Assessing the accuracy factors in the determination of postoperative acetabular cup orientation using hybrid 2D-3D registration

[Three-axis displacement classification of pelvic fracture and its reduction principles]

Objective

Based on images of pelvic CT three-dimensional reconstruction, to establish three-dimensional coordinate system of pelvis and investigate the three-axis displacement classification of pelvic fracture and its reduction principles.

Methods

Between June 2015 and May 2016, 21 cases of normal pelvic CT data were included in the study, and the mean pelvic three-dimensional model was established. The pelvic three-dimensional axis was established by defining the origin as the midpoint of the anterior superior iliac spine. Based on this coordinate system, a three-axis displacement classification of pelvic fracture were built. To assess the clinical guidance value of the three-axis classification, 55 cases (29 males and 26 females, aged 11-66 years with an average of 35.6 years) of pelvic fractures were analyzed by this classification, and replaced and fixed according to the principles of the reverse reduction.

Results

According to the theory of three-axis, pelvic fractures were divided into x-axis positive displacement/negative displacement, positive rotation/negative rotation; y-axis positive displacement/negative displacement, positive rotation/negative rotation; z-axis positive displacement/negative displacement, positive rotation/negative rotation. The average incision of included patients with pelvic fractures was 7.1 cm. The average reduction time was 12.2 minutes and the average radiation time was 55.3 s. The average time of screw implantation was 27.2 minutes. Postoperative pelvic X-ray films or three-dimensional CT showed all pelvic fracture was reducted well and the screw or plate was implanted correctly. The average intraoperative blood loss was 96.5 mL, the average operation time was 2.1 hours, and the average hospitalization time was 18.7 days. All patients were followed up 6-53 months (mean, 16.7 months). At last follow-up, according to Matta standard by pelvic radiography evaluation, there were excellent in 39 cases, good in 13 cases, and fair in 3 cases, the excellent and good rate was 94.55%.

Conclusion

Based on three-dimensional coordinate system, three-axis displacement classification of pelvic fracture can illustrate the displacement mode of patient simply and accurately, and can also guide the intraoperative reduction precisely.

Computer-aided pelvic reduction frame for anatomical closed reduction of unstable pelvic fractures

Traditional closed reductions of unstable pelvic fractures are mainly performed by surgeons using manual manipulation and subjective verification based on intra-operative roentgenography. It is difficult to perform an accurate closed reduction because of a lack of adequate knowledge of the displacement patterns and an inability to apply the reduction in correct direction. Using the concept of the remote center of motion mechanism and computer-aided design software, we developed a pelvic reduction frame for use in anatomical closed reductions of unstable pelvic fractures. With three-dimensional reconstruction technique and the matrix algorithm, the spatial orientation of the displaced hemipelvis can be calculated and deconstructed into several rotational and translational movements that can be completed with the frame. To verify the accuracy of this system, the rotations were repeated 10 times in arbitrary degrees and directions. After the matrix is calculated, the displaced hemipelvis can be reduced to the anatomical position using our frame. The maximum residual translational and rotational displacements were less than 5 mm and 4 degrees, which indicated the accuracy of this system. The maximum average residual translation and rotation were 1.87 mm in Z-axis (ranging: 4.63-0.1 mm) and 1.1 degrees around Y-axis (ranging: 3.81-0.13 degrees), respectively. Only the Z-axial translation showed a statistically significant difference (p < 0.05). In conclusion, the proposed pelvic reduction frame could be a useful tool for the anatomical reduction of unstable pelvic fractures.