This study aimed to predict the modification of the alignment between the tibial tubercle (TT) and the trochlear groove (TG) that occurs during femoral or tibial hemiepiphysiodesis. MRI scans of 541 knees were retrospectively reviewed to determine the distances between the cranial insertion of the patellar tendon on the TT and the femoral physis (FP)/tibial physis (TP). Thereafter, we developed a trigonometric formula to calculate the predicted change of the TT–TG distance that occurs during hemiepiphysiodesis around the knee using both the planned angular correction as well as the length between the physis (both distal femoral and proximal tibial) and the insertion of the patellar tendon of the TT. This study showed that TT–FP and TT–TP distances vary very little with sex and age during growth and the mean values of FP–TT and TP–TT distances (55 and 7 mm, respectively) can thus be used in clinical settings for calculating a rough estimate of the translation of the TT position that will occur during ‘guided growth’. On this subject, one can expect a 1 mm simultaneous lateral or medial transfer of the TT for every 1° of angular correction during distal femoral hemiepiphysiodesis. For proximal tibial hemiepiphysiodesis, an angular correction of 8° should roughly translate into a simultaneous 1 mm transfer of the TT. This study puts forward the hypothesis that a simultaneous modification of the TT–TG distance has to be expected following hemiepiphysiodesis, whether femoral or tibial. Level of evidence: III.