A novel surgical technique in transforaminal lumbar interbody fusion by the bone graft delivery device: evaluation of therapeutic effect in patients with minimally invasive spine surgery

Lumbar interbody fusion (LIF) is increasingly recognized as a feasible and effective approach for the management of a wide range of spine disorders including degenerative lumbar diseases and spondylolisthesis [1]. It has been commonly indicated in patients with symptomatic low back pain and/or disability after failure of conservative management [2]. Historically, posterior lumbar interbody fusion (PLIF) was the gold standard approach for spine fusion; however, transforaminal lumbar interbody fusion (TLIF) has emerged as a popular variant of PLIF that is based on lateral access of the intervertebral body, complete discectomy, and bone graft placement transforaminal [3]. The TLIF poses several advantages over the traditional posterior approach including easier access, lack of ligament damage, and the need for a single unilateral incision to allow bilateral anterior column support [4, 5]. Previous reports demonstrated that TLIF is associated with lower risk of spinal nerve damage compared to the traditional posterior approach [6]. Additionally, with the introduction of minimally-invasive surgery (MIS), the intra- and postoperative outcomes of TLIF have improved significantly in terms of muscle injury, bleeding, hospital stay, and functional recovery [1].

The delivery of autologous bone graft or bone substitutes is a major concern during the TLIF approach. It is well-established that a sufficient amount of bone graft ≧ 50% of the cross-sectional area of the gap must be placed to achieve a successful interosseous fusion. Failure to do so has been shown to decrease the success of fusion and negatively affect clinical outcomes [7]. Yoo et al. has reported a significantly increased fusion rate in MIS-TLIF with the increased bone graft volume [8], however, the limited visual field and the minimal destruction in MIS often poses a challenge to the amount of bone graft delivered. The minimal amounts of bone graft transplanted not only compromises the fusion rate but also can lead to increased intraoperative time and a higher risk of iatrogenic injury due to the repeated filling procedure [9]. Improved bone graft delivery tools allowing better visualization of the cannula tip and complete filling of the prepared disk space and thus reducing the risk accompanied by the repeated filling process has been published [10]. Shau et al. also demonstrated that an articulated delivery arm system for interbody graft placement facilitated increased segmental lordosis [11]. However, the inserted graft volume could be discrepant from the desired volume and was subjected to individual surgeons as delivering bone graft by these devices relied mainly on manual filling. The challenges remain as participants reported no significant improvement in the radicular pain and the fusion rate, which is directly related to the treatment efficacy, is left undetermined.

In light of the need to effectively deliver sufficient bone graft, we introduced a novel MIS bone graft delivery device (MIS device) with an integrated threaded rod. When connected to the surgical drill, it drives the granule bone graft into the filler tube and moving forwards during spinning. This MIS device allowed surgeons to directly deliver the graft into the deep side of patients in a safe, stable, continuous, and effective approach.

In this study, we have demonstrated an improved bone healing capacity and fusion time with increased volumes of bone grafts in a MIS-TLIF procedure by using our novel MIS bone graft delivery device in the comparison with traditional tool. MIS-TLIF has shown less intraoperative blood loss, reduced postoperative narcotic use, fewer complications, and shorter hospital stay as well as recovery time compared to traditional open TLIF [4]. MIS-TLIF is sometimes preferred over other minimally invasive LIF given the disease and patients’ conditions. For instance, a better ODI, VAS pain, and complication rate were reported in treating degenerative lumbar disease when compared with MIS lateral lumbar interbody fusion (MIS-LLIF) [14]. Research also showed that MIS-TLIF is especially advantageous for obese patients [15]. One of the potential risks of MIS-TLIF is the inserted cannula that can damage the endplate or skate off to an undesired location. Repeat filling of the bone graft using traditional cannula also increases the risk of iatrogenic injury. Under the same minimally invasive procedure, the MIS device used in this study significantly reduced the filling time to only 20 s in addition to the subsequent blood loss volume. We also observed less blood loss when using the MIS device during the procedure.

Postoperative fusion rate along with clinical satisfaction indexes has been the gold standard of assessing LIF surgery efficacy. The postoperative fusion rate ranges from 70 to 95% depending on the patient-based factors, surgical techniques, and graft materials [16‐20]. Although evidence have shown that graft materials influence LIF fusion rate, TLIF generally yields a high fusion rate despite the graft materials used [21]. A substantial effort was put for better postoperative lumbar stability which is reported to be related to the fusion rate [22]. The articulating cages or expandable cages give the surgeon better insertion control and increased footprint size on endplate and anterior contact in the disc space during MIS-TLIF. The use of these cages resulted in longer-lasting disc height restoration, improved lumbar lordosis and stability, providing a stable fusion environment [23, 24]. The relation between solid fusion and postoperative lumbar stability provided by pedicle screws were also investigated. While no surgical or clinical outcomes differences were found, Ren et al. demonstrated that bilateral pedicle screw fixation is preferred over unilateral fixation for less cage migration and higher fusion rate [25]. We used bilateral pedicle screw fixation for greater postoperative lumbar stability in this study, minimizing the possibility of skewed fusion rate due to lumbar instability.

To achieving successful bone fusion, the three pillars of bone regeneration including osteogenesis, osteo-induction, and osteo-conduction are critical [26]. The amount of transplanted bone graft serving the 3 properties is thus relatable to the fusion rate. Indeed, DiGiovanni et al. recently demonstrated that an adequate volume of bone graft between the osseous surface was needed for successful fusion in their ankle fusion study [7]. Although it is logical to assume the criticalness of adequate bone graft volume in lumbar fusion surgeries, the principle was only confirmed in MIS-TLIF by Yoo et al. [8]. Given the fact that less than 50% of the disc area is actually grafted in most LIF surgeries, increasing the graft volume transplanted is believed to be the desired strategy [27].

As the concept was demonstrated relatively recent, only a few studies have explored the possibility of transplanting greater bone graft volume using novel devices in LIF surgeries. An in-situ cage filling system was investigated using an in vitro lateral lumbar interbody fusion (LLIF) model, in which additional bone graft could be delivered after the pre-packed cage was inserted [28]. The delivery system significantly increased the cage volume with graft compared with the traditional cage filling methodology, however, it was difficult to predict the functionality of the novel system in a living human spine. The compressible liquid consistency of the bone graft poses a challenge during the filling process as the pressure applied by the plunger usually preferentially drives out the liquid part of the mixture. Kleiner et al. thus aimed to improve the graft flow and the volume delivered to the disc space by using a modified cannula during the MIS-TLIF [10]. The enlarged cross-sectional area of the cannula with outlets on the sides improved the flow of bone graft, allowing complete filling of the disc space. The authors, however, emphasized the relationship between the disk material removed and the graft volume for better graft volume prediction required during the procedure. The amount of bone graft in effect of fusion rate remained unknown. In this study, we integrated the threaded rod into the cannula that surgeons can connect to an electric surgical drill. The threaded rod concept not only resolved the shortfall of manual plungers that liquid graft mixture being the primary transplanted part, it also provided a stable and continuous filling process when in combination with the surgical drill. The MIS device delivered approximately 3 times more of the graft than that of using traditional funnel and cannula.

Notably, our study showed relatively small graft volumes were transplanted to the disc space compared with Yoo et al. [8] and Kleiner et al.’s studies [10]. Yoo et al. indicated a graft volume > 12 mL is considered to be sufficient for improved fusion rate, while the average bone graft delivered in Kleiner’s study was 9.2 mL. However, we demonstrated a higher than average fusion rate for MIS-TLIF with only an average of 6.72 mL bone graft volume. The difference may result from the mechanical design of the cannulas. The voids between bone graft granules can be diminished or collapsed during graft insertion, leading to a skewed final volume. Instead of compressing the graft granules manually through syringe and cannula, the threaded rod design moves the graft through the cannula and to the disc space without collapsing the granules and thus resulting in a small transplanted graft volume.

In MIS fusion surgeries, the biological environment may be different from the traditional open fusion surgeries, which are inherently more advantageous for fusion due to greater surface area and bigger cage for more graft materials. While the limitation of the MIS technique to completely remove disc space was reported to compromise the fusion rate, MIS-TLIF results in comparable outcomes as open TLIF in terms of solid fusion [29, 30]. Our study not only showed a higher fusion rate with the MIS device than other MIS-TLIF studies but also reaffirmed the notion of improved fusion rate with increased bone graft volume. Furthermore, this study is the first of its kind to elaborate the relationship between transplanted graft volume and the progression of fusion rate during the follow-up period. The use of the MIS device resulted in greater graft volume transplanted, leading to more than 88% fusion rate in the first year whereas the conventional methodology yielded only 75% solid fusion at the 2-year time point.

The use of novel MIS bone graft delivery device significantly improves bone healing capacity and bone fusion time by increasing the amount of bone graft in patients receiving MIS-TLIF. The MIS device delivered 3 times more bone graft into the disc space and significantly reduced the intraoperative blood loss as well as the filling time to 20 s. This technique provides an efficient, safe, and time-saving surgical approach for surgeons in the field of MIS lumbar fusion surgery.