1. Introduction
Spinopelvic dissociation (SPD) is associated with transverse sacral fractures, which cause the dissociation of the sacrum from the pelvis [
1,
2]. It is associated with 3% of transverse sacral fractures and 3% of sacral fractures are associated with pelvic ring injuries [
3]. SPD is well known for its high mortality and comorbidities such as nerve root injuries [
4]. When SPD is correctly diagnosed and appropriately treated, patient outcomes can be optimized [
5]. However, a high level of consensus and a unified approach for dealing with this complex issue are lacking.
The traditional fixation methods for the posterior pelvic ring include tension band transiliac plate fixation, local plate fixation, open or percutaneous ilio-sacral screw fixation, and transiliac bars, which do not guarantee postoperative stability and may result in fixation failures [
6,
7]. In recent years, surgeons have used triangular osteosynthesis (TOS) in combination with the surgical technique of unilateral L5 fixation using S2AI or iliac screws for SPD treatment, and the literature indicates that these patients show satisfactory postoperative function and radiological outcomes [
8]. With or without a combination of bilateral or dual iliac screw fixation techniques [
9], TOS is a reliable form of fixation that enables early weight-bearing while preventing the loss of reduction [
9,
10,
11,
12]. In addition, compared with traditional surgical methods, its complication rate is low [
3,
5] (
Figure 1A–F).
Currently, a radiographic assessment remains the standard peri-operative measurement for displacement and reduction in studies of pelvic fractures. However, there is still a lack of research investigating the relationship between peri-operative SPD and prognosis from the perspective of radiology in patients with SPD who underwent reduction and fixation by TOS using S2AI screws. Though the measurement of outcomes is difficult and the level of evidence in this area is poor, this article revealed three such methods for measuring radiographic displacement [
11,
13,
14].
This study aimed to investigate the recovery time course and imaging parameters relevant to the functional recovery of patients with SPD treated by TOS.
3. Results
As shown in
Table 2, a total of 23 patients were enrolled in this study. There were 15 men and 8 women (65.2% vs. 34.8%), and the mean age was 47.8 (19.3) years. More than half of the patients were in the 61C1 (60.9%) category according to the AO 2018 classification, followed by those in the 61C3 (26.1%), 61C2 (8.70%), and 62C2 (4.35%) categories.
As time progressed, the functional outcomes improved, and the patients returned to a near-normal life within one year. The EQ−5D−5L score increased with time, from 0.14 at 6–8 weeks to 0.94 at one year. The differences for the time trend were 0.32 in the crude GEE model (95% confidence interval [CI]: 0.25, 0.39) and 0.31 in the adjusted GEE model (95% CI: 0.25, 0.37) (
Table 3,
Figure 6). The EQ−VAS and Majeed pelvic scores also increased with time. The differences for the time trend were 0.17 for the EQ−VAS (95% CI: 0.14, 0.30) and 0.20 for the Majeed pelvic score in the adjusted GEE model (95% CI: 0.18, 0.22).
In this study, three image-evaluation methods, including the measurement of the inlet–outlet ratio, the cross−measurement method, and ADM, were used pre-and postoperatively. The association between the EQ−5D−5L score and the radiographic displacement measurement is presented in
Table 4. For the preoperative radiographic displacement measurements, the EQ−5D−5L score increased by 2.141 per outlet ratio unit (95% CI: 0.041, 4.241). In the postoperative period, the EQ−5D−5L score increased by 1.359 per inlet ratio unit and 1.804 per outlet ratio (95% CI: 1.301, 2.307) but decreased by 0.01 per HD (95% CI: −0.018, −0.002) after adjusting for age, gender, and the follow-up time. This shows that changes in the horizontal direction are more correlated with EQ−5D−5L recovery.
The association between the EQ−VAS score and the radiographic displacement measurements is shown in
Table 5. The association was significant only with the inlet ratio in the postoperative period. The EQ−VAS score increased by 1.270 per inlet ratio (95% CI: 0.093, 2.447) in the adjusted GEE model. However, there were no significant associations between the Majeed pelvic score and any of the radiographic displacement measurements (
Table 6).
4. Discussion
The present study revealed that the displacement of SPD in spinopelvic fixation provides good vertical reduction results. During surgery, a reduction in the vertical direction is easier to achieve by fluoroscopy. A vertical anatomical reduction is often mentioned and highlighted for the treatment of unequal feet. A vertical displacement causes differences in the lower extremities, abnormal motor gaits, and lower Majeed scores.
It is sometimes difficult to achieve a perfect horizontal reduction due to comminuted sacroiliac fractures or an indirect reduction in the sacroiliac joints with complex anatomical structures radiologically.
Regarding horizontal reduction, the analysis showed that patients with a short-term follow-up showed a lower tolerance for postoperative horizontal displacement. Only a few studies have focused on the relationship between the inferior quality of horizontal displacement reduction and unsatisfying functional outcomes. We believe that the inferior quality of the horizontal reduction results in a change in the lever arm of the peak moment of the hip, which causes greater work in terms of hip abduction, adduction, flexion, and extension in the affected side in patients with SPD (
Figure 7). As a result, the centroid experiences a mid-lateral shift, which may increase the metabolic cost and mechanical work of the lower extremities [
20,
21]. With rehabilitation, patients improved their function over time, but the change of the lever arm may contribute to unsatisfaction, increasing metabolic costs, and increased mechanical work in the short term postoperatively. No significant correlation was found in the asymmetric index. This could be because the integrity of the pelvic ring was restored postoperatively, while the SI joint was left without a complete reduction.
Our results revealed that the patient’s function will return to normal in one year. The postoperative gait analyses of patients with various pelvic ring fractures by Kubota et al. [
22,
23] showed that there was a complete recovery of peak hip abduction, and a partial recovery of peak hip extension and hip strength were noted at the 12-month follow-up. The horizontal displacement of the pelvis may affect the offset change of the hip joint, which is associated with abductor function. Dean et al. [
24] concluded that patients with type C pelvic fractures had weaker hip abductor strengths, lower peak hip abduction moments, slower walking speeds, lower peak hip abductions, and lower peak hip extensions in the short-term after the surgery; however, at the 12-month follow-up, the bilateral hip strength (abduction, adduction, flexion, and extension), bilateral peak hip moment (abduction, adduction, flexion, and extension), peak hip power, or walking speed did not differ between groups. We reasoned that an insufficient hip abduction strength may in turn lead to differences in short-term functional outcomes [
22,
23,
24].
There is no perfect assessment tool, and the measurements of pelvic radiographs have not yet been well validated [
25]. This is the first study to connect functional outcomes to radiological assessments. We found that the questionnaires and assessment tools for functional outcomes were often subjective and generalized; therefore, currently, we can hardly ascribe the unsatisfying hip function to the postoperative horizontal residual displacement. Although reduction is important, the evaluation of the association between the radiological displacement and functional outcomes requires better tools. In patients with SPD, there will be multi-axial displacements, including horizontal, vertical, and rotational displacements. The plain radiographs could only reveal the measurement of horizontal or vertical displacements, whereas the rotational displacement could be assessed by CT. It is reported that CT or three-dimensional reconstruction-based displacement measurements of pelvic ring injury displacement may provide a more accurate assessment [
26].
This study had some limitations. First, a tomographic analysis is warranted to assess the rotation, but a customized view along the long axis of the pelvic bone is required for a correct assessment. Second, this was a single-center, observational, retrospective study with a small number of participants. However, as patients suffering from pelvic fractures with SPD are relatively rare, greater-scale research is difficult to carry out. To improve patients’ functional outcomes and satisfaction, this study sets a template for future research focusing on this topic. Further studies with more patient data would help to improve the understanding of the correlation between the functional outcomes and reductions in different dimensions.