Advantages and disadvantages of MIS-TLIF have been controversially discussed, as the results vary widely in the literature and publication bias may be an potential issue . Before minimally invasive techniques in spinal surgery gained popularity, the traditional open approach represented the “gold standard” for lumbar instrumented fusion. This open approach involves large incisions and deattachment of healthy muscle tissue from bone resulting in increased muscle trauma, blood loss and associated postoperative pain . This muscle trauma leads to muscle edema, decreased muscular performance and potentially denervation [25, 32]. These findings might be associated with prolonged hospital stays and increased postoperative complications [37, 47]. To overcome these problems, the minimally invasive approach for lumbar instrumented fusion has become increasingly attractive, as it provides potential benefits in terms of reduced paraspinal muscle trauma and resulting in decreased loss of blood, faster recovery rates and reduced surgical site infection rates [40, 46, 56]. In contrast, the long-term observations comparing minimally invasive to open TLIF procedures failed to reveal significant differences, so that the beneficial effect of MIS procedures seem to be in the first weeks after surgery. Some studies examined functionality and pain after 3 months [36, 44], but the short-term period with analysis of multiple time points during the first 12 weeks postoperatively is still underreported. One study reported the average time until walking or standing-up postoperatively with MISS was 3.2 days compared to 5.4 days with open surgery . Another study reported a significant reduction of muscle injury and systemic inflammatory markers during the acute postoperative period with MISS . The authors suggested that MISS may play an important role in preventing medical morbidity after spinal surgery . Others reported decreased pain, stress, fatigue, and mood disturbance six weeks postoperatively  and reduced surgical site infections [6, 38, 50] compared to patients who underwent open surgery. Postoperative narcotic use and return to work were found to be the most clinically relevant factors both reduced 2-fold by MISS . Despite these few reports about the postoperative short follow-up, there is no comparative study, that focusses on the first 12 weeks postoperatively in detail.
Our study reports advantages of the MIS-TLIF procedure regarding operative time, intraoperative as well as postoperative blood loss and some functional outcome scores. In terms of VAS and NRS, the MIS group revealed significantly more leg pain but a trend towards faster improvement of back pain compared to the open procedure. The patient-reported outcome measures (PROMs) demonstrated no statistically significant intergroup difference after 12 weeks.
Significantly more leg pain (VAS and NRS leg) of patients treated with MIS-TLIF was appearent at nearly all postoperative time points within the evaluated period of 12 weeks. The cause of increased leg pain in the MIS cohort may be explained by the greater nerve root retraction due to the minimally invasive approach. In case of an open TLIF procedure the facet joint is commonly widely resected and Kabin`s triangle may be approached with less retraction of the traversing and exiting nerve root as well as the dural sac without a predefined lateromedial angulation of a minimally invasive installed retractor . By choosing a too medial MIS-TLIF approach the dural sac and the traversing nerve root might be stressed. In case of a far lateral approach, the Kambin’s triangle is approached in a flat angle leading to a small working corridor for cage implantation as well as potential irration of the exiting nerve root. Two previous studies examined VAS leg after 6 and 24 months without significant differences [2, 39]. In contrast, immediate postoperative low back pain showed no significant differences between the two cohorts, even though back pain tended to improve faster in the minimally invasive treatment group potentially related to less muscle trauma. Our results are in line with the existing literature observing the long-term follow-up, as most studies have observed less postoperative pain in the MIS TLIF cohort [11, 40] than in the open group mostly, without statistically differences.
In addition to that, the learning curve of MIS procedure can not be neglected. MIS-TLIF requires mastering new techniques, which is thought to increase the length of operation due to the learning curve and also effects other perioperative factors such as complication rate [27, 35] .
Functionality and disability are important factors after spinal surgery, so that most of the published studies evaluated the Oswestry disability index between MIS and Open procedures between 3 and 24 months postoperatively . To summarize their long-term findings (3 and 24 months postoperatively), MIS patients showed lower postoperative ODI scores compared to open treated patients at nearly all study timepoints [12, 36, 48]. We can mostly agree with these findings, but a 1:1 comparison could not be drawn due to the lack of studies evaluating the short postoperative course. Focussing on the fact that the ODI depends on patients’ responsiveness we additionally used the TUG test. There are recent studies demonstrating the reliability and the ease use of the TUG test [19, 52] for the measurement of functionality in case of spinal disorders. Considering the TUG, the MIS cohort also tended to outperform the open cohort. These findings might be also explained by the minimized surgical muscle trauma leading to reduced postoperative low back pain and consequently to a higher functionality state .
Summarizing the majority of studies in terms of surgical time, there were no significant differences between MIS and Open TLIF [16, 27, 55], which coincide with our observation. Nevertheless, duration of surgery might only have a limited significance on the superiority of one technique, as it strongly depends on the surgeon’s routine .
Regarding peri- and postoperative blood loss as well as complication rate, we can support the results of prior studies [11, 23, 27], as in our observation the MIS cohort had a significantly reduced blood loss and no statistically significant difference in terms of complication rate. However, it should be mentioned that there are also many cofactors (multi-level fusion, preoperative hemoglobin, male gender and body mass index) related to a higher loss of blood .
In terms of PROMs, they are less frequently recorded in the existing literature regarding the comparison of MIS and open TLIF procedures. In general, the most common PROMs consist of ODI, COMI, GDS, health status questionnaire as well as the EQ-5D. Recent studies only reported on the Short-Form 36 (SF-36) [28, 45, 47], the Short-Form12 (SF-12)  and the EQ-5D [2, 38]. These findings showed no statistical significance in any of the reviewed studies in the long term follow up . This observation also applied for the short term follow up within 12 weeks, without significant differences between both groups.
Our results are limited by the lack of randomization and by the small patient cohort. In view of strengths, there is no other prospective cohort study that conducted such early regular postoperative visits during the first 12 postoperative weeks and thus highlights the differences in this vulnerable phase especially in elderly and patients with comorbidities.