Session I - Hip Fractures

Fixation Stability of Displaced Femoral Neck Fractures in Synthetic Model

Matthew J. Weresh, MD, Mark T. Cutright, MD, Marnie O. Njus, MS, Glen O. Njus, PhD, Gregory A. Vrabec, MD

Akron General Medical Center, Akron, OH

Purpose: To answer the question of how to maximize fixation stability for displaced femoral neck fractures, we have studied the effect of increased screw thread contact width (tcw) and the effect of the number of screws placed across the fracture on femoral neck fracture stability. As called upon by Swiontkowski and colleagues in their previous study, to eliminate the biomechanical variability of cadaveric specimens we have done this experiment in a reproducible synthetic femur developed by Pacific Research Laboratories, Vashon, WA.

Methods: Femoral neck fractures were created in 40 synthetic femurs. This was done with an oscillating saw. A biting forceps was used to remove cortical continuity of the posterior aspect of the fracture. The anterior one-third of the cortical circumference was cut but left well opposed. These forty specimens were divided into five groups of eight. In each group the fractures were fixed by screws of varying number or varying screw thread contact width (tcw = thread diameter minus drill hole diameter). The screw type, number, and orientation were: group I - two 6.5mm solid cancellous screws in the sagittal plane (tcw = 3.3mm), group II - three 6.5mm solid cancellous screws in equilateral triangle configuration (tcw = 3.3mm), group III - three cannulated 7.0mm cancellous screws in equilateral triangle configuration (tcw - 2.5mm), group IV - three 7.3mm cannulated cancellous screws in equilateral triangle configuration (tcw - 2.3mm) and group V - four 6.5mm solid cancellous screws in quadrilateral configuration (tcw - 3.3mm). We used the Instron machine to gather data on the following aspects of fixation stability: torsional stiffness, torsional strength to failure, axial bending stiffness, and axial bending strength to failure.

Results: Groups with three screw configurations were more stable than those with four or two screw configurations. Group II was significantly stiffer in bending than all other groups.

Discussion: Failure of fixation of unstable femoral neck fractures has been identified to occur at the bone/screw interface as opposed to mechanical failure of the screw itself. It has also been found that by increasing screw thread contact area the bone/screw interface strength is improved in torsion and axial loading. It therefore stands with reason that by using screws with larger screw thread contact area, femoral neck fracture fixation stability is improved. Despite this, cannulated screw design has tended toward larger screws with a larger core and decrease screw thread contact width and thread surface area. In this study we have paid particular attention to this concept of thread contact area by measuring the difference in screw thread width in screws with identical pitch and rise. We have also measured the effect of varying the number of screws across the fracture and its relation to fixation stability. The fact that we have used a reproducible synthetic bone model allows us to measure the biomechanical differences in our constructs. Cadaveric studies have not been able to reliably do this.

Conclusion: Femoral neck fracture fixation stability in bending is improved with the use of screws with increased thread contact width. A three screw construct is superior to a four or two screw construct.