Pain is the common denominator for many complications following TKA.
Identifying the underlying cause is challenging. For example, stiffness and knee pain can be the cause and the result of the other, and they are self-reinforcing. Working through the web of differential diagnoses is critical because performing revision TKA to treat pain without an underlying cause has only a 40% improvement.
There are some details about the symptom of pain that offer clues to the underlying diagnosis.
Timing of pain.
A knee that is painful since surgery is associated with instability (such as component malalignment or unbalanced gaps), acute infection, soft tissue impingement (from component malpositioning or incorrect sizing), or from an external source (such as the ispilateral hip or back, which is hopefully a secondary source of pain, and not the true pain misdiagnosed as an arthritic knee).
A knee that initially improved and then became painful is associated with component loosening or infection. Additional information, such as start up pain suggest component loosening while continuous pain or night pain suggests infection, tumor, or ARDS. A recent history of trauma may also make one think of loosening or periprosthetic fracture.
Physical examination offers other insights.
Point tenderness can indicate an area of impingement (such as pes bursa inflammation from medial overhang of the tibial component), or soft tissue reaction from retained cement. Point tenderness over the medial metaphysis (at least 1 cm below the joint line) can also indicate component loosening or stress fracture. Joint effusion is very nonspecific and results from hemarthrosis, synovitis, ligament instability, poly wear, infection…you get the idea). Evaluating TKA stability and patellar tracking helps to identify issues with malpositioning.
Laboratory and radiographic work up should always be part of the evaluation for TKA related pain.
Inflammatory markers should be ordered in most cases unless there is an obvious etiology for pain. The ESR and CRP are standard initial tests to evaluate for infection. If positive, a follow up aspiration is recommended for cell count.
AP and Lateral X-ray will provide information about component positioning (see post-op x-ray evaluation), loosening, and periprosthetic fracture. Comparison to prior x-rays is helpful when looking at osteolysis and loosening. A high quality AP and Lateral x-ray is critical to evaluate component positioning and alignment. Further imaging studies such as CT or MRI (look for osteolysis/loosening, fracture, position malalignment), and bone scan (look for loosening) can be considered will be discussed in the following sections.