OTA 2005 Posters
Scientific Poster #61 Basic Science
Proximal Tibia Locking Plates: Bicortical versus Unicortical Screw Placement
Purpose: The use of locking plates for unstable bicondylar proximal tibia fractures (OTA classification 41-C2) has become a popular method of treatment because the screws can be set at a fixed angle, acting as a rigid strut such as with a blade plate. It has been recommended that the proximal screws are placed in a unicortical manner. Biomechanical comparison of unicortical screw placement versus bicortical screw placement for treating these unstable fractures has not been well studied. Our hypothesis is that with proximal locking plate screws, bicortical screw placement is stronger than unicortical screw placement.
Methods: Osteotomies were performed in the proximal tibia to reproduce a 41-C2 bicondylar fracture pattern. The 4.5-mm proximal tibia periarticular locking plates were applied to the tibiae using four proximal bicondylar locking screws and three screws distal to the fracture site.
We varied only the proximal bicondylar screws - one group consisted of bicortical screws and the second consisted of unicortical screws. Unicortical screws were placed 5 mm less than the medial cortex to minimize the effects of a longer lever arm. Alternate sides were used for each type of screw to minimize right/left variation.
Five pairs of elderly thawed fresh frozen cadaveric tibiae (four female and one male, ages 63 to 104 years) were used. The tibia were potted for use in the materials testing machine (Instron, Canton, MA). The medial tibia condyle was compressed up to 20 mm at the displacement rate of 0.417 mm/s. Comparison of stiffness, yield load, and maximum load was done between the unicortical/bicortical constructs using the paired Student's t-test.
Results: When compared to unicortical screw placement, bicortical screw placement was significantly stronger for both yield load (unicortical 164.30±52.23 N versus bicortical 318.02 ±61.67 N, P<0.001) and maximum load (unicortical 269.15±72.17 N compared to bicortical 501.12±87.92 N, P<0.005). Stiffness was not significantly different between the two constructs (unicortical 41.22±5.28 N/mm versus bicortical 45.96±2.08 N/mm, P=0.181).
Conclusions/Significance: Locking plates are frequently used for treatment of elderly patients with proximal tibia fractures. Our study suggests that if increased strength of a construct is needed, longer screws with bicortical purchase should be used.