A Biomechanical Comparison of Two Triple-Screw Methods for Femoral Neck Fracture Fixation in a Synthetic Bone Model

Background: Fixation of femoral neck fractures is often accomplished by several screws inserted along the neck axis. Alignment and number of screws remain at the discretion of the surgeon. Two fracture repair methods were compared. Methods: Sixteen large, left, adult, synthetic femurs, known as Third Generation Composite Femurs (Pacific Research Laboratories, Vashon, WA), were osteotomized with a transverse cut perpendicular to the neck axis. Fractures were reduced and repaired using method 1 (n = 8) or method 2 (n = 8) cannulated cancellous screw methods. Method 1 screws were inserted parallel in an upside-down triangle configuration and abutted against the cortical walls inferiorly, anteriorly, and posteriorly. Method 2 screws were placed adjacent to one another as often done clinically. Femoral shafts were positioned in a nonclinical vertical orientation to obtain conservative "lower bound" measurements. Specimens were tested for torsional and axial stiffness using subclinical loads followed by axial failure tests. Results: Method 1 showed statistically higher values compared with method 2 for torsional stiffness (9.9 vs. 7.9 Nm/deg, method 1/method 2 ratio = 1.25, p = 0.018), axial stiffness (1469.0 vs. 1278.1 N/mm, method 1/method 2 ratio = 1.15, p = 0.023), and axial failure load (3493.5 vs. 2863.5 N, method 1/method 2 ratio = 1.22, p = 0.000). However, there were no statistical differences in axial failure displacement (10.9 vs. 16.9 mm, method 1/method 2 ratio = 0.64, p = 0.101) or axial failure energy (29.9 vs. 35.9 J, method 1/method 2 ratio = 0.83, p = 0.453). For both methods, femoral heads move distally while screw shafts cut through the spongy cancellous matrix of the femoral neck in knife-like fashion. Conclusions: Method 1 was more mechanically stable than method 2 in femoral neck fracture fixation as detected by three of five biomechanical measurements and equivalent to method 2 for two of five biomechanical measurements.