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Learning Curve of a Complex Surgical Technique Minimally Invasive Transforaminal Lumbar Interbody Fusion (MIS TLIF) Kong Hwee Lee, MBBS, MRCSEd, William Yeo, MPhty (Manips), Henry Soeharno, MBBS, MRCSEd, and Wai Mun Yue, MBBS, FRCSEd, FAMS (Ortho Surgery)
Study Design: Prospective cohort study. Objective: This study aimed to evaluate the learning curve of minimally invasive transforaminal lumbar interbody fusion (MIS TLIF). Summary of Background Data: Very few studies have evaluated the learning curve of this technically demanding surgery. We intend to evaluate the learning curve of MIS TLIF with a larger sample size and assess surgical competence based not only on operative time but with perioperative variables, clinical and radiologic outcomes, incidence of complications, and patient satisfaction. Materials and Methods: From 2005 to 2009, the first 90 singlelevel MIS TLIF, which utilized a consistent technique and spinal instrumentation, performed by a single surgeon at our tertiary institution were studied. Variables studied included operative time, perioperative variables, clinical (Visual Analogue Scores for back and leg pain, Oswestry Disability Index, North American Spine Society Scores for neurogenic symptoms) and radiologic outcomes, incidence of complications and patient rating of expectation met, and the overall result of surgery. Results: The asymptote of the surgeon’s learning curve for MIS TLIF was achieved at the 44th case. Comparing the early group of 44 patients to the latter 46, the demographics were similar. For operative parameters, only 3 variables showed differences between the 2 groups: mean operative duration, fluoroscopy duration, and usage of patient-controlled analgesia. At the final follow-up, for clinical outcome parameters, the 2 groups were different in 3 parameters: VAS scores for back, leg pain, and neurogenic symptom scores. For radiologic outcome, both groups showed similar good fusion rates. For complications, none of the MIS TLIF cases were converted to open TLIF intraoperatively. In the early group, there were 3 complications: 1 incidental durotomy and 2 asymptomatic cage migrations; and in the latter group, there was 1 asymptomatic cage migration.
Received for publication April 4, 2013; accepted January 29, 2014. From the Singapore General Hospital, Singapore, Singapore. This study has been IRB approved by Singhealth Singapore. W.M.Y. is a consultant to Medtronics and Depuy Synthes. The remaining authors declare no conflict of interest. Reprints: Wai Mun Yue, MBBS, FRCSEd, FAMS (Ortho Surgery), Department of Orthopaedic Surgery, Singapore General Hospital, Outram Road, Singapore 169 608, Singapore (e-mail: yuewm@ singnet.com.sg). Copyright r 2014 by Lippincott Williams & Wilkins
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Conclusions: In our study, technical proficiency in MIS TLIF was achieved after 44 surgeries, and the latter patients benefited from shorter operative duration and radiation, less pain, and more relief in their back, leg, and neurogenic symptoms. Key Words: learning curve, minimally invasive, spinal surgery, transforaminal lumbar interbody fusion (J Spinal Disord Tech 2014;27:E234–E240)
F
or 3 decades, open transforaminal lumbar interbody fusion (TLIF) has proven to be a safe and effective technique in achieving lumbar fusion while treating degenerative, traumatic, and neoplastic conditions of the lumbar spine.1,2 However, studies have reported deleterious effects of extensive and prolonged muscle ischemia in the open approach, affecting adversely both short-term and long-term patient outcomes.3–6 Less invasive surgical techniques and exposures have been developed with the aim of decreasing the complications related to the traditional open approaches, while maintaining equivalent or better surgical outcomes. A good example in the field of surgery will be laparoscopic cholecystectomy, which has replaced open cholecystectomy as the gold standard in the management of gallstone disease.7 In the field of orthopedics, an example would be the arthroscopic meniscectomy in the management of meniscal pathologies of the knee.8 At present, these minimally invasive procedures have largely replaced their open counterpart. Minimally invasive (MIS) TLIF is made possible with advancements in spinal instrumentation and radiologic imaging, and it provides surgeons a viable alternative in achieving solid fusion with significantly less soft tissue injury. Foley et al9 first described this novel technique, which utilized tubular retractors inserted serially under radiologic guidance by a muscle dilating approach, thus reducing the amount of iatrogenic muscle and soft tissue injuries. It is postulated that biomechanically, in MIS TLIF, with the intact posterior lumbar musculature, patients can expect more physiologically normal lumbar motion postoperatively. However, at present, no studies have conclusively proven this theory.10 MIS TLIF must be learned, and we would like to evaluate this steep learning curve based not only on operative duration but also on perioperative parameters, clinical and radiologic outcomes, incidence of complications, and J Spinal Disord Tech
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patient satisfaction. A surgeon’s mastery of a surgical technique is achieved when there is a plateau in his operative duration and accompanied by improved perioperative results and fewer complications.
MATERIALS AND METHODS Data Collection After obtaining Institutional Review Board’s approval, the first 90 consecutive records of patients receiving single-level MIS TLIF performed by our senior author (W.M.Y) from June 2005 to June 2009 were retrieved from our prospectively collected Singapore General Hospital Spine Outcome Registry. The subject inclusion criteria for this study were (1) single-level MIS TLIF and (2) MIS TLIF cases utilizing a consistent set of percutaneous pedicle screw system, polyether-etherketone cage, and bone grafts (autogenous or allogenic). The exclusion criteria were (1) previous spinal instrumentation, (2) spinal tumor pathologies, (3) spinal infections, and (4) acute spinal trauma. All patients included had preoperative evaluation with detailed neurologic examination and radiologic imaging, which involved static (anterior-posterior and lateral) and dynamic (flexion and extension) plain lumbar spine radiographs, magnetic resonance imaging, and/or computed tomography scans. The indications for surgery were spondylolisthesis (grade 1 and 2), spinal stenosis with potential for intraoperative instability, and degenerated collapsed disk, all presenting with radicular pain refractory to medical therapy of at least 6 months’ duration. After discharge from the hospital, patients were followed up regularly by the surgeon (2 wk, 3 mo, 6 mo, 1 y, and annually thereafter) and monitored closely for complications. Plain lumbar spine radiographs and, if indicated, magnetic resonance imaging or computed tomography scans, were ordered to assess and confirm complications. Complications were categorized into clinical and technical aspects. Clinical complications included new or worsening neurologic deficit(s) and wound infections. Technical complications included cage migrations and malpositioned screws. For clinical outcome measures, the patients were evaluated by independent assessors at our Orthopedic Diagnostic Centre (ODC) before surgery and at 6 months and 2 years postsurgery. Our ODC (where the Singapore General Hospital Spine Outcome Registry is managed) has been systematically tracking and administering patient-reported outcomes for all elective spine surgeries at various time points since August 1998. When patients did not return, follow-up was obtained through personal telephone calls. With these outcome data collected by ODC and stored in standardized digital formats in our hospital’s computer server, future data retrieval and analysis on the prospectively collected data can be performed.
Surgical Technique All the MIS cases were performed by a single spine surgeon with prior 4 years of experience with open TLIFs. r
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The surgeon first observed the MIS TLIF procedure from 2 spine surgeons and familiarized the technique on 3 cadavers before embarking on the procedure with patients. The initial technique was subsequently refined and modified along the way, resulting in the current technique being significantly different from the initial operative technique, mainly in terms of workflow and ergonomics.11 The surgeon performed the initial approach from the more symptomatic side, and fluoroscopy was used to determine the operative level. A paravertebral incision was made approximately 3–5 cm lateral to the midline, extending between the cephalad and caudad pedicles at the targeted disk level. Sequential soft tissue dilators were then inserted through the incision down to the facet complex until the desired working diameter was achieved. Using a high-speed drill, the surgeon performed facetectomy from lateral to medial to expose the posterolateral aspect of the disk. Discectomy, endplate preparation, and disk space distraction were performed for cage placement. After selecting the appropriate sized cage and filling it with bone graft, the surgeon would impact the disk space with the remnant bone graft before inserting the cage. The bone graft consisted of autograft with a mixture of iliac crest and local bone, augmented by demineralized bone matrix or recombinant human bone morphogenetic protein-2. Fluoroscopy was used to confirm satisfactory positioning of the cage. The lateral margin of the ligamentum flavum was resected to expose the ipsilateral exiting and traversing nerve roots. In addition, if more extensive stenosis were present, the tubular retractor would be angled medially with the patient tilted laterally to facilitate direct decompression of the central canal and contralateral stenosis. Once adequate decompression had been carried out, the tubular retractor was removed and an ipsilateral pedicle screw-rod construct was placed through the same incision. Pedicle screws and rods were inserted percutaneously over K-wires and guided with fluoroscopy. Before final tightening, compression was applied to the construct to recreate the lumbar lordosis. A contralateral pedicle screw-rod construct was placed through a contralateral incision. Before skin closure, a final fluoroscopic check (both anterior-posterior and lateral views) was conducted to ensure that the cage and screw-rod construct were still in a satisfactory position. No surgical drains were used postoperatively.
Methods The patient case records were arranged sequentially in order of operation date and the trend in operative duration was evaluated using piecewise regression analysis (Fig. 1). This piecewise regression analysis was performed using R statistical software (version 2.15.1). The underlying premise of piecewise regression assumes that the line of best fit in a scatterplot comprises 2 (or more) straight lines connected at breakpoint(s). The breakpoint estimate and its 95% confidence limits are estimated using the mathematical algorithm described by Muggeo.12 The www.jspinaldisorders.com |
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variables with nonparametric data (such as body mass index, duration of operation, intraoperative blood loss, fluoroscopy time, use of patient-controlled analgesia, length of stay, time to ambulation, back and leg pain VAS, SF-36 Mental Health Subscale, post-operation ODI, NSS, and SF-36 Physical Function subscale) were performed using the Mann-Whitney U test. For comparisons of categorical variables such as sex, race, operated level, radiologic outcome (Bridwell Fusion Grading), and patient satisfaction (expectation met for surgery and overall result of surgery), w2 and the Fisher Exact tests were utilized.
RESULTS
FIGURE 1. The bar above the x-axis represents the breakpoint estimate (44) and its 95% confidence limits (35–54 patients).
case number at breakpoint would serve as the point of reference between the early learning phase and latter steady state. Cases in the 2 groups would then be compared for the various parameters to elucidate if improvement in operative duration was accompanied by better patient outcomes. The authors focused on 3 broad categories of parameters, namely: perioperative, clinical, and radiologic. Perioperative parameters included patient demographics, operative duration, fluoroscopy time, intraoperative blood loss, length of hospitalization stay, time to ambulation, and the use of patient-controlled analgesia. Clinical outcome parameters included visual analogue scores (VAS) for back and leg pain, Oswestry Disability Index (ODI), North American Spine Society score for neurogenic symptoms (NSS), SF-36 health-related quality of life (HRQOL) outcomes, and patient rating of the expectation met for surgery and the overall result of surgery. For radiologic outcome parameter, both static and dynamic plain lumbar radiographs taken at 6 months and 2 years postsurgery were utilized to assess fusion. The fusion grading criteria was based on Bridwell interbody fusion grading system, and the assessments were performed by 2 independent assessors, with a third assessor available for adjudication.13
Statistical Analysis All data were collected prospectively and retrospective review of the data was performed. Statistical comparisons between the early and latter cases were performed using statistical software SPSS version 17.0. In all analyses, statistical significance was defined as P < 0.05. Comparisons of continuous variables with parametric data (such as age, preoperative ODI, NS, and SF-36 Physical Function subscale) were performed using the independent t test, whereas comparisons of continuous
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From the piecewise regression analysis, the surgeon’s operative duration for performing single-level MIS TLIF was estimated to stabilize after performing the 44th operation (95% CI, 35–54). Using the 44th operation as the reference point, the early phase group would consist of 44 patients and the latter steady state group consisting 46 patients. Comparing the early group of 44 patients to the latter 46, there was no significant difference in patient demographics, namely age, sex, race, operated level, and body mass index (Table 1). Analyzing the perioperative parameters, 3 variables showed statistical differences between the 2 groups, namely mean operative duration, mean fluoroscopy duration, and mean usage of patientcontrolled analgesia (Table 2). The mean operative duration in the early group was 187.16 ± 50.29 minutes, decreasing significantly to 132.28 ± 18.17 minutes in the latter group (P = 0.000). Similarly for intraoperative fluoroscopy, the mean fluoroscopy duration was 74.44 ± 31.41 seconds in the early group and decreased to a mean of 29.88 ± 10.83 seconds in the latter (P = 0.000). The mean usage of patient-controlled analgesia (morphine) was 5.59 ± 12.48 mg in the first group and decreased to a mean of 0.87 ± 2.73 mg (P = 0.010) in the latter group. For intraoperative blood loss, although blood loss in the latter group (107.61 ± 36.47 mL) was less than the early group (153.41 ± 171.65 mL, P = 0.151), the improvement was not statistically significant. For both groups, the patients took about the same time to start ambulating and be discharged from the hospital. For clinical outcome parameters, both groups exhibited similar preoperative baseline pain and disability (except for SF-36 mental health) and, postoperatively, all 90 patients had clinical improvements. At the final follow-up 2 years postsurgery, the 2 groups were significantly different in 3 parameters: VAS scores for back and leg pain, and neurogenic symptom scores (Table 3). For VAS back pain, the early group patients rated a mean score of 2.88 ± 3.30 versus the latter group’s mean score of 1.34 ± 2.32 (P = 0.016). For VAS leg pain, the early patients rated a mean score of 2.28 ± 3.05 versus the latter group’s mean scoring of 0.48 ± 1.34 (P = 0.001). For neurogenic symptom score, the early patients rated a mean score of 22.87 ± 27.74 versus 7.80 ± 17.40 in the latter group (P = 0.001). However, there was no difference between the 2 groups in terms of ODI and r
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Learning Curve of MIS TLIF
TABLE 1. Patient Demographics Demographics Age (y) Sex (male:female) Race (Chinese:Malay:Indian:Others) Operated level L23:L34:L45:L56:L5S1 Body mass index (kg/m2)*
Early 44 Cases
Later 46 Cases
P
49.70 ± 11.69 11:33 (25%:75%) 31:9:2:2 (70%:20%:5%:5%) 0:2:32:0:10 (0%:4%:73%:0%:23%) 26.28 ± 5.06
54.61 ± 16.00 15:31 (33%:67%) 36:4:2:4 (78%:9%:4%:9%) 1:4:27:1:13 (2%:9%:59%:2%:28%) 24.35 ± 3.40
0.101 0.490 0.404 0.487 0.064
*Nonparametric data.
SF-36. In assessing patients’ satisfaction, at both early and final follow-up, the latter group has a larger portion of patients rating their surgical experiences more favorably than their earlier counterpart (P > 0.05) (Table 4). For radiologic outcome, early follow-up at 6 months showed a significant difference between the 2 groups, with the latter group exhibiting a higher proportion of Bridwell 1 solid fusion (Table 5). However, at the final follow-up, both groups showed similar good fusion rates. For complications, none of the MIS TLIF cases were converted to open TLIF intraoperatively. In the early group, there were 3 complications. Patient 30 had an incidental durotomy at the anterior surface of the left L5 root axilla that was promptly repaired with commercially available collagen patch and fibrin glue. Patients 11 and 39 had asymptomatic cage migration noted at the final follow-up. In the latter group, there was only 1 complication with patient 59 having an asymptomatic cage migration noted at a routine 6-month follow-up. In this cohort study, the authors managed to achieve a high percentage of patient follow-up. At 6 months, 95.8% of patients returned for their follow-up attendance (3 patients were foreigners and had returned to their countries at that time). At 2 years, the same patients returned for follow-up.
There are several papers addressing the learning curves in spinal procedures, for example, microscopic discectomies and laminectomies,14–16 but there are few literature available detailing the process in which a spinal surgeon becomes competent with MIS TLIF.17,18 Unfortunately, these few studies are mostly anecdotal, looking mainly at operative timing as basis of learning curve and did not include patients’ long-term (2 y) outcomes and satisfaction. Neal and Rosner performed a retrospective review analyzing the learning curve for MIS TLIF of a neurosurgical resident and noted the learning curve based on operative timing plateau after 15 cases. The authors also recognized that shorter operative duration does not necessarily correlate with better patient outcomes, and a surgeon’s mastery of technique is dependent on evaluating other factors such as complications, readmissions, patient satisfaction, and long-term outcomes.17 Lee et al18 did a similar retrospective study with 86 MIS TLIF (60 single level) patients; using logarithmic curve-fit analysis on the surgeons’ operative durations, they hypothesized that the asymptote of their MIS TLIF learning curve was reached after performing about 30 TABLE 3. Clinical Outcome Parameters Early 44 Cases Later 46 Cases
DISCUSSION MIS TLIF is a surgically demanding and challenging operation, and the inherent difficulties in mastering this technique include limited 2-dimensional view of the surgical field, greater surgical finesse needed with longer surgical instruments, reduced tactile feedback, greater demands of eye-hand coordination and motor dexterity. TABLE 2. Perioperative Parameters Early 44 Cases
Later 46 Cases
Duration of operation (min)* 187.16 ± 50.29 132.28 ± 18.17 Intraoperative blood loss 153.41 ± 171.65 107.61 ± 36.47 (mL)* Fluoroscopy time (s)* 74.44 ± 31.41 29.88 ± 10.83 Patient-controlled 5.59 ± 12.48 0.87 ± 2.73 analgesia—morphine (mg)* Length of stay (d)* 2.96 ± 1.24 2.85 ± 1.20 Time to ambulate (d)* 1.16 ± 0.48 1.15 ± 0.47 *Nonparametric data. wP < 0.05, significant.
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P 0.000w 0.151 0.000w 0.010w 0.744 0.941
Back pain (VAS) Preoperation* 6 mo* 2 y* Leg pain (VAS) Preoperation* 6 mo* 2 y* Neurogenic symptom score Preoperation 6 mo* 2 y* Oswestery Disability Index Preoperation 6 mo* 2 y* SF-36—physical function Preoperation 6 mo* 2 y* SF-36—mental health Preoperation* 6 mo* 2 y*
P
6.34 ± 2.88 3.51 ± 2.81 2.88 ± 3.30
5.96 ± 2.64 1.91 ± 2.56 1.34 ± 2.32
0.399 0.004w 0.016w
6.16 ± 3.07 2.35 ± 2.83 2.28 ± 3.05
5.04 ± 3.43 0.52 ± 1.44 0.48 ± 1.34
0.135 0.000w 0.001w
49.35 ± 28.60 19.32 ± 25.18 22.87 ± 27.74
49.20 ± 27.21 12.58 ± 20.72 7.80 ± 17.40
0.981 0.075 0.001w
48.06 ± 16.75 27.42 ± 18.93 24.94 ± 23.27
44.38 ± 21.16 16.38 ± 14.59 14.62 ± 14.46
0.364 0.004w 0.095
43.41 ± 25.92 62.56 ± 26.24 65.81 ± 29.50
44.89 ± 27.68 69.55 ± 20.88 73.30 ± 21.57
0.794 0.278 0.386
63.27 ± 22.13 67.53 ± 20.88 70.98 ± 20.55
73.83 ± 21.36 77.55 ± 17.83 79.27 ± 17.13
0.013w 0.015w 0.050
*Nonparametric data. wP < 0.05, significant.
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TABLE 4. Patient Satisfaction Early 44 Cases
Later 46 Cases
P
Has the surgery met your expectation? (Yes, totally:Yes, almost totally:Yes, quite a bit:More or less:No, not quite:No, far from it:No, not at all) 6 mo 7:13:8:4:7:1:3 (16.3%:30.2%:18.6%:9.3%:16.3%:2.3%:7.0%) 13:18:6:3:3:1:0 (29.6%:40.9%:13.6%:6.8%:6.8%:2.3%:0.0%) 0.267 2y 9:15:8:5:1:2:3 (20.9%:34.9%:18.6%:11.6%:2.3%:4.7%:7.0%) 19:17:3:3:1:1:0 (43.2%:38.6%:6.8%:6.8%:2.3%:2.3%:0.0%) 0.134 How would you rate the overall result of surgery? (Excellent:very good:good:fair:poor:terrible) 6 mo 5:15:8:9:6:0 (11.6%:34.9%:18.6%:20.9%:14.0%:0.0%) 10:18:11:3:2:0 (22.7%:40.9%:25.0%:6.8%:4.6%:0.0%) 0.116 2y 8:12:9:9:4:1 (18.6%:27.9%:20.9%:20.9%:9.3%:2.4%) 14:18:9:2:1:0 (31.8%:40.9%:20.5%:4.5%:2.3%:0.0%) 0.073 *P < 0.05, significant.
cases. Comparing the early to latter patients, the latter patients had shorter operative duration, less blood loss, and were able to ambulate earlier. However, at 1-year follow-up, both groups reported similar ODI and VAS (back and leg) scores; thus, the authors concluded that mastering this technique had a small effect on their patient’s 1-year clinical outcomes. In our study, a formal statistical method (piecewise regression analysis) was utilized to determine the breakpoint in the learning curve, which occurred at the 44th case. In addition to operative duration, the authors aimed to portray a more comprehensive review of the learning curve of this technically demanding operation; thus, a larger variety of parameters, such as perioperative variables, clinical, and radiologic outcomes were analyzed. To reduce the number of confounding factors in the study, one senior spine surgeon performed all the MIS cases using only one type of pedicle screw-rod instrumentation and interbody cage; only single-level TLIF patients were included in this single center study. We believe this study is the first to report the learning curve of MIS TLIF, which utilized only 1 type of instrumentation by a single surgeon. To further reduce biasness, independent assessors performed the data collection and analysis, and the surgeon was not involved in the process. After achieving technical competency, the surgeon was able to complete the procedure in just over 2 hours, and this average duration was 55 minutes faster compared with the duration in the early group, representing a substantial 29% improvement in operative timing. Even though the surgeon played a significant role in this reduction, other factors were also important in improving the operative duration. The first assistant, scrub nurses, and radiographer made dedicated efforts to improve surgical efficiency by familiarizing the operative steps and modifying the workflow ergonomics, for example, optimal
TABLE 5. Radiologic Outcome Parameters Early 44 Cases
Later 46 Cases
P
Bridwell fusion grading (1:2:3:4) 6 mo 21:21:2:0 38:4:1:0 0.000* (47.7%:47.7%:4.6%:0.0%) (88.4%:9.3%:2.3%:0.0%) 2y 40:0:0:2 42:0:0:0 0.494 (95.2%:0.0%:0.0%:4.8%) (100%:0%:0%:0%) *P < 0.05, significant
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positioning of the microscope and fluoroscopy machine in the operating theater before the start of the surgery. For fluoroscopy duration, the surgical team and the radiographer were able to obtain the required intraoperative fluoroscopy images more efficiently in the latter patients (early: 77.44 ± 31.41 s vs. latter: 29.88 ± 10.83 s; P = 0.000). This 61% improvement in efficiency could be explained by the reasons detailed above. In addition to contributing to the shorter operation, this reduction of 46 seconds of fluoroscopy time could potentially benefit both the patient and operative team; however, because of the recent adoption of MIS TLIF, the side effects of additional radiation exposure have not been thoroughly evaluated. The latter patients also required significantly less patient-controlled analgesia (morphine), about 84.4% less than the early group. This significant reduction in pain could be explained by 2 factors: firstly, with a shorter operative duration, the latter patients were exposed to a shorter duration of the noxious surgical stimuli. Secondly, as the surgeon improved on his surgical technique, the improvement in soft tissue handling and dissection resulted in less paraspinal tissue trauma and damage, translating to less pain. Although there was no significant difference in intraoperative blood loss between the 2 groups, the surgeon’s fastidious attention to soft tissue handling and hemostasis could yield potential further benefits to the patients. All 90 patients did not have postoperative surgical drains, thus minimizing potential postoperative morbidities, for example, infection. As compared with the latter group, the outliers in the early group contributed to the group’s large SDs for operative time, blood loss, fluoroscopy time, and PCA usage. These outliers occurred as the surgeon performed cases that required more decompression while familiarizing the surgical technique. The removal of these outliers will likely narrow the SD for these 4 parameters and reduce the differences between the early and latter groups. However, the inclusion of these outliers will portray a more comprehensive picture of the learning curve of this technique. At the final 2-year follow-up, even though all patients had clinical improvements, the latter group had significantly less clinical symptoms as compared with the early group in terms of back pain, leg pain, and NSS. The latter patients had about 53% less back pain, 79% less leg r
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pain, and 66% less NSS (P < 0.05). NSS is a widely accepted disease-specific tool to assess the outcome of elective spinal surgeries, and a change of Z20% in the score is considered a clinically significant difference.19 This positive difference in the latter group could be attributed to the surgeon’s more thorough decompression of the stenotic neuroforamen while handling the paraspinal tissues better. Interestingly, there was no statistical difference between the 2 groups in patient satisfaction, ODI, and SF-36 health-related quality of life (HRQOL) outcomes. This meant although the latter patients had better clinical symptoms postoperatively, this symptomspecific benefit was not translated to a significantly better quality of life. This could be explained by the multifaceted nature of spinal pathologies. For fusion, at 6 months, the authors observed a greater proportion of solid fusion in the latter group, but this advantage was not maintained at 2 years. The initial advantage could be explained by a more thorough disk space preparation. One major consideration in performing MIS TLIF is the amount of extraradiation exposed as compared with open TLIF.20–22 MIS pedicle screw placement is technically challenging, and the spine surgeon requires intraoperative fluoroscopy to assess the starting point of screw entry, trajectory, and subsequent placement. In the guideline “Standards for protections against radiation” (title 10, part 20), the nuclear regulatory commission recommends that for radiation workers, the annual total effective dose equivalent for the whole body should not exceed 5000 mrem (5 rem) and 50,000 mrem (50 rem) to the hands. Using a regular C-arm fluoroscopy, the exposure rate to the surgeon is estimated to be 20 mrem/min (without lead shield protection) or 2 mrem/min (with protection) to the torso and 30 mrem/min to the hands.23 In our study, fluoroscopy time for the early group (74.4 s) was about 2.5 times longer than that of the latter group (29.9 s). Thus, after achieving proficiency in MIS TLIF, the average amount of radiation to the surgeon’s protected torso dropped from 2.48 to 1 mrem per case, whereas radiation to the surgeon’s hand dropped from 37.2 to 14.9 mrem per case. With the 60% reduction in radiation exposure, the surgeon could theoretically perform up to 5000 cases per year (torso exposure limit) or 3333 cases per year (hand exposure limit). There are several limitations and assumptions in this study. Firstly, the results are based on a single spine surgeon’s experience. Secondly, this study assumed that the surgeon was technically competent in TLIF after performing open TLIF for 4 years. Thirdly, this study assumed that 90 cases would be of sufficient sample size to elucidate meaningful interpretations in all 15 variables. Fourthly, the surgeon also performed other minimally invasive spinal procedures during the study period, for example, microscopic discectomies, microscopic laminectomies, and these additional microsurgical surgeries could possibly improve his MIS TLIF operative proficiency. In conclusion, achieving shorter operative duration may not necessarily equate to surgical competence and r
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better patient outcomes. To illustrate a more comprehensive picture of the learning curve of MIS TLIF, clinical and radiologic outcomes were included in this study. On the basis of our surgeon’s experience, MIS TLIF is a technically demanding surgery but with prior experience in Open TLIF, good outcomes can be achieved even in the early phase of adopting the MIS TLIF technique. In our study, technical proficiency in MIS TLIF was achieved after 44 surgeries, and the latter patients benefited from shorter operative duration and radiation, less pain, and more relief in their back, leg, and NSS. Lastly, the surgeon started learning and exploring the technique when it was not an established one. Perhaps the asymptote could be achieved earlier if a surgeon has been exposed to the technique earlier in his training, for example, as part of his residency. ACKNOWLEDGMENT The authors thank Mr Pua Yong Hao (PhD), principal physiotherapist, of Singapore general hospital physiotherapy department for his assistance with the piecewise regression analysis. REFERENCES 1. Rosenberg WS, Mummaneni PV. Transforaminal lumbar interbody fusion: technique, complications, and early results. Neurosurgery. 2001;48:569–575. 2. Lowe TG, Tahernia AD, O’Brien MF, et al. Unilateral trans-foraminal posterior lumbar interbody fusion (TLIF): indications, technique, and 2-year results. J Spinal Disord Tech. 2002;15: 31–38. 3. Gejo R, Matsui H, Kawaguchi Y, et al. Serial changes in trunk muscle performance after posterior lumbar surgery. Spine. 1999;24:1023–1028. 4. Rantanen J, Hurme M, Falck B, et al. The lumbar multifidus muscle five years after surgery for a lumbar intervertebral disc herniation. Spine. 1993;18:568–574. 5. Sihvonen T, Herno A, Paljiarvi L, et al. Local denervation atrophy of paraspinal muscles in postoperative failed back syndrome. Spine. 1993;18:575–581. 6. Styf JR, Willen J. The effects of external compression by three different retractors on pressure in the erector spine muscles during and after posterior lumbar spine surgery in humans. Spine. 1998;23: 354–358. 7. Keus F, de Jong JA, Gooszen HG, et al. Laparoscopic versus open cholecystectomy for patients with symptomatic cholecystolithiasis. Cochrane Database Syst Rev. 2006;4:CD006231. 8. Northmore-Ball MD, Dandy DJ, Jackson RW. Arthroscopic, open partial, and total meniscectomy. A comparative study. J Bone Joint Surg Br. 1983;65:400–404. 9. Foley KT, Holly LT, Schwender JD. Minimally invasive lumbar fusion. Spine. 2003;15(suppl):26–35. 10. Patwardhan AG, Meade KP, Lee B. A frontal plane model of the lumbar spine subjected to a follower load: implications for the role of muscles. J Biomech Eng. 2001;123:212–217. 11. Peng CW, Yue WM, Poh SY, et al. Clinical and radiological outcomes of minimally invasive versus open transforaminal lumbar interbody fusion. Spine. 2009;34:1385–1389. 12. Muggeo VMR. Estimating regression models with unknown breakpoints. Stat Med. 2003;22:3055–3071. 13. Bridwell KH, Lenke LG, McEnery KW, et al. Anterior fresh frozen allografts in the thoracic and lumbar spine. Spine. 1995;20: 1410–1418. 14. Wang B, Lu¨ G, Patel AA, et al. An evaluation of the learning curve for a complex surgical technique: the full endoscopic interlaminar approach for lumbar disc herniations. Spine J. 2011;11:122–130.
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15. Rong LM, Xie PG, Shi DH, et al. Spinal surgeons’ learning curve for lumbar microendoscopic discectomy: a prospective study of our first 50 and latest 10 cases. Chin Med J (Engl). 2008;121: 2148–2151. 16. Parikh K, Tomasino A, Knopman J, et al. Operative results and learning curve: microscope-assisted tubular microsurgery for 1- and 2-level discectomies and laminectomies. Neurosurg Focus. 2008;25: E14. 17. Neal CJ, Rosner MK. Resident learning curve for minimal-access transforaminal lumbar interbody fusion in a military training program. Neurosurg Focus. 2010;28:E21. 18. Lee JC, Jang HD, Shin BJ. Learning curve and clinical outcomes of minimally invasive transforaminal lumbar interbody fusion: our experience in 86 consecutive cases. Spine. 2012;37:1548–1557.
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19. Daltroy LH, Cats-Baril WL, Katz JN, et al. The North American spine society lumbar spine outcome assessment Instrument: reliability and validity tests. Spine. 1996;21:741–749. 20. Rampersaud YR, Foley KT, Shen AC, et al. Radiation exposure to the spine surgeon during fluoroscopically assisted pedicle screw insertion. Spine. 2000;25:2637–2645. 21. Bindal RK, Glaze S, Ognoskie M, et al. Surgeon and patient radiation exposure in minimally invasive transforaminal lumbar interbody fusion. J Neurosurg Spine. 2008;9:570–573. 22. Mroz TE, Abdullah KG, Steinmetz MP, et al. Radiation exposure to the surgeon during percutaneous pedicle screw placement. J Spinal Disord Tech. 2011;24:264–267. 23. Singer G. Occupational radiation exposure to the surgeon. J Am Acad Orthop Surg. 2005;13:69–76.
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RIGINAL
ARTICLE
Learning Curve of a Complex Surgical Technique Minimally Invasive Transforaminal Lumbar Interbody Fusion (MIS TLIF) Kong Hwee Lee, MBBS, MRCSEd, William Yeo, MPhty (Manips), Henry Soeharno, MBBS, MRCSEd, and Wai Mun Yue, MBBS, FRCSEd, FAMS (Ortho Surgery)
Study Design: Prospective cohort study. Objective: This study aimed to evaluate the learning curve of minimally invasive transforaminal lumbar interbody fusion (MIS TLIF). Summary of Background Data: Very few studies have evaluated the learning curve of this technically demanding surgery. We intend to evaluate the learning curve of MIS TLIF with a larger sample size and assess surgical competence based not only on operative time but with perioperative variables, clinical and radiologic outcomes, incidence of complications, and patient satisfaction. Materials and Methods: From 2005 to 2009, the first 90 singlelevel MIS TLIF, which utilized a consistent technique and spinal instrumentation, performed by a single surgeon at our tertiary institution were studied. Variables studied included operative time, perioperative variables, clinical (Visual Analogue Scores for back and leg pain, Oswestry Disability Index, North American Spine Society Scores for neurogenic symptoms) and radiologic outcomes, incidence of complications and patient rating of expectation met, and the overall result of surgery. Results: The asymptote of the surgeon’s learning curve for MIS TLIF was achieved at the 44th case. Comparing the early group of 44 patients to the latter 46, the demographics were similar. For operative parameters, only 3 variables showed differences between the 2 groups: mean operative duration, fluoroscopy duration, and usage of patient-controlled analgesia. At the final follow-up, for clinical outcome parameters, the 2 groups were different in 3 parameters: VAS scores for back, leg pain, and neurogenic symptom scores. For radiologic outcome, both groups showed similar good fusion rates. For complications, none of the MIS TLIF cases were converted to open TLIF intraoperatively. In the early group, there were 3 complications: 1 incidental durotomy and 2 asymptomatic cage migrations; and in the latter group, there was 1 asymptomatic cage migration.
Received for publication April 4, 2013; accepted January 29, 2014. From the Singapore General Hospital, Singapore, Singapore. This study has been IRB approved by Singhealth Singapore. W.M.Y. is a consultant to Medtronics and Depuy Synthes. The remaining authors declare no conflict of interest. Reprints: Wai Mun Yue, MBBS, FRCSEd, FAMS (Ortho Surgery), Department of Orthopaedic Surgery, Singapore General Hospital, Outram Road, Singapore 169 608, Singapore (e-mail: yuewm@ singnet.com.sg). Copyright r 2014 by Lippincott Williams & Wilkins
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Conclusions: In our study, technical proficiency in MIS TLIF was achieved after 44 surgeries, and the latter patients benefited from shorter operative duration and radiation, less pain, and more relief in their back, leg, and neurogenic symptoms. Key Words: learning curve, minimally invasive, spinal surgery, transforaminal lumbar interbody fusion (J Spinal Disord Tech 2014;27:E234–E240)
F
or 3 decades, open transforaminal lumbar interbody fusion (TLIF) has proven to be a safe and effective technique in achieving lumbar fusion while treating degenerative, traumatic, and neoplastic conditions of the lumbar spine.1,2 However, studies have reported deleterious effects of extensive and prolonged muscle ischemia in the open approach, affecting adversely both short-term and long-term patient outcomes.3–6 Less invasive surgical techniques and exposures have been developed with the aim of decreasing the complications related to the traditional open approaches, while maintaining equivalent or better surgical outcomes. A good example in the field of surgery will be laparoscopic cholecystectomy, which has replaced open cholecystectomy as the gold standard in the management of gallstone disease.7 In the field of orthopedics, an example would be the arthroscopic meniscectomy in the management of meniscal pathologies of the knee.8 At present, these minimally invasive procedures have largely replaced their open counterpart. Minimally invasive (MIS) TLIF is made possible with advancements in spinal instrumentation and radiologic imaging, and it provides surgeons a viable alternative in achieving solid fusion with significantly less soft tissue injury. Foley et al9 first described this novel technique, which utilized tubular retractors inserted serially under radiologic guidance by a muscle dilating approach, thus reducing the amount of iatrogenic muscle and soft tissue injuries. It is postulated that biomechanically, in MIS TLIF, with the intact posterior lumbar musculature, patients can expect more physiologically normal lumbar motion postoperatively. However, at present, no studies have conclusively proven this theory.10 MIS TLIF must be learned, and we would like to evaluate this steep learning curve based not only on operative duration but also on perioperative parameters, clinical and radiologic outcomes, incidence of complications, and J Spinal Disord Tech
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patient satisfaction. A surgeon’s mastery of a surgical technique is achieved when there is a plateau in his operative duration and accompanied by improved perioperative results and fewer complications.
MATERIALS AND METHODS Data Collection After obtaining Institutional Review Board’s approval, the first 90 consecutive records of patients receiving single-level MIS TLIF performed by our senior author (W.M.Y) from June 2005 to June 2009 were retrieved from our prospectively collected Singapore General Hospital Spine Outcome Registry. The subject inclusion criteria for this study were (1) single-level MIS TLIF and (2) MIS TLIF cases utilizing a consistent set of percutaneous pedicle screw system, polyether-etherketone cage, and bone grafts (autogenous or allogenic). The exclusion criteria were (1) previous spinal instrumentation, (2) spinal tumor pathologies, (3) spinal infections, and (4) acute spinal trauma. All patients included had preoperative evaluation with detailed neurologic examination and radiologic imaging, which involved static (anterior-posterior and lateral) and dynamic (flexion and extension) plain lumbar spine radiographs, magnetic resonance imaging, and/or computed tomography scans. The indications for surgery were spondylolisthesis (grade 1 and 2), spinal stenosis with potential for intraoperative instability, and degenerated collapsed disk, all presenting with radicular pain refractory to medical therapy of at least 6 months’ duration. After discharge from the hospital, patients were followed up regularly by the surgeon (2 wk, 3 mo, 6 mo, 1 y, and annually thereafter) and monitored closely for complications. Plain lumbar spine radiographs and, if indicated, magnetic resonance imaging or computed tomography scans, were ordered to assess and confirm complications. Complications were categorized into clinical and technical aspects. Clinical complications included new or worsening neurologic deficit(s) and wound infections. Technical complications included cage migrations and malpositioned screws. For clinical outcome measures, the patients were evaluated by independent assessors at our Orthopedic Diagnostic Centre (ODC) before surgery and at 6 months and 2 years postsurgery. Our ODC (where the Singapore General Hospital Spine Outcome Registry is managed) has been systematically tracking and administering patient-reported outcomes for all elective spine surgeries at various time points since August 1998. When patients did not return, follow-up was obtained through personal telephone calls. With these outcome data collected by ODC and stored in standardized digital formats in our hospital’s computer server, future data retrieval and analysis on the prospectively collected data can be performed.
Surgical Technique All the MIS cases were performed by a single spine surgeon with prior 4 years of experience with open TLIFs. r
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Learning Curve of MIS TLIF
The surgeon first observed the MIS TLIF procedure from 2 spine surgeons and familiarized the technique on 3 cadavers before embarking on the procedure with patients. The initial technique was subsequently refined and modified along the way, resulting in the current technique being significantly different from the initial operative technique, mainly in terms of workflow and ergonomics.11 The surgeon performed the initial approach from the more symptomatic side, and fluoroscopy was used to determine the operative level. A paravertebral incision was made approximately 3–5 cm lateral to the midline, extending between the cephalad and caudad pedicles at the targeted disk level. Sequential soft tissue dilators were then inserted through the incision down to the facet complex until the desired working diameter was achieved. Using a high-speed drill, the surgeon performed facetectomy from lateral to medial to expose the posterolateral aspect of the disk. Discectomy, endplate preparation, and disk space distraction were performed for cage placement. After selecting the appropriate sized cage and filling it with bone graft, the surgeon would impact the disk space with the remnant bone graft before inserting the cage. The bone graft consisted of autograft with a mixture of iliac crest and local bone, augmented by demineralized bone matrix or recombinant human bone morphogenetic protein-2. Fluoroscopy was used to confirm satisfactory positioning of the cage. The lateral margin of the ligamentum flavum was resected to expose the ipsilateral exiting and traversing nerve roots. In addition, if more extensive stenosis were present, the tubular retractor would be angled medially with the patient tilted laterally to facilitate direct decompression of the central canal and contralateral stenosis. Once adequate decompression had been carried out, the tubular retractor was removed and an ipsilateral pedicle screw-rod construct was placed through the same incision. Pedicle screws and rods were inserted percutaneously over K-wires and guided with fluoroscopy. Before final tightening, compression was applied to the construct to recreate the lumbar lordosis. A contralateral pedicle screw-rod construct was placed through a contralateral incision. Before skin closure, a final fluoroscopic check (both anterior-posterior and lateral views) was conducted to ensure that the cage and screw-rod construct were still in a satisfactory position. No surgical drains were used postoperatively.
Methods The patient case records were arranged sequentially in order of operation date and the trend in operative duration was evaluated using piecewise regression analysis (Fig. 1). This piecewise regression analysis was performed using R statistical software (version 2.15.1). The underlying premise of piecewise regression assumes that the line of best fit in a scatterplot comprises 2 (or more) straight lines connected at breakpoint(s). The breakpoint estimate and its 95% confidence limits are estimated using the mathematical algorithm described by Muggeo.12 The www.jspinaldisorders.com |
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variables with nonparametric data (such as body mass index, duration of operation, intraoperative blood loss, fluoroscopy time, use of patient-controlled analgesia, length of stay, time to ambulation, back and leg pain VAS, SF-36 Mental Health Subscale, post-operation ODI, NSS, and SF-36 Physical Function subscale) were performed using the Mann-Whitney U test. For comparisons of categorical variables such as sex, race, operated level, radiologic outcome (Bridwell Fusion Grading), and patient satisfaction (expectation met for surgery and overall result of surgery), w2 and the Fisher Exact tests were utilized.
RESULTS
FIGURE 1. The bar above the x-axis represents the breakpoint estimate (44) and its 95% confidence limits (35–54 patients).
case number at breakpoint would serve as the point of reference between the early learning phase and latter steady state. Cases in the 2 groups would then be compared for the various parameters to elucidate if improvement in operative duration was accompanied by better patient outcomes. The authors focused on 3 broad categories of parameters, namely: perioperative, clinical, and radiologic. Perioperative parameters included patient demographics, operative duration, fluoroscopy time, intraoperative blood loss, length of hospitalization stay, time to ambulation, and the use of patient-controlled analgesia. Clinical outcome parameters included visual analogue scores (VAS) for back and leg pain, Oswestry Disability Index (ODI), North American Spine Society score for neurogenic symptoms (NSS), SF-36 health-related quality of life (HRQOL) outcomes, and patient rating of the expectation met for surgery and the overall result of surgery. For radiologic outcome parameter, both static and dynamic plain lumbar radiographs taken at 6 months and 2 years postsurgery were utilized to assess fusion. The fusion grading criteria was based on Bridwell interbody fusion grading system, and the assessments were performed by 2 independent assessors, with a third assessor available for adjudication.13
Statistical Analysis All data were collected prospectively and retrospective review of the data was performed. Statistical comparisons between the early and latter cases were performed using statistical software SPSS version 17.0. In all analyses, statistical significance was defined as P < 0.05. Comparisons of continuous variables with parametric data (such as age, preoperative ODI, NS, and SF-36 Physical Function subscale) were performed using the independent t test, whereas comparisons of continuous
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From the piecewise regression analysis, the surgeon’s operative duration for performing single-level MIS TLIF was estimated to stabilize after performing the 44th operation (95% CI, 35–54). Using the 44th operation as the reference point, the early phase group would consist of 44 patients and the latter steady state group consisting 46 patients. Comparing the early group of 44 patients to the latter 46, there was no significant difference in patient demographics, namely age, sex, race, operated level, and body mass index (Table 1). Analyzing the perioperative parameters, 3 variables showed statistical differences between the 2 groups, namely mean operative duration, mean fluoroscopy duration, and mean usage of patientcontrolled analgesia (Table 2). The mean operative duration in the early group was 187.16 ± 50.29 minutes, decreasing significantly to 132.28 ± 18.17 minutes in the latter group (P = 0.000). Similarly for intraoperative fluoroscopy, the mean fluoroscopy duration was 74.44 ± 31.41 seconds in the early group and decreased to a mean of 29.88 ± 10.83 seconds in the latter (P = 0.000). The mean usage of patient-controlled analgesia (morphine) was 5.59 ± 12.48 mg in the first group and decreased to a mean of 0.87 ± 2.73 mg (P = 0.010) in the latter group. For intraoperative blood loss, although blood loss in the latter group (107.61 ± 36.47 mL) was less than the early group (153.41 ± 171.65 mL, P = 0.151), the improvement was not statistically significant. For both groups, the patients took about the same time to start ambulating and be discharged from the hospital. For clinical outcome parameters, both groups exhibited similar preoperative baseline pain and disability (except for SF-36 mental health) and, postoperatively, all 90 patients had clinical improvements. At the final follow-up 2 years postsurgery, the 2 groups were significantly different in 3 parameters: VAS scores for back and leg pain, and neurogenic symptom scores (Table 3). For VAS back pain, the early group patients rated a mean score of 2.88 ± 3.30 versus the latter group’s mean score of 1.34 ± 2.32 (P = 0.016). For VAS leg pain, the early patients rated a mean score of 2.28 ± 3.05 versus the latter group’s mean scoring of 0.48 ± 1.34 (P = 0.001). For neurogenic symptom score, the early patients rated a mean score of 22.87 ± 27.74 versus 7.80 ± 17.40 in the latter group (P = 0.001). However, there was no difference between the 2 groups in terms of ODI and r
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Learning Curve of MIS TLIF
TABLE 1. Patient Demographics Demographics Age (y) Sex (male:female) Race (Chinese:Malay:Indian:Others) Operated level L23:L34:L45:L56:L5S1 Body mass index (kg/m2)*
Early 44 Cases
Later 46 Cases
P
49.70 ± 11.69 11:33 (25%:75%) 31:9:2:2 (70%:20%:5%:5%) 0:2:32:0:10 (0%:4%:73%:0%:23%) 26.28 ± 5.06
54.61 ± 16.00 15:31 (33%:67%) 36:4:2:4 (78%:9%:4%:9%) 1:4:27:1:13 (2%:9%:59%:2%:28%) 24.35 ± 3.40
0.101 0.490 0.404 0.487 0.064
*Nonparametric data.
SF-36. In assessing patients’ satisfaction, at both early and final follow-up, the latter group has a larger portion of patients rating their surgical experiences more favorably than their earlier counterpart (P > 0.05) (Table 4). For radiologic outcome, early follow-up at 6 months showed a significant difference between the 2 groups, with the latter group exhibiting a higher proportion of Bridwell 1 solid fusion (Table 5). However, at the final follow-up, both groups showed similar good fusion rates. For complications, none of the MIS TLIF cases were converted to open TLIF intraoperatively. In the early group, there were 3 complications. Patient 30 had an incidental durotomy at the anterior surface of the left L5 root axilla that was promptly repaired with commercially available collagen patch and fibrin glue. Patients 11 and 39 had asymptomatic cage migration noted at the final follow-up. In the latter group, there was only 1 complication with patient 59 having an asymptomatic cage migration noted at a routine 6-month follow-up. In this cohort study, the authors managed to achieve a high percentage of patient follow-up. At 6 months, 95.8% of patients returned for their follow-up attendance (3 patients were foreigners and had returned to their countries at that time). At 2 years, the same patients returned for follow-up.
There are several papers addressing the learning curves in spinal procedures, for example, microscopic discectomies and laminectomies,14–16 but there are few literature available detailing the process in which a spinal surgeon becomes competent with MIS TLIF.17,18 Unfortunately, these few studies are mostly anecdotal, looking mainly at operative timing as basis of learning curve and did not include patients’ long-term (2 y) outcomes and satisfaction. Neal and Rosner performed a retrospective review analyzing the learning curve for MIS TLIF of a neurosurgical resident and noted the learning curve based on operative timing plateau after 15 cases. The authors also recognized that shorter operative duration does not necessarily correlate with better patient outcomes, and a surgeon’s mastery of technique is dependent on evaluating other factors such as complications, readmissions, patient satisfaction, and long-term outcomes.17 Lee et al18 did a similar retrospective study with 86 MIS TLIF (60 single level) patients; using logarithmic curve-fit analysis on the surgeons’ operative durations, they hypothesized that the asymptote of their MIS TLIF learning curve was reached after performing about 30 TABLE 3. Clinical Outcome Parameters Early 44 Cases Later 46 Cases
DISCUSSION MIS TLIF is a surgically demanding and challenging operation, and the inherent difficulties in mastering this technique include limited 2-dimensional view of the surgical field, greater surgical finesse needed with longer surgical instruments, reduced tactile feedback, greater demands of eye-hand coordination and motor dexterity. TABLE 2. Perioperative Parameters Early 44 Cases
Later 46 Cases
Duration of operation (min)* 187.16 ± 50.29 132.28 ± 18.17 Intraoperative blood loss 153.41 ± 171.65 107.61 ± 36.47 (mL)* Fluoroscopy time (s)* 74.44 ± 31.41 29.88 ± 10.83 Patient-controlled 5.59 ± 12.48 0.87 ± 2.73 analgesia—morphine (mg)* Length of stay (d)* 2.96 ± 1.24 2.85 ± 1.20 Time to ambulate (d)* 1.16 ± 0.48 1.15 ± 0.47 *Nonparametric data. wP < 0.05, significant.
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P 0.000w 0.151 0.000w 0.010w 0.744 0.941
Back pain (VAS) Preoperation* 6 mo* 2 y* Leg pain (VAS) Preoperation* 6 mo* 2 y* Neurogenic symptom score Preoperation 6 mo* 2 y* Oswestery Disability Index Preoperation 6 mo* 2 y* SF-36—physical function Preoperation 6 mo* 2 y* SF-36—mental health Preoperation* 6 mo* 2 y*
P
6.34 ± 2.88 3.51 ± 2.81 2.88 ± 3.30
5.96 ± 2.64 1.91 ± 2.56 1.34 ± 2.32
0.399 0.004w 0.016w
6.16 ± 3.07 2.35 ± 2.83 2.28 ± 3.05
5.04 ± 3.43 0.52 ± 1.44 0.48 ± 1.34
0.135 0.000w 0.001w
49.35 ± 28.60 19.32 ± 25.18 22.87 ± 27.74
49.20 ± 27.21 12.58 ± 20.72 7.80 ± 17.40
0.981 0.075 0.001w
48.06 ± 16.75 27.42 ± 18.93 24.94 ± 23.27
44.38 ± 21.16 16.38 ± 14.59 14.62 ± 14.46
0.364 0.004w 0.095
43.41 ± 25.92 62.56 ± 26.24 65.81 ± 29.50
44.89 ± 27.68 69.55 ± 20.88 73.30 ± 21.57
0.794 0.278 0.386
63.27 ± 22.13 67.53 ± 20.88 70.98 ± 20.55
73.83 ± 21.36 77.55 ± 17.83 79.27 ± 17.13
0.013w 0.015w 0.050
*Nonparametric data. wP < 0.05, significant.
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TABLE 4. Patient Satisfaction Early 44 Cases
Later 46 Cases
P
Has the surgery met your expectation? (Yes, totally:Yes, almost totally:Yes, quite a bit:More or less:No, not quite:No, far from it:No, not at all) 6 mo 7:13:8:4:7:1:3 (16.3%:30.2%:18.6%:9.3%:16.3%:2.3%:7.0%) 13:18:6:3:3:1:0 (29.6%:40.9%:13.6%:6.8%:6.8%:2.3%:0.0%) 0.267 2y 9:15:8:5:1:2:3 (20.9%:34.9%:18.6%:11.6%:2.3%:4.7%:7.0%) 19:17:3:3:1:1:0 (43.2%:38.6%:6.8%:6.8%:2.3%:2.3%:0.0%) 0.134 How would you rate the overall result of surgery? (Excellent:very good:good:fair:poor:terrible) 6 mo 5:15:8:9:6:0 (11.6%:34.9%:18.6%:20.9%:14.0%:0.0%) 10:18:11:3:2:0 (22.7%:40.9%:25.0%:6.8%:4.6%:0.0%) 0.116 2y 8:12:9:9:4:1 (18.6%:27.9%:20.9%:20.9%:9.3%:2.4%) 14:18:9:2:1:0 (31.8%:40.9%:20.5%:4.5%:2.3%:0.0%) 0.073 *P < 0.05, significant.
cases. Comparing the early to latter patients, the latter patients had shorter operative duration, less blood loss, and were able to ambulate earlier. However, at 1-year follow-up, both groups reported similar ODI and VAS (back and leg) scores; thus, the authors concluded that mastering this technique had a small effect on their patient’s 1-year clinical outcomes. In our study, a formal statistical method (piecewise regression analysis) was utilized to determine the breakpoint in the learning curve, which occurred at the 44th case. In addition to operative duration, the authors aimed to portray a more comprehensive review of the learning curve of this technically demanding operation; thus, a larger variety of parameters, such as perioperative variables, clinical, and radiologic outcomes were analyzed. To reduce the number of confounding factors in the study, one senior spine surgeon performed all the MIS cases using only one type of pedicle screw-rod instrumentation and interbody cage; only single-level TLIF patients were included in this single center study. We believe this study is the first to report the learning curve of MIS TLIF, which utilized only 1 type of instrumentation by a single surgeon. To further reduce biasness, independent assessors performed the data collection and analysis, and the surgeon was not involved in the process. After achieving technical competency, the surgeon was able to complete the procedure in just over 2 hours, and this average duration was 55 minutes faster compared with the duration in the early group, representing a substantial 29% improvement in operative timing. Even though the surgeon played a significant role in this reduction, other factors were also important in improving the operative duration. The first assistant, scrub nurses, and radiographer made dedicated efforts to improve surgical efficiency by familiarizing the operative steps and modifying the workflow ergonomics, for example, optimal
TABLE 5. Radiologic Outcome Parameters Early 44 Cases
Later 46 Cases
P
Bridwell fusion grading (1:2:3:4) 6 mo 21:21:2:0 38:4:1:0 0.000* (47.7%:47.7%:4.6%:0.0%) (88.4%:9.3%:2.3%:0.0%) 2y 40:0:0:2 42:0:0:0 0.494 (95.2%:0.0%:0.0%:4.8%) (100%:0%:0%:0%) *P < 0.05, significant
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positioning of the microscope and fluoroscopy machine in the operating theater before the start of the surgery. For fluoroscopy duration, the surgical team and the radiographer were able to obtain the required intraoperative fluoroscopy images more efficiently in the latter patients (early: 77.44 ± 31.41 s vs. latter: 29.88 ± 10.83 s; P = 0.000). This 61% improvement in efficiency could be explained by the reasons detailed above. In addition to contributing to the shorter operation, this reduction of 46 seconds of fluoroscopy time could potentially benefit both the patient and operative team; however, because of the recent adoption of MIS TLIF, the side effects of additional radiation exposure have not been thoroughly evaluated. The latter patients also required significantly less patient-controlled analgesia (morphine), about 84.4% less than the early group. This significant reduction in pain could be explained by 2 factors: firstly, with a shorter operative duration, the latter patients were exposed to a shorter duration of the noxious surgical stimuli. Secondly, as the surgeon improved on his surgical technique, the improvement in soft tissue handling and dissection resulted in less paraspinal tissue trauma and damage, translating to less pain. Although there was no significant difference in intraoperative blood loss between the 2 groups, the surgeon’s fastidious attention to soft tissue handling and hemostasis could yield potential further benefits to the patients. All 90 patients did not have postoperative surgical drains, thus minimizing potential postoperative morbidities, for example, infection. As compared with the latter group, the outliers in the early group contributed to the group’s large SDs for operative time, blood loss, fluoroscopy time, and PCA usage. These outliers occurred as the surgeon performed cases that required more decompression while familiarizing the surgical technique. The removal of these outliers will likely narrow the SD for these 4 parameters and reduce the differences between the early and latter groups. However, the inclusion of these outliers will portray a more comprehensive picture of the learning curve of this technique. At the final 2-year follow-up, even though all patients had clinical improvements, the latter group had significantly less clinical symptoms as compared with the early group in terms of back pain, leg pain, and NSS. The latter patients had about 53% less back pain, 79% less leg r
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pain, and 66% less NSS (P < 0.05). NSS is a widely accepted disease-specific tool to assess the outcome of elective spinal surgeries, and a change of Z20% in the score is considered a clinically significant difference.19 This positive difference in the latter group could be attributed to the surgeon’s more thorough decompression of the stenotic neuroforamen while handling the paraspinal tissues better. Interestingly, there was no statistical difference between the 2 groups in patient satisfaction, ODI, and SF-36 health-related quality of life (HRQOL) outcomes. This meant although the latter patients had better clinical symptoms postoperatively, this symptomspecific benefit was not translated to a significantly better quality of life. This could be explained by the multifaceted nature of spinal pathologies. For fusion, at 6 months, the authors observed a greater proportion of solid fusion in the latter group, but this advantage was not maintained at 2 years. The initial advantage could be explained by a more thorough disk space preparation. One major consideration in performing MIS TLIF is the amount of extraradiation exposed as compared with open TLIF.20–22 MIS pedicle screw placement is technically challenging, and the spine surgeon requires intraoperative fluoroscopy to assess the starting point of screw entry, trajectory, and subsequent placement. In the guideline “Standards for protections against radiation” (title 10, part 20), the nuclear regulatory commission recommends that for radiation workers, the annual total effective dose equivalent for the whole body should not exceed 5000 mrem (5 rem) and 50,000 mrem (50 rem) to the hands. Using a regular C-arm fluoroscopy, the exposure rate to the surgeon is estimated to be 20 mrem/min (without lead shield protection) or 2 mrem/min (with protection) to the torso and 30 mrem/min to the hands.23 In our study, fluoroscopy time for the early group (74.4 s) was about 2.5 times longer than that of the latter group (29.9 s). Thus, after achieving proficiency in MIS TLIF, the average amount of radiation to the surgeon’s protected torso dropped from 2.48 to 1 mrem per case, whereas radiation to the surgeon’s hand dropped from 37.2 to 14.9 mrem per case. With the 60% reduction in radiation exposure, the surgeon could theoretically perform up to 5000 cases per year (torso exposure limit) or 3333 cases per year (hand exposure limit). There are several limitations and assumptions in this study. Firstly, the results are based on a single spine surgeon’s experience. Secondly, this study assumed that the surgeon was technically competent in TLIF after performing open TLIF for 4 years. Thirdly, this study assumed that 90 cases would be of sufficient sample size to elucidate meaningful interpretations in all 15 variables. Fourthly, the surgeon also performed other minimally invasive spinal procedures during the study period, for example, microscopic discectomies, microscopic laminectomies, and these additional microsurgical surgeries could possibly improve his MIS TLIF operative proficiency. In conclusion, achieving shorter operative duration may not necessarily equate to surgical competence and r
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Learning Curve of MIS TLIF
better patient outcomes. To illustrate a more comprehensive picture of the learning curve of MIS TLIF, clinical and radiologic outcomes were included in this study. On the basis of our surgeon’s experience, MIS TLIF is a technically demanding surgery but with prior experience in Open TLIF, good outcomes can be achieved even in the early phase of adopting the MIS TLIF technique. In our study, technical proficiency in MIS TLIF was achieved after 44 surgeries, and the latter patients benefited from shorter operative duration and radiation, less pain, and more relief in their back, leg, and NSS. Lastly, the surgeon started learning and exploring the technique when it was not an established one. Perhaps the asymptote could be achieved earlier if a surgeon has been exposed to the technique earlier in his training, for example, as part of his residency. ACKNOWLEDGMENT The authors thank Mr Pua Yong Hao (PhD), principal physiotherapist, of Singapore general hospital physiotherapy department for his assistance with the piecewise regression analysis. REFERENCES 1. Rosenberg WS, Mummaneni PV. Transforaminal lumbar interbody fusion: technique, complications, and early results. Neurosurgery. 2001;48:569–575. 2. Lowe TG, Tahernia AD, O’Brien MF, et al. Unilateral trans-foraminal posterior lumbar interbody fusion (TLIF): indications, technique, and 2-year results. J Spinal Disord Tech. 2002;15: 31–38. 3. Gejo R, Matsui H, Kawaguchi Y, et al. Serial changes in trunk muscle performance after posterior lumbar surgery. Spine. 1999;24:1023–1028. 4. Rantanen J, Hurme M, Falck B, et al. The lumbar multifidus muscle five years after surgery for a lumbar intervertebral disc herniation. Spine. 1993;18:568–574. 5. Sihvonen T, Herno A, Paljiarvi L, et al. Local denervation atrophy of paraspinal muscles in postoperative failed back syndrome. Spine. 1993;18:575–581. 6. Styf JR, Willen J. The effects of external compression by three different retractors on pressure in the erector spine muscles during and after posterior lumbar spine surgery in humans. Spine. 1998;23: 354–358. 7. Keus F, de Jong JA, Gooszen HG, et al. Laparoscopic versus open cholecystectomy for patients with symptomatic cholecystolithiasis. Cochrane Database Syst Rev. 2006;4:CD006231. 8. Northmore-Ball MD, Dandy DJ, Jackson RW. Arthroscopic, open partial, and total meniscectomy. A comparative study. J Bone Joint Surg Br. 1983;65:400–404. 9. Foley KT, Holly LT, Schwender JD. Minimally invasive lumbar fusion. Spine. 2003;15(suppl):26–35. 10. Patwardhan AG, Meade KP, Lee B. A frontal plane model of the lumbar spine subjected to a follower load: implications for the role of muscles. J Biomech Eng. 2001;123:212–217. 11. Peng CW, Yue WM, Poh SY, et al. Clinical and radiological outcomes of minimally invasive versus open transforaminal lumbar interbody fusion. Spine. 2009;34:1385–1389. 12. Muggeo VMR. Estimating regression models with unknown breakpoints. Stat Med. 2003;22:3055–3071. 13. Bridwell KH, Lenke LG, McEnery KW, et al. Anterior fresh frozen allografts in the thoracic and lumbar spine. Spine. 1995;20: 1410–1418. 14. Wang B, Lu¨ G, Patel AA, et al. An evaluation of the learning curve for a complex surgical technique: the full endoscopic interlaminar approach for lumbar disc herniations. Spine J. 2011;11:122–130.
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19. Daltroy LH, Cats-Baril WL, Katz JN, et al. The North American spine society lumbar spine outcome assessment Instrument: reliability and validity tests. Spine. 1996;21:741–749. 20. Rampersaud YR, Foley KT, Shen AC, et al. Radiation exposure to the spine surgeon during fluoroscopically assisted pedicle screw insertion. Spine. 2000;25:2637–2645. 21. Bindal RK, Glaze S, Ognoskie M, et al. Surgeon and patient radiation exposure in minimally invasive transforaminal lumbar interbody fusion. J Neurosurg Spine. 2008;9:570–573. 22. Mroz TE, Abdullah KG, Steinmetz MP, et al. Radiation exposure to the surgeon during percutaneous pedicle screw placement. J Spinal Disord Tech. 2011;24:264–267. 23. Singer G. Occupational radiation exposure to the surgeon. J Am Acad Orthop Surg. 2005;13:69–76.
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