Distribution of in vivo and in vitro range of motion following 1-level arthroplasty with the CHARITE artificial disc compared with fusion

Lumbar Arthroplasty: Past, Present, and Future

Lumbar degenerative disc disease is a pathologic process that affects a large portion of our aging population. In the recent past, surgical treatment has involved fusion procedures. However, lumbar disc arthroplasty and replacement provides an alternative for carefully selected patients. It provides the major advantage of motion preservation and thus keeps adjacent segments from significantly progressive degeneration. The history of lumbar disc replacement has roots that start in the 1960s with the implantation of stainless-steel balls. Decades later, multiple implants with different material design and biomechanical properties were introduced to the market. New third-generation implants have made great strides in improved biomechanics and clinical outcomes. Although there is room for further advancement and studies are warranted to assess the long-term durability and sustainability of lumbar disc arthroplasty, it has certainly proven to be a very acceptable alternative within the surgical armamentarium that should be offered to patients who meet indications. In this review we present an overview of lumbar disc arthroplasty including its history, indications, biomechanics, challenges, and future directions.

Five-Year Reoperation Rates of 2-Level Lumbar Total Disk Replacement Versus Fusion: Results of a Prospective, Randomized Clinical Trial

STUDY DESIGN

Long-term analysis of prospective randomized clinical trial data.

SUMMARY OF BACKGROUND DATA

Lumbar total disk replacement (TDR) has been found to have equivalent or superior clinical outcomes compared with fusion and decreased radiographic incidence of adjacent level degeneration in single-level cases.

OBJECTIVE

The purpose of this particular analysis was to determine the incidence and risk factors for secondary surgery in patients treated with TDR or circumferential fusion at 2 contiguous levels of the lumbar spine.

METHODS

A total of 229 patients were treated and randomized to receive either TDR or circumferential fusion to treat degenerative disk disease at 2 contiguous levels between L3 and S1 (TDR, n=161; fusion, n=68).

RESULTS

Overall, at final 5-year follow-up, 9.6% of subjects underwent a secondary surgery in this study. The overall rate of adjacent segment disease was 3.5% (8/229). At 5 years, the percentage of subjects undergoing secondary surgeries was significantly lower in the TDR group versus fusion (5.6% vs. 19.1%, P=0.0027).Most secondary surgeries (65%, 17/26) occurred at the index levels. Index level secondary surgeries were most common in the fusion cohort (16.2%, 11/68 subjects) versus TDR (3.1%, 5/161 subjects, P=0.0009). There no statistically significant difference in the adjacent level reoperation rate between TDR (2.5%, 4/161) and fusion (5.9%, 4/68). The most common reason for index levels reoperation was instrumentation removal (n=9). Excluding the instrumentation removals, there was not a significant difference between the treatments in index level reoperations or in reoperations overall.

CONCLUSIONS

There were significantly fewer reoperations in TDR patients compared with fusion patients. However, most of the secondary surgeries were instrumentation removal in the fusion cohort. Discounting the instrumentation removals, there was no significant difference in reoperations between TDR and fusion. These results are indicative that lumbar TDR is noninferior to fusion.

Why Lumbar Artificial Disk Replacements (LADRs) Fail

STUDY DESIGN

A retrospective review of prospectively collected data.

OBJECTIVE

To determine why artificial disk replacements (ADRs) fail by examining results of 91 patients in FDA studies performed at a single investigational device exemption (IDE) site with minimum 2-year follow-up.

SUMMARY OF BACKGROUND DATA

Patients following lumbar ADR generally achieve their 24-month follow-up results at 3 months postoperatively.

MATERIALS AND METHODS

Every patient undergoing ADR at 1 IDE site by 2 surgeons was evaluated for clinical success. Failure was defined as <50% improvement in ODI and VAS or any additional surgery at index or adjacent spine motion segment. Three ADRs were evaluated: Maverick, 25 patients; Charité, 31 patients; and Kineflex, 35 patients. All procedures were 1-level operations performed at L4-L5 or L5-S1. Demographics and inclusion/exclusion criteria were similar and will be discussed.

RESULTS

Overall clinical failure occurred in 26% (24 of 91 patients) at 2-year follow-up. Clinical failure occurred in: 28% (Maverick) (7 of 25 patients), 39% (Charité) (12 of 31 patients), and 14% (Kineflex) (5 of 35 patients). Causes of failure included facet pathology, 50% of failure patients (12 of 24). Implant complications occurred in 5% of total patients and 21% of failure patients (5 of 24). Only 5 patients went from a success to failure after 3 months. Only 1 patient went from a failure to success after a facet rhizotomy 1 year after ADR.

CONCLUSIONS

Seventy-four percent of patients after ADR met strict clinical success after 2-year follow-up. The clinical success versus failure rate did not change from their 3-month follow-up in 85 of the 91 patients (93%). Overall clinical success may be improved most by patient selection and implant type.

Fluoroscopic assessment of lumbar total disc replacement kinematics during walking

STUDY DESIGN

Descriptive.

OBJECTIVE

The purpose of this study was to determine the in vivo kinematics of functional spinal units, during gait, in individuals with a single-level lumbar total disc replacement (TDR).

SUMMARY OF BACKGROUND DATA

TDR is a motion preservation technology that offers an alternative to spinal fusion for treatment of degenerative disc disease. The aim of TDRs is to replicate motion of the functional spinal units, which may protect adjacent intervertebral discs against accelerated degeneration. At present, there is limited understanding of the in vivo motion of TDRs, particularly during dynamic activities such as gait. Such information is important for understanding the wear characteristics of TDRs and furthering design rationale of future implants.

METHODS

TDR motions were obtained from 24 participants who underwent implantation with single-level L4-L5 or L5-S1 CHARITÉ or In Motion TDRs. Video fluoroscopy was used to obtain measurements in the frontal and sagittal planes during fixed speed treadmill walking.

RESULTS

The mean range of motion between the upper and lower lumbar TDR endplates during walking was 1.6° and 2.4° in the frontal and sagittal planes, respectively. These values were significantly different from zero and corresponded to 19% of the maximum static range of motion in each plane.

CONCLUSION

Lumbar TDRs provide a degree of motion preservation at the operative level during moderate speed walking. The distribution of lumbar TDR motions during walking presented here will inform relevant standards for conducting standardized tests of lumbar TDRs, particularly wear assessments, and, hence, enable more realistic mechanical and computer-based wear simulations to be performed.

LEVEL OF EVIDENCE

N/A.

ISASS Policy Statement - Lumbar Artificial Disc

PURPOSE

The primary goal of this Policy Statement is to educate patients, physicians, medical providers, reviewers, adjustors, case managers, insurers, and all others involved or affected by insurance coverage decisions regarding lumbar disc replacement surgery.

PROCEDURES

This Policy Statement was developed by a panel of physicians selected by the Board of Directors of ISASS for their expertise and experience with lumbar TDR. The panel's recommendation was entirely based on the best evidence-based scientific research available regarding the safety and effectiveness of lumbar TDR.

Intraoperative determination of lumbar prosthesis endplate lordotic angulation to improve motion

The aim of total disc replacement (TDR) is to restore and maintain closer-to-physiology motion. Therefore, the factors that influence postoperative intervertebral motion have to be controlled. Factors such as disc height (DH), postoperative segmental lordosis (SL), implant design and positioning are still recognized to be influent. Otherwise, range of motion (ROM) distribution, between flexion and extension, appear to be influenced by obtaining parallel bearing surfaces, which depends on prosthesis endplate lordotic angulation. To assess in vivo the correlation between an intraoperative parameter (intraoperative segmental lordosis: ISL) and a postoperative parameter (postoperative segmental lordosis: PSL). To determine the advantage of ISL measurement on the improvement of the prosthetic endplate lordotic angulation choice. Radiological comparison between intraoperative and postoperative segmental parameters. Fifty-seven patients who received a TDR at one level, L4-L5 or L5-S1, with different prosthetic endplate lordotic angulations (0°, 5°, and 10°). Twenty-one consecutive patients underwent intraoperative measurement (ISL) on a lateral view, with a spacer at the mid-vertebral bony endplates (Group 1). ISL was correlated using a linear correlation test with PSL. Group 1 postoperative prosthesis endplate lordosis (PEL: angle between the bearing surfaces) were compared to those of 46 patients without intraoperative measurement (Group 2). The mean ISL and PSL angles were 12.2° (7-21°) and 13.9° (8-23°), respectively. We observed a strong linear correlation between ISL and PSL (r=0.78, P <0.006). In Group 1, PEL varied between -1° and 11°, and between -3.7° and 17.8° in Group 2. For 80% of the patients in Group 1, the PEL was less than 5°, versus 33% of the patients in Group 2. Only prostheses with PEL less than 5° had a preserved extension curve in ROM distribution (+3°). Intraoperative measurement of ISL has emerged as a key factor in predicting PSL in TDR. The percentage of parallel bearing surfaces was increased by a prosthesis endplate lordotic angulation choice guided by ISL measurement. This study confirmed the advantage of choosing the adequate lordotic angulation of the prosthesis endplate to restore a physiological motion distribution between flexion and extension.

Quantification of intradiscal pressures below thoracolumbar spinal fusion constructs: is there evidence to support "saving a level"?

STUDY DESIGN

In vitro cadaveric study.

OBJECTIVE

The purpose of this study was to quantify the relative biomechanical protection resulting from "saving a level" in long spinal fusions.

SUMMARY OF BACKGROUND DATA

"Saving levels" in spinal deformity surgery is desirable. Constructs with lowest instrumented vertebra (LIV) in the lumbar spine may increase loads on unfused lumbar intervertebral discs, leading to accelerated disc degeneration. No study to date has quantified the relative pressure changes that occur in the unfused caudal discs with progressively longer fusions.

METHODS

We used a validated in vitro cadaveric long fusion model to assess intradiscal pressures (IDPs) below simulated fusions. Eight fresh frozen T8-S1 specimens were instrumented from T8 to L5. A follower-type loading system and 7.5-N·m moments were applied in flexion and extension. IDP profiles were assessed with a pressure transducer. After acquiring IDP measurements at a given construct length, the rod was cut 1 level higher until LIV = T12. IDP data from each unfused disc were averaged and normalized to the mean value of the disc when immediately subjacent to the LIV.

RESULTS

In both flexion and extension, the mean normalized IDP of the unfused discs below the LIV increased with increasing fusion length. For each 1-level increase in construct length, pressure increased by 3.2% ± 4.8% in flexion and 4.3% ± 4.5% in extension for each unfused disc. Although the differences in pressure for a given unfused disc with differing LIV were not significant, there were significant differences between unfused discs at a given LIV. With shorter fusion lengths, pressure in the disc immediately subjacent to the fusion was consistently greater than for the caudal-most discs.

CONCLUSION

Unfused caudal lumbar discs experienced increased IDPs with increasing length of instrumentation, most notably at the subjacent discs closest to the LIV.

Segmental contribution toward total cervical range of motion: a comparison of cervical disc arthroplasty and fusion

STUDY DESIGN

Prospective radiographic evaluation of patients that underwent cervical total disc replacement (TDR-C) or anterior cervical discectomy and fusion (ACDF) for one-level cervical disc disease.

OBJECTIVE

To evaluate the following: (1) total cervical range of motion (ROM) from C2 to C7, and (2) the relative contribution to total cervical ROM from the operative level and each adjacent level after one-level TDR-C or ACDF.

SUMMARY OF BACKGROUND DATA

The development of symptomatic adjacent segment disease after ACDF has served as the impetus for the development of motion-preserving alternatives, most notably cervical disc arthroplasty. While previous reports have evaluated device-level and total cervical motion, no study has comprehensively quantified the relative contribution made from each of the adjacent levels to total cervical ROM in TDR-C and ACDF.

METHODS

Radiographic review of 187 randomized patients from a multicenter, prospective, randomized trial comparing TDR-C with ACDF for one-level cervical disc disease. There were 93 TDR-C and 94 ACDF patients included. ROM measurements were performed independently using quantitative motion analysis, a custom digitized image stabilization software program, to evaluate total cervical ROM and relative contribution to total ROM from each level from C2 to C7 preoperatively and at 24 months.

RESULTS

The most common operative level was C5/C6 (57%) followed by C6/C7 (34%). At 2 years, the TDR-C group underwent a statistically greater improvement in total cervical ROM (+5.9°) compared with ACDF (-0.8°, P = 0.001). In TDR-C, the relative contributions to total cervical ROM from the operative level and each caudal and cranial adjacent level were statistically equivalent from baseline to 24 months. In contrast, ACDF patients had significantly reduced contribution to total cervical ROM from the operative level (by 15%, P < 0.001), and significantly elevated contribution from the caudal adjacent level (by 5.9%, P < 0.001), first cranial adjacent level (by 3.3%, P < 0.001), second cranial adjacent level (by 5.3%, P < 0.001), and third cranial adjacent level (by 3.0%, P < 0.001).

CONCLUSION

Compensation for the loss of motion at the operative level in ACDF is seen throughout the unfused cervical spine. Cervical disc arthroplasty, however, increases total cervical ROM compared with ACDF and maintains a physiologic distribution of ROM throughout the cervical spine at 2 years, potentially lowering the risk for adjacent segment breakdown.

Adjacent level disk disease--is it really a fusion disease?

Adjacent segment degeneration (ASD) is a relatively common phenomenon after spinal fusion surgery. Whether ASD is a consequence of the previous fusion or an individual's predisposition to continued degeneration remains unsolved to date. This article summarizes the existing biomechanical and clinical literature on the causes and clinical impact of ASD, as well as possible risk factors. Further, the theoretical advantage of motion-preserving technologies that aim to preserve the adjacent segment is discussed.

Effect of sagittal alignment on kinematic changes and degree of disc degeneration in the lumbar spine: an analysis using positional MRI

STUDY DESIGN

Retrospective analysis using positional MRI.

OBJECTIVE

To determine the effects of total sagittal lordosis on spinal kinematics and degree of disc degeneration in the lumbar spine.

SUMMARY OF BACKGROUND DATA

Changes in sagittal lordosis alter the load on the spine and may affect spinal mobility. There is increasing recognition of the clinical impact that sagittal alignment has on back pain, especially its possible role in accelerating adjacent segment degeneration after spinal fusion. However, its relationship to segmental mobility and degeneration of the lumbar spine has yet to be determined.

METHODS

Four hundred and thirty patients who had low back pain with or without leg pain (241 males and 189 females) with a mean age of 42.98 years (range, 16-85 years) were included. Total sagittal lordosis (T12-S1) was divided into three groups; Group A: Straight or Kyphosis (<20°, n = 84), Group B: Normal lordosis (20-50°, n = 294), and Group C: Hyperlordosis (>50°, n = 52). The degree of disc degeneration was graded using midsagittal T2-weighted MR images. Segmental mobility, including translational motion and angular variation, was measured using positional MRI. Their relationship with total segmental lordosis was identified.

RESULTS

When compared with group B, the segmental motion in group C tended to be lower at the border of lordosis and higher at the apex of lordosis, with a significant difference in angular motion at L2-L3. The contrary finding was identified in group A, which had a higher segmental motion at border segments and lower motion at apical segments of lordosis, with significant difference of translational motion at L3-L4 and angular motion at L1-L2. Apical segments contributed more, whereas border segments contributed less to the total angular mobility in more lordotic spines. The opposite was seen in more kyphotic spines. Disc degeneration tended to be greater at all levels in group C, and at L1-L2 and L5-S1 in group A.

CONCLUSION

Changes in sagittal alignment may lead to kinematic changes in the lumbar spine. This may subsequently influence load bearing and the distribution of disc degeneration at each level. Sagittal alignment, disc degeneration, and segmental mobility likely have a reciprocal influence on one another.

Prospective, randomized, multicenter Food and Drug Administration investigational device exemption study of the ProDisc-L total disc replacement compared with circumferential arthrodesis for the treatment of two-level lumbar degenerative disc disease: results at twenty-four months

BACKGROUND

Disc replacement arthroplasty previously has been shown to be an effective alternative to spine fusion for the treatment of single-level lumbar degenerative disc disease. The purpose of the present study was to determine the twenty-four-month results of a clinical trial of the ProDisc-L total disc replacement as compared with spinal fusion for the treatment of degenerative disc disease at two contiguous vertebral levels from L3 to S1.

METHODS

A total of 237 patients were treated in a randomized controlled trial designed as a non-inferiority study for regulatory application purposes. Blocked randomization was performed with use of a 2:1 ratio of total disc arthroplasty to circumferential arthrodesis. Evaluations, including patient self-assessments, physical and neurological examinations, and radiographic examinations, were performed preoperatively, six weeks postoperatively, and three, six, twelve, eighteen, and twenty-four months postoperatively.

RESULTS

At twenty-four months, 58.8% (eighty-seven) of 148 patients in the total disc replacement group were classified as a statistical success, compared with 47.8% (thirty-two) of sixty-seven patients in the arthrodesis group; non-inferiority was demonstrated. The mean Oswestry Disability Index in both groups significantly improved from baseline (p < 0.0001); the mean percentage improvement for the total disc replacement group was significantly better than that for the arthrodesis group (p = 0.0282). An established clinical criterion for success, a ≥15-point improvement in the Oswestry Disability Index from baseline, occurred in 73.2% (109) of 149 patients in the total disc replacement group and 59.7% (thirty-seven) of sixty-two patients in the arthrodesis group. The Short Form-36 physical component scores were significantly better for the total disc replacement group as compared with the arthrodesis group (p = 0.0141 at twenty-four months). Visual analog scale scores for satisfaction significantly favored total disc replacement from three to twenty-four months. At twenty-four months, 78.2% (111) of 142 patients in the total disc replacement group and 62.1% (thirty-six) of fifty-eight patients in the arthrodesis group responded "yes" when asked if they would have the same surgery again. Lumbar spine range of motion on radiographs averaged 7.8° at the superior disc and 6.2° at the inferior disc in patients with total disc replacement. Reduction in narcotics usage significantly favored the total disc replacement group at twenty-four months after surgery (p = 0.0020).

CONCLUSIONS

Despite the relatively short duration of follow-up and design limitations, the present study suggests that two-level lumbar disc arthroplasty is an alternative to and offers clinical advantages in terms of pain relief and functional recovery in comparison with arthrodesis. Longer-term follow-up is needed to determine the risks for implant wear and/or degenerative segment changes.

Use of a personalized hybrid biomechanical model to assess change in lumbar spine function with a TDR compared to an intact spine

Total disc replacements (TDRs) have been employed with increasing frequency in recent years with the intention of restoring natural motion to the spine and reducing adjacent level trauma. Previous assessments of the TDRs have subjectively measured patient satisfaction, evaluated sagittal range of motion via static imaging, or examined biomechanical loading in vitro. This study examined the kinematics and biomechanical loading of the lumbar spine with an intact spine compared to a TDR inserted at L5/S1 in the same spine. A validated biologically driven personalized dynamic biomechanical model was used to assess range of motion (ROM) and lumbar spine tissue forces while a subject performed a series of bending and lifting exertions representative of normal life activities. This analysis concluded that with the insertion of a TDR, forces are of much greater magnitude in all three directions of loading and are concentrated at both the endplates and the posterior element structures compared to an intact spine. A significant difference is seen between the intact spine and the TDR spine at levels above the TDR insertion level as a function of supporting an external load (lifting). While ROM within the TDR joint was larger than in the intact spine (yet within the normal ranges under the unloaded bending conditions), the differences between spines were far greater in all three planes of motion under loaded lifting conditions. At levels above the TDR insertion, larger ROM was present during the lifting conditions. Sagittal motions were often greater at the higher lumbar levels, but there appeared to be less lateral and twisting motion. Collectively, this analysis indicates that the insertion of a TDR significantly alters the function of the spine.

Arthroplasty in the military: a preliminary experience with ProDisc-C and ProDisc-L

OBJECT

The introduction of cervical and lumbar arthroplasty has allowed for management of cervical radiculopathy and lumbar degenerative disease in patients with the preservation of motion at the affected segment. While the early clinical outcomes of this technology appear promising, it remains unclear what activity limitations should be imposed after surgery in patients with these implants. This is of particular interest in military personnel, who may be required to return to a rigorous level of activity after surgery. The goals of the FDA trials evaluating various disc arthroplasty devices were to establish safety, efficacy, and equivalency to arthrodesis. Information regarding the level of physical performance attained and restrictions or limitations is lacking, as these were outside the objectives of these trials. Nevertheless, there data are essential for the military surgeon, who is tasked with guiding the postoperative management of patients treated with arthroplasty and returning them to full duty. While there is a single report of clinical results of lumbar arthroplasty in athletes, at this writing, there are no reports of either cervical or lumbar arthroplasty in active duty military personnel.

METHODS

The surgical database at a single, tertiary care military treatment facility was queried for all active-duty patients who underwent placement of either a cervical or lumbar arthroplasty device over a 3-year period. The authors performed a retrospective chart review to collect patient and procedural data including blood loss, length of hospital stay, tobacco use, age, rank, complications, and ability to return to full unrestricted active duty. Arthroplasty cohorts were then compared to historical controls of arthrodesis to ascertain differences in the time required to return to full duty.

RESULTS

Twelve patients were identified who underwent cervical arthroplasty. All patients returned to unrestricted full duty. This cohort was then compared with 12 patients who had undergone a single-level anterior cervical discectomy and fusion. The average time to return to unrestricted full duty for the arthroplasty group was 10.3 weeks (range 7-13 weeks), whereas that in the fusion group was 16.5 weeks. This difference between these 2 groups was statistically significant (p = 0.008). Twelve patients were identified who underwent lumbar arthroplasty. Ten (83%) of 12 patients in this group returned to unrestricted full duty. In patients who returned to full duty, it took an average of 22.6 weeks (range 12-29 weeks). This cohort was then compared with one in which patients had undergone anterior lumbar interbody fusion. Eight (67%) of 12 patients in the lumbar arthrodesis group returned to unrestricted full duty. In patients who returned to full duty, it took an average of 32.4 weeks (range 25-41 weeks). This difference was not statistically significant (p = 0.156).

CONCLUSIONS

The preliminary experience with cervical and lumbar arthroplasty at the authors' institution indicates that arthroplasty is comparable with arthrodesis and may actually expedite return to active duty. Patients are capable of returning to a high level of rigorous training and physical performance. There are no apparent restrictions or limitations that are required after 3 months in the cervical patient and after 6 months in the lumbar patient. Further prospective studies with long-term follow-up are indicated and will be of value when determining the role of arthroplasty compared to arthrodesis in the active-duty population.

Rheology of intervertebral disc: an ex vivo study on the effect of loading history, loading magnitude, fatigue loading, and disc degeneration

STUDY DESIGN

An ex vivo biomechanical study on the rheological properties of healthy porcine and degenerated human intervertebral disc.

OBJECTIVE

To quantify the effect of loading history, loading magnitude, fatigue loading, and degeneration on disc rheology.

SUMMARY OF BACKGROUND DATA

Disc rheological parameters, i.e., the aggregate modulus (HA) and hydraulic permeability (k) regulate the mechanical and biologic function of disc. The knowledge of effects of loading condition and degeneration on disc rheology can be beneficial for the design of new disc/nucleus implants or therapy.

METHODS

The following 4 phases of experiments were conducted to find the changes of disc rheological properties: (1) Effect of loading history during 1-hour creep (640 N) and relaxation (20% strain) test. (2) Effect of loading magnitude (420 N vs. 640 N) during the creep test. (3) Effect of fatigue loading (420 N, 5 Hz for 0.5, 1, and 2 hours) on the creep loading behavior. (4) Difference of healthy porcine and degenerated human discs during creep loading. The experimental data were fitted with linear biphasic model.

RESULTS

The aggregate modulus increased but hydraulic permeability decreased during creep loading. The aggregate modulus decreased but the hydraulic permeability did not change significantly during relaxation loading. The higher creep loading increased the aggregate modulus but decreased the hydraulic permeability. The fatigue loading did not change the aggregate modulus significantly, but decreased hydraulic permeability. Comparing the degenerated human disc to the healthy porcine disc, the aggregate modulus was higher but the hydraulic permeability was lower.

CONCLUSION

The external loading and degeneration induce disc structural changes, e.g., the disc water content and interstitial matrix porosity, hence affect the disc rheological properties. The increase of aggregate modulus may be due to the reduction of disc hydration level, whereas the decrease of hydraulic permeability may be because of the shrinkage of disc matrix pores.

Normal functional range of motion of the lumbar spine during 15 activities of daily living

STUDY DESIGN

Prospective clinical study.

OBJECTIVE

The purpose of this investigation was to quantify normal lumbar range of motion (ROM) and compare these results with those used to perform 15 simulated activities of daily living (ADLs) in asymptomatic subjects.

SUMMARY OF BACKGROUND DATA

Previous studies reporting the ROM of the lumbar spine during ADLs have been limited, only focusing on 4 ADLs. The purpose of this investigation was to quantify the extent of normal lumbar ROM and determine how much motion is necessary to perform 15 simulated ADLs.

METHODS

A noninvasive electrogoniometer and torsiometer were used to measure the ROM of the lumbar spine. The accuracy and reliability of the devices were confirmed by comparing the ROM values acquired from dynamic flexion/extension and lateral bending radiographs with those provided by the device that was activated while the radiographs were obtained. Intraobserver reliability was established by calculating the intraclass correlation coefficient for repeated measurements on the same subjects by 1 investigator on consecutive days. These tools were employed in a clinical laboratory setting to evaluate the full active ROM of the lumbar spines (ie, flexion/extension, lateral bending, and axial rotation) of 60 asymptomatic subjects (30 women and 30 men; age 20 to 75 y) and to assess the functional ROM required to complete 15 simulated ADLs.

RESULTS

When compared with radiographic measurements, the electrogoniometer was found to be accurate within 2.3+/-2.6 degrees (mean+/-SD). The intraobserver reliabilities for assessing full and functional ROM were both excellent (intraclass correlation coefficient of 0.96 and 0.88, respectively). The absolute ROM and percentage of full active lumbar spinal ROM used during the 15 ADLs was 3 to 49 degrees and 4% to 59% (median: 9 degrees/11%) for flexion/extension, 2 to 11 degrees and 6% to 31% (6 degrees/17%) for lateral bending, and 2 to 7 degrees and 6% to 20% (5 degrees/13%) for rotation. Picking up an object from the ground, either using a bending or squatting technique, required the most ROM of all the ADLs. Squatting required a significantly less amount of sagittal motion compared with bending at the waist (42 vs. 48 degrees, P=0.003). No difference was seen in both lateral and rotation motion between these 2 techniques (11 vs. 11 degrees and 6 vs. 6 degrees, respectively). Both ascending and descending stairs required equivalent amounts of total motion in all 3 motion planes. As a whole, personal hygiene ADLs (hand washing, washing hair, shaving, and make-up application) required a similar amount of motion compared with the 3 locomotive ADLs (walking, up and down stairs).

CONCLUSIONS

By quantifying the amounts of lumbar motion required to execute a series of simulated ADLs, this study indicates that most individuals use a relatively small percentage of their full active ROM when performing such activities. These findings provide baseline data that may allow clinicians to accurately assess preoperative impairment and postsurgical outcomes.

Effect of lumbar disc replacement on the height of the disc space and the geometry of the facet joints: a cadaver study

In a study on ten fresh human cadavers we examined the change in the height of the intervertebral disc space, the angle of lordosis and the geometry of the facet joints after insertion of intervertebral total disc replacements. SB III Charité prostheses were inserted at L3-4, L4-5, and L5-S1. The changes studied were measured using computer navigation software applied to CT scans before and after instrumentation. After disc replacement the mean lumbar disc height was doubled (p < 0.001). The mean angle of lordosis and the facet joint space increased by a statistically significant extent (p < 0.005 and p = 0.006, respectively). By contrast, the mean facet joint overlap was significantly reduced (p < 0.001). Our study indicates that the increase in the intervertebral disc height after disc replacement changes the geometry at the facet joints. This may have clinical relevance.

The effect of different design concepts in lumbar total disc arthroplasty on the range of motion, facet joint forces and instantaneous center of rotation of a L4-5 segment

Although both unconstrained and constrained core lumbar artificial disc designs are in clinical use, the effect of their design on the range of motion, center of rotations, and facet joint forces is not well understood. It is assumed that the constrained configuration causes a fixed center of rotation with high facet forces, while the unconstrained configuration leads to a moving center of rotation with lower loaded facets. The authors disagree with both assumptions and hypothesized that the two different designs do not lead to substantial differences in the results. For the different implant designs, a three-dimensional finite element model was created and subsequently inserted into a validated model of a L4-5 lumbar spinal segment. The unconstrained design was represented by two implants, the Charité disc and a newly developed disc prosthesis: Slide-Disc. The constrained design was obtained by a modification of the Slide-Disc whereby the inner core was rigidly connected to the lower metallic endplate. The models were exposed to an axial compression preload of 1,000 N. Pure unconstrained moments of 7.5 Nm were subsequently applied to the three anatomical main planes. Except for extension, the models predicted only small and moderate inter-implant differences. The calculated values were close to those of the intact segment. For extension, a large difference of about 45% was calculated between both Slide-Disc designs and the Charité disc. The models predicted higher facet forces for the implants with an unconstrained core compared to an implant with a constrained core. All implants caused a moving center of rotation. Except for axial rotation, the unconstrained and constrained configurations mimicked the intact situation. In axial rotation, only the Slide- Disc with mobile core reproduced the intact behavior. Results partially support our hypothesis and imply that different implant designs do not lead to strong differences in the range of motion and the location of center of rotations. In contrast, facet forces appeared to be strongly dependent on the implant design. However, due to the great variability in facet forces reported in the literature, together with our results, we could speculate that these forces may be more dependent on the individual spine geometry rather than a specific implant design.

Effect of lumbar total disc arthroplasty on the segmental motion and intradiscal pressure at the adjacent level: an in vitro biomechanical study

Object

The artificial disc has been proposed as an alternative to spinal fusion for degenerative disc disease. The primary aim of this biomechanical study was to compare motion and intradiscal pressure (IDP) in a ball-and-socket artificial disc–implanted cadaveric lumbar spine, at the operative and adjacent levels, using a displacement-controlled setup. A secondary comparison involved a “salvage” construct, consisting of pedicle screws (PSs) added in supplementation to the artificial disc construct.

Methods

Ten human cadaveric lumbosacral spines (L2–S1) were potted at L-2 and S-1. All measurements were initially made in the intact spine, followed by implantation of the artificial disc, and finally by the salvage PS condition. For the artificial disc condition, a Maverick ball-and-socket artificial disc was implanted at L4–5. For the PS condition, CD Horizon PSs were placed at L4–5, and the artificial disc was left in place. A displacement-controlled, custom-designed testing apparatus was used to impart motion in the sagittal and coronal planes. Motion at both the implanted level (L4–5) and immediately adjacent levels (L3–4 and L5–S1) was measured. Intradiscal pressure at the rostral adjacent level (L3–4) was also measured. The Tukey test was used for statistical analysis (p < 0.05).

Results

In flexion, no significant difference was noted between the artificial disc and the intact spine with regard to motion at the operative level, motion at adjacent levels, or IDP. In lateral bending, while the artificial disc significantly decreased operative-level motion (p < 0.05), no significant difference was noted in adjacent-level motion or IDP. With regard to extension, the artificial disc significantly increased operative level motion and decreased the rostral adjacent level (L3–4) motion and IDP (p < 0.05). Caudal adjacent-level (L5–S1) motion was not significantly different.

In flexion and lateral bending, the addition of PSs significantly decreased motion at the implanted level when compared with the intact spine and the artificial disc (p < 0.05). This decrease in motion at the index level was associated with a compensatory increase in motion at both adjacent levels in flexion only (p < 0.05), but not in lateral bending (p > 0.05). The IDP was significantly increased in lateral bending but not in flexion. With regard to extension, the significant decrease in IDP that was noted with the artificial disc persisted despite the addition of PSs (p < 0.05).

Conclusions

The artificial disc either maintains or reduces adjacent-level motion and pressure, compared with the intact spine. The addition of PSs to the artificial disc construct leads to significantly increased motion at adjacent levels in flexion and significantly increased IDP in lateral bending. At the operative level, the artificial disc is associated with hypermobility in extension, which is restored to the intact state after the addition of supplementary PSs.

Segmental contribution toward total lumbar range of motion in disc replacement and fusions: a comparison of operative and adjacent levels

STUDY DESIGN

Radiographic results from a prospective, randomized, multicenter trial assessing patients who underwent lumbar total disc replacement (TDR) or circumferential arthrodesis for 1-level degenerative disc disease.

OBJECTIVE

To quantify the relative segmental contribution to total lumbar range of motion (ROM) at the operative level at each adjacent level in lumbar TDR and arthrodesis.

SUMMARY OF BACKGROUND DATA

Although previous studies have evaluated ROM in TDR and fusion, no study has quantified or compared the relative segmental contribution to total lumbar ROM. Further, no study to the best of our knowledge has evaluated the kinematic profile of the more cranial adjacent segments (i.e., the second or third adjacent levels) following implantation of either TDR or fusion.

METHODS

Radiographic data collected from all randomized 1-level degenerative disc disease patients operated at L4/5 or L5/S1 that participated in the multicenter, prospective, randomized IDE study comparing ProDisc-L with circumferential lumbar arthrodesis were evaluated before surgery and at 24 months. Radiographic measurements were performed independently using custom digitized image stabilization software to generate ROM at the operative level, and at each cranial and caudal adjacent level.

RESULTS

There were 200 total patients included (155 ProDisc-L, average age 39 years; 45 arthrodesis, average age 40 years). At 24 months, the L4/5 TDR group experienced a significant improvement in total lumbar ROM from baseline (+6.3 degrees ), whereas there was no change seen with L5/S1 TDR or any fusion group. Between-group comparisons from baseline to 24 months postoperatively revealed: (1) significantly more contribution from the operative level towards total lumbar range in TDR at operative level L4/5 (TDR: -2.5%, fusion: -16.8%, P = 0.006), and operative level L5/S1 (TDR: -5.1%, fusion: -15.9%, P < 0.001), and (2) the relative contribution towards total lumbar range of motion from the first cranial adjacent segment to fusion at L5/S1 increased by 12.1%, compared with -1.2% seen in TDR (P = 0.03). There were elevated contributions from the more cranial adjacent levels to a fusion when compared with TDR, however, these differences were not statistically significant. At operative level L4/5, there was significantly increased ROM from the first caudal segment below TDR (6%, P = 0.03), but not below fusion (3.1%, P = 0.59).

CONCLUSION

In conclusion, patients with TDR lost slight relative contribution to total lumbar motion from the operative level which was mostly compensated for by the caudal adjacent level (if operated at L4/5). In contrast, the significant loss of relative range of motion contribution from the operative level in fusions was redistributed among multiple cranial adjacent levels, most notably at the first cranial adjacent level.

Design concepts in lumbar total disc arthroplasty

The implantation of lumbar disc prostheses based on different design concepts is widely accepted. This paper reviews currently available literature studies on the biomechanics of TDA in the lumbar spine, and is targeted at the evaluation of possible relationships between the aims of TDA and the geometrical, mechanical and material properties of the various available disc prostheses. Both theoretical and experimental studies were analyzed, by a PUBMED search (performed in February 2007, revised in January 2008), focusing on single level TDA. Both semi-constrained and unconstrained lumbar discs seem to be able to restore nearly physiological IAR locations and ROM values. However, both increased and decreased ROM was stated in some papers, unrelated to the clinical outcome. Segmental lordosis alterations after TDA were reported in most cases, for both constrained and unconstrained disc prostheses. An increase in the load through the facet joints was documented, for both semi-constrained and unconstrained artificial discs, but with some contrasting results. Semi-constrained devices may be able to share a greater part of the load, thus protecting the surrounding biological structure from overloading and possible early degeneration, but may be more susceptible to wear. The next level of development will be the biomechanical integration of compression across the motion segment. All these findings need to be supported by long-term clinical outcome studies.