A green light indicates a good chance of improvement after revision surgery. As good reasons for revision surgery, polyethylene wear, loosening, malposition, component overhang/oversizing, instability, malalignment of TKR, progression of OA, and infection can be considered [7, 21–23].

An orange light indicates a reasonable to moderate chance of improvement after revision surgery. Acceptable reasons for revision surgery such as painful stiffness, patella dislocation, allergy and unresurfaced patella can be considered [4, 16, 22].

A red light indicates a poor chance of improvement after revision surgery. As poor reasons for revision surgery, unexplained pain, chronic regional pain syndrome, progressive neurological disorders, depression, anxiety disorders, and uncontrolled sepsis should be considered [2, 22]. Here a realistic patient’s expectation is of utmost importance.

It is general consensus among most orthopedic surgeons that revision surgery should be performed only if the cause of the patient’s problems is unambiguously identified [1, 5, 8, 12, 15, 27]. Only if the mechanical failure mechanism is understood by the treating orthopedic surgeon revision surgery will lead to a significant relief of the patient’s pain [12]. Therefore, a standardized diagnostic algorithm should be followed in each of those patients including extended patient’s history, detailed clinical examinations, laboratory testing, conventional radiographs, stress radiographs, CT, SPECT/CT, and other examinations [5, 10, 12, 27]. Such an algorithm is described in detail in Chap.​ 32.

However, even after following such an extensive algorithm, there remains a group of patients, in which no clear cause of the pain and problems after TKR is identified [5, 9, 11].

Typically these desperate patients:

In summary these patients with unexplained chronic painful TKRs remain a major challenge for every surgeon. Many authors have emphasized how difficult diagnosis and failure analysis in these “challenging knees” can be [6, 10, 12, 28].

When no cause for the patient’s pain is identified, it has been shown that about half of the patients could improve with a conservative approach [6]. Within the first 3 years after TKR, it is most likely that a significant improvement of function and pain levels can be achieved [6]. From 3–5 years after TKR, the pain levels can still improve, but in a lesser degree [6].

Mont et al. investigated 27 knees which underwent exploratory revision surgery for unexplained pain [20]. The patients were divided into two groups. One complained about unexplained pain and the other one about flexion of less than 80°. In the group with flexion deficit, 60 % had a good or excellent outcome from revision surgery and showed improved ability to flex their knee. In contrast, in the unexplained knee pain group, only 17 % improved by revision surgery.

The exploration of the outcome of revision TKR in 28 patients showed that 83 % of patients in which a clear cause of the pain had been identified achieved excellent or good results following revision surgery [14]. In contrast, those with unexplained pain showed no improvement. Moreover, in a study of Elson et al., 2 of 24 knees were slightly more painful than before revision surgery [6].

The evidence is unambiguously clear. Hence, it is recommended to treat patients with unexplained pain nonsurgically. Patients who still want to undergo revision arthroplasty for unexplained pain must be advised that their outcome may not improve.