Fri., 10/19/07 Basic Science, Paper #43, 4:48 pm OTA-2007
How Much Does the Addition of Locked Screws Add to the Stability of “Hybrid” Plate Constructs in Osteoporotic Bone?
Andy L. Freeman, MSME1 (n); Paul Tornetta III, MD2 (e-Smith+Nephew);
Andrew Schmidt, MD3 (e-Smith+Nephew); Joan Bechtold, PhD1 (n);
William Ricci, MD4 (e-Smith+Nephew); Mark Flemming, MD3 (n);
1Gustilo Medical Education Center, Minneapolis, Minnesota, USA;
2Boston University Medical Center, Boston, Massachusetts, USA;
3Hennepin County Medical Center, Minneapolis, Minnesota, USA;
4Washington University, St. Louis, Missouri, USA
Introduction: Hybrid plating refers to the use of locked and unlocked screws in one construct. It has been shown that the use of all locked screws is not an advantage in normal bone, but creates a stiffer construct with better fatigue properties in osteoporotic bone. We sought to determine the contributions of locked screws placed to create a hybrid construct on the fatigue properties in an osteoporotic bone model.
Materials: A prefabricated osteoporotic model was used for reproducibility (composite cylinders 35 mm in diameter and consisting of a 2.5-mm fiberglass shell filled with 10 lb/ft3 polyurethane). A 5-mm gap model was used, and fixed with a 12-hole plate (Smith & Nephew, Memphis, TN). Four constructs were tested. All screws were bicortical and placed with 4 Nm of torque. Baseline removal (loosening) torque was recorded for each screw and compared with final removal torque after cyclic loading. Testing was performed with ±8 Nm of torsional load after prestress and run to 100,000 cycles. Stiffness of each construct was measured at 10,000 cycle increments and the removal torque of each screw was recorded at the conclusion of the 100,000 cycles.
Results: Two-way analysis of variance testing was used to compare the stiffness and removal torque for all data points (to account for multiple comparisons). The exchange of one of the unlocked screws in favor of one locked screw on each side of the gap (construct #2 vs construct #1) did not increase the stiffness of the construct (P = 0.34), or the removal torque of the screws (P = 0.3-0.9). The addition of one locked screw on each side (construct #3) increased the stiffness by 25% (P <0.001) and increased the removal torque of the screws adjacent to the locked screws, particularly in the last hole (P = 0.02). Finally, the exchange of one unlocked screw for a locked screw and the addition of a second locked screw (construct #4) resulted in the stiffest construct (P <0.001) and also the least loosening over the 100,000 cycles. The stiffness for construct #1 and #2 decreased by 25% over the first 20,000 cycles and 35% over 100,000 cycles compared with only 14% and 21% for constructs #3 and #4. Removal torque for the unlocked screws closest to the fracture site were the lowest, and this value did not vary between the constructs (P >0.9), indicating that these screws bear significant stress and are not protected by locked screws farther from the gap, unlike the most distal screws, which are shielded by the locked screws that are closer to the gap.
Conclusions: In a consistent osteoporotic model, the addition of one locked screw on each side of the gap did not appreciably increase the stiffness of the construct or removal torque of the screws as compared with the unlocked situation. The addition of a second locked screw on each side substantially increased the stiffness, maintained stability, and prevented loosening of all of the screws as compared with the other constructs. We recommend the use of at least two locked screws in the diaphysis in hybrid constructs to maximize the advantages of this plating technique.
If noted, the author indicates something of value received. The codes are identified as a-research or institutional support; b-miscellaneous funding; c-royalties; d-stock options; e-consultant or employee; n-no conflicts disclosed, and *disclosure not available at time of printing.
• 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.