OTA 1997 Posters - Scientific Basis for Fracture Care

Improved Fixation in Osteoporotic Bone: A Novel Locked Supracondylar Buttress Plate

Jordan A. Simon, MD, James Hale, BS, Fred Kummer, PhD, Kenneth J. Koval, MD

New York, New York, USA

Purpose: There has been recent interest in the use of plates with locked screws to improve fracture fixation. Locking the screws to the plate facilitates the use of unicortical fixation and allows the plate/screw construct to act as a fixed angle device. Toggling of the screws within the plate is prevented, thus decreasing the risk of fracture displacement.

This laboratory study was initially performed to compare fixation stability of a standard condylar buttress plate to the same plate with distal locked screws in an unstable distal femoral fracture model. Later, this locked plate was similarly compared to a standard 95° blade plate.

Methods: Twelve matched pairs of mildly to moderately osteoporotic femurs were selected from a large cadaver harvest. Initially, six matched femoral pairs were randomly assigned to one of two methods of instrumentation: 1) a standard six hole lateral condylar buttress plate using four distal 6.5 mm. cancellous screws; 2) a modified lateral condylar buttress plate made by welding four tapped nuts into the screw holes of the distal plate flange in the identical pattern used for the standard buttress plate to create a locked screw system. After instrumentation, a one centimeter section of bone was removed using a thin blade reciprocating saw to create a gap 6 centimeters proximal to the lateral joint line.

The bone/implant constructs were mounted on an MTS and compressive load to 1000 N applied to the femoral head at a rate of 100 N/sec; fracture gap displacement was measured using an electronic displacement gauge. The specimens were also tested in bending/torsion by applying an anterior load to the femoral head to simulate rising from a seated position. Fixation stability in bending/torsion and axial loading was defined as the slope of load-deflection curves normalized for neck length. The femurs were then subjected to an axial cyclic load of 1000N for 10 (to 5th power) cycles (3Hz) to approximate early weight bearing. Load-deflection curves and fracture displacements in axial loading and bending/torsion were again acquired to determine the stability of the constructs post cycling.

The experiment was repeated with six additional matched femoral pairs to compare stability of the locked plate to a standard 95° blade plate with one additional 6.5 mm. distal cancellous screw inserted into the distal fragment. The testing protocol was identical to the previous comparison. Statistical analysis of the stiffness and displacement values for the paired specimens was performed using Student t-tests. A p value less than .05 was considered statistically significant.

Results: The average fracture displacements measured in axial loading at 1000N for both the standard and locked condylar buttress plate constructs were 1.34 mm for the locked plate and 3.48 mm for the standard plate (1.52 vs 4.30 after cycling). The locked plate construct was significantly more stable (i.e. smaller displacement at a given load) to axial loading than the standard plate construct both prior to and after cyclic loading. The locked buttress plate construct was significantly more stable than the 95° blade plate in axial loading at 1000N both prior to and after cyclic loading. The displacement of the fracture gap was 1.42 mm for the locked plate and 3.20 mm for the blade plate (1.6 vs 3.65 after cycling). The locked plate was approximately 50% more resistant to bending/torsional loading compared to both the standard buttress and blade plate but these differences were not statistically significant due to their large standard deviations.

The distal fragment of five of the standard buttress plate and three of the blade plate specimens exhibited gross loosening after cycling. The locked condylar buttress plate did not exhibit this loss of distal fixation in any specimen.

Discussion: The increased stability of the locked condylar buttress plate as compared to the standard condylar buttress plate can be attributed to the absence of toggling motion at the screw/plate interface. None of the locked plates exhibited visible loosening at this interface after axial cycling, nor was there significant gross displacement during the axial preload.

The 95° blade plate achieves fixation of the distal fragment from its blade inserted into a single region of the distal fragment with one screw inserted in an oblique fashion. Any widening of the blade channel in the bone during insertion or due to compaction of the osteopenic cancellous bone upon loading will lead to motion at the blade/bone interface. The locked plate achieves its purpose in a multiple regions of bone which, in this study led to a more stable construct.

A limitation of the locked plate concept is that the locked screws are set in a fixed orientation and cannot be arranged to accommodate different conformations.

Conclusion: In this study the locked condylar buttress plate provided significantly greater fixation stability than both a standard buttress plate and 95° blade plate in a distal femoral gap osteotomy model. These results show that a condylar buttress plate using locked screws is a valid concept to maximize fixation stability in osteopenic distal femur fractures.