Loosening of tibial component of knee arthroplasty with medial impression fracture defined by bone SPECT/CT

A 50-year-old man with a history of osteoarthritis of the right knee underwent medial hemi-arthroplasty of the right-knee joint. Following hemi-arthroplasty, the patient noted persistent pain in the joint for which he underwent a ^99mTc-HDP SPECT/CT scan (Figure 1), which suggested loosening of the tibial component. The patient subsequently received a revision total knee arthroplasty (TKA).

Three years following the revision, the patient presented again with persistent pain in the prosthetic right-knee joint. Routine radiographs were unremarkable. The patient was referred for a ^99mTc-DPD bone scintigraphy to evaluate for prosthetic joint pathology.

The study was performed on a Symbia Intevo™ 16 SPECT/CT scanner.

Following intravenous (IV) injection of 600 MBq of ^99mTc-DPD, a 3-phase bone scan was completed. Initial dynamic planar perfusion images were followed by planar blood-pool images of both knee joints. Delayed-phased planar whole-body images were acquired 3 hours post injection, followed by SPECT/CT of both knees. CT and fused SPECT/CT images were reviewed together for final evaluation.

The planar blood-pool and delayed images (Figure 2) suggest medial tibial compartment overload, with possible loosening of the tibial component of the TKA prosthesis. As evident from the SPECT/CT images (Figures 2-8), the primary site of stress overload is localized to the edge of the medial tibial compartment and clearly reflects overload stress with an impression fracture secondary to the chronic varus deformity of the right-knee joint after TKA.

Alignment measurements obtained from dedicated third-party 3D orthopedic software, using CT data generated from SPECT/CT, show excessive (9 degrees) varus deformity of the right knee along with slightly increased flexion, which explains the medial tibial compartment overload stress (Figures 9-11). Medial compartment stress is the cause of the impression fracture of the medial edge of the tibial condyle adjacent to the edge of the tibial component of the TKA prosthesis, defined as focal hypermetabolism on SPECT/CT. This is accompanied by loosening of the tibial component, as shown by the gap between the prosthetic edge and adjacent bone, as well as reactive bony stress around the stabilization screw as seen on SPECT/CT. The medial aspect of the posterior margin of the right patella shows patellar-overload-related hypermetabolism, possibly related to a medial shift of the patella or patellar maltracking. Thus the overall impression of tibial component loosening with medial tibial compartment overload, along with medial patellar overload—all caused by excessive varus deformity of the right knee following TKA—is clearly evident from the SPECT/CT findings.

This case illustrates how bone SPECT/CT, in combination with bone-alignment analysis using CT performed by dedicated third-party 3D orthopedic software, can correctly identify the site of excess bone stress and accurately explain the causative malalignment or post-operative deformity. The primary cause of pain in this revision TKA was the significant (9 degrees) varus deformity, which developed following surgery. This varus deformity led to chronic stress on the medial tibial compartment as well as the medial aspect of the tibial component of the TKA prosthesis. This leads to loosening of the tibial component, excessive overload stress on the medial tibial compartment, which in turn lead to an impression fracture. A ^99mTc-HDP bone SPECT/CT was instrumental in accurately defining the focal point of excessive bony stress as well as in defining the impression fracture and prosthetic loosening. The CT images from the SPECT/CT study were used by third-party 3D orthopedic software to measure mechanical alignment of the femoral and tibial components and measure the deviation from the femoral axis to compare it to the normal side, thereby demonstrating the excessive varus deformity of the right-knee joint following the prosthesis insertion.

In a similar study involving 37 patients with TKA who underwent bone SPECT/CT, the location, pattern, and intensity of uptake were compared between symptomatic and asymptomatic prosthetic knee joints.¹ Intensity and location of skeletal hypermetabolism correlated with abnormal joint alignment, particularly excess varus and flexion deformities. Symptomatic TKAs were more flexed with greater internal rotation and showed higher intensity of tracer uptake as compared to asymptomatic knees. The highest bony hypermetabolism was found around the tibial stem with significantly higher intensity in symptomatic knees. The intensity of tracer uptake in the medial compartment correlated with the mechanical varus alignment of the knee. This clinical case has a similar finding where the medial compartment overload is associated with excess varus deformity. Excess varus may lead to medial shift of the patella, which is reflected by the medial patellar overload with hypermetabolism in the medial part of the posterior patellar surface. Tibial internal rotation accompanying the varus may also cause patellar maltracking, resulting in patellar overload stress.

Bone SPECT/CT can be used to accurately define bone stress in symptomatic prosthetic knee joints. In this case, SPECT/CT helped to confirm absence of any other significant bony stress in the femoral component within the periprosthetic region. Since accurate fusion of the focal hypermetabolism and bony morphology is key 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 the proper evaluation of this case. High-quality CT was also a key requirement for the evaluation of mechanical alignment of the femoral and tibial components and measurement of the prosthetic component’s relationship with the femoral axis by dedicated third-party 3D orthopedic software. This provided accurate varus/valgus, flexion/extension, and internal/external rotation for evaluation of the true varus deformity and correlation with the location and intensity of tracer uptake on SPECT/CT for correct assessment of the pathology and focal points of overload stress.