Introduction
The traditional answer for a painful deteriorated joint in orthopaedics has historically been a fusion procedure. By immobilizing a joint and obliterating it, the pain from abnormal motion is eliminated. Sometimes nature accomplishes this at the end stage of degenerative joint disease (DJD), which ends up in the ankylosed painless joint. The rationale of doing a fusion has been to speed up the natural process of ankylosis. The price is great however as the motion of the fused joint is lost which overloads the adjacent joints as well as diminishes mobility. We rarely see hip and knee fusions now because we have discovered other ways to stabilize joints and reduce the amount of inflammatory tissue. In the hip and knee the answer to end-stage arthritis has been total and partial joint replacements for the last several decades. These entail much quicker healing time than fusions, restoration of approximate joint function and removal of deteriorated joint tissues which are a source of pain and inflammation. Both total hip replacement (THR) and total knee replacement (TKR) are remarkably common and represent some of the most successful procedures in orthopaedics. It has been appropriate to apply some of the principles of THR and TKR, including the biomaterials, concepts of constraint and modes of fixation to the spine. This heralded the new era of spinal arthroplasty and motion preservation, which started in Europe and subsequently spread all over the world. In United States spine surgery in recent years, we have entered new vistas of technology that will hopefully be advantageous to fusions in patient morbidity, safety and efficacy. The motion-preserving devices that are already in clinical use include total disc replacements, both cervical and lumbar, the X STOP device, the Dynesis device and some others.

Lumbar Spine Fusion Alternatives
The main indications for a fusion in the adult lumbar spine include degenerative instability (degenerative spondylolysthesis or multiple (2 or more) recurrences of herniated nucleus pulposus at the same level), acute traumatic instability (2 or 3 column- injury), instability from chronic trauma (isthmic spondylolysthesis), developmental (dysplastic spondylolysthesis), infectious and neoplastic processes, as well as symptomatic coronal (scoliosis) and saggittal (kyphosis) plane deformity. Severely symptomatic one- or two-level disk degeneration (discogenic low back pain) is also generally considered to be an indication for a fusion in the absence of psychiatric co-morbidities (e.g. somatization and depression) and secondary gain issues, but still remains a somewhat controversial issue mostly due to the lack of accurate diagnostic testing. The technologies discussed below are mostly applicable to the adult degenerative disk disease and spinal stenosis with or without spondylolysthesis, which do represent a substantial majority of cases encountered in the adult spine surgery practice.

1. Total Disk Replacement
The natural extrapolation of the total hip and knee replacement to the spine has been development of total disk replacement (TDR). The main rationale for using TDR’s is not as much preservation of motion as avoidance of adjacent level degeneration, which can lead to additional surgery in up to 3% per year of patients undergoing fusion. It is still controversial whether this number represents a truly increased breakdown at the adjacent levels or just a natural history of spine DJD. Currently there are 4 main TDR systems in the clinical use in the U.S.: Charite III (DePuy, FDA-approved), ProDisk II (Synthes, Figure 1, FDA-approval expected this year), Maverick (Sofamor Danek, investigational) and Flexicore (Stryker Spine, Figure 2, investigational). TDR’s are not created equal and have some dramatic design differences, which are likely to have clinical consequences. With regard to constraint (limitation of motion), TDR’s can be unconstrained (Charite), semi-constrained (ProDisc and Maverick) and constrained (Flexicore). The early concerns with unconstrained TDR include device dislodgement, which has already been reported. The concerns with semi-constrained devices include overloading the posterior elements, causing fractures and accelerated facet arthrosis. With regard to bearing surfaces, the TDR’s can be divided onto polyethylene-on-metal (Charite and ProDisc) and metal-on-metal (Maverick and Flexicore) articulations. The concerns with the former include polyethylene-wear debris and aseptic loosening. The concerns with the latter include some rare cases of metal allergy.

Figure 2.

The lumbar total disk replacement has been performed in Europe for more than 20 years for a variety of indications. In general, there is a lack of level I and II data summarizing that experience. Recently, however, there has been a requirement for FDA mandated rigorous U.S. studies regarding those prostheses.

Lemaire et al. in the most favorable of the several clinical series on the Charite device have reported clinical and radiological outcomes for the artificial disc in 107 patients at minimum follow-up of 10 years.1 A total of 147 prostheses were implanted with 54 one-level and 45 two-level procedures and 1 three-level procedure. The prostheses were placed through a standard anterior retroperitoneal approach. Clinically, 62% had an excellent outcome, 28% had a good outcome, and 10% had a poor outcome. Of the 95 eligible to return to work, 88 (91.5%) either returned to the same job as prior to surgery or a different job. Mean flexion/extension motion was 10 degrees for all levels. 5 patients required a secondary posterior fusion.

Guyer et al. in the U.S. FDA study investigated whether the patients with symptomatic degenerative disc disease treated with Charite artificial disc (DePuy Spine, Raynham, MA) arthroplasty would show significant improvement in functional outcome measures and to compare these results to BAK cage fusions.2 They reported on a consecutive series of 144 patients randomized using a 2:1 ratio of Charite versus BAK (Zimmer Spine, Minneapolis, MN). All patients were being treated for single-level discogenic pain confirmed by plain radiography, MRI and provocative discography. The mean Oswestry Disability Index score for the BAK group was 69 preoperatively and 27 at 24-month follow-up (p<.001). The corresponding mean Oswestry score for the Charite disc patients was 71 preoperatively and 30 at 24-month follow-up (p<.001). The authors have concluded that total disc replacement appears to be a viable alternative to fusion for the treatment of single-level symptomatic disc degeneration unresponsive to nonoperative management. Results from other FDA pivotal trials on the newer artificial discs will be forthcoming.

In summary, lumbar TDR is a new and promising surgical technique. More studies are needed to clarify the optimal type of constraint in those devices as well as the best biomaterials to be used.

2. Interspinous Process Decompression (IPD)
Spacers placed between the lumbar spinous processes represent a promising surgical treatment alternative for a variety of spinal pathologies. Intuitively they provide a flexion-distraction force and have a potential to relieve the symptoms of neurogenic intermittent claudication (NIC), associated with spinal stenosis. The first IPD device to be used in the U.S. is the X STOP device, which was FDA-approved for the treatment of patients with spinal stenosis in November. The X STOP (Figures 3-4, St. Francis Medical Technologies Inc., Alameda, CA) was developed to treat NIC from spinal stenosis with minimal morbidity and intervention. The ideal patient for the X STOP implantation has predominantly lower extremity complaints with or without back symptoms secondary to lumbar spinal stenosis at one or two levels. The clinical diagnosis of spinal stenosis should be confirmed with either MRI or CT myelogram. The symptoms should be relieved with flexion or sitting.

Figures 3-4.

The X STOP fills the large void of treatment options between the safer, yet less effective conservative care, and the riskier, but more effective surgical decompression with or without fusion. The X STOP limits terminal extension movement at only the individual level(s) that provokes symptoms, while allowing unrestricted movement of the remaining motion axes of the treated level(s) and the untreated levels. Biomechanical studies have shown that the X STOP significantly increases the spinal canal, subarticular recess and neuroforaminal size; limits terminal extension; and reduces intradiscal pressure and facet loading.

Zucherman et al. have demonstrated that IPD with the X STOP is superior to non-operative therapy in patients with neurogenic intermittent claudication secondary to spinal stenosis in the multi-center randomized study at 1 and 2 years post-operatively. 3-4 It was the first study to provide the level 1 data for surgical and non-surgical treatment of spinal stenosis. At 2-year follow-up, 57% of the patients reported a clinically significant improvement in their physical function compared to 15% of the control based on the Zurich Claudication Questionnaire, a validated outcome tool for neurogenic claudication. 73% of the X STOP patients were at least somewhat satisfied compared to 36% of the non surgically treated patients. At all follow-up time points, the X STOP group scored significantly better than the control group in every physical domain. Kondrashov et al. have reported on 18 X STOP patients at the average 4.2-year follow-up.5 Using a 15-point improvement from baseline ODI score as a success criterion, 14/18 patients (78%) had successful outcomes at follow-up, demonstrating that intermediate-term clinical outcomes of X STOP surgery are stable over time.

Hannibal et al. have compared the hospital costs of IPD with X STOP device to those of laminectomy for the treatment of lumbar spinal stenosis (LSS).6 29 patients with LSS treated surgically were matched for age, length of follow-up and preoperative Oswestry scores. 18/29 had X STOP implantation and 11/29 had laminectomy. The average follow-up was 51 months in both groups. The Oswestry improvement was 29 points in the X STOP group and 10 in the laminectomy group. Average hospital costs for 1 level X STOP and laminectomy group were $17,059 and $45,302 respectively. Average hospital costs for 2 level X STOP and laminectomy groups were $24,353 and $45,739 respectively. The main savings in the X STOP group were in OR costs (shorter operative time), hospital charges (X STOP is an outpatient procedure) and anesthesia charges (X STOP is placed under local anesthesia).

In summary, interspinous process decompression with the X STOP device is a new effective treatment option for surgical treatment of lumbar spinal stenosis with or without degenerative spondylolysthesis. The existing level I data suggest that it is at least as effective as lumbar decompressive surgery at 2-4 year follow-up and offers significant savings in direct hospital costs over standard laminectomy. Its main advantages include avoiding the general anesthesia in the elderly patients, avoiding the iatrogenic nerve root injury, dural tears and epidural scarring, and other risks associated with laminectomy. It also obviates the need for a fusion in a subset of patients with degenerative instability. The current research is focusing on using X STOP for the discogenic low back pain and other as well as modifying it for the cervical spine.

3. Dynamic Stabilization: The Dynesis system
Whenever pathological motion from either trauma (fracture) or arthritis (degenerative instability) has to be obliterated, one can use either external stabilization (i.e. a cast or a brace) or internal fixation (i.e. rods, screws, plates, pins, etc.). Historically, whenever internal stabilization/fixation have been utilized, the orthopaedists have also usually performed fusions, since any metallic hardware has a limited number of cycles before fatigue failure or hardware loosening in the bone ensues. Therefore any fusion has always been a race between a solid bony union and a failure of the metallic fixation. A new concept has been introduced into the spine surgery about a decade ago: stabilization without a fusion or dynamic stabilization, best exemplified by Dynesys system (Zimmer Spine, Minneapolis, MN). The Dynesys system (Figure 5) is a pedicle screw-based system with a polyethylene cord and a polyurethane spacer connecting the screws instead of the conventional metal rods. The cord and the spacer do permit some motion (hence the term "dynamic") but with certain restrictions (hence "stabilization"). With the Dynesys system, no bone grafting is necessary, therefore, donor site morbidity can be avoided. The procedure can be revised to a fusion by changing the cords to rods and adding some bone graft as long as the screw to bone fixation holds.

Figure 5.

The European experience with Dynesys has been marked by initial great enthusiasm, followed by some skepticism, once the intermediate-term data became available. Schnake et al. have reported on the German experience in 26 patients (mean age 71 years) with lumbar spinal stenosis and degenerative spondylolisthesis who underwent lumbar decompression and dynamic stabilization with the Dynesys system at a minimum follow-up of 2 years.7 Mean leg pain decreased significantly (P < 0.01), and mean walking distance improved significantly to more than 1000 m (P < 0.01). There were 5 patients (21%) who still had some claudication. A total of 21 patients (87.5%) would undergo the same procedure again. The implant failure rate was 17%, and none of them were clinically symptomatic. The authors have concluded that in elderly patients with spinal stenosis with degenerative spondylolisthesis, dynamic stabilization with the Dynesys system in addition to decompression leads to similar clinical results as seen in established protocols using decompression and fusion with pedicle screws.

Schwarzenbach et al. have cautioned, however, that a dynamic stabilization device has to provide stability throughout its lifetime, unless it activates or allows reparative processes with a reversal of the degenerative changes.8 They emphasized that anchorage to the bone is crucial, at least for pedicular systems. This is a great demand on spinal implants and assumes rest and motion going together. Their Swiss experience has shown that Dynesys has limitations in elderly patients with osteoporotic bone or in patients with a severe segmental macro-instability combined with degenerative olisthesis and advanced disc degeneration. Such cases have an increased risk of failure. The authors have called for controlled prospective randomized studies to prove the safety, efficacy, appropriateness, and economic viability of dynamic stabilization.

In summary, the dynamic stabilization of the lumbar spine with Dynesys and similar concepts may be a promising alternative to a fusion with some reservations about the longevity and loosening of the screws. Level 1 data is still lacking for this device to support the indications for its use, which currently include degenerative spondylolysthesis and as an adjunct to a diskectomy.

Summary
In summary, recently there has been an explosion of new motion-preserving techniques and devices available in lumbar spine surgery. They pose a tempting alternative to spinal fusion. One has to be careful interpreting the clinical data and study design and pay particular attention to the definition of success, quoted in the studies.

References

1. Lemaire JP, Carrier H, Sariali el-H, Skalli W, Lavaste F. Clinical and radiological outcomes with the Charite artificial disc: a 10-year minimum follow-up. J Spinal Disord Tech. 2005 Aug;18(4):353-9.
2. Guyer RD, McAfee PC, Hochschuler SH, Blumenthal SL, Fedder IL, Ohnmeiss DD, Cunningham BW. Prospective randomized study of the Charite artificial disc: data from two investigational centers. Spine J. 2004 Nov-Dec;4(6 Suppl):252S-259S.
3. Zucherman JF et al. A multi-center, prospective, randomized trial evaluating the X STOP interspinous process decompression system for the treatment of neurogenic intermittent claudication. Spine 2005;30:1351-1358.
4. Zucherman JF et al. A prospective randomized multi-center study for the treatment of lumbar spinal stenosis with the X STOP interspinous implant: 1-year results. Eur Spine J. 2004 Feb;13(1):22-3.
5. Kondrashov D, Hannibal M, Hsu K, Zucherman J. Interspinous Process Decompression with the X STOP Device for Lumbar Spinal Stenosis: a 4-year follow-up study. Accepted for publication. J Spinal Disord Tech. 2006.
6. Hannibal M, Kondrashov D, Hsu K, Zucherman J. X STOP Interspinous Process Decompression Versus Laminectomy for Treatment of Lumbar Spinal Stenosis: Economic Analysis. Poster presented at the annual meeting of the International Society for the Study of Lumbar Spine, Bergen, Norway, 2006.
7. Schnake KJ, Schaeren S, Jeanneret B. Dynamic stabilization in addition to decompression for lumbar spinal stenosis with degenerative spondylolisthesis. Spine. 2006 Feb 15;31(4):442-9.
8. Schwarzenbach O, Berlemann U, Stoll TM, Dubois G. Posterior dynamic stabilization systems: DYNESYS. Orthop Clin North Am. 2005 Jul;36(3):363-72.