In our study, the length of acetabular posterior column corridor and the maximum screw diameter in males were greater than those in females, which might be caused by broader pelvis in males. The acetabular posterior column corridor demonstrated an irregular shape on the perspective, which had a triangular-prism shape from inferior of acetabulum to the greater sciatic notch. The diameters of acetabular posterior column corridor were 17.21 ± 1.41 mm and 15.54 ± 1.51 mm in males and females, respectively, which is sufficient to accommodate a virtual screw with a diameter of 7.3 mm. Furthermore, three 7.3 mm screws could be inserted in this corridor in males and three 6.5 mm screws were feasible for females in theory (Fig. 7), which suggested that there was enough space in the acetabular posterior column corridor to accommodate screws with larger diameters. Therefore, we recommended that a 7.3 mm screw can be used in both genders for the posterior column fixation.

The acetabular posterior column is fixed by three lag screws

What calls for special attention is that there were three sites with a high risk of penetrating the cortices, which were located at the transition between the posterior acetabular wall and the ischium, the middle of the acetabulum, and 1 cm below the greater sciatic notch, respectively. Correspondingly, when the screw penetrates the posterior acetabular wall at the transition between the posterior acetabular wall and the ischium, it is potential to damage the sciatic nerve; When the screw penetrates the cortex of middle acetabulum, the screw is easy to perforate the hip joint; Besides, there is a risk of damaging the superior gluteal nerve and vessels when screw penetrates the greater sciatic notch.

The placement of acetabular posterior column screw is greatly affected by the intraoperative body position. For the placement of antegrade posterior column lag screw, the ilioinguinal approach with patient supine is preferred. For the placement of retrograde posterior column screw through the ischial tuberosity, the screw can be inserted in prone or supine, even lateral position, with flexible hip and knee. The advantages of knee and hip joints flexion include straightforward access to the ischial tuberosity, relaxing sciatic nerve, and convenience of intraoperative fluoroscopy [5, 7, 9]. The virtual screw entry point located at the midline is the ischial tuberosity. The distance between the entry point and the distal ischial tuberosity was around 1.3 cm. The retrograde posterior column screw direction was lateral inclination (10.52 ± 3.04)° in males, and (7.72 ± 2.99)° in females. Correspondingly, the angle between the screw and the coronal plane was anterior inclination (15.00 ± 4.92)° in males, and (12.94 ± 4.72)° in females, 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. At the same time, due to the individual difference of the screw insertion angle, an individual preoperative planning is recommended for each patient. Completing three-dimensional CT reconstruction of the pelvis, simulating the operation in Mimics system, and measuring the entry point and direction of screw will be helpful for the accurate individualized treatment.

The pelvic anteroposterior view, 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 [7, 10, 11]. However, the acetabular posterior column structure is quite irregular, the above-mentioned fluoroscopic methods cannot fully reflect whether the screw penetrates the joint, the quadrilateral plate or the posterior acetabular wall. The posterior column is adjoining with sciatic nerve, superior gluteal nerve and vessels, and pelvic viscera; therefore a misdirected or misplaced screw during the surgery may cause serious complications [6, 12]. We found that the acetabular posterior column corridor is similar to the shape of a triangular prism, and the tangent positions of the three sides of the triangular prism can be used to assess whether the screw is safely inserted. When the ilium is obliquely positioned 10° for fluoroscopy, the quadrilateral plate can be roughly overlapped into a line. At this time, we can evaluate whether the screw penetrates the quadrilateral plate. In the same way, when the ilium is obliquely positioned 60°or the obturator foramen is obliquely positioned 60°, we can evaluate whether the screw penetrates the hip joint or posterior acetabular wall (Fig. 8). Wei Chen et al. [3] found that the posterior acetabular wall was abducted at 60° with the sagittal plane, and the quadrilateral plate was adducent at 9° with the sagittal plane. These results are similar to our study. Therefore, we can obtain the three tangent positions by moving the C arm according to the angles mentioned above, namely iliac oblique 10°, iliac oblique 60°, and obturator oblique 60° views, but further clinical verifications are needed. Nowadays, computer-assisted fluoroscopy-navigated percutaneous screw fixation technique has been advocated for its accuracy and versatility in acetabular posterior column fractures. However, this technique has often been frustrated in many hospitals primarily because of the high cost. In fact, the standard C-arm fluoroscopy is the most frequent image-guided technique in percutaneous screw fixation [13, 14].

The optimal fluoroscopic views. ①The right iliac oblique 10° view; A: acetabular quadrilateral plate; ②The right iliac oblique 60°; B: acetabular dome; ③The right obturator oblique 60°; C: posterior acetabular wall

Acetabular posterior column screws were commonly used in clinics including long thread screws (32 mm threaded), partial thread screws (16 mm threaded), and full thread screws. When fixing acetabular posterior column fractures, the threaded head of the screw should completely traverse the fracture in order to achieve stable fixation; therefore the selection of screws may affect the fixation range. Biomechanical studies revealed that long threaded screws and fully threaded screws showed significantly higher stiffness and less failure compared to conventional partial threaded screws [15, 16]. In this study, we simulated the retrograde fixation of the acetabular posterior column with a virtual screw with a 16 mm thread, and we found that 16 mm threaded retrograde acetabular posterior column screws through ischial tuberosity can fix all the acetabular posterior column fractures which were not 4 cm above the femoral head center. Therefore, fully thread screws, long thread screws, or partial thread screws can be chosen to fix the low acetabular posterior column fractures, and all these kinds of screws can provide enough holding force. For high acetabular posterior column fractures, we can choose short-threaded screws in order to obtain a larger fixation range, but the risk of internal fixation failure caused by insufficient holding force should be taken into account. However, fully threaded screws can obtain the maximum fixation range and keep the stability of fixation. For fractures that are 4 cm above the femoral head center, antegrade screws are recommended for a reason that retrograde screws cannot provide enough holding force and stable fixation. In short, the fixation range of retrograde posterior column screws is affected by the type of screws, and the screw entry point and direction are quite vital for fracture fixation. Furthermore, preoperative Mimics virtual surgery and intraoperative computer navigation-assisted screw placement system can increase the safety and success rate of screw insertion, and reduce surgical risks and complications.