ALIF WITH AUTO-LOCKING CAGE WITHOUT SUPPLEMENTATION – TOMOGRAPHIC ANALYSIS OF INTERBODY BONE FUSION ALIF

Objective: The objective of this work is to study the fusion rate and complications of the mini-ALIF with an auto-locking device at the L5-S1 level. Methods: Retrospective and radiological study. The inclusion criteria were mini-ALIF in L5-S1 with auto-locking cage, DDD and/or low grade spondylolisthesis. The exclusion criteria were posterior/anterior supplementation; lack of 12-month follow-up images, and previous surgery at L5-S1 level. The primary endpoint was fusion assessed in CT images and/or lateral lumbar flexion/extension radiographs. The secondary endpoint was the revision surgery due to device movement/migration or pseudoarthrosis. Lumbar TCs and radiographs were analyzed during 12 months of follow-up. Fusion was defined according to Bridwell/Lenke classification. Results: Sixty-one cases were included in this study. Complete or ongoing fusion was found in 57 cases (93%). Forty-two of the 61 levels (65%) were completely fused after 12 months. Fifteen levels (28%) had evident bone growth, two levels (3%) showed lysis lines around the implant, and two levels (3%) presented lysis lines and depression. Reoperation for pedicular screw supplementation was necessary in two cases (3%), one with vertebral sliding progression (12 months), and one with symptomatic micro-movement (six months). No implant has undergone migration or expulsion of the disc space. Conclusions: Mini-ALIF in L5-S1 level using an auto-blocking interbody implant construction in cases of low segmental instability results in good interbody fusion index and low failure rate, even without the need for further supplementation, but should not be applied indiscriminately. Evidence Level: IV. Type of study: Case series.


INTRODUCTION
Interbody fusion is a surgical procedure in which a discectomy and the placement of a cage at the level of interest are performed with the goal of fusing the vertebrae and thus immobilizing the segment.The purpose of the procedure is to cure or minimize the pain caused by a pathological movement and to restore disc height, thus promoting indirect decompression. 1 Several surgical techniques can be used to achieve this fusion, among them anterior lumbar interbody fusion (ALIF). 2and is one of the most frequent causes of disability.Lumbar spondylosis may result in mechanical back pain, radicular and claudicant symptoms, reduced mobility and poor quality of life.Surgical interbody fusion of degenerative levels is an effective treatment option to stabilize the painful motion segment, and may provide indirect decompression of the neural elements, restore lordosis and correct deformity.4] However, there are several disadvantages to this approach.][4][5] In an attempt to reduce morbidity and interoperative complications from the surgical approach, less invasive techniques like as the anterior retroperitoneal approach for lumbar fusion (mini-ALIF) have evolved.7] These factors not only reduce the possible complications, but also sustain a good rate of surgical success.
In general, after the discectomy and introduction of the cage, pedicle screws are usually used posteriorly for the fixation of the operated level.However, although quite stable, this option has disadvantages when compared to the stand-alone alternative (without supplementation). 1 Currently, there are biomechanically viable options with cages with auto-locking screws that provide enough stability for use without the need for additional fixation. 8Despite well-established biomechanical data, few studies have evaluated the clinical and radiological advancements in anterior approach interbody fusion using auto-locking devices.Thus, the objective of this study was to evaluate solid fusion of the L5-S1 level through mini-ALIF with an auto-locking device.

METHODS
This was a retrospective radiological study conducted in a single center.It was analyzed and approved by the Institutional Review Board (IRB no.52909516.3.0000.5551)and the ICF requirement was waived.
The primary endpoint was fusion evaluated in computed tomography (CT) images and/or in lateral lumbar flexion/extension radiographs.The secondary endpoint was the presence/absence of revision surgery for the movement/migration of the device or for pseudoarthrosis.Lumbar radiography and tomography studies were analyzed after 12 months.Fusion was defined according to the Bridwell/Lenke classification. 9

RESULTS
Table 1 shows the data from the group studied in this work.Sixtyone patients with an average age of 44.3 years, 37 (61%) of them women, were included.Of these, 46 cases (75%) had been diagnosed with DDD and 15 (25%) with low-grade spondylolisthesis.All cases underwent interbody fusion at level L5-S1 by single access anterior approach.Auto-locking cages with cortical screws were used for fixation without the necessity for posterior approach.All cases had a minimum follow-up time of 12 months.
Table 2 displays the results for fusion rates according to the Bridwell-Lenke classification.Complete or ongoing fusion was observed in 57 cases (93%).According to the Bridwell/Lenke classification, 42 levels (65%) were judged completely fused in the CT 12 months following surgery (grade I) (Figures 1 and 2).Fifteen levels (28%) had ongoing bone growth (grade II), two levels (3%) showed lucency and collapse (grade IV).
Posterior surgical reapproach for supplementation with pedicle screws (Figure 3) was necessary in two cases (3%) -one with progression of vertebral slippage (at 12 months) and the other with symptomatic micromovement (at six months).No interbody implant suffered migration or expulsion for the disc space.

DISCUSSION
This study evaluated interbody fusion in patients who underwent single access anterior mini-ALIF with an auto-locking cage at level L5-S1 without any additional posterior fixation.In total, the rate of complete or ongoing fusion (grade I and II) was 93% and only 3% experienced adverse events requiring a surgical reapproach.None of the implants migrated or was displaced from the location where it had been fixed.
4] This technique has gained many adepts over the years and, in parallel, interbody spacers have evolved from the bone graft blocks used in the early days of the technique, to the first synthetic threaded cages (BAK), to the auto-locking PEEK (polyetheretherketone) and titanium alloy cages currently being used. 15elf-locking cages with screws directed to the adjacent vertebral bodies offer stability comparable to an ALIF with fixation with pedicle screws in movements of flexion, extension, and lateral inclination and they present superior biomechanical properties in rotation. 16dditionally, a study observed that these auto-locking cages could distribute the load like an intact lumbar disc. 8uto-locking cages enable interbody fusion for a single access and can deliver better clinical outcomes and a similar fusion rate when compared to anterior approach arthrodesis in combination with posterior fixation with pedicle screws in single level cases. 17The percentage of fusion observed in our study is comparable to others reported in the literature.Allain et al. conducted a prospective study in which 65 patients were followed-up for 12 months and the authors observed a fusion rate at the end of this period of 93.3%. 18Likewise, Rao et al. reported a fusion rate of 91% in patients submitted to ALIF for the treatment of spondylolisthesis 19 and a death rate similar to that of our study.
This fusion rate is comparable to that of techniques widely used in clinical practice, such as TLIF (Transforaminal Lumbar Interbody Fusion) with the presence of pedicle screws for posterior. 20However, the success rate observed in these studies is not as high in more extensive arthrodeses, and so it is not recommended without additional posterior supplementation. 21he observation of both growth and solid bone fusion has been put in check because of the lack of a simple and direct correlation with better clinical outcomes.However, complementary to this discussion, evolution with instability, migration of the device, or pseudoarthrosis may indeed be deleterious in certain cases.Importantly, in this study there were no cases of migration or breakage of the implanted cages, although two cases required surgical reapproach for posterior pedicle fixation, one due to pseudoarthrosis, and the other to the presence of micromovement causing pain.
Two limitations of this study should be highlighted: the retrospective design and limited number of cases analyzed.Prospective, multicenter studies with a greater number of cases should be conducted with more data about achieving bone fusion using auto-locking implants for anterior approach lumbar fusion.

CONCLUSIONS
The interbody fusion of L5-S1 using mini-ALIF with an auto-locking implant without the use of pedicle screws can achieve a good bone fusion rate and a low reoperation rate in selected cases.Single access for the fusion of L5-S1 can be performed without the need for posterior supplementation in cases of low segmental instability.
Author Luiz Pimenta declares that he receives royalties from the manufacturer (MDT Implantes) of a prosthesis similar to that presented in this article.The other authors declare no potential conflict of interest related to this article.

Figure 1 .
Figure 1.Example of a case showing solid bone fusion (grade I) at the 12-month postoperative follow-up.

Figure 2 .
Figure 2. Example of a case showing solid bone fusion (grade I) at the 12-month postoperative follow-up.

Figure 3 .
Figure 3. Example of a case requiring stabilization with transpedicle screws.

Table 1 .
Demographic data of the group studied.