Thurs., 10/13/11 Basic Science, Paper #39, 4:59 pm OTA-2011
Do Locked Screws Work in Bent Plates?
Christina L. Boulton, MD1; Hyunchul Kim, MS2; Swapnil B. Shah, MD3; Scott P. Ryan, MD1; Thomas A. Metzger2; Adam H. Hsieh, PhD2; Robert V. O’Toole, MD1;
1R Adams Cowley Shock Trauma Center, Dept. of Orthopaedics, University of Maryland
School of Medicine, Baltimore, Maryland, USA;
2Orthopaedics Mechanobiology Lab, Dept. of Bioengineering, University of Maryland,
College Park, Maryland, USA;
3Alameda County Medical Center, Oakland, California, USA
Purpose: Locking plates often require bending to contour the plate to bony anatomy; however, little is known about the subsequent effects on locked screw function. The purpose of our study was to determine whether plate bending significantly changes the biomechanical properties of a locked screw-hole unit.
Methods: Small fragment 3.5-mm locking compression plates were used and a single coronal plane bend was placed in each test plate with the bend apex at a locking hole. Contoured test samples were created with bends at 5°, 15°, or 45° and a 45° test group was created in which a locking screw-hole insert was placed prior to bending. Ten plates were tested for each group and compared to non-bent controls in a step-wise cyclic loading protocol for a total of 10,000 cycles or until failure.
Results: Outcome variables included screw failure and number of cycles survived before failure. The Fisher exact test was used to compare survival between groups. There was a statistically significant difference between control and both the 15° (P = 0.006) and the 45° (P = 0.0007) groups. The 5° group showed a trend towards poorer survival but this did not reach statistical significance (P = 0.17). The average cyclic load (# cycles) survived was also poorer for all bent plate groups. The difference between controls (mean, 9004.5) and the 15° (mean, 5691.4; P = 0.027) and 45° (mean, 3827.6; P = 0.0002) groups was statistically significant. The mean cycles to failure in the 5° group was 16% lower than controls (mean, 7537.5 vs 9004.5); however, this did not reach statistical significance (P = 0.37). Analysis of variance comparing groups showed a statistically significant effect (P = 0.0002). The additional test group bent to 45° after placement of a locking screw-hole insert showed no significant difference from the regular 45° group in mean number of cycles survived or in protocol survival.
Conclusion: Plate bending at a locking hole of over 5° results in a statistically significant decrease in survival of the corresponding locked screw during a cyclic loading protocol. This effect cannot be prevented by the placement of a locking screw-hole insert prior to bending.Alphabetical Disclosure Listing (628K PDF)
• 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.