Tibial fractures can now be treated successfully in the majority of patients, yet nonunions of the tibia are not uncommon. They may result in significant morbidity, require numerous operative procedures to treat, and leave the patient with functional deficits.
The subcutaneous position of the tibia results in a greater incidence of open fractures and provides less soft-tissue coverage, factors that produce a higher incidence of nonunion and infected nonunion. Although appropriate and prompt treatment is needed to treat tibial injuries successfully, the incidence of a nonunion is more closely related to the fracture characteristics than subsequent treatment. Establish realistic expectations of the outcome with the patient as early in the treatment course as possible, preferably prior to treatment intervention. 
Historically, the definition of delayed union and nonunion have been based on time from the onset of injury. Currently, the exact time frames are considered to be less important. Fracture healing is a dynamic, progressive process, and intervention is warranted within 3-5 months after injury if monthly radiographic studies do not show progression of fracture healing. 
Typically, the term delayed union is used for a fracture that has not united within a period that would be considered adequate for bone healing. Delayed union suggests that union is slow but will eventually occur without additional surgical or nonsurgical intervention. The time frame is different for different fractures. Tibial diaphyseal fractures that do not show enough bridging callus to achieve clinical stability by 16 weeks are considered to be delayed union fractures. 
The term nonunion refers to a fracture that will not unite without additional surgical or nonsurgical intervention (usually by 6-9 months).
The development of a tibial nonunion is related most often to the type and degree of injury, but several additional factors may predispose a patient to a tibial nonunion, such as the degree of fracture comminution and bone loss, whether the fracture is open, and the degree of soft-tissue injury. Subsequent complications, such as infection or compartment syndrome may play a role. 
In a prospective observational study of 416 patients from 41 trauma centers who underwent operative treatment of tibial shaft fractures, delayed healing or nonunion occurred in 13%. Open fractures with injuries less than 5 cm were 3.6 times as likely to have delayed healing or nonunion as closed fractures; for open fractures greater than 5 cm, the likelihood of delayed healing or nonunion was 5.7 times greater than that for closed fractures. Healing problems were twice as great for distal shaft fractures and fractures with a postoperative diastasis. [4, 5]
In an observational study involving 200 patients who experienced tibial fractures, Fong et al found that nonunion was more likely to occur in fractures with less than 25% cortical continuity.  The presence of a fracture gap after fixation, open fractures, and transverse fracture type were also associated with nonunion. The highest risk of nonunion occurred in cases involving an open fracture in conjunction with a fracture gap.
The patient profile also contributes to the incidence of nonunion. Cigarette smoking is well documented to place the patient at higher risk for delayed healing or nonunion.  The use of nonsteroidal anti-inflammatory drugs (NSAIDs) may inhibit bone healing, as can impaired patient nutritional status and inadequate compliance with the postoperative regimen.
Finally, prompt and appropriate treatment is needed because iatrogenic injury to the soft-tissue envelope (ie, excessive periosteal stripping), distraction across the fracture site, inadequate immobilization or fixation, and the splinting effect of an intact fibula may contribute to the development of a nonunion.
Tibial nonunions are estimated to constitute 2-10% of all tibial fractures. The incidence is greater with high-energy injuries and open fractures. The National Center for Health Statistics has reported that close to 500,000 tibia and fibula fractures occur each year in the United States.
Little documentation exists in the literature regarding the functional outcomes of patients treated for tibial nonunions.  Discussing the potential limitations in future functional abilities with the patient is critical. Fracture healing does not mean full function, and residual weakness, pain, and limitations in function are common, even in appropriately treated patients with clinically successful outcomes.