Effects of surgical errors on small fragment screw fixation

Objectives: The purpose of this study is to determine the effects of technical errors that occur during the application of small fragment screw fixation and to assess which screw holes can be salvaged. Intervention: Testing of screw pullout from a bone substitute model on a universal testing instrument (Instron Corp., Canton, MA). Outcome Measurements: Testing was performed on 9 sets of 12 small fragment screws applied to a bone substitute model using the instruments available in a small fragment set (Synthes, Paoli, Pa). In the first 2 sets, 3.5-mm cortical screws and 4.0-mm cancellous screws were placed using the proper instrumentation according to recommended AO/ASIF techniques. The other 7 sets were inserted using "incorrect" methods: a single step was altered intentionally to assess its influence on fixation strength. The third set of screws included 3.5-mm cortical screws placed after drilling the pilot hole with a 3.5-mm drill. For the fourth set, the 2.5-mm drill was used, but the hole was tapped using the 4.0-mm cancellous tap before insertion of a 3.5-mm cortical screw. In set five, 4.0-mm cancellous screws were placed after tapping the hole with a 3.5-mm cortical tap. Set 6 included cancellous screws placed without tapping. The seventh set included 3.5-mm cortical screws that were placed according to recommended methods, and then removed and replaced into the screw hole. Set number 8 included 3.5-mm cortical screws, which were inserted correctly and then stripped by overtightening. The ninth set included 3.5-mm cortical screws that were stripped as those in set 8; the stripped screws were removed, the holes were packed with bone material, and the screws were replaced. All screws were inserted to a thread depth of 32mm. Results: Drilling a 3.5-mm pilot hole for a 3.5-mm cortical screw and "stripping" the screw by overtightening resulted in 76% and 82% less pullout strength, respectively, than when the proper technique was used (P < 0.01). Use of the wrong tap before placement of a 3.5-mm cortical or 4.0-mm cancellous screw decreased pullout strength by 12% and 11%, respectively (P < 0.01). Exchanging screws of similar geometry had no significant effect on screw pullout strength (P > 0.1). Inserting a 4.0-mm cancellous screw without tapping actually increased pullout strength by 4%'(P < 0.01). Conclusions: Alterations from recommended techniques for the placement of orthopedic screws had varying effects on screw fixation, as assessed by the pullout strength. Clinically, these findings indicate that, in some cases, a screw hole that was not initially placed according to the optimal technique may be salvaged. Finally, the authors recommenend that careful vigilance be maintained at all times in surgery and that fixation be applied according to sound principles in an effort to avoid some of these problems.