The focus of this study was to compare clinical and radiological results following posterior open or minimally invasive percutaneous thoracolumbar spinal fracture fixation. In contrast to earlier studies our work focuses on one specific fracture type (AOSpine type A3) limited to the thoracolumbar junction (T11 to L2) [14, 16,17,18] .
Both treatment groups in this study were comparable in terms of age, gender and other demographic data, fracture level as well as surgical technique and fixation materials. Regarding fracture pattern the OS group included more fractures of a higher McCormack score than the MIS group without any significant difference.
Regarding functional outcome, the ODI Score which was defined as our primary functional outcome parameter did not demonstrate significant differences between both groups. Our results only demonstrated a significant difference in the MCS Score of the SF-36 and in the VAS Score with lower scores in the MIS group.
Bisegmental kyphosis angle was defined as our primary radiological outcome parameter. In terms of achieved reduction analyzing kyphosis angle pre- and postoperatively both techniques revealed good reduction without any significant difference.
Loss of reduction at time of follow-up at least 24 months after initial surgery demonstrated a significant difference between open and percutaneous techniques.
In the subgroup that received additional anterior fusion the MIS group demonstrated significantly less loss of reduction compared to the OS group. These results have to be interpreted carefully due to low patient numbers and minimal differences in terms of p-values.
The sagittal alignment parameters (PI, PT, SS, LL, TK and C7-Lot) were comparable in both groups demonstrating no significant difference.
The duration of surgery was significantly lower for posterior stabilization, anterior fusion, and implant removal in the MIS group compared to the OS group. Multiple other studies also reported significantly shorter surgery times for the percutaneous technique [8, 17, 18]. The comparably less extensive surgical approach in the minimally invasive technique may reduce operation time for primary stabilization and for implant removal.
Summarizing our main findings, a major difference comparing the open versus the percutaneous approach could not be demonstrated in the current study.
Data comparing functional outcomes of both techniques is rare due to different patient populations and operative approaches. Pishnamaz et al. included 43 patients with open and 29 with percutaneous posterior stabilization of fractures of the thoracolumbar spine. The majority were AOSpine type A3 (n = 29) fractures, but also type A2, A4, and B1 fractures were found. They did not detect significant differences in radiological or functional outcome comparing the open to the percutaneous approach [18]. Fitschen-Oestern et al. compared 104 patients between the age of 15 and 86 including vertebral bodies T9-L3 as well as fracture classifications of all types. In the subgroup with A3 fractures (n = 35) they could not find significant differences in postoperative kyphosis angle between minimally invasive and open posterior stabilization. Functional outcome was not evaluated [17].
In contrast to open procedures, surgical tools for secure reconstruction of the physiological alignment of the thoracolumbar spine are limited using percutaneous techniques. In this study, both techniques obtained significant reduction of kyphosis angle preoperatively versus postoperatively. In the MIS group, this could be achieved mainly by optimized positioning of the patient including ventral sagging and lifting of the upper body. Using the percutaneous technique, reconstruction of the physiological spinal kyphosis in our patient group was sufficient in contrast to comparable earlier trials e. g. provided by Grass et al. 2006 who reported a relatively high amount of cases with incomplete vertebral body reduction [8]. The difference might be related to improved surgical instruments and methods during recent decades. Especially the use of monoaxial screw systems and stiffer chrome-cobalt rods have led to more stable constructs as biomechanical studies have demonstrated [28, 29]. The consistent use of monoaxial screws and tools is therefore recommended as mandatory precondition for precise percutaneous reduction of posttraumatic kyphosis after thoracolumbar burst fractures [12].
Our data demonstrated a moderate loss of reduction level in both groups with a significant difference (− 3.2° vs. -5.6°). Within the subgroups the patients that received open posterior stabilization and additional anterior fusion did not show a difference to those who received open posterior stabilization only. However, patients who received the percutaneous technique and additional anterior fusion had significantly less loss of reduction than those with dorsal percutaneous stabilization only (1.0° vs. 5.6°). These findings suggest that in indicated cases additional anterior fusion should be performed to support the posteriorly achieved reduction. There is a controversial debate about the best timepoint to perform additional anterior fusion. Spiegl et al. reported that in patients that received posterior monoaxial stabilization and additional delayed anterior fusion after 6 weeks did not lead to more reduction loss [30]. On the other hand, Sander et al. reported a comparably high rate of traumatic vertebral disc lesions 1 year after trauma without any disc pathology in the initial MRI following trauma [31]. Thus, delayed MRI might be more sensitive in terms of identifying traumatic vertebral disc lesions. Thereby, delayed anterior fusion aims to reduce the number of anterior fusions. This concept was performed in trauma center A in patients without indirect signs of vertebral disc lesions and McCormack scores lower than 7 [30]. Therefore, the period between posterior stabilization and additional anterior fusion was significantly longer in trauma center A.
An advantage of early additional anterior fusion might be the lower surgical effort due to less sclerosis and therefore easier surgical preparation when performed at an earlier timepoint. This is supported by the significantly lower surgery time in our data (99.5 vs. 136.5 min). Especially for patients that are working early anterior fusion might have economic advantages because return to professional life is sooner than in delayed anterior fusion. Disadvantages include that some patients who might have been stable enough with posterior stabilization only possibly received an unnecessary additional anterior fusion when operated at an earlier point of time.
Reported disadvantages of percutaneous posterior stabilization include higher rates of radiation exposure and difficulty to control fracture reduction and to maintain lordosis [8, 32, 33]. Advantages of the percutaneous technique such as protection of autochthonous back muscles, less blood loss, shorter operation time, lower risk of infection, shorter duration of hospital treatment, less postoperative pain levels, earlier pain relief and improved clinical outcomes have been reported in literature [14, 34,35,36]. Our data supports many of these findings. As mentioned above we also found a significantly shorter surgery time for the MIS group (63.5 min versus 106.7 min). Surprisingly, the current study revealed equal duration of hospital stay for patients who received posterior stabilization only. Pishnamaz et al. [18] also reported almost equal lengths of hospital stay while Fitschen-Oestern et al. [17] reported shorter times for the percutaneous technique. Other reported advantages such as less blood loss have not been evaluated in this study. Suction devices are rarely used for the percutaneous technique making it difficult to assess intraoperative blood loss.
Malpositioning of pedicle screws in thoracolumbar spinal fracture fixation is not avoidable completely but considered to be safe also in percutaneous techniques [37]. In this study, one pedicle screw was placed too close to the vertebral disc in the MIS group, but no surgical revision was necessary due to malpositioning of pedicle screws in both groups. The majority of percutaneous stabilizations have been controlled intraoperatively by 3D-scan. This as well as surgical experience may explain the good positioning of pedicle screws in the percutaneous technique in our data set.
Study limitations
We acknowledge some limitations of the present study. First of all, this was a retrospective study design with all its limitations. Furthermore, duration of intraoperative radiological imaging and blood loss have not been assessed. Previous studies have shown that radiological imaging in minimally invasive stabilization may be more extensive and blood loss significantly less [8, 33, 34, 36]. Focusing on one specific fracture configuration limited to vertebral bodies T11 to L2 lead to smaller patient groups even though the study was performed at two high-volume level I trauma centers, but higher numbers of included patients would have been desirable.
