Ratcheting-fatigue behavior of trabecular bone under cyclic tensile-compressive loading

The failure mode of a mechanically loaded equine medial femoral condyle analog with a void and the impact of lag and neutral screw placement

OBJECTIVE

To determine the failure method of simulated equine medial femoral condyle (MFC) subchondral bone defects under compression and the influence of screw placement on failure resistance.

STUDY DESIGN

In vitro study.

SAMPLE POPULATION

Composite disks (CD) simulating the moduli of yearling bone in the MFC.

METHODS

Four CD conditions were tested, all with a 12.7 mm void (n = 6 per condition): intact (no void), void only, void with a 4.5 mm screw placed in neutral fashion, and void with a 4.5 mm screw placed in lag fashion. Composite disks of each condition were tested under monotonic compression to 6000 N and cyclic compression to 10 000 cycles. Observable failure, load at first observable failure, and displacement at peak 2000 N load were compared among conditions.

RESULTS

Specimens failed by cracking at the superior aspect of the void or the screw exit hole. After monotonic loading, cracks were observed 6/6 CD with a void, 6/6 CD with a void/lag screw, and 5/6 CD with a void/neutral screw. After cyclical testing, cracks were noted only on the superior aspect of 6/6 CD with a void and 3/6 CD with a void/lag screw. Displacement at peak load was 0.06 mm (intact), 0.32 mm (void), 0.24 mm (void/lag screw), and 0.11 mm (void/neutral screw).

CONCLUSION

Model MFC voids failed by superior cracking that was resisted by lag and neutral screw placement.

CLINICAL SIGNIFICANCE

Neutral screws may be an acceptable treatment for subchondral lucencies in the MFC.