Although a large number of studies have reported the surgical treatment of thoracolumbar burst fractures combined with osteoporosis, there is no consensus on the surgical treatment strategy8–11,23−24. The current consensus is to correct kyphosis, rebuild spinal stability, and restore sagittal balance 25. Posterior pedicle screw fixation is considered an effective treatment for TLBF 8–9, 11. however, screw loosening occurs in more than 60% of patients with thoracolumbar fractures associated with osteoporosis 5. PMMA augmented hollow pedicle screws are considered as an alternative technique with broad application prospects 26,27. In recent years, some researchers have applied it in the treatment of thoracolumbar burst fracture with osteoporosis 11,28, but more attention has been paid to the improvement of clinical symptoms and correction of kyphosis. Less attention has been paid to the spino-pelvic parameters and their possible effects on clinical symptoms and kyphosis.
For osteoporotic TLBF patients, the mechanical stability of CICPS is a common concern. In the study, the difference of D-a/s was statistically significant at 3 months after surgery compared with that after surgery. However, there was no significant difference in D-a/s between 3 months after operation and the last follow-up (Fig. 2A-B), indicating that the screw displacement tended to be stable. The distance of some CICPS was greater than 1mm at the last follow-up (Fig. 3). Although the displacement distance of some CICPS was greater than 1mm at the last follow-up, there was no double ring sign on anteroposterior radiograph and no screw loosening on CT scan. We believe that the main reason for this phenomenon is that there is a loss of reduction in the anterior and middle part of the fractured vertebrae after surgery, resulting in the relative displacement of CICPS. In the study, the anterior-middle portion of the fractured vertebra showed varying degrees of height loss at the 3-month follow-up (Table 2). Loss of fractured vertebral height after surgery has also been found in some studies 24,29–31, and in particular Ulrich J et al. 30–31also found varying degrees of reduction loss in the anterior and central parts of the fractured vertebral body after surgery, where the tips of CICPS are located. Wu et al. 32,33 pointed out that in osteoporotic burst fractures reduced by position or instrumentation, the area within the vertebral body is almost empty. Hu et al. 10 pointed out that a gap may be formed between the cement and bone by using a screw-rod system for distraction reduction after the cement is cured. Due to the absence of support, the reduced fractured vertebra may further collapse, which to some extent may cause the loss of early postoperative correction. Therefore, the greater height loss of the anterior and middle part of the fractured vertebra in the short term may be due to the insufficient support of the anterior column of the vertebral body. However, Cai et al. 34 pointed out that cement has the innate advantage of inducing bone growth, which is beneficial for the recovery of burst fractures, which may imply that CICPS may achieve better clinical outcomes in the presence of effective anterior column support.
Some scholars 24,29,31,35 have reported a loss of Cobb angle reduction after surgery .This study similarly found a loss of Cobb angle correction after surgery, and it is worth noting that the loss of Cobb angle seemed to be most pronounced at 3 months and stabilized thereafter (Table 2). The reasons for the large loss of Cobb angle in the short term are as follows: The first is the loss of the height of the injured vertebra. In the study, a large loss of the height of the anterior and middle part of the injured vertebra occurred at 3 months after surgery, which coincided with the loss of the reduction of the Cobb angle at the time point. Second, intervertebral disc degeneration and loss of height also increase the incidence of kyphosis 36, 37 ,especially in the upper intervertebral disc of the fractured vertebra. Ulrich J et al. 24,29,35 both found intervertebral disc degeneration and height loss in the upperintervertebral disc of the injured vertebra during their study follow-up and pointed out that this may be one of the reasons for the loss of correction. In addition, some scholars 38,39 have also pointed out that after the use of bone cement, the upper or lower endplate blood decreases, accelerating the degeneration of the corresponding segment. Schwab et al. 40,41 pointed out that sagittal spinal parameters are correlated with the postoperative quality of life of patients, and that SVA ≥ 50mm and excessive PI-LL values will aggravates functional disability in patients with spinal deformity. Despite the loss of Cobb angle reduction during the study, there was no corresponding change in spinal sagittal balance (Table 2). We speculated that the patient had postural compensation through lumbar lordosis (LL, final follow-up VS postoperative, p < 0.05) and pelvic retroversion (PT, final follow-up VS postoperative, p < 0.05). This may also be the reason why the reduction of Cobb angle was lost, but the VAS score and ODI score still improved significantly at the last follow-up.
With the rise of surgical techniques in recent decades, proximal junctional kyphosis (PJK) has emerged as a new complication after spinal instrumentation 42–44. Smith et al. 43 noted that the prevalence of PJK in patients who have undergone adult degenerative scoliosis (ASD) correction is generally considered to be between 17% and 39%, with some studies reporting an incidence as high as 69% at 3 years of follow-up. The incidence of PJK in this study was 10.3% (6/58). The incidence of PJK is lower than that reported in traditional studies, which may be mainly due to the following reasons: First, compared with ASD correction surgery, the number of patients with short-segment fixation in the study was larger. Except for less damage to local soft tissue, it has less effect on the amplitude of spinal deformity reduction and stiffness of local structures. Some studies have also identified perispinal soft-tissue destruction45,46, the extent of deformity correction 47, and structural stiffness 48 as risk factors for PJK. Secondly, the overall sagittal balance of the spine was relatively well maintained during the follow-up (Table 2). Lafage et al. 49,50 also explained the importance of the sagittal balance of the spine in the prevention of PJK, and pointed out that PI-LL mismatch was found in PJK patients. Finally, some studies 51,52 have pointed out that PMMA augmented screws may lead to the aggravation of adjacent intervertebral disc degeneration and increase the stress of adjacent vertebral bodies, leading to vertebral fracture and then PJK. However, this does not seem to explain the large number of subsequent fractures in noninstrumented adjacent vertebrae. Therefore, under the premise of restoring the sagittal balance of the spine, selecting appropriate fixation segments and reducing the number of CICPS are of positive significance to reduce the incidence of PJK.
This study has several limitations. Firstly, this study is not a randomized controlled study, which contributed to a low level of evidence. Secondly, the sample size is small and the follow-up time is short, thus a study with more patients and longer follow-up duration is required to further confirm our results. Finally, standard application protocol of CICPS should be developed to avoid the catastrophic consequences of excessive application, which will be our future research focus.