Effect of Bidirectional Insertion of External Skeletal Fixation Pins on Axial Pullout Strength in Canine Cadaveric Bone

Introduction: Effect of bidirectional insertion on axial pullout strength of tapered run out (TRO) and traditional 1/8” negative profile (TNP) pins was evaluated. Our null hypothesis was that no difference existed in axial pullout strength of TRO or TNP pins when inserted bidirectionally to the desired position (DP) (360° of thread exiting the trans cortex), remaining within the threads of the pin.

Materials and Methods: TRO pins were inserted unidirectional to DP (G1), bidirectional past DP, then backed up to DP (G2), and bidirectionally as described for G2, repeated twice (G3). TNP pins were placed similarly (G4–6). Additionally, TNP pins were driven unidirectionally to one thread before the thread/shaft interface (G7). A servohydraulic testing machine measured (1 mm/sec) and recorded peak axial pullout strength (N). One-way analysis of variance (ANOVA) was used for statistical evaluation (p < 0.05).

Results: Eighty-four pins were placed and extracted (n = 12/group). There was no significant difference in axial pullout strength with any insertion technique using a TRO or TNP pin. Unidirectional TRO pins (G1 = 3075.03N, SE 292.34N) had significantly greater axial pullout strength than TNP pins (G4 = 2365.96N, SE 143.41N) [p = 0.0199]. TRO pins, irrespective of pin insertion method, had greater axial pullout strength than TNP pins [p = 0.0161].

Discussion/Conclusion: Axial pullout strength can be preserved if the cis cortex is not violated with the pin shaft. TRO pins offered a stronger bone-pin interface than TNP pins. This supports pin adjustment during ESF placement.

Acknowledgement: Supported by Cohn Family Chair for Small Animals and IMEX Veterinary, Inc., Longview, TX, United States.

No conflict of interest has been declared by the author(s).