Elsevier

Clinical Biomechanics

Volume 24, Issue 8, October 2009, Pages 637-641
Clinical Biomechanics

The strength of polyaxial locking interfaces of distal radius plates

https://doi.org/10.1016/j.clinbiomech.2009.06.004Get rights and content

Abstract

Background

Currently available polyaxial locking plates represent the consequent enhancement of fixed-angle, first-generation locking plates. In contrast to fixed-angle locking plates which are sufficiently investigated, the strength of the new polyaxial locking options has not yet been evaluated biomechanically. This study investigates the mechanical strength of single polyaxial interfaces of different volar radius plates.

Methods

Single screw–plate interfaces of the implants Palmar 2.7 (Königsee Implantate und Instrumente zur Osteosynthese GmbH, Allendorf, Germany), VariAx™ (Stryker Leibinger GmbH & Co. KG, Freiburg, Germany) und Viper™ (Integra LifeSciences Corporation, Plainsboro, NJ, USA) were tested by cantilever bending. The strength of 0°, 10° and 20° screw locking angle was obtained during static and dynamic loading.

Findings

The Palmar 2.7 interfaces showed greater ultimate strength and fatigue strength than the interfaces of the other implants. The strength of the VariAx™ interfaces was about 60% of Palmar 2.7 in both, static and dynamic loading. No dynamic testing was applied to the Viper™ plate because of its low ultimate strength. By static loading, an increase in screw locking angle caused a reduction of strength for the Palmar 2.7 and Viper™ locking interfaces. No influence was observed for the VariAx™ locking interfaces. During dynamic loading; angulation had no influence on the locking strength of Palmar 2.7. However, reduction of locking strength with increasing screw angulation was observed for VariAx™.

Interpretation

The strength of the polyaxial locking interfaces differs remarkably between the examined implants. Depending on the implant an increase of the screw locking angle causes a reduction of ultimate or fatigue strength, but not in all cases a significant impact was observed.

Introduction

Fractures of the distal radius account about one-sixth of all treated fractures and are therefore among the most common skeletal injuries (Graff and Jupiter, 1994, Jupiter, 1991). Anatomical reconstruction is occasionally demanding but essential for good functional results (Jupiter and Lipton, 1993). Therefore, the aim of the surgical treatment is proper anatomical reduction and subsequent stable fixation to promote functional post-surgical treatment. In the recent years promising results were reported for open reduction and internal fixation using uniaxial volar locking plates (Murakami et al., 2007, Orbay, 2005, Osada et al., 2008, Oshige et al., 2007). The major disadvantage of these first-generation locking plates is the predetermined screw path that inhibits customization to fracture pattern or to anatomically aberrance of the radius. Meanwhile, newer polyaxial locking plates are available, allowing a limited variation of the angle of screw insertion (angulation) to meet the surgeons requirements e.g. to catch specific bone fragments.

By now, a number of different polyaxial implants for the distal radius are available. Unfortunately, manufacturers usually do not reveal the actual stability of the polyaxial locking interfaces of their implants. Hence, the aim of our study was to investigate the strength of different polyaxial locking interfaces and to evaluate the influence of different locking screw angulation in regard to the locking strength. Therefore, we have comparatively tested three different polyaxial locking interfaces and three different screw insertion angles of 0°, 10° or 20°.

Section snippets

Implants

In our study we tested the polyaxial interfaces of three different volar distal radius plates shown in Fig. 1.

The locking screws of the implant Palmar 2.7 (Königsee Implantate und Instrumente zur Osteosynthese GbmH, Allendorf, Germany) have threaded conical heads. Polyaxial locking is achieved by discontinuous multiple threads in the holes on the plate allowing up to 20° tilting of the screw during insertion.

For the second implant VariAx™ (Stryker Leibinger GmbH & Co. KG, Freiburg, Germany)

Results

All results are summarized in Table 1.

Discussion

Uniaxial locking plates have acquired great popularity in orthopaedic surgery. The superior mechanical strength of these first generation locking plates in contrast to conventional plating was revealed in a variety of biomechanical investigations (Boswell et al., 2007, Kandemir et al., 2008, Levin et al., 2008). The relatively new polyaxial locking plates represent the consequent enhancement of the uniaxial, first-generation locking plates. Like in conventional plating they allow intraoperative

Conclusions

In this study we could show that the strength of the polyaxial locking interface greatly depends on the implant design. The design with multiple threads, although multiple recessed, is still almost twice as strong as the single threaded interface. Both of these form-closed designs are superior to the friction-locked interface. However, further investigation is needed to scrutinize the clinical relevance of our findings.

Conflict of interest

This study was supported by Königsee Implantate und Instrumente zur Osteosynthese GmbH, Allendorf, Germany with funding of 1000 Euro (approx. 1300 $) and reimbursement of all tested implants.

However, this involvement had no influence on study design, interpretation of data or the writing of the manuscript. The authors declare that they do not have any personal relationship to this company.

Acknowledgement

I would like to thank Andrea Just and Helena Dickinson for proofreading my English.

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