Thurs., 10/14/10 Basic Science, Paper #34, 4:17 pm OTA-2010
A Comparison between the Use of Unicortical versus Bicortical Medial Malleolar Fixation with Lag Screws
Robert O. Crous, MD; Matthew P. Willis, MD; Timothy B. Ervin, BS; Peter J. Nowotarski, MD;
University of Tennessee, College of Medicine Chattanooga,
Dept. of Orthopaedic Surgery, Chattanooga, Tennessee, USA
Purpose: This study is a biomechanical assessment to test the hypothesis that a bicortical fixation would be stiffer and stronger than unicortical cancellous insertion. If this was indeed so, then bicortical fixation would be the preferred method for medial malleolar fixation, but especially so for fixation where the bone quality was poor.
Methods: 40 synthetic tibias with osteoporotic bone density were used for testing. A jig was constructed to create a reproducible oblique shear fracture of the medial malleolus (OTA type 44-A2.1). The bones were then divided into 2 groups of 20 and were fixated with 1 of 2 techniques. Group 1 was stabilized with two 3.5-mm fully threaded cortical screws and a bicortical lag screw technique was used. Group 2 was stabilized with two 4.0-mm partially threaded cancellous screws and a unicortical lag screw technique was used. Each group was split into two sets: in one set, force was applied in offset axial loading, and in the other, it was applied in offset transverse loading. Force was applied at a rate of 10 N per second and the construct was loaded to 800 N, 2 mm of displacement, or until catastrophic failure. The two groups were compared using Welch’s t test with 95% confidence intervals.
Results: Our results show statistically significant increases in construct stiffness for bicortical versus unicortical fixation in offset axial loading (685.56 N/mm vs 359.70 N/mm; P = 0.018) and max load to failure (770.14 N vs 539.62 N; P = 0.0126). The results also show statistically significant increases in construct stiffness for bicortical versus unicortical fixation in offset transverse loading (186.56 N/mm vs 102.75 N/mm; P < 0.0001) and max load to failure (370.49 N vs 184.26 N; P = 0.002).
Conclusion: On the basis of the positive results of this study, we recommend the use of bicortical fully threaded 3.5-mm screws inserted in a lagged manner. The bicortical purchase, as well as the increased thread engagement length, gives excellent and firm fixation in almost all cases. This is especially important in fractures with questionable bone quality, ie, osteoporotic fractures in the elderly and comminuted fractures in the young. In these cases, micromovement and the subsequent formation of fibrous rather than bony ingrowth are likely to be significantly decreased by the use of bicortical rather than the traditional unicortical screws, thus increasing the likelihood of a positive outcome.
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• The FDA has not cleared this drug and/or medical device for the use described in this presentation (i.e., the drug or medical device is being discussed for an “off label” use). ◆FDA information not available at time of printing. Δ OTA Grant.