Tibial tubercle torsion is associated with patellar height when measured by computed tomography

Abstract

Purpose: To establish an average tibial tubercle (TT) torsion angle from computed tomography (CT) scans of patients without known patellofemoral instability and investigate whether TT torsion angles would differ based on demographics, tibial tubercle-trochlear groove (TT-TG) distance and patellar height.

Methods: The Stryker Orthopaedics Modeling and Analytics (SOMA) database was queried for patients with CT scans and available measures related to patella and TT position. The mean TT torsion angle was compared in patients with an increased and normal TT-TG distance (≥20 vs. <20 mm) and patellar height (Insall-Salvati [IS] index ≥1.3 vs. <1.3). Measurements of sulcus angle, patellar inclination angle, congruence angle, trochlear groove depth and long limb axis alignment were assessed.

Results: A total of 886 knees from 499 patients within the SOMA database were included, with a mean age of 59.4 ± 16.5 years and 238 (48%) females. The mean TT torsion angle for all patients was 24.7 ± 5.2°. Females had a significantly higher mean IS index (1.24 vs. 1.18), TT-TG distance (13.8 mm vs. 11.8 mm) and TT torsion angle (25.5° vs. 24.0°) compared to males. The mean TT torsion angle for patients with a TT-TG distance ≥20 mm and <20 mm was 24.7° in both groups (p = n.s.). There was a significantly greater TT torsion angle in patients with an IS index ≥1.3 (26.6°) compared to those with an IS index <1.3 (24.0°) (p < 0.001). A weak and positive correlation was found between TT torsion angle and IS index (r = 0.242, p < 0.001), but not with other measurements.

Conclusion: The mean TT torsion angle for patients without known patellofemoral instability was 24.7° and increased TT torsion angles were associated with increased patellar height. An association between TT torsion and TT-TG was not found. Findings of the current study describe the relationship between morphologic assessments of the patellofemoral joint using CT.

Level of evidence: Level IV, cohort study.

Keywords: CT; Insall‐Salvati index; TTTG; patellar height; patellar instability; tibial tubercle torsion.

Figure 1

Calculation of the tibial tubercle (TT) torsion angle. (a) A patient image is used to manually calculate TT torsion using an axial computed tomography (CT) slice. (1) The cut in which the posterior femoral condyles are best visualized is used to create the posterior femoral condylar axis. (2) To determine the TT axis, a line is drawn through the intersection of two points marking the medial and lateral borders of the tibial tubercle. (θ) The TT torsion angle is then calculated on the medial side of the two axes. (b) Example of TT torsion calculation using three‐dimensional bone modelling of CT scans from SOMA automated software (24.2°).

Figure 2

Demographic comparisons of radiographic measurements. (a) Sex: male (n = 463), female (n = 423). (b) Age in years: <18 (n = 8), 18–39 (n = 103), 40–64 (n = 371) and ≥65 (n = 367). *Significantly higher value detected compared to males (p < 0.05). CT, computed tomography; IS, Insall‐Salvati; TT, tibial tubercle; TT‐TG, tibial tubercle‐trochlear groove. Measurements described for all included CT scans (not individual patients).