Spinal arthrodesis has long been the gold standard for surgical treatment of discogenic low back pain when conservative management has failed to alleviate symptoms. Donor-site morbidity, pseudoarthrosis, and adjacent segment disease continue to dampen the success of surgical outcomes. Total disc arthroplasty (TDA) is a motion-preserving technology that theoretically bypasses these potential complications. Despite some initial signs of success, it remains unclear whether TDA is a cost-effective, durable, and clinically superior treatment option for discogenic low back pain.
Disc arthroplasty is indicated for isolated discogenic back pain without instability.
Disc arthroplasty is contraindicated for stenosis, facet arthrosis, spondylosis/spondylolisthesis, radiculopathy secondary to herniated nucleus pulposus (HNP), scoliosis, osteoporosis, obesity, chronic steroid use, pregnancy, and previous lumbar fusion/fracture/infection.
Anterior retroperitoneal approach is utilized to gain adequate exposure.
Proper sizing and positioning of prosthesis and restoration of lordotic angle is essential during procedure.
Potential complications include loosening, device migration, polyethylene wear, osteolysis, and infection.
A bump may provide local lordosis, which will assist in discectomy and implant insertion.
During the approach, care must be taken to avoid injury to sympathetic and parasympathetic nerves.
Intact end plates are essential in obtaining good fixation and avoiding subsidence.
The surgeon must restore appropriate lordosis and end plate coverage. Spacer should fit snug but should not overdistract.
Proper position of the prosthesis should be verified in both the anteroposterior (AP) and lateral plains with intraoperative radiographs. It should be midline in the AP view and slightly posterior in the lateral view.
Implant should be impacted parallel to the end plate to avoid fracture.
Procedure does not address HNP with radicular symptoms, facet arthrosis, or central or lateral recess stenosis.
Positioning in hyperlordosis can cause tension on retroperitoneal vessels and should be avoided.
Violation of end plates while burring or curetting will increase chance of subsidence.
Disc space distraction is essential in completely excising disc. It may be necessary to release posterior longitudinal ligament and posterior osteophytes. Mobilization allows proper placement and sizing of implant. Incomplete seating of the polyethylene inlay must be avoided.
End plate fracture can be avoided by impacting implant parallel to end plate.
Postoperative radicular pain may result from component malposition or traction of the nerve root during distraction. It typically resolves in 6 months without treatment.
Ideal implant should restore but not significantly exceed normal disc height. The soft tissue should not be tensioned because this could reduce range of motion after surgery.
Symptomatic lumbar degenerative disc disease is a widespread and debilitating condition commonly treated by the spine surgeon. Arthrodesis has long been the gold standard for surgical treatment when conservative management has failed to alleviate symptoms of instability and mechanical low back pain. Because low back pain is such a difficult clinical entity to characterize and treat, even carefully selected patients do not always ensure a successful surgical outcome. Several potential complications stand in the way. Pseudoarthrosis may compromise the results of spinal fusion in the short term, requiring further surgery. In the long term, donor-site morbidity and adjacent segment disease may be the cause of failure. Although the use of biologic growth factors and the development of more rigid constructs have improved fusion rates, the spine community is still searching for a viable alternative to traditional arthrodesis. Total disc arthroplasty (TDA), developed in the 1950s and utilized since the mid-1980s in Europe, has just recently garnered attention in the United States. TDA is a motion-preservation technology, which possesses several theoretical advantages. By eliminating the need for fusion, pseudoarthrosis is no longer a potential complication. In addition, donor-site morbidity is removed. Finally, because functional motion is restored, this procedure should theoretically avoid future adjacent-level breakdown. Short-term studies comparing TDA with arthrodesis have shown generally equivalent outcomes, with authors suggesting TDA is a reasonable alternative to treatment of discogenic low back pain. Analysis of literature suggests that a stronger correlation exists between fusion and adjacent segment disease compared with TDA. Despite some initial signs of success, long-term studies are still needed. In addition, with the advent of new technology comes new potential complications, and TDA is not without its problems. For one, the learning curve is steep; hospitals and surgeons with greater volume tend to have better surgical outcomes. Potential problems with the procedure itself include subsidence, polyethylene wear, loosening, osteolysis, component migration, and infection, among others. The cost-effectiveness of these implants is also an issue that needs to be addressed. Nevertheless, with recent design improvements and careful patient selection, TDA remains a potential surgical alternative for the treatment of discogenic low back pain.
INDICATIONS AND CONTRAINDICATIONS
The success of TDA is highly dependent on strict adherence to patient selection criteria. In the United States, lumbar TDA is approved for treatment of isolated discogenic back pain without instability. Objective evidence of disease should be displayed on computed tomography or magnetic resonance imaging. Provocative discography is helpful for confirmation and to verify that adjacent segments are normal. Surgery should be reserved for one-level disease in a patient aged 18 to 60 years who has not responded positively to at least 6 months of conservative therapy. Because of the fact that all other potential pain sources must be ruled out, the list of contraindications is long. Exclusion criteria include central or lateral recess stenosis, or both; facet arthropathy; spondylosis or spondylolisthesis; radiculopathy secondary to herniated nucleus pulposus; scoliosis; osteoporosis; chronic steroid use; previous lumbar fusion, fracture, or infection; obesity; and pregnancy.
SURGICAL TECHNIQUE: PEARLS AND PITFALLS
Although the most important aspect to a successful lumbar TDA is strict criteria for patient selection, the first step for a skillfully performed lumbar TDA is patient positioning. The positioning is akin to that for an anterior lumbar interbody fusion, straight supine, with a few extra key steps. The patient is positioned onto a radiolucent table to ensure device placement. The fluoroscopy screen is usually placed at the foot of the operating table. Arms are placed away from the patient’s body on arm boards. If a conventional left-sided retroperitoneal approach is planned, a pulse oximeter is placed onto the left great toe to monitor blood flow during great vessel retraction. A well-padded bump under the sacrum tends to reduce lumbar lordosis and bring the L5-S1 disc space into a more accessible vertical orientation. A sacral bump can also limit the tendency for the disc spaces to excessively open anteriorly and not allow the implant to seat properly. One must be careful to keep the bump under the sacrum, because a common error is to place it under the lumbar spine and accentuate lordosis.
Surgical access to the anterior lumbar spine was first described by Ito et al. in the 1930s for the treatment of tubercular infections. Since then, both retroperitoneal and transperitoneal approaches with a variety of incisions have been described. The transperitoneal approach has largely been abandoned for a primary approach because of the violation of the peritoneal cavity and high incidence of retrograde ejaculation in male patients, but it remains a viable option in revision settings. Lumbar TDAs are approved for single-level implantation from L3 to S1; therefore, surgical planning and approaches can be divided according to level treated, L3 through L5 and L5-S1. The L3-4 and L4-5 disc spaces are accessed lateral to the descending aorta and inferior vena cava, whereas the L5-S1 disc space is accessed below the bifurcation of the great vessels into the left and right common iliac vessels.
Midlumbar Approach (L3-4 and L4-5)
The midlumbar spine can be approached through either a midline rectus (MR) or a paramedian lateral rectus (PLR) approach. Bendo et al. described the three major differences in the approaches, and evaluated quality and consistency of device positioning with either approach. The MR approach uses a vertical midline incision, whereas the PLR approach uses a left-sided longitudinal incision based over the lateral border of the left rectus muscle. The MR approach longitudinally divides the rectus fascia down the midline linea alba, and the PLR approach incises the external oblique fascia along the lateral margin of the left rectus abdominus muscle. The preperitoneal space is reached by retracting both rectus muscles laterally in the MR approach and by retracting the left rectus muscle medially in the PLR approach. Once in the preperitoneal space, hand and blunt dissection is used working to the patient’s left to bring the peritoneal envelope medially. The ipsilateral ureter, distinguished by its characteristic peristalsis, should be identified, protected, and brought medially with the peritoneum. Entering into the retroperitoneum from the left side, the more anterior and left-sided aorta and left common iliac artery is first encountered. The aorta and inferior vena cava are retracted to the right to expose the midlumbar discs. The mobility of the great vessels is often restricted by segmental arteries and the ascending iliolumbar vein. A circumferential, articulated retractor system is commonly used to facilitate working access to the disc and to protect vital structures. These should be clipped and divided. Bendo et al. report improved implant placement with the MR approach and have subsequently abandoned the PLR approach.
Lumbosacral Approach (L5-S1)
Access to the L5-S1 disc space can be achieved with either a midline, infraumbilical incision or low transverse (Pfannenstiel) incision. Subcutaneous dissection is carried down to the external rectus fascia; if a midline incision was used, the fascia is divided at the aponeurotic linea alba and the rectus muscles are retracted laterally to expose the preperitoneal fat and peritoneum. If a low transverse incision is used, the rectus fascia also is divided transversely and the exposed rectus muscles are retracted laterally. The peritoneum and ureter are retracted medially over the left and right common iliac vessels. The middle sacral artery is then clipped and divided, which exposes the anterior surface of the L5-S1 disc. The overlying sympathetics are carefully cleared using a Kittner dissector to limit risk for retrograde ejaculation.
Surgical Strategies for Revision Anterior Surgery
If a lumbar TDA has failed, the first choice is often to perform a posterolateral fusion to obviate the need for a revision anterior exposure. However, there are incidences when this is necessary, such as a migrated device threatening or compressing the great vessel, infection, or periprosthetic fracture. If a revision anterior approach is necessary, most recommend preoperative vessel imaging and placement of ureteral stent for intraoperative palpation. If a left-sided retroperitoneal approach was used in the index procedure, a right-sided approach can be used. A transperitoneal approach can also be used. Once the back of the peritoneum is reached, it must be excised over the L5-S1 disc for access.
Discectomy and Disc Space Preparation
Careful bunt dissection underneath the great vessel is often required to safely retract them more laterally. Any tethering branches, such as spinal segmental or ascending iliolumbar vein, should by clipped or tied and divided. Excessive traction can result in vessel avulsion and result in massive hemorrhage that can be difficult to control and repair. After the disc is clearly exposed, a true anteroposterior (AP) fluoroscopy shot is obtained and the midline marked. An annulotomy is created and a thorough discectomy performed making sure not to violate the vertebral end plates. Under fluoroscopic control, a lamina-type spreader is positioned on the posterior margins of the vertebral bodies to gradually mobilize the segment. Avoid positioning the spreader at mid-disc to minimize risk for end plate fracture. Sequential resection of the posterior annulus and posterior longitudinal ligament is performed as needed to achieve middle column mobilization and parallel disc space distraction. Both Food and Drug Administration (FDA)–approved devices stress importance of end plate integrity but do allow for minimal shaping at the margins to remove posterior osteophytes to achieve parallelism.
Sequential posterior release leads to parallel distraction. Much like hip and knee arthroplasty, the implant should reconstruct native anatomy. Although the Charité (DePuy Spine, Raynham, MA) and ProDisc-L (Synthes, West Chester, PA) have design differences, they share this restorative goal. The implant trial footprint should be selected to maximize coverage of the vertebral bodies; its height and lordosis are commonly based of the adjacent, nondiseased disc. On a lateral fluoroscopic view, the center of rotation of the device should be at to 2 mm posterior to the midvertebrae. The ProDisc-L used a midline keel cut into the superior and inferior end plates based on the trial ( Fig. 47-1 ). The Charité Disc uses teeth to grip the end plate ( Fig. 47-2 ). For both systems, the end plates are inserted first, then distracted, and a polyethylene core is slid in between. The polyethylene is captured when distraction is released. Fluoroscopy ensures implant placement and restoration of disc height and lordosis ( Fig. 47-3 ). Rundell et al. have shown through finite element modeling that appropriate posterior positioning of the TDA results in an unloading of the facets during extension and more physiologic load transfer to the vertebral bodies.