Does One Size Fit All: Variation In Tibial Tubercle Trochlear Groove (TTTG) Measurements As A Function Of Age, Size, And Patellar Instability

OBJECTIVES: The tibial tubercle trochlear groove (TTTG) measurement was developed to quantify rotational abnormalities about the knee associated with patella instability and to help guide surgical decision making. The purpose of this study was to assess variations in TTTG as a function of patient age and size in a population of patients with patella instability compared to those with no instability. METHODS: This IRB approved retrospective study assessed all patients under the age of 21 undergoing a surgical procedure for patella instability from 2010 to 2012. A cohort of 180 patients with no history of patella instability or patellofemoral complaints was used as a control group. TTTG was measured in all patients on MR axial images using the centers of the tibial tubercle and the trochlear groove. Data was then normalized based on patient height, weight, BMI, and femur width. Statistical analysis was performed with an alpha set at p<0.05 to declare significance. RESULTS: The average age of the 45 patella instability patients was 15.4 years (range 10 - 18), their mean TTTG was 16.3 mm (range 6.5 - 26), and 51% were male. By comparison, the mean age of the control was 16 years (range 10-19), the mean TTTG was 11.7 mm (range 3 - 22), and 58% were male. The TTTG and the normalized TTTG for height, weight, BMI, and femur width were all greater in the patella instability group compared to the control group (p≤0.001). 31% of patients had a TTTG greater than 20 mm in the instability group compared to 2.7% in the control group (p<0.05, Specificity 97%, Sensitivity 31%). TTTG was found to increase as a function of height in both groups (r =0.14, p=0.04) and decreased with age only in the instability group (r = -0.3, p=0.04). There was no correlation between TTTG and age in the non-instability group (r= -0.04, p=0.5). TTTG was not associated with patient gender, weight, or BMI (p>0.05). CONCLUSION: Much like adult patients, an elevated TTTG is associated with patella instability both in pediatric and adolescent patients. However, this measurement varies as a function of patient age and height. Normalization of TTTG to patient height may control for size variations and should be undertaken in the work-up and management of patients with patella instability. Given the dependence of TTTG on patient age and height, care should be taken when using absolute values or a singular critical cutoff in the adolescent or pediatric population in the surgical decision process.