OTA 2005 Posters

Scientific Poster #66 Basic Science

Small Fragment Screw Fixation of the Femoral Condyles for Supracondylar-Intercondylar Fractures of the Distal Femur

Afshin Khalafi, MD (a-AO North America Resident Trauma Research Grant); Shane Curtiss, AS (n); Scott Hazelwood, PhD (n);

Philip R. Wolinsky, MD (n);

University of California, Davis, Sacramento, California, USA

Purpose: The advent of the less invasive stabilization system (LISS) plate, with its wider "footprint" and locked screw constructs, has made the placement of large diameter screws for the fixation of the intercondylar portion of a supracondylar-intercondylar femur fracture more difficult. This study compares the stability of two 6.5-mm screws to two or four 3.5-mm screws for the fixation of the femoral condyles.

Methods: The study design was biomechanical testing of synthetic composite bone-screw constructs. A fracture model was created to simulate an AO/OTA 33-B1 fracture. Fixation was achieved using three different screw constructs (n=30): two 6.5-mm cancellous screws inserted using the lag technique; two 3.5-mm cortical screws inserted using the lag technique; and four 3.5-mm cortical screws, with two inserted using the lag technique and two as position screws. After reduction and fixation, the constructs were axially loaded in a material testing machine.

Results: The main outcome measurement, stiffness of the fixation method, was determined by mean load at 1 mm and 2 mm of fracture displacement. The 6.5-mm screw construct required 56% more load to displace the fracture fragment 1 mm than the two 3.5-mm screw construct (P<0.0001), and 40% more load than the four 3.5-mm screw construct (P<0.0001). At 2 mm of fracture displacement, these differences increased to 62% (P<0.0001) and 48% (P<0.0001), respectively. In addition, compared to the two 3.5-mm group, the mean load to displace the fracture for the four 3.5-mm screw construct was 28% higher at 1 mm (P=0.003) and 27% higher at 2 mm (P=0.03). Furthermore, the mean stiffness for the 6.5-mm group (1312.5 N/mm) was significantly higher than both the four 3.5-mm construct (784.2 N/mm; P<0.0001) and the two 3.5-mm screw construct (409.4 N/mm; P<0.0001). The difference in stiffness between the two 3.5-mm groups was also significant (P<0.0001).

Conclusion/Significance: Stabilization of the intercondylar portion of a supracondylar-intercondylar femur fracture with the two 6.5-mm cancellous screws provides the most rigid and stable fixation. When using a LISS plate, four 3.5-mm cortical screws should be used to approach the biomechanical stability of two 6.5-mm screws.