Ex vivo comparison of one versus two distal screws in 8 mm model 11 interlocking nails used to stabilize canine distal femoral fractures.
OBJECTIVE
To compare the structural properties of an 8 mm model 11 interlocking nail (IN) with 2 proximal and 2 distal screws (2/2) to 2 proximal and 1 distal screws (2/1) in an unstable canine fracture model.


METHODS
Ex vivo biomechanical investigation.


METHODS
Eight pairs of adult canine femurs.


METHODS
A simple transverse distal metaphyseal femoral fracture with a 1 cm gap was created. The unstable fracture in 1 femur was repaired with a nail with 2 distal and 2 proximal screws and the paired femur with a nail with 1 distal and 2 proximal screws. Cyclic mechanical testing in torsion was performed to assess fatigue life, peak torque, stiffness, and mode of failure.


RESULTS
All 2/1 IN-femoral constructs, but only 2 of eight 2/2 constructs, failed before completion of 50,000 loading cycles. The 2/2 constructs had significantly greater peak torque to failure (P = .002) and longer fatigue life (P = .00003) compared with 2/1 constructs. There were no significant differences in stiffness between 2/2 and 2/1 constructs when the non-failed constructs were compared (P > .5). All constructs failed by screw deformation.


CONCLUSIONS
An 8 mm model 11 IN used for fixation of unstable canine distal femoral fractures has a longer fatigue life and is stronger under torsional loads when 2 rather than 1 distal screws are placed.


CONCLUSIONS
When repairing unstable canine distal femoral fractures with an IN system, 2 distal screws should be inserted to avoid catastrophic implant failure before bone healing is achieved.
Department of Clinical Sciences, Colorado State University, Fort Collins, CO, USA.
Michael R
MR
Reems
G Elizabeth
GE
Pluhar
Donna L
DL
Wheeler
D000818:Animals; D001696:Biomechanical Phenomena; D001858:Bone Nails / Q000662:veterinary*; D001863:Bone Screws / Q000662:veterinary*; D004285:Dogs / Q000601:surgery*; D004867:Equipment Design / Q000592:standards / Q000662:veterinary; D005264:Femoral Fractures / Q000601:surgery / Q000662:veterinary*; D016268:Internal Fixators / Q000662:veterinary*; D013522:Surgery, Veterinary / Q000295:instrumentation*; D016896:Treatment Outcome