The Effect of Location of a Unicortical Defect on the Mechanical Properties of Rabbit Tibiae: A Model of the Distal Jig Pin Hole in Tibial Plateau Levelling Osteotomy.

Objective:  The aim of this study was to determine the effect of a unicortical defect at either the mid-diaphysis (MD) or distal metaphysis (DM) on the torsional properties of tibiae in an rabbit model, and to further examine optimal distal jig pin position for the canine tibial plateau levelling osteotomy (TPLO) procedure.

Study Design:  Thirty-eight tibiae from 19 skeletally mature female New Zealand White rabbits were assigned to one of three groups; Group 1: intact, Group 2: MD defect and Group 3: DM defect. Defects were created using a 1.6 mm Ellis pin. Pure torsion was applied to each sample and peak torque and angular displacement recorded.

Results:  All tibiae fractured in a spiral configuration. Fracture lines involved the defect in 33% of the MD samples and 0% of the DM samples. No differences were detected for peak torque and stiffness between groups. However, energy (mean ± standard deviation) was significantly reduced ( = 0.028) in the MD group (0.18 ± 0.07) relative to the intact tibia group (0.31 ± 0.14). Angle was also significantly reduced ( = 0.040) in the MD group (0.17 ± 0.05) compared with the intact group (0.23 ± 0.07). Placement of a DM defect had no significant effect on mechanical properties of the rabbit tibiae.

Conclusion:  Defects placed in the MD significantly reduced energy and angle in comparison to intact samples. No significant difference in peak torque or stiffness was observed between groups. If canine tibiae were similarly affected, our findings suggest jig pin placement in the DM to have a lesser effect on the torsional properties of the tibiae.