When performing a primary THA for a patient with Crowe IV DDH, the acetabular reconstruction especially presents considerable technical challenges to orthopedic surgeons. Further, Component revision in high dislocated hips was significantly more associated with the acetabular osteolysis, cup instability and severe polyethylene wear both in mid-term and long-term clinical studies[21, 9, 22, 23]. During the prior surgery, we noted that the acetabular anatomy and bone stock distribution was relatively constant in patients with Crowe IV dysplastic dislocation. Several studies[15, 13, 4] have reported that 3D simulation not only can provide high resolution visualization of morphological changes in skeletal disorders, but also can predict the postoperative rotation center and the orientation of prosthesis with high validity and accuracy. In the present study, we presented the first quantitative 3D morphological to determine feasibility, availability and optimal placement of standard-sized cup for acetabular reconstruction in Crowe IV hip, with excellent midterm outcomes while applying this technique.
Compared with other Crowe types, the true acetabulum in Crowe IV was thoroughly separated from the dislocated female head, resulting in remarkably dysplastic and deformed. Similar with previous description[7], Crowe IV acetabulum presented as a narrow, shallow and low-volume socket. Worth to mention, the acetabular volume in triangular acetabulum was approximately one fifth of normal one, resulting in an extremely smaller component size and lower implanted height. Due to the lack of articular contact, the Harris fossa is covered by few osteophytes and hard to identify. Further, correct recognition of the inferior edge of the true acetabulum is of particular importance for anatomic implantation intraoperatively[24, 25].
After simulated implantation, the Crowe IV acetabulum tended to be sharply abductive, with smaller Cup-CE and larger Cup-Sharp. Fujii et al[26] suggested that Cup-CE angle by host bone should be greater than 0° for satisfactory bony fixation in a five year follow-up study. Herein, the Cup-CE was 23.44 ± 7.62 according to the lateral edge of the host bone which provides possibility that there is no need for structural bone graft in most Crowe IV acetabular reconstruction[27, 9, 28]. In this study, autogenous bone augmentation was used in 9 hips (21.43%) and no implant failure was found. Moreover, the dysplastic acetabulum had a more excessive anteversion and thicker medial wall, similar as we reported previously[4]. In the present study, component was uniformly oriented at 20 anteversion and tangent to the inner cortex of the medial acetabular wall. Surgeons generally apply the technique of combined anteversion and acetabular cotyloplasty for better bony coverage. Although there was relatively sufficient bone stock medially, spongy bone condition and inadequate bone stock in the anteroposterior direction should also be taken careful consideration during acetabular reaming.
Regarding the limitation of the thin polyethylene liner and extremely small femoral head, standard-sized component with ceramic-on-ceramic bearing is recommended in recent studies[13, 12, 10]. Quantitative coverage analysis suggested that segmental deficiency located at the entire acetabular rim, excepted for the posterior direction. In comparison, the most severe dysplastic bone stock was in the anterosuperior direction, whose sector angle decreased more than 35° than normal hips. Due to the great anteversion, there is no significant difference in the posterior cover angles between Crowe IV and control group. Further, posterosuperior deficiency was also less severe than that in the anterosuperior and superior direction, which is consistent with the description of abnormal bone distribution in the previous studies[7, 5]. A careful reaming for protection of the anterior acetabular wall is of particular important for stable acetabular component fixation, especially in Crowe IV hips[29, 5].
Reliable bony landmark can not only provide surgeon accurate identification but avoid intraoperative complications as well[29]. Based on the component opening plane, useful bone landmarks were projected by 3D simulation. Our data showed the optimal component center mainly located at the midpoint between the superolateral and posteroinferior points in the coordinate system, which was also proved to have good feasibility and repeatability intraoperatively (Fig. 5). The average distance between selected point and component center was only 4.12 mm, with 2.53 mm horizontally and 0.38 vertically. Similarly, Xu et al[13] suggested the component center in Crowe IV should be placed posterosuperiorly, described to be 4.55 mm superior and 4.37 mm posterior to the center of true acetabular circumcircle. Yoshitani et al[5] revealed that ideal center position located at the upper third point of posterior bony wall. In contrast, the mean distance in our data was smaller and variation range was within 10 mm in all hips. Compared with acetabular wall, the extreme poles of the acetabulum had better recognition and lower risk of abrasion and impingement. Midterm results also showed satisfactory outcomes in this study.
Limitations
The limitations of our study should be noted. First, the sample size was relatively small. However, the Crowe IV hips are uncommon and the results of statistical analysis indicated reliable reproducibility. Second, all the components were oriented at 40 abduction and 20 anteversion during preoperative planning, while implantation may be adjusted to be more abductive and anteverted individually in actual operation and implanted orientation cannot precisely match the preoperative planning without intraoperative navigation. Third, since the actual weight-bearing area was difficult to define in the 3D environment, we chose to weight the uncovered portions equally in the coverage evaluation.