Session VI - Basic Science
Sat, 10/9/04 Basic Science, Paper #33, 9:34 am
Plate Length Matters but Are Locked Screws Helpful? A Biomechanical Study of the Effect of Plate Length and Locked versus Unlocked Screws on the Fixation of Fractures
Purpose: We sought to identify the relative contributions of plate length, locked versus unlocked fixation, and number of cortices of fixation in a comminuted fracture model.
Methods: Twenty matched pairs of formalin-fixed cadaveric ulnas were divided into four groups of five pairs, with one specimen of each pair randomly assigned to be the control or experimental. Specimens were instrumented with an 8- or 10-hole small-fragment combiplate (Synthes) which allows screws to be placed in either a locked or unlocked configuration at each hole. A 1-cm ostectomy was performed in the center of the specimen to simulate a comminuted fracture. Control specimens were instrumented with an 8-hole combiplate with six cortices of fixation (unlocked) on each side of the fracture to replicate standard fixation. Experimental groups consisting of fixation with a near-far technique, filling the screw hole closest to the ostectomy site and the hole furthest away on each side of the fracture. Four groups were tested: group 1, 8-hole combiplate with four unlocked cortices on each side of the fracture; group 2, 10-hole combiplate with four unlocked cortices on each side; group 3, 8-hole combiplate with two screws (four cortices) locked on each side of the fracture; group 4, 10-hole combiplate with two screws (four cortices) locked. Specimens were tested in torsion to 5° of displacement and in four-point bending to failure with use of an MTS machine. Stiffness in torsion, four-point bending, maximum load at failure, and displacement at yield in four-point bending were recorded and compared between experimental and control groups as well as among the experimental groups.
Results: Plate length was the most important factor in stiffness and yield strength. The 10-hole unlocked plate was significantly stiffer then the 8-hole locked plate (P <0.001) and the 10-hole locked plate (P <0.009). Both of the 10-hole plates (locked and unlocked) had a higher load to failure (P <0.03) then the 8-hole plate and the control groups. There was no significant difference in torsion stiffness between any of the four groups. Locked fixation was less stiff but showed no difference in yield strength versus standard fixation. The near-far technique was equivalent to the standard technique for number of cortices of fixation.
Conclusion/Significance: In this comminuted ulnar fracture model, the use of longer plates with fewer screws yielded a stronger overall construct. The use of locking screws did not increase the overall strength of the construct and actually exhibited less stiffness then in the unlocked group. The results of this study would indicate that the length of plate and the screw configuration are more important then either number of screws providing fixation or use of locked screws. This finding has an application in indirect reduction techniques through small incisions with less soft tissue stripping, exposure, and stress risers from additional screw holes. In this model, the use of locking screws does not add benefit in strength or stiffness.