High incidence of acute and recurrent patellar dislocations: a retrospective nationwide epidemiological study involving 24.154 primary dislocations.

Gravesen KS, Kallemose1 T, Blønd L, Troelsen A, Barfod KW. Knee Surg Sports Traumatol Arthrosc 2017 Jun 23. doi: 10.1007/s00167-017-4594-7. [Epub ahead of print] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/28646382 ]

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The authors undertook this retrospective study to determine the incidence in the Danish population of acute and recurrent patellar dislocation, and to see if their investigations might help to improve the diagnostic process and treatment in patients before and after patellar dislocation.

They explored the Danish National Patient Registry for the years 1994 to 2013. The results were expressed in the usual way for such epidemiological data, that is the number of people who had been registered with these problems per 100,000 person-years.

The incidence rate of primary patellar dislocation of 42/100,000 was considered to be high, and of these young females aged 10-17 were especially prone. In the whole population studied a primary patellar dislocation had occurred, there was a risk of the problem becoming recurrent in that knee of 22.7% (36.8% in females aged 10-17, and up to 72% if they also had trochlear dysplasia) and in the opposite knee of 5.8% (11.1% in patients aged 10-17).

The authors felt that this high risk of the problem becoming recurrent should be taken into consideration by the clinician when deciding on treatment, especially in young patients.

The authors discussed the increasing awareness over the past decade of biomechanical factors affecting the patellofemoral joint which can predispose to patellar dislocation.

The major factors included trochlear dysplasia, patella alta, femoral antetorsion and an increased tibial tubercle-trochlear groove distance.

Dislocation often happens during sporting activities, when the knee is slightly flexed, rotated inwards, in valgus position and with the foot fixed to the surface.

landing in valgus

Landing with knee in valgus and foot planted