History
A 52-year-old woman with a history of right total-knee arthroplasty (TKA) presented with persistent right knee pain. An inconclusive radiographic led to a 99mTc-MDP three-phase bone scan, which was followed by SPECT/CT imaging to evaluate knee pathology.
After an intravenous injection of 600 MBq of 99mTc-MDP, the threephase bone scan was initiated. Initial dynamic planar perfusion images were followed by planar bloodpool images of both knee joints. Delayedphase planar whole-body images were acquired three hours post injection accompanied by a SPECT/CT of both knees, which was performed on a Symbia Intevo™ 6. CT and fused SPECT/CT images were reviewed together for final evaluation.
Findings
The prior radiographic information was inconclusive (Figure 1) since there was no sign of loosening (lysis), periprosthetic fracture, or particle disease. In patients with painful joints after arthroplasty, functional evaluation is key to identifying the cause of pain.
The perfusion and bloodpool images show synovitis and osseous hypervascularization of the right knee, right patella, and medial tibial epipysis (along the medial tibial plateau). In correlation, late-phase planar and SPECT/CT images demonstrate osseous hypermetabolism in the right patella and the medial femoro-tibial joint reflecting bone stress and bone marrow edema. Varus deformity of the right knee joint with the mechanical axis of the right femur (line between head of the right femur and midpoint between two femoral condyles) is slightly in variance to the tibial shaft axis (line through the tibial shaft). Varus deformity developing following TKA leads to overload stress at various points in the right knee, especially the medial femoral and tibial compartments, as is reflected in the pattern of hypermetabolism.
SPECT/CT delineated the overload stress pattern secondary to a varus deformity delineated on the whole-body planar images. The various orientations of CT and SPECT/CT images (Figures 4 to 7) clearly demonstrate the bonyoverload stress in the patella, medial femoral condyle, and medial tibial plateau. The CT images show osteolysis in the patella, medial femoral condyle, and cystic zone of osteolysis in the lateral femoral condyle, reflecting periprosthetic bone loss. There is no visible evidence of prosthetic loosening, misalignment of articular surfaces, or patellar misalignment.
The synovitis involving the entire knee joint is reflected in the mild hyperperfusion and hypervascularity seen in the dynamic perfusion and bloodpool images. The Insall-Salvati (IS) ratio, as well as tibial tuberosity (TT)-trochlear groove (TG) distance of the right knee, is within normal limits (Figures 8 and 9), which reflects absence of misalignment of patella and prosthetic articular components. This finding, along with absence of loosening, confirms the mild varus deformity of the right knee to be the principal cause of the patellar and medial femoral condylar and medial tibial plateau overload stress, which are identified as the source of the pain.
Discussion
The most frequent cause of pain and implant loosening in arthroplasty patients is overload/misload of periprosthetic bone and patella, which is reflected as hypermetabolism related to bone stress on bone SPECT/CT. In this case this was instrumental in accurately defining the focal areas of periprosthetic bony stress in the right knee joint as related to overload stress caused by mild varus deformity secondary to TKA in absence of any prosthetic loosening, malalignment, or articular displacement or periprosthetic fracture. Bony stress-related hypermetabolism in the medial femoral condyle and medial tibial plateau delineated on SPECT/CT correlates with varus deformity visible on the planar whole-body and coronal SPECT/CT images, which causes atypical stress to the medial compartment of the right knee joint.
The level of hypermetabolism is lower than that of the patella and is likely not the principal cause of pain. The patellar hypermetabolism in absence of any patellar misalignment, malrotation, or displacement suggest patellar overload secondary to genu varum deformity. The normal IS ratio and TT-TG distance in the right knee also suggests stable patella with proper position of patellar articular surface in the trochlear groove of the femoral component of the TKA prosthesis. This rules out patellar malalignment or maltracking as a cause of the patellar hypermetabolism, thereby confirming patellar overload stress caused by varus deformity of right knee.
Patellar overload without malposition is most likely due to scarring of the patellar ligaments, the likely reason for acute pain. The CT shows perfect alignment of the prosthetic margins with periprosthetic bone without any gap—which rules out loosening. Even without loosening, the osseous overload of medial part of the tibial component with genu varum deformity is likely to be the biomechanical reason for eventual loosening. The reasons for patella overload are frequently axial patella shift (medial or lateral), high or low riding patella, and tibial malrotation to the femur that leads to patella maltracking) or scarring of the patella ligaments. Various measurements in the SPECT/CT images show the orthopedic surgeon how to correct the mal-positioning. Comparison with the other side (knee) is essential because knee alignment, patella positioning, and tilt varies from patient to patient. Measurements of patella shift, tilt, vertical position, and TT-TG are key to explain the reasons for overload. Chronic stress to the arthroplasty will eventually lead to loosening with CT evidence of lysis, which reflects instability caused by a moving prothesis. in such situations the management decision is between waiting until the arthroplasty becomes loose or earlier realignment of the joint. This depends on the pain profile of the patient.
Conclusion
Since focal bony stress may be associated with changes visualized on CT, like focal lysis or sclerosis (which are non-specific), evaluation of bone metabolism in the joint—with correlation to evidence of instability, deformity, malalignment, or loosening using bone SPECT/CT—is vital for proper evaluation of post-arthroplasty pain or movement restriction. Although an MRI can delineate bone edema related to focal bone stress, it is often difficult to interpret in the presence of a prosthesis. As shown in this particular case study, SPECT/CT with accurate co-registration of bony metabolic abnormalities to joint and prosthetic morphology can be of vital clinical value. The high-quality CT, combined with the high-resolution SPECT provided by the Symbia Intevo are key to diagnostic accuracy and confidence.