According to earlier research, the proximal epiphysis of children's femur was separated into three sections: the longitudinally growing epiphyseal plate, the greater trochanter-growing epiphyseal plate, and the isthmus-growing epiphyseal plate. The longitudinal growth epiphyseal plate mainly promotes the elongation of the femoral neck, the greater trochanter growth 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 epiphysis, 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 epiphysis 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 ossification 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 include CLD, 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/CLD 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.
According to our findings, among normal children aged 5–14 years, there is no significant difference in the mean ATD at different ages. The mean ATD for males is 23.51 mm, and the 95% confidence interval is (14.96,31.60) mm, while that for females is 21.40 mm (14.01,30.41) mm. Figure 5 also showed that LTA and TTD generally increase with age, and the distance between them was stable. Showed TTD/CLD and TTD/LTA in Figs. 3 and 4 also increase with age. It is worth noting that TTD/CLD gradually approaches 1 with age and remains stable.
We discovered that the ATD values 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 values remained unchanged at different age stages, with the average value of 20 mm, which was 16.1mm and 22.8mm for men and women, respectively5. Langenskiöld and Salenius had also reported differences in ATD between men and women, 23 ± 4mm and 16 ± 3.6mm for men and women, respectively6. Lawrence J.Iwersen mentioned that in children with unilateral DDH and ischemic necrosis, the ATD of unaffected side of women was 21.9mm and that of men 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 also been reported in pathological 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. 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 not the same as that of unaffected side, but it can make up for the lack of control in bilateral children. It had been pointed out that the development of the healthy hip will be affected to some extent in children with different lower limb lengths because of different walking forces. Therefore, we put forward an average of ATD at the age of 5–14 years to better judge the overgrowth of the greater trochanter and the shortening of the affected limb in children with hip diseases.
At present, there is not other relevant literature to report the cut off value of ATD requires surgical treatment. 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 greater the 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 is no significant difference in the growth rate of the greater trochanter when the male is 7 years old and the female is 6 years old (p > 0.05). It is worth noting that there is no significant difference in the growth rate of the greater trochanter when k < 6. Whether k < 6 can be used as a surgical opportunity for greater trochanter arrest needs further research to confirm. Therefore, we anticipate that our research will provide a better basis and reference for timing of surgery in the future.
In this study, we didn't 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)18. 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 position19. 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 advantage20. 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 men were around 14 years old and women were 13 years old. Another reason was that children under 14 years old were mainly treated in children's hospitals in China.
The ATD value we acquired differs from the true ATD value because children's proximal femurs include cartilage, which cannot 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.