Baker’s cyst was first reported by Adams in 1840, in detail by Baker in 1877, and in even more detail in various later articles. Infection of a Baker’s cyst is very rare (21 reported cases) [3]. Most such infections developed in immunosuppressed patients, such as those with myelodysplastic syndrome, psoriasis, rheumatoid arthritis, or Epstein-Barr virus infection [2, 4].
On the first postoperative day, the CRP was 9.62 mg/dL and it decreased to 2.67 mg/dL by 2 weeks postoperatively. The ESR remained at 120 mm/h continuously. Two months postoperatively, the patient had no knee symptoms, but the CRP had increased to 4.07 mg/dL. During evaluation of the cause, rheumatoid arthritis was diagnosed based on a 2010 American College of Rheumatology/European League Against Rheumatism classification criteria total score of 7. However, even with a diagnosis of rheumatoid arthritis and planned TKA, we would not have considered evaluating a Baker’s cyst infection.
The clinical manifestations of a Baker’s cyst are ischemic pain, and claudication caused by compression of the popliteal artery if the cyst is large. Also, knee joint movement is limited. To date, no symptoms distinguishing a typical Baker’s cyst from an infected cyst have been described. Rupture of an infected Baker’s cyst can trigger signs of infection, and symptoms similar to those of deep vein thrombosis may develop because of pain in the popliteal area [3]. To confirm infection of a Baker’s cyst, fine-needle aspiration is required. The aspiration fluid is usually synovial fluid-like in appearance, but the fluid from an infected Baker’s cyst is characteristically pyogenic in appearance [5]. However, invasive examination of the surgical site is generally avoided by clinicians planning TKA.
Periprosthetic joint infection occurs in 0.8–1.9% of patients after TKA [6]. Periprosthetic joint infection requires a long hospital stay incurring major costs, upsetting both the clinician and patient. Thus, before TKA, it is important to conduct a careful preoperative assessment to prevent postoperative infection. Kong et al. [7] reported that age, obesity, operation time, drain usage, diabetes mellitus, urinary tract infection, and rheumatoid arthritis were risk factors for periprosthetic joint infection. Ratto et al. [6] reported that careful management of glucose level, general nutrition and body weight, as well as smoking cessation, reduced infection rates. However, Baker’s cysts may disappear naturally after surgery and, even if they persist, no clinical manifestations are evident. Thus, clinicians usually do not treat a cyst or evaluate it in detail preoperatively.
In this case, had we performed the TKA as initially planned, periprosthetic joint infection was inevitable. Of course, if the CRP level is high, the preoperative assessment is more rigorous, but it may remain difficult to identify an infected Baker’s cyst. If rheumatoid arthritis had been diagnosed preoperatively, we would have attributed the elevated CRP to this, and would have proceeded with TKA associated with a very high risk of periprosthetic joint infection.
Although generalization is impossible given that we describe only one case, patient evaluation before TKA requires a detailed examination of a previously aspirated Baker’s cyst that remains symptomatic, especially in the presence of altered blood tests, given the potential severity of complications if the cyst is infected.