Complications of Total Disc Arthroplasty

With the recent FDA Approval of the CHARITE Artificial Disc, and ongoing studies with a number of other artificial disc designs nearing completion, artificial disc replacement is fast becoming the leading edge of spinal surgery, an area that spine surgeons and patients alike are eagerly embracing. The anterior approach to the spine has been used for many years for fusions and subsequent redo operations, `and the complications of these operations are well known to spine and access surgeons. Artificial Disc replacement is a new technique that is just beginning to be performed more widely, and the number of revision or redo surgeries for these discs is small at this time. There is little doubt however that these numbers will soon be increasing, and judging from early evaluation of these cases, it may be that re-operating on total discs may present its own unique challenges separate from routine fusion cases.

Redo surgery is always challenging, and each case has its own unique issues to consider. These include length of time since the original procedure, whether or not the initial level of surgery needs to be directly addressed or if the approach will be to an adjacent level, whether infection is involved, and what type of device was used if any. With the artificial disc, our revisions and the few reported cases of revision have to do with malfunction or misplacement of the disc and require a direct approach to the original level.

Anterior spinal surgeons are familiar with the complications of this approach including injuries to surrounding structures: nerves, arteries, veins and the ureter. With redo surgeries, because of the subsequent scarring, the risk of these injuries increases. It has been our experience in more than 150 artificial discs and a few revisions , that the reaction surrounding an artificial disc that has dislodged is much more extensive, making exposing that level much more treacherous. The following discussion will go into detail regarding these complications and strategies for avoiding them in the first place, and ways of dealing with them if they should occur.

The initial consideration in a redo operation is the approach. If the surgery occurs within a few days of the original surgery, returning through the retroperitoneal approach is usually possible. When one is further out from the initial surgery, it becomes more difficult. I usually try this first as occasionally it will work, and one trick is to start the retroperitoneal dissection a little above or below the initial area, entering the retroperitoneal space in virgin territory and working back to the previous area of dissection... If it is too difficult, then a trans- abdominal approach is used, using self retaining retractors to retract the bowel.

The ureter is attached to the peritoneal sack, and in an initial surgery, it stays with the peritoneal sack as it is being dissected off the spine. One does not need to dissect it out at all. The surgeon just needs to be careful not to compress it too strongly with the retractors. The blood supply to the ureter involves small vessels coursing around the structure without a lot of collateral, so retracting it strongly can cause local ischemia with resulting necrosis and disruption. In a redo operation the ureter can be very stuck to the spine and can be disrupted with too vigorous of blunt dissection. Whether one is able to stay in a retroperitoneal approach or not, the ureter has to be identified and protected.

The preoperative placement of ureteral stents will allow you to feel the location of the ureter as you are exposing the spine. At times the adhesions can be so dense that recognizing individual structures is very difficult. You can also give methylene blue (10cc IV) so that any small laceration of the ureter can be noticed at the time of the injury. Any injury should be repaired immediately. An unrecognized laceration or late disruption will result in a urinoma and will require drainage and repair. These are rarely infected and usually walled off from the implant.

In the initial anterior approach to the spine, the arteries and veins are mobilized off the spine to gain direct access to the discs. Depending on the level, the vessels are dissected sufficiently to allow moving them well off the anterior spine. This initial dissection is usually without complication if done carefully, though the vessels can be torn or ligatures pulled off side branches with over vigorous retraction. If there is preexisting atherosclerosis, the mobility of the artery will be greatly diminished, and the risk of dissection or thrombosis goes way up. If an artery is hardened with plaque, it may need to be dissected out more so that it can be retracted without too much tension. Once retracted, the presence of distal flow should be checked frequently with a Doppler. In a redo procedure the risk of arterial injury goes way up secondary to the adhesions of the vessel to the spine. Again is may be impossible to see clearly the walls of the vessels. If the implant is displaced interiorly, it is imperative to know where the vessels lie in relationship to it. A preoperative angiogram should be performed to assess this. The dissection of the vessels in the face of dense scar tissue is tedious and must be done with great care. A Vascular surgeon is usually necessary for this. If a laceration occurs, the vessel needs to be repaired immediately (techniques for repair will be discussed separately). The veins present their own additional set of complications. DVT is a known complication of any spinal surgery. In a redo situation, it is necessary to know if DVT is present before surgery via Doppler or CT angio. If it is, a venacava filter should be placed preoperatively. In a preemptive effort to make dissection of the vessels off the spine easier, we have begun placing a double patch of Dacron (hemacarotid) over the disc, loosely attached above and below to the anterior ligament to separate the vessels from the implant. If we half to go back in this area, the plan will be to dissect between the two layers, safely lifting the vessels off the implant. To date, there have been no problems from the placement of this barrier, and none of these patients have required a redo operation.

Fortunately, nerve injuries related to the exposure are rare, even with redo procedures. The ilio-femoral nerve lying anteriorly on the psoas muscle can be injured or irritated causing referred pain to the lower abdomen and inguinal area. This usually resolves on its own. The sympathetic chain can be cut, causing a warm leg, this affect is usually temporary and no treatment is necessary. Damage to the parasympathetics in the dissection of the iliac vessels can result in retrograde ejaculation in males. This fortunately is a rare complication, but all males undergoing anterior spinal surgery need a very informed consent on this problem. The dissection around the iliac vessels should be kept to a minimum, and the bovey should not be used to minimize this complication.

Though our experience with redo disc replacement is limited, we have noted that the retroperitoneal scarring with these devises is quite intense, and the surgery requires careful dissection. Careful preoperative assessment and preparation will hopefully make redo surgery a safer procedure. As more discs are implanted, and subsequent redo's are performed, it will be necessary to continue close evaluation of these cases to development ongoing safe strategies for these difficult approaches.

Disarticulation

Implant-specific issues.

Pro-Disc. The poly component is seated in the lower endplate and held in position by a small bump. Attention by the surgeon is necessary to make sure that the poly insert is fully seated in the device and flush with the anterior border of the lower endplate. This requires direct visualization and palpation after placement of the device.

Inadequate posterior release will prevent proper seating of the poly as the posterior aspect of the interspace will not open up fully, allowing the poly to be seated. Too anterior position of the device may increase shear stresses on the poly and possibly cause anterior migration as well as limited motion. Care must be taken to scrutinize the position of the patient on the table. Lumbar hyperextension closes the posterior aspect of the interspace and makes it difficult to obtain adequate posterior release, and may encourage the surgeon to take off more posterior endplate than he desires.

With the FlexiCore device, the smallest size currently available is 13 mm. This may be too large for a few of the patients.

The Charite has minimal shear resistance built into its design, with the only resistance being the shallow depressions of the polyethylene disc in the upper and lower endplates. The endplates themselves have no ingrowth mechanism in the U.S. model. Shear stresses after implantation will pass through the interspace with minimal resistance from the prosthesis itself. The force will be taken up mostly by the pedicle and facet joints. If either of these fail, dislocation of the poly may occur. We know of 2 pedicle fractures with dislocations.

Generic implant issues

Prevention of dislocation/subluxation.

Proper sagittal alignment allows for the devices to function as they are intended with maximum motion. This also reduces the forces on the implant and on the posterior elements. As mentioned above, adequate posterior vertebral height is mandatory, and unless the disc height is unusually high, typically the entire posterior annulus is loosened or transected, as well as the PLL. Anesthesia muscle relaxation is necessary, especially for more heavily muscled patients. Some lumbosacral flexion facilitates posterior placement of the implant and is desirable. Magnification when removing the disc posteriorly has been very helpful in our hands.

Similarly, coronal alignment is equally important. Non-rotated flouro views must be obtained to accurately mark the midline, and the midline must be attended to throughout the procedure. The implant should be a size large enough to reach the sagittal plane of the pedicles, ideally bisecting the pedicle on each side. Having implant off-center predisposes to subarticular entrapment, lateral angulation, eccentric settling, and anterior sagittal malplacement. Attention should be made to the soft tissue release so that it is symmetric.

Sizing of the implant is also important. As mentioned, the implant should be the largest that fits within the disc. The implant should be snug enough so that there is no anterior migration in the interspace; however, if placed in too tightly, motion will be diminished.

Wear issues.

There are some recent reports of pain from metal-on-metal arthroplasty hypersensitivity reactions in total joint replacements, and this may be an occasional factor in predisposed patients. Wear issues involving the polyethylene are likely related to positioning of the device, as a malpositioned polyethylene metal or metal metal device will be expected to generate more wear particles and also suffer from more plastic deformation or creep.

Subsidence.

Subsidence can certainly destroy the biomechanical effect of the implant, cause pain, subluxation and promote autofusion. The surgeon, of course, should be aware of osteoporosis risk factors, and bone density should be performed on overweigh women, smokers, women over 55, and individuals with other risk factors. Vertebral endplate configurations in which there is prominent “fishmouthing” or concavity usually require some resection of the posterior endplate, which can be a factor in weakening the posterior vertebral body and possibly causing settling of the posterior aspect of the prosthesis. One should keep the device co-linear with the endplates as much as possible. Using the largest prosthesis the disc will accept also discourages subsidence. Especially at the L5-S1 level, flexion of the hips is helpful in keeping the lumbosacral angle less hyperextended and enabling adequate release with as little resection of the posterior endplates as possible.

Postoperative leg pain.

Postoperative leg pain is noted to be more common in previously markedly collapsed interspaces, especially those that have had previous laminotomies at the same segment. This is not surprising that some stretch injuries may occur to the nerve in this situation. In that case, there is usually pain immediately after the surgery. This usually fades away, in our experience. In three of 150 motion segments, we have noted delayed-onset radicular symptoms on standing and walking. Two of these had posterior decompressions without apparent stenosis present but with subsequent relief after the procedures. The other patient is currently being scheduled. As mentioned, we were not able to identify nerve entrapment at the time of surgery. Whether this is a dynamic factor relating to motion of the implants or an anomaly of the patient population is not clear at this point.

Strategies for revision

When faced with a failure of the implants, timing is of paramount importance in terms of the relationship to the date of the initial surgery. If it is within two to three weeks, the approach is relatively easy, as the early wound healing frequently does not prohibit going in through the same plane. From three weeks to six months is most difficult, as the tissue is friable and can be densely adherent at the same time. Retroperitoneal fibrosis around the great vessels and ureters represents a formidable technical challenge. Beyond six months it is still difficult, but not as treacherous as the intermediate period.

If there is no neural entrapment and no vascular impingement, posterior fusion in situ is a reasonable safe option, and if the timing is early, revision of the device can also be considered.

When there is neural entrapment, if this can be correctible posteriorly, this is preferable for safety reasons. Depending on the timing of the problem's occurrence, anterior revision is another option, if postoperatively early enough, and depends on the source of the neural compression.

With vascular impingement revision or removal of the implant must always be considered. If there is coronal malalignment, I personally prefer posterior reduction and subsequent anterior removal and fusion. With coronal malposition, the segment may have lost its intrinsic stability, and is therefore too unstable to revise. If there is only sagittal malalignment, then revision or fusion anteriorly done with anterior plate can be considered, or subsequent posterior pedicle fixation if a fusion is desired. Posterior fusion with subsequent anterior fusion is also a viable option.

As discussed by Dr. Bailey, if there is expulsion anteriorward, it is helpful to know as best as possible where the vein is and whether the vein is compressed. Venogram and ultrasound can be helpful in this regard.

Since we do not have good data on long-term follow-up on these cases and know some instances of expulsion and failure occur, a prophylactic approach would make sense. We use a tight-weave vascular tubular graft or a large enough piece to be folded over so that there are two layers of graft, one opposite the great vessels, one opposite the disc operative site, extending at least halfway up the vertebral bodies, cephalad and caudad. These are anchored into position to the prevertebral fascia. If revision has to occur in the future, dissection can safely be done between the layers of graft material, and the posterior layer can easily and safely be cut to expose the interspace, while the anterior vascular graft layer can be used to shield the vessels and facilitate retraction.

Susan Bailey MD Vascular Surgeon
James Zucherman Orthopedic Spine Surgeon
Zuchermanj@aol.com
St. Mary's Spine Center #450
San Francisco, CA 94117
415-750-5825
415-750-8103 (fax)