Midline lumbar fusion, providing comparable biomechanical purchase and clinical outcomes, is a relatively emerging technique and an efficient alternative option in the treatment of lumbar degenerative disease. However, whether it is a surgically challenging and familiarity demanding operation? What about the safety and effectiveness during the learning process? There are few literatures available detailing the process in which a spinal surgeon becomes competent with MIDLF. The outcomes, accuracy and complications during the learning curve remain vague.
Attempting to characterize the safety and effectiveness during one surgeon’s initial learning phase with MIDLF procedures by freehand technique, a formal statistical method (piecewise regression analysis) was utilized to determine the breakpoint in the learning curve, which occurred at the 40th case. To reduce the confounding factors in the study, one senior spine surgeon performed all the single-level MIDLF cases in this single center study. And independent assessors excluding the surgeon performed the data collection and analysis to further reduce biasness.
More familiar with surgical procedures: a natural process
In our study, the intraoperative outcomes improved with the increasing case numbers. Significant changes were identified between the two groups in average surgical time, intraoperative blood loss and length of incision. The results were consistent with that in similar cohorts[31]. The average surgical time ranged from 117.8min to 237.6min[8, 20], and 119ml to 276.5ml[21, 32] for intraoperative blood loss in previous studies of single level cortical screw fixation surgery[21, 32-34]. The reduction of duration of surgery should be attributed to technical competence achievement. Surgeon’s familiarity of anatomy and key surgical procedures played an important role in the improvement. Additionally, the tacit cooperation of surgery team made dedicated efforts to raise surgical efficiency by familiarizing surgery steps and modifying the workflow.
Purposeful attention to soft tissue handling yields visible benefits
The late experience cohort had significantly less intraoperative blood loss, about 47% less than the early group. The advantage was due to two reasons: firstly, on account of a shorter operative time, patients in the late group were exposed in a shorter duration. Moreover, as the surgeon gains familiarity with surgical technique and anatomy, the improvement in soft tissue management and paraspinal muscles dissection resulted in less tissue damage, translating to less blood loss. Furthermore, thanks to this familiarity and improvement of surgery technique, a significantly shorter invasion was required in patients of the late group.
As the indicators of postoperative recovery, the less postoperative drainage, duration of drainage and postoperative hospitalization means the better and faster recovery. It should be noted that the drain tube is usually removed after it is less than 50ml every 24 hours in our institution. Although there was no significant difference in postoperative drainage between the 2 groups, duration of drainage and postoperative hospitalization in the late group was significantly less than the early. The surgeon’s purposeful attention to soft tissue handling, protection of muscles and hemostasis led to less trauma and damage, which could yield visible short-term interest and potential further benefits to the patients[8].
The valid procedure of MIDLF could be mastered early: the decompression and insertion accuracy
Although the function and pain of patients in both cohorts got huge improvement comparing with preoperative status according to VAS and ODI, however, no significant difference was demonstrated in either the preliminary outcomes or the mid-term during the follow-up. This lack of significant differences between the 2 groups illustrated that the valid decompression procedure of MIDLF could be efficiently mastered relatively early.
CBT screws rely on four-point purchase between the dorsal cortex at the site of insertion, the medial and lateral cortex of pedicle wall and the curvature of the vertebral body wall, to maximize the cortical bone contact to improve purchase strength[18], but inaccurate placement may give rise to suboptimal fixation even nerve injury. We took five types misplacement into account in the study. There were 6 (6/360, 1.67%) screws penetrating medial cortex of pedicle, 35 (35/360, 9.72%) screws penetrating lateral cortex, 2 (2/360, 0.56%) endplate tresis and 3 (3/360, 0.83%) foraminal tresis respectively. A totally breach incidence of 12.78% was presented, which was a little bit higher than Wray’s 11.4%, Kasukawa’s 9.5% and Dayani’s 7%[35-37].
The authors assume that the difference could be partly explained by the freehand screws insertion and the diverse evaluation system and method used in different studies. All screws in this study were placed without any intraoperative navigation system, however, in Kasukawa’s and Dayani’s research, all patients underwent cortical screw placement using fluoroscopic guidance or intra-operative CT guidance. Comparing with taking intraoperative guidance as reference, our surgeon located the entry point and inserted screws carefully by scrutinizing spine landmarks only, yet osteoproliferation, synostosis and other malformations were more likely came into being in these landmark structures in patients with lumbar degenerative disease. Therefore, screws misplacement rate tended towards higher since the surgeon was misguided. Additionally, one comprehensive and detailed evaluation method based on CT scan images including five kinds of misplaced insertion was utilized to assess the placement accuracy, we believe that the absence of intraoperative navigation system and the utilization of precise evaluation system could make contribution to the higher breach incidence somehow.
A brief comparison for misplacement rate was conducted between the early and late groups. Significant difference was identified in lateral pedicle breach (24 vs. 11, p=0.003) and total misplacement (30 vs. 13, p=0.000). The late phase group had 54.17% less lateral pedicle breach rate and 56.67% less total misplacement rate than the early phase. The significant improvement suggested that there was an explicit learning curve associated with the placement accuracy. In the initial phase of the cohort, the surgeon was not familiar with the medial-to-lateral trajectory for the relatively emerging technique. According to the anatomical characteristic of fleeing away from nerve root, the surgeon assumed that it is much safer to cannulate the screw more laterally, which could explain the incidence of lateral pedicle breach was quite higher than the medial kind, even in the late phase. Over time and with increasing experience, even freehand insertion technique could be modified and proficiently executed to prevent pedicle from being penetrated.
Practice makes perfect: the complications
Although MIDLF is less invasive and offers comparable outcomes to traditional fixation, complications exist. The occurrence of perioperative complications in the early group did not differ significantly from that in the late groups respectively. These perioperative complications highlight the intrinsic peculiar property of lumbar decompression and fusion procedure, even as technical competency is achieved and efficiency is improved. The incidence of intraoperative complications presented in our study was consistent with the finding of Sakaura’s[19] and Patel’s[38]. During at least 12 months’ follow-up, none hardware malfunction, one case of pseudarthrosis (in the early group) and similar fusion rate were found in all patients, as evaluated by flexion-extension X-rays and CT. The result was consistent with Ninomiya’s study, indicating that rare radiology evidence of hardware complications[39], which is the intrinsic advantage of CBT in osteoporosis patients[7]. The coherence of complications incidence in different studies demonstrated the safety in one’s initial phase experience of MIDLF procedures.
After the overall complications were summarized, the significant lower incidence of overall complications (17.50% vs. 2.00%, p=0.028) was shown in the late cohort. The result demonstrated that as the surgeon gains familiarity with the procedure these potential complications can be minimized to some extent.
There was a significant higher incidence of complication due to medially placed screws than caused by laterally inserted (33.33% vs. 0.0%, p=0.018). The result was consistent to a certain degree to the assumption we introduced before, that it is much safer to cannulate the screw more laterally. The laterally placed screws may negatively affect the biomechanical properties and purchase strength of cortical fixation, yet we have no evidence and reference. Therefore, we recommend the practicing surgeons to select a lateral relatively trajectory if they cannot ensure the accurate screws insertion.
We recognize that our study has its limitations. Firstly, the results are based on the experience of only one single spine surgeon in one institution, the results may not be broadly applicable, future studies should be conducted to portray a surgical learning curve of multiple surgeons. Secondly, with a small sample size, the study is underpowered to elucidate meaningful interpretations in all variables. Thirdly, the surgeon also performed other lumbar surgeries during the study period, these additional procedures could possibly improve his MIDLF operative proficiency. And lastly, retrospective study carries inherent bias in nature, even though we attempt to avoid this bias.