This is the editor's interpretation of a paper published in the orthopaedic literature in 2017 (and online in 2016) - our attempt to make relevant medical articles accessible to lay readers. If you wish to read the original it is easy to ask your librarian to obtain a reprint for you from any medical library.

Morphology and Anatomic Patellar Instability Risk Factors in First-Time Traumatic Lateral Patellar Dislocations: A Prospective Magnetic Resonance Imaging Study in Skeletally Immature Children.

Askenberger M, Janarv PM, Finnbogason T, Arendt EA. Am J Sports Med. 2017 Jan;45(1):50-58 [Pubmed: ]

First time patellar dislocation is worrying for both parent and child. The anxiety revolves around whether this is going to recur and whether there are underlying structural problems that caused the first dislocation.

This paper compares the anatomy around the kneecap (or patella) in two groups of children - those that have experienced a 'first-time' dislocation of their patella and a control group of those that have not had such a problem. Because it is an 'MRI imaging' study, the parameters explored are limited to the structures that can be seen via MRI scans.

An exploration such as this should allow the identification of 'at risk' children. It should also give the parents and doctors an idea of whether a particular child suffering a first time patellar dislocation has underlying structural problems that might predispose to further incidents.

First time patellar dislocation - the study

A prospective series of 103 skeletally-immature children aged 9 to 14 years with an MRI-confirmed primary lateral patellar dislocation were matched with a control group of 69 children. Over a period of two and a half years, the patella and underlying groove in which it tracks (trochlear groove of femur bone) were scanned using standardised MRI from the side and also horizontally (sagittal and axial views). Particular attention was paid to assessing the proposed anatomic risk factors of trochlear dysplasia, abnormal lateral patellar tilt, elevated TT-TG distance and patella alta.


The findings

What the researchers found is that there was a significant difference in mean values of all established anatomic risk factors between the children with a first-time lateral patellar dislocation and the controls.

The main risk factor identified was trochlear dysplasia. The presence of trochlear dysplasia together with lateral patellar tilt (≥20°) had the strongest association with lateral patellar dislocation in these children.

The researchers clarify that the incidence of primary lateral patellar dislocation in children aged 9 to 14 years is 0.6 to 1.2 per 1000.

Anatomic Patellar Instability Risk Factors explored included -

  • trochlear dysplasia
  • abnormal lateral patellar tilt
  • elevated tibial tubercle-trochlear groove [TT-TG] distance
  • patella alta

The TT-TG distance is a sensitive indicator of potential instability. It requires a calculation from two axial slices. Basically it is a calculation of how out of alignment the tibial attachment of the patellar tendon is.