A randomised controlled trial of transforaminal endoscopic discectomy vs microdiscectomy

Open spinal surgery comes at a cost of approach related morbidity. The importance of reducing muscle damage, particularly to the multifidus muscle that maintains segmental stability, has become well-recognised [1]. The concept that less aggressive decompression may yield better results has led to the development of minimally invasive techniques with microdiscectomy becoming commonplace in most centres.

In 1990, Kambin highlighted the potential access to the lumbar disc via the ‘safe-working’ zone or ‘Kambin triangle’ [2]. This precipitated interest in the transforaminal approach to the disc from within the canal [3, 4, 5], as an alternative to minimally invasive central nucleotomies (blind percutaneous endoscopic, automated percutaneous and laser discectomy) that lacked evidence of efficacy [6]. The ability to safely resect disc tissue by this method from inside the disc out [7], or outside in [8, 9], has been recognised. Whether the theoretical advantages of transforaminal endoscopic (TED) surgery for symptomatic lumbar disc herniation are borne out by patient outcomes remains unclear. A systematic review has highlighted a paucity of evidence for the TED technique [9], in comparison to data now available in respect of microendoscopic approaches [10, 11]. In view of the increasing use of transforaminal approaches to the spine, there is clearly a need for high-quality randomised controlled trials (RCTs) to determine whether TED is as effective as traditional microdiscectomy.

The aim of this prospective randomised controlled trial was to determine whether TED leads to equivalent, better or worse outcomes than the current ‘gold standard’ of microdiscectomy. A power sensitivity analysis suggested 140 patients were needed to detect a 10% difference in Oswestry Disability Index (ODI) improvement at 5% significance, with 80% power and 10% loss to follow-up. Ethical approval was obtained and the study protocol published (http://www.controlled-trials.com/ISRCTN11556571).

The inclusion criteria were: age 25–70, a single level prolapse with exiting and/or traversing nerve root compression and failure of conservative management (>6 weeks of physiotherapy). Exclusion criteria were: upper lumbar disc disease (L2/3 or above), previous disc prolapse, massive sequestered disc prolapse, weight >115 kg, and malignancy.

Patient-reported outcomes following TED were not inferior to those following Micro and some were better. This supports a previous study comparing outcomes from a cohort study of TED and Micro data from the Swedish Spine Registry [20]. Pain in the affected leg was significantly better in the TED group at 2 years. This is in contrast to the findings of the 2014 Cochrane Review of all forms of minimally invasive discectomy (11 studies) that indicated that patients had less ongoing leg pain following open discectomy [21]. However, sample sizes were small and many of the studies were of questionable quality. We found no difference in back pain between treatments. Improvements in ODI at 12 months for both TED and Micro were similar to those reported in the SPORT trial following Micro [22]. Patient-reported outcomes for TED were similar to those reported by Ahn et al. [23] in their recent retrospective analysis of outcomes following endoscopic discectomy (Yeung Endoscopic Spine System technique) in young soldiers.

In this trial, only patients with non-sequestered herniations are included. Distally migrated discs are amenable to transforaminal surgery [24] but it is technically difficult. Inclusion of these patients was thought at the trial outset to potentially introduce bias. A lower age limit of 25 was selected to exclude ‘juvenile’ type prolapses which have a different natural history.

Symptom duration prior to surgery displayed a broad range. Sub-analysis of the SPORT trial [22] has indicated that patients with greater than 6 months of symptoms risk inferior results. Most of our patients reported a history of some symptoms at a mean of 2 years before surgery and we were not able to discriminate ‘acute’ symptoms with accuracy. However, symptom duration was similar in both groups. Once in contact with the surgical service, the preoperative rehabilitation regime was standardised for all patients. We cannot comment on the effect or nature of any other prior non-operative management. Following surgery, patients were referred for physiotherapy if they complained of stiffness. Alternative therapies were not prescribed, but if received, were sourced independently by the patient and may represent a possible source of bias between groups.

We recognise that anaesthetic methods differed between the treatment arms in this trial with conscious sedation rather than GA chosen for TED as a ‘patient reporting’ safeguard against nerve root injury. Although the difference in anaesthesia did not materially alter the length of the surgical procedure (the longer GA induction for the Micro group was balanced by a longer theatre ‘set-up’ time for TED) it should be accepted that a shorter ‘wake-up’ after surgery would have contributed to the reduced hospital stay noted in the TED group. This difference may not have been as pronounced if GA had been used in both groups. Radiation doses for both procedures were well within safe ranges (<3% of accepted Dose Area Product threshold of 500 Gy cm²) [25].

The site of disc prolapse was similarly distributed in both treatment arms. The transforaminal approach is particularly suited to excision of ‘far-out’ prolapse and widening of the foramen for the exiting nerve root [26]. In contrast, Micro provides easier access to the lateral recess of the canal, although possibly with greater epidural scarring [27]. Access to the L5/S1 disc during TED may be difficult if the patient’s pelvic crest is high. In this series, L5/S1 disc access was facilitated by careful patient positioning using a ‘broken’ operating table and no patient required intra-operative conversion to Micro.

Transforaminal endoscopic surgery is associated with a significant learning curve [28]. All five revisions in the TED arm of the trial were within the first two-thirds of the study. As surgical experience with TED increases and technology advances, the incidence of revision would be expected to decline.

The combined rate of revision surgery at 5% was within that expected from most reported studies of discectomy. Of those revised, five opted for Micro as a second operation. One patient chose to have repeat TED achieving a good outcome. The reported absence of scarring at revision of TED [29] has important implications for longer-term outcomes in terms of the ease of revision surgery. The reduction in tissue disruption provided by the TED technique may also have important effects on outcome via a reduction in cytokine release lessening the overall inflammatory response to surgery [30]. In this trial, patients who underwent revision were included in our outcome data only up to the point of revision. This point was considered to represent their worst outcome scores as all patients improved following revision surgery. Had outcome scores subsequent to revision procedures been included this may have falsely improved 1 and 2 year results. On this basis, we found no significant difference in quality of life improvement between the treatment arms at 2 years.

Weaknesses of this study include the non-blinded nature of the trial. Both surgeon and patient were aware of their treatment and the senior surgeon acknowledges a specific interest in endoscopy which may introduce bias. However, all outcomes were collected independently and are patient reported. The data were scrutinised by all authors. Different anaesthetic techniques were used which may favour shorter length of stay in the TED group. This was pragmatic as it was considered safer to perform TED under conscious sedation. Though length of stay was significantly shorter in the TED group, this was a secondary outcome measure and the study was not powered to detect differences therein. No record was made of any litigation pertaining to any presenting injury. Finally, data were analysed “as treated” not as “intention-to-treat”. This was considered acceptable as only one case crossed over between treatment arms and this was due to equipment failure not clinical choice. Thirteen patients (9.3%) were lost to follow-up by 2 years. This was within the 10% allowed by our power calculation and was significantly less than the 20% required by a level 1 trial.

Transforaminal endoscopic discectomy and microdiscectomy give comparable outcomes. A reduced length of hospital stay and less leg pain at 2 years following TED are offset by a greater requirement for repeat MR scans and revision surgery. Although a learning curve applies for this technique, the lack of requirement for general anaesthesia and a mean length of stay under 1 day makes TED an ideal technique for use in day-case units.