ORIGINAL PAPER Knee size chart nomogram for evaluation of tibial tuberosity-trochlear groove distance in knees with or without history of patellofemoral instability Jacques Hernigou 1 & Esfandiar Chahidi 2 & Medhi Bouaboula 3 & Eric Moest 1 & Antoine Callewier 1 & Theofylaktos Kyriakydis 4 & Dimitrios Koulalis 5 & Olivier Bath 1 Received: 7 January 2018 /Accepted: 18 February 2018 # SICOT aisbl 2018 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 coeffi- cient = 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. Keywords Patellofemoral instability . Tibial tuberosity-trochlear groove distance . Tibial tubercule transfer . Patellofemoral dislocation . Knee size Introduction Acute patellofemoral dislocation is estimated to 3% of all knee injuries. Its incidence is six per 100,000 individuals [1–3]. One of the consequences of a patellofemoral dislocation or repeated occurrence is cartilage damage of the patellofemoral compartment but it can also lead to generalized knee osteoar- thritis [4–7]. Furthermore, pain associated with patellofemoral knee instability without dislocation is frequent in physically active patients [8]. Both dislocation and pain are attributed to multiple factors. One of them is increase distance between tibial tubercle and trochlear groove [8, 9]. Lateralization of the tibial tubercle (TT) was the first bone- mark used in assessing patients with patellofemoral instability. To evaluate this lateralization of the TT, Goutallier [10] in * Jacques Hernigou jacques.hernigou@gmail.com 1 Department of Orthopaedic and Traumatology Surgery, Louis Caty hospital, Epicura Baudour, Saint-Ghislain, Belgium 2 Department of Orthopaedics, School of Medicine, Free university of Brussels, Brussels, Belgium 3 Department of Radiology, George Pompidou European hospital, Paris, France 4 Department of Orthopaedic and Traumatology Surgery, Erasme Hospital, Brussels, Belgium 5 1st Department of Orthopaedics, School of Medicine, University of Athens, Athens, Greece International Orthopaedics https://doi.org/10.1007/s00264-018-3856-4