Introduction
The Tibial-Tuberosity-Trochlear-Groove (TTTG) distance is an important parameter to measure the position of tibial tuberosity (TT) in relation to the trochlear groove (TG) in patella instability cases. This measurement allows to determine if medialization of the tibial tuberosity is needed or not. It is also a known fact that tibia rotates externally in the last 15° of extension and locks on femur, called as screw-home mechanism. On flexion, tibia again rotates internally and rolls over the femoral condyles. If tibial rotation takes place with flexion-extension, then position of TT cannot be static in relation to TG. The purpose of this study is to determine the dynamic relationship between TT and TG on Dynamic 3D Printed Knee models in various range of movements and to compare the data with CT and computer aided design (CAD) measurement controls. The hypothesis is that routinely measured TTTG in neutral extension is a mis-guiding parameter in clinical settings.
Type of Study: Prospective comparative analysis
Methods
In order to minimize the cost and radiation hazard to the patients, only five prospective patients suffering from recurrent dislocation of patella due to complex soft tissue and bony pathologies and who agreed to undergo 3D printing were selected. The CT scan was performed in 0° extension and 30° flexion in all the cases. In addition, one more CT scan was done either in hyperextension or in 60° extension depending on clinically dislocating patella either in extension or in flexion respectively. The collected data in different desired degrees of flexion was transferred to OrthoCAD lab at XXX Institute of Technology (Name withheld), where a team of dedicated mechanical and bio-medical engineers performed the data analysis and converted CT DICOM images to 3D CAD models and then 3D processing of images was done. Polymer printing of the processed images was done on a 3D printer (Fortus 380mc, Stratasys Ltd, USA-Israel) using ABS (Acrylonitrile Butadiene Styrene, Stratasys Ltd) material.
Results
On Dynamic 3D printed knee models, the mean reduction in TTTG distance from hyperextension to extension was 18.45%, from extension to 30° flexion was 39.71%, from 30° to 60° was 8.57%. These values were compared using CT and CAD 3D model controls retrospectively. On CT scan, the mean reduction in TTTG distance from hyperextension to extension was 11.1%, from extension to 30° flexion was 36.18%, from 30° to 60° was 5.16%; while on CAD 3D models, the mean reduction in TTTG distance from hyperextension to extension was 5.94%, from extension to 30° flexion was 34.78%, from 30° to 60° was 13.85%.
Discussion
Though original authors realised importance of change of TT position in early flexion (Goutallier 1978, DeJour H, 1994), TTTG is measured in full extension. All three methods observed reduction in TTTG distance from hyper-extension to neutral extension, to 30° flexion, and to 60° flexion. The maximum difference in reduction was noted from 0° to 30° degree, which was 39.71% on Dynamic 3D printed knee models and was comparable to 36.18% and 34.78% on CT scan and OrthoCAD measurements controls respectively. A significant reduction of 39.71% in TTTG from 0° to 30° flexion confirms that measuring TTTG in full extension is meaningless. The flexion range of 0-30° is crucial because at 30°, TT pulls patella and engages it into the trochlea; thus, making measurement of TTTG in 30° more logical.
Conclusion
The dynamic 3D printed Knee models confirmed that TTTG reduces mean 39.71% from full extension to 30° flexion and hence TTTG measured in 0° is meaningless. If at all TTTG needs to be measured, it should be measured in 30° flexion.