The strength of polyaxial locking interfaces of distal radius plates
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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