Fluoroscopically guided acetabular posterior column screw fixation via an anterior approach

Objective Safe posterior column screw fixation via an anterior approach under two-dimensional fluoroscopic control. Indications Anterior column with posterior hemitransverse fractures (ACPHF); transverse fractures; two-column fractures and T‑type fractures without relevant residual displacement of the posterior column after reduction of the anterior column and the quadrilateral plate. Contraindication Acetabular fractures requiring direct open reduction via a posterior approach; very narrow osseous corridor in preoperative planning; insufficient intraoperative fluoroscopic visualization of the anatomical landmarks. Surgical technique Preoperative planning of the starting point and screw trajectory using a standard pelvic CT scan and a multiplanar reconstruction tool. Intraoperative fluoroscopically controlled identification of the starting point using the anterior–posterior (ap) view. Advancing the guidewire under fluoroscopic control using the lateral–oblique view. Lag screw fixation of the posterior column with cannulated screws. Postoperative management Partial weight bearing as advised by the surgeon. Postoperative CT scan for the assessment of screw position and quality of reduction of the posterior column. Generally no implant removal. Results In a series of 100 pelvic CT scans, the mean posterior angle of the ideal posterior column screw trajectory was 28.0° (range 11.1–46.2°) to the coronal plane and the mean medial angle was 21.6° (range 8.0–35.0°) to the sagittal plane. The maximum screw length was 106.3 mm (range 82.1–135.0 mm). Twelve patients were included in this study: 10 ACPHF and 2 transverse fractures. The residual maximum displacement of the posterior column fracture component in the postoperative CT scan was 1.4 mm (0–4 mm). There was one intraarticular screw penetration and one perforation of the cortical bone in the transition zone between the posterior column and the sciatic tuber without neurological impairment.


Introductory remarks
In acetabular fractures involving one column only, either a single anterior or a single posterior approach is required for internal fixation. However, in acetabular fractures involving both columns, such as transverse fractures, T-type fractures, anterior column with posterior hemitransverse fractures and two-column fractures, various strategies for surgical stabilization may be considered. Some of these fractures may be treated via a single approach without plate fixation of the other column, while others may require an extended or a combined anterior and posterior approach. The latter increases surgical time, blood loss and morbidity due to the need for a second approach [1][2][3][4].
An alternative option for selected acetabular fractures involving both columns is open reduction and plate fixation of the anterior column via an anterior approach combined with a fluoroscopically controlled lag screw fixation of the posterior column via the same single approach [5][6][7]. One prerequisite of this technique, however, is that the posterior column fracture is either nondisplaced or adequately reduced after reduction of the anterior column and the quadrilateral plate. As a consequence, T-type fractures or two-column fractures, in general, are less often amenable to this technique, whereas transverse fractures and anterior column with posterior hemitransverse fractures (ACPHF; . Fig. 1) appear to be a favorable indi-cation for this fixation strategy. ACPHF and transverse fractures constitute up to 31% of acetabular fractures in patients aged 55 years and older and about 12% of fractures in younger patients, while T-type and two-column fractures represent more than one third of fractures in both older and younger patients [8,9].
Although surgical techniques such as CT-controlled and navigated screw fixation may be capable of reducing the rate of screw malpositioning in pelvic and acetabular surgery [10][11][12], these techniques are not widely available due to their high costs and skills required. The aim of this article therefore is to present a simple preoperative planning technique for the estimation of the starting point and screw trajectory of acetabular posterior column screw fixation via an anterior approach and to delineate its intraoperative application under fluoroscopic control.

Surgical principles and objective
Safe posterior column screw fixation via an anterior approach under two-dimensional fluoroscopic control in the following two steps: First step. Preoperative planning of the starting point and screw trajectory for posterior column screw placement via an anterior approach using a standard pelvic CT scan and a commonly available multiplanar reconstruction tool.

Second step.
Intraoperative fluoroscopically controlled identification of the starting point in the pelvic anterior-posterior (ap) view and advancing the guidewire/ screw under fluoroscopic control using a lateral-oblique view in order to prevent intraarticular screw penetration and lesions of the sciatic nerve.  ACPHF typically result from force transmission via the greater trochanter and the femoral neck with the hip joint in extension. Due to the anteversion of the femoral neck the anterior column is affected first and frequently shows a multifragmentary fracture pattern.Further protrusion of the femoral head leads to a simple posterior hemitransverse fracture and a fracture component in the transition zone between the anterior column and the quadrilateral plate. The quadrilateral plate therefore remains in osseous continuity with the posterior column.These two fracture components allow for an internal rotation of the posterior column as a result of the medial protrusion of the femoral head. Accordingly, the quadrilateral plate is not separated from the two acetabular columns.It is internally rotated in osseous continuity with the posterior column.b Fracture components of ACPHF. The injury mechanism described in a results in the typical fracture patterns of ACPHF with the following fracture components and characteristics: multifragmentary or comminuted anterior column fracture; simple posterior hemitransverse fracture; internal rotation of the posterior column and the attached quadrilateral plate; impactionofthe articularsurface ofthe superomedial dome ("gull sign"). c"Gull sign": impaction of the articular surface of the superomedial dome results from force transmission via the femoral head. The radiological appearance of this impaction has been described as "gull sign" referring to children's style of drawing sea gulls. The "gull sign" is associated with a poor outcome after open reduction and internal fixation of acetabular fractures [13][14][15]

Preoperative work up
The preoperative planning is the major step for safe fluoroscopically controlled posterior column screw placement and is therefore described in detail (. Fig. 2 and 3). Pelvic CT scans with a slice depth of 0.6 mm are recommended for preoperative planning. CT scans with slice depths >0.6 mm are also applicable, but may result in inferior image quality during the reformation process. Any imaging software, which supports two-dimensional multiplanar reformation (MPR), is suitable for preoperative assessment. There was one intraarticular screw penetration and one perforation of the cortical bone in the transition zone between the posterior column and the sciatic tuber without neurological impairment.  We highly recommend to assess the ideal posterior column screw trajectory using the uninjured contralateral acetabulum because screw trajectory analysis is easier and more precise in absence of fracture lines and displacement. The uninjured contralateral acetabulum can be reliably used as a template for preoperative planning as the left and right posterior column anatomy (screw insertion angles, screw starting point and screw length) do not significantly differ within the same pelvis (see Results).In case of no or only minor displacement, however, preoperative planning may be also performed on the injured side. The starting point of the posterior column screw is located in the transition zone between the supraacetabular region and the iliac wing on the inner cortex of the iliac bone.The screw trajectory is oriented from cranial-anterior-lateral to caudal-posterior-medial (a, b). The ideal starting point and screw trajectory are assessed by using native axial CT images and a two-dimensional multiplanar software reconstruction tool. Therefore, the axes of coordinates are translated and the axes itself rotated to assess the ideal entry point and screw trajectory. In the present case, the medial angle of the ideal posterior column screw trajectory is 14°to the sagittal plane (a) with a maximum screw length of 130 mm from the startingpoint tothe endpoint (cortexofthe sciatic tuber).Furthermore, the posterior angle of this screw trajectory is 28°to the coronal plane (b) and obviously shows the same maximum screw length in this second reformation plane In transverse fractures, the single fracture line is addressed from anterior. In anterior column with posterior hemitransverse fractures (ACPHF), the posterior hemitransverse fracture is reduced by reduction of the quadrilateral plate, which is attached to the posterior column as outlined above. After reduction and anterior column fixation, the posterior column screw fixation is performed via the lateral window of the ilioinguinal approach. If a modified Stoppa approach is used, this approach is frequently combined with an Olerud approach for anterior column fixation (a). In this case, the posterior column lag screw can be easily placed via the Olerud approach. If only a modified Stoppa approach is used for the fixation of the anterior column, a second small incision at the iliac crest ("Mini Olerud approach") is applied to approach the entry point It is advisable to switch to the lateral-oblique view when approaching the hip joint. After predrilling, a cannulated 6.5 mm screw with a 32 mm thread and a length of 115 mm was inserted.The screw length must not be longer than the maximum length determined in preoperative planning, but may be shorter provided that the screw thread completely passes the fracture line.The fluoroscopic control shows a screw trajectory as preoperatively planned (c). e Intraoperative application-fluoroscopic control in lateral-oblique view.The left hip joint is located next to the radiation source which results in a "larger" left hip joint.Additionally, the radiation source is tilted 15°upwards resulting in a more posterior projection of the left hip.

Keywords
The lateral-oblique view shows that thescrewdoes not penetratethehip joint anddoes not perforate thecortical boneofthe posteriorcolumnandthesciatic tuber. fPostoperative CTcontrol. The correct screw trajectory isconfirmedinthe postoperative CTscan. The posteriorhemitransverse fracture isanatomically reduced Operative Orthopädie und Traumatologie 6 · 2019 509 Fig. 7 9 Screw misplacement. a Preoperative planning for a 56-year-old man who sustained a typical anterior column with posterior hemitransverse fracture (ACPHF) on the right side. The preoperative planningreveals a relatively narrow osseous corridor for a posterior column screw. b Fluoroscopic control in lateral-oblique view. The image quality is moderate in general. Additionally, the lateral-oblique view was not sufficiently oblique resulting in a poor differentiation between the left and the right hip and sciatic tuber (upper image). The screw does not perforate the cortical bone at the sciatic tuber, but is tangent to the projection of the hip joint line (lower image). c Postoperative CT scans revealed adequate reduction of the posterior hemitransverse fracture via the quadrilateral plate. The posterior column screw, however, penetrates the cortex of the acetabular fossa. The hip joint motion was not restricted and the patient refused revision surgery tive CT scans available for review were included. There were 10 ACPHF and 2 transverse fractures. The residual maximum displacement of the posterior column fracture components was 1.4 mm (range 0-4 mm). Beside the case shown in . Fig. 7, there was one case with a perforation of the cortical bone in the transition zone between posterior column and sciatic tuber (maximum screw protrusion of 5 mm) without neurological impairment.