Anatomic patellar instability risk factors in primary lateral patellar dislocations do not predict injury patterns: an MRI-based study

  • Marc A. Tompkins
  • Sara R. Rohr
  • Julie Agel
  • Elizabeth A. ArendtEmail author



The primary goal was to describe the injury patterns in a population of primary (first time) lateral patellar dislocators (LPD) to lend clarity to commonly held notions about injury patterns in this population.


A prospective study identifying patients presenting with LPD between 2008 and 2012. Inclusion criteria were a history and physical exam consistent with primary LPD, and an MRI consistent with the diagnosis without other significant ligamentous injury. On MRI, location of cartilage, medial patellofemoral ligament (MPFL) injury, and bone bruising were noted. Severity was categorized as partial or complete for MPFL and cartilage lesions. Anatomic patellar instability risk factors (patella alta, trochlear dysplasia, increased TT-TG, and lateral patella tilt) were recorded and compared to the injury patterns.


This study involved 157 patients; 107 patients were skeletally mature. Of the 157 patients, 26 had surgery for this injury due to clinician-perceived need for cartilage debridement. MPFL injury severity was complete rupture (N = 69, 44%), partial (N = 67, 43%), and none (N = 19, 13%). MPFL injury location was isolated femoral (N = 16, 10%), isolated patella (N = 26, 17%), isolated mid-substance (0%), multiple locations (N = 95, 61%), and none (N = 20, 13%). Chondral injury location was patella (N = 67, 43%), lateral femoral condyle (N = 11, 7%), multiple locations (N = 53, 34%), and none (N = 26, 17%). A majority (61%) of patellar chondral lesions were at its inferomedial aspect; all medial patellar retinacular partial injuries involved the inferomedial aspect of the patella, consistent with the insertion of the medial patellotibial ligament (MPTL). Skeletally immature patients had a greater risk of isolated patellar MPFL and chondral injury. No clear relationship was found between/across the location and/or severity of bone bruising, MPFL, or chondral injury.

Clinical relevance

Underlying anatomic patellar instability risk factors defined by MRI, do not predict injury patterns. MPFL and chondral injury, as well as bone bruising, are common following LPD. The medial patellotibial ligament is torn in patellar-based medial retinacular injuries, based on MRI injury location. Skeletal immaturity plays a role in the location of the injury pattern with isolated patellar-based MPFL/chondral injury being more common in the skeletally immature patient. Sex does not appear to be a factor in injury patterns after primary LPD. Knowledge of these injury trends will help focus the clinician in injury evaluation when managing primary patellar dislocations.

Level of evidence

Case series, Level IV.


MRI Lateral patellar dislocation Bone bruising Patellar instability 


Complaince with ethical standards

Conflict of interest

The authors have no conflicts of interest.


There was no funding for this study.

Ethical Approval

Institutional Review Board (IRB) approval was obtained from the IRB (study number 1005E82296) prior to initiation of this study.

Informed consent

Informed consent was not obtained for this study. This research involved the study of existing data.


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Copyright information

© European Society of Sports Traumatology, Knee Surgery, Arthroscopy (ESSKA) 2017

Authors and Affiliations

  • Marc A. Tompkins
    • 1
    • 2
  • Sara R. Rohr
    • 3
  • Julie Agel
    • 1
  • Elizabeth A. Arendt
    • 1
    • 2
    Email author
  1. 1.Department of Orthopaedic SurgeryUniversity of MinnesotaMinneapolisUSA
  2. 2.TRIA Orthopaedic CenterBloomingtonUSA
  3. 3.Center for Diagnostic ImagingSt. Louis ParkUSA

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