The horizontal distance between the tibial tubercle and the centre of the groove (TT-TG) is an important determinant in the treatment of patellar instability. We set out investigate whether it could be measured in the outpatient setting using a length of string lined up between a proximal reference point, the centre of the patello-femoral groove and the tibial tubercle. The technique was employed in 24 knees of patients awaiting patellar realignment and the results compared with linked MRI slices through the trochlear and tibial tubercle. With the knee at 90 degrees of flexion, the centre of the trochlear groove lines up with tibial tubercle in patients with both objective and potential instability, making visual inspection of the knee in this position unhelpful in deciding whether to transfer the tibial tubercle. In the extended knee, the technique failed to detect the large TT-TG distance demonstrated on the MRI in the objectively unstable group. Visual inspection of the leg is unreliable in assessment of the position of the tibial tubercle relative to the trochlear groove. The knee should be scanned before a decision is taken to transfer the tibial tubercle medially.