Figure. Computed tomography images of the femoral trochlea and tibial tubercle.
A 26-year-old man presents to the office with recurrent instability of the right patella. In the past year, the patient has had 2 patellar dislocations during athletic activities, both of which were treated with closed reduction in the emergency department. The knee instability has failed to improve with conservative treatment such as physical therapy and bracing. Superimposed axial computed tomography (CT) images of the femoral trochlea and tibial tubercle are obtained (Figure). The vertical white lines in the image show the tibial tubercle-trochlea groove (TT-TG) distance.
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Patella instability may be due to an incompetent medial patellofemoral ligament (MPFL), trochlear dysplasia, patella alta, vastus medialis insufficiency, and a lateralized tibial tubercle. A TT-TG measurement is an important tool to guide treatment of patellofemoral instability.1
The TT-TG distance is measured by superimposing an axial CT or magnetic resonance imaging (MRI) image of the femoral trochlea and tibial tubercle.2 Lines are then drawn vertically through the shallowest part of the trochlea and the most anterior part of the tibial tubercle. The presence of tibial tubercle lateralization as measured by TT-TG shows the patella being pulled laterally in the trochlea resulting in an increased likelihood of lateral patellar dislocation.2
The TT-TG distance is significantly greater in patients with recurrent dislocations. A TT-TG distance of 20 mm or more is widely used as an indication for surgical medialization of the tibial tubercle to reduce pressure on the patella tendon. A distance between 15 mm and 20 mm is considered a relative indication for surgery.2
The goal of an anterior medialization (AMZ) procedure is to move the tibial tubercle medially to reduce the TT-TG ratio to between 10 mm and 15 mm. An AMZ procedure involves an osteotomy of the tibia tubercle, which is shifted medially and fixed with 2 screws directed anterior to posterior. The TT-TG ratio does not change with age and an AMZ is generally performed when patients reach skeletal maturity.1
1. Dickens AJ, Morrell NT, Doering A, Tandberg D, Treme G. Tibial tubercle-trochlear groove distance: defining normal in a pediatric population. J Bone Joint Surg Am. 2014;96(4):318-324. doi:10.2106/JBJS.M.00688
2. Tensho K, Shimodaira H, Akaoka Y, et al. Lateralization of the tibial tubercle in recurrent patellar dislocation: verification using multiple methods to evaluate the tibial tubercle. J Bone Joint Surg Am. 2018;100(9):e58. doi:10.2106/JBJS.17.00863