Optimal pin position in supraacetabular external fixation

Abstract

Background: This study focuses on (i) the length of the intraosseous part of the supraacetabular pin using the insertion technique from the spina iliaca anterior inferior to the cortical part of the incisura ischiadica major, (ii) the angle of insertion of the supraacetabular pin in the transversal plane and (iii) gender-specific differences of the measured results.

Methods: Images of uninjured pelves from 49 patients (64-line computed tomography scanner) were evaluated, and virtual external fixator pins were positioned using a three-dimensional reconstructions of computed tomography scans. The length of the pins and the insertion angle were investigated. Descriptive statistics were used, and gender-specific differences were calculated. A P-value of <0.05 was considered statistically significant.

Results: The results showed significant differences between male and female pelves concerning both pin length and insertion angel. For male pelves, the mean screw length was 82.7 mm (SD 5.1; range 72.9-94.3). For females, this was statistically significantly shorter (P ≤ 0.001), with an average of 74.1 mm (SD 5.0; range 63.1-81.9). In the male subgroup, the insertion angle was a mean of 22.6° (SD 3.4; range 12.4-31.8), and the female pelves had an average angle of 19.7° (SD 4.0; range 11.7-24.5). These values differed statistically significantly (P = 0.0032).

Conclusion: Based on our measurements, we can confirm that both the length of the Schanz screws and the angle of insertion for the supraacetabular external fixator show a statistically significant difference between males and females.

Keywords: gender difference; pelvic ring fracture; pin position; supraacetabular external fixation.

Fig. 1

Mimics interface.

Fig. 2

Determination of screw length (ac1_screw) between the points ‘ac1_ant’ and ‘ac1_post’.

Fig. 3

Determination of insertion angle.

Fig. 4

Construction of the screw plane.