase in both maximum compressive force and torque with increasing density of the test material (Table 2).

Compressive force

The generated compressive force increases significantly (p < .001) for screws CS 1.25, CS 2.5 and SLN as the density of the test material increases. DTS shows a significant increase in compression force only when the test material density increases by at least 10 pcf. SLN shows superior compression in all densities over all other types of screws (Fig. 3).

Interpretation

The tested lag screws for the treatment of odontoid fractures type II according to Anderson and De'Alonzo (Anderson and D'Alonzo, 1974) show different characteristics in the phase of optimal fragment compression as well as a different vulnerability to overtightening and screw stripping.

Limitations of the experimental technique

The test material used has a very homogeneous structure, which only resembles ideal cancellous bone. A possible contact of the cancellous bone thread with cortical or sclerosed bone is not taken into account. Likewise, a possible bicortical implantation of the cancellous bone screw is not included in the test series. The manual insertion of the screw allows the determination of the moment of cutting the screw only in steps of 90°. The rotational stability of the connection between the two test

Conclusions

  • The maximum compression force increases with the screw diameter.

  • The quotient of maximum torque and plateau torque can be improved by optimizing the pre-drilling.

  • The application torque can be indicative of the stripping torque.

  • Compression force and torque are only correlated for CS 1.25, CS 2.5 and SLN.

  • DTS and SLN show a higher tolerance to screw tearing than classic CS.

  • SLN is superior in compression and as robust as DTS against screw stripping.

  • It is unclear how much compression is required for

Funding

No funding was received for this research.

Studies with human participants

This article does not contain any studies with human participants performed by any of the authors.

Declaration of Competing Interest

Jan-Uwe Müller:

Dr. Müller is consultant for Signus and received financial support for travel costs and honoraria for consultation.

Dr. Müller has no other financial relationship to Signus, no direct or indirect profit sharing arrangements, rebates, commissions or compensations in any form.

All other authors:

All other authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in

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