According to earlier research, the proximal epiphyseal plate of children's femur was separated into three sections: the femoral head epiphyseal plate, the greater trochanter epiphyseal plate, and the femoral neck isthmus epiphyseal plate. The femoral head epiphyseal plate mainly promotes the elongation of the femoral neck, the greater trochanter epiphyseal plate mainly promotes the growth of the greater trochanter, and the femoral neck isthmus epiphyseal plate mainly promotes the thickening of the femoral neck3.
ATD is defined as the distance between the tip of the greater trochanter and the articular surface of the femoral head on the long axis of the femoral shaft in X-ray, which represents the relative positional relationship between the greater trochanter and the femoral head. When femoral head disease affected its epiphyseal plate, the growth of the femoral neck will be slowed down and the greater trochanter will grow excessively, so that the ATD will decrease. Some researchers believed that ATD was influenced by the epiphyseal plate of femoral head and greater trochanter, and that TTD was only affected by the growth of the greater trochanter, which would allow TTD to more accurately reflect the growth of the greater trochanter in radiographs4. TTD is the distance from the tip of the lesser trochanter to the tip of the greater trochanter. Because the epiphysis of the lesser trochanter grows laterally rather than longitudinally, the change of TTD directly represents the growth of the greater trochanter. LTA is the distance from the tip of the lesser trochanter to the articular surface of the femoral head, which is influenced by the growth of the epiphyseal plate and the ossification center of the femoral head. In order to eliminate the influence of the ossification center of the femoral head on the measurement of the femoral neck growth, we further included ELD, which is the distance from the tip of the lesser trochanter to the midpoint of the epiphyseal plate and represents the growth of femoral neck. At the same time, we introduced TTD/ELD to explore the relative growth rate of greater trochanter and femoral neck. The results of TTD/LTA were also presented for the reference of follow-up research. Furthermore, In order to reduce the error caused by the posture when shooting, all the measured values were measured on the long axis of the femoral shaft.
According to our findings, among normal children aged 5–14 years, there was no significant difference in the mean ATD at different ages. The mean ATD for male was 23.51 mm, and the 95% confidence interval was (14.96,31.60) mm, while that for female was 21.40 mm (14.01,30.41) mm. The minimum ATD for male and female aged 5–14 years were 8.6mm and 8.2mm respectively. Children from 6 months to 4 years old were not included in the results. We believe that in this period, the ossification center of children's femoral head is in a process of rapid ossification, and it on X-ray changes obviously with the increase of age, which would make the ATD of this period change obviously. Figure 4 also shows that LTA and TTD generally increase with age, and the distance between them is stable. Tables 3 and 4 also shows TTD/ELD and TTD/LTA increase with age. It is worth noting that TTD/ELD gradually approaches 1 with age and remains stable.
We discovered that the ATD in our investigation were similar to the examined in some articles. In 2008, James J. McCarthy & Dennis S. Weiner reported in the literature that among their 35 children with LCPD, ATD remained unchanged at different age stages, with the average of 20 mm, which was 16.1mm and 22.8mm for male and female, respectively5. Langenskiöld and Salenius had also reported differences in ATD between male and female, 23 ± 4mm and 16 ± 3.6mm for male and female, respectively6. Lawrence J.Iwersen mentioned that in children with unilateral DDH and ischemic necrosis, the ATD of unaffected side of female was 21.9mm and that of male was 22.8 mm7. Some scholars even included 30 bodies of children aged 5–10 years old to measure, and the obtained mean ATD was 21.84 ± 3.96 mm8.
The decrease of ATD had been reported in dislocation of hip9, DDH, SCFE and LCPD. Rajiv M. Merchant once mentioned that in children with unilateral DDH, the growth of femur was the main factor that causes the difference in Leg Length Discrepancy (LLD). When the measured length was less than the unaffected side, the shortened part mainly focuses on the difference of ATD10. Some scholars had studied the difference of ATD of hip in children with unilateral SCFE, and thought that the reduction of ATD was a simple and sensitive method for early diagnosis of SCFE, which was better than other classic signs such as Trethowan sign, Loss of epiphyseal height etc 11. Similarly, the difference of bilateral ATD had been proved to have a strong correlation with LLD in SCFE children. Unfortunately, bilateral SCFE children weren’t included in this research12. In the LCPD, K. W. Park believed that the greater trochanter overgrowth was defined by the ATD index < 0.8 (ATD index refers to the ATD of the affected side/the healthy side)13 and H. Kitoh put forward that ATD < + 5mm should be taken as the standard for the greater trochanter overgrowth14. According to our results, ATD less than 8.6 mm in male and 8.2 mm in female aged 5–14 years is considered as the greater trochanter overgrowth. Meanwhile, male TTD/ELD over 0.97 in 4–6 years old, over 1.17 in 7–10 years old and over 1.23 in 11–14 years old are also considered as greater trochanter overgrowth. For female, the similar inference could not be made according to our results. The notable thing is that most articles only include children with unilateral lesions and exclude children with bilateral lesions when studying the decline of ATD. At present, it is generally believed that in children with unilateral lesions, the healthy side is considered as the best control of the affected side, while bilateral lesions are excluded because of lack of control. We believe that when using the data in this paper as a control, the accuracy of unilateral side is inferior to that of unaffected side, but it can provide a good reference for the lack of control in bilateral children.
At present, it is generally believed that the effect of trochanter arrest surgery before 6–8 years old is better than that after 8 years old15–17, which is similar to the results of k− male and k− female proposed by us. K represents the growth rate of the greater trochanter at all ages. The larger k means that the greater trochanter grows faster and the better effect of the greater trochanter arrest, the lower the k represents the slower the growth rate and the less satisfactory the effect of the operation. According to the results of our study, there was no significant difference in the growth rate of the greater trochanter when the male is over 8 years old and the female is over 6 years old (p > 0.05). It is worth noting that both k− male and k− female are less than 6 during this period, which means epiphysiodesis may with an unsatisfied outcome when k < 6.
In this study, we have compared the ATD to chronological age and not skeletal age. Researches at present show that there are differences between chronological age and skeletal age18, but a high correlation between chronological age and skeletal age was provded19,20. We choose the chronological age because it is easier to obtain and more practical in clinical work. we did not take the posture of the radiograph into consideration. In a study comparing supine and standing pelvic radiographs, it was found that there were clear differences between the positive rates of the supine and standing pelvic radiographs for the crossover sign, ischial spine sign, acetabular depth, and lateral center edge angle (LCEA)21. At the same time, the photos taken in supine position may not be absolutely symmetrical, because it is easier to control the slight change of hip rotation in standing position; However, it was difficult to control the symmetry and rotation between the left and right femurs in supine position22. Therefore, when selecting X-ray films, we paid attention to the fact that bilateral pelvic symmetry, bilateral obturator symmetry and bilateral trochanters are clearly displayed. Another cause of left-right asymmetry may be the defect of lower limb advantage, usually the left-leg advantage23. After analyzing the left-right differences of ATD, TTD and LTA at different ages, the results show that there is no statistical significance. In this study, we failed to include the hip radiographs of teenagers aged 15–18. The author considered that the hip joint basically developed when male were around 14 years old and female were 13 years old. Another reason was that children under 14 years old were mainly treated in children's hospitals in China.
The ATD we acquired differs from the true ATD because children's proximal femurs include cartilages, which can not be seen on X-rays. This study mainly evaluated the relative position of the greater trochanter and femoral head in children through X-ray, so as to provide a comparative basis for clinical evaluation of the greater trochanter over growth in children, which is more practical in practical work.