Percutaneous cannulated screw placement (PCSP) is a common method of fixation. In pelvic trauma neurovascular structures are in close proximity to the screw path. Pre-operative planning is needed to prevent injury. This study aims to the safety margin and accuracy of screw placement with computer navigation (CAS).

A control had no pathology in the pelvis but CT scans were performed for suspected trauma. The treated group had pelvic and acetabular fractures and were treated with CAS PCSP at our institution. Using a new technique involving CT 3D modelling of the whole (3D) safe corridor, the dimensions of the Posterior elements (PE) of the pelvic ring and the anterior column of the acetabulum (AC) were measured in the control group.

The accuracy of screw placement (deviation between the actual screw and planned screw) was measured in treated patient using a screenshot method and post-operative CTs. There were 22 control patients and 30 treated patients (40 screws).

The mean ± (standard deviation, SD) minimum measurement of the safe corridor at the PE was 15.6 ± 2.3 mm (range 11.6 mm to 20.2 mm) and at the AC was 5.9 ±1.6 mm (range 3.0 mm to 10.0 mm). The mean ± (SD) accuracy of screw placement was 6.1 ± 5.3 mm and ranged from a displacement of 1.3 mm to 16.1 mm. There was a notable correlation between Body Mass Index, duration of surgery and inaccuracy of screw placement in some patients. The largest inaccuracy of screw placement was due to reduction of the fracture during screw insertion, causing movement of the bone fragments relative to the array and therefore also the computerised screw plan.

There were no screw breakages, non-unions, neurological or vascular complications.

CAS PCSP is a safe and accurate technique. However, the safe corridor is variable and often very narrow. We recommend that the dimensions of the safe corridor be assessed pre-operatively in every patient using 3D modelling to determine the number and size of screw that can be safely placed.

Introduction: Percutaneous cannulated screw placement (PCSP) is a safe method of internal fixation, indicated for pelvic ring fractures. Due to the close proximity of neurovascular structures to the path of the screw placed in either the Posterior elements (PE) or Anterior column (AC), pre-operative planning is needed to prevent injury.

This study aims to develop a pre-operative protocol for the Australian population, regarding the safe number of screws and size of screw that may be placed. Additionally, results from the study may help identify patients at increased risk of injury during PCSP.

Methods: All patients were from the Australian population and had been admitted into the emergency department at The Royal Melbourne Hospital. Control patients had no pathology in the pelvis, while treated group patients had pelvic ring fractures and were treated with PCSP.

Safe corridor measurements of the PE and AC were taken in the control patients. Pelvic CT scans, taken as part of trauma protocol, were reconstructed using 3D modelling and the dimensions of the whole (3 dimensional) safe corridor measured.

The accuracy of screw placement was determined in each treated patient. Accuracy was assessed by the screenshot method, the post-operative CT method or by both methods. In both methods, accuracy was taken as the deviation between the positions of the actual screw and planned screw.

Results: There were 22 control patients and 12 treated patients.

The mean ± (standard deviation, SD) minimum measurement of the safe corridor at the PE was 15.6 ± 2.3 mm (range 11.6 mm to 20.2 mm) and at the AC was 5.9 ±1.6 mm (range 3.0 mm to 10.0 mm).

The mean ± (SD) accuracy of screw placement was 6.1 ± 5.3 mm and ranged from a displacement of 1.3 mm to 16.1 mm.

Conclusion: The minimum dimensions of the safe corridor and the accuracy of screw placement occurred over a wide range. We recommend that dimensions of the safe corridor be assessed pre-operatively in every patient using 3D modelling to determine the safe number and size of screw that can be placed.