A 40-year-old male patient was referred for magnetic resonance imaging (MRI) of the right knee after a knife attack. On physical examination, two-incision scars were observed on the anterior aspect of the knee. Both active and passive joint extensions were restricted.
Radiographic examination showed thickening of prepatellar soft tissue, partially obliterated infrapatellar bursal fat pad just above the tibial tuberosity and an incomplete fracture line at the dorsal cortex of patella (Figure 1). On the sagittal T1-weighted MR image complete transection of the patellar tendon is observed (Figure 2). Additionally, the sagittal T2-weighted fat-saturated(fs) MR image shows edematous signal changes in the Hoffa’s fat pad, subcortical patellar bone marrow edema and periosteal oedema corresponding to the radiographic findings at respective locations (Figure 3). Micrometalic artifacts, anterior of the patella, and the patellar tendon were seen on sagittal fs-T2-weighted gradient-echo images (Figure 4). Discontinuity in the patellar tendon, soft-tissue oedema in the prepatellar area, and bone marrow oedema in the patellar bone were seen on consecutive axial proton density fs MR images.
Although the patellar tendon with its bone to bone attachment classifies this structure as a ligament, it is commonly referred to like the patellar tendon. It constrains patellar motion together with the patellotibial ligament, and the retinacula. Primary supporting muscles of the patellofemoral joint are rectus femoris, vastus-lateralis, vastus-medialis, and vastus-intermedius which join caudally to form the quadriceps tendon .
The most common cause of patellar tendon tear is a weakened patellar tendon due to overuse traumas, chronic diseases, steroid use, and tendinopathy . Patellar tendon tear due to penetrating sharp object trauma is rare.
The most common site for a tear is avulsion of the tendon from the inferior pole of the patella. Other particular sites include midsubstance tear and avulsion from the tibial tubercle .
In this case, two separate incisions were present, one at the patellar level and the other in the infrapatellar area. Trauma at the patellar level caused damage to the quadriceps tendon, patellar cortex, and dorsal periosteum, trauma at the infrapatellar level caused mid-substance patellar tendon discontinuity and incomplete fracture of the proximal tibia.
Findings of complete tear on radiographs include high riding patella, blurring of the posterior margin of the patellar tendon into Hoffa's fat pad and an avulsion fracture .
Ultrasonography is a cost-effective and highly sensitive method that is used as a first-line diagnostic modality for patellar tendon pathologies. High-frequency linear transducers provide high spatial resolution [3,4].
MRI can further demonstrate the whole extent of the patellar tendon, peritendinous soft tissue, bone injuries and outlines possible co-existing intraarticular lesions . Due to the presence of an open wound in the prepatellar area, MRI was preferred as the first-line diagnostic modality in the presented case.
Magic angle artifact which is most pronounced in the proximal patellar tendon on MR should be distinguished from true tendon disorders. True pathologic signal changes due to degeneration or partial tears require a close comparison between T1 and T2-weighted images. Findings of tendon thickening and associated fluid may secondarily help to discriminate true tendon disorders from magic angle artifact .
Complete patellar tendon tears require surgical treatment, either by primary repair or tendon reconstruction based on the type of tear. With complete tears, primary repair is performed. The end-to-end repair is preferred for midsubstance tears, as in our case. The prognosis is better with prompt surgical repair .
Written informed patient consent for publication has been obtained.
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