Clinical performance of an elastomeric lumbar disc replacement: Minimum 12 months follow-up.

BACKGROUND: Elastomeric disc replacements have been developed to restore normal shock absorption and physiologic centers of rotation to the degenerated disc. The Physio-L Artificial Lumbar Disc is an elastomeric disc which uses a compliant polycarbonate-polyurethane core with enhanced endurance properties. The objective of this study was to evaluate the safety and efficacy of the Physio-L through a 12-month follow-up period in a prospective, nonrandomized clinical trial.
METHODS: Twelve patients who met the inclusion/exclusion criteria were enrolled in the study. Eight patients received a single implant (L5-S1) and 4 received a 2-level implantation (L4-5 and L5-S1). Patients were assessed preoperatively and postoperatively at 6 weeks and 3, 6, and 12 months. Primary outcomes included the VAS, ODI, a radiographic analysis of implant condition, incidence of major complications, and reoperations. Secondary outcomes included SF-36, ROM at index and adjacent levels and disc height.
RESULTS: All patients completed the 12-month follow-up evaluations. Through 12 months, the Physio-L devices have remained intact with no evidence of subsidence, migration, or expulsion. VAS low-back pain and ODI scores improved significantly at all follow-up periods compared to preoperative scores. The range of motion of 13.3° ± 5.5° at the index level was considered normal. Overall, patients were satisfied with an average score of 83.5 ± 26.8 mm. When comparing the device to other artificial discs, the current device showed a clinically relevant improvement in both ODI and VAS scores at all follow-up time points. Statistically significant improvements in both scores were observed at 12 months (P < .05).
CONCLUSION: The Physio-L is safe and efficacious, as demonstrated by improved pain relief and functional recovery without any implant failures, significant device related complications, or adverse incidents. The clinical results for VAS and ODI were superior to other marketed artificial lumbar discs such as the Charité and ProDisc-L at the same follow-up timeframes.

Artificial disc replacements (ADR) have been under development for over 20 years for use as motion preservation alternatives to fusion in the treatment of chronic disabling low back pain caused by degenerative disc disease (DDD). Current designs of spinal disc prostheses typically achieve their desired motion by having one surface slide relative to another in a similar manner to total hip and total knee prostheses.[1-6] These rigid sliding surfaces are constructed from metal, polymer, or ceramic with differing types of motion dependent on specific designs. It is clear, however, that many of these designs lack resistance to motion and the ability to provide shock absorption. In recent years, mounting concern has been registered in the literature concerning accelerated facet joint degeneration at the index level, an increased rate of adjacent level degeneration after ADR, and stress fractures of the pars or pedicle at the index levels. These untoward effects may be related to the nonphysiologic nature of the design of these disc prostheses.[7-10]

To overcome these concerns, the use of elastomeric disc prostheses has been proposed to mimic physiologic levels of shock absorption and flexural stiffness; however, an earlier elastomeric disc design using a polyolefin core was developed and examined in clinical trials but was eventually abandoned due to material failure.[11, 12] With the recent availability of better fatigue-resistant and bio-stable polymers, a new generation of elastomeric disc prostheses has been developed. The current study evaluates the safety and efficacy of a new generation disc replacement through the 12-month follow-up period.

Materials and methods

The disc prosthesis

The Physio-L (Nexgen Spine, Whippany, NJ) lumbar artificial disc uses a compliant polycarbonate polyurethane that is securely attached to 2 titanium endplates by injection molding the polymer through perforated plates. This provides a purely mechanical attachment that employs no adhesives. This design allows the restoration of the normal range of motion and function of a healthy disc to the involved level (Fig. 1). The domed endplates are manufactured from medical grade titanium alloy and are porous coated with titanium beads to promote bone in-growth and long-term prosthesis-bone interface stability.

Fig. 1

Physio-L artificial lumbar disc.

Study design

A prospective, nonrandomized clinical trial was conducted on 12 patients at 2 clinical sites to evaluate the safety of the artificial lumbar disc. All surgeries were performed by 1 of 2 surgeons between March and August 2007. Patients presenting with low-back pain caused by degenerative disc disease (DDD) were enrolled in the study after failing to respond to nonoperative treatment for a minimum of 6 months. Degenerative disc disease at 1 or 2 levels between L3-S1 was confirmed by patient self-reporting of back pain, MRI, and discography. All patients reported in this group fit within defined inclusion/exclusion criteria (Table 1). Contraindications included active systemic infection or localized infection near the implant site, isolated radicular compression symptoms due to disc herniation, allergy or sensitivity to implant materials, osteoporosis, lumbar stenosis, facet joints arthritis, osteopenia, pars defects, instability and/or deformity. Patient evaluations occurred preoperatively (within 3 months of surgery) and postoperatively at 6 weeks and 3, 6, and 12 months. Patients will be followed subsequently at 24 months.

Table 1

Inclusion/exclusion criteria

Inclusion

Exclusion

Skeletally mature (age, 18 –70)BMI <35Degenerative Disc Disease at or levels between L3-S1 confirmed by: 1) Self reports of greater amount of back pain compared with leg pain; 2) MRI or discography with concordant pain within 6 months.Self reports preoperative Visual Analog Scale score for back pain ≥40%Self reports preoperative Oswestry Disability Index score 40%Documented symptoms for minimum of monthsPatient is competent and has signed the informed consent

Symptomatic DDD at more than levels between L3-S1.Patient has experienced previous abdominal surgeries which could compromise surgical exposure.Patient is pregnant or seeking to become pregnant during the study.Previous diagnosed Paget's disease, osteomalacia, other metabolic bone disease or is insulin-dependent.Bone density measurements indicating osteopenia or osteoporosis (DEXA score of −1 or less)Significant facet joint arthritisActive infectionChronic steroid useKnown history of metal or plastic allergySpondylolysis at index levelActive malignancySpinal tumorPreviously diagnosed autoimmune disorders or immune compromisedSpondylolysthesis ≥ 3mm (Grade 1)Significant instability ≥ 3mm indicated by F/radiographLumbar scoliosis > 10%Mid-sagittal stenosis < 11mm (by CT or MRI)Prior fusion adjacent to the index level.Positive single or bilateral straight leg raise test.Previous surgical procedures at the index level that would preclude the placement of the device such as extensive posterior decompression procedures.Current treatment for psychosocial disorders (eg, chemical dependence).Currently smoker.Return visits burdensome to patient.Currently participating in another investigational study that could interfere with the outcome measurements of this study.

Patient demographics

Eleven patients were male and 1 was female, with an average age of 40.6 ± 8.4 years (range, 25–55) and an average BMI of 26.3 ± 3.5 (range, 20.9–31.6). Of these patients, 8 received a single implant (L5-S1 level) and 4 received a 2-level implantation (L4-5 and L5-S1) (Fig. 2). A total of 16 artificial discs were implanted. All patients returned for each follow-up visit up to 12 months.

Fig. 2

Lateral radiographs showing A) a patient implanted with a single Physio-L at L5-S1, and B) a patient implanted with two Physio-L devices at L4-L5 and L5-S1 at the 12 month follow up time point.

Surgical technique

The patient was positioned in the supine position and underwent anterior disc removal through an anterior retroperitoneal exposure of the lumbo-sacral spine, which is similar to other artificial disc replacement surgeries. Endplate preparation was performed using contoured bone rasps to closely match the specific dome shape of the metal endplates. A keel cutter was used to cut the channels on vertebral endplates for the central keel without violating the anterior cortex. Following endplate preparation, the artificial disc was inserted as a single unit.

Clinical outcome measures

Patient self-assessment outcome measures included the oswestry disability index (ODI), the visual analog scale (VAS) for back pain, and the Short Form 36 (SF-36) Health Survey questionnaire. A 10-point decrease in ODI scores and 18-point decrease in VAS scores were considered a minimal, clinically important difference (MCID).[13] Mental and physical component summary scores (MCS and PCS, respectively) were calculated from the SF-36 Quality of Life questionnaires. A 5-point increase in SF-36 scores was considered a clinically significant improvement.[14] Additionally, work status was collected at all follow-up evaluations and patient satisfaction was collected at 6 months and 12 months.

Radiographic analysis

Radiographs were analyzed using an independent radiologist and QMA™ Software (Medical Metrics, Inc., Houston, TX) to evaluate flexion/extension range of motion, disc height, loosening, subsidence, migration, and expulsion.[15]

Single versus 2-level implantation

Eight patients received the artificial disc at a single L5-S1 level and 4 patients received a 2-level L4-S1 implantation. Patient self-assessment scores of VAS back pain and ODI were compared between these 2 groups.

Comparison to other devices

The present study was conducted using similar protocols and outcome measures to previous reports involving the Charite (DePuy Spine, Raynham, MA) and ProDisc-L (Synthes, West Chester, PA) devices.[16-19] As a result, direct comparison of the clinical results for the Physio-L to these devices and their fusion controls is possible. Because no individual subject data was available for the referenced studies, only the reported means and standard deviations of the ODI and VAS scores were used for the comparisons, using 1-way ANOVA (applied to summary statistics). The fusion groups from the Charite and Prodisc-L studies were pooled in the analysis.

Results

Surgery

Mean operative time was 157.5 ± 46.3 minutes (range, 90–240), mean total blood loss was 439 ± 555 cc (range, 70–2000), and the mean average length of hospital stay was 1.2 ± 0.4 days (range, 1–2) for all patients regardless of the number of levels treated. Surgical data was separated to determine the effect of patients treated for single level or 2-level disc disease and results are reported in Table 2.

Table 2

Operative data for single and two-level Physio-L implantations

Number of levels treated	Operative time (min)	Blood loss (cc)
Single level	135.0 ± 32.1	303 ± 315
Single level Two levels	202.5 ± 37.7	713 ± 864

Using a paired t test, no significant differences were observed in the total operative time, blood loss, or length of hospital stay for patients receiving a single level replacement versus a 2-level replacement (P < .05). Two approach- related adverse events were recorded during or immediately after the surgery. One patient receiving a 2-level implantation experienced clinically significant blood loss greater than 1500cc, requiring transfusion. This adverse event was resolved without further incident. One patient experienced retrograde ejaculation between the 3- and 6-month follow-up evaluations, and this event was resolved spontaneously by the 12-month follow-up evaluation.

Clinical outcomes

ODI

Mean preoperative ODI scores were 54.3 ± 11.1, and decreased to 17.2 ± 14.6, 15.8 ± 13.9, 13.5 ± 13.9, and 12.7 ± 14.8 at 6 weeks and 3, 6, and 12, respectively. This corresponds to an overall decrease of 75.2% at 6 months and 76.7% at 12-month follow-up (Fig. 3). A statistically significant decrease in mean ODI scores (P < .05) was calculated at all follow-up evaluations when compared with the preoperative scores. Additionally, these results indicate that a clinically relevant difference in function has occurred when comparing the preoperative to all postoperative scores, with a mean decrease in ODI greater than 10 points.

Fig. 3

Mean ODI scores at all follow up evaluations. Error bars indicate standard deviations. * indicates statistically significant differences (p < 0.05) from the pre-operative scores. The dotted line indicates the minimal clinically important difference (MCID) of a 10 point decrease from the pre-operative score.

VAS

Mean preoperative VAS scores were 76.0 ± 14.3 and decreased to 18.8 ± 21.4, 18.3 ± 16.3, 21.8 ± 21.5, and 16.5 ± 25.1 at 6 weeks and 3, 6, and 12 months, respectively. This corresponds to an overall decrease of 71.4% at 6 months and 78.3% at 12-month follow-up (Fig. 4). A statistically significant decrease in mean VAS scores (P < .05) was calculated at all follow-up evaluations when compared with the preoperative scores. Additionally, these results indicate that a clinically relevant difference in low back pain has occurred when comparing the preoperative to all postoperative scores with a mean decrease in VAS greater than 18 points.

Fig. 4

VAS scores at all follow up evaluations. Error bars indicate standard deviations. * indicates statistically significant differences (p < 0.05) from the pre-operative scores. The dotted line indicates the MCID of an 18 point decrease from the pre-operative score.

SF-36

Mean preoperative PCS and MCS scores were 37.7 ± 4.4 and 39.6 ± 15.0, respectively. The mean values of both scores increased by greater than 5.42 points at all follow-up time points when compared to the preoperative values (Fig. 5), indicating that the quality of life of the patients has been improved.

Fig. 5

Mean SF-36 scores at all follow up evaluations. Error bars indicate standard deviations.

Patient satisfaction

Ten out of 12 patients (83%) indicated that they would definitely or probably recommend this treatment to others, and 2 patients would not recommend the treatment after 12 months. Overall, all patients were satisfied with their treatment, as shown by an average score of 83.5 ± 26.8 mm on the patient satisfaction scale.

Work status

Prior to surgery, 3 patients (25%) were not working and 7 patients (58%) were able to work but with restrictions. Only 2 patients (17%) were able to work without restrictions. At the 12-month follow-up, all patients were working. Ten patients (83%) were able to work without any restrictions and 2 patients (17%) were working with restrictions.

Radiographic outcomes

Range of motion

The total range of motion at the index level was 12.0° ± 6.2° preoperatively and 13.3° ± 5.5° at the 12-month follow- up. The total range of motion at the adjacent level was 10.8° ± 5.5° preoperatively and 13.3° ± 5.0° at the 12- month follow-up. The range of motion has been maintained from the baseline values (Table 3). This range of motion is within the normal values previously reported.[19]

Table 3

Range of motion pre-operatively and at the 12 month follow up evaluation for operative and adjacent levels

Description	Operative level	Adjacent levels
Total ROM, baseline	12.0° 6.2°	10.8° 5.5°
Total ROM, 12-month follow-up	13.3° 5.5°	13.3° 5.0°

Disc height

Disc height (DH) was measured preoperatively and immediately postoperatively (baseline) for each level implanted (16 levels).

The change in disc height (ΔDH) between the pre-op and baseline values and the change between the baseline and last follow-up was calculated (results listed in Table 4). The disc height was restored and maintained to within 1 mm through the last follow-up time point.

Table 4

Change in disc height

Operative level	Preop DH (mm)	Baseline DH (mm)	ΔDH, preop to baseline	ΔDH, baseline to 12-month follow-up
L4-(4) (n = 4)	8.5 ± 2.7	13.9 ± 1.6	5.4 mm	0.1 mm
L5-S1 (n = 12)	8.7 ± 1.5	15.7 ± 1.4	7.0 mm	−0.6 mm

Subsidence, migration, expulsion, loosening

There was no incidence of subsidence, migration, or expulsion for any of the implanted devices.

Fourteen of the 16 implanted devices exhibited no radiolucent lines along the device/implant interface. Mild radiolucency was observed at the 3- and 6-month follow-ups for 2 patients with a 2-level implantation. In both cases, radiolucency was noted at the L4-5 level. At 12 months, this mild radiolucency was only observed in 1 of these patients. Additionally, at 12 months, moderate radiolucency was noted at the L4-L5 level of another patient with a 2-level implantation.

At the 12-month follow-up time point, VAS low back pain scores were 8.9 ± 7.6 and 31.8 ± 41.4 for the single and 2-level implantation groups, respectively. ODI scores were 8.0 ± 8.6 and 22.0 ± 21.3 for the single and 2-level implantation groups, respectively. These results were not statistically significant for either outcome.

For all treatment modalities (Physio-L, Charite, Prodisc- L and fusion), a minimal clinically important difference (MCID) was noted between the preoperative and postoperative ODI and VAS scores at all follow-up time points. Moreover, compared to all other treatments, the current device showed a statistically significant and clinically relevant improvement in both ODI and VAS scores at all follow-up time points when compared to the fusion and ProDisc-L treatment modalities (P < .05). A statistically significant difference was seen between the current device and the Charite at 6 weeks and 3 and 12 months (Fig. 6). There was no significant difference in preoperative scores between the current device and any other treatment modality.

Fig. 6

Comparison of the clinical outcomes for the Physio-L versus the Charite, ProDisc-L, and Fusion for A) ODI scores and, B) VAS low back pain scores.

Discussion

This is the first report on an elastomeric lumbar artificial disc using a new generation high endurance polyurethane material system, and shows that, at least after 12 months, the devices have remained intact and are performing as intended. Furthermore, the patients have improved pain relief and function, and all have returned to work.

The present investigation employed a clinical protocol similar to those of previous category 1 studies and included validated clinical outcome measures.[16-19] Consequently, it is possible to compare the current data with that previously reported for the Charite and ProDisc-L discs as well as their fusion controls. These short-term results and clinical comparisons are encouraging and warrant further investigation of the device in a larger study with longer-term follow-up results.

Short-term clinical results appear to be predictive of long-term outcomes in similar studies. A retrospective analysis using the Charite and ProDisc-L IDE studies was performed to compare the 6 month mean VAS low back pain and ODI outcomes measures to the same measures at 12, 24, and 60 months.[16, 17, 19] No statistically significant difference was found between the 6-month follow-up time point and the 12-, 24-, or 60-month follow-up time point for either the VAS or ODI.

The superior results for the current device, compared to the Charite, ProDisc-L, and fusion, may be explained by the more physiologic compliance of the elastomeric core. The concept of disc prostheses made from elastomeric materials capable of providing physiologic levels of shock absorption and resistance to motion has been in development almost as long as the development of the current generation “sliding bearing” disc prostheses.[20] It is postulated that these types of discs may reduce or eliminate the concerns expressed over sliding bearing disc designs, including the fixed center of rotation and increased facet joint loading.[21, 22] Historically, however, the availability of fatigue-resistant and biostable materials and the method of attaching the elastomer to a metal endplate were difficult obstacles to the design of a successful device.

Early designs of elastomeric disc prostheses resulted in many failures. The C-Flex material used by Lee et al lacked bio-stability resulting in environmental stress cracking and fatigue failure.[20] Two separate clinical trials were conducted to assess the safety of the Acroflex elastomeric artificial disc design (Acromed Corporation, Cleveland, OH). This disc was manufactured using a polyolefin rubber core material. Prior to initiating the clinical studies, extensive biocompatibility and biomechanical tests were conducted on the polyolefin core material including cytotoxicity, compressive creep, peel strength at the titanium-rubber interface and compression, torsion, and shear endurance testing.[12, 23, 24] The Acroflex disc was implanted into 28 patients at 1 or 2 levels between L4-S1.[11, 12] Ten patients (36%) experienced tears in the polyolefin material, a condition which leads to revision surgery in many affected patients between 2 and 4 years following surgery. The most common mode of failure was anterior-inferior peripheral tears in the rubber that are characteristic of material failure during flexion-extension bending. The authors found no reference that describes any bending endurance testing that was conducted in the laboratory prior to the clinical trial.

More recently, a different family of polymers, polycarbonate polyurethanes, has become available, with longer fatigue life and high resistance to environmental stress cracking.[25-28] Notably, polycarbonate polyurethane has been incorporated successfully in various load bearing nonfusion spinal devices such as the Bryan cervical disc (Medtronic, Memphis, TN), Dynesys posterior dynamic fixation (Zimmer, Minneapolis, MN), and the M6 cervical disc prosthesis (Spinal Kinetics, Sunnyvale, CA). The current device also employs such a polycarbonate polyurethane. Prior to clinical implantation, the current device was endurance- tested at least 10 million cycles in multiple modes of testing including flexion-extension, lateral bending, and combined bending to ensure that the previously observed types of failures did not occur.[29]

Conclusion

That the current device is safe and effective has been demonstrated by improved pain relief and functional recovery without any implant failures, significant device related complications, or adverse incidents at the 12-month follow- up time point. The clinical results for VAS and ODI are superior to other marketed artificial lumbar discs at the same follow-up timeframes. It is postulated that this clinical difference may be due to the compliant nature of the elastomer used in this new generation of elastomeric discs.