Eur Spine J DOI 10.1007/s00586-015-3876-3
The learning curve of lateral access lumbar interbody fusion in an Asian population: a prospective study Chong Leslie Lich Ng1 • Boon Chuan Pang2 • Paul Julius A. Medina3 Kimberly-Anne Tan4 • Selvaraj Dahshaini1 • Li-Zhen Liu5
Received: 13 September 2014 / Revised: 13 February 2015 / Accepted: 26 February 2015 Springer-Verlag Berlin Heidelberg 2015
Abstract Purpose Lateral access lumbar interbody fusion (LLIF) is a minimally invasive technique that has an increasing popularity. It offers unique advantages and circumvents risk of certain serious complications encountered in other conventional spinal approaches. This study provides a statistical analysis defining the lateral access learning curve in the Asian population. Methods This prospective study included 32 consecutive patients who underwent LLIF from April 2012 to August 2014. The surgeries were performed by two senior spine surgeons and follow-up was conducted at 6 weeks, 3, 6, 9 months and 1 year post-operation. Results The breakpoint in operating time occurred at the 22nd level operated, from a mean of 71 min in the early phase group to a mean of 42 min in the steady state group.
& Chong Leslie Lich Ng [email protected]
Paul Julius A. Medina [email protected]
Kimberly-Anne Tan [email protected]
Spine Surgery, Deptartment of Orthopedics, Tan Tock Seng Hospital, Jalan Tan Tock Seng, Singapore 308433, Singapore
National Neuroscience Institute, Jalan Tan Tock Seng, Singapore 308433, Singapore
Department of Orthopedics, Tan Tock Seng Hospital, Jalan Tan Tock Seng, Singapore 308433, Singapore
Faculty of Medicine, UNSW Medicine, University of New South Wales, UNSW, Sydney 2052, Australia
Tan Tock Seng Hospital, Jalan Tan Tock Seng, Singapore 308433, Singapore
LLIF at L4/5 level is technically more demanding but technically feasible as competency is achieved. Conclusions During the learning process, there was no compromise of perioperative or clinical outcomes. It should be feasibly incorporated into a spine surgeon’s repertoire of procedures for the lumbar spine. Keywords Lateral access lumbar surgery Minimally invasive surgery Lumbar spine Learning curve
Introduction Lateral access spine surgery is a minimally invasive, transpsoas, retroperitoneal approach that is ligament sparing and allows for use of larger intrinsically stable implants in the intervertebral disc space [1–8]. Indications for lateral spine access have expanded from lumbar interbody fusion for degenerative disc disease to vertebral corpectomy and deformity correction such as in adult scoliosis [7, 9]. Thus, the imperative to define the learning curve of lateral access spine surgery [4, 10]. Although published outcomes thus far have been positive and demonstrated safety comparable to standard TLIF [11, 12], results could mostly be from surgeons with relative mastery of the technique. To date, existing learning curve studies have only been retrospectively reported and there remains a call for the formal definition and statistical analysis of this subject [13–15]. Unfamiliarity with the approach and its perceived high technical demands, particularly at L4/L5 level, are possible reasons why lateral access surgery has yet to be more widely practiced. This study establishes a learning curve (as exemplified by the MIS TLIF study by Lee et al. ) for lateral lumbar interbody fusion (LLIF) based on operating time and associated perioperative parameters.
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Methods and materials
We conducted a prospective cohort study, approved by the Hospital Research/Development Office and Ethics Committee. All patients were recruited from the senior author’s spine specialist clinics. A research coordinator interviewed the patients and collected all pre-, peri- and postoperative outcome measures. An independent statistician from the Hospital Research/Development Office conducted the data analysis. The inclusion and exclusion criteria are shown in Table 1. The objectives of lateral access surgery include: (1) Achievement of stability through intervertebral body fusion, (2) indirect decompression of the lumbar spinal canal with reduction of listhesis and enlargement of the neuro-foramen, (3) correction of lumbar degenerative scoliosis through insertion of intervertebral body spacer. To confirm diagnoses and study eligibility, all patients underwent pre-operative magnetic resonance imaging (MRI) and erect static and dynamic (flexion and extension) plain lumbar radiographs. Such imaging was essential for identifying specific neural, vascular, and bony anatomy during pre-operative planning . Our learning curve measures fall into two categories: Measures of surgical process in terms of operating and intra-operative fluoroscopy time and measures of patient outcomes. It is recognized that a surgeon’s mastery of a surgical technique is achieved when there is a plateau in operative duration and a corresponding improvement in the patient’s functional outcomes.
Perioperative parameters included operative duration, fluoroscopy time, use of patient-controlled analgesia (PCA) and length of hospitalization. Clinical outcomes were recorded pre-operatively and assessed at 6 weeks, 3, 6, 9 months and 1 year postoperatively. These consisted of the Visual Analogue Scale (VAS) for back pain and leg pain, Oswestry Disability Index (ODI), Short-Form 36 Physical Component Score (SF-36 PCS) and SF-36 Mental Component Score (MCS).
Table 1 Study inclusion and exclusion criteria Inclusion criteria Patients undergoing single- or multiple-level XLIF who: 1. Presented with uni-/bilateral radicular lower limb pain due to central/lateral lumbar stenosis between L1 and L5 2. Were unresponsive to prior conservative management lasting a minimum of 6 months Exclusion criteria Diagnosis of malignancy High-riding iliac crestsa Pregnancy Previous spinal surgery Previous abdominal surgery High-grade spondylolisthesis Spinal infection
Surgical technique The surgeries were performed by the senior authors Ng CL and Pang BC. All patients underwent lumbar lateral access surgery with NuVasive’s Extreme lateral interbody fusion system (XLIF NuVasive San Diego CA) with supplementary posterior stabilization. The procedures were carried out on a breakable radiolucent table with the patient in lateral decubitus position and iliac crest coinciding with the table break. Increasing the table break to 20o opens targeted lumbar intervertebral spaces and tilts the pelvis away from the spine. This is especially pertinent when accessing L4/5. An image intensifier is used to guide the marking of access points. This is followed by a two-incision technique as described by Ozgur et al. . When involving multiple levels, an additional direct lateral incision is made for each level being accessed and the posterolateral incision is made at a location equidistant from the most cranial and most caudal levels being accessed. NVM5 neuro-monitoring is used throughout sequential dilatation of the working channel, up till the radiographical verification and guidewire marking of the target disc location. The retractor assembly, with its light source attachment is then applied and direct visualization annulotomy proceeds. Next, an Osteocel Plus-filled PEEK–OPTIMA CoRoent interbody cage is placed, followed by a NuVasive eXtreme Lateral Plate (XLP) locking side plate if necessary. A deep drain is placed in retroperitoneal space to detect any possible retroperitoneal bleed. The patient is then transferred onto a Jackson table with a Wilson frame for posterior stabilization if indicated . Posterior stabilization implants include minimally invasive percutaneous pedicle screws (SpheRx NuVasive), standard open pedicle screws (Armada NuVasive), Affix plate or cortical MIS PLIF NuVasive screws. Final anteroposterior and lateral view radiographs are taken to ensure proper placement of implants before skin closure.
Injury sustained from high-energy trauma Vertebral body fracture
Ongoing work compensation/litigation a
Iliac spine crossing [50 % of L4 vertebral body on plain lateral lumbar radiograph
Patient case records were arranged by operation date, then further divided sequentially into individual operated levels.
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Operative time per level was measured from the cutting of the direct lateral skin mark to the completion of placement of the XLIF intervertebral cage. Piecewise regression analysis of individual operated levels and their operative time was performed using R statistical software (version 2.15.1) to define the breakpoint on the learning curve, applying the same algorithm described by Muggeo  and implemented by Lee et al. . Operated levels were then split into two groups, the early phase group (EPG) and the steady state group (SSG), depending on whether they occurred before or after the curve’s breakpoint. We assume that competence is obtained when operating time reaches steady state. The groups were then compared to ascertain if reduced operative time related to improved patient outcomes. Data comparison between the EPG and the SSG was performed using SPSS version 17.0 and statistical significance was defined as p \ 0.05 for all analyses. Clinical outcomes were analysed at each pre-operative and follow-up time point using independent T tests by an independent statistician. Overall comparison of clinical outcomes across the entire follow-up duration was conducted using area-under-graph analysis.
Results 32 consecutive patients were recruited by the senior authors between April 2012 and August 2014. A scatterplot was generated using individual vertebral level operating times. Piecewise regression analysis showed that operating time decreased, then stabilized after XLIF had been performed on the 21st vertebral level (Breakpoint = 21.64; 95 % CI, 14.52–28.76) (Fig. 1a). Therefore, by rounding up to use the 22nd operated level as the reference point, the initial 22 operated levels composed the EPG, while the latter 25 composed the SSG. Six more vertebral levels were later operated upon and added to the SSG, while all analyses remained based off the original
calculated breakpoint of 21.64. However, this addition was noted to cause the breakpoint to shift to an earlier vertebral level of 19.46 (Fig. 1b). This suggests that the learning curve could be yet faster than is reported in this study. The EPG and SSG patient’s demographics are shown in Table 2. Radiological outcomes were compared across these two groups. However, clinical outcomes of VAS back pain, VAS leg pain, ODI, SF-36 PCS and SF-36 MCS, were analysed by splitting patients, rather than vertebral levels, into two groups to avoid double counting in cases where patients underwent XLIF at more than one level. As the 22nd level XLIF was performed on the 13th patient, the EPG comprised patients 1–13, while the SSG comprised patients 14–32 (Fig. 2). No significant differences between patient groups were found when analysing amount of PCA used, length of hospitalization and fluoroscopy time (Table 3). There were no statistical significant differences found between the EPG and SSG for all pre-operative parameters. We also noted the functional improvement in both groups at all follow-up time points. Although there were statistically significant differences in VAS back pain and leg pain and ODI at 1 year review (Table 4), we do not consider these differences to be clinically significant. We found that the results were contradictory when we used area-under-graph analysis to compare the overall outcome across the entire follow-up period. SSG patients had statistically significant better VAS back pain and SF36 MCS outcomes than EPG patients. EPG patients reported significantly better overall ODI and SF-36 PCS outcomes than SSG patients. We do not find the differences to be clinically significant. No significant difference in overall VAS leg pain was found between groups (Table 5). There was a statistically and clinically significant decrease in operative time for L4/5 between EPG levels and SSG levels (Table 6). A decrease between groups in operative time per level for L2/3 and L3/4 cases also occurred, but these differences were not statistically significant.
Fig. 1 XLIF learning curve: a Original breakpoint occurred at the 22nd vertebral level attempted, b six additional vertebral levels attempted caused breakpoint to shift earlier
Eur Spine J Table 2 Patient demographics Early phase group (patients 1–13)
Steady state group (patients 14–32)
56.85 ± 12.10
61.26 ± 8.96
Gender (male: female)
5: 8 (38.5 %: 61.5 %)
11: 8 (61.1 %: 44.4 %)
Body mass index (kg•m-2)
25.73 ± 4.80
25.50 ± 3.03
Race (Chinese: Malay: Indian)
10: 2: 1 (76.9 %: 15.4 %: 7.7 %)
19: 0: 0 (100 %: 0 %: 0 %)
No. of levels operated (single: double: triple)
5: 7: 1 (38.5 %: 53.8 %: 7.7 %)
10: 8: 1 (52.6 %: 42.1 %: 5.3 %)
Early phase group (patients 1–13)
Steady state group (patients 14–32)
Fig. 2 Pre-operative baseline (0 weeks) and postoperative (6 weeks, 3, 6, 9 months and 1 year) outcomes of early phase group versus steady state group patients: a Average Visual Analogue Scale back pain, b Average Visual Analogue Scale leg pain, c Average Oswestry Disability Index, d Average SF-36 Physical Component Score, e Average SF-36 Mental Component Score
Table 3 Perioperative parameters
Fluoroscopy time (s) Patient-controlled analgesia—morphine (mg) Length of stay (days)
3.65 ± 2.33
3.60 ± 2.71
36.20 ± 34.11 7.15 ± 5.15
32.76 ± 42.09 6.21 ± 6.01
Eur Spine J Table 4 Pre-operative clinical baseline and clinical outcomes at 6 months and 1 year
Mean score for early phase group (patients 1–13)
Mean score for steady state group (patients 14–32)
Visual Analogue Scale for back pain Pre-operation
4.31 ± 3.54
4.81 ± 3.51
1.77 ± 2.74
2.92 ± 1.98
2.46 ± 3.07
Visual Analogue Scale for leg pain Pre-operation
4.38 ± 3.57
6.38 ± 2.90
0.31 ± 0.75
1.77 ± 2.62
1.46 ± 2.22
Oswestry Disability Index Pre-operation
46.49 ± 12.79
43.18 ± 21.39
6 months 1 year
19.49 ± 13.90 14.65 ± 11.39
26.59 ± 21.67 0
SF-36 Physical Component Score Pre-operation
27.79 ± 6.66
32.00 ± 14.28
44.30 ± 7.98
39.48 ± 11.96
50.79 ± 6.92
52.67 ± 1.72
51.38 ± 14.44
SF-36 Mental Component Score Pre-operation
43.63 ± 14.24
55.98 ± 10.00
54.82 ± 12.33
57.69 ± 5.95
60.77 ± 8.05
* Significant difference
Table 5 Summary of area-under-graph analysis for overall comparison of clinical outcomes across entire follow-up duration Area-under-graph for early phase group (Patients 1–13)
Area-under-graph for steady state group (Patients 14–32)
Visual analogue scale for back pain
Visual analogue scale for leg pain
Oswestry disability index
SF-36 physical component score
SF-36 mental component score
Mean operative time per level across the 24 L4/L5 cases was 43 min while that of L2/L3 and L3/L4 was 22 min. Two out of the 24 (8.33 %) cases had to be converted to transforaminal lumbar interbody fusion (TLIF) due to technical difficulties later described. Pre-operative data are 100 % complete while follow-up data for 6 weeks, 3, 6, 9 months and 1 year are 93.75, 87.5, 81.25, 78.13, respectively, and 50 % complete as patients have yet to reach the corresponding postoperative marks.
Discussion Obenchein  described laparoscopic lumbar discectomy that revolutionized the development of other minimally invasive (MIS) procedures. From this development came
the idea of a novel approach of a lateral lumbar interbody fusion described by Pimenta et al.  and modified by Ozgur et al.  that combines the concepts of MIS with a direct visualization of the surgical site, thereby theoretically lessening the learning curve. Formica et al. and Rodgers et al. have also demonstrated the safety of this technique, even when used in an octogenarian population [5, 11]. Similar to other MIS procedures, LLIF takes advantage of lesser surgical trauma and improved comfort, which hastened return to activities of daily living. However, issues of LLIF’s safety by learning surgeons have yet to be addressed leading to calls for definition of its learning curve. The objective of this study was to analyse the LLIF learning curve, especially in the Asian population, via piecewise regression analysis in correlation with operative
Eur Spine J Table 6 Level-specific comparison of operative time per level between early phase and steady state groups No. of levels out of 53 levels operated upon
Operative time per level (min) Early phase group (operated levels 1–22)
Steady state group (operated levels 23–53)
4 (7.5 %)
16.15 ± 39.90
5.74 ± 17.32
23 (43.4 %)
43.46 ± 29.96
28.47 ± 21.94
24 (45.3 %)
60.38 ± 48.37
30.84 ± 20.17
* Significant difference
Fig. 3 When performing XLIF at L4/5, retractors may be pushed cranially by the iliac crest, resulting in guidewire placement at a less than ideal angle
time, blood loss and perioperative clinical outcomes. Though Ozgur appreciated the need for establishment of a learning curve, it was not defined in his publication . Hyde and Aichmair et al. described the linear decrease in operative time of two subgroups of 10 patients and studied the occurrence of complications (e.g. thigh pain, sensory deficits and motor deficits) in three chronological subgroups of patients operated upon in different years, respectively, without consideration of a break point [13, 14].
The breakpoint in our study occurred at the 22nd level operated. This is earlier than the breakpoint of other MIS procedures with similar indications such as the 44th level for MIS-TLIF as reported by Lee  and the 25th level for microscopic-assisted percutaneous nucleotomy as reported by Franke et al. . From the breakpoint, we noted a significant decrease in the operative time from 71 min in the EPG levels versus 43 min in the SSG levels. The average operating time, when competence has been achieved
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in this study, is lower than the average time reported by Hyde for a single-level LLIF of 119 min . We agree that the application of LLIF to L4/L5, despite being done in the same amount of time as the overall average operation time in this study, is technically more demanding compared to more cranial lumbar levels. This is attributable to the anatomy of this region. The retractors are often pushed cranially at an angle by the abutting iliac crest (Fig. 3). In addition, the placement of the cage is also partly dictated by the position of the lumbar plexus across the L4/L5 disc space (Fig. 4). For 8.33 % of our patients, LLIF at the L4/L5 were aborted because of the more ventral positioning of the lumbar plexus relative to the psoas and the presence of grade I degenerative spondylolisthesis. We believe that anterolisthesis further narrows the ‘window’ for disc space preparation in the lateral position. However, with greater familiarity in handling the instruments and increasing proficiency in neuro-monitoring, LLIF at L4/L5 is technically feasible as demonstrated by the improvement in operative time between the EPG and SSG. Throughout the learning curve, the blood loss remained negligible and fluoroscopy times remained statistically the same regardless of the length of operating time. Based on Singer’s calculations, we estimate the radiation exposure to be well within the guidelines stipulated by the Nuclear Regulatory Commission in ‘‘Standards for protection against radiation’’ (title 10, part 20) . There were also four cases of ALL rupture and a case of retroperitoneal hematoma, all of which did not require surgical revision. These details exemplify the safety of the procedure’s
conduct to both the patient and the surgeons themselves, even when competence has yet to be fully achieved by the latter. Though postoperative clinical outcomes showed statistically contradictory discrepancies, especially in the area-under-graph analysis, the differences did not translate clinically and these discrepancies could be attributable to the sample size (Type 1 error). For the same reason, there was no comparison done on the outcomes of LLIF patients to open decompression patients. Patients from both the EPG and the SSG showed statistically (by T Test) and clinically significant improvement from their pre-operative values to the postoperative scores of their SF-36 PCS, SF-36 MCS, ODI, and VAS leg pain regardless of their group. VAS back pain scores, despite showing a similar pattern of improvement, however, were significantly better in the SSG patients at 1 year post-operation. Decreased duration of muscular retraction due to shorter operative times may account for this corresponding decrease in back pain with increasing LLIF competence . One patient with ALL rupture during surgery was noted to have an asymptomatic ventral displacement of cage during follow-up, but this did not require surgical revision. However, no statistical pattern of complications can be drawn from the study for the reasons that there were only a few noted, the distribution does not signify anything as the sample population is small and there remains a need for a longer period of follow-up. These data support the safety of the procedure as previously described [11, 12, 25]. The study is also limited as it represents a two-surgeon series
Fig. 4 XLIF is not accomplishable if the lumbar plexus resides too closely to the dilators or retractors. This is most pertinent at the L4/5 level because the lumbar plexus migrates ventrally relative to the disc space as it descends caudally [20–23]
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learning curve. A single surgeon learning curve may potentially be longer.
Conclusions LLIF has an acceptable, relatively short learning curve, with technical competency being achieved by the 22nd level operated. The positive clinical outcomes and nonremarkable perioperative parameters throughout this series reassure that patient safety and quality of care remain uncompromised during the learning process.
Acknowledgments We would like to acknowledge Richard Tjahjono, medical illustrator, for his drawings used in this article. 14. Conflict of interest of interest.
The authors declare that they have no conflict 15.
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