Knee size chart nomogram for evaluation of tibial tuberosity-trochlear groove distance in knees with or without history of patellofemoral instability.

ABSTRACT

PURPOSE: Lateralized tibial tubercle is a cause of patellar instability. Before proceeding with reduction of the tibial tubercle-trochlear groove (TT-TG) distance, surgeons prefer to know whether this distance is pathologic. However, the pathological value remains discussed and may vary with the size of the knee. METHODS: We sought to determine variability in the traditional TT-TG distance versus the anthropometric knee size, using dimensions of the distal part of the femur and proximal part of the tibia of 85 CT scans of the knees in two groups of knees, one normal group without history of patellofemoral instability and one pathologic group with history of instability. RESULTS: The average TT-TG distance measured 13 mm in normal knees and 16.4 mm in pathologic knees. The variability in measurements between normal and pathologic knees varied respectively between ± 5 and ± 15 mm, with as consequence absence of threshold value between normal and pathologic knees. These measurements were supplemented by an analysis of a size ratio coefficient. In the normal group without history of instability, linear regression analysis showed that patients with larger knees tended to have higher TT-TG distances and that the values are associated with the mean ML femoro-tibial width (p = 0.014; Pearson coefficient = 0.4). The knees with history of instability also keep proportional increase of TT-TG with the size of the knee as the knees without history of instability. We developed a nomogram to more appropriately represent the normal values for a given size of the knee. Application of the nomographic model on the CT scan TT-TG data of the patients who have knee instability allows the orthopaedic surgeon to associate the TT-TG distance with the knee size and to evaluate the medial transfer corresponding to the knee size. CONCLUSIONS: The average TT-TG distances in normal and pathologic knees were not identical for each size of the knees.