Fixation of the femoral condyles: a mechanical comparison of small and large fragment screw fixation.

BACKGROUND To compare the stability achieved using two 6.5-mm screws versus two or four 3.5-mm screws for the fixation of a unicondylar distal femur fracture. METHODS A fracture model was created in femoral synthetic composite bones to simulate a lateral femoral condyle fracture (AO/OTA 33-B1). Fixation was performed using three different types of screw constructs: 1) two 6.5-mm cancellous screws inserted using the lag technique, 2) two 3.5-mm cortical screws inserted using the lag technique, and 3) 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. Main outcome measurements were the mean load required to displace the osteotomy site 1 and 2 mm as well as the mean stiffness of the different fixation methods. RESULTS The 6.5-mm construct required 56% more load to displace the osteotomy fragment 1 mm than the two 3.5-mm construct required (p < 0.0001), and 40% more load than the four 3.5-mm construct required (p < 0.0001). At loads that caused 2 mm of osteotomy displacement, these differences increased to 62% (p < 0.0001) and 48% (p < 0.0001), respectively. The mean loads needed to displace the osteotomy site were 28% higher for 1 mm of displacement (p = 0.003) and 27% higher for 2 mm of displacement (p = 0.03) for the four 3.5-mm screw construct compared with those needed for the two 3.5-mm group. The mean stiffness for the 6.5-mm group (1312.5 N/mm) was significantly higher than for 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 3.5-mm groups was significant as well (p < 0.0001). CONCLUSION Stabilization of a unicondylar distal femur fracture with two 6.5-mm cancellous screws provides the most rigid and stable fixation. If small fragment screws are used, a minimum of four 3.5-mm cortical screws should be used to approximate the mechanical stability of two 6.5-mm screws.