"Stress taper" fixation increases torsional failure strength in a cadaveric femur model.

BACKGROUND

To potentially limit peri-implant fractures our institution commonly implements a "stress-taper" fixation construct in which the screw lengths towards the proximal end of a construct are incrementally decreased, in order to avoid a focal stress-riser when loaded. To assess this construct, we asked: 1) Does the stress taper strategy increase torsional strength than the bicortical locking construct when biomechanically tested in a cadaveric femur model? 2) Does it fail in a less comminuted fracture pattern?

METHODS

Seven matched pairs of cadaveric femora were randomly assigned to one of two distal femur fixation groups: plating with stress taper strategy or bicortical fixation. Specimens were first cyclically loaded, then axially rotated to failure under 800 N of compression. Peak torque at failure, degrees of rotation at failure, and energy to failure were calculated and compared using paired t-tests. Fractures were categorized with the assistance of fluoroscopy according to the Orthopedic Trauma Association classification, 32.

FINDINGS: There was significantly greater peak torque (110.6 ± 49.7 Nm vs. 80.6 ± 35.2 Nm), rotation at failure (23.8 ± 5.3° vs 18.9 ± 4.5°) and energy to failure (25.3 ± 15.7 J vs. 14.1 ± 8.3 J) in the stress-taper group as compared to the bicortical group (p = 0.0424), (p = 0.0213) and (p = 0.0460), respectively. 6/7 fractures in the stress-taper group were classified 32 A1 with 1/7 classified A2. 5/7 fractures in the bicortical group were classified B1 and 2/7 classified A2.

INTERPRETATION: 'Stress taper fixation' in distal femurs may be protective against peri-implant fractures compared to traditional bicortical fixation. The 'stress taper' concept can increase torsional failure strength in an in vitro model.