Open reduction and internal fixation of acetabular fractures provide the most consistent functional results when an anatomic restoration and a congruent articular surface can be achieved[10]. Infra-acetabular screw firstly mentioned by Culemann[3] strengthens the fixation dramatically by constituting periacetabular fixation frame with both osseous columns, the ilioinguinal plate, and supra-acetabular screw fixation. In several biomechanical analysis studies[11, 12], some authors found that additional placement of an infra-acetabular screw significantly increased the fracture fixation strength and reduced the displacement of the fracture. However, compared with the acetabular anterior and posterior corridors, the infra-acetabular corridor is significantly narrower, which may be even more significant for Asians. Additionally, infra-acetabular corridor is surrounded by obturator nerves and vessels[13], which increases the risk of iatrogenic injury when screw is inserted. Therefore, it is essential to conduct a detailed study on the placement method and anatomical parameters of the infra-acetabular screw.
- Placement method for infra-acetabular screw
In the original article by Gras et al[6], they found the mean IACD of males and females were 7.7 mm and 6.9 mm respectively, the corridor with a diameter of at least 5 mm existed in 93% of cases (90% in females versus 94% in males) and the mean corridor length of males and females were 106.4mm and 96.2mm. However, in our study, the mean IACD of males and females were (5.15±1.25) mm and (4.42±1.01) mm respectively, the corridor with a diameter of at least 5 mm existed in 71% of cases (66% in females versus 76% in males), and the mean corridor length of males and females in all-in screw group were (99.43±4.04)mm and (88.83±3.71)mm and those data in in-out-in screw group were (98.19±4.37)mm and (87.31±4.48)mm. Obviously, the diameter and length of infra-acetabular corridor studied by Gras et al are greater than the results in our study, which may be caused by the racial difference. The infra-acetabular corridor is narrow and long and the safety range is limited. Therefore, tiny angular deviation may cause iatrogenic damage to the peripheral neurovascular bundles and hip joint[14]. Cai XH et al[15] have shown that when 1/3~1/2 of the screw diameter is located in the bone of the quadrilateral plate, it can effectively resist the separation of fracture fragment,provide rigid fixation and avoid the screw from penetrating the joint. In our study, we found that the minimum ICAD of males and females were respectively 4.09mm and 3.72mm, and it was difficult to insert 3.5mm screws in this situation. Whereupon we proposed that the screw placement method can be selected according to IACD. When IACD was ≥5mm,the 3.5mm all-in screw was selected; When IACD was <5mm, we inserted the 3.5mm in-out-in screw and made 1/2 of the screw diameter exposed out of the quadrilateral plate cortex.
Arlt et al[16] found that the infra-acetabular corridor showed a double-cone shape with the isthmus located in the region of the acetabular fovea as the limiting anatomical structure. Additionally, the body weight, body height, and the diameter of Köhler’s teardrop in the anteroposterior X-ray view showed significant positive correlations with the corridor volume. In our study, IACD had a positive correlation with MTMAW. Hence, based on the regression equation, we can calculate the IACD by the MTMAW which can be measured by CT scan, thereby the screw placement method can be selected preoperatively. When the MTMAW is greater than 3.15mm, all-in screw can be inserted.
- infra-acetabular screw entry point and orientation
Culemann et al[3] described that the entry point for the infra-acetabular screw is 1 cm caudal of the iliopectineal eminence in the mid-width of the pubic ramus. Gras et al[4]measured the distance between anterior superior iliac spine and pubic symphysis and the distance between pubic symphysis and screw entry point, they found the mean ratio of these two distances is 1.36. Whereas these two methods are not utility in clinical practice. Baumann[17] found that the entry point is located at the posteromedial of apex of iliopubic eminence. The relationship between the entry point and the iliopubic eminence has no relevance with gender, age, or body type. In our study, the apex of iliopubic eminence and true pelvic rim were regarded as the reference point to measure the entry point, which could be confirmed by palpation[18] and was convenient to locate the entry point during the operation. We found that the all-in screw entry point was closer to the apex of iliopubic eminence than the results in the study by Baumann, which may be caused by the racial differences. The all-in screw exit point was roughly located at the medial of the middle ischial tuberosity. The infra-acetabular corridor was surrounding by obturator nerves and vessels, the in-out-in screw entry point was located at lateral of the obturator groove in order to avoid iatrogenic injury. Compared with all-in screws, the in-out-in screw entry point was round 2mm outwards and backwards, and closer to true pelvic rim(Fig.8). When in-out-in screw is applied, the periosteum dissector can be used to push away the obturator nerves and vessels to protect them. 90% of length of in-out-in screw in the sclerotin can ensure the stability the screw fixation.
According to the data in our study, there is no statistical significance on the difference of optimum screw angle between groups in same gender. The angle between screw and the sagittal plane was more medial tilted in males than that in females in both groups, which may be caused by the morphological differences of pelvis in genders. The pelvis is “funnel-shaped” in males, however that is “barrel-shaped” in females. The infra-acetabular corridor was nearly parallel to the sagittal plane and the medial inclination angle was round (0.42±6.49) ° in males. However, the angle was lateral inclined in females, which was round (8.09±6.33) °. Additionally, the screw was (54.06±7.37) ° posterior inclination with the coronal plane in both genders.
- Method of intraoperative fluoroscopy to determine the screw position
The inlet, outlet, iliac oblique and obturator oblique views are commonly used to verify whether the screw perforates the joint or cortex during the process of screw placement[19, 20]. However, the anatomy peri infra-acetabular corridor is quite complicated, and the above-mentioned fluoroscopic methods cannot fully assess whether the screw penetrates the joint, the quadrilateral plate and the posterior of obturator. If the screw posterior inclination is too small, it is easy for the screw to penetrate the posterior of obturator and damage the obturator nerve and vascular bundles. The infra-acetabular corridor is narrow and long and the safety range is limited. If the angle between the screw and the sagittal plane is too large during the screw placement process, there is a risk of penetrating the acetabular joint. In the original research by Culemann et al[3],c-arm rotated to the injured side and tilted 30°caudally in the obturator oblique and outlet views with patient supine for the control of correct screw path(Fig.9A). In our study, we trend to rotate the c-arm to the injured side and tilted 50°-55° cranially. The infra-acetabular screw in males is nearly parallel to the sagittal plane, however it is slightly lateral tilted in females. According to the orientation of screw, we adjust the position of c-arm to eliminate the angle between the screw and the coronal and sagittal planes. In this way, the screw becomes a dot in the perspective view. We can verify whether the screw penetrate the joint or quadrilateral plate through this view(Fig.9B).
The infra-acetabular corridor is quite narrow, which makes a challenge for the orthopedists to insert the screw. Hence, Gras et al[21] recommended inserting the infra-acetabular screw under the guidance of 3D navigation system. Lehmann et al[18] designed a new electromagnetic navigation system and completed infra-acetabular screw placement for 22 of 24 patients with the help of this new system. However, these technologies are demanding for the hospital facilities, which makes a difficulty to widely apply around the world, especially in the developing countries. Therefore, if a portable infra-acetabular screw sighting device could be designed based on the results in this study, the safety and effectiveness of the operation will be improved.