Acute traumatic patellar dislocation is a common injury with majority of them occurring in young adults less than 20 years old. Traumatic patellar dislocation is mostly associated with trauma sustained during physical or sports activity. Strong patellar displacement leads to injury involving medial stabilizing structures, notably the medial patella-femoral ligament (MPFL), most frequently at its femoral attachment. Displacement involving more than 50% of the patellar width is considered abnormal. X-ray and computed tomography (CT) are helpful in diagnosing bony risk factors for patellar dislocation and to detect small osteochondral bodies. Magnetic resonance (MR) imaging is the primary diagnostic modality of choice and helpful in detecting capsular, ligamentous, cartilaginous, and bone injuries and thus can help in deciding the course of management. Here in we review the typical MRI imaging findings in the knee after acute traumatic patellar dislocation.

Keywords: Magnetic Resonance Imaging; Trauma; Patellar; Ligament; Dislocation.

1. Sillanpää P, Mattila VM, Iivonen T, Visuri T, Pihlajamäki H. Incidence and risk factors of acute traumatic primary patellar dislocation. Med Sci Sports Exerc.2008 Apr;40(4):606-11.
2. Fithian DC, Paxton EW, Stone ML, Silva P, Davis DK, Elias DA. Epidemiology and natural history of acute patellar dislocation. Am J Sports Med.2004 Jul-Aug;32(5):1114-21.
3. Atkin DM, Fithian DC, Marangi KS, Stone ML, Dobson BE, Mendelsohn C. Characteristics of patients with primary acute lateral patellar dislocation and their recovery within the first 6 months of injury. Am J Sports Med.2000 Jul-Aug;28(4):472-9.
4. Harilainen A, Myllynen P, Antila H, Seitsalo S. The significance of arthroscopy and examination under anaesthesia in the diagnosis of fresh injury haemarthrosis of the knee joint. Injury. 1988 Jan;19(1):21-4.

5. Stefancin JJ, Parker RD. First-time traumatic patellar dislocation: a systematic review. Clin Orthop Relat Res.2007 Feb;455:93-101.
6. Hawkins RJ, Bell RH, Anisette G. Acute patellar dislocations: the natural history. Am J Sports Med.1986 Mar-Apr;14(2):117-20.
7. Diederichs G, Issever AS, Scheffler S. MR imaging of patellar instability: injury patterns and assessment of risk factors. Radiographics. 2010 Jul-Aug;30(4):961-81. 

8. Elias DA, White LM, Fithian DC. Acute lateral patellar dislocation at MR imaging: injury patterns of medial patellar soft-tissue restraints and osteochondral injuries of the inferomedial patella. Radiology 2002 Dec;225(3):736–43.
9. Guerrero P, Li X, Patel K, Brown M, Busconi B. Medial patellofemoral ligament injury patterns and associated pathology in lateral patella dislocation: an MRI study. Sports Med Arthrosc Rehabil Ther Technol.2009 Jul 30;1(1):17.
10. Kirsch MD, Fitzgerald SW, Friedman H, Rogers LF. Transient lateral patellar dislocation: diagnosis with MR imaging. AJR Am J Roentgenol.1993 Jul;161(1): 109-13.
11. Lance E, Deutsch AL, Mink JH. Prior lateral patellar dislocation: MR imaging findings. 1993 Dec;189(3):905-7.
12. Nomura E, Horiuchi Y, Inoue M. Correlation of MR imaging findings and open exploration of medial patellofemoral ligament injuries in acute patellar dislocations. 2002 May;9(2):139-43.
13. Sanders TG, Paruchuri NB, Zlatkin MB. MRI of osteochondral defects of the lateral femoral condyle: incidence and pattern of injury after transient lateral dislocation of the patella. AJR Am J Roentgenol.2006 Nov;187(5):1332-7.
14. Virolainen H, Visuri T, Kuusela T. Acute dislocation of the patella: MR findings 1993 Oct;189 (1):243–6.
15. Duthon VB. Acute traumatic patellar dislocation. Orthop Traumatol Surg Res.2015 Feb;101(1 Suppl):S59-67.
16. Hinton RY, Sharma KM. Acute and recurrent patellar instability in the young athlete. Orthop Clin North Am.2003 Jul;34(3):385-96.
17. Smirk C, Morris H. The anatomy and reconstruction of the medial patellofemoral ligament. 2003 Sep;10(3):221-7.
18. Dejour H, Walch G, Nove-Josserand L, Guier C. Factors of patellar instability: an anatomic radiographic study. Knee Surg Sports Traumatol Arthrosc.1994;2(1):19-26.
19. Carrillon Y, Abidi H, Dejour D, Fantino O, Moyen B, Tran-Minh VA. Patellar instability: assessment on MR images by measuring the lateral trochlear inclination—initial experience. 2000 Aug;216(2):582-5.
20. Pfirrmann CW, Zanetti M, Romero J, Hodler J. Femoral trochlear dysplasia: MR findings. 2000 Sep;216(3):858-64.
21. Neyret P, Robinson AH, Le Coultre B, Lapra C, Chambat P. Patellar tendon length: the factor in patellar instability? 2002 Feb;9(1):3-6.
22. Salzmann GM, Weber TS, Spang JT, Imhoff AB, Schöttle PB. Comparison of native axial radiographs with axial MR imaging for determination of the trochlear morphology in patients with trochlear dysplasia. Arch Orthop Trauma Surg. 2010 Mar;130(3):335-40. 23. Schweitzer ME, Falk A, Berthoty D, Mitchell M, Resnick D. Knee effusion: normal distribution of fluid. AJR Am J Roentgenol. 1992 Aug;159(2):361-3.

Manik Mahajan

House no 109, Sector-7, Channi Himmat, Jammu (J&K), India - 180015