r e v b r a s o r t o p . 2 0 1 5; 5 0(2) :159–163 www.rbo.org.br Original article Trochlear dysplasia and patellar instability in patients with Down syndrome  Tiago Amaral Rebouc ¸as Moreira, Marco Kawamura Demange * , Riccardo Gomes Gobbi, Zan Mustacchi, José Ricardo Pécora, Luis Eduardo Passarelli Tírico, Gilberto Luis Camanho Institute of Orthopedics and Traumatology, Hospital das Clínicas, School of Medicine, Universidade de São Paulo, São Paulo, SP, Brazil a r t i c l e i n f o Article history: Received 23 August 2013 Accepted 7 March 2014 Available online 30 March 2015 Keywords: Patellar dislocation Down syndrome Knee a b s t r a c t Objective: To analyze occurrences of trochlear dysplasia in patients with Down syndrome in the presence and absence of femoropatellar instability. Methods: Eleven knees with stable patellae and thirteen with unstable patellae in patients with Down syndrome were compared. Radiographs were produced to evaluate patellar height, trochlear angle and femoropatellar congruence angle. Results: The prevalence ratio for a high patella between the unstable and the stable patients was 1.01 using the Insall–Salvati index and 0.68 using the Caton–Deschamps index. For an abnormal congruence angle, the prevalence ratio was 2.04. An increased congruence angle was only found in four cases, all presenting instability. Conclusions: Trochlear dysplasia was only found in cases of instability. The trochlear groove angle and the femoropatellar congruence angle correlated with the presence of patellar instability. © 2015 Sociedade Brasileira de Ortopedia e Traumatologia. Published by Elsevier Editora Ltda. All rights reserved. Displasia da tróclea e instabilidade patelar em pacientes com síndrome de Down Palavras-chave: Luxac ¸ão patelar Síndrome de Down Joelho r e s u m o Objetivo: Analisar displasia troclear em pacientes portadores de síndrome de Down na presenc ¸a e na ausência da instabilidade femoropatelar. Métodos: Comparac ¸ão de 11 joelhos com patelas estáveis e 13 joelhos com patelas instáveis em portadores de síndrome de Down. Foram feitas radiografias em que foram avaliados altura patelar, ângulo da tróclea e ângulo de congruência femoropatelar.  Work developed in the Knee Group, Institute of Orthopedics and Traumatology, Hospital das Clínicas, School of Medicine, Universidade de São Paulo (HC/FMUSP), São Paulo, SP, Brazil. ∗ Corresponding author. E-mail: demange@me.com (M.K. Demange). http://dx.doi.org/10.1016/j.rboe.2015.03.005 2255-4971/© 2015 Sociedade Brasileira de Ortopedia e Traumatologia. Published by Elsevier Editora Ltda. All rights reserved.