Lower back pain is one of the most common ailments faced by the general adult population at a prevalence rate of 84%.1 Up to half of these cases may be attributable to degenerative disc disease (DDD).2 For those that fail conservative measures, surgical options include discectomy, fusion procedures, and disc replacement. Lumbar disc replacement (LDR) or arthroplasty involves removal of the diseased disc and replacement with an artificial mechanical one developed with hopes to avoid undesirable consequences of fusion such as immobility and increased risk of adjacent segment degeneration (ASDeg). In this review, we go over the history of lumbar arthroplasty, available devices, and their associated clinical trials, its indications, biomechanics, complications, and future directions.
HISTORY OF LUMBAR ARTHROPLASTY
The elimination of motion through a fusion procedure has been the primary treatment for degenerative spine disease since the early 20th century. The origins of LDR started in the 1960s with Fernström3 who implanted a stainless-steel ball within 191 lumbar and 13 cervical discs spaces of 125 patients with clinical outcomes similar to fusion. However, the ball was associated with significant complications caused by subsidence and extrusions. A couple of decades later, in the early 1980s, at Charité Hospital, Dr Karin Buettner-Janz, an orthopedic spine surgeon as well as former Olympic gymnast, and Kurt Schellnack, an engineer, published their first experience with the original Charité artificial disc for the lumbar spine, which ushered in the modern era for lumbar arthroplasty.4 The device went through revisions over the next 6 yr, resulting in the SB Charité III, and the first clinical experience was published in 1994 using the final version of the SB Charité III (DePuy Spine Inc, Raynham, Massachusetts).5 The clinical trial in the United States for Food and Drug Administration (FDA) approval began in 2000, and the device was cleared for use in 2004. Since then, multiple other lumbar arthroplasty devices have been developed and have become available in the United States and Europe. Following Charité’s United States approval in 2004, the second generation of artificial disc design, ProDisc-L (Centinel Spine, West Chester, Pennsylvania), was granted FDA approval in 2006, followed by a third-generation artificial disc design, activL (Aesculap Implant Systems, Center Valley, Pennsylvania) in 2015. In addition to these devices, Acroflex (Acromed Corporation, Cleveland, Ohio), the Maverick (Medtronic, Dublin, Ireland), Kineflex (Spinal Motion, Mountainview, California), FlexiCore (Stryker, Kalamazoo, Michigan), LP-ESP (FH Orthopedics, Heimsbrunn, France), and M6-L (Orthofix, Lewisville, Texas) lumbar discs have either completed their trials, are actively ongoing or have been discontinued or withdrawn without FDA approval. All of these devices are available outside of the United States. Additionally, XL TDR (NuVasive, San Diego, California) and Triumph (Globus Medical, Audubon, Pennsylvania), which offered insertion techniques via lateral and posterior approaches, respectively, are only investigational at this point or available outside the United States. A timeline of key events in the history of lumbar arthroplasty is shown in Figure 1.
There had been a rise in disc arthroplasty in the years following FDA approval of the Charité, with the largest rise in the first year from 2004 to 2005. However, enthusiasm for arthroplasty was tempered the following decade. Early problems with implantation of the Charité artificial discs by the general spine surgeon community resulted in a number of reported serious adverse events to the FDA and numerous litigation cases. Also, the Centers for Medicare and Medicaid Services issued a National Coverage Determination in 2006, which negatively impacted people's access to the procedure for those who were over the age of 60. As a result, the number of United States lumbar arthroplasty procedures dropped from 3650 in 2005 to 1863 in 2010, whereas revision procedures increased to 499 from 420 during the same time interval.6 Additionally, improvements in pain management, fusion techniques, and less-invasive methods also contributed to reductions in arthroplasty.
In 2011, Johnson & Johnson acquired Synthes, which included the ProDisc-L, and by 2012, they stopped worldwide sales of the Charité artificial disc. Although ProDisc-L was still available, surgeon interest in arthroplasty was poor following the Charité experience. A slow, but steady, increase in lumbar arthroplasty interest and utilization has occurred following the introduction of the third-generation device, activL, in 2015. Currently, 65% of insurance providers now cover single-level lumbar arthroplasty compared to 25% in 2015 and 13% in 2012. All current studies have generated a large body of evidence on the safety and efficacy of arthroplasty and have demonstrated overall noninferiority to fusion. Spinal surgeons should be aware that arthroplasty is a very acceptable alternative within the surgical armamentarium and should not shy away from utilizing arthroplasty when indicated.
LUMBAR DISC REPLACEMENT DEVICES
LDRs are required by the FDA to complete randomized, controlled studies known as Investigational Device Exemption (IDE) trials prior to market approval in the United States. Thus, LDRs have undergone step-wise clinical evaluation. There are now prospective randomized trials and long follow-up studies, which have produced level 1a evidence for both fusion and arthroplasty in the treatment of DDD.8 Table 1 provides a summary of the different LDRs.
TABLE 1. -
Summary of Artificial Lumbar Disc Implants
||IDE trial or largest study
||Blumenthal et al10 2005
||McAfee et al11 2005
||Zigler et al21 2007
||Garcia et al31 2015
||Sasso et al38 2008
||Guyer et al36 2014
||Elastic core, Constrained
||Gornet et al35 2011
||IDE trial complete, OUS only
||One piece, laterally placed
||Tohmeh et al39 2015
||One piece, elastomeric
||Pimenta et al40 2010 (Brazil)
||No IDE trial, OUS only
||One piece, elastomeric
||Schätz et al42 2014 (Germany)
||No IDE trial, OUS only
||One piece, elastomeric
||Rischke et al43 2015 (Switzerland)
||IDE trial incomplete
||One piece, elastomeric
||Lazennec et al41 2018 (France)
||No IDE trial, OUS only
MoP: metal-on-polymer; MoM: metal-on-metal.
Charité Artificial Disc
The Charité artificial disc (Figure 2A) endplates are made of a cobalt-chrome-molybdenum alloy with a convexity to better match the concavity of the vertebral endplates. The unconstrained center is made of an ultra-high molecular weight polyethylene sliding core that articulates between these endplates. Endplate fixation is obtained with 3 fixation teeth anteriorly and 3 posteriorly, each measuring 2.5 mm in height.
The Charité artificial disc went through transformations in the 1980s, which led to the SB LINK Charité III, the first modern LDR device to be widely used and FDA approved.9 The prospective randomized, controlled IDE trial for Charité III artificial disc was initiated in 2000 and completed in 2003. The 2-yr study included 205 arthroplasty and 99 anterior lumbar interbody fusion (ALIF) with BAK cage and autograft patients and was presented in 2 manuscripts: one focused on clinical outcomes10 and the other on radiographic outcomes.11 Short-term papers presented early analyses from single sites,12,13 but in the completed trial at 2 yr, authors reported no device-related complications and a reoperation rate of 5.4% (vs 9.1% in the control arm). There was a 48.5% reduction in Oswestry Disability Index (ODI) vs 42.4% in the control group and a 40.6-point reduction in Visual Analogue Score (VAS) vs 34.1 reduction in the control group. Clinically, mean ODI and VAS scores were significantly lower in the Charité group at all time-points, except at 24 mo. Hospital stay was also significantly shorter as compared to the fusion group.10,11
The FDA IDE trial follow-up period was extended and the Charité 5-yr ODI, and VAS results were similar to the 2-yr follow-up results.14,15 The 5-yr outcomes of the study were limited by the low follow-up rates (43%). In a separate study, a subgroup analysis was performed of patients having prior surgery or discectomy and then being part of the trial with a trend toward lower clinical outcomes in the fusion group compared with arthroplasty.14 Despite the promising prospective results, critics formulated objections to these results.16 Some have criticized the study because the fusion group had a high failure rate of a procedure that had for the most part been abandoned, and because clinical success was indicated by a modest 25% improvement in the ODI score, no device failure, no major complication, and no neurological deterioration. A total of 57% of the LDR group met these criteria vs 46% for the fusion group, and a significant percentage of the patients were still using narcotics at the end of the 2-yr follow-up.17
Lemaire18 reported 10-yr follow-up results in 100 patients. A total of 54 of these patients were operated on at 1 level, 45 at 2 levels, and 1 patient operated on at 3 levels. Overall, the authors reported excellent or good clinical outcomes in 90% of follow-up cases. In a second long-term study, David19 presented 10-yr data on 106 patients with 1-level surgeries. Excellent or good clinical outcome were obtained in 82.1% patients. Both papers concluded that arthroplasty was a viable long-term option for DDD.
The initial ProDisc lumbar artificial disc was developed by Thierry Marnay20 in 1989, and was used clinically in the early 1990s. Subsequently, the second generation, ProDisc II (Figure 2B) was developed in 1999 with cobalt chrome endplates and constrained polyethylene core and approved for commercial use in Europe the same year. Two years after publication of the Charité IDE trial in the United States, in 2007, the results of the ProDisc IDE trial comparing LDR to a circumferential fusion as a control were published.21 Short-term studies disclosed early findings from single sites involved in the trial.22-26
The completed trial at 2-yr follow-up included 161 arthroplasty and 75 fusion patients.21 Complication rate was noted to be at 9% at 8.7 yr follow-up, and there was a reoperation rate of 3.7%.27 Overall success in the IDE trial was defined as a 15-point improvement in ODI score, no revision, improvement in Short Form 36 score, absence of neurological events, and radiographic success (no migration, subsidence, radiolucency, or loss of disk height, and maintenance of range of motion [ROM]). Trial data reported improvements in the LDR group in VAS for pain by an average of 39 points and improvement by 28 points in ODI. It is worth noting, however, that the ODI tool used in this trial was not the validated ODI.28 Additionally, there was a clear difference in the success rate between the sponsor criteria and the FDA criteria. Using the sponsor's criteria, 63.5% of the LDR patients vs 45.1% of the control group achieved success.21 The results of this study were maintained after 5 yr without significant clinical advantage between LDR and fusion; ROM was decreased by 0.5 degrees and radiographic analysis showed a significantly lower rate of adjacent-level degenerative changes (9% in the LDR group and 28% in the fusion group). Reoperation rates were not significantly different at the index level (8% for LDR and 12% for fusion) or adjacent levels (1.9% for LDR and 4% for fusion).21 Intraoperative times and estimated blood loss were significantly less in the LDR group. The 5-yr follow-up results presented consistent outcomes in terms of ODI score, maintenance of pain control, and quality of life 5 yr after index surgery.29
The most recently FDA approved artificial lumbar disc is the activL (Figure 2C), which was approved in 2015, although it has been available in Europe since 2005.30 It is comprised of an inferior and superior cobalt-chromium plate anchored in the endplate and an ultra-high molecular weight polyethylene inlay that may translate 2 mm in the anterior-posterior direction on the inferior endplate. This third-generation device is semiconstrained with a controlled mobile core. Endplates feature a spike or central keel, based on surgeon choice and patient anatomical characteristics, to anchor the device to the vertebral bodies.
The activL IDE trial compared it to the other FDA-approved artificial discs, Charité and ProDisc-L, becoming the first arthroplasty device to prospectively be compared to other arthroplasty devices in a randomized trial. A total of 218 patients were randomized to the activL group and 106 to other artificial discs. activL patients had higher rates of success and ODI scores. Return to work was 1 mo earlier in the activL group and adverse events were less common.31 The majority of activL implants (90%) chosen in the trial had a height of 8.5 mm, whereas the smallest ProDisc-L core is 10 mm. Preliminary results from a 5-yr follow-up show persistent positive results and safety and efficacy compared to the other 2 artificial discs.32 Freedom from reoperation for adjacent segment disease (ASDis) was 99% at 5 yr for activL and an explant rate of 0.04% based on the ongoing FDA mandated enhanced safety surveillance study. Since Charité was discontinued in 2012, the activL remains the only other FDA-approved artificial lumbar disc in addition to ProDisc-L. Most recently, a network meta-analysis of data from comparisons of multiple device implants, fusion, and conservative treatment showed that activL had the highest probability for being the best treatment option in DDD as measured by improvements in pain, disability, and quality of life.33
The Maverick (Figure 2D) device is a two-piece metal-on-metal (cobalt-chromium) semiconstrained ball and socket lumbar prosthesis. When first commercialized in the early 2000s, a 2-yr prospective study with significant results from France was published in 2005. It was based on 64 patients from one institution. The efficacy of arthroplasty was demonstrated as ODI scores decreased by an average of 20.7 points and VAS scores by 4.4 points.34 Maverick was studied in an IDE trial published in 2011 comparing it to stand alone ALIF. Success at 2 yr was noted at 73.5% for the Maverick group and 55.3% for the fusion group.35
Kineflex (Figure 2E) is another artificial lumbar disc designed in 2004. Two-year results of their prospective, randomized controlled IDE trial were published in 2014 comparing it to another artificial disc, Charité artificial disc.36 The trial demonstrated noninferiority of Kineflex compared to Charité and these results were maintained in the 5-yr follow-up study.37 Patients in both the Charité and Kineflex group experienced significant clinical improvements measured by ODI and VAS scores. The design, unlike the Charité disc, is a semiconstrained device. Like the Maverick, it is also a metal-on-metal design. Therefore, patients were also tested for serum iron levels because of concern from potential wear and tear from a metal-on-metal device.
Flexicore developed as another metal-on-metal intervertebral disc composed of a retained ball and socket device positioned between 2 base plates. Only 1 paper has been published.38 The study describes the clinical outcomes of 44 patients, of whom only 6 were available for 2-yr follow-up. Operative time and length of stay were decreased. Further follow-up was not published and the implant was discontinued.
XL TDR is an LDR implant that allows for placement via lateral approach. It is only available outside the United States. Its IDE trial showed good outcomes with low complications in 64 patients with 3-yr follow-up; however, it was not compared to any other device or procedure and the device never obtained FDA approval.39 Other devices such as the Physio-L, M6-L, Freedom Disc (Figure 2F), and LP-ESP are one-piece elastomeric devices that were developed in the latter half of the 2000s and show studies with satisfactory results, but none of which have completed IDE trials or are approved for use within the United States.40-43
One of the most critical factors of success for arthroplasty is patient selection. Even precise placement of the implant and excellent surgical technique will not overcome poor patient selection for the procedure.
Indications and Contraindications
The ideal candidate for LDR is generally early on the Kirkaldy–Willis degenerative timeline than a typical patient for fusion. During the first Lumbar Total Disc Replacement Summit, a list of indications for disc replacement was outlined including the following: failure of 6 mo of conservative therapy, symptomatic DDD at one level (L3-S1), skeletally mature patient, and no more than a grade I spondylolisthesis.44 Several studies have broadened the indications to include patients with prior surgery, such as microdiscectomy,45 prior fusion with ASD, and disc replacement below a previous long-segment fusion for scoliosis.46
Contraindications to lumbar arthroplasty should also be considered. The presence of spondylolysis, spondylolisthesis, spinal fracture, posterior element disease such as a significant facet joint arthropathy or previous facet joint removal, central or lateral recess stenosis, extruded nucleus pulposus with a radiculopathy, a nonmobile segment, osteoporosis, metal allergy (for metal-on-metal implants), or infection should be considered contraindications to an arthroplasty. Additionally, issues such as obesity and psychosocial conditions should be addressed.47
Siepe et al45 suggested that the best results are obtained in patients who are younger than 40 yr of age, with single-level disc degeneration. Sott and Harrison48 challenged such statements, showing that age has no significant influence on the outcomes of LDR. The number of levels treated has also been studied with promising results found for both 1- and 2-level disc replacement, but with 2-level replacements having higher revision rates.49 Some have also presented combination treatment with fusion and disc replacement that would have better results compared to 2-level fusions.50
Subsets were analyzed including those over age 60, those who underwent single-level vs multilevel replacement, and smokers vs nonsmokers, all of which showed no major differences between subgroups.51-54 In all these studies, authors reiterated the common theme of importance of proper patient selection. Using the Charité IDE trial patient population, subanalyses by patient type were published stratifying patients by age at surgery (18-45 yr vs 46-60 yr) or whether they had had prior surgery, showing no difference in both subgroup analyses.55,56 Similarly, Geisler et al57 also evaluated the clinical outcomes of patients from the Charité randomized, controlled trial who did not improve with arthroplasty and needed revision surgery to a fusion. These patients (7.1% of all arthroplasty cases) did not improve, despite the revision surgery, further highlighting the importance of proper patient selection and, possibly, the fact that patient selection still remains an imprecise science.
Two-year results of a prospective trial studying ProDisc-L at 2 levels were published in 2011, demonstrating good functional outcome and improved pain and SF36 scores.58 Additionally, ProDisc-L was evaluated for treatment of DDD from ASDis after prior fusion, showing improvement in pain and ODI scores.59
Other considerations include disc height and vertebral endplate morphology. Radiographically, patients with at least 50% remaining disc height obtained higher satisfaction rates compared to those with less than 50%, specifically showing satisfactory outcomes for disk height >4 mm.51,60 Flat-end plates (type I, as described by Yu-Bertagnoli classification)61 make implantation easier. End plates that exhibit a type II or type III morphology require, respectively, either a keel or a spike mode of endplate stabilization.
A spinal motion segment is composed of an intervertebral disc and 2 posterior facet joints. The kinematics of the spine with its multiple motion segments is different from that of a unicompartmental hip or knee joint limiting comparisons between disc replacement and joint replacement. Also, although the facet joints are true synovial joints, the intervertebral disc is a synchondrosis. The biomechanics of intervertebral discs adjacent to fusion vs disc replacements has been studied showing that stress profiles adjacent to an artificial disc were more similar to the intact discs compared to that of a fusion model.62
Implant designs are divided based on composition: metal-on-metal vs those with a polyethylene core. Furthermore, this core can be more mobile (unconstrained) or fixed to one of the end plates (constrained). Biomechanical studies comparing the 2 show no significant advantage of constrained (ProDisc-L) over an unconstrained device (Charité). Unconstrained devices had higher ROM for axial rotation and lateral bending and lower ROM for flexion and extension.63
It has been described that the constrained prosthesis with a small radius of curvature would cause increased facet joint loads in extension while unconstrained prostheses that are free to translate during extension might find an equilibrium point between facet joint compression and capsuloligamentous tension.64 Also, it is theorized that the constrained device would cause increased facet loads caused by impingement during flexion and extension of the motion segment, whereas an unconstrained device would unload the facet joint. On the other hand, it has been postulated that loading forces would be absorbed in a constrained prosthesis, resulting in long-term facet preservation, whereas an unconstrained LDR, lacking inherent shear stability, would make the facets and posterior ligaments subject to increased forces resulting in an increased incidence of degenerative facet changes.65 More recent studies of the semiconstrained activL device show decreased facet strain with the activL compared to baseline measurements in human cadaveric spines.66
Range of Motion
Using ProDisc-L cases, it has been shown that a ROM of at least 4.6 degrees must be observed to be 95% confident that a segment has any sagittal motion.67 Additionally, at least 9.6 degrees must be measured for ROM to confirm a positive dynamic change in the segment.68 Flexion-extension ROM results from the 2-yr ProDisc-L IDE trial were determined at an average of 7.7 degrees and characterized as a normal ROM. This was preserved in 94% of patients 24 mo out.21 However, at 8.7 yr after surgery, Huang et al69 reported ROM measurements less than that reported in control group of asymptomatic patients, with an average motion of 3.8 degrees and 66% had measurable flexion-extension motion of at least 2 degrees. More importantly, ROM greater than 5 degrees has been associated with better ODI outcomes.70 Another retrospective study on 26 patients concluded that sagittal balance and ROM significantly improved after lumbar arthroplasty. Specifically, lumbar lordosis increased from 15.8 to 23.2 degrees on average and mean ROM increased from 11.4 to 14.6 degrees at 2-yr follow-up.71 More recently, a 5-yr follow-up study shows drop off of index level ROM as well as compensatory reduction in lordosis of the adjacent segment in patients who underwent ProDisc-L implantation at L5-S1.72
Using Charité disc, McAfee et al11 had a 13.6% increase in motion from preoperative to the 2-yr follow-up. The ROM also correlated to device placement, with poor device placement correlating with statistically significant reduction in motion at that segment. In 2 different 10-yr follow-up studies, average ROM of 10.1 and 10.3 degrees were reported.18,19 This preservation was further demonstrated with dynamic imaging while speed walking, showing that after single-level disc replacement there is a degree of motion preservation at the index level.73
A study comparing the Charité, ProDisc-L, and Maverick discs found no significant difference in ROM at the index level between the different devices and further observed maintenance of sagittal balance with all devices.74 The activL showed significantly more ROM at 5 yr compared to the ProDisc-L (7.9 degrees vs 6.2 degrees, respectively).75 There is significant interdependence between disc space height and ROM after disc replacement and increase in ROM correlated to clinical outcomes as measured by ODI and VAS scales. The L5-S1 level was noted to be least mobile after disc replacement. One can minimize loss of ROM at the L5-S1 by using an unconstrained artificial disc or a third-generation device—activL maintained near baseline motion at 5 yr following implantation.75,76 Figure 3 demonstrates pre and postoperative images of a patient who underwent total disc replacement with an activL implant for DDD and stenosis. Postoperative flexion and extension radiograph one year out demonstrate motion preservation and hardware stability.
Adjacent Segment Changes
One of the reasons lumbar arthroplasty gained traction was a growing concern for ASDis with fusion. The artificial lumbar disc has advantages of restoring or maintaining normal motion, height, and lordosis, preserving paravertebral muscles, and overall decreasing the forces on the adjacent levels. In Vivo fluoroscopy studies showed that compared to circumferential fusions, for disc replacement with the ProDisc-L there is steeper motion profiles at the adjacent segment in fusion cases and disc replacement maintains more physiologic motion profile.77
Long-term analyses have been done with ProDisc-L at 8.7 yr and Charité at 10-yr follow-up. In the ProDisc-L study, 24% of patients developed ASDeg. A correlation was noted between a low ROM and the prevalence of ASDeg. Specifically, all patients with ASDeg had ROM less than 5 degrees, whereas only 59% of patients without ASDeg had ROM less than 5 degrees.78 In the long-term studies with the Charité, there was an incidence of 2% ASDeg with 11% case of facet arthrosis in 1 study18 and 2.8% adjacent-level degeneration with 4.7% cases of facet arthrosis in another study.19
In 1 systemic review of over 1732 arthrodesis patients compared to 758 arthroplasty patients, there was a significant difference between the arthroplasty group compared arthrodesis group in ASDeg as well as a stronger correlation with ASDis with arthrodesis. This review of class II and IV studies yielded a grade 1C recommendation for using arthroplasty over arthrodesis when indicated to decrease risk of ASDis.79 A more recent meta-analysis of 13 studies, including 2 randomized controlled trials shows that the difference in ASDeg becomes more significant after 5-yr follow-up and that within 5 yr, the rates overall are similar.80 In a post hoc analysis of patients from the activL IDE trial, patients with evidence of radiographic ASDeg did not lead to clinical symptoms and there was no significant progression. Overall change in ASDeg was 9.7% and at 5-yr follow-up, there was a 2.3% reoperation rate at the adjacent level for combined activL and ProDisc-L patients.81 Additionally, there was an inverse linear relationship with the development of adjacent segment changes and the preservation of motion; 10.6% with 0 to 1 degrees of motion which went down to 0% with preservation of 6 degrees or more of motion.81
Although not FDA approved, disc replacements at multiple levels and their effect on adjacent segment biomechanics has also been studied. There is an increase in mobility as well as facet forces in flexion and extension with multilevel LDR.82 Additionally, in Vitro study using disc replacement at L3-4 and L4-5 compared to arthrodesis and normal intact spines show similar intradiscal pressures and ROM with intact spines.83 Evaluating secondary surgery rates from a prospective randomized cohort undergoing 2-level LDR vs 2-level arthrodesis, reoperation rates were higher in the arthrodesis group at 5-yr follow-up.84
The biomechanics of facet joints is also important to analyze given that one goal of arthroplasty is to preserve the integrity of the functional spinal unit. Although arthroplasty has proven beneficial effects on adjacent-level disc preservation, index level facet joint degeneration was higher than other levels with ProDisc at an average of 53 mo follow-up. The rate of facet degeneration was significantly higher at the L5-S1 level than other levels. This progression of facet degeneration had a negative impact on ROM and ODI scores.85
In an early review of complications following Charité disc replacement, it was noted that the main cause of the unsatisfactory results were degeneration of facet joints at the index levels or neighboring levels, in addition to subsidence and migration of the prosthesis.86 Using an artificial ball and socket disc in cadaver models, studies showed anteriorly placed disc replacements loaded the facets 2.5 times more than the intact segment, and posteriorly situated devices correlated with smaller facet loads.87 Anterior and posterior shifts influence muscle forces significantly more than lateral shifts.88 In a study using Maverick implants, it was shown that disc replacements can be done at levels with grade 1 or 2 facet arthrosis with favorable results and a posterior center of rotation lightens facet load.89
Geisler90 provided a detailed account of the surgical technique for the Charité and Tropiano et al91 provided details of surgical technique for the ProDisc-L. Overall, the approach and most technical notes are identical to that of an ALIF procedure.
Patient is placed in supine position. Fluoroscopic x-rays are obtained to identify the disc space. The approach surgeon may obtain access via either a transperitoneal or retroperitoneal approach using various skin incisions.92,93 Once exposure is complete, and the level is confirmed with lateral x-ray, an AP x-ray is done to identify midline.
Discectomy is performed and the endplates are prepared with a variety of curettes and Kerrison rongeurs. The cartilaginous endplate should be removed as completely as possible because it can impede osteointegration of the implant to the endplates. However, too extensive of a preparation and thinning of the bony endplates can increase the risk of implant subsidence and migration or cause heterotopic ossification. A central and lateral decompression is performed.
The interspace is widened and mobilized by using disc space distractors. Implant trials are placed to determine size of the artificial disc. The size of the trial implant and correct positioning of the implant is confirmed with fluoroscopy. Finally, appropriate midline identification and positioning of the device is critical.
With procedures involving new techniques and devices, some of these complications may not have been encountered before. Overall complications can be divided into short-term or long-term. With regards to LDR, they can be divided into technique-related, approach-related, or device-related categories. A summary of reported complications in the literature are outlined in Table 2.
TABLE 2. -
Summary of Reported Complications From LDR in the Literature
||Shim et al95 2005
||Case series of 2 patients
||Putzier et al96 2006
||23% of 53 patients with 17 year f/u
||Devin et al102 2008
||Schulte et al100 2007
||Bilateral pedicle fractures
||Mathew et al34 2005; Harrison et al48 2013
||Facet joint disease
||Siepe et al85 2010
||20% (44/220) at average of 4.4 years f/u
||Tortolani et al103 2007; Park et al101 2011
||4.3% (12/276) with 2-year f/u 50% (30/60) with average 8-year f/u
||Harrop et al79 2008; Zigler et al81 2018
||9-9.7% ASDeg 1% ASDis
||Chung et al76 2009
||Coronal deformity in 4 patients with 2-level arthroplasty
||Mayer et al49 2002; van Ooij et al86 2002; Aunoble et al94 2004
||Fragmented device parts or dislocation of device components
||Devin et al102 2008
||Device wear, burnishing, and debris
||van Ooij et al105 2007; Kurtz et al106 2009; Choma et al114 2009; Lebl et al116 2012; Veruva et al106 2015
||Different degrees of severe wear, endplate burnishing, and oxidation in explanted devices
||François et al108 2007; Zeh et al107 2007; Gornet et al109 2013
||Increased cobalt/chromium serum ions and local metallosis
||Cabraja et al116 2012
||Local granulomatous inflammatory mass
||van Ooij et al86 2003; Punt et al113 2007
||52-59% of revisions related to subsidence
||Superior hypogastric plexus injury
||Sasso et al112 2003; Härtl et al111 2014
||4.1%, more with transperitoneal approach
||Brau et al113 2008
||Other: dural tear, postop hematoma, ileus
||Holt et al103 2007
Possible technique-related complications include excessive facet distraction, malpositioning, endplate violation, pedicle fracture, device dislocations, and vertebral body split fractures (specifically with devices that have keels).94,95 There is also an association of development of scoliotic deformities and spontaneous fusion secondary to malpositioned implants.96 Revisions were often found to be associated with technical errors, such as errors in positioning or sizing of the implant.
Several studies have shown that proper positioning of the implant has a direct correlation with its postoperative mobility, and all efforts should be made to have a centralized device.97,98 If the implant selected is too small, there is increased risk of migration. Ideally, an implant has maximum coverage over the endplates.99 Subsidence can also occur as a result of inadequate bone density. In addition to size, it is important to select the appropriate height to preserve mobility and prevent over distraction of the facet, which can irritate the facets and nerve roots. The stretch and traction on the dorsal rami from over distraction are suspected sources of pain.
Despite having the impetus of preserving motion, LDR does not mimic the exact motion profiles of a normal healthy spinal segment with its constantly changing center of rotation. The new forces applied with a disc replacement can place abnormal stresses on the facet joints and intervertebral ligaments, resulting in further strain and degeneration. Thus, facet joint degeneration and acquired spondylolysis is a recognized complication, especially with over distraction leading to stress on the zygapophysial joints. During physiologic activities, the lower lumbar spine experiences axial and anteriorly directed forces, particularly at L5-S1. The facet joints assist the disc in resisting these forces and preventing forward translation.100 For the most part, these changes can be due to improper implant size and positioning and eventually will require revision surgery to either replace the implant or fuse the segment.
Heterotopic ossification is another complication that will lead to reduced mobility and unwanted fusion. Improper patient selection and aggressive endplate preparation can increase the risk of ossification.101 In 2003, McAfee et al102 studied this type of ossification after disc replacement and introduced a method to characterize heterotopic ossification. This was applied in a retrospective review of 276 arthroplasty patients from the Charité IDE trial. A total of 4.3% of cases of heterotopic ossification were noted. However, heterotopic ossification was not related to ROM, and there was no significant difference in VAS s or ODI at 24 mo after surgery. These changes also rarely result in neurologic complications.103
Device-specific complications have been noted as well, especially for older, first-generation devices that have had more follow-up studies. For example, it has been documented with the ProDisc-L that a major complication is a vertical split fracture of the vertebral body. In a case report of 2 patients, no operative intervention was pursued, but the patients experienced prolonged back pain.95 For the Charité artificial disc, key complications were observed on the earlier first-generation devices, because of gamma sterilization in which air led to potential for oxidation of the core polyethylene nucleus.86,104 This was resolved when gamma sterilization was done with nitrogen starting in 1998. A cause of chronic failure of the Acroflex artificial disc 19 yr after implantation was reported in a case report showing osteolysis and metallic disc material invading the bone. Multiple cases of Charité artificial disc and 2 cases of osteolysis in ProDisc implants have also been noted and it is now suspected that polyethylene particles may cause osteolysis.105,106
For the Maverick disc, there are reports of early removal 1 yr after implantation owing to severe persistent back pain and pathology showed gross metallosis around the articulation of the device. Metallosis was thus cited as a potential complication for devices consisting of a metal-on-metal design.107,108 Metal-on-metal devices can lead to metallic debris and ion release, which may lead to adaptive host responses, hypersensitive reactions, and pseudotumor formation. A prospective study measured serum metal ions after a metal-on-metal implant at 12 mo and 24 mo showing that levels were lower than previously reported and significantly lower than hip metal-on-metal replacements.109
Approach-related complications are the same as those for an ALIF and include retrograde ejaculation, ureteral injury, and vascular injury. These have been mostly minimized by choice of approach and by the experience of an approach surgeon.110
Meta-analysis concerning complications of the anterior approach found an incidence of 9% for all types of neurological events, including retrograde ejaculation.111 Injury to the superior hypogastric plexus can result in retrograde ejaculation, which has been reported to occur in between 0.42% and 6% of cases.112 The plexus is located in the retroperitoneal space in close proximity to the lumbosacral junction and overlies the common iliac artery, making it vulnerable to injury during dissection, mobilization, and retraction.
The incidence of vascular injuries increases with prolonged retraction, multilevel surgeries, and in patients with previous abdominal surgeries.91 Excessive retraction may lead to thrombosis or tearing of the vessel. Major vein lacerations have occur in between 1.4% and 3% of cases, although some incidences may go unreported.113 Patients with prior anterior approaches as well as those with posterior manipulation of the disc space can have adhesions of the iliac vessels making retraction difficult thus increasing the risk of vascular injury. Approach related complications are higher with revisions. When surveyed, 52.7% of surgeons expressed worry about the technical challenges of revision surgery after LDR.7
CONCLUSION AND FUTURE DIRECTIONS
Both fusion and LDR have been widely studied as standards of care for lumbar DDD patients. All meta-analyses reporting on disability, pain, and patient satisfaction demonstrate that LDR significantly improves these outcomes at 2 yr in contrast to surgical fusion for the treatment of lumbar DDD.114-119 Additionally, multiple long-term studies of LDRs have helped to address concerns showing improved clinical and safety benefits with LDR at 5 yr and beyond. Table 3 outlines long-term outcomes data comparing lumbar arthroplasty to fusion in terms of ODI scores and back pain scores. These studies also showed significant differences in reoperation rates and patient satisfaction favoring arthroplasty to arthrodesis.14,29,120,121
TABLE 3. -
Comparison of Outcomes of ODI and Back Pain Score Between Arthrodesis and Arthroplasty
||ODI success (>15 pt improvement)
||Back pain score mean (SD)
|Gornet et al120 2010
|Guyer et al14 2009
|Sköld et al121 2013
|Zigler and Delamarter29 2012
Additionally, intervertebral disc replacements for the lumbar spine have been under constant design improvements for over 4 decades and the latest design offers safe and effective motion preservation for carefully selected patients suffering from DDD. Maintaining motion clearly reduces the development of ASDis and need for additional surgical intervention on long-term follow-up. Questions regarding lifelong durability and consequences of the devices remain and will be only gleaned over time, but experience with current discs is promising.
The third generation of LDR design has resulted in substantial improvement over previous designs, evolution is natural and inevitable. We are moving toward a consensus that LDR is a durable, well-tested, and appropriate surgical option for lumbar DDD.122 Lessons learned from cervical arthroplasty will likely influence future lumbar device design. Choice of materials may change with newer devices as may the mechanics. Utilizing preoperative planning and three-dimensional printing to generate patient specific implants to better conform endplates with native anatomy could help improve device function and longevity and decrease subsidence issues. Matching device design to the specific requirements above a fusion could help promote arthroplasty as the procedure of choice for ASDis. Revisiting a laterally placed artificial disc could increase arthroplasty for the L3-4 and L2-3 levels. Whether or not a viable posteriorly placed arthroplasty device can be developed remains to be seen but there is certainly interest and potential to see this happen.
Dr Sandhu is a consultant for Aesculap. Dr Garcia receives royalties and is a consultant for Aesculap. The other author has no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article.
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This is a very well written and well referenced review article summarizing the history of lumbar arthroplasty devices and the evidence supporting their use. Although, there has been a significant decline in their use especially following the reported complications with the Charite disk device, newer iterations and modifications of these devices may offer a motion sparing option for well selected candidates with the added advantage of reduction in the rates of adjacent segment disk degeneration and disease.
Nader S. Dahdaleh