Knee Surgery, Sports Traumatology, Arthroscopy

, Volume 26, Issue 12, pp 3593–3600 | Cite as

Patellar tracking should be taken into account when measuring radiographic parameters for recurrent patellar instability

  • Si Heng Sharon Tan
  • Mazen M. Ibrahim
  • Zhaojie Joel Lee
  • Yen Kit Michael Chee
  • James H. Hui



To date, many radiographic parameters on patellar instability have their measurements taken statically, and have not been studied in various degrees of flexion according to the patellar tracking. There are also limited data regarding the use of these parameters in predicting recurrent patellar dislocation. The current study aims to review the radiographic parameters of the patellofemoral joint in different degrees of knee flexion and to correlate them with the presence of recurrent instability.


A 10-year retrospective study was conducted on all patients who had computed tomography patellar-tracking scan done for patellar instability when aged 18 years or younger. The computed tomography patellar-tracking scans were performed with the knee in extension, 10° flexion, and 20° flexion. The axial radiographic parameters were evaluated at the patellar equator, roman arch, and distal patellar pole. Sagittal and coronal parameters were noted. Radiographic parameters were then correlated with recurrent patellar instability.


The femoral sulcus angle and trochlear groove depth at the distal patellar pole in 10° knee flexion (p value 0.04 and 0.03, respectively) and patellar equator in 20° knee flexion (p value 0.02 and 0.03, respectively) had the most significant clinical correlations with recurrent instability on multivariate analysis. Other radiographic parameters found to have significant clinical correlation on univariate analysis include the patellar tilt angle, congruence angle, femoral sulcus angle, trochlear groove depth, and Wiberg’s classification.


As per the knee dynamics, axial radiographic parameters had the most significant correlation with recurrent patellar instability when measured at the distal patellar pole in 10° knee flexion and at the patellar equator in 20° knee flexion. Future axial radiographic evaluation of patellofemoral instability should then be performed at these degrees of knee flexion and axial cuts. Trochlear dysplasia, as measured by the femoral sulcus angle and trochlear groove depth, was the most significant predictor of recurrent patellar instability in the skeletally immature. Wiberg’s classification was also a novel factor found to have clinical correlation with patellofemoral instability.

Level of evidence



Recurrent patellar instability Patellar dislocation Patellar tracking Patellar kinematics 


Author contributions

TSHS, MMI, and HHP were involved in the design of the study. TSHS, MMI, LZJ, and CYK were involved in the data acquisition and drafting of the manuscript. All authors read and approved the final manuscript.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflicts of interest.


The authors received no financial support for the research, authorship and/or publication of this article.

Ethical approval

The study was approved by the Ethical committee of the institution.

Informed consent

Informed consent was obtained and research approved by the Institutional Review Board.


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Copyright information

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

Authors and Affiliations

  • Si Heng Sharon Tan
    • 1
  • Mazen M. Ibrahim
    • 1
    • 2
  • Zhaojie Joel Lee
    • 1
  • Yen Kit Michael Chee
    • 1
  • James H. Hui
    • 1
  1. 1.Department of Orthopaedic Surgery, University Orthopaedic, Hand and Reconstructive Microsurgery ClusterNational University Health System (NUHS)SingaporeSingapore
  2. 2.Department of Orthopaedic SurgeryHelwan UniversityCairoEgypt

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