The most important finding of the present study is that the ideal entry point for an infra-acetabular screw is 10.2 mm caudal and 10.4 mm medial of the IPE. This is the first study to report on the ideal screw position in relation to directly palpable landmarks via an intra-pelvic approach. Additionally, we referenced the angle of the screw to the pelvic inlet plane which is visible intraoperatively. This referencing in relation to visible anatomic landmarks may allow the surgeon to reproducibly place the infra-acetabular screw in an optimal position.
Demographic changes lead to a rising number of geriatric acetabular fractures involving anterior acetabular structures (anterior wall/anterior column) [6, 19, 20, 23]. Baseline data of our patient population regarding age and physical dimensions reflect the characteristics of a typical patient population for fractures involving anterior acetabular elements. This is concordant with other studies on the configuration of the infra-acetabular corridor [5, 9].
Historically, the posterior Kocher/Langenbeck approach was the standard approach to stabilize acetabular fractures [13, 16]. During the last 20 years, fracture patterns have changed so that anterior approaches are used most frequent today [8]. This trend initiated further advancement of anterior approaches and operative techniques via anterior approaches [7, 11, 14, 18, 21, 24]. There is an ongoing discussion about the “safe zones” and “dangerous zones” in acetabular surgery [25]. Due to the variance in pelvic anatomy, the safe zones are often relatively far away from the acetabulum, making it difficult to obtain adequate peri-acetabular stability. In 2011, Culemann et al. [5] proposed a modified quadrilateral screw inserted in the region of the Koehler’s teardrop. This so-called infra-acetabular screw was intended to minimize the risk of intra-articular placement of the screw. The entry point for this modified screw was reported to be 1 cm caudal to the IPE and in the middle of the pubis ramus. When this infra-acetabular screw position was introduced, the anterior ilio-inguinal approach which allows direct access to the pubic ramus circumference was the most frequent anterior approach [19]. However, identification of the middle of the pubic ramus can be difficult via an intra-pelvic approach, which is the standard anterior approach today [8]. Therefore, the instructions to identify the ideal entry point determined by Culemann et al. [5] are not applicable via the intra-pelvic approach. We investigated the ideal screw position in CT-based 3D-models and determined the relation to anatomic landmarks which are intra-operatively palpable via an intra-pelvic approach. We indicate a reproducible guideline for the placement of an infra-acetabular screw via an intra-pelvic approach.
The major advantage of the intra-pelvic approach is a direct inspection of the quadrilateral surface which is often involved in geriatric fractures [1,2,3, 8, 10, 19, 25]. In cases of reduced bone quality, a simple fall can cause a fracture of the anteromedial acetabulum or the quadrilateral surface by load transmission through the major trochanter [4]. A lack of support of the quadrilateral surface has been identified as a risk factor for a secondary dislocation leading to a central subluxation of the femoral head [6, 11, 20, 24]. In recent years, different concepts were introduced to increase medial support of the quadrilateral surface to prevent this complication [3,4,5, 7, 10, 11, 18, 21, 24].
Letournel was the first to describe the construct of a peri-acetabular frame to improve stability in acetabular fracture fixation [16]. They recommended a screw placement through the acetabular fossa. Culemann et al. [5] modified the position of this quadrilateral surface screw to the infra-acetabular corridor to minimize the risk of an intra-articular placement of the screw. The diameter of the drill hole for the infra-acetabular screw is 2.5 mm. The standard 3.5-mm fully threaded screw does not apply any inter-fragmentary compression force to the fracture and, therefore, only works as a distance screw. However, the main functions of this screw are a fixation of the anterior column to the posterior column and to prevent a dislocation of the quadrilateral surface [5, 18]. Gras et al. [11] found in a biomechanical cadaveric study that an additional placement of an infra-acetabular screw significantly increases the stability of the fracture fixation compared to a standard plate fixation.
Recently, a large biomorphometric CT-based study identified a viable infra-acetabular corridor with a diameter over 5 mm in 93% of specimens [9]. This study measured the entry point in relation to the pubic symphysis. The pubic symphysis is well-accessible via an intra-pelvic approach. However, they found a large variance for this distance of 54–91 mm [9]. This variance is due to variation of the configuration of the pubic rim and makes this reference not useful for clinical practice. This study measured the angle of the screw to the anterior pubic plane. The APP is well-established in the determination of the pelvic tilt in hip arthroplasty [15]. It can be identified in a prone position and provides accurate and reproducible referencing plane in 3D-analyses. However, several studies have demonstrated an impairment in clinical reproducibility of the APP leading to inaccuracy of the implant position [22]. We intended to orientate the angle of the screw to a plane that is clearly visible for the surgeon via an intra-pelvic approach. Therefore, we chose the pelvic inlet plane as the reference plane which can be easily identified by the ilio-pubic brim and the symphysis. We could confirm the sex-specific differences in angulation of the screw reported by Gras et al. [9]. We found a mean length of 75 mm in female and 84 mm in male patients respectively. The screw length in our patient population was therefore somewhat shorter than that in other studies [5, 9].
This study has some limitations. The guideline for placement of the infa-acetabular screw is based on the identification of the IPE. This anatomic landmark is a large-area bony bump rather than a sharply marked spike. However, via an intra-pelvic approach, it is well-palpable and its tip can easily be identified [25]. Another limitation to transfer this guideline into clinical practice is an insufficient reduction of the fracture. Incongruence of the infra-acetabular corridor can lead to narrowing of the designated screw canal or even to occlusion of the safe zone for screw placement. An intra-articular screw position may be the consequence. Therefore, an accurate radiographic review of the screw position is mandatory. Lastly, the limited number of cases reduces the generalizability of the guideline.
