What is the tooth sign on the skyline view of the patella?
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Answer:
The tooth sign is a bony spur projecting from the superior pole of the patella at the insertion of the quadriceps tendon, seen on the skyline or axial patellar view as a discrete tooth-like process rising from the patellar margin. It is an enthesophyte, that is, new bone formed within the tendon at its attachment in response to chronic traction, and it is therefore a marker of quadriceps enthesopathy rather than of joint disease. It is common with increasing age and with patellofemoral osteoarthritis, and when symptomatic it accompanies quadriceps tendinopathy, which ultrasound or MRI demonstrates as tendon thickening, loss of the normal fibrillar pattern, hypoechoic change or intrasubstance signal alteration at the insertion. The findings that should be assessed with it are the other features of patellofemoral osteoarthritis, lateral facet joint space narrowing, subchondral sclerosis and trochlear osteophytes, and the alignment measures such as patellar tilt and the trochlear sulcus angle. Distinguish an enthesophyte from a syndesmophyte or the shaggy, fluffy enthesitis of the seronegative spondyloarthropathies and DISH, in which multiple entheses at the pelvis, calcaneus and patella are involved together, and from an old avulsion fragment or a dorsal defect of the patella, a well-corticated superolateral lucency that is a normal variant and must not be called a lesion.
Why is it called so?
The spur rises from the smooth superior patellar margin like a single tooth from a gum line.
Pathophysiology
The enthesis of the quadriceps tendon is a fibrocartilaginous attachment that concentrates tensile load at a small area of bone. Repetitive traction produces microinjury at the fibrocartilage, and the reparative response includes endochondral ossification within the tendon substance, so new bone extends from the patella into the tendon along its line of pull. This is the same mechanism that produces calcaneal enthesophytes at the Achilles and plantar fascia insertions. Ossification is a response to mechanical load and is therefore usually asymptomatic, which is why the finding correlates poorly with pain and must be interpreted against the clinical examination and soft tissue imaging.
Alternative names: Superior patellar enthesophyte; patellar tooth
Other associated named signs: Patellar spur at the inferior pole, the dorsal defect of the patella, and calcaneal enthesophytes as the analogous finding elsewhere
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