The diagnosis of nonunion is made both clinically and radiographically (see Plate 9-13). On clinical examination of a nonunion, the fracture fragments are still mobile after the appropriate healing time. Radiographs show no bony trabeculae spanning the fracture gap in the anteroposterior, lateral, or both oblique views. If the fracture was treated with an internal fixation device, diagnosis is based entirely on the radiographic evidence.

The causes of nonunion include inadequate reduction or immobilization of the fracture, interposition of soft tissue in the fracture gap, significant soft tissue loss or vascular damage at the time of the original injury, and osteomyelitis at the fracture site. In some cases, the cause remains unknown.

Several types of nonunion exist, and each nonunion must be classified to select the appropriate treatment. Three main types of nonunion include hypertrophic nonunion, atrophic nonunion, and pseudarthrosis. All three types of nonunion can additionally be described as infected if osteomyelitis is present. In most cases of nonunion, histologic examination shows a gap between the fracture fragments that is filled with a combination of fibrous, cartilaginous, and bony tissue. Because the fibrinous tissue usually dominates in hypertrophic nonunions, these were historically called a fibrous nonunion. Hypertrophic nonunions are characterized by excessive amounts of bony callus formation and occur when there is excessive fracture movement (due to inadequate fixation) in a healthy bone-healing environment. Atrophic nonunions are characterized by minimal to no bony callus formation but may also have small amounts of fibrous tissue present. Atrophic nonunions are the result of a poor healing environment devoid of proper biologic requirements of fracture healing, likely owing to soft tissue stripping and devascularization of the bone. Atrophic nonunions can actually also be the result of internal fracture fixation that is too rigid. Studies have shown that fractures require a small degree of “micromotion” to stimulate healing. Extremely rigid fixation devices such as locking plates may actually lead to atrophic nonunion in certain circumstances.

In about 12% of nonunions, however, the histologic composition of the fracture gap is quite different. The nonunion site is a cleft filled with fluid and lined with a synovial-like membrane. This type of nonunion, called synovial pseudarthrosis, can also be caused by excessive movement of the fracture fragment during the healing process.