Session I - Hip Fractures

Effect of Posterior Comminution on the Fixation Stability of Femoral Neck Fractures

Jeffrey I. Kauffman, MD, Kenneth J. Koval, MD, Charles J. Pearlman, MD, Fred J. Kummer, PhD, Joseph D. Zuckerman, MD

Hospital for Joint Diseases, New York, NY

Introduction: It is generally accepted that nondisplaced and impacted femoral neck fractures can be adequately stabilized using three cancellous lag screws or pins placed in parallel. Previous laboratory work has further suggested that for noncomminuted displaced femoral neck fractures, three parallel screws provide fixation stability similar to that provided by four screws. Some authors, however, advocate fixation using four screws when there is comminution of the posterior femoral neck. The present laboratory study was performed to determine whether this technique provides enhanced fixation of displaced femoral neck fractures with posterior comminution.

Methods and Materials: Nine pairs of mildly osteogenic embalmed femurs (based on Singh index and DEXA scan) were selected and randomized into three matched pair groupings: (1) a reproducible subcapital fracture with a posterior defect fixed with three 6.5-mm cannulated cancellous screws in a triangular configuration; (2) the same fracture pattern as in #1 fixed with four screws in a diamond configuration; and (3) a noncomminuted subcapital fracture fixed with three screws. Intact femurs were mounted on an MTS servohydraulic testing machine to simulate one-legged stance and axially loaded in increments to establish control values. Simulated subcapital fractures were made by scoring the femoral neck and completing the fracture with a mallet. In twelve specimens, a standardized large posterior defect was created. The specimens were then instrumented and nondestructive axial load testing repeated, followed by anterior and torsional loading; fracture displacements as a function of load were obtained. Cyclic axial loading (1000 N) was then performed for 10,000 cycles and displacements remeasured. The femurs were finally axially loaded until failure. Statistical analysis was performed to compare fracture displacements for the various loading schemata and ultimate failure loads, with p <.05 considered significant.

Results: In axial loading, three-screw fixation of the comminuted fracture allowed significantly more fracture displacement (~150%) than did either four-screw fixation of this fracture or three-screw fixation of the noncomminuted fracture. There was no significant difference between the stability provided by three-screw fixation of the noncomminuted fracture and four-screw fixation of the comminuted fracture. Similar trends (not statistically significant) were found for all test groups in anterior loading and torsion. Under cyclic loading, the comminuted fractures that were stabilized with three screws displaced ~60% more than the other two test groups. Comminuted fractures fixed with three screws sustained significantly less load to failure (2600 N) than comminuted fractures fixed with four screws (3300 N) or noncomminuted fractures fixed with three screws (3600 N). There was no significant difference between the failure loads of the latter two groups.

Discussion and Conclusion: In the comminuted femoral neck fracture model, four cancellous lag screws provided significantly greater fixation stability and strength then did three screws. The fixation afforded by four screws in the comminuted model was similar to that seen in the noncomminuted model fixed with three screws. Internal fixation of displaced femoral neck fractures with posterior comminution should be performed with four cancellous lag screws.