Imaging After a Total Knee Arthroplasty
Total Knee Arthroplasty (TKA) is the most commonly performed joint replacement surgery in the United States (1,2) with an estimated 4 million patients walking around with a replaced knee (3). Failure rates of TKAs can be secondary to prosthetic loosening, infection, instability, periprosthetic fracture, arthrofibrosis, polyethylene wear, malalignment, extensor mechanism deficiency, patellar subluxation/fracture, among other less common causes. Detecting these complications requires a clinical acumen, knowing what you are looking for, and a broad understanding of the capabilities and limitations of the various imaging modalities available. A recent review article on the appropriateness of imaging published in the journal of the American College of Radiology (ACR) looked at over 168 publications to determine the appropriateness of a variety of post-TKA imaging (4). In this review article, the authors looked at 12 common clinical scenarios of a post op TKA patient with the imaging recommendations rated by appropriateness by the American College of Radiology (4).
Several studies indicate that radiographs are not needed until first outpatient follow up appointment unless the procedure was complicated. Kosashvili et al surveyed orthopedic surgeons and estimated of the 65,910 TKAs performed, only 8 required same day revision based off of radiographs (5). On outpatient visit, some providers recommend getting a baseline hip to ankle x-ray which may be better in calculating tibiofemoral angle to assess coronal plane alignment of the joint, but other studies show a standard AP is a valid alternative (6). An AP image may prove to be beneficial in future as a comparison if malalignment becomes an issue.
It is generally recommended and accepted that every 1 to 2 years, the patient should get an x-ray of their knee in an asymptomatic patient. However, there is no concrete guidelines or recommendations.
Pain from prosthesis
Patients can present with pain and symptoms related to the wear and degeneration of the polyethylene articular surface. This is best assessed with 3 view radiographs which can show joint space narrowing, varus or valgus deformity, or patellar tilt (4). Fluoroscopy can allow the provider to evaluate the joint under varus and valgus stress which can help improve the evaluation of polyethylene thickness (7). US is under investigation as an emerging modality that may be able to assess component wear, but currently only has limited evidence.
Evaluating for periprosthetic infection
Pain after a TKA, especially early after a TKA but can occur at any time, a periprosthetic infection must be suspected and ruled out. A chronic smoldering infection can present with pain only, without obvious swelling, redness, warmth, fever, or a leukocytosis on laboratory work. The American Academy of Orthopedic Surgeons (AAOS) guidelines recommends the use of ESR and CRP testing as a screening tool and if both are negative, infection is unlikely (8). There is also emerging evidence of synovial alpha-defensin (peptide made by neutrophils) demonstrating 97% sensitivity and 96% specificity (9). This may be an emerging useful tool in increasing sensitivity of arthrocentesis as some studies show only a 75% sensitivity in joint aspiration (10). Laboratory testing, joint aspiration, and x-rays are the main stay in working up a joint for infection.
Positive synovial fluid cultures
When a patient is diagnosed with an infected joint, there is no subsequent imaging that is needed and treatment should be started promptly. MRI with and without contrast can be considered to assess for osteomyelitis or fluid collections. CT scan with IV contrast and US can also assess for edema or fluid collections, however knowing that the studies may be limited due to metal artifact (4).
Negative cultures, suspected infection
In a patient with negative or inconclusive studies, but infection is still suspected, a repeat joint aspiration is the next best study. It is important to remember that cultures can be negative up to 7% of the time and this may be associated with antibiotic exposure prior to culture (11). If the data is still inconclusive, then a combined marrow imaging and tagged WBCs bone scan can be used to assess for inflammation and infection with a sensitivity of 96%, specificity of 87%, with an accuracy of 91% in correctly identifying infections (12, 13). However, it is important to note that marrow imaging, bone scans, and tagged WBC scans when performed alone have been shown to be less reliable. Bone scans specifically can show increased uptake in a normal post surgical knee for several years and a tagged WBC scan also can be falsely positive in a patient with aseptic loosening. These tests have large amounts of radiation, are costly, complicated with small margin for error due to timing. It is more reasonable to repeat arthrocentesis prior.
Negative cultures, suspect aseptic loosening
Aseptic loosening is the major cause of TKA failure more than 2 years out from surgery (10). A CT scan is one of the best tests to assess for lucent zones that may be less apparent on radiographs. Fluoroscopy is also reasonable as it allows the provider to manipulate the knee looking for lucent lines that may be obscured on standard views. Bone scans can have some utility, but it is important to note that 20% of asymptomatic patients will have positive bone scans 1 year out from surgery and 12.5% 2 years out (14). In a patient several years out from surgery, this may be more appropriate after infection is ruled out.
Negative cultures, suspect granuloma or osteolysis
Osteolysis occurs secondary to macrophage phagocytosis of particle debris from the polyethylene, cement, and metal causing cell mediated inflammatory response (15). This can be asymptomatic at first and progress to symptoms as it becomes more severe. If there is concern of osteolysis on radiographs, it is reasonable to get a CT scan without contrast to further assess. Fluoroscopy can also be useful in assessing for osteolysis that may have been obscured in traditional views.
Concern for instability
Instability of a prosthetic is usually due to surgical error and poor prosthesis selection causing displacement of the articular surfaces (16). In assessing an unstable joint, hip to ankle radiographs can help assess for malalignment as that is often associated with instability. This is where it may be helpful to have an initial hip to ankle x-ray on initial follow up to establish a baseline and a comparison. Fluoroscopy while performing valgus, varus, and anterior/posterior strain can show dynamic motion of the joint. CT scan and MRI can sometimes be appropriate when wanting to assess rotational position of the prosthetic which can impact stability in flexion (17).
Suspect periprosthetic fracture
Periprosthetic fractures can occur during surgery or from low energy trauma, especially in older adults with lower bone density. Patellar fractures are rare, but can be associated with rheumatoid arthritis, steroid use, osteonecrosis, and malalignment (4). Radiographs are always taken first, but it is reasonable to perform a CT scan without contrast to furth assess for fracture.
Suspect complications related to patella
Patellar complications after a TKA include dislocation, fracture, prosthetic loosening or wear, impingement, and osteonecrosis (18, 19, 20). Radiographs are the most appropriate image to assess the patella, specifically the axial (sunrise) view that can be done both weight bearing and non weight bearing to further assess patella mechanics (21).
Measuring component rotation
In a patient with patellofemoral pain or patella complications, it may be reasonable to assess prosthetic component rotation as this can lead to patellar complications (22). The best way to assess for measuring axial malrotation is a CT scan without contrast.
Suspect periprosthetic soft tissue dysfunction unrelated to infection
When soft tissue abnormality, tendinopathy, post operative arthrofibrosis, patellar clunk, or impingement of nerves is suspected, US and MRI without contrast are the most appropriate next best imaging modalities to order. US can be used to evaluate for quadriceps/patellar tendinopathies, post surgical arthrofibrosis, and periarticular soft tissue masses as seen in patellar clunk syndrome (23, 24). US can be limited by provider experience, but otherwise is equally appropriate studies with US being much more cost efficient and convenient for patients.
– More Knee Pain: https://www.sportsmedreview.com/by-joint/knee/
– Knee pain @ Wiki Sports Medicine: https://wikism.org/Knee_Pain_Main
This review was performed by Dr. Michael Schoeller a second year internal medicine resident at NCH Healthcare in Naples, Florida. He will be applying for a primary care sports medicine fellowship next year.
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