Implant material and design alter construct stiffness in distal femur locking plate fixation: a pilot study

Abstract

Background: Construct stiffness affects healing of bones fixed with locking plates. However, variable construct stiffness reported in the literature may be attributable to differing test configurations and direct comparisons may clarify these differences.

Questions/purposes: We therefore asked whether different distal femur locking plate systems and constructs will lead to different (1) axial and rotational stiffness and (2) fatigue under cyclic loading.

Methods: We investigated four plate systems for distal femur fixation (AxSOS, LCP, PERI-LOC, POLYAX) of differing designs and materials using bone substitutes in a distal femur fracture model (OTA/AO 33-A3). We created six constructs of each of the four plating systems. Stiffness under static and cyclic loading and fatigue under cyclic loading were measured.

Results: Mean construct stiffness under axial loading was highest for AxSOS (100.8 N/mm) followed by PERI-LOC (80.8 N/mm) and LCP (62.6 N/mm). POLYAX construct stiffness testing showed the lowest stiffness (51.7 N/mm) with 50% stiffness of AxSOS construct testing. Mean construct stiffness under torsional loading was similar in the group of AxSOS and PERI-LOC (3.40 Nm/degree versus 3.15 Nm/degree) and in the group of LCP and POLYAX (2.63 Nm/degree versus 2.56 Nm/degree). The fourth load level of > 75,000 cycles was reached by three of six AxSOS, three of six POLYAX, and two of six PERI-LOC constructs. All others including all LCP constructs failed earlier.

Conclusions: Implant design and material of new-generation distal femur locking plate systems leads to a wide range of differences in construct stiffness.

Clinical relevance: Assuming construct stiffness affects fracture healing, these data may influence surgical decision-making in choosing an implant system.

Fig. 1

Type AO/OTA A3-33 fracture of the distal femur is demonstrated with a supracondylar comminution zone.

Fig. 2A–D

(A) Test setup for eccentric axial loading using a cardanic joint distally and proximally. (B) Deformation of the construct resulting from axial loading. Note the inclined plane of 33° for anatomical fit of the plates. (C) Torsional loading was realized with a cardanic joint distally and two horizontal linear bearings proximally. (D) Example of the standardized screw configuration in the shaft area of the bone implant constructs with four bicortical screws: 1 = most proximal plate hole; X = bicortical locking screw; O = no screw/empty plate hole; 8 = distal.

Fig. 3

Box-and-whisker plot of the axial construct stiffness (N/mm) under static loading. The box shows the interquartile range (IQR) and the whiskers extend to the smallest and largest values except data points with distance greater than 1.5 times IQR. The black line within the box represents the median. Outliers are data points lying between 1.5 times IQR and three times IQR. Extremes are data points beyond three times IQR and were marked as asterisks (*). All data points were included for statistical analysis. The highest stiffness was found for AxSOS followed by PERI-LOC, LCP, and POLYAX (n = 6 per implant system).

Fig. 4

Box-and-whisker plot of the torsional construct stiffness (Nm/degree) under static loading. The box shows the interquartile range (IQR) and the whiskers extend to the smallest and largest values except data points with distance greater than 1.5 times IQR. The black line within the box represents the median. Outliers are data points lying between 1.5 times IQR and three times IQR and were marked as circles (°). Extremes are data points beyond three times IQR and were marked as asterisks (*). All data points were included for statistical analysis. The highest stiffness was found for AxSOS followed by PERI-LOC, LCP, and POLYAX (n = 6 per implant system).

Fig. 5

The load cycle and peak load at the time of failure (10° displacement) are demonstrated for each BIC (n = 6 per implant system, n = 5 for LCP).

Fig. 6

Box-and-whisker plot of the load cycle integral of all implant systems. The box shows the interquartile range (IQR) and the whiskers extend to the smallest and largest values except data points with distance greater than 1.5 times IQR. The black line within the box represents the median. Outliers are data points lying between 1.5 times IQR and three times IQR. Extremes are data points beyond three times IQR and were marked as asterisks (*). All data points were included for statistical analysis. We found differences between all systems except for PERI-LOC and POLYAX and for LCP and POLYAX.