Applied Radiology

Diagnosis

Infected nonunion of the tibial shaft fracture with pin-tract osteomyelitis. AP and lateral views of the left tibia and fibula reveal ununited fractures, medial-posterior angulation of the distal fragments, sclerosis at the fracture margins, and a multilaminar periosteal response (figure 1). In addition to generalized osteopenia, there are multiple focal lucencies within the tibia surrounding the sclerotic "rings" of prior pin tracts (figure 2). These findings are consistent with infected nonunion and pin-tract osteomyelitis, signified by ring sequestra.¹ Note the proximity of the inferior tibial pin-tract relative to the fracture site. Physical examination in the emergency room revealed a cutaneous fistula at the anterior lower leg, with extrusion of bony fragments. The differential diagnosis is limited. Other, less likely diagnostic considerations include myeloma, lymphoma, adamantioma, or pathologic fracture associated with metastatic disease. The "end on" appearance of these lesions simulates the nidus of osteoid osteoma, although the broad surrounding lucency and clinical features exclude this lesion.

Discussion

Percutaneous external pin fixation is a commonly used method for fracture reduction and stabilization because it allows transmission of forces across the fracture without violating the fracture site.² Minor pin-tract infections with purulent drainage that respond to antibiotic therapy and local wound care are seen in 5% to 10% of patients. A major complication of pin fixation is pin-tract osteomyelitis, which may be encountered in up to 4% of patients.³ Factors predisposing to pin-tract osteomyelitis include poor hygiene, taut skin surrounding the pin penetration site, intracortical placement, pin motion, pins-in-plaster, and immune compromise.^3,4 Flat bones (e.g. pelvis) are affected less commonly than long, tubular bones.⁵ Thermally-induced osteocytic death with subsequent medullary bone repair is a normal osseous response to pin placement. It is seen radiographically as a zone of sclerosis surrounding the pin tract. Cell death increases with high speed drills and blunt pin tips, which cause higher heat production compared to hand drilling with sharper drill bits.³ In supervening pin-tract infections, the zone immediately surrounding the pin tract becomes necrotic and is separated from viable bone by inflammatory tissue. This phenomenon is seen on the radiograph as a ring sequestrum. Additional radiographic findings suggesting infection include periosteal reaction and soft tissue swelling.¹ Detection of ring sequestra requires a radiographic projection with the pin tract approximately parallel to the central ray. Fluoroscopically-guided positioning may be helpful. Thermal osteonecrosis is seen as a broad zone of pin-tract sclerosis, with an occasional subtle radiolucent halo. Normal pin tracts may show a focal, central radiodensity after pin removal. Pin loosening appears as a lucency between the metallic pin and the sclerotic pin tract. Common infecting organisms are Staphylococcus aureus, Pseudomonas aeruginosa, Proteus mirabilis and Streptococcus marcescens.Staphylococcal osteomyelitis is more common in short-term fixator use (less than four months), while gram-negative rod infections are more often associated with long-term use.⁵ Therapy includes removal of pins, appropriate antibiotic therapy, and over-drilling of the infected tract with or without delayed autologous bone grafting.^3,5

1. Nguyen VD, London J, Cone RO:Ring sequestrum: Radiographic characteristics of skeletal fixation pin-tract osteomyelitis. Radiology 158:129-131, 1986.
2. Sisk TD:General principles and techniques of external skeletal fixation. Clinical Orthop Rel Res 180:96-100, 1983.
3. Green SA: Complications of external skeletal fixation. Clin Orthop Rel Res 180:109-116, 1983.
4. Selegson D, Harman K:Negative experiences with pins-in-plaster for femoral fractures. Clin Orthop Rel Res 138:243-145, 1979.
5. Green SA, Ripley MJ:Chronic osteomyelitis in pin-tracts. J Bone Joint Surg 66:1092-1098, 1984.