Quantitative analysis of dynamic patellar tracking in patients with lateral patellar instability using a simple video system

• Patellar tracking during active knee extension was quantitatively evaluated with a newly developed digital video camera system.
• Lateral patellar translation in both knees with and without lateral patellar dislocation in patients.
• The patellar tracking in patients with an unstable patella differed significantly from that in controls.

BackgroundAs patellar dislocation occurs during activity, it is more important to assess the behavior of the patellofemoral joint under dynamic conditions. The aim of this study was to compare patellar tracking between knees with and without patellar dislocation in patients with an unstable patella and healthy controls using a simple video technique.MethodsTwenty-three knees with patellar dislocation (dislocated group), 23 contralateral knees without dislocation (non-dislocated group), and 23 healthy knees (control group) were examined. Those with skin markers on anatomical landmarks were made to extend their knees actively, and skin markers were attached to the examiner's fingertips and the patella was followed by pinching. The knee during active knee extension was recorded with digital video cameras. The patella was tracked on imaging software, and the mediolateral patellar position (% patellar position: %PP) was calculated in reference to the knee width consecutively.Results%PP was significantly different between the dislocated and control groups, from 30° (mean ± SD: 58.9 ± 6.2%, 54.6 ± 4.7%) to 5° (64.2 ± 5.2%, 55.2 ± 5.2%). It was also significantly different between the non-dislocated and control groups, from 25° (58.9 ± 7.1%, 54.5 ± 4.6%) to 5° (63.8 ± 6.5%, 55.2 ± 5.2%). No significant difference in %PP was found between the dislocated and non-dislocated groups.ConclusionWith the new video system, patellar tracking during active knee extension was successfully quantified. The tracking patterns were the same in knees with and without patellar dislocation, and the tracking in patients significantly differed from that in the controls at lower knee flexion angles.Clinical relevanceThe development of a quantitative examination technique for dynamic patellar tracking, which is easy to use and repeatedly applicable in a clinical situation, could help to follow-up the time-dependent changes and analyze the treatment effect on an unstable patella.