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Today's VeTerinary PracTice | september/october 2015 | tvpjournal.com a PracTiTioner's Guide To FracTure ManaGeMenT Peer reviewed 28 Implant Failure Implants are under constant load while the bone is healing and are at risk of implant failure when put through cyclic strain due to excessive motion of fracture fragments, overuse of the limb, or poor confnement of the patient. Implant failure can occur in the form of screw back out or breakage, plate bending or breakage, bending of IM pins, breakage of ILN screws or bolts, breakage or migration of cerclage wire, or loosening of external fxator pins. Implants can also fail due to infection of the implants, or poor healing qualities of the bone (Figure 4). Poor Bone Healing Poor healing of the bone can lead to delayed union, nonunion, or malunion. 7 Delayed unions can be caused by too much motion or poor healing properties of the bone at the fracture site, which causes the healing to be prolonged (normal healing is approximately 8–12 weeks). Nonunions—in which the fracture fails to progress regardless of healing time—occur when delayed unions are not managed appropriately. Based on radiographic evaluation, nonunions can be classifed as viable or nonviable. Viable nonunions are classifed as: • Hypertrophic: Considerable callus and "elephant's foot" appearance on radiographs • Moderately hypertrophic: Lesser degree of callus formation and "horse's foot" appearance on radiographs • Oligotrophic: No radiographic evidence of bone healing; hard to distinguish from a nonviable nonunion. Nonviable nonunions are classifed as: • Dystrophic: Nonviable bone on either side of the fracture • Necrotic: An infected section of bone, specifically a sequestrum, which is biologically dead • Defect: Gap at the fracture site that is too large for normal healing to occur • Atrophic: Nonviable nonunions in which dead bone is removed by the host without a healing response. Malunions result when fractures heal with the limb in improper alignment and can manifest by shortening of the limb, malalignment of proximal and distal joints relative to each other, rotational abnormalities, and varus or valgus deformities. FIGUR e 4. Lateral and craniocaudal radiographs after patient presented for sudden onset of right hindlimb lameness approximately 4 weeks after fracture repair (A). Note the IM pin was removed approximately 1 week prior to the sudden onset of right hindlimb lameness because it was migrating and causing irritation; on the craniocaudal view, note that all of the proximal screws have broken, leaving the fracture unstable. Immediate postoperative lateral and craniocaudal radiographs ( B ) of a revision that replaced screws in the proximal segment and augmented the fxation with a second plate for additional stability. Four-month postoperative lateral and craniocaudal radiographs (C) revealing fracture healing; note that the bone altered its shape during healing; however, due to maintenance of axial alignment, the alteration did not create a clinically signifcant problem. A B C Further information on orthopedic complications is available in the article, Orthopedic Follow- Up Evaluations: Identifying Complications (september/october 2014), available at tvpjournal.com.