Sat., 10/10/09 Basic Sci./Injury Prevent./Spine, Paper #69, 10:50 am OTA-2009
A Biomechanical Testing Model That Matches Clinical Failure Modes of Locked Plate Constructs
William M. Ricci, MD1 (3, 4, 5A, 7-Smith &Nephew; 4, 7-AO, 4, 5A-Wright Medical
Technology, Inc.; 5A, 7-Synthes); Jacob Cartner BA2 (5-Smith &Nephew);
Yanming Zheng, PhD3 (5-Smith &Nephew); Bob Jones, BA3 (5-Smith &Nephew);
Zane Hartsell3 (5-Smith &Nephew);
Paul Tornetta, III, MD2 (3,5A, 7-Smith &Nephew; 8-Exploramed);1Washington University School of Medicine, Saint Louis, Missouri, USA;
2Boston University School of Medicine, Boston, Massachusetts, USA;
3Smith &Nephew, Inc., Memphis, Tennessee, USA
Purpose: Biomechanical evaluations of plate-screw constructs generally produce failure modes that are different than those seen clinically. Static testing to failure may provide insight regarding ultimate construct strength, but results of such tests are less relevant to failures seen clinically that occur after repetitive cyclic loading. The vast majority of in vitro cyclic loading studies utilize low-cycle counts and loading in a single orientation. These typically lack catastrophic failure or result in failure modes that are uncommonly encountered clinically. The purpose of this study was to compare clinical failure behaviors of a specific locked plating construct to a similar construct tested biomechanically under a physiologically relevant high-cycle combined loading condition of simultaneous axial load, bending, and torsion in an osteoporotic human cadaveric distal femoral fracture model.
Methods: Failure modes after locked plate fixation of comminuted supracondylar fractures in a clinical cohort and a biomechanical study were compared. A retrospective review (IRB-approved) included 34 consecutive patients >60 years of age with comminuted supracondylar fractures (OTA 33A3, 33C2, 33C3) treated with a distal femoral locking plate. The biomechanical evaluation included 8 osteoporotic cadaveric femora with an extra-articular gap created to simulate metaphyseal comminution. All specimens were instrumented with a distal femoral locking plate construct consisting of four 4.5-mm proximal cortical screws and five 5.7-mm cannulated locked distal screws. The left and right femurs of matched pairs were randomly assigned to either locked or nonlocked diaphyseal fixation. After implantation, the femur was resected below the lesser trochanter and potted such that femur motion was not restricted. Custom-made loading fixtures were used to facilitate a physiological combined loading configuration (axial load, varus bending, and torsion) based on a standard test (ISO 7206-4) used in evaluations of hip arthroplasty stems. Specimens were subjected to a staircase cyclical load starting at 150 lbf, with an increased load increment of 25 lbf every 25,000 cycles until failure.
Results: The clinical modes of failure were mimicked by the biomechanical model. 11 of the 34 patients showed evidence of construct failure. Fixation failure was in the proximal fragment in 8 cases (screw loosening and/or screw fracture), the distal fragment in 2 (screw fracture), and over the zone of comminution in 1 (plate fracture). In the biomechanical evaluation, proximal screw failure was also the most common location of failure, occurring in 4 specimens (diaphyseal fixation: 1 locked, 3 nonlocked). Plate fracture over the zone of comminution occurred in 1 specimen with locked diaphyseal fixation. In 3 specimens, the plate construct did not fail; rather, the bone fractured outside of the zone of fixation.
Conclusions: Biomechanical tests that attempt to evaluate plate construct integrity after repetitive loading are helpful only if they reproduce clinical modes of failure. This study, in contrast to most available studies, replicated clinically relevant modes of failure for locked distal femur fracture fixation by utilizing a high-cycle combined axial, bending, and torsional loading scenario. Further, these results provide clinically relevant evidence that locked diaphyseal screw fixation constructs are advantageous to maintain stability in poor-quality bone.
• 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