Variation in tibial tubercle-trochlear groove measurement as a function of age, sex, size, and patellar instability.

BACKGROUND: The tibial tubercle-trochlear groove (TT-TG) measurement was developed to quantify morphologic abnormalities about the knee associated with patellar instability and to help guide surgical decision making.

PURPOSE: To assess variations in TT-TG as a function of patient age and size in a population of patients with patellar instability compared with those with no instability.

STUDY DESIGN: Cross-sectional study; Level of evidence, 3.

METHODS: All patients younger than 20 years who underwent surgery for patellar instability from 2010 to 2012 were included in this retrospective study. A total of 180 patients with no history of patellar instability or patellofemoral complaints were used as a control group. The TT-TG was measured on magnetic resonance axial images using the centers of the tibial tubercle and the trochlear groove. Data were normalized based on patient height, weight, body mass index (BMI), and femur width. Alpha was set at P < .05 to declare significance.

RESULTS: The average age of the 45 patellar instability patients was 15.4 years (range, 10-18 years), their mean TT-TG was 16.3 mm (range, 6.5-26 mm), and 51% were male. By comparison, the mean age of the control group was 16 years (range, 10-19 years), the mean TT-TG was 11.7 mm (range, 3-22 mm), and 58% were male. The TT-TG and the normalized TT-TG for height, weight, BMI, and femur width were all greater in the patellar instability group compared with the control group (P ≤ .001). Thirty-one percent of patients had a TT-TG greater than 20 mm in the instability group compared with 3% in the control group (P < .05, specificity 97%, sensitivity 31%). The TT-TG was found to increase as a function of height in both groups (r = 0.14, P = .04) and decreased with age only in the instability group (r = -0.3, P = .04).

CONCLUSION: An elevated TT-TG is associated with patellar instability both in pediatric and adolescent patients. However, this measurement varies as a function of patient age and height, with each centimeter in height increasing the TT-TG by 0.12. Normalization of TT-TG to patient height may control for size variations and should be undertaken in the work-up and management of patients with patellar instability.