Objectives: Following a patella dislocation event, the patella can remain unstable, leading to patellofemoral instability (PI). There are various anatomic risk factors for PI that help guide surgical treatment including the tibial tubercle to trochlear groove (TT-TG) distance. However, to the authors knowledge, no study has analyzed the temporal changes in TT-TG prior to any surgical intervention. This study, therefore, sought to understand the variations in TT-TG for pediatric patients suffering from patellar instability prior to any surgical intervention. The authors hypothesized that the TT-TG would not substantially change between timepoints.
Methods
Patients undergoing medial patellofemoral ligament (MPFL) reconstruction using CPT code 27427 between January 1, 2014 and December 31, 2019 by one of two orthopedic surgeons were identified. Patients were included if they 1) had two pre-operative MRIs performed of the same knee within 8 months of each other prior to any surgical intervention and 2) had an initial TT-TG between 10 mm and 20 mm. Exclusion criteria were previous ipsilateral knee surgery or planned MPFL reconstruction with tibial tubercle osteotomy. Initial and subsequent TT-TG measurements were compared using paired-samples t-tests with two-tailed statistical significance set at p =0.05. IBM SPSS Statistics version 22 (Armonk, NY) was used for all statistical analysis.
Results
After considering 251 patients for inclusion, 26 patients met inclusion criteria and were analyzed. Mean age of the cohort was 14.5±2.4 years and 42.3% were male. TT-TG was initially noted to be 15.3±1.8 mm. At mean time between sequential MRI’s of 4.8±1.9 months, TT-TG was noted to be 17.1±3.5 mm. The differences between initial and subsequent TT-TG ranged from a 21% decrease to a 61% increase, with a mean difference of a 12.1% increase. Comparison between initial and subsequent TT-TG values demonstrated a significant difference (p=0.006).
Conclusion
At a mean time between MRIs of 4.8 months, variations in TT-TG ranged from a decrease of 21% to an increase of 61%. These findings suggest that TT-TG measurements may vary in patients on sequential MRIs and could be due to variations in tibiofemoral rotation during imaging. Surgeons should be aware of these variations when planning surgical correction for patellar instability.