Development of a novel radiographic measure of lumbar instability and validation using the facet fluid sign.

BACKGROUND: Lumbar spinal instability is frequently referenced in clinical practice and the scientific literature despite the lack of a standard definition or validated radiographic test. The Quantitative Stability Index (QSI) is being developed as a novel objective test for sagittal plane lumbar instability. The QSI is calculated using lumbar flexion-extension radiographs. The goal of the current study was to use the facet fluid sign on MRI as the "gold standard" and determine if the QSI is significantly different in the presence of the fluid sign. METHODS: Sixty-two paired preoperative MRI and flexion-extension exams were obtained from a large FDA IDE study. The MRI exams were assessed for the presence of a facet fluid sign, and the QSI was calculated from sagittal plane intervertebral rotation and translation measurements. The QSI is based on the translation per degree of rotation (TPDR) and is calculated as a Z-score. A QSI > 2 indicates that the TPDR is > 2 std dev above the mean for an asymptomatic and radiographically normal population. The reproducibility of the QSI was also tested. RESULTS: The mean difference between trained observers in the measured QSI was between -0.28 and 0.36. The average QSI was significantly (P = 0.047, one-way analysis of variance) higher at levels with a definite fluid sign (2.3±3.2 versus 0.60±2.4). CONCLUSIONS: Although imperfect, the facet fluid sign observed may be the best currently available test for lumbar spine instability. Using the facet fluid sign as the "gold standard" the current study documents that the QSI can be expected to be significantly higher in the presence of the facet fluid sign. This supports that QSI might be used to test for sagittal plane lumbar instability. CLINICAL RELEVANCE: A validated, objective and practical test for spinal instability would facilitate research to understand the importance of instability in diagnosis and treatment of low-back related disorders.

Optimizing success with lumbar disc arthroplasty.

PURPOSE: This study aimed at determining the variables that may prove useful in predicting clinical outcomes following lumbar disc arthroplasty. METHODS: Pre- and post-operative imaging assessments were obtained for 99 single-level lumbar disc arthroplasty patients from a prospective IDE study. The assessments and patient demographics were tested to identify variables that were significantly associated with clinical outcomes. RESULTS: Clinical outcome data were available for 85 % of patients at the 5-year follow-up. Numerous assessments made from the pre-operative imaging were found to have statistically significant associations with clinical outcomes at 2 and 5 years. The most notable factors were related to the amount of degeneration at the index level, with patients achieving better outcome scores at 5 years if they have higher grades of degeneration preoperatively. CONCLUSIONS: Several variables may prove effective at optimizing clinical outcomes including a preoperative disc height <8 mm, Modic type 2 changes adjacent to the target disc, a low amount of lordosis present at the treatment level, low levels of fatty replacement of the paraspinal musculature, a prominent amount of facet joint or disc degeneration, and the presence of flat or convex vertebral endplates. There were also post-operative findings associated with better patient outcomes including a larger percent of the endplate covered with the implant, larger implant heights, greater increases in disc space heights, and a larger increase in index level lumbar lordosis. These variables could be explored in other clinical studies to facilitate meta-analyses that could identify effective strategies to optimize clinical outcomes with lumbar disc arthroplasty.

Asymmetric motion distribution between components of a mobile-core lumbar disc prosthesis: an explanation of unequal wear distribution in explanted CHARITÉ polyethylene cores.

BACKGROUND: The biconvex mobile core of the CHARITÉ lumbar disc prosthesis forms two joints (spherical bearings) with the metal end plates. We quantified the intra-prosthesis motion to test the hypothesis that the total prosthesis motion would not be equally distributed between the two bearings of implanted CHARITÉ discs, which might explain the unequal wear distribution reported in explanted cores. METHODS: The hypothesis was tested by studying the flexion-extension motion responses of (1) twenty-six monosegmental CHARITÉ III discs implanted in nineteen human cadaveric lumbar spines, and (2) twenty-one CHARITÉ III discs (fifteen monosegmental, six bisegmental) implanted in eighteen patients in other published clinical studies. Intra-prosthesis motions were quantified with use of a radiographic image analysis technique. RESULTS: Eighty-eight percent of the CHARITÉ discs implanted in cadaveric specimens exhibited larger motion at the superior bearing, with 54% demonstrating more than twice as much motion at the superior bearing as at the inferior bearing. The ratio of motion at the superior bearing to motion at the inferior bearing averaged 2.68 ± 1.84, which was significantly larger than 1.0 (p < 0.001). Ninety percent of prostheses implanted in patients showed larger motion at the superior bearing. The motion ratio averaged 2.39 ± 2.47 for monosegmental cases and 2.55 ± 2.66 for all cases; both ratios were significantly larger than 1.0 (p < 0.05). CONCLUSIONS: We found preferentially larger motion at the superior bearing of the CHARITÉ discs implanted in human cadaveric lumbar spines and in patients, regardless of the implanted level.

Sagittal alignment after Bryan cervical arthroplasty.

STUDY DESIGN: Prospective, randomized, Food and Drug Administration Investigational Device Exemption trial from one study site. OBJECTIVE: Examine the radiographic sagittal alignment of the Bryan cervical disc for one-level disease. SUMMARY OF BACKGROUND DATA: Prospective, randomized studies demonstrate Bryan arthroplasty provides statistically better functional outcomes than anterior cervical discectomy and fusion. Uncontrolled case reports describe kyphosis after disc replacement. No prospective study has critically assessed sagittal alignment after cervical arthroplasty. METHODS: Forty-eight patients reviewed with a minimum follow-up of 2 years. Quantitative motion analysis determined the change in overall (C2-C7) and treatment-level sagittal alignment, disc space heights, and range of motion. RESULTS: Preoperatively, overall sagittal alignment was equivalent in the two groups. At 24-month follow-up, overall lordosis for the cohorts was not statistically different from preoperative values for each group. In addition, overall lordosis was not significantly different at 24 months when comparing Bryan patients with the fusion patients. The average change in disc angle from preoperative to immediate postoperative at the treated level in the Bryan disc group was a nonsignificant increase in lordosis of 0.92°. The anterior disc height was the same at all time points, but the posterior disc height was slightly (0.7 mm) more in the Bryan than in the fusion patients (P = 0.04). The angular range of motion in the Bryan group was statistically equivalent at all time points. At the fused levels, average range of motion decreased from 6.4° to 0.9° at 24 months (P < 0.0001). CONCLUSION: With the Bryan disc, there was an insignificant increase in lordosis of 0.9° at immediate postoperative time point. Overall cervical sagittal alignment is not different between the experimental and control populations. This prospective study does not demonstrate a clinically significant increase in segmental kyphosis after Bryan disc arthroplasty. Global cervical lordosis is statistically equivalent between arthroplasty and fusion groups at 2 years follow-up.

ProDisc-C: an in vivo kinematic study.

STUDY DESIGN: A prospective study of 22 patients with single-level cervical spondylosis. OBJECTIVE: To quantify the changes in the biomechanics of the cervical spine after the insertion of a ProDisc-C (Synthes Spine, Paoli, PA) artificial disc. SUMMARY OF BACKGROUND DATA: Cervical arthroplasty is designed to maintain cervical motion of the functional spinal unit (FSU) after cervical discectomy. The impact of the ProDisc-C on in vivo kinematics and sagittal alignment requires further assessment. METHODS: Flexion/extension lateral radiographs of the cervical spine were prospectively collected and reviewed in 22 patients preoperatively and after the placement of ProDisc-C. Disc height (DH), FSU angle, range of motion (ROM), and center of rotation (COR) were calculated at each time point using validated, computer-assisted methods. Preoperative values were compared with the postoperative values using paired student t tests. RESULTS: Mean follow-up time was 11.0 months (SD, 2.4 mo). At the surgical level, the ProDisc-C produced increased segmental ROM (P = 0.03), an anterior shift of 1.1 mm in COR X (P = 0.004), and increased DH both anteriorly (P < 0.0001) and posteriorly (P < 0.0001). At the inferior adjacent level, anterior DH decreased (P < 0.05) and posterior DH increased (P = 0.02) after surgery. The FSU angle at the surgical level increased (P < 0.0001), but the inferior adjacent-level FSU angle decreased (P = 0.002). No significant changes were observed at the superior adjacent level. CONCLUSIONS: The ProDisc-C increased segmental ROM and shifted the COR anteriorly at the surgical level. DH increased at the surgical level. The surgical level became more lordotic, whereas the inferior adjacent level became more kyphotic after ProDisc-C implantation.

Prospective analysis of imaging prediction of pseudarthrosis after anterior cervical discectomy and fusion: computed tomography versus flexion-extension motion analysis with intraoperative correlation.

STUDY DESIGN: A prospective comparison of quantitative motion analyzed (QMA) flexion/extension radiographs versus computed tomography (CT) as an analytical predictor of cervical pseudarthrosis. Intraoperative confirmation of the fusion was performed. OBJECTIVE: To prospectively compare motion analyzed flexion/extension radiographs to CT to predict pseudarthroses. Define motion thresholds on flexion/extension radiographs to define pseudarthroses. SUMMARY OF BACKGROUND DATA: Assessment of postoperative fusion success is an important factor in assessing success after anterior cervical spine fusion. Gross intervertebral motion can be used as a measure; however, the current "gold standard" for determining fusion status is a CT to assess bridging bone. Defining the amount of intervertebral motion at the fusion site has been previously addressed and definitions have varied widely. METHODS: Data were analyzed at 47 fusion segments. Intervertebral motion at the fusion site was measured from flexion/extension radiographs taken at least 1 year after the cervical spine fusion. Motion was quantified from digitized radiographs by an independent researcher using proprietary quantitative motion analysis (QMA) software. CT scans on all patients were analyzed for fusion status by a neuroradiologist. Those patients determined to have a symptomatic pseudarthrosis were revised and intraoperative motion at the facet joints was documented. Correlation between intraoperative findings, CT and QMA was performed. RESULTS: Using greater than 4° of measured motion on flexion/extension radiographs resulted in a Spearman correlation P-value of 0.096 (95% confidence interval: -0.06 to 0.66). Using greater than 1° of motion, the Spearman correlation P < 0.0001 (95% CI: 0.54-0.90). The positive predictive value (PPV) using 4° of motion as the criterion was 100%, indicating a high specificity. The negative predictive value (NPV) was 52%, indicating a low sensitivity. Using greater than 1° of motion, the PPV was 100% and the NPV was 73%. Findings from CT showed an identical PPV and NPV to assessments made using greater than 1° of rotation. Specificity and positive predictive value were 100% for all criteria. Using a lack of bridging on CT or more than 1° of intervertebral motion during flexion/extension increased the sensitivity to 85% and the negative predictive value to 85%. CONCLUSION: A threshold level of 4° of motion is commonly used to identify a pseudarthrosis. Our prospective study suggests that this value has a high PPV, but a low specificity and would miss many of the pseudarthroses that have angular motion less than 4° (sensitivity 23%). By lowering the threshold for angular motion to 1°, the sensitivity improves to 77%. CT scan has been touted as the gold standard, and it has a high positive predictive value of 100%. However, its NPV was slightly lower than using 1° of motion on QMA analyzed flexion-extension films (73% vs. 79%). In conclusion, although CT scan has long been regarded as the gold standard for determining a pseudarthrosis in the cervical spine, the interpretation is subjective and vulnerable to both type I and type II errors. Analysis of motion using Quantitative Motion Analysis is seemingly less subjective than CT and in our prospective study was more predictive of an operatively confirmed pseudarthrosis.

Determination of the in vivo posterior loading environment of the Coflex interlaminar-interspinous implant.

BACKGROUND CONTEXT: The in vivo loading environment of load-bearing implants is generally largely unknown. Loads are typically approximated from cadaver tests or biomechanical calculations for the preclinical assessment of a device's safety and efficacy. PURPOSE: To determine the actual in vivo loading environment of an elastic interlaminar-interspinous implant (Coflex). STUDY DESIGN: A retrospective radiographic study to noninvasively measure the in vivo implant loads of 176 patients. METHODS: For this study, neutral, flexion, and extension radiographs were quantitatively analyzed using validated image analysis technology. The angle between the Coflex arms was measured for each radiograph and statistically evaluated. Separately, the Coflex implant was characterized using mechanical test data and finite element analysis, which resulted in a load-deformation formula that describes the implant load as a function of its size and elastic deformation. Using the formula and the elastic implant deformation data obtained from the radiographic analysis, the exact implant load was calculated for each patient and each posture. For statistical analysis, the patients were grouped by indication and procedure, which resulted in 12 different groups. The determined loads were compared with the strength of the posterior lumbar spinal elements obtained from the literature and with the static and dynamic mechanical limits of the Coflex interlaminar-interspinous implant. RESULTS: The force data were independent of implant size, diagnosis (with one exception), number of levels of the decompression procedure, number of levels of implantations (one or two), and follow-up time. The median compressive force acting on the Coflex implant was found to be 45.8 N. The maximum load change between flexion and extension was 140 N; the maximum overall load exceeded 239 N in extension. CONCLUSIONS: The average loads exerted by the Coflex implant on the spinous process and lamina are 11.3% and 7.0% of their respective static failure load. The implant fatigue strength is significantly higher than the measured median force, which explains the extremely rare observation of a Coflex fatigue failure.

Sagittal balance influences range of motion: an in vivo study with the ProDisc-C.

BACKGROUND CONTEXT: Cervical arthroplasty is designed to maintain cervical motion of the functional spinal unit after cervical discectomy. The impact of the ProDisc-C (Synthes Spine, Paoli, PA) on in vivo kinematics and sagittal alignment requires further assessment. PURPOSE: The purpose of this study is to test the hypothesis that the ProDisc-C increases range of motion (ROM) in flexion and extension at the surgical level, and assess its impact on cervical alignment. STUDY DESIGN: Clinical study. PATIENT SAMPLE: Fifteen patients with a mean age of 49 years were included in this study. Each patient had a single-level arthroplasty. Patients with multilevel arthroplasty, previous cervical spine surgery, and length of follow-up less than 6 months were excluded from this study. OUTCOME MEASURE: Lateral dynamic radiographs of the cervical spine were analyzed using quantitative measurement analysis (QMA) preoperatively and postoperatively. QMA software was also used to determine the ROM and sagittal translation at the surgical level. METHODS: Flexion/extension lateral radiographs of the cervical spine were prospectively collected and reviewed in 15 patients preoperatively and at early (1-3 months) and late (6-14 months) follow-up after placement of the ProDisc-C. Shell angle (SA) and C2-C7 Cobb angles were measured. Sagittal translation and ROM were calculated at each time point. Preoperative values were compared with early and late follow-up values using paired Student t tests and Pearson's correlation. RESULTS: The C2-C7 Cobb angle did not change significantly after surgery. Segmental ROM increased significantly from neutral to flexion (p=.02) and neutral to extension (p=.002) at late follow-up. SA correlated significantly with ROM from neutral to extension (Pearson's r=-0.55; p=.02) and translation from neutral to extension (Pearson's r=-0.58; p=.02). CONCLUSIONS: The ProDisc-C increased overall segmental ROM. A lordotic SA may be associated with restricted segmental ROM and translation in extension. This study did not detect any change in overall cervical spinal alignment after insertion of the device.

Short-term comparison of cervical fusion with static and dynamic plating using computerized motion analysis.

STUDY DESIGN: This study is a retrospective review of fusion rates for cervical plates, analyzed by means of computerized analysis. OBJECTIVES: This study compares the fusion rates for two-level anterior cervical discectomy and fusion between patients with static versus dynamic plates. SUMMARY OF BACKGROUND DATA: Anterior cervical plating has been shown to decrease the pseudarthrosis rate. However, static plates, which have been successful in reducing nonunion rates, may be "too rigid" in certain situations, leading to pseudarthrosis in some patients. Recently, some surgeons have begun using dynamic plate constructs to avoid this problem. METHODS: A retrospective review was performed of patients having a two-level anterior cervical discectomy and fusion performed either with a static or dynamic plate. A computerized method for evaluating the presence of a solid fusion was used with a criterion of <2 degrees of motion considered a solid fusion. RESULTS: The follow-up time period averaged 10 months (range, 5.8-13 months) for the static plate group and 9.5 months (range, 5.8-13 months) for the dynamic plate group. Based on a motion threshold of 2 degrees, the rate of fusion per level for patients in the static plate/autograft group was 87.8%, resulting in an overall fusion rate of 76.2%. The rate for fusion per level for patients treated with a dynamic plate and allograft was 89.8%, with an overall fusion rate of 81.8%. There was no statistically significant difference between the two groups (P = 0.469). The fusion rate increased during the follow-up period: In the 6- to 9-month interval, the static plate/autograft group had a 62.5% fusion rate, versus 75% for the dynamic plate/allograft group. In the 10- to 13-month interval, the fusion rate had increased to 84.7% for the static plate/autograft group and 90% for the dynamic plate/allograft group. CONCLUSIONS: Computerized evaluation of digitized films can improve the accuracy and reproducibility of the analysis of anterior cervical fusion. An angular threshold of 2 degrees was selected for this purpose. This study showed that the rate of fusion with a dynamic plate was similar to that of the static plate despite the use of allograft bone with the dynamic plate. In addition, this study found that successful fusions continued to evolve throughout the first year following surgery.

Defining pseudoarthrosis in the cervical spine with differing motion thresholds.

STUDY DESIGN: Calculate the apparent pseudarthrosis rate for anteriorly plated cervical spine fusions using different threshold levels of intervertebral rotation. OBJECTIVES: Determine whether the apparent pseudarthrosis rate depends on the amount of intervertebral rotation used to characterize a fusion as a pseudarthrosis. SUMMARY OF BACKGROUND DATA: Intervertebral motion at a spine fusion site is an important outcome measure in most spine fusion research studies. Intervertebral motion measures are used to determine if a device can be sold for use in spine fusion surgery. There is no consensus on how much motion is too much motion at a spine fusion site, and the intervertebral motion threshold that is used to define a pseudarthrosis is arbitrary. There are only limited data on the dependence of the pseudarthrosis rate on the threshold used to define a pseudarthrosis. METHODS: Intervertebral motion at the fusion site was measured from flexion extension radiographs taken 1 year after cervical spine fusion using anterior plates and allograft in 200 patients. The number of cases where intervertebral motion at the fusion site exceeded thresholds between 1 degrees and 4 degrees were calculated. RESULTS.: The apparent pseudarthrosis rate varied from 6% when the intervertebral motion threshold was 4 degrees to 44% when the intervertebral motion threshold was 1 degrees . CONCLUSIONS: The apparent pseudarthrosis rate was highly dependent on the threshold of motion used to define a pseudarthrosis.