Study design: Cross-sectional study of 10 patients to measure the serum levels of cobalt and chromium after TDA. Summary of Background Data: In total hip endoprosthetics and consequently for TDA (total disc arthroplasty), metal-on-metal combinations are used with the aim of reducing wear debris. In metal-on-metal TDA the release of metal ions has until now been secondary to the main discussion. Objectives: To investigate the release of cobalt and chromium ions into the serum following implantation of the metal-on-metal MaverickTM type artificial lumbar disc. Materials and methods: We investigated the serum cobalt and chromium concentration following implantation of 15 Maverick™ TDAs (monosegmental L5/S1 n=5; bisegmental L4/5 and L5/S1 n=5; average age = 36.5 years). 5 healthy subjects (no metal implants) acted as a control group. The measurements of the metals were carried out using the HITACHI Z-8200 AAS polarized Zeeman atomic absorption spectrometer after an average of 14,8months. Results: The concentrations of cobalt and chromium ions in the serum amounted on average to 4.75μg/l (standard deviation 2,71) for cobalt and 1.10μg/l (standard deviation 1,24) for chromium. Compared to the control group, both the chromium and cobalt levels in the serum showed significant increases (Mann-Whitney Rank Sum Test, p=0.0120). At follow-up the Oswestry Disability Score was on average significantly decreased by 24.4 points (L5/S1) (t-test, p < 0.05) and by 26.8 points (L4-S1)(t-test, p < 0.05). The improved clinical situation is also represented by a significant decrease of the Visual Analog Pain Scale of 42,2points after the follow-up (t-test, p< 0,05). Conclusion: Significant systemic release of Cr/Co was proven in the serum compared to the control group. The concentrations of Cr/Co measured in the serum are similar in terms of their level to the values measured in THA metal-on-metal combinations or exceed these values given in the literature. Long-term implication of this metal exposuere in unknown and should be studied further

Introduction: ACDF involves cancellation of the diseased spinal motion segment, the neighboring spinal segments take the burden of excessive compensatory spinal movements and strain resulting in early degeneration. Adjacent segment degeneration with new, symptomatic radiculopathy occurs after ACDF in 2–3% of patients per year on cumulative basis. An estimated 15% of patients ultimately require a secondary procedure at an adjacent level. An alternative to fusion is total disc arthroplasty (TDA). The key advantage of this promising technology is restoration and maintenance of normal physiological motion rather than elimination of motion. We describe 4 patients with a serious complication observed following implantation of the Bryan disc prosthesis in our cohort of 48 patients. Material and Results:. Patient #1: 43 M, with neck pain & left brachalgia, with left C6 dermatome signs, with MRI findings of C5/6 disc prolapse with left C6 root impingement, undergoing C5/6 Bryan TDA in April 2004, with treatment recommendation of C3/4 and C6/7 Bryan TDA in January 2006. Patient #2: 47 M, with worsening gait over 2 years with right brachalgia, with findings of progressive cervical myelopathy with right C5 radiculopathy, with MRI findings of severe C5/6 disc degeneration with spinal cord compression, undergoing C5/6 Bryan TDA in January 2003, with MRI FU findings after 16/12 with new left C6/7 disc prolapse and left C6 radiculopathy, with treatment recommendation of C6/7 Bryan TDA, on waiting list. Patient #3: 45 F, 6 years of neck pain with right brachialgia, with right C5 dermatome signs, with MRI findings of C5/6 central disc herniation with cord compression, undergoing C5/6 Bryan TDA in December 2000, with FU MRI showing after 5 years and 7/12 (67/12) new C6/7 canal narrowing with right C6 radiculopathy, and treatment recommendation of C6/7 Bryan TDA. Patient #4: 38 M, worsening gait over 5 years and exam findings of progressive cervical myelopathy, with MRI findings of severe C5/6 disc degeneration with spinal cord compression, undergoing C5/6 Bryan TDA in August 2003, with FU MRI showing after 3 years new C4/5 disc prolapse with C5 radiculopathy, followed by treatment recommendation of C4/5 Bryan TDA. Discussion & Conclusions: Bryan TDR did not prevent the development of accelerated ASD. Evidence from in vivo X ray studies suggested that the range of motion across the operated levels did not match the physiological ROM. Despite the MRI images preoperatively, it is difficult to exclude the natural progression of degeneration as a reason for ASD

Purpose: To investigate subsidence of the Charite total disc arthroplasty (TDA) and to identify if a discrepancy between vertebral endplate and the Charite footprint predispose to subsidence. Method: Between July 2001 and May 2008, 69 patients underwent a Charite TDA (DePuy Spine, Raynham, MA). They were prospectively followed at 3, 6, 12 months, and once a year thereafter. The following measurements were performed on the replaced motion segment using a lateral radiograph:. The anterior-posterior (AP) dimension of the end plates. Amount of subsidence. The distance between the TDA and the posterior and anterior borders of the vertebra bodies (to represent the extent of uncoverage of the endplate by the TDA). The AP dimension of the TDA metal end-plate. The ratio between the actual and radiographic AP length of the metal endplate was calculated and utilized as the correction factor for the error of magnification on all other radiographic measurements. Results: At L5-S1 the mean subsidence was 1.87 mm and occurred exclusively at the posterior part of the inferior end plate of L5. The mean posterior uncoverage was 3.5 mm (L5) and 0.27mm (S1). At L4-L5 the mean subsidence was 1.48 mm (L4) and 0.56 mm (L5). Posterior uncoverage of L4 and L5 vertebrae were 4.81 and 2.22 mm, respectively. Subsidence of more than 1 mm was present in all cases where the posterior uncoverage of the end plate with the TDA was more than 2 mm (odds ratio: 5.7). Subsidence was non – progressive in all cases. An anatomic mismatch exists between L5 and S1 endplates in the AP dimension; in more than half the patients S1 is shorter than L5. Conclusion: The radiographic measurements suggest an increased likelihood of subsidence with more than 2 mm of posterior uncoverage of the end plate by the TDA. The endplate AP length of S1 is frequently less than that of L5. Implant selection based on the smaller S1 endplate may produce worrisome uncoverage of the L5 inferior endplate leading to an increased risk of subsidence and possible catastrophic failure. TDA design should afford modularity to compensate

Purpose: To investigate subsidence of the Charite total disc arthroplasty (TDA) and to identify if a discrepancy between vertebral endplate and the Charite footprint predispose to subsidence. Methods: Between July 2001 and May 2008 64 patients underwent a Charite TDA (DePuy Spine, Raynham, MA). They were prospectively followed at 3, 6, 12 months, and once a year thereafter. The following measurements were performed on the replaced motion segment using a lateral radiograph:. The anterior-posterior (AP) dimension of the end plates. Amount of subsidence. The distance between the TDA and the posterior and anterior borders of the vertebra bodies (to represent the extent of uncoverage of the endplate by the TDA). The AP dimension of the TDA metal endplate. The ratio between the actual and radiographic AP length of the metal endplate was calculated and utilized as the correction factor for the error of magnification on all other radiographic measurements. Results: At L5-S1 the mean subsidence was 1.87 mm and occurred exclusively at the posterior part of the inferior end plate of L5. The mean posterior uncoverage was 3.5 mm (L5) and 0.27 mm (S1). At L4–L5 the mean subsidence was 1.48 mm (L4) and 0.56 mm (L5). Posterior uncoverage of L4 and L5 vertebrae were 4.81 and 2.22 mm, respectively. Subsidence of more than 1 mm was present in all cases where the posterior uncoverage of the end plate with the TDA was more than 2 mm (odds ratio: 5.7). Subsidence was non – progressive in all cases. An anatomic mismatch exists between L5 and S1 endplates in the AP dimension; in more than half the patients S1 is shorter than L5. Conclusion: The radiographic measurements suggest an increased likelihood of subsidence with more than 2 mm of posterior uncoverage of the end plate by the TDA. The endplate AP length of S1 is frequently less than that of L5. Implant selection based on the smaller S1 endplate may produce worrisome uncoverage of the L5 inferior endplate leading to an increased risk of subsidence and possible catastrophic failure. TDA design should afford modularity to compensate. No financial benefits or funding has been received for the completion of this study

Introduction: Degenerative disc desease is one of the most frequently encountered spinal disorders. The intervertebral disc is a complex anatomic and functional structure, which makes the development of an efficient artificial disc a challenge [. 1. ]. Based on the complexity of the anatomical structures and the nearly unknown loading conditions at the moment only contradictory knowledge exists about the kinematics after TDA and in particular the location of the center of rotation in the human lumbar spine [. 2. ]. The objective of our study was to evaluate the kinematics of the human lumbar spine and the ability of TDA to restore the native conditions in regard to range of motion (ROM), neutral zone (NZ) and center of rotation (COR). Material and Methods: In-vitro flexibility testing on functional spinal units (FSU) out of 12 fresh frozen lumbar spines has been performed. The FSU (L2/L3 and L4/L5) were tested first in the native condition, followed by nucleotomy and partial annulus resection and also after TDA with activ L (lumbar artificial disc, Aesculap Germany). Therefore a spinal simulator has been customized, applying pure moments for flexion/extension, lateral bending and axial rotation (+/−7.5Nm) and axial preload (FP=400N) with a defined velocity (1°/s). The instantaneous COR has been calculated based on the velocity pole method using a 3D ultrasonic motion analysis system, measuring the twelve components of motion. Results: The TDA with activ L leads to a good restoration of ROM and NZ in all loading directions under in-vitro flexibility testing. The instantaneous COR is exemplary described for the native condition under flexion/ extension in the sagittal plane. For the native condition the COR is located in the center of the inferior vertebral endplate. After nucleotomy the COR shifts dorsally into the region of the spinal cord and a significant grade of instability has been measured. After insertion of the lumbar artificial disc the instability can be reduced to the native grade of motion and the COR is located again in the main axis of the spinal column in the upper third of the inferior vertebra. Conclusion: The instantaneous COR has been estimated in-vitro for the different loading situations in the human lumbar spine before and after TDA. Based on the newly introduced method further optimizations of TDA devices can be undergone in regard to the particular aspect of physiological kinematics

Purpose: To report the clinical and radiographic prospective results of a consecutive series of patient with a minimum two year follow-up with the Charite Total Disc Arthroplasty (TDA). Method: Between 2001 and 2005, sixty patients underwent a Charite TDA (Depuy Spine, Raynham, MA) at either L4-5 or L5-S1. The primary indication for surgery was discogenic low back pain confirmed by provocative discography. Clinical assessment was carried out preoperatively and postoperatively at 3, 6, 12 months, and once a year thereafter using the Oswestry Disability Index (ODI), Visual Analogue Scale (VAS) for back and leg pain, and SF-36. Radiographic analysis included: angle of sagittal rotation, translation of the rostral vertebra onto the caudal vertebra, anterior vertical motion (AVM), middle vertical motion (MVM), posterior vertical motion (PVM), pre- and post-operative lumbar lordosis, disc height and subsidence of the TDA. The radiographic measurements were performed using the GE Medical Systems Centricity PACS Software Version 1.0. Results: There were 36 female and 24 male patients with a mean age of 39 (range 21–59). The mean duration of low back pain was 70 months. Twenty-five percent claimed work compensation status. The mean postoperative hospital stay was 4.8 days. A statistical significant improvement was demonstrated between the mean pre-operative ODI (50) and all post-operative intervals (p< 0.0001) which had declined to 27.7 by one year. Similarly, pre-operative VAS back pain (8.0), leg pain (6.1), SF-36 physical component summary score (33.5) and mental component summary score (41.8) remained improved (p< 0.0001) by three months (4.1, 3.1, 51.7, 62.0 respectively). One patient with an L5-S1 TDA has since undergone a posterolateral instrumented fusion. The mean pre- and post-operative lumbar lordosis was 34.58 and 53.48 respectively. The mean saggital rotation was 6.5 degrees at 5 year follow-up, while the mean translation was 0.83 mm. The mean AVM, MVM and PVM were 0.59 mm, −3.96 mm and 3.69 mm respectively at 5 year follow-up. Conclusion: This study demonstrates satisfactory clinical results in carefully selected patients. The radiographic assessment confirmed preservation of movement at the replaced disc during flexion and extension of the lumbar spine

Introduction Current evidence suggests that, in the short and medium term, total disc arthroplasty (TDA) is at least as good as fusion for the management of discogenic low back pain. The optimum kinematic design of TDA, however, at this stage has not been identified. This paper compares the clinical outcomes of two TDA prostheses with different kinematic properties. Methods A randomised comparative clinical study with prospective data collection was performed. Two groups of patients with low back pain who were treated surgically with TDA were compared. All patients had primary symptoms of low back pain without significant facet degeneration. All patients had MRI evidence of disc degeneration and positive discography at the target level. The two surgical groups were treated consecutively by the one surgeon. The first chronological group consisted of 23 cases of SB Charité (CHR) prostheses (DePuy Spine) and the second group consisted of 18 cases of Maverick (MAV) disc replacements (Medtronic Sofamor Danek). The CHR group consisted of 16 males and 7 females with an average age of 37.1 years. The MAV group consisted of 10 males and 8 females with an average age of 41.3 years. 17 (74%) of the CHR group were single level cases. 6 (24%) cases were performed at the same time as an adjacent level anterior fusion. There were no two level prostheses performed. One case in the CHR group was lost to follow-up. 10 (55%) of the MAV group were single level cases. 7 (38%) of the MAV group were double level prosthesis cases. There was 1 case performed adjacent to a fusion in this group. Patients were assessed by visual analogue pain scores (VAS), Low-Back outcome score (LBOS), and SF12 health outcomes that were collected preoperatively and at 6 months. Results There was a mean improvement of 3.4 VAS points with the CHR group compared to a mean improvement of 5.1 points with the MAV group (p=0.039). There was a mean percentage improvement in VAS scores of 42.1% with the CHR group compared to a 70.1% improvement in the MAV group (p=0.003). The mean LBOS scores improved by 12.3 points in the CHR group, compared to an improvement of 25.6 points in the MAV group (p=0.016). Percentage improvement in LBOS score was 67.8% for the CHR group compared to 210% for the MAV group (p=0.006). The mean improvement in the PCS score of the SF12 measure was 7.3 for the CHR group compared to 15.6 for the MAV group (p=0.055). The percentage improvement in PCS scores was 23.1% for the CHR group compared to 56.0% for the MAV group (p=0.047). There was no significant difference in the improvement in MCS score of the SF12 measure (6.3 compared to 8.1 p=0.7). Discussion On all outcome measures of pain and physical disability the MAV cases improved by a statistically greater amount than the CHR cases. This difference appears to be clinically important. There are a number of possible reasons for this difference, including the fact that the cases were not performed concurrently (learning curve affect) and that there were a number of fusions performed simultaneously in the CHR group. However one possible explanation is the difference in the kinematic behaviour of the two prostheses

Introduction Central placement of a total disc arthroplasty (TDA) in the coronal plane will result in equivalent facet joint loading, less tendency for lateral core migration, optimum kinematics, and better outcomes. This study was performed to determine which of the radiographic markers – the vertebral body, the pedicles, or the spinous process – provides the most accurate guide to the coronal midline, so to optimise coronal TDA. The coronal midline was defined as the perpendicular bisector of a line drawn between the midpoints of the two facet joints. Methods Axial CT images were reconstructed from 35 abdominal CT’s to show the relevant anatomy at L4, L5, and S1. Measurements were taken comparing the consistency of the midpoints of the vertebral body, the pedicles, and the spinous processes, in relation to the coronal midline. Results The mean distance from the coronal midline to the vertebral body midpoint was 0.55mm (SD 0.45), to the interpedicular midpoint was 0.19mm (SD 0.40), and to the spinous process midpoint was and 1.30mm (SD 1.30). 16% of the distances from the coronal midline to the spinous process midpoint were greater than or equal to 3mm, compared with 0% of the distances to the interpedicular midpoint or the vertebral body midpoint. The interpedicular midpoint was significantly closer to the coronal midline than the spinous process midpoint or the vertebral body midpoint at all levels (p< 0.001). Discussion The interpedicular midpoint is the most accurate guide to the coronal midline. We recommend this landmark be used in preference to the spinous processes or the vertebral body midpoint when placing the implant in TDA. The close location of the interpedicular midpoint to the implant, compared with the more posteriorly located spinous process, means the likelihood of parallax error, by rotation of the patient or the C arm, is reduced using the interpedicular midpoint

Purpose: To report the clinical and radiographic prospective results of a consecutive series of patient with a minimum two year follow-up with the Charite Total Disc Arthroplasty (TDA). Methods: Between 2001 and 2005 sixty patients underwent a Charite TDA (Depuy Spine, Raynham, MA) at either L4–5 or L5-S1. The primary indication for surgery was discogenic low back pain confirmed by provocative discography. Clinical assessment was carried out preoperatively and postoperatively at 3, 6, 12 months, and once a year thereafter using the Oswestry Disability Index (ODI), Visual Analogue Scale (VAS) for back and leg pain, and SF-36. Radiographic analysis included: angle of sagittal rotation, translation of the rostral vertebra onto the caudal vertebra, anterior vertical motion (AVM), middle vertical motion (MVM), posterior vertical motion (PVM), pre- and post-operative lumbar lordosis, disc height and subsidence of the TDA. The radiographic measurements were performed using the GE Medical Systems Centricity PACS Software Version 1.0. Results: There were 36 female and 24 male patients with a mean age of 39 (range 21–59). The mean duration of low back pain was 70 months. Twenty-five percent claimed work compensation status. The mean post-operative hospital stay was 4.8 days. A statistical significant improvement was demonstrated between the mean pre-operative ODI (50) and all post-operative intervals (p< 0.0001) which had declined to 27.7 by one year. Similarly, pre-operative VAS back pain (8.0), leg pain (6.1), SF-36 physical component summary score (33.5) and mental component summary score (41.8) remained improved (p< 0.0001) by three months (4.1, 3.1, 51.7, 62.0 respectively). The mean pre and post-operative lumbar lordosis was 34.58 and 53.48 respectively. The mean sagittal rotation was 6.5 degrees at 5 year follow-up, while the mean translation was 0.83 mm. The mean AVM, MVM and PVM were 0.59 mm, −3.96 mm and 3.69 mm respectively at 5 year follow-up. Conclusion: This study demonstrates satisfactory clinical results in carefully selected patients. The radiographic assessment confirmed preservation of movement at the replaced disc during flexion and extension of the lumbar spine. Interest Statement: No financial benefits or funding has been received for the completion of this study

School of Mechanical Engineering, University of Birmingham, Birmingham, UK. This study investigated the effects on friction of changing the dimensions of a ball-and-socket Total Disc Arthroplasty (TDA). A generic ball-and-socket model was designed and manufactured based on the dimensions and geometry of a metal-on-metal Maverick (Medtronic, Minneapolis, USA) device. Keeping the radial clearance similar to the Maverick, the ball and socket dimensions varied between 10 to 16 mm and 10.015 to 16.015 mm, respectively, in order to enable the comparison between different dimensions. The implants were made out of Cobalt Chrome Molybdenum alloy, with a surface roughness of 0.05 μm. A Bose spine simulator (Bose Corporation, ElectroForce Systems Group, Minnesota, USA) was used to apply an axial compressive force to the TDA. Axial rotation of ±2° was then applied at various frequencies and the resulting frictional torque measured. The tests were performed under an axial load of 50, 600 and 1200 N and frequencies of 0.5, 1.0, 1.5 and 2.0 Hz, for four different samples of radii 10, 12, 14 and 16 mm (48 combinations in total). The results showed variation of frictional torque in different frequencies for all four samples under constant axial load. It was observed that the frictional torque had the lowest value for the implant with ball radius of 16mm. It might be concluded that the implant with larger ball radius may create less friction and hence offer a longer life

Background: Spinal fusion for degenerative disc disease remains a controversial issue due to adjacent segment disease. Several alternatives have been evaluated; including Dynamic Neutralisation System for Spine and artificial disc arthroplasty. In our cohort study, we are highlighting advantages of DYNESYS over TDA and fusion as being a simple and reversible procedure if removal of implants is necessary. Materials and Methods: Prospective cohort study on 415 patients underwent DYNESYS. The implant has to be removed in 43 patients. Patients were evaluated preoperatively using Oswestry Disability Index, SF 36 and Visual Analogue Scores together with plain imaging and MRI scanning. Preoperative discography was performed in 81% of patients. Questionnaires were evaluated on subsequent follow up at 3 & 6 months, 1 year then at annual intervals. Results: Average treatment period was 26 months with a range from 3/12 to 60/12. The main reason for DYNE-SYS removal was screws’ failure or loosening followed by infections. The implant was used to treat single disc space in 20.9%, two spaces in 67.4% and three disc spaces in 9.3%. 81% of the screws were not Hydroxy-appetite coated. No loosening was detected among HA coated screws. Outcome assessment revealed significant improvement in SF36 even after removal of stabilising system while no worsening of ODI or VAS in comparison with preoperative status. Conclusions: DYNESYS can maintain disc height and restricts movements exceeding physiological limits. HA coated screws have superior biomechanical characteristics in preventing screw loosening and subsequent spinal spasms. If necessary, removal of implants is a simple procedure

Background: Spinal fusion for degenerative disc disease remains a controversial issue due to adjacent segment disease. Several alternatives have been evaluated; including Dynamic Neutralisation System for Spine and artificial disc arthroplasty. In our cohort study, we are highlighting advantages of DYNESYS over TDA and fusion as being a simple and reversible procedure if removal of implants is necessary. Material & Methods: Prospective cohort study on 415 patients underwent DYNESYS. The implant has to be removed in 43 patients. Patients were evaluated preoperatively using Oswestry Disability Index, SF 36 and Visual Analogue Scores together with plain imaging and MRI scanning. Preoperative discography was performed in 81% of patients. Questionnaires were evaluated on subsequent follow up at 3/12, 6/12 then at annual intervals. Results: Average treatment period was 26 months with a range from 3/12 to 60/12. The main reason for DYNE-SYS removal was screws’ failure or loosening followed by infections. The implant was used to treat single disc space in 20.9%, two spaces in 67.4% and three disc spaces in 9.3%. 81% of the screws were not Hydroxy Appetite coated. No loosening was detected among HA coated screws. Outcome assessment revealed significant improvement in SF36 even after removal of stabilising system while no worsening of ODI or VAS in comparison with preoperative status. Conclusions: DYNESYS can maintain disc height and restricts movements exceeding physiological limits. HA coated screws have superior biomechanical characteristics in preventing screw loosening and subsequent spinal spasms. If necessary, removal of implants is a simple procedure

Purpose: Cervical arthroplasty is an emerging technology with the potential of motion preservation and reduced adjacent level disease. However, the factors that influence postoperative range of motion (ROM) and patient satisfaction is not fully understood. The aim of this study was to evaluate the influence of disk height on the postoperative motion as well as clinical outcomes. Method: 167 patients from a multi-center prospective randomized FDA trial with single level ProDisc-C arthroplasty performed were evaluated radiographically utilizing Medical Metrics (QMATM, Medical Metrics, Inc.). Preoperative and postoperative disk height and ROM were measured from standing lateral and flexion-extension radiographs. Of these 167 patients, 19 patients from a single center had clinical outcomes based on ODI and VAS scores evaluated pre and postoperation with a mean follow-up of 22 months. Two-tailed student’s T-test and Spearman’s Rho tests were performed in order to find out if there was any correlation or “threshold” effect between the disk height and ROM. Results: Patients with less than 4 mm of preoperative disc height had a 1.8° increase in their flexion-extension ROM after TDA as compared to no change in ROM in patients with more than 4 mm of preoperative disc height (p=0.04). Patients with more than 5mm of postoperative disc height have significantly higher postoperative flexion-extension ROM (10.1°) than those with less than 5mm disc height (8.3°, p=0.014). However, patients with more than 7mm of postoperative disc height have significantly lower postoperative lateral bending ROM (4.1°) than those with less than 7mm disc height (5.7°, p=0.04). It appears that the optimal postoperative disc height is between 5 to 7 mm for increased ROM on flexion-extension and lateral bending. No correlation could be found between clinical outcomes and disc height. Similarly, no threshold effect could be found between any specific disc height and ODI or VAS. Conclusion: Patients with greater disc collapse benefit more in ROM from a TDR. The optimal range to maximize ROM for postoperative disc height is between 5 to 7mm. This optimal range did not translate into better clinical outcome at 2 year follow-up. A longer follow-up will yield if less ROM ultimately leads to worse clinical outcomes

Introduction There is presently great interest throughout the world for total disc arthroplasty (TDA). This paper aims to present techniques that make it safer, easier and faster for the surgeon to perform the anterior, retroperitoneal approach to the lumbar spine necessary for the implantation of these artificial discs (Brau SA, et al ; The Spine Journal, 2004). Methods The author has performed over 200 approaches for TDA since May of 2001. In addition he has performed approaches for over 2000 arthrodesis cases since August of 1997. Analysis of these cases has revealed techniques that can make the approach faster and easier while helping to prevent complications. Results Prevention of complications starts with the pre-operative evaluation of the patient. Individuals with a BMI of 40 or over and patients with vascular calcifications have potential for increased vascular complications. Pedal pulses must be evaluated in every patient prior to surgery. The lateral X-ray of the lumbar spine must be seen to evaluate for the presence of osteophytes and calcified vessels, both of which increase the morbidity of the operation. A pulse oxymeter should be placed on the left great toe to evaluate the status of the left iliac artery during the procedure. Disc level localization by fluoroscopy is necessary for the placement of the recommended transverse incision in optimal location. Circumferential left rectus mobilization for single level cases will help keep the incision small. The ureter and the superior hypogastric plexus should be mobilized with the peritoneum to keep them from injury. Distal mobilization of the left iliac artery will help reduce the incidence of thrombosis of this vessel and ligation of the ileo-lumbar vein will help reduce the chances of left iliac vein laceration when exposing L4-5. Reverse lip retractors blades help to keep the incision small and to protect the vessels from injury. Discussion These techniques have helped reduce the incidence of left iliac artery thrombosis to 0.45% (6 in 1310) and of major vein laceration to 1.4% (19 in 1310) in a series of patients published in 2004. The incidence of ureteral injury and of retrograde ejaculation in these same patients was reduced to 0. The techniques should, therefore, be helpful to the access surgeon in performing the approach safely and more expeditiously

Introduction: Vertebral end plate is both inervated and has been shown to be a source of pain. Clinical experience shows some degree of end plate subsidence, usually posteriorly, in cases of total disc arthroplasty (TDA). It follows therefore that this may be a cause of pain. It is theorized that in quiet standing, a prosthesis with a posteriorly placed centre of rotation (COR) will have maximal end plate stresses posteriorly unless the centroid of the prosthesis end plate lies over the COR of the prosthesis. The rationale is that a posteriorrly weighted non uniform static end plate stress distribution will be produced in order to satisfy the static requirement that the sum of all moments be zero. It is further theorized that removal of portions of anterior end plate to move the centroid closer to the pivot point will make the stress distribution more normal. Methods: A ball and socket prosthesis with a posterior articulation was subjected to static compression with an Instron testing machine against a foam block (Sawbones 1522-11) with the lower part of the specimen on rollers to allow lateral translation. Load was applied normal to the lower end plate in displacement control at a constant rate of 3 mm/min. Load displacement curves, lateral translation of the lower endplate and subsidence angle into the foam was measured. The experiment was repeated with various shaped end plates with the same outside footprints, though with various cutouts of the endplate footprint, such that the footprint area was reduced though with the centroid now lying over the prosthesis COR. Results: With the standard prosthesis subsidence was noted to be associated with a translatatory movement of the inferior assembly and tilting, with the posterior portion subsiding more than the anterior portion. The prosthesis continued to subside with an increase in the tilt angle and liftoff of the anterior portion of the end plate until the calculated centroid of the subsidence footprint in the foam was over the prosthesis COR. With an end plate cutout the inferior assembly did not translate and the prosthesis subsided in a parallel fashion. This behavior was unchanged by varying the geometry of the cutout. The area of the modified cutout prosthesis was 69% (564 sq mm cf 817 sq mm) of the area of the original end plate, while the yield load was 89% (2.34 kN cf 2.62 kN) of the original. The Yield stress was higher in the modified prosthesis (4.13 MPa cf 3.21 MPa). Discussion: A potential reason for end plate subsidence in TDA’s may be mismatch between the position of the end plate centroid and the COR. In static loading this causes a tendency to posterior subsidence and tilting. Removal of part of the end plate footprint such that the centroid is moved closer to the COR results in less of reduction in yield load than would be anticipated by loss of surface area alone. There is also abolition of subsidence by tilting

Introduction: Cervical disc replacement has emerged as an alternative to the gold standard ACDF in ordered to preserve the motion of the cervical spine, and reduce the risk of adjacent degenerative changes. However, little data actually exists to support or refute the effect of symptoms chronicity on TDA for the treatment of degenerative cervical spine. Material and Methods: We prospectively investigated 130 consecutive patients who have undergone a Prestige LP cervical disc replacement for degenerative disease causing chronic neck pain and radiculopathy. The pre and post-operative NDI, SF-36, Visual Analogue Score (VAS), HDS and HAS were recorded. Results: There were 130 patients in total. There were 66 males and 64 females, with an mean age at surgery was 51.12 +/− 0.84. Seventy five patients had one level disc replacement, and 56 had 2 levels. The mean follow up was 28+/−0.35 months, and the mean duration of symptoms was 34.46 +/−3.8 months. Mean length of stay 1.75+/−0.11 (1–4). There was no correlation between the DOS and any of the functional outcome:. Improvement Neck disability index (r=−0.181, p=0.134). Improvement in HDS (r=−0.126, p=0.296). Improvement in HAS (r=0.00, p=0.99). Improvement in SF-36 bodily pain (r=−0.011, p=0.925). Improvement in SF-26 mental health (r=0.042, p=0.324). Improvement in VAS neck pain (r=−0.0120, p=0.324). Improvement in VAS arm pain (r=0.0178, p=0.141). Dividing the DOS into more or less than 12 months shows significant improvement only in the NDI: less than 12 months DOS (31.85±3.209) and in patients with more than 12 months DOS (19.71±2.164), p=0.002. While there was no statistical difference in the other outcomes. Dividing the DOS into more or less than 24 months showed significant improvement the out come in patients with less than 24 months in the following outcomes:. Improvement in NDI: less than 24 months (27.35±2.714), more thank 24 months (19.72+2.435), p=0.04. Improvement in VAS arm pain: less than 24 months (4.6000±0.34446), more than 24 months (2.7414±3.7236), p=0.001. Improvement in hospital depression score: less than 24 months (5.37±0.589), more than 24 months (3.60±0.47), p-0.023. Improvement in hospital anxiety score: less than 24 months (4.28±0.49), more than 24 months (2.45±0.48), p=0.009. While there was no significant difference in the other outcomes. Dividing the DOS into less or more than 36 months showed no statistical difference in any of the outcome,. That was also the case when the DOS was divided into more or less than 48 months. Conclusion: Duration of symptoms of more that 24 months was associated with less favorable outcome, that was evident in improvement in NDI, HDS, HAS and VAS arm pain. Patients should be counseled when they present with DOS more than 24 months

Introduction: Total disc arthroplasty’s (TDA) fall into two groups – constrained ball and socket and sliding core devices. It is commonly theorized that sliding core devices offer the advantage of being able to adapt to varying centres of rotation (COR) of the functional spinal unit (FSU), however no rigorous justification has, so far, been tendered for this. Despite the perceived advantage, differing clinical results have been reported in the lumbar spine, possibly with better results with ball and socket devices. Furthermore abnormal motion with a large hysteresis effect has been identified in in vitro flexibility testing with a physiological preload in the lumbar spine. The purpose of this paper was to develop an understanding of the kinematics of sliding core TDA’s, their ability to match variable COR’s of a normal FSU, and to gain an understanding of theoretical load displacement behaviour when implanted. Methods: The motion of a biconvex sliding core prosthesis was observed to define the motion as a linked kinematic chain. By the use of sequential multiplication by appropriate transformation matrices that described this kinematic chain, equations for the position and orientation of the upper vertebrae were established. By a similar method equations for the position and orientation of the upper vertebrae were developed for a physiological simple rotation around the FSU COR. Attempts were made to solve these two sets of equations simultaneously to see if motion of the biconvex core prosthesis could match either the position, orientation or both position and orientation of the normal physiological motion. Functions defining the length of the load vector through the COR were obtained. By considering a physiological load in the direction of this vector, a function describing potential energy was defined. This was further modified by the addition of ligament constraints with a “J” shaped non linear load displacement behaviour that approximated normal ligament stiffness. Sensitivity analysis was then performed to establish the behaviour of the prosthesis under differing loads, ligament strains and malplacements and the outcomes were compared to published in vitro results. Results: The motion of the device could be modeled as a ‘two bar linkage’. Attempts to find simultaneous solutions for the equations for the two bar linkage and physiological movement showed that a solution was possible when matching either position or orientation but not both. The biconvex core prosthesis best approximated the normal motion by a change in the length of the vector joining the FSU COR to the vertebrae above. When the potential energy caused by this change in length was plotted as a two dimensional surface, a ‘saddle shape’, indicating an unstable high energy equilibrium position at neutral was found. The addition of functions to simulate ligament structures showed a ‘metastable’ energy surface with two stable minimum equilibrium positions with an intervening unstable high energy equilibrium position. Sensitivity analysis showed that the prosthesis could adapt quite well to changes in vertical position of the FSU COR though had limited ability to adapt to anteroposterior malplacement. Discussion: The theoretical potential energy function for a biconvex core prosthesis predicts significant hysteresis with a high energy unstable central position. The equations predict abnormal load behaviour that is similar to observed in vitro testing. This may explain the difference in clinical results