Acetabular fractures are severe injuries, often occurring in polytrauma patients as a result of a high-energy trauma such as motor vehicle accidents or falls from a height . Less often they occur as a result of a minor trauma in older patients presenting with osteopenic bone .
Anatomic reduction of the acetabulum and stable fixation are primary goals in acetabular trauma surgery. Open reduction and internal fixation with several available approaches [3–5, 7, 8] remains the standard for definitive treatment, while in recent years less invasive modifications and minimally invasive percutaneous techniques have been developed [9–12].
Definitive treatment with open reduction and internal fixation typically is performed three to five days after the injury to prevent excessive bleeding that can be found in acute pelvic surgery . This implies that there is enough time for meticulous preoperative surgical planning.
Cimerman et al. introduced a surgical planning software for pelvic and acetabular fractures with a mouse-based CAD-style interface . In contrast, the presented tool was designed with a virtual reality-style visuo-haptic interface, generating an artificial sense of touch for the surgeon to more naturally interact with fracture fragments in a 3D environment and to simulate relevant steps of the operative procedure. Despite the rapid advances in radiology and computer technology in the last years and developments in minimally invasive surgery, surgical simulation and planning is rarely used in clinical routine. There are different reasons for the slow adoption of such technologies. One important factor may be the reservation of surgeons to explore new technologies as they are devoted to their technical skills and performance. Yet we think that with the maturing of a new generation of surgeons amenable for new technologies and with the introduction of tools implementing more intuitive interfaces, the integration of such technologies will accelerate.
The emphasis in designing the presented tool was not on execution of a surgical technique, but on supporting the preoperative surgical planning. The developed planning software consists of three consecutive steps: virtual fracture reduction and internal fixation using patient-specific CT data as well as measurement and documentation.
The tool enabled fast and reliable virtual fracture reduction. Interactive manipulation of the fracture fragments gave the surgeon insight into their spatial relation and helped in choosing the operative approach. Citak et al. showed that virtual planning of acetabular fracture reduction helps in understanding the fracture morphology and leads to more accurate and efficient reductions .
Virtual internal fixation allowed contouring models of osteosynthesis implants currently used at our hospital to the reduced pelvis. According to measured bending and torsion angles between plate segments the surgeon could bend the fixation plates preoperatively. The use of prebent fixation plates adjusted to the patient-specific anatomy and fracture pattern was found to be extremely helpful in guiding fracture reduction especially of severely comminuted acetabular injuries, pushing the fracture fragments into their anatomic position while tightening the screws.
Finally the tool also supported us in planning minimally invasive percutaneous screw fixations in selected fracture patterns. Screws should be placed as perpendicular as possible to the fracture plane while maintaining a safe distance to the hip joint. To enable the most accurate application of minimally invasive planning in the operating room, different measurements like angles and lengths in 3D space were taken in relation to specific landmarks visible or palpable on the pelvic bone.
In this study, the planned fracture fixation was followed completely in six cases and partially in one case with a good to satisfactory radiographic result according to Matta  in all cases. In the cases a good correlation between preoperative planning and respective postoperative follow-up CT scans was found. In particular no case with inadvertent penetration of the hip joint was observed. In one case the surgical planning partially failed due to the impossibility to execute fracture reduction as planned preoperatively. In a further case the fixation plate could not be placed on the quadrilateral surface exactly as planned, because of soft tissue interfering with the placement of the screws. The plate consequently had to be tilted slightly with screw trajectories directed more caudally as planned (Figure 4 and 5).
A first limitation of this study is the limited number of patients. Also due to the variability of injury patterns, it is difficult to make definite quantitative conclusions. This study therefore only is able to show initial experiences and a larger patient population is requested to further assess the presented tool.
A second limitation is the time-consuming segmentation of the pelvic bones and fracture fragments for the generation of the patient-specific models, requiring manual refinements especially in osteopenic bone or severely impacted fractures. In this study, segmentation was performed by a radiologist but could also be performed by a trained technician or surgeon. In addition, further developments in segmentation algorithms will accelerate or even automate this task.
As a final limitation, we did not simulate interfering soft tissues with the current design of the presented tool. Soft tissue structures like muscles and tendons inserting into pelvic bones, blood vessels and pelvic organs were not modelled. In reality these structures interfere with fracture reduction and narrow down the working space or can even render a desired fracture fixation impossible.
In conclusion, the presented prototype software tool for surgical planning of acetabular fractures gives visual and haptic information about the injury and allows a patient-specific adaptation of osteosynthesis implants to the virtually reduced pelvis. Manual prebending of fixation plates according to the procedure plan can guide fracture reduction especially in severely comminuted injuries.
In future the coupling of the presented planning tool with an intraoperative guiding system will be planned, enhancing the transfer of the surgical planning into the operating room.
In addition the information of the shape of the planned plate can be exported in STL-format enabling to order a prebent plate from dedicated companies.