Today, transcutaneous spinal fixation is efficient in terms of solidity and reduction. Achieving real fusion remains a problem, however, as does the successful navigation of each surgical approach. Fusion takes place, after all, at the hardest point to reach before actually opening the spine, writes Jean-Paul Steib.
Generally an interbody technique is used for spinal fusion. Approaching interbody fusion through the foramen requires the subtle navigation of nerve roots, whilst an anterior approach requires the surgeon to deal with the psoas muscle and the aorta and vena cava vessels.
There is certainly a place for a new posterior approach for interbody fusion. For the insertion of bone graft, it is possible to use the pedicle to access the vertebral body using the same path as the screw. Here we can turn the needle upwards to join the disc, transpiercing the endplate. This transpedicular approach is safe, avoiding any contact with the nerves or the vessels. The trans-lumbar perforation of the vertebral endplates will bring vascularisation to the bone graft.
The surgery is performed under general anaesthesia, with the patient in prone position. Under fluoroscopy, a bone marrow biopsy needle is inserted into each pedicle. Through the needle, a pedicle guidewire is positioned at the central part of the vertebral body, under fluoroscopic control in sagittal view. Needles are replaced by tubes to protect the surrounding tissues. The pedicles are then drilled using the guidewires. Straight cannulas are tightened to the posterior wall of the vertebral body, before the insertion of a curved nitinol guide pin into the cannula, perpendicular to the superior endplate of the vertebra. Once out of the cannula, the inserter recovers its shape and transpierces the endplates. A flexible drill is then lowered along the curved guide. Drilling is performed until the desired tunnel size is reached. Both sides are done in the same way. The guided pins are then removed, and bone graft or substitutes injected through an injection cannula, and pushed into the drilled tunnel to bridge the two vertebrae. The surgery is finished by a posterior fixation using pedicle screws and rods. The total procedure is performed by a percutaneous approach.
This new procedure, which is performed under fluoroscopy, enables instrumentation and fusion by a single transpedicular approach, without requiring any new surgical positions or next-step surgery. The surgeon must be able to set a Jamshidi needle (CareFusion) in the pedicle transcutaneously, which can be helped by navigation. A tunnel is dug, only in the bone, to reach the intervertebral disc. This working channel is completely safe, without the risks of bleeding or Dural tear inherent in other techniques. The choice of the graft—derived from iliac crest, heterologous bone, substitutes or bone morphogenetic protein—is up to the surgeon.
The key aim is to fit the cavity with a structure known to provide a good environment for bone. Good vascularisation provides a fast and effective fusion. The decortication of the endplate and the contact with cancellous bone should facilitate fusion by the issuance of blood and bone cells in the created cavity. I think that we will obtain a better rate of fusion for with this method of vascularisation, but we have not yet shown this with data. The procedure is safe and fast, and can be performed on one or several levels in lumbar and thoracic spine. It can be used in treatment of discal or articular back pain, in fractures where the opening of the canal is not required, or with a small traditional targeted open approach if needed.
I am convinced that the future of spinal surgery will be in less aggressive techniques; but to obtain a really good fusion will remain a goal in cases where arthroplasty is not required. In this direction, the transpedicular approach to interbody fusion is promising.
Jean-Paul Steib is the head of Spine Surgery at the Spine Unit, University Hospital, Strasbourg, France