The purpose of the DDH treatment is to reconstruct the normal biomechanical environment of the hip joint, which should be realized by THA (2). However, overestimate or underestimate of FO would be the barrier of the way towards this purpose. THA is the ultimate treatment for DDH patients with old age, severe hip pain and secondary osteoarthritis, whose effectiveness has been widely confirmed. FO is the vertical distance from the center of the femoral head to the anatomical axis of the femur (11). The biomechanical model of the hip joint is a lever and the abductor muscle arm is positively correlated with FO (17). The FO of DDH patients is generally less than that of normal people. Hence the restoration of FO is fatal in THA of DDH patients. Unreasonable FO would have adverse effects on postoperative hip function and prosthesis condition (4; 24).
The underestimate of FO could affect the function of hip. Small postoperative FO directly affects hip abductor muscles. Yamaguchi T et al demonstrated that small postoperative FO led to the insufficient lever arm of the hip abductor muscles, which were composed of the gluteus medius, gluteus medius, and tensor fasciae latae, resulting in high abductor muscle load and low abductor muscle strength (34). Moreover, the low abductor muscle strength affects the abduction function of the hip joint. McGrory BJ et al observed that the low strength of the abductor muscles and the lack of soft tissue tension due to little postoperative FO together led to a considerable reduction in hip abduction range (17). These changes will lead to gluteus medius gait. The study of Mahmood SS et al described that low abductor muscle strength and hip abduction function caused by small postoperative FO gave rise to insufficient abduction torque support on body weight. The insufficient torque brought out the gait support period to be compensatory tilted to the affected side, that is, the gluteus medius gait (15). At the same time, the injuries caused by small FO not only increase the probability of dislocation of the hip joint towards different directions, but also led to soft tissue collapse which cause pain in the affected hip after THA (13; 23; 31).
These undesirable hip function which was induced by small FO can damage the prosthesis condition as well. In the study of Matziolis G et al, changed abduction torque due to small FO led to increased stress between the femoral head and acetabular cup of the prosthesis, resulting in increased wear of the prosthetic polyethylene liner (16). Besides, the loosening of prostheses was also a negative effect of small FO. In the uncemented THA study, Kochbati R et al observed that the prosthesis suffered abnormal stress at the prosthesis-bone contact surface when FO is small. The long-term accumulation of this stress resulted in difficulty in bone ingrowth and loosening of prostheses (9).
Compared with the effect of small FO, although large postoperative FO has less negative impacts, the impacts cannot be ignored in clinical practice. Bjordal F et al demonstrated that the large postoperative FO caused the abductor muscle and soft tissue long-term stretched, resulting in high tension. The immense tension led to postoperative pain and even affected hip function (2). Abnormal stress direction can cause the loosening of the prosthesis. In the bone cement THA of Harrington M et al ‘s study, large FO caused the stress direction of the prosthesis deviated, resulting in increased stress at the prosthesis-cement interface, leading to early dissociation, prosthesis loosening and even osteolysis. In the uncemented THA of Otani T et al ‘s study, large FO caused stress abnormality at the prosthesis-bone interface, resulting in difficulty of bone ingrowth and loosening of the prosthesis(6; 21). In terms of prosthesis wear rate, the study of Dastane M et al illustrated that when the FO is large, the increased torque caused excessive stress on the interface between femoral head and acetabulum of the prosthesis, increasing the wear of polyethylene liners (5).
In a word, accurate measurement of FO is necessary before THA (3). In this study, compared with X-ray, CT and EOS measurement, the 3D-CT measurement method of this study is not only accurate but also brief, convenient and popularizable. In terms of accuracy，according to the results of this study, a significant difference between 3D-CT measurement and X-ray measurement was found (p < 0.05)（2.4% in normal people and 6.9% in DDH patients）, which was caused by different measurement theory. The X-ray method is to find the approximate femoral head rotation center and the approximate femoral shaft anatomical axis on the X-ray film. After measurement of the vertical distance between them, the actual FO is estimated according to the scale on X-ray film. Although X-ray measurement method has excellent accessibility, its low measurement accuracy has been demonstrated by numerous studies (1; 8; 18; 19; 22; 26; 27; 33). That is the reason why X-ray measurement is limited to a two-dimensional plane, which is greatly influenced by the choice of imaging plane and the X-ray beam divergence (20; 30). Briefly, Actual FO is a 3D distance between point and line, but X-ray measurement was conducted on 2D planes to simulate the 3D distance. As mentioned above, FO measure by X-ray could be underestimated or overestimated due to the femoral anteversion and divergence of X-ray beams. Compared with X-ray measurement, CT measurement provides an opportunity to view the femur and pelvis from positive, lateral and axial perspectives, which lead to high accuracy. The methods of Sariali E et al, Pasquier G et al, and Weber M et al were conducted in the same theoretical framework(22; 27; 33). The CT measurement method are still conducted on two-dimensional planes, which lack the intuitiveness of 3D femur reconstruction. Thus, the anatomical features of hip joint and femur cannot be completely restored. Fortunately, these errors could be overcome by 3D-CT measurement. The 3D model reconstructed by Mimics software reproduces the actual shape of the femur based on CT data. The 3D measurement of FO was performed on the stereo model intuitively, which simulate the real FO in the maximum extent, improving the accuracy of the measurement.
For DDH patients, the 3D-CT method has higher measurement accuracy and more considerable clinical significance. From the results of this study, the error of X-ray in DDH patients is higher than that in normal people (1.68 ± 0.50mm, p < 0.05). The error rate of DDH patients (the percentage of the error value in the actual value) is 15.19%, while that of normal people is 9.09%. The error rate of DDH patients increased by 67.11% compared with normal people. Therefore, for measurement in DDH patients, the error would be more serious, which is more likely to cause various clinical problems. The reason for this phenomenon could be that the femoral developmental malformation of DDH patients may cause the secondary hip-spine syndrome, that is, secondary spinal deformity (12). This deformity increases the angle of torsion in DHH patients compared with normal people, which lead to a greater extent of femoral anteversion, resulting in lower accuracy. Besides, the flat shape of the femoral head in DDH patients is different from the ordinary round femoral head. Therefore it is hard for the auxiliary circle to fit the DDH femoral head, which leads to proximal or distal deviation of the simulant femoral head center, resulting in X-ray error (25). The 3D model reconstructed by 3D-CT measurement directly displays the actual shape of the femur in DDH patients. The center of deformity femoral head can be defined by Mimics software. The choice of landmarks and the measurement of distances are conducted in a 3D perspective, which avoids anatomical distortion in the 2D plane. The 3D-CT method can availably avoid the enormous error of X-ray measurement of FO in DDH patients. Hence, 3D-CT measurement has clinical significance to a certain extent when measuring FO before THA in DDH patients.
In consideration of the superiority of the 3D-CT method in measuring FO, the FO values of DDH patients (including Hartofilakidis-I, II, III) and normal people were compared based on 3D-CT measurements. The FO of normal people was higher than that of DDH patients (p < 0.05), indicating a general trend that FO of DDH patients is lower compared with normal people. Additionally, the FO of Hartofilakidis-I patients is higher than the FO of Hartofilakidis-II patients（p < 0.05）. According to the Hartofilakidis classification, this result represented that the FO of simple acetabular dysplasia is greater than the FO of the incomplete dislocation of the hip joint. The probable mechanism of the lower FO value in incomplete dislocation hip joint was shown as follows. The posterior acetabular tilt collides with the femoral neck, resulting in a flat femoral head(28). Flat femoral head shortens the distance from the center of rotation of the femoral head to the anatomical axis of the femur, resulting in a lower FO. Besides, in the previous studies, the absolute value was not used to represent the X-ray error. However, FO obtained from X-ray might be either large or small. Hence directly calculating the mean value would underestimate the error value. In this study, the absolute value was used to represent the X-ray error value, which presents the error more comprehensively. The FO data measured by X-ray and 3D-CT of all groups in this experiment could provide a more accurate reference range for THA preoperative design in Northeast China.