In the literature, the location of the TT related to the TW will migrate laterally following chronology [16, 17]. Anatomically, the PT inserts on the TT to aid extension of the knee. The anatomic alignment of the knee is 5o-7o valgus and the TT is more laterally located [3, 4]. If the patella is not laterally subluxed, a normal Q-angle is reported of 8o-20o in clinical measurement [18].
Bone is not an inert tissue and has turnover at all times [19, 20]. In principle, the components of bone increase following compressive stresses due to osteoblast accumulation [21]. On the other hands, the components of bone decrease under tensile stresses due to osteoclast accumulation [21]. The PT has 2.3 cm width and the non-linear traction by the quadriceps femoris will introduce inconsistent traction stresses on the TT. The final results may cause the TT deviating from the center of PT.
Clinical measurement of the lower arm of a Q-angle normally uses palpation on the apex of TT [2]. Ideally, the resultant force of PT action should be at the center of PT. Whether the apex of TT can be used to represent the traction fulcrum of the PT should be clarified. In the current study, with MRI evaluation the apex of TT and the center of PT are almost without discrepancy (0.64 mm of separation). Therefore, clinical use of the TT for quadriceps angle measurement may be believable,
Although the apex of TT and the center of PT are very close, in the literature which is medial or lateral is still debated. Using computed tomography (CT) in studying TT-TG distance, the values are generally larger as compare to MRI studies (12–19 mm versus 8–16 mm, Table 2) [13, 16, 22–27]. Using both CT and MRI studies of TT-TG distance in the same group of patients, the CT values are also larger [28–30]. However, in the current study with MRI measurement, the apex of TT is medial to the center of PT by 0.64 mm. Because the two parameters are very close, they may be regarded as identical and specifically distinguishing the two may be practically unnecessary.
Statistically, there is high significance (p < 0.001) between the sexes for the TW and PT width. The cause may be men to be taller [31]. Bone and the PT are consequently larger. However, this difference does not influence the relative location of the apex of TT and the center of PT between sexes (p > 0.05).
The TT-TG distance had been numerously reported to affect PM. The value of excessing 2 cm is suggested an absolute indication for medial transfer osteotomy for the TT [11, 12, 32]. Recently, a relative value is reported with consideration of the size effect of the knee [33]. Practically, measuring the TT-TG distance may avoid the interference of an unstable patella. However, it can only provide one of contributing factors in causing PM. In the literature, imbalance of peripatellar soft tissue tension is considered the most important factor in causing PM [34, 35].
The high correlation (r = 0.84) between the apex of TT and the center of PT may reveal that the latter may be evaluated reliably with clinical measurement of the former alone. Concomitant measurement of the two parameters may be unnecessary. Therefore, clinical measurement of a Q-angle may become convenient.
The limitations of the current study may include (1) all study samples are patients with knee injuries. Practically, persuading normal individuals to accept MRI examinations for pure studies are difficult. Theoretically, patients in the current study will not interfere the investigations due to without bone involvement. The results should be reliable. (2) The MRI slices are 4 mm separated. The end of the PT may be not just at the TT. Predictably, the exact TT may be more distal. The apex of TT in the current study may be at the upper part of the exact TT. However, this discrepancy may not greatly hinder the measurement.