Evaluation of the Tibial Tubercle to Posterior Cruciate Ligament Distance in a Pediatric Patient Population

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Evaluation of distal extensor mechanism alignment continues to evolve in children with patella instability. Prior studies support the use of the tibial tubercle to trochlear groove (TT-TG) distance but limitations exist for this measurement including: changes in the TT-TG distance with knee flexion, difficulty with finding the deepest part of a dysplastic trochlea, and limitations regarding identification of the site of the anatomic abnormality. The tibial tubercle-posterior cruciate ligament (TT-PCL) distance has been introduced as an alternative measure to address the shortcomings in the TT-TG distance by quantifying the position of the TT independent of the trochlea and with respect to the tibia only. The objectives of this study were to (1) confirm that TT-PCL measurements in the pediatric population are reliable and reproducible; (2) determine whether normal TT-PCL distance changes with age; and (3) compare TT-PCL distances in patients with and without patellar instability to assess its utility in the workup of pediatric patellar instability.


All knee magnetic resonance imaging performed for patients from birth to 15.9 years of age at our institution between December 2004 and February 2012 were retrospectively collected (total 566). Eighty-two patients had patellar instability and 484 patients did not have patellar instability. Two magnetic resonance imaging reviewers measured TT-PCL distance on T2-weighted axial images in a blinded manner. Intraobserver and interobserver agreement was measured. Correlation between TT-PCL distance and age as well as group differences between mean TT-PCL distances was evaluated.


Intraobserver and interobserver agreement was excellent (0.93) and very good (0.80), respectively. The mean TT-PCL distance was 20.1 mm with a range of 5.8 to 32.1 mm. The mean age was 12.6 years with a range of 0.8 to 15.9 years. The average TT-PCL distance was 21 mm for the instability group and 19.9 mm for the control group. TT-PCL distance increased significantly as subject age increased; however, there was no significant measurement difference shown between the patellar instability group and the control group.


TT-PCL distance increased with age in the pediatric population but did not correlate with recurrent patella instability in this pediatric cohort.

Level of Evidence:

Level III—diagnostic.

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