A new minimally invasive technique for L5-S1 interbody lumbar fusion
A NEW MINIMALLY INVASIVE TECHNIQUE FOR L5-S1 INTERBODY FUSION
Lumbar spinal fusion has been a well accepted treatment option for degenerative disc disease for well over three decades. The clinical indications and various surgical techniques have provided mixed patient outcomes as the debates over proper patient selection continue. However the morbidity associated with the traditional muscle cutting open approach to the spine has been a significant motivating factor in the development of minimally invasive techniques.
Parviz Kambin and other early pioneers of the minimally invasive approach were able using cadaver studies, to identify a safe transforaminal approach to the intervertebral disc space.
The evolution of the MIS specialty has involved a close relationship between improving microsurgical technology and the clinical understanding of back pain.
The initial use of this technique was primarily for disc decompression and fragmentectomies. The possibility of performing lumbar fusions was entertained briefly by Parviz Kambin in 1973 but was not advanced due to limitations of the instruments available at the time. However the conceptual framework was established and it became clear that if the same goals of endplate preparation and implant placement could be adequately achieved via a minimally invasive approach then the overall morbidity of spinal fusion would be reduced.
In the late 1990’s Spineology commenced work on developing a system for the implantation of bone allograft intrabody implants for the correction of VCF. The successful patient outcomes prompted some surgeons to broaden its scope of use to include lumbar fusions. The posterolateral approach via Kambin’s triangle provides an acceptable technique for accessing the intervertebral spaces from L1-2 to L4-5. However due to the anatomical constraints often created by a high pelvic brim, the L5-S1 space is difficult to reach therefore potentially limiting the use of the minimally invasive approach at this frequently affected level.
This article will describe a new surgical technique for a minimally invasive interlaminar L5-S1 interbody fusion using the Spineology optimesh system.
Description of Technique
The patient is positioned prone on a Kambin frame, which helps minimize the lumbar lordosis thereby facilitating access to the disc space. All the routine operative precautions are taken with regard to monitoring and pressure point protection. The choice of anesthetic can include local with sedation, epidural or general anesthesia and will depend amongst other things on the patient’s medical condition and the surgeon’s level of expertise.
The use of biplanar fluoroscopy is mandatory and it is essential that the operating surgeon be very familiar with the radiological interpretation of key anatomical landmarks. The ability to mentally re-create a 3-D image from 2-D representations is vital to the safe and successful execution of the surgery. It is important to align the patient at 90 degrees to the fluoroscopy unit at the start of the procedure and to strictly maintain this orientation. Failure to do so by even a few degrees will lead to instrument misplacement and possibly fatal consequences. The C arm should only be rotated between the lateral and AP. It is these two views that provide the radiological guide to the operation.
Neurological monitoring is another essential component of the surgery. SSEP and EMG will provide an almost continuous assessment of neural status and will alert the surgeon to any situation that potentially threatens nerve integrity. This importance of this tool cannot be stressed enough and it is one if the technical details that allows the surgeon to safely navigate the spinal canal without traumatizing the neural components.
The patient’s back is prepped and draped in a sterile fashion and the patient is given a pre-operative dose of IV antibiotics.
The midline is marked out and the lumbar levels are clearly identified by counting down from the 12th rib. Radiological inspection of the L5-S1 interlaminar space with the identification of its superior, inferior and lateral margins provides the initial basis for location of the entry point (see Fig. 1). It is evident from the diagram that this is usually found within the infero-lateral portion of the space i.e. the widest path between the thecal sac and the exiting nerve roots.
Having identified the entry point, 1% lidocaine is then used to infiltrate the skin and subcutaneous tissues. An 18 gauge Tuhoy needle is then advanced under fluoroscopic control to the ligamentum flavum where using the loss of resistance technique the epidural space is identified. 5 cc of contrast is then injected into the epidural space to outline the location of the thecal sac and the exiting nerve roots. The tuhoy needle is then removed and an 18 gauge spinal needle is then advanced under fluoroscopic control along the same track. Having radiologically marked the location of the neural elements, the needle is carefully guided through the spinal canal while referring to the neurological monitor to ensure no neurological trauma occurs. It is necessary to confirm the needle position in both the AP and lateral views to ensure optimal instrument placement within the disc (see Fig. 2)
Once the needle has reached the posterior margin of the disc it is carefully advanced through the annulus with a slow steady force and frequent fluoroscopic views to ensure it does not enter the great vessels or the bowels. 5 cc of contrast solution is then injected into the disc to confirm correct needle placement.
The stylet is removed and a guidewire is inserted through the needle. A 1cm incision is then made in the skin and a dilator is inserted over the guidewire advancing it carefully with fluoroscopic guidance through the spinal canal and up to the posterior margin of the disc. A mallet is then used to advance the dilator through the annulus and into the disc cavity.
Once the dilator is securely within the disc, a working sheath is advanced over the dilator. It is important to confirm that the dilator is positioned at the outer margin of the disc (see Fig. 3) and then using a depth gauge the depth lock on the sheath is positioned to ensure that no surgical instruments advance past the anterior margin of the disc. Using a pituitary rongeur the nuclear contents are then removed and the space irrigated and suctioned to ensure the evacuation of any free fragments.
A disc shaver is then inserted through the sheath. The shaver has a unique design feature that allows the sequential discharge of a progressively larger blade width (see Fig 4) which enables preparation of the endplates. Irrigation and suction of the space confirms adequate blood flow from the debrided endplates thus ensuring optimal conditions for a successful fusion. The working sheath is then secured in place by attaching it to the frame shown if Fig. 5. This ensures that tamping of the allograft bone will not dislodge the sheath.
Having decompressed and prepared the intervertebral space the optimesh is advanced into the disc space using the mesh driver. Fluroscopic views will confirm correct placement of the mesh. The driver is secured to the sheath and then allograft bone is tamped into the mesh. The bone is mixed with a lubricant to facilitate passage through the container tubes. As the bone begins to fill the space it becomes evident radiologically which allows the surgeon to assess the quality of implant positioning. Additionally once the space has been optimally filled the tamp will encounter resistance signifying the end of the bone filling phase.
The mesh is then disconnected from the driver using the instrument shown in Fig. 6. and after releasing the sheath from the anchoring frame it is removed from the patient’s back.
The entry wound can be closed with either one interrupted 3-0 prolene suture or just a band-aid.
The patient is usually discharged within 2-4 hours post operatively, and is instructed to wear a hard brace for the next two weeks. Additionally he or she is given strict instructions regarding activity limitation and load bearing. The patients are followed up at 1 week post op and then every 2 months for the next year.
The advent of minimally invasive spine surgery has provided a much overdue addition to the field of back care. The advantages of this approach in terms of reduced morbidity are clear. The minimization of muscle destruction and bone resection lead to a much lower incidence of post operative scarring which is believed to be a major cause of failed back syndrome. Compared to an open procedure there is significantly less blood loss and anesthesia time, which in conjunction with less post-operative pain enables the patient to be discharged on the same day.
In critically evaluating emerging technologies efficacy and risk are assessed and compared with the accepted gold standard. Currently lumbar fusions can be carried out using a posterior, transforaminal or anterior approach. Which technique is used will depend on the surgeon’s preference and his diagnostic assessment of the disc pathology. The success of a fusion is evaluated primarily using clinical criteria e.g. SF-36, Oswestry, VAS and secondarily with radiological markers.
Fusion as a concept for the management of axial and radicular pain makes both mechanical and biological sense. The removal of disc pain generators, the restoration of vertebral height and neuroforaminal decompression act to reduce back and leg pain. Graft positioning and fusion rate are factors involved in the overall surgical outcome. A large component of the morbidity with the open approach stems from the aggressive muscle cutting and bone resection that are necessary to adequately perform the surgery. Therefore the risk benefit ratio is unfavorable when compared to the minimally invasive approach, which enables the surgeon to carry out a fusion using a 1cm muscle sparring technique.
The optimesh provides a large surface area interface between itself and the vertebral endplate which encourages faster bony in growth. Additionally the containment of the allograft bone within a Dacron mesh creates a local environment that concentrates the fusion force vectors in an optimal fashion (see. Fig. 6).
The use of posterior instrumentation will depend on the clinical assessment of instability. The fact that the patient’s posterior column is not disrupted is significant in that this will provide stabilization to the implant when no translational instability is detected pre-operatively. The rationale for the use of posterior instrumentation in an open approach takes into account the potentially destabilizing effect bony resection has on the spinal column and thus on fusion outcome. This situation is not created with the minimally invasive approach and therefore in our opinion the utilization of instrumentation is more questionable.
However if the indication for rods and screws is clear, as would be the case with an associated spondylolisthesis, then they are inserted using the minimally invasive Pathfinder system from Spinal Concepts thus retaining all the advantages of the muscle and bone sparring approach.
The argument in favor of the minimally invasive spinal approach is in its infancy and as the collection of clinical data continues it’s efficacy compared to the open approach will become clearer. However it would be reasonable to assume that with regards to morbidity the MIS approach is superior, and there is already a significant body of evidence supporting this position. The crux of the ongoing debate relates to its efficacy, and with fusion the controlling factors are careful patient selection and a good surgical technique. To date 15 one and two level fusions at the L4-5 and L5-S1 levels have been performed. Clinical and radiological data collection is ongoing with future studies planned.