Session VIII - Femur/Knee

*Biomechanical Evaluation of the Less Invasive Stabilization System (LISS), Angled Blade Plate, and Retrograde Intramedullary Nail for the Fixation of Distal Femur Fractures: An Osteoporotic Cadaveric Model

Michael Zlowodzki, MD; Randall Scott Williamson, BS; Lyle D. Zardiackas, PhD; Philip J. Kregor, MD; Vanderbilt University Medical Center, Nashville, Tennessee, USA; and the University of Mississippi Medical Center, Jackson, Mississippi, USA (a-Synthes, AO Research Foundation)

Purpose: The most commonly used devices for fixation of distal femur fractures are the retrograde intramedullary femoral nail (IMN) and the 95° angled blade plate. However, loss of distal fixation occurs, especially in the setting of a very distal fracture or osteoporotic bone or both. The Less Invasive Stabilization System (LISS) provides a fixation construct for supracondylar/intracondylar distal femoral fractures, with features including submuscular fixation and percutaneous placement of self-drilling unicortical, fixed-angled screws. Clinical trials have demonstrated the efficacy of the LISS for fixation of distal femur fractures, but there is concern regarding the stability of the LISS, particularly with the proximal unicortical screws. The purpose of this study was to evaluate the stability of the retrograde femoral nail, angled blade plate, and the LISS in catastrophic axial and torsional loading in an osteoporotic cadaveric femoral model.

Methods: Twenty-four matched pairs of fresh frozen human femora (donor age >70 years) were used. DEXA scanning was performed in the supracondylar region to assure the presence of osteoporosis and the uniformity of specimens. Four groups of six pairs each were tested: LISS versus blade plate (axial), LISS versus IMN (axial), LISS versus blade plate (torsional), and IMN (torsional). A fracture model was created to simulate an AO/OTA 33-A3 fracture 6 cm above the intercondylar notch. In each pair, fixation was randomly assigned to the LISS for one femur and the blade plate or IMN for the other. Axial loading was conducted with a pre-load of 100 N at a rate of 10 mm/min and in the direction of the mechanical axis of the femur. Torsional loading was conducted at a rate of 10°/min.

Results:

LISS vs. blade plate: In axial testing, loss of distal fixation occurred in two of six femora instrumented with the blade plate, and none of the six femora instrumented with the LISS. The mode of failure in axial loading with the LISS was plastic deformation of the implant. The average axial load to failure (yield point) was 34% higher for the LISS compared with the blade plate (1046 N vs. 783 N, P = 0.03). No differences were seen in torsional testing, with the average torque at failure being 39.5 Nm for the LISS and 43.8 Nm for the blade plate (P = 0.58).

LISS vs. IMN: In axial testing, loss of distal fixation occurred in six of six femora instrumented with an IMN and in one of six instrumented with the LISS. The typical mode of failure with the IMN was gross medial displacement of the distal femoral "block" relative to the intramedullary nail (varus collapse). The yield point for the LISS occurred with subtle plastic deformation of the implant in five of six cases, not with loss of distal fixation. The average load to failure (yield point) was 24% higher for the LISS vs. IMN (1054 N vs. 848 N, P = 0.15). In torsional testing, the average torque at failure was higher for the IMN vs. LISS (55 Nm vs. 37 Nm, P = 0.03).

Discussion and Conclusion: Of significant interest to the surgeon is potential loss of fixation in catastrophic loading of a supracondylar femoral fracture fixation construct. The blade plate and retrograde intramedullary nail are accepted treatment modalities for distal femoral fractures. Little information exists, however, regarding catastrophic one-time testing of cadaveric bone constructs that use these modalities. The LISS has proven clinical efficacy in the treatment of distal femoral fractures. However, there is considerable concern regarding possible proximal screw pullout, given their unicortical design. The LISS has multiple distal, fixed-angled locked screws that lock into the fixator and can be thought of as having multiple "mini-blade plates" distally.

In conclusion, biomechanical testing demonstrated 1) superior fixation of the distal femoral "block" of the LISS vs. blade plate in axial loading, 2) equivalent torsional strength between the blade plate and the LISS, 3) better purchase of the distal femoral "block" of the LISS vs. IMN, 4) improved torsional strength of the IMN compared with the LISS, possibly due to the unicortical proximal screws used with the LISS. The results further indicate that catastrophic axial loading of the LISS fixation construct will ultimately result in fixator plastic deformation, rather than screw pullout.