A 74-year-old male patient underwent left total knee arthroplasty (TKA) at our institution for degenerative knee osteoarthrosis. He had an uneventful postoperative course with no history of delayed wound healing or persistent drainage. Four years later, he presented with a 2-month history of a gradually developing painless swelling over the anterior aspect of the operated knee; the swelling was associated with a small sinus that was extruding a straw-coloured fluid. He had no history of fever, decreased appetite, or weight loss. He had no other musculoskeletal, respiratory, or systemic symptoms of note. He had no history of antecedent trauma, recent travel, or contact with infectious diseases. The patient is a known hypertensive, but the blood pressure was well controlled with treatment, and he is otherwise healthy. He is a retired teacher with no history of involvement in activities requiring excessive kneeling. He is ambulatory in his community and can walk comfortably with the assistance of a cane.
The patient's general physical examination results were within normal limits; positive physical findings were limited to the involved knee. There was an anterior knee swelling involving mainly the prepatellar area, approximately 7 cm in diameter, fluctuant, and not tender to palpation, with minimal surrounding erythema; the erythema was present mainly at the punctum. The punctum was draining a yellowish discharge on pressure (Figure 1(a)). There was no bony tenderness at the patella, distal femur, or proximal tibia. There was no detectable knee effusion, instability, or crepitus. The range of motion was well preserved (5-110 ), as it was a prosthetic knee. It was only painful at the end of flexion as this movement compressed the prepatellar bursa.
Plain radiographs of the knee showed a prepatellar soft tissue swelling (Figure 2(a)). There were no obvious bony changes, osteolysis, or loosening at the bone-prosthesis interface. Needle aspiration of the prepatellar bursa yielded 50 mL of slightly turbid straw-coloured yellowish fluid that was sent as per our protocol for cell count determination, microcrystal analysis, Gram staining, and Ziehl-Neelsen (ZN) staining for acid-fast bacilli (Figure 1(b)). Cultures for aerobic and anaerobic bacteria, Mycobacterium tuberculosis, Brucella, and fungi were requested.
Considering the patient's age, chronic presentation, and the presence of sinus, he was admitted with a diagnosis of infected prepatellar bursitis. The primary aims of admission were wound dressing, awaiting aspiration results, and further work-up. Aspiration results revealed normal cell counts, no crystals, and no organisms on Gram stain. To our surprise, ZN stain revealed acid-fast bacilli consistent with typical tuberculous infection; the bacterium was confirmed on culture 6 weeks later as Mycobacterium tuberculosis-sensitive to rifampicin, isoniazid, ethambutol, and streptomycin. Cultures for bacteria, Brucella, and fungi were negative. Aspiration was repeated and yielded similar results. The blood work-up showed normal total and differential white blood cell count and slight elevation of both erythrocyte sedimentation rate (ESR) (80 mm/hr; normal: 30-70 mm/hr) and C-reactive protein (CRP) (8 mg/L; normal: 0-4 mg/L). Tuberculin skin test revealed a negative result (<5 mm induration at 72 hrs), and further work-up including chest radiograph and echocardiogram revealed no evidence of systemic disease. A triple three-phase bone scan displayed normal uptake both at the bone-prosthesis interface and at the patella. An infectious disease consult was obtained. The patient was started on rifampicin (600 mg/day), isoniazid (300 mg/day), pyrazinamide (1500 mg/day), and ethambutol (800 mg/day) for 2 months and continued on the same dosage of rifampicin and isoniazid to complete a 6-month course.
Three weeks later, the swelling significantly subsided in size and the sinus healed; therefore, the patient was discharged. He was reviewed at 6-week intervals at both the orthopaedic and infectious disease clinics for clinical progression and for any side effects of the medication. At the most recent 6-year follow-up, he was doing well with no evidence of local recurrence or prosthetic loosening (Figure 2(b)).