DYNAMIC LOCKING SCREW (DLS) – BIOMECHANICAL TESTING OF A NEW SCREW SYSTEM

Abstract

Introduction: Standard treatment for distal tibia fractures is the fixation with locking compression plates. Locking plate fixation has revolutionized fracture treatment in the last decade and may be ideally suited for a bridging plate osteosynthesis. This technique allows some controlled axial fracture motion, what essential for secondary bone healing is. A disadvantage of the locking plate technique seems to be an unsymmetrical micro motion along the fracture gap. The micromotion at the far cortex side is much larger than at the near cortex side (near the plate). It is supposed to be that the fracture movement on the near cortex is too small.

To increase the motion at the near cortex side a new kind of screws has been developed. In this study we examined the micromotion using normal locking head screws versus the new dynamic locking head screws.

Materials and Methods: A simplified fracture model was created by connecting 2 plastic cylinders (POM C, EModul: 3.1GPa) with a standard 11-holes Locking Compression Plate (Synthes). The fracturegap (between the two cylinders) amounted 3mm. Three kinds of fracture models were constructed: The model of a transverse fracture, an oblique fracture and a spiral fracture. An axial load from 0N up to 200N was applied with a testing machine (Zwick). The motion of the fracture model was measured in three dimensions using the optical measurement system PONTOS 5M (GOM, Braunschweig, Germany). The accuracy of the optical measurement system was about 5 micrometers.

Results: A total of 72 measurements were compared. Using the new screw, axial stiffness was decreased for 16% and micromotion was up to 200 μm higher in comparison to the old screw.

Discussion: Using the new dynamic locking head screw it’s possible to increase interfragmentary motion up to 200μm on the near cortex side (plate side).