Keywords
Skeletal scintigraphy - suprapatellar bursitis - unilateral “hot” patella sign
Introduction
Bursae are fluid filled sacs lined by synovial membrane that reduce friction and cushion
pressure points between muscles and skin, bones and tendons. They are usually present
adjacent to joints. Suprapatellar bursa refers to synovial space located posterosuperior
to patella in-between tendon of rectus femoris muscle and femur. It usually communicates
with knee joint in most of the individuals.[[1]] Inflammation involving this bursa refers to suprapatellar bursitis.
Case Report
A 52-year-old female presented to our department with dull aching pain in the left
anterior knee for 4 days. On clinical examination, the pain was not associated with
any external signs of inflammation along with an unrestricted range of motion in the
left knee joint. Three-phase 99mTc-methylene diphosphonate (MDP) bone scintigraphy done for further evaluation revealed
increased tracer activity in the suprapatellar region of the left knee (black arrows)
in early anterior flow [[Figure 1a]], anterior blood pool images [[Figure 1b]] along with minimal increased tracer uptake in the corresponding left anterior knee
region in the delayed whole-body image [[Figure 1c]] and planar static image [[Figure 1d]], black arrow]. Axial, sagittal, coronal computed tomography (CT) [[Figure 1e]],[[Figure 1f]],[[Figure 1g]] and fused single-photon emission CT/CT images [[Figure 1h]],[[Figure 1i]],[[Figure 1j]] localize the increased tracer uptake in the left knee to patella along with fluid
collection in the suprapatellar bursa (white arrows). No morphological abnormality
is noted in the patella in the corresponding CT images. Based on scan findings of
increased flow, blood pool activity in the suprapatellar region, a diagnosis of suprapatellar
bursitis was considered. Increased osteoblastic activity involving patella predominantly
along the superior rim in the delayed phase imaging is possibly due to reactive increased
blood flow to the patella.
Figure 1 Three phase skeletal scintigraphy shows increased flow, (a) blood pool (b) involving
the left suprapatellar region along with increased tracer uptake in the left knee
in delayed images (c,d). SPECT-CT images (e-j) localize increased tracer uptake to
left patella along with fluid collection in the left suprapatellar bursa.
Discussion
Various other causes of “hot” patella sign include fracture, chondromalacia patellae,
malignancy, Paget's disease, osteomyelitis, and patellofemoral arthritis following
total knee replacement.[[2]],[[3]],[[4]] Chondromalacia patellae often presents as bilateral “hot” patella sign compared
to other causes.[[3]] Patella bone can be site of primary bone tumor apart from metastatic spread from
various cancers such as the breast, lung, kidney, esophagus, and cervix along with
its involvement in multiple myeloma.[[5]],[[6]] Absence of lytic-sclerotic component, erosions, osteophytes, and sclerosis on CT
images along with fluid collection in the suprapatellar space in our case excludes
mitotic disease, osteomyelitis, osteoarthritis, and Paget's disease favoring a diagnosis
of suprapatellar bursitis. In addition, the absence of tenderness over the patella
clinically excludes patellar osteomyelitis in our case. Even though monostotic Paget's
disease can present as a unilateral “hot” patella sign, it presents with diffuse intense
tracer uptake in the patella in the late phase of bone scan compared to our case.[[7]] Patellar metastases usually present as tracer avid lytic-sclerotic lesion involving
part of the patella on 99mTc-MDP bone scan along with increased osteoblastic activity in rest of the lesions
in the skeleton.[[8]] Ahmad et al. reported the association of “hot” patella sign with patellofemoral arthritis and
anterior knee pain in the setting of total knee replacement.[[9]] Patients having “hot” patella sign in that study showed clinical improvement following
secondary patellar resurfacing.
Conclusion
This index case emphasizes the importance of three-phase bone scan in musculoskeletal
inflammation and lists out various other causes of “hot” patella sign along with its
clinical significance on skeletal scintigraphy.
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