Osteomyelitis associated with an orthopedic implant

A 7-year-old, 28 kg, intact female crossbreed dog was admitted to the Veterinary Hospital due to chronic draining fistulas from the bone to the skin on the left rear limb. According to the owner, the dog had an accident that fractured both femurs when it was 3 mo old. The right femoral fracture was reduced by using an intramedullary pin; it was removed 1 y later. The left femoral fracture was reduced by using an intramedullary pin and cerclage wire, but the pin was not removed because it had become fully embedded in the bone. Approximately 5.5 y after the surgery, the dog developed several draining fistulas in the left hindlimb and, in the past year, had received several types of antibiotics and antiinflammatory drugs and had undergone exploratory surgery, none of which was successful in correcting the problem.

On physical examination, the dog was severely lame on the left hind limb, its range of movement in the stifle and hock was decreased, and it had hyperextension of the hock joint. Muscle atrophy and serosanguineous draining fistulas on the lateral and medial aspects of the thigh were observed (Figure 1A). A hemogram and the temperature were normal. Radiographs of the left femur showed loss of the normal trabecular pattern, osteopenia, periosteal reaction, calcification of soft tissue, presence of the cerclage wire and of the intramedullary pin buried inside medullary canal, and a well-defined area of lucency located in the distal part of the diaphysis (Figure 1B). Positioning the dog for radiographic examination was difficult due to loss of mobility in the articulations.

Figure 1. (A) Muscle atrophy and draining fistulas on the left thigh are observed. (B) Craniocaudal radiograph of the left femur showing periosteal reaction, osteopenia, calcification of the soft tissue, presence of cerclage wire and intramedullary pin buried inside medullary canal, and a well-defined area of lucency located in the distal part of the diaphysis.

Figure 1. (A) Muscle atrophy and draining fistulas on the left thigh are observed. (B) Craniocaudal radiograph of the left femur showing periosteal reaction, osteopenia, calcification of the soft tissue, presence of cerclage wire and intramedullary pin buried inside medullary canal, and a well-defined area of lucency located in the distal part of the diaphysis.

The dog was anesthetized and the femur approached from the craniolateral aspect. Multiple fibrous adhesions associated especially with draining tracts were observed. The cerclage wire was removed. The stainless steel intramedullary pin was observable through the osteolytic defect, but a hole had to be drilled through the medial femoral condyle in order to remove it. The area was flushed with isotonic saline solution and the surgical approach was closed. Aerobic and anaerobic cultures of samples taken from the medullary canal and implant yielded Staphylococcus aureus that was resistant to ampicillin and penicillin G. Based on the culture results, cefalexin (Keflex; Eli Lilly, São Paulo, Brazil), 30 mg/kg body weight (BW), q8h, for 30 d, and metronidazol (Flagyl; Aventis Pharma, São Paulo, Brazil), 50 mg/kg BW, q24h, for 5 d, were administered. After 10 d of treatment, the draining tracts had healed. On physical examination, 1.7 y after implant removal, there was no evidence of infection. The dog was moderately lame on the left hind limb, and the range of motion in the stifle and hock was diminished. Radiographs showed that the osteopenia, periosteal reaction, calcification of the soft tissue, and area of lucency located in the distal part of the diaphysis were still present, but less evident; that there was a radiolucent area in the medial femoral condyle, where the hole had been drilled to remove the intramedulary pin (Figures 2A and 2B); and that the stifle had signs of articular degenerative disease.

Figure 2. (A) Appearance of the left limb without draining tracts 1.7 y after implant removal. (B) Lateral and craniocaudal views showing the bone lesions and the radiolucent area located in the medial femoral condyle corresponding to the hole drilled to remove the intramedullary pin.

Figure 2. (A) Appearance of the left limb without draining tracts 1.7 y after implant removal. (B) Lateral and craniocaudal views showing the bone lesions and the radiolucent area located in the medial femoral condyle corresponding to the hole drilled to remove the intramedullary pin.

Infection, fistula, inflammatory reaction, and hypersensitivity are among the complications related to implants (1). Metallosis could be considered as one in this case; however, there were no signs of metal corrosion or local staining of soft tissue, or of contact between the pin and cerclage wire (2). Fixation devices are foci for bacterial colonization, but there are differences, depending on the type of material used in the implant (3) and how receptive its surface is to bacteria and adhering to it (4,5). Bacteria may be introduced during the surgical procedure or the traumatic incident, hematogenously after the insertion of the implant, or by contiguous spreading (4,6). Sometimes, it is not possible to determine, based on history and physical examination, when and how the infection occurred, as in this case, (7). Staphylococus aureus is the most common bacterium involved in osteomyelitis and it may be resistant to antibiotics due to B-lactamase production, intrinsic resistance, or tolerance (5,8); it also stimulates bone resorption (5).

Cryptic infections may be quiescent for weeks even up to years (8). Suddenly bacteria may become active and induce local osteomyelitis and systemic disease. In this dog, the process was considered focal, because no signs of bacteremia or toxemia were observed. These infections are, in general, associated with biofilm production that prevents the action of antibiotics and phagocytes (4,5,6). A hypometabolic state may be another important factor, since glycocalyx development is not necessary for antimicrobial resistance (7). Removal of the implant is very important for resolution of the osteomyelitis (9), as occurred in this case, because it eliminates adherence sites for bacteria and restores local host defense (5).