Minimally invasive tubular versus conventional open microsurgery of the lumbar spine for degenerative disorders

Conventional open microsurgery (COMS) for lumbar degenerative disorders (LDD) such as lumbar spinal stenosis (LSS) and lumbar disc herniation (LDH) includes longer skin incisions and complete or partial laminectomy [1‐4]. This surgical approach offers clear visualization of the operating field during surgery for various indications. Intraoperative blood loss, surgical trauma and postoperative segmental instability might be of concern [5].

With the rapid advancement of the technology as well as need and expectations of the both, patient and surgeon for a fast recovery, return to a normal life and less postoperative morbidity, minimally invasive techniques have been developed and gradually implemented in a daily praxis of the spine surgeons. Firstly, minimally invasive tubular microsurgery (MITM) for the lumbar spine was described by Foley and Smith in 1997 [6]. Initially, an endoscope was utilized to enable visualization, but the technique was soon modified to become a microscope-assisted procedure. In this technique via interlaminar route, the surgical level is detected with the C-arm in lateral projection. Then a 1.5–2 cm midline skin incision is performed. The muscle dilators are then inserted, aiming at the lateral hemilamina and the medial facet joint. A 16- to 20-mm tubular retractor is then inserted. The retractor is always initially docked on the facet joint and hemilamina since these are safe bony landmarks for the muscle dilation process. Lateral and antero-posterior X-rays are then taken to ensure the optimal final position of the working channel along the craniocaudal axis as well as its position with respect to the facet joint, the pedicle, hemilamina and midline. The tube should be aimed at the causative pathology, but it should also keep bone structures within view as these offer safe planes for preparation. The operating microscope is used for visualization. The muscle fibres within the operating field are cauterized and removed. A drill is then used for the hemilaminotomy and if needed to remove part of the medial facet joint. Once the recess is decompressed and the flavectomy is performed, the dural sac and the nerve root can be identified. Using this technique, the recessal stenosis can be decompressed, the LDH can be removed and the contralateral side can be decompressed as well as moving the tubular retractor at various angles known as wanding allows the surgeon to reach different areas within a spinal segment. All of the surgical manoeuvres used in a standard open microdiscectomy can be performed through the tubular retractor under microscopic view. After the successful decompression and/or sequestrectomy and adequate haemostasis within the entire operating field, the tube is smoothly withdrawn and microbleedings of muscle fibres in the different layers of the paravertebral musculature are microscopically identified and cauterized. This procedure is a recommended precaution to be taken to avoid re-haemorrhage. Subfascial drainage is not necessary according to our experience. The incision is then closed with subcutaneous sutures and skin glue. In spite of the more restricted visualization and the steeper learning curve associated with the technique, MITM has increased in popularity with many surgeons and patients. This led to many studies and gathering more information about the technique, pearls and pitfalls. Approach-related trauma, blood loss and length of hospital stay were reduced according to some authors [7‐11]. The superiority of the MITM technique was questioned by the others as the patient-related factors (blood loss, clinical outcomes, complication rate, pain) [12‐16] as well as hospital-related factors (surgical time, length of stay, re-admission rate) of MITM were not in favour of the minimally invasive technique absolutely. This diverged literature does not provide a clear recommendation and leaves considerable number of spine surgeons somewhat undecided and sceptical towards MITM.

While most publications concentrate on LDD [15, 17, 18], for which the technique was first described, the evolution of MITM has seen it applied in a wide variety of indications such as dorsal cervical spine, tumour microsurgery, paediatric surgery as well as fusion surgery [19‐22]. Nevertheless, the investigated were rather smaller patient groups and not big cohorts which does not offer an answer for generalizability in respect of indications.

Lastly, the surgeons related factor are also very important. The restricted visualization of the surgical field can lead to more intraoperative X-ray usage. Because of this, paired with a broader overview on the whole surgical field, some surgeons continue to prefer COMS. To date, however, while X-ray exposure in MITM has been addressed in fusion procedures, it has not been investigated in any of the large cohort studies.

The aim of this retrospective observational comparative study was to investigate all three—patient-related variables, hospital-related variables and surgeon-related variables as well as generalizability of MITM compared to COMS in patients undergoing microsurgery for LDD in our single-center cohort.

The study was a retrospective observational comparative single-centre study, carried out between 09/2018 and 09/2021. All patients were seen preoperatively in an outpatient clinic or emergency room (ER). All consecutive patients with LDD such as LDH or LSS who had given informed consent, were due for microsurgery and completed both follow-up time points were included in the study. Unilateral transmuscular (MITM, METRx® System, Medtronic, USA) or subperiosteal (COMS) approaches were used in all cases. Surgical approach was decided according to surgeon’s preference.

Primary endpoints were intraoperative estimated blood loss (EBL), surgical time, X-ray exposure (measured in mGy, mGy cm² and exposure time in s) and pain measured on a visual analogue scale (VAS Back/VAS Leg) at baseline, 6 weeks and 1 year postoperatively. Secondary endpoints were the re-admission rate, complication rate and patient outcome (quality of life), determined by the modified Macnab criteria at 6 weeks and 1 year follow-up postoperatively. All patients from both study groups were followed either in an outpatient clinic visit (at 6 weeks) or contacted per telephone (at 1 year).

A total of 403 (116 MITM, 287 COMS) patients were enrolled. 121 levels in MITM (L1/2: 2, L2/3: 13, L3/4: 22, L4/5: 48, L5/S1: 36) and 293 levels in COMS group (L1/2: 12, L2/3: 43, L3/4: 52, L4/5: 93, L5/S1: 87) were operated. The mean age was 57 years (SD 14.6) in the MITM and 55 years (SD 15.2) in the COMS group. There was no difference between the groups in respect of baseline demographics, indications and comorbidities (all p > 0.05). The further analyses of subgroups according to indication within both study groups confirmed no statistically significant difference between subgroups in respect of sex, age, BMI and comorbidities (all p > 0.05).

Detailed demographic and clinical variables of the study population are summarized in Table 1.

Mean Rx-Dosis (mGy and Gy.cm² resp.) and mean exposure time (s) were 1.1 (1.1), 0.3 (0.3) and 3.6 (2.0) in MITM group and 1.6 (2.1), 0.5 (0.7) and 4.55 (3.0) in COMS group. Mean blood loss, surgery time and nights in hospital were further stratified according to indications and showed a significant difference between both groups (all p < 0.05; Table 2).

Complications until follow-up at 1 year are listed in Table 2. Overall as well as stratified according to indication, there was no significant relationship between the type of microsurgical approach and the number of complications (all p > 0.05). 3 (2.5%) patients in the MITM group and 33 (11.3%) patients in COMS group had a recurrent LDH (p = 0.004). No significant relationship was found between the type of microsurgical approach overall (p = 0.081; Table 2) as well as for stratified indications and hospital re-admissions/revisions (all p > 0.05).

Clinical outcome at 6 weeks as well as at 1-year follow-up was measured by the modified Macnab criteria and VAS Scale (VAS Leg/VAS Back). An independent t-test showed overall significant difference in mean clinical outcomes between both groups for both the Macnab criteria and VAS Scale (VAS Leg/VAS Back), at 6 weeks and at 1 year postoperatively (Macnab 6W vs. 1y: p = 0.02 vs. p = 0.002, VAS Leg 6W vs. 1y: p = 0.005 vs. p ≤ 0.001 and VAS Back 6W vs. 1y: p ≤ 0.001 vs. p ≤ 0.001). The results of further stratified analyses for every indication are in Table 3.

Longitudinal analysis of VAS Scale (VAS Leg/VAS Back in Table 4, Fig. 1) and clinical outcome measured by modified Macnab criteria (Table 5, Fig. 2) showed significant improvement at each follow-up point for both groups (p < 0.001). Intraoperative view of both microsurgical techniques is shown in Fig. 3.

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In our hands, MITM had a shorter surgical time, compared to COMS (by 9.5 min). The results from the literature are mixed. However, Ryang and colleagues demonstrated no difference in operating times [16] while Righesso and colleagues, Huang and colleagues, and Teli and colleagues showed that COMS had a significantly shorter operating time in surgeries in which MITM was used as originally described [9, 13, 23].

In our study, blood loss was also lower in the MITM group (by 28 ml), in line with the findings of Pintea and colleagues and Huang and colleagues and supported by retrospective studies [13‐15, 24]. In contrast, Ryang and colleagues and Righesso and colleagues demonstrated no difference in estimated blood loss between the MITM and COMS groups [13, 16]. Altogether, the amount of 28 ml difference between groups is clinically irrelevant and when intraoperative bleeding appears, it likely results from epidural veins or the segmental artery in foraminal explorations, which can bleed similarly in both groups. Approach-related significant haemorrhage is rare for both types.

The mean length of hospital stay (LOS) was 4.3 nights in the MITM group, which was significantly lower than in the COMS group (5.2 nights; p = 0.001). Huang and colleagues and Righesso and colleagues also reported a shorter hospital stay for patients treated with the MITM approach [9, 13]. There was no difference in the LOS in the studies carried out by Lau and colleagues [14] and Arts and colleagues [12].

With regard to complications, our study found no significant relationship between the microsurgical approach and the number of complications (p = 0.5). Nor was there any significant relationship found in our study between the microsurgical approach and the re-admission rate (p = 0.08). This applies for the overall as well as stratified analyses of the results for each of the indications except for LDH recurrence. The findings of our study relating to safety and feasibility are also in accord with those of Pintea and colleagues who, with a prospective analysis of MITM vs. COMS, reported that MITM is safe and effective, and those of Clark and colleagues who found no significant differences between complication rate and the microsurgical approach [15, 25, 26].

Subgroup analysis of complications showed a significantly lower rate of LDH recurrence in the MITM group (p = 0.004). In contrast, Ryang and colleagues as well as Huang and colleagues demonstrated no difference in recurrent LDH and cerebrospinal fluid leaks [16] or complications in general [13] while Teli and colleagues reported higher LDH recurrence rates in MITM patients [6]. The remaining complications were equally distributed and showed no significant differences.

All re-admitted patients had revision surgery. Detailed analysis of re-admitted patients in the MITM group vs. COMS group showed re-admissions/revisions due to: recurrence, 2 (1.6%) vs. 17 (5.8%); cerebrospinal fluid leak, 1 (0.8%) vs. 4 (1.4%); wound infection, 1 (0.8%) vs. 4 (1.4%); instability, 2 (1.6%) vs. 5 (1.7%), which was not significant. Differences between the groups according to different diagnoses were not significant either. This supports the hypothesis that these complications may rather occur due to the underlying pathology and are not related to the microsurgical approach [5, 27].

Mean clinical outcome was significantly better in the MITM group for both the Macnab criteria and VAS Scale (VAS Leg/VAS Back) at 6 weeks as well as 1 year postoperatively for any indication. This supports the original hypothesis that the surgical approach does not affect the natural course of disease, but it can reduce the secondary morbidity caused by surgery itself and can contribute to better clinical outcome. As the stratified analyses showed no different results comparing to combined MITM and COMS groups, the longitudinal analyses were performed for these groups combined with no further stratification for better visualization. The longitudinal analysis of VAS Back/VAS Leg showed a significant improvement from baseline to 6 weeks and from 6 weeks to 12 months follow-up. Interestingly, mean VAS Back in the COMS group at 6-week follow-up (3.2 [SD 1.1]) was significantly higher than in the MITM group (1.4 [SD 1.1]), which represents a clinically significant improvement [6] and corresponds with a slightly lower mean Macnab score (1.7 in COMS vs. 1.5 in MITM). This can be explained by the typically longer incision and likely a greater tissue trauma caused by COMS. Splitting the muscles by MITM leads to building of less scar tissue and thus faster healing. Psychological factors might also play big role as the patients aware of having only small incision trust themselves to be more active, less self-limiting, and therefore more satisfied. Patients from the MITM group in our sample felt better and performed better at the 6-week follow-up. This important finding can be helpful in deciding which type of approach to perform in specific types of patients.

The differences in the Macnab scores and both VAS pain scales between both study groups are no-longer as distinct at 12 months postoperative but still showing slight superiority of MITM to COMS in respect of VAS Score and Macnab scores, which is in contrary to two big RCT trials conducted by Pintea and colleagues [15] and Arts and colleagues [12] that showed an inferiority of MITM to COMS on that matter.

A possible explanation for the findings relating to blood loss and surgery time between the approaches may lie in the level of experience and skill of the surgeons. For every surgeon, the learning curve is an important process at the end of which are more challenging operations and settings. In the study department, the MITM approach is performed by both older residents and attending neurosurgeons, while COMS is one of more frequent teaching operations performed by younger, less experienced colleagues at the beginning of their training so that we cannot fully exclude a systematic bias resulting from the experience of the primary surgeon.

Our results must be interpreted in line with consideration that we are a tertiary teaching hospital for residents in high-income European country with a high standard of care and resources which plays an important role in decision-making, timing of surgery and perioperative treatment as well as postoperative follow-up.

There are no reported values from large, randomized studies that consider the X-ray exposure in the MITM approach. In our study, we saw that the intraoperative mean Rx-dose measured in mGy (1.1 [SD 1.1]) and Gy.cm² (0.3 [SD 0.3]), as well as mean exposure time (3.6 s [SD 2.0]) was significantly lower in the MITM group (all p < 0.05).

Overall, acceptance and implementation of a new surgical technique into daily praxis is a long process, which requires a thorough investigation providing a valid information on the safety and benefit for patients, institution and surgeons. The results of our study showed that MITM is a safe, feasible and beneficial for all three regardless of indication and confirms a nowadays’ trend towards the minimally invasive surgical approaches in spine surgery [28‐30].

The strength of this study is that it was a large cohort, included various indications and a long-term follow-up. However, there are some limitations of our study that need to be addressed. First, we acknowledge the retrospective observational design of the study, which cannot eliminate (selection) bias, false or incomplete documentation or missing data. A prospective, randomized study would be a more rigorous tool for comparative studies such as ours. Secondly, as mentioned above, the MITM approach was preferably performed by experienced residents and attendings, whilst COMS is the standard “teaching approach” for operations performed by residents in training and that this could influence the results in terms of surgery time and blood loss.

The minimally invasive tubular approach is a safe and effective approach from both the surgeon’s as well as the patient’s perspective and should be considered for a wide range of indications for microsurgical lumbar spine surgery. The technical considerations, as well as re-admission rates and complications are equivalent or slightly better in MITM compared to COMS. MITM showed somewhat better clinical outcomes in both short- and long-term follow-up, albeit small between-group differences.