Mid-term follow-up of far lateral microdiscectomy—surgical technique, outcomes and reoperation rates

Introduction

Background

Far lateral disc herniations (FLDHs) are foraminal and extra-foraminal disc prolapses that comprise only 7–12% of all lumbar disc herniations (1). Compared with the more common paracentral/posterolateral lumbar disc herniations (PLDHs), FLDHs are more likely to occur in older patients (2-4), and are associated with more severe pain and neurological deficit preoperatively (3). FLDHs most commonly occur at L4/5 and L3/4 (5-8) and cause extreme radicular pain due to direct compression of the exiting nerve root, typically the upper nerve root at the index level, as well as the dorsal root ganglion (DRG).

First-line management of FLDHs is non-invasive and includes the use of physical therapy, non-steroidal anti-inflammatory drugs (NSAIDs), and epidural corticosteroid injections. If these conservative measures fail, a far lateral microdiscectomy (FLMD) may be required. FLMD is the preferred surgical intervention, yet there are numerous other approaches reported in the literature, each with their own advantages and disadvantages.

Leon Wiltse in 1968 (9) described a muscle-splitting approach via a paramedian incision, which provides optimal exposure of the lateral and foraminal disc to enable discectomy with minimal facet joint removal and preservation of the pars interarticularis. Results via this approach have showed improved function and minimal complications post-operatively (10). Other approaches, including midline (8,11), endoscopic (12,13) and tubular (5,14), have also been described, reporting favourable outcomes in terms of pain reduction and improved function.

Rationale and knowledge gap

Reporting of complications following FLMD is limited, but includes infection, seroma/hematoma, dural tear, residual disc, and dysesthesias (5,14-16). Reoperation rates range from 2–20% (3,5,15,17-20), most commonly due to reherniation at the same level (15,18,20,21) and recurrence is usually managed with redo FLMD or interbody fusion. If required, reoperation is most likely to occur within 1 year of the index case (5,14,15,18-20). Reoperation rates following lumbar microdiscectomy (all approaches) are 7.5% (22), with the reported mean interval between primary and revision surgeries ranging between 15 weeks and 7.6 years (23).

There remains wide variation in the utilization and experience of an extra-foraminal approach between regions, institutions and surgeons. Most studies on FLDHs report follow-up of between 6 weeks and 12 months, with few comprehensively reporting outcomes, complications, and reoperations, especially in the mid-to-long term. There is a paucity of data on the longer-term outcomes, recurrence rates and reoperation rates following a FLMD operation.

Objective

This study describes the FLMD technique and the differences in approach at each anatomical level. We report long-term follow-up in a large sample of patients (n=50) undergoing FLMD for patient satisfaction, complications, recurrence and reoperation. We present this article in accordance with the STROBE reporting checklist (available at https://jss.amegroups.com/article/view/10.21037/jss-25-10/rc).

Methods

Study design

This was a single centre (Epworth Hospital, Richmond, Australia), single surgeon (G.M.M.), retrospective analysis of all consecutive patients undergoing a lumbar FLMD via a paramedian Wiltse approach from January 2010 to December 2021. The minimum follow-up was 3 years.

The standard indications for surgery were symptomatic extra-foraminal disc prolapses causing severe radicular pain or motor radiculopathy that failed to respond to physical therapy, opiate analgesia, or epidural injections for a minimum of 6 weeks. Contraindications for FLMD included a previous lateral surgery, severe foraminal stenosis (whereby fusion is probably required to open the foramen), or difficult access (L5/S1 can be difficult to access in some patients due to narrow pelvis with high iliac crest).

Indications

A FLMD via Wiltse approach may be indicated in several instances. Firstly, if the predominantly affected root was the upper one for the level in question. Additionally, the disc prolapse was foraminal or extra-foraminal. Previous surgery in the spinal canal increases concerns about scar tissue, risking dural tear or damage to the unaffected lower nerve root. Lastly, anatomical reasons such as protecting the pars interarticularis, especially at upper lumbar levels.

Outcome measures

Patient-reported outcome measures (PROMs) included visual analogue scale (Yingsakmongkol, #24840) back and leg, Oswestry Disability Index (ODI) and short-form-12 (SF-12). PROMs and patient satisfaction using Odom’s outcome criteria were collected preoperatively, at 6 weeks postoperatively, and at last follow-up (minimum 3 years).

The persistence or recurrence of leg pain from recurrent disc prolapse, foraminal stenosis, or both was recorded. Any reoperation included revision FLMD, pedicle screw-rod fixation +/− interbody fusion at the index surgical level.

Surgical technique

Patients were anaesthetised (general anaesthetic) and placed prone on a Wilson frame or Jackson table or equivalent. A preoperative fluoroscopic level check was performed, and the midline was marked. An example of a left L4/5 FLDH is shown in Figure 1 via axial (Figure 1A) and sagittal (Figure 1B) T2-weighted magnetic resonance imaging (MRI) images. Depending on the level, a paramedian incision of approximately 4 cm length was marked in the range of 2.5–4.5 cm from the midline. For example, 2.5 cm at L2/3, 3 cm at L3/4, 3.5 cm at L4/5 and 4 cm at L5/S1. Adjustments were made depending on patient weight, musculature, fat distribution and with reference to the preoperative MRI measurements from the midline to the outer edge of the superior articular process (SAP) of the caudal vertebra. Often, we added 0.5 cm to these measurements due to thickness of fat above lumbar fascia.

Figure 1 T2-weighted MRI images showing left L4/5 far lateral disc herniation. (A) Axial view. (B) Lateral view. MRI, magnetic resonance imaging.

Dissection down to lumbar fascia was performed. This was incised and then blunt dissection (digital) was used to create a plane between longissimus and multifidus muscles. The transverse processes of the level above and below were identified by palpation, and muscle was stripped off by digital blunt dissection, then Cobb retractor and monopolar diathermy (it is important at this stage to identify the caudal facet joint and the lamina more superficial and medial to it. In obese patients, it is possible to incorrectly identify the lamina as the transverse process, and this is a trap for inexperienced surgeons).

The assistant used a deep Langenbeck retractor, or large bayonetted nerve root retractor to retract the muscles medially while this preparation was being done. The retractor tip was placed on top of the lower facet joint (it can be held there safely by the assistant with significant force but as muscle is being retracted against the midline bone it is important not to be too lateral with the incision—otherwise there is too much muscle bulk).

An initial Aesculap microdiscectomy retractor was placed, flat blade facing laterally, curved ones superior and inferior. Flat lateral blade was usually about 1–2 cm longer than cranio-caudal blades to help retract muscle over inter-transverse membrane.

A second lateral blade from another Aesculap retractor (Figure 2) was then added to hold the medial muscle clear over the distal facet joint. This blade was shorter as it sits atop the facet joint. A pair of artery forceps was used to stabilise it under the arm of the other lateral blade.

Figure 2 Intraoperative image of double Aesculap retractor in situ during FLMD. FLMD, far lateral microdiscectomy.

Once a good exposure was achieved and the correct level confirmed with fluoroscopy (with an instrument such as a Watson Cheyne dissector in the target foramen), an operating microscope was used.

Most muscle over the intertransverse membrane was retracted by the lateral blade. Any residual was then removed with rongeurs or equivalent until the intertransverse membrane (Figure 3A) was clearly identified. Any vessels coursing medial to lateral across it were divided after bipolar cautery. The muscle/membrane was opened, with any of a blunt hook, knife, rongeur or punch. Immediately deep to the intertransverse membrane, fat and fibrous tissue were encountered (Figure 3B), then branches of the pars vessel, which were divided and cauterised immediately superficial to the nerve root (Figure 3C). Due to vessel branches in this area, it often had to be cauterised several times.

Figure 3 Illustration of intertransverse membrane and pars vessels encountered during FLMD. (A) Pars vessels overlying intertransverse membrane. (B) Fat and fibrous tissue deep to intertransverse membrane. (C) Exiting nerve root immediately deep to fat and fibrous tissue. Nerve descends inferiorly after passing underneath the pedicle. This figure was commissioned by Professor Gregory Malham from Taehoon Gwag. FLMD, far lateral microdiscectomy.

Once in this deeper space and with good vision (with operating microscope), a Watson Cheyne or blunt hook was used to confirm the anatomy. Starting distally and working off the transverse process of the lower level onto the side of the lower vertebra and then onto the disc. By staying inferior we found the disc prolapse was usually encountered before the nerve, as most commonly the disc pushes the nerve root superiorly (Figure 4A). Occasionally the nerve was found to be tented tightly over the prolapse and very thinned out just lateral to the ganglion. If uncertain, we dissected further medially to find the medial edge of the root, confirmed by easy passage of a hook back into the foramen below the root.

Figure 4 Intraoperative microscope images of exiting left L5 nerve root relative to prolapsed disc material. (A) Left L5 root overlying disc prolapse. (B) Retraction of nerve root to provide better access to disc material for removal. SAP, superior articular process.

At the upper lumbar levels, we found it possible to complete the discectomy with minimal bone removal. In cadaver dissections we were able to expose all the levels L2/3, L3/4 and L4/5 without any fact joint or pars removal, made easier by a longer incision and better view. However, at L5/S1 (technically most difficult level) it was always necessary to remove some of the lateral facet joint of L5/S1, specifically the SAP of S1, in order to access the disc and nerve. In practice, we usually removed part of the lateral aspect of the caudal facet joint at all levels mostly removing bone from the SAP of the caudal vertebra so as to better expose the lateral aspect of the disc—20–30% was usually sufficient. This also provided a better approach angle to the disc, requiring less root retraction to access the disc.

Having identified the disc coming from below the root, a ball hook or similar was used to confirm the root. It was often identified by feel, before it was clearly visualised. Once identified and mobilised, the assistant could gently retract the root superiorly to provide safer access to the disc (Figure 4B). However, this was sometimes difficult due to the tight working space between the lower pedicle and exiting nerve root. Often the surgeon retracted the root with a Rhoton sucker, while removing the disc prolapse with a rongeur, or ball hook.

Finally, a hook was passed around the root from below and above, to be sure there was no residual fragment, then the disc space was flushed with saline to remove any loose disc material. It was rare to find any fragments above the root, as most prolapses pushed the root superiorly.

Following completion of the discectomy the root was always dramatically freer than it was at the start and often the ganglion was readily identified. The root was then bathed in a mix of bupivacaine and dexamethasone, with or without a haemostat such as Gelfoam and the retractor was removed. Due to the muscle splitting incision, there was rarely any bleeding at this point and the wound was closed in a routine fashion.

Ethical statement

The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. This was a review of cases collected under a standard privacy disclosure to patients that their information will be used for ongoing evaluation of outcomes and their identity will be protected in any publication arising from this. Verbal informed consent was obtained at last follow-up. The project was reviewed against the Human Research Ethics and classified as a low-risk research project in accordance with section 5.1.19 of the National Statement on Ethical Conduct in Human Research (2023). This study involves no more than minimal risk to participants, in accordance with the institutional review board (IRB) guidelines for minimal risk research [45 CFR 46.102(i)]. The research involves only the collection of de-identified data, with no potential for physical, emotional, or social harm. The waiver is not required as the study could practicably be conducted without it and will not adversely affect the rights and welfare of the participants. Additionally, when appropriate, participants will be provided with relevant information after their participation, as per the guidelines outlined in 45 CFR 46.116(d).

Statistical analysis

Statistical analyses were performed using R version 4.2.2 (R Foundation for Statistical Computing, Vienna, Austria). Descriptive statistics were calculated for demographic and clinical characteristics, with continuous variables presented as means and standard deviations (Falavigna, #24759) and categorical variables summarized as frequencies and percentages.

The primary analysis employed paired sample t-tests to evaluate changes in clinical outcomes before and after FLMD procedures. This statistical approach was selected to account for the within-subject design, where each patient served as their own control. Paired t-tests were specifically used to compare pre-operative and post-operative scores on the VAS (Yingsakmongkol, #24840) for pain, Oswestry Disability Index (ODI), and SF-12 physical and mental component summaries.

Prior to analysis, the assumptions for paired t-tests were verified. Normality of the differences between paired measurements was assessed using the Shapiro-Wilk test, and visual inspection of Q-Q plots. For any outcome measures that violated the normality assumption, a Wilcoxon signed-rank test was applied as a non-parametric alternative.

For all statistical tests, a P value <0.05 was considered statistically significant. Effect sizes were calculated using Cohen’s d to quantify the magnitude of observed changes. Missing data were handled using complete case analysis, with sensitivity analyses performed to assess the impact of missing values on the overall findings.

A priori power analysis indicated that our sample size provided 80% power to detect a clinically meaningful difference in the primary outcome measures with an alpha level of 0.05.

Results

Fifty patients underwent FLMD with a mean age of 60.6 years (range, 35–86 years) and body mass index (BMI) of 27.9±2.6 kg/m², with an equal gender distribution (50.0% male) (Table 1). The most frequently operated levels were L3/4 (36.5%) and L4/5 (34.6%), followed by L5/S1 (17.3%) and L2/3 (11.5%). The mean operation time was 77±17 minutes. The mean follow-up duration was 5.2 years (range, 3–14 years).

Clinical outcome measures including VAS (Yingsakmongkol, #24840), ODI, and SF-12 scores all demonstrated statistically significant improvement from pre-operative consultation to final post-operative follow-up (Table 2). Patient satisfaction was high, with 42 patients (84%) reporting excellent or good outcomes.

Eight patients required reoperation, yielding a reoperation rate of 16%. Two patients (4%) needed early revision surgery (within 2 weeks) for symptomatic recurrent far lateral disc prolapses causing radiculopathy, requiring redo FLMD at the index level. Six patients (12%) required later reoperation between 0.17 and 3 years postoperatively (mean 1.9 years) for symptomatic disc degeneration, foraminal stenosis, or both. These later reoperations consisted of four lateral and two posterior interbody fusions.

Complications were minimal, with persisting leg pain reported in 3 patients (6%). No patients experienced dural tears, nerve root injuries, infections, iatrogenic pars defects, or facet fractures.

Discussion

In a cohort of 50 FLMD patients with a mean follow-up of 5.2 years, the reoperation rate was 16%. A mid-term follow-up was performed given the limited literature data on mid-term and long-term follow-up outcomes of FLMD surgery.

The mean operation time of approximately 80 minutes is consistent with operative times reported in other series and is longer than those reported for a standard posterolateral microdiscectomy (24). FLMD is a more technique challenging procedure with particular care taken to avoid damage to the target DRG, exiting nerve root and segmental pars vessels. The most common level for a far lateral disc prolapse was equally L3/4 and L4/5, in contrast to the standard posterolateral disc prolapse which predominantly occurs at L4/5 and L5/S1 (4,25).

The mean follow-up of 5.2 years with range of 3–14 years is among the longer reported follow-up times for FLMD surgeries in the literature. The reoperation rate of 16% (8/50) was consistent with the reoperation rates in other mid- and longer-term follow-up papers (6,17). Redo FLMD for symptomatic recurrent disc prolapses occurred early (within 2 weeks), but the need for interbody fusion (6/50, 12%) most commonly occurred later (between 1–3 years). Interbody fusions undertaken via lateral or posterior approach were due to symptomatic disc degeneration, foraminal stenosis or both, but not for instability (Table 3). Only five studies report mid-to-long-term follow-up of FLMD reoperation rates, ranging from 5–12.2 years (2,6,8,15,17), but with limited reporting of indications for reoperation. A recent systematic review and meta-analysis of 1,527 patients with lumbar disc herniations reported a recurrence rate of 6.6%, which was higher in patients receiving a transforaminal endoscopic discectomy (8.6%) compared with a conventional microdiscectomy (5.5%) (26). In contrast, a large-scale population study of 13,654 patients in the United States reported a 12.2% reoperation rate at 4 years after a single-level lumbar discectomy (27).

Over 80% of patients in this study reported excellent or good outcomes, consistent with most other reported series on far lateral microdiscectomies, ranging from 58–100% (2,6,12,18,19,28). Complication rates were low, with 6% (3/50) reporting persisting leg pain, and no deep or superficial surgical site infections. This is consistent with other series also reporting low rates of infection, ranging from 0.6–2.2%, and postoperative neuropathic pain (5,19). It can be postulated that the Wiltse incision may be less prone to infection due to reduced skin pressure from underlying bone when supine (during recovery) when compared to a midline incision. No patients in this study suffered a dural tear, consistent with the extremely low rates of dural tear/pseudomeningocele reported in the literature, ranging from 0.6–6.5% (7,14). A recent systematic review of randomised controlled and prospective cohort studies including 2,730 patients receiving microdiscectomies for lumbar disc herniation reported a durotomy incidence of 2.3% (22). The absence of dural tears in the present series is likely due to the FLMD being performed in the lateral half of the foramen or outside the foramen (extraforaminal). This compares to central/paracentral disc herniations, where a conventional midline microdiscectomy would be performed, which has an increased risk of durotomy.

It is reasoned that recurrent far lateral lumbar disc prolapses are more likely to be symptomatic due to compression of the DRG, the lack of posterior longitudinal ligament to help contain disc material, and the reduction in foraminal height that inevitably follows prolapse, causing foraminal stenosis. The more horizontal course of the exiting root also means that a greater length of nerve root is in contact with the disc prolapse, compared to the descending root in the central canal and lateral recess. This may account for why patients with a far lateral disc prolapse report increased radicular pain and more severe radiculopathy than patients with a medial or posterolateral disc prolapse. Being cognisant of this permits a surgeon to counsel patients about the higher risk of recurrent disc prolapse and the potential need for either redo FLMD or later interbody fusion. Hence, in cases of primary far lateral disc prolapse with concurrent severe foraminal stenosis, a surgeon may sensibly recommend interbody fusion as the primary surgical management.

The paramedian Wiltse approach is a safe and effective technique for the management of symptomatic far lateral disc prolapses. Spine surgeons would benefit from consideration of this technique and undergoing appropriate training and mentorship. This technique may be a safer approach to far lateral disc prolapses in terms of reducing the risk of iatrogenic instability due to pars defects, especially at the upper lumbar levels. As with any technique, there is a learning curve. However, if taught well, and if key guidelines are adhered to, the procedure can be performed safely even by surgeons only performing small numbers of cases per year.

These recommendations have been summarised in Table S1.

Several limitations of this study warrant acknowledgment. All procedures were performed by a single senior spine surgeon at a single institution, which eliminates inter-surgeon variability but potentially limits the external validity of our findings. The relatively small sample size (50 patients) and retrospective design are additional limitations that may introduce bias. Despite these limitations, our study provides valuable mid-term outcome data that contributes to the limited body of literature on long-term results following FLMD. We describe our FLMD technique and outcome without a comparative cohort to other techniques such as transforaminal endoscopic or tubular discectomy surgeries.

Conclusions

FLMD via a Wiltse approach is safe and effective for decompressing the exiting nerve root and the DRG, providing excellent visualization of the pathology and the exiting root, while requiring minimal bone removal. This mid-term follow-up (over 3 years), demonstrated over 80% of patients reported excellent or good outcomes. The reoperation rate (16%) is higher than what is reported in the literature for conventional microdiscectomies following posterolateral disc prolapses. Recurrent far lateral disc prolapses in this series occurred early (within 2 weeks), and subsequent interbody fusions most commonly occurred later (between 1–3 years) due to symptomatic disc degeneration, foraminal stenosis or both, rather than for instability.

Acknowledgments

Figure 3 was commissioned by Professor Gregory Malham from Taehoon Gwag.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://jss.amegroups.com/article/view/10.21037/jss-25-10/rc

Data Sharing Statement: Available at https://jss.amegroups.com/article/view/10.21037/jss-25-10/dss

Peer Review File: Available at https://jss.amegroups.com/article/view/10.21037/jss-25-10/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jss.amegroups.com/article/view/10.21037/jss-25-10/coif). G.M.M. serves as an unpaid editorial board member of the Journal of Spine Surgery. G.M.M. has disclosures of Globus Medical (consultancy), LifeHealthcare (travel), and Australian Biotechnologies (consultancy) and Medtronic (consultancy and travel). The other authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The project was reviewed against the Human Research Ethics and classified as a low-risk research project in accordance with section 5.1.19 of the National Statement on Ethical Conduct in Human Research (2023). The waiver is not required. This was a review of cases collected under a standard privacy disclosure to patients that their information will be used for ongoing evaluation of outcomes and their identity will be protected in any publication arising from this. Verbal informed consent was obtained at last follow-up.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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