Symptomatic synovial cyst formation is a rare, late occurrence after total knee arthroplasty (TKA); these cysts are generally discovered by chance. If they enlarge, they can result in significant pain and disability. A few case reports have described the development of very large cysts that required revision knee surgery. In this patient, polyethylene wear disease after TKA resulted in a massive synovial cyst that extended into the posterior compartment of the leg, as well as a progressive peripheral neuropathy. Revision of a loose patella component and worn polyethylene liner with complete synovectomy, plus decompression of the cyst via needle aspiration, resulted in an excellent short-term outcome.
To the author’s knowledge, this is the first case report of peripheral neuropathy of the tibial nerve secondary to a massive Baker cyst after total knee replacement. The patient provided written informed consent for print and electronic publication of this case report.
Case Report
The patient was a 65-year-old woman with a complex medical history and multiple left knee surgeries, including a high tibial osteotomy and subsequent cemented TKA performed in the mid-1990s. She presented to the orthopedic department at a university hospital with complaints of knee pain 13 years after TKA. Observation was recommended; however, she was lost to follow-up.
The patient presented to her primary care physician (PCP) 16 years after TKA with a very large, painful mass in the back of her left leg. An ultrasound showed a large Baker cyst, and the patient was sent to interventional radiology. A few months later, she had an ultrasound-guided aspiration into the left calf, which produced 300 mL of thick synovial fluid. A cell count was not performed, but bacterial cultures were negative. Immediately after the aspiration, the pain was relieved.
Approximately 3 months after the aspiration, she presented again to her PCP with re-accumulation of fluid in the back of her left leg and severe leg pain. She was referred to a different orthopedic surgeon who determined that the risk of surgery was too great given her complex medical history.
The woman’s PCP referred the woman to our office 6 months after the aspiration. On presentation, her pain was localized to the posterior left leg. She reported the pain level as a constant 9 out of 10 on the visual analog scale, despite ingesting high doses of narcotics, including oxycontin and morphine. Her physical examination was remarkable for an ill-defined large calf mass. The posterior compartment of her left leg was firm and severely tender, similar to the characteristic findings seen in acute compartment syndrome.
Radiographs showed evidence of asymmetric polyethylene wear on the medial side of the knee (Figures 1A, 1B). Serum labs were ordered to evaluate for infection. C-reactive protein was mildly elevated at 5.5 mg/L (normal range, 0-5 mg/L); however, the erythrocyte sedimentation rate was normal at 12 mm/h (normal range, 0-20 mm/h). Magnetic resonance imaging of the left lower extremity with intravenous contrast showed the presence of a very large Baker cyst contained within the posterior compartment of the knee and a smaller surrounding cyst adjacent to the popliteal neurovascular bundle (Figure 2).
The Baker cyst was re-aspirated in the office. The automated synovial fluid cell count could not be performed because of high fluid viscosity. However, a manual review of the fluid specimen under light microscopy revealed proteinaceous, viscous tan-colored fluid containing no neutrophils and a few macrophages. Fluid cytology was also sent for review under polarizing light microscopy as described by Peterson and colleagues.1 Scattered fragments of polarizable foreign material were consistent with polyethylene debris (Figure 3).
The patient was counseled about the risks and benefits of surgery and was offered revision TKA with polyethylene liner exchange and synovectomy, only after complete cessation of smoking. She underwent serum nicotine monitoring to ensure tobacco cessation; however, she also reported the onset of a progressive sensory deficit over her left foot during this period. Although her medical history was remarkable for spinal stenosis, she noted a progressive decline in sensory function and new-onset paresthesia of her left foot.
An urgent consult to neurology was requested for nerve conduction studies. According to the electrodiagnostic study, the patient had a moderately severe left tibial neuropathy, likely at the popliteal fossa or distal to it. The nerve conduction study showed a chronic tibial nerve peripheral compressive mononeuropathy, and she was immediately scheduled for revision knee surgery with decompression of her Baker cyst to prevent further neurologic deficit.
During surgery, the knee joint exhibited hypertrophic synovitis with a characteristic pale-yellowish discoloration secondary to significant polyethylene wear disease (Figure 4). The polyethylene liner was severely worn with pitting, cracking, and delamination (Figure 5). While the patellar component was grossly loose, the tibial and femoral components were stable. After a complete synovectomy, the loose patellar component and tibial polyethylene liner were replaced. Osteolytic areas within the tibia underwent curettage and allograft impaction grafting. Lastly, decompression of the ruptured Baker cyst was performed via a 16-gauge needle placed in the posterior compartment of the left leg. The calf was gently squeezed with a “milking” maneuver, which yielded approximately 200 mL of thick, mucoid yellowish-brown synovial fluid resembling tapioca pudding (Figure 6).
