Published data exist regarding any of the normal foot radiographic angles. Although these studies are commonly used and cited in orthopedics, they describe few normal ankle radiographic angles. Also, although some studies have described the normal radiographic angles of ankle, a paucity of study has been published that conveys the normal radiographic angles for the ankle as thorough and comprehensive as the data in the present study[5–12].
The foundation of radiographic ankle angles is that the relative radiographic definitions. A bone’s anatomic axis is the mid-diaphyseal line. And the joint orientation angle is the angle by between the joint line and a bone’s anatomic axis or the opposite joint line[4, 14].
The present study does have its limitations that should be mentioned. First, our data were only measured by two panelists. Ideally, measurements should have more team members to do. A second limitation is that this study was limited to a set of the AP and lateral X-ray of the ankle. Because we want to minimize the potential risk to the volunteers. Therefore, this study not only collected the large data but also had a lot of participants. We believe that these angles and reference points not previously reported, which can be valuable in clinical practice.
The TAS angle is very significant for the preoperative planning of ankle osteoarthritis. TAS is an angle formed between the anatomic axis of the tibia and the joint orientation line of the distal tibia. In varus ankle osteoarthritis(OA), the varus of tibial plafond is increasing as the stage of OA deteriorates[15]. The TAS angle can indicate the degree of the tibial varus. Nevertheless, we found it is different of the TAS angle compared with the previous study. The TAS angle has been measured as 92.4 ± 3.1 degrees (range ,88.0 -100.0 degrees) by Hinterman et al [16]. But the TAS angle has been measured at 88.3 ± 3.3 degrees (range,87.6–88.9 degrees) in our study. Hinterman et al[16] collected a cohort of 93 asymptomatic control subjects. And our study has 100 normal ankles were measured. Therefore, the sample could not have played role in the difference in the TAS angle. As far as we know, our study is the first measurement of the TAS angle about China. Maybe there are differences in the angles of ankle among different race.
The TLS angle is formed between the anatomic axis of the tibia and the joint orientation line of the distal tibia which is drawn between the anterior and posterior margins. The TLS angle can illustrate the position of the tibial plafond on lateral X-ray. Changjun Guo et al[11] reported the measurement of the TLS angle. In their study, three observers measured the TLS angle. The TLS angle has been measured firstly as 81.9 ± 1.92 degrees, 78.3 ± 3.64 degrees and 79.1 ± 3.67 degrees, respectively. In our study, the TLS angle has been measured as 79.7 ± 4.07 degrees. The data of the TLS angle are very close. Maybe there are similar in the angles of ankle among one ethnic. However, these need a large of a sample to prove.
The present study also researched the other angles to measure the data and reference points. Overall, this study finishes a systematic comprehensive measurement system of the normal ankle. These data will be valuable for surgeons and clinicians in the assessment of the ankle.
In conclusion, the data collected in this study have presented a comprehensive and valuable set of standard radiographic angles and reference points. These data will be useful in preoperative planning and intraoperative adjustment and postoperative evaluations of the ankle. A total of 9 angles of ankle was measured in our study, which can illustrate the ranges of the ankle position in the normal Chinese.