SPECT/CT in evaluation of anterior knee pain following patellofemoral arthroplasty

A 50-year-old man with a history of patella-femoral arthroplasty for patella-femoral arthritis of the right knee joint presented with persistent anterior knee pain, which accentuated on flexion and lateral rotation of the knee. Routine radiographs were unremarkable, with the exception of a wide patellar tilt. The patient was referred for a ^99mTc-DPD bone scintigraphy to evaluate joint pathology.

A three-phase bone scan was performed on a Symbia Intevo™ SPECT/CT after an intravenous (IV) injection of 600 MBq of ⁹⁹mTc-DPD. Initial dynamic planar perfusion images were followed by planar bloodpool images of both knee joints. Delayed-phase planar whole-body images were acquired three hours post injection, followed by a SPECT/CT of both knees in which CT and fused SPECT/CT images were reviewed together for final evaluation.

Radiography in this patient shows an abnormal gap between the flat patellar-articular surface with sclerosis following resurfacing and the anterior margin of the patella-femoral arthroplasty prosthesis, which is attached to the trochlear groove with suggestion of exaggerated tilt of the right patella. The mild hyperemia of the right patella seen in the bloodpool images correlates with the lateral patellar hypermetabolism seen on the planar whole-body image, which reflects patellar overload secondary to
patella-femoral arthroplasty (Figures 1 to 3). Although the mechanical axis of the right femur and tibia are aligned and no valgus or varus deformity is visualized in the right knee, the minor hypermetabolism in the medial femoro-tibial compartment probably reflects slight medial femoro-tibial overload. In this context, bone SPECT/CT is key to the accurate identification of focal stress overload and evaluation of patellar, articular and prosthetic instability, displacement, loosening, periprosthetic fracture, and malalignment.

As evident from the SPECT/CT images (Figures 4 to 8), the primary site of stress overload is localized to the lateral part of the right patella but without any evidence of patellar fracture, prosthetic loosening, or periprosthetic focal bony stress or fracture. Predominant lateral patellar stress in absence of any other evidence of significant femoro-tibial focal hypermetabolism suggests presence of patellar overload with a lateral patellar tilt, which may reflect incorrect positioning of the prosthesis with the patellar-articular surface.

The indices of knee joint stability such as the IS ratio and TT-TG distance (Figures 9 and 11) as calculated from the CT data are normal for the right knee, thereby confirming absence of any prosthetic displacement, malalignment, or joint instability. The larger-than-normal right patellar tilt (Figure 10) measured on the axial CT images clearly shows the lateral tilt of the right patella, which may have an impact on the lateral patellar overload.

Anterior knee pain is one of the most common causes of persistent problems after implantation of a total or partial knee replacement. It can occur in patients with or without patellar resurfacing. As a result of the surgical procedure itself many changes can occur, which may affect the delicate interplay of the components in the patello-femoral joint. These causes include offset errors, oversizing of prosthetic component, rotational errors of femoral or tibial component. Additionally, instability, maltracking, chondrolysis, patella baja, and aseptic loosening can result as well. In these cases, reoperation or revision is often necessary.

Patello-femoral instability is a common cause of post-operative pain and functional limitations in the joint, which may lead to revision surgery. Patello-femoral instability or maltracking is usually diagnosed by conventional X-ray (merchant view), which can demonstrate patella lateralization, tilting of the patella, or a lateral osteophyte. Pain related to patellar overload stress secondary to patellofemoral instability is usually associated with hypermetabolism in the related patellar articular surface, as reflected in this case example.

This case illustrates how bone SPECT/CT can accurately define the cause of post-arthroplasty pain in presence of inconclusive radiographic and CT findings. CT shows mild sclerosis in the lateral part of right patella and abnormal lateral tilt with a larger gap between the patellar articular surface and the prosthetic anterior surface on the medial aspect with a large patellar tilt angle of 12%. However, in absence of any evidence of fracture, loosening, joint instability, or mal-alignment, CT findings alone do not confirm presence or absence of focal joint stress and its location nor the likely cause. Only with bone SPECT/CT is it possible to accurately define the patellar overload stress with the focal point in the lateral patella, which correlates with the medial patellar shift and large patellar tilt. The SPECT/CT scan also confirmed lack of any other significant bony stress in the femoral and tibial condylar region or articular or periprosthetic region with normal joint stability and absence of any prosthetic displacement or periprosthetic or patellar fracture. Accurately defining the patellar overload as the origin of pain with SPECT/CT is key to correct management. Since accurate fusion of the focal hypermetabolism and bony morphology is fundamental to SPECT/CT evaluation, the high resolution SPECT imaging and high CT quality (with absence of displacement between CT and SPECT acquisition planes) provided by Symbia Intevo was instrumental in proper evaluation of this case.