Summary

This study seperated elevated TT-TG found in patellofemoral instability patients in a morpholgical or tibiofemoral component and takes a closer look at its root cause.

Abstract

Background

Tibial tuberosity to trochlear groove (TT-TG) distance serves as one of the main metrics clinicians use to understand how a patient’s osteological morphology leads to patellofemoral instability (PFI). TT-TG is dependent both on morphological variation, such as lateralization of the tibial tuberosity (i.e., translational component), and on external tibiofemoral rotation (i.e., rotational component). Established surgical methods address either the translational (e.g., tibial tuberosity transfer osteotomies) or rotational (e.g., derotational osteotomies) component.

Purpose

The purpose of this study is to split TT-TG into its rotational and translational components, to elucidate how they relate to each other, and to determine how they differ between recurrent PFI patients and controls.

Methods

CT-scans of recurrent PFI patients, with at least two reported dislocation events and seen by physicians in our institution’s orthopedic department, were retrospectively acquired. The control cohort’s CT-scans were acquired from the Multicenter Osteoarthritis Study (MOST). CT-scans of both groups were segmented and 3D landmarks placed on the distal femur and the proximal tibia. TT-TG and its rotational and translational components and tibiofemoral rotation were algorithmically calculated.. The two cohorts’ means were compared using Mann-Whitney U-tests. Pearson coefficients were used to evaluate the correlation between rotational and translational components of TT-TG. The reliability of the measurement methods was evaluated with interclass correlation coefficients (ICC). The required sample size for a power level of 0.80 was calculated with an a priori sample size calculation.

Results

A total of 26 PFI (female: 46.2%; age: 24.6±10.0 years) and 303 control knees (female: 51.4%, age: 51.5±5.7 years) were analyzed. Significant differences between patients and controls for means in TT-TG (18.7±4.8 vs. 12.1±3.5 mm, p<0.001), rotational (5.3±2.5 vs. 1.0±2.6 mm, p<0.001) and translational components of TT-TG (13.7±3.7 vs. 12.0±2.5 mm, p=0.016), and tibiofemoral rotation (10.7±4.7 vs. 3.9±4.8 deg, p<0.001) were found. No significant correlation between the translational and rotational components of TT-TG was found (p=0.16). 21 (81%) patients had a TT-TG of more than 15 mm; of those, 11 (42%) patients had only the rotational component above a defined threshold of 3.2 mm. Interrater reliability predictive ICCs for the four measurements ranged from 0.82 – 0.99 (i.e., good to excellent).

Conclusion

TT-TG can be split into a translational component, primarily dependent on bony morphology, and a rotational component, caused by external tibiofemoral rotation, both of which can lead to an elevated TT-TG measurement independent from each other. The rotational component is the primary factor for differences observed between patellofemoral instability patients and controls. Our findings emphasize the importance of personalized treatment strategies tailored to individual patient profiles in treating patellar instability, and will aid in more targeted selection of surgical methods addressing either translational or rotational components of TT-TG.
Key Terms
Patellar instability, Tibial Tuberosity to Trochlear Groove distance (TT-TG), Three-dimensional (3D) analysis, Tibiofemoral rotation