Acetabular fractures still are among the most challenging fractures to treat because of complex anatomy, involved surgical access to fracture sites and the relatively low incidence of these lesions [1], resulting in long learning curves. Primary goals of acetabular surgery are anatomic reduction of the articular surface with attention to careful soft tissue management, facilitating rapid postoperative recovery with early rehabilitation and a long-term functioning hip joint [2]. Proper evaluation and surgical planning is necessary to achieve these goals [1].

The ilioinguinal and the posterior Kocher-Langenbeck approaches with or without surgical hip dislocation are the most commonly used operative approaches for the treatment of pelvic and acetabular fractures [3–5]. In 1994 Cole introduced the modified Stoppa approach as an alternative for the ilioinguinal approach, allowing access to essentially the entire pelvic ring through a single window [6–8]. Some centres have developed less invasive modifications of these approaches or implemented percutaneous screw fixation techniques following open or closed reduction for distinct fracture patterns [9–12], reducing damage from soft tissue dissection. Especially when using such minimally invasive techniques a careful planning of the operative approach as well as type, size and placement of osteosynthesis implants is crucial and may decrease the operative time. Patients may also benefit from decreased blood loss, decreased fluoroscopy radiation exposure, more accurate plate and screw placement and lowered incidence of neurovascular complications.

Today high scanner speeds and diagnostic accuracy superior to other modalities has made computed tomography (CT) imaging the standard for evaluation of blunt trauma to the pelvis [13, 14]. Multiplanar reformatted images and volume rendered views [15] of the CT datasets are readily available on current workstations. These 2D and 3D visualizations are complementary in fracture classification, identifying the main fracture fragments and recognizing their displacement and rotation as well as their spatial relation. But because of their static nature they may give only limited insight into the optimal choice of surgical approach and osteosynthesis implants for internal fixation [16, 17]. Thus surgeons still have to make some important decisions based on the mental combination of available imaging studies, or sometimes intraoperatively after fracture fragment reduction, using intraoperative fluoroscopy as a flexible, yet limited 2D imaging modality.

During the last years a few preoperative planning tools specific to acetabular fractures have been developed, leveraging advances in radiology and computer technology. Cimerman et al. reported favorable results in the preoperative planning of pelvic and acetabular fracture reduction and osteosynthesis using a commercially available tool with a mouse-based interface comparable to Computer-Aided Design (CAD) software [18]. The surgeons performed the virtual operations themselves after patient-specific virtual models had been built from CT datasets by computer engineers. Brown et al. fabricated life-size wax stereolithographic replica of the fractured hemipelvis and the reversed non-fractured hemipelvis to prebend fixation plates and to produce methyl methacrylate drill guidance templates matching the planned screw trajectories [16]. They could achieve accurate plate and screw placement using this technique.

The goal of this study was to test the feasibility of preoperative surgical planning in acetabular fractures using a new prototype planning tool based on an interactive virtual reality-style environment, including fracture reduction, fixation and measurement.

According to the Letournel classification [21] there were 4 cases with both column fractures, 2 cases with anterior column fractures and 1 case with a T-shaped fracture including a posterior wall fracture (Table 1).

Preoperative planning and operative outcome

A Stoppa approach combined with the first window of the ilioinguinal approach was planned and executed in five cases and an ilioinguinal approach in one case. In one case a combined Stoppa and posterior approach was planned and executed (Table 2). The planned fracture fixation was followed completely in six cases and partially in one case (case 5).

Case	Surgical approach	Fixation	Articular dis-placement (mm)
1	Stoppa, first window of ilioinguinal approach	prebent 14-hole and 5-hole plates	3
2	Ilioinguinal approach	prebent 9-hole plate, 7.3 mm lag screw (acetabular dome)	1
3	Stoppa, first window of ilioinguinal approach	prebent 12-hole plate, two 7.3 mm lag screws (posterior column)	1
4	Stoppa, first window of ilioinguinal approach	prebent 12-hole plate, 7.3 mm lag screw (acetabular dome)	1
5	Stoppa, additional posterior approach	prebent 9-hole and 7-hole plates, additional 5-hole plate	3
6	Stoppa, first window of ilioinguinal approach	prebent 12-hole and 9-hole plates	2
7	Stoppa, first window of ilioinguinal approach	prebent 12-hole and 9-hole plates	2

In case 6 (Figure 4, 5 and 6), placement of the fixation plate on the acetabular dome shows a very good match between planning and actual execution while the second plate on the quadrilateral surface could not be placed exactly as planned. Because soft tissue was interfering with the placement of the screws, the plate had to be tilted slightly.

Special attention was given to complement conventional internal fixation with percutaneous screw fixations. For example fixation of a posterior column fracture after reduction from an anterior approach was performed, avoiding an additional posterior approach. In one case percutaneous screw fixation of the posterior column (case 3) and in three cases of the dome of the acetabulum was successfully planned and performed (cases 2, 4 and 6). In case 4 complementary screw fixation of the acetabular dome after a both column fracture was performed (Figure 7, 8, 9 and 10).

Prebent fixation plates were used in all cases. In four cases (cases 2, 3, 6 and 7) with severely comminuted injuries to the pelvis this tremendously helped in guiding the fracture reduction.

Comparing the postoperative follow-up CT scans to respective preoperative planning, a good correlation was found in six of seven cases. The remaining case (case 5) partially failed due to the impossibility to reduce the fracture in the planned manner.

Postoperative congruence of the acetabular joint surface as determined according to Matta [5] in the follow-up CT was anatomic in three cases (43%) and satisfactory in four cases (57%) (Table 2). There was no case with inadvertent penetration of the hip joint.

We found no serious postoperative complications such as deep infections or failure of osteosynthesis implants. Analysis of functional outcome, for example occurrence of posttraumatic osteoarthritis of the hip joint, has not been included in this study because of the absence of long-term follow-up.

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 [22]. Less often they occur as a result of a minor trauma in older patients presenting with osteopenic bone [23].

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 [8]. 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 [18]. 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 [17].

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 [5] 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.