Delayed Union, Nonunion, and Malunion

Chapter 122 Delayed Union, Nonunion, and Malunion



Randy J. Boudrieau



DELAYED UNION AND NONUNION


Healing times for similar fractures in any single group of patients are fairly uniform; however, a small number of fractures have longer than normal healing times, or may fail to heal at all. The particular type of fracture (comminuted or simple), the bone involved and its location (e.g., distal radius/ulna in small-breed dogs), the age of the animal, and the type of fixation use all influence normal healing times.


• Classification:
• When the fracture requires longer than normal time to heal but shows definitive signs of progression in healing, it is classified as a delayed union.

• A fracture that does not heal over a similar period and that has no tendency toward further healing is classified as a nonunion (Table 122-1).

Table 122-1 EXPECTED APPROXIMATE HEALING TIMES OF UNCOMPLICATED DIAPHYSEAL FRACTURES WITH MINIMAL LOSS OF CORTICAL BONE























Age of Animal External Skeletal and Intramedullary Pin Fixation Bone Plate Fixation*
<3 mo 2–3 wk 4 wk
3–6 mo 4–6 wk 2–3 mo
6–12 mo 5–8 wk 3–4 mo
>1 yr 7–12 wk 5–8 mo

* Fractures stabilized by this method may not be considered clinically healed (have sufficient strength) as early as fractures stabilized by other means of fixation, because direct cortical union (primary bone healing by Haversian remodeling) is not supported by periosteal callus. This is of primary importance when considering timing of implant removal. Clinical function is not adversely affected by this method of fixation because plates provide rigid fixation.


Other classifications are based on fracture site, fragment displacement, and presence or absence of infection, but are not routinely used.



Definitions



Delayed Union


Radiographic evaluation of the fracture site will reveal callus formation and progressive bone healing, but complete healing has yet to occur over a longer than expected time frame. Delayed union usually needs no other therapy than continuation of ongoing treatment of the fracture. Continued immobilization (assuming stable fixation) allows healing to occur in the majority of cases. A delayed union may, however, be preliminary to a nonunion.



Nonunion


Radiographic evaluation of the fracture site reveals a lack of progression of fracture healing (i.e., bone healing has stopped). Variable amounts of callus may be present depending upon a further subclassification of viable (biologically active) or non-viable (biologically inactive) nonunion (Table 122-2). These nonunions can be classified into two groups: those with callus formation (the hypertrophic viable nonunions) and those without callus formation (both viable oligotrophic and non-viable nonunions).



Table 122-2 VIABLE AND NON-VIABLE NONUNION



Viable Nonunion (Biologically Active)


Hypertrophic (“elephant foot,” or abundant callus)


Moderately hypertrophic (“horse’s hoof,” or moderate callus)


Oligotrophic (little or no callus)



Non-viable Nonunion (Biologically Inactive)


Dystrophic (poor vascularity of one or both sides of the fracture)


Necrotic (avascular areas, or bone fragments, within the fracture, i.e., sequestra)


Defect (large bone defect at the fracture)


Atrophic (defect at the fracture with resorption of the adjacent bone)


Most fractures unite within a reasonable time despite systemic factors such as malnutrition, generalized metabolic or endocrine abnormalities, and acute or chronic generalized disease states.




Key Point


Nonunion results from local factors at the fracture site.


Most commonly these local factors can be identified as inadequate fracture fixation, resulting in instability (Table 122-3). Motion within a fracture site creates interfragmentary strain at the site, and if this strain exceeds tissue tolerance, the tissue will not form within the gap. For example, essential fragile capillaries will not be able to cross the fracture gap within the early granulation tissue formation, or later, with the subsequent stages of tissue differentiation (cartilage and bone).


• This concept has mistakenly been thought to be a greater problem in highly comminuted fractures as opposed to simple fractures. The problem has greater significance in a two-piece fracture. This situation most often occurs in transverse fractures, where greater difficulty is encountered in attaining appropriate stability. This is important in fracture fixation and the achieving of stability when anatomically reconstructing a fracture.

• A two-piece fracture has the interfragmentary strain concentrated at the single fracture site.

• A comminuted fracture has this same amount of interfragmentary strain distributed throughout the many fracture sites (or less strain at each individual site).

• The result is that in a single fracture there is high strain, whereas in a comminuted fracture there is low strain at each fracture site.

• Resultant motion in a two-piece fracture will have potentially greater adverse effects on bone healing than in a more comminuted fracture.


Table 122-3 FACTORS ASSOCIATED WITH DELAYED UNION AND NONUNION



Local Factors*


Fracture location


Fracture gap


Soft tissue interposition

Bone loss secondary to trauma

Soft tissue trauma


Loss of blood supply as a result of initial trauma

Contamination, infection


Neoplasia



Treatment Factors†


Malposition (inadequate reduction)


Fracture gap


Soft tissue interposition

Distraction (by implants or external fixation devices)

Bone loss due to intraoperative removal

Soft tissue trauma


Loss of blood supply due to surgical trauma


Inadequate Fixation (Internal or External)‡


Instability

Postoperative infection



* Related to the fracture.


† Related to the reduction and fixation.


‡ Most common factor.


The most common local factor is a fracture gap (with or without interposition of soft tissues) that exceeds the regenerative capacity of the bone. There is a critical distance over which bone will not form within a gap, resulting in a nonunion.




Key Point


Soft tissue trauma also is an important local factor for bone healing, as damage to the vascular supply will impede healing.


This may occur at the time of fracture, but also at the time of surgery. The importance of the surrounding soft tissues cannot be overemphasized as it is these tissues that are the source for the early revascularization of the bone (transient extraosseous circulation). Therefore, the surgical approaches must be anatomic and atraumatic in nature in order to best preserve this surrounding soft tissue envelope.


Finally, a very common local factor identified in the etiology of nonunions is in miniature or toy breed dogs, where there is a limited vascular supply to the distal radius. Fractures of this bone in these breeds of dogs have a high propensity for developing into a nonunion. Therefore, obtain rigid fixation using bone plates and place cancellous bone grafts.




Key Point


Regardless of the etiology of the nonunion, it requires some form of surgical intervention in order for healing to progress.



Problems Related to Non-healing Fractures


Patients with non-healing fractures may have additional problems related to function, such as disuse muscle atrophy, decreased range of joint motion and stiffness related to scar tissue contraction, neurovascular dysfunction, and limb angulation and/or shortening. These functional deficiencies will be the ultimate determinant of the success or failure of treatment, not whether the bone eventually can be made to unite.



Bone Healing



• Bone heals by either primary (Haversian remodeling) or secondary (periosteal callus) union.

• Secondary bone healing, with formation of visible periosteal callus, begins with connective tissue formation that progresses to form fibrocartilage and finally bone.

• Primary bone healing occurs without formation of connective tissue; bone disposition occurs directly (direct Haversian remodeling, or by gap healing, with fracture gaps <0.8 mm) without any visible callus. Familiarity with these concepts allows accurate sequential evaluation of the healing process.


Histology



• The most notable histologic feature of a delayed union or nonunion is increased periosteal cartilage in the callus in lieu of bone formation.

• Persistent fracture gaps at the bone ends (sometimes greater than that present at the time of the original fracture) are filled with fibrous tissue or fibrocartilage callus.

• The characteristics of a nonunion include chondroid tissue, areas of degeneration with necrosis, and fibrous connective tissue.

• Sclerosis of the bone ends at the fracture site also may occur, effectively sealing the medullary cavity (and access to the medullary circulation).


Clinical Signs



• The patient is usually lame on the affected limb, and may be non–weight bearing.

• Muscle atrophy and joint stiffness are likely sequelae to limb disuse.

• Pain may or may not be present at the level of the fracture.

• Movement (instability) of the fracture may or may not be detected clinically.

• Signs of infection and resultant pain may be present.

• Palpable enlargement of the fracture area may be present.


Diagnosis



Diagnostic Imaging



• Radiographic signs vary, depending on the extent of healing (delayed union versus nonunion); classification of these events is the basis for treatment.

• Radiography allows sequential evaluation of healing.
• Delayed union: There is continued healing, albeit slow, as indicated by progressive callus formation and resorption of dead bone. A persistent fracture line with evidence of some non-bridging callus is characteristic. The marrow cavity remains open without evidence of significant sclerosis of the bone ends.

• Nonunion: There is no evidence of progression of fracture healing (i.e., little or no change on sequential radiographic evaluation over a 3-month evaluation period). Smooth fracture surfaces are typical (no periosteal “irritation”), with evidence of sclerosis at the bone ends and sealed marrow cavities. Sequestra or avascular bone fragments may be observed.

• Radiography may reveal the etiology of the problem.
• Evaluate the “treatment factors” (see Table 122-3) that may be contributing to the delayed union or nonunion.

• The fracture fixation is the most common source of the problem, with visible evidence of instability and implant loosening. For example, a small, radiolucent halo around any of the implants (screws, wires, etc.) indicates a loose device. A change in the position of the implants on serial radiographs also indicates implant instability.

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Aug 27, 2016 | Posted by in SMALL ANIMAL | Comments Off on Delayed Union, Nonunion, and Malunion

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