Biomechanics of a Fixed–Center of Rotation Cervical Intervertebral Disc Prosthesis

Biomechanical evaluation of a metal-on-metal cervical intervertebral disc prosthesis

The aim of this is to evaluate the biomechanical performance of double-level semirigid pedicle screw fixation and artificial intervertebral disc replacement in lumbar spine. Ti6Al4V and CFR-PEEK material are used for pedicle screw fixation and artificial disc replacement. In the present study, pedicle screw fixation and artificial intervertebral disc replacement are carried out between L3-L4-L5 regions under the application of moment 6,8,10 Nm and range of motion is compared during flexion, extension, and right-left lateral bending. Two-level pedicle screw fusion and total disc replacement are developed in the L3-L4-L5 of the lumber spine vertebrae. Carbon fiber reinforced (CFR-PEEK) and ultra-high molecular weight polyethylene (UHMWPE) are considered for the spinal fusion and the core part of the artificial disc respectively. Afterwards, applying the finite element analysis, it is detected that CFR-PEEK rod is able to increase range of motion at the implanted level in comparison to Ti6Al4V rod for both flexion–extension and lateral bending. In case of artificial intervertebral disc replacement hypermobility was observed. Hence, it is significant that rod material with CFR-PEEK is a better alternative for the treatment of degenerative diseases.

This study presents data on 67 patients from 2 study sites involved in the multicenter, prospective, randomized, controlled investigational device exemption study of FlexiCore artificial disc replacement versus fusion with a 2-year follow-up. To compare the outcomes achieved with the FlexiCore disc replacement versus standard circumferential fusion for the treatment of discogenic pain due to single level degenerative disc disease (DDD). The FlexiCore Intervertebral Disc (Stryker Spine, Allendale, NJ) is a metal-on-metal artificial lumbar disc replacement device currently being studied for the treatment of DDD under an investigational device exemption protocol approved by the United States Food and Drug Administration. Artificial disc replacement in the spine is designed to preserve motion at the intervertebral segment and subsequently reduce adjacent segment degeneration. This is the first study to compare a metal-on-metal artificial lumbar disc replacement with circumferential fusion. Here we report the 2-year follow-up results. Forty-four patients were treated with the FlexiCore and 23 patients were treated with fusion. The FlexiCore treatment group consisted of 23 men and 21 women, with an average age of 36 and an average body mass index of 28. Thirty-two of the FlexiCore surgeries were performed at L5-S1, and 12 were performed at L4-L5. The control treatment group consisted of 10 men and 13 women, with an average age of 41 and an average body mass index of 28. Seventeen of the control surgeries were performed at L5-S1, 5 were performed at L4-L5, and 1 was a 2-level fusion performed at L4-L5 and L5-S1. Prospective data were collected before surgery and after surgery at 6 weeks, and at 3, 6, 12, and 24 months. Disability and pain were assessed using the Oswestry Disability Index and the Visual Analog Scale. Range of motion was determined by independent radiologic assessment of flexion/extension and lateral bending radiographs. The mean Oswestry Disability Index scores were 62 (FlexiCore) and 58 (control) before surgery, 36 (FlexiCore) and 50 (control) at 6 weeks, and 6 (FlexiCore) and 12 (control) at 2 years. The mean Visual Analog Scale scores were 86 (FlexiCore) and 82 (control) before surgery, 32 (FlexiCore) and 43 (control) at 6 weeks, and 16 (FlexiCore) and 20 (control) at 2 years. The FlexiCore group's angular rotation averaged 2.8 degrees before surgery and 3.8 degrees at 6 weeks after surgery. The group's lateral bending averaged 4.7 degrees before surgery and 4.2 degrees at 6 weeks after surgery. The average operative time (skin to skin) was 82 minutes for the FlexiCore group versus 179 minutes for the control group (P < 0.001). The average estimated blood loss was 97 mL for the FlexiCore group versus 179 mL for the control group (P < 0.02). The average hospital stay was 2 days for the FlexiCore group versus 3 days for the control group (P < 0.005). These initial results from 2 study sites demonstrate that the FlexiCore compares very favorably to circumferential fusion for the treatment of lumbar DDD unresponsive to conservative treatment. These results are not intended to represent the overall study results.

Titanium alloy-based Pedicle screw-rod fusion is a very common technique to provide higher fusion regularity than other methods. In recent times, Carbon-fibre-reinforced (CFR)-PEEK rod is used to better reduce the fusion rate. Alternatively, total disc replacement (TDR) is also very common for the non-fusion treatment method for degenerative disc disease (DDD). This study aims to investigate flexibility (ROM), stability, stress condition in implant, implant adjacent bone of the implanted lumbar spine during different physiological movements and loading environments. The finite element (FE) intact model of the lumbar spine (L2-L5) with two-level pedicle screw-rod fusion at L3-L4-L5 and two-level artificial disc replacement was developed. CFR-PEEK was taken for rod for semi-rigid fusion. UHMWPE was taken as core part of the artificial disc. The FE models were simulated under 6, 8 and 10 Nm moments in left right lateral bending, flexion and extension movements. The total ROM was reduced for two-level pedicle screw fixation and increased for the artificial disc replacement model during flexion extension compared to the intact spine. The total ROM was reduced by around 54% and 25% for two-level fixation and increased by 30% and 19.5% for artificial disc replacement spine, under flexion-extension and left-right lateral bending respectively. For screw fixation, the ROM increased by 15% and 18% reduced by 4.5% and 14% for disc replacement at the adjacent segments for flexion-extension and left-right lateral bending.

This in vitro experimental study was conducted to investigate the initial biomechanical effect of artificial intervertebral disc replacement in the cervical spine. The multidirectional flexibility of replaced and adjacent spinal segments were analyzed using a cadaveric cervical spine model. The following three cervical reconstructions were sequentially performed at the C5-6 level after anterior discectomy in seven human cadaveric occipitocervical spines: anterior artificial disc replacement with a bioactive three-dimensional (3D) fabric disc (FD); anterior iliac bone graft; and anterior plate fixation with iliac bone graft. Six unconstrained pure moments were applied with a 6-df spine simulator, and 3D segmental motions at the operative and adjacent segments were measured with an optoelectronic motion measurement system. The 3D FD group demonstrated statistically equivalent ranges of motion (ROMs) when compared with intact values in axial rotation and lateral bending. The 45% increase in flexion-extension ROM was demonstrated in 3D FD group; however, neutral zone analysis did not reach statistical significance between the intact spine and 3D FD. The anterior iliac bone graft and iliac bone graft reconstructions demonstrated statistically lower ROMs when compared with 3D FD in all loading modes (p < 0.05). The adjacent-level ROMs of the 3D FD group demonstrated nearly physiological characteristics at upper and lower adjacent levels. Excellent stability at the interface was maintained during the whole testing without any device displacement and dislodgment. The stand-alone cervical 3D FD demonstrated nearly physiological biomechanical characteristics at both operative and adjacent spinal segments in vitro, indicating an excellent clinical potential for cervical artificial disc replacement.

Back pain and associated maladies can account for an immense amount of healthcare cost and loss of productivity in the workplace. In particular, spine related injuries in the US affect upwards of 5.7 million people each year. The degenerative disc disease treatment almost always arises due to a clinical presentation of pain and/or discomfort. Preferred conservative treatment modalities include the use of non-steroidal anti-inflammatory medications, physical therapy, massage, acupuncture, chiropractic work, and dietary supplements like glucosamine and chondroitin. Artificial disc replacement, also known as total disc replacement, is a treatment alternative to spinal fusion. The goal of artificial disc prostheses is to replicate the normal biomechanics of the spine segment, thereby preventing further damage to neighboring sections. Artificial functional disc replacement through permanent metal and polymer-based components continues to evolve, but is far from recapitulating native disc structure and function, and suffers from the risk of unsuccessful tissue integration and device failure. Tissue engineering and regenerative medicine strategies combine novel material structures, bioactive factors and stem cells alone or in combination to repair and regenerate the IVD. These efforts are at very early stages and a more in-depth understanding of IVD metabolism and cellular environment will also lead to a clearer understanding of the native environment which the tissue engineering scaffold should mimic. The current review focusses on the strategies for a successful regenerative scaffold for IVD regeneration and the need for defining new materials, environments, and factors that are so finely tuned in the healthy human intervertebral disc in hopes of treating such a prevalent degenerative process.

Finite element model of L3-S1 segment and confirmatory cadaveric testing were used to investigate the biomechanical effects of a mobile core type artificial disc (Charité artificial disc; DePuy Spine, Raynham, MA) on the lumbar spine. To determine the effects of the Charité artificial disc across the implanted and adjacent segments. Biomechanical studies of artificial discs that quantify parameters, like the load sharing and stresses, are sparse in the literature, especially for mobile-type core artificial disc designs. In addition, there is no standard protocol for studying the adjacent segmental effects of such implants. Human osteo-ligamentous spines (L1-S1) were tested before and after L5-S1 Charité artificial disc placement. The data were used to validate further an intact 3-dimensional (3-D) nonlinear L3-S1 finite element model. The model was subjected to 400-N axial compression and 10.6 Nm of flexion/extension pure moments (load control) or pure moments that produced the overall rotation of the L3-S1 Charité model equal to the intact case (hybrid approach). Resultant motion, load, and stress parameters were analyzed at the experimental and adjacent levels. Finite element model validation was achieved only with the load-controlled experiments. The hybrid approach, believed to be more clinically relevant, revealed that Charité artificial disc leads to motion increases in flexion (19%) and extension (44%) at the L5-S1 level. At the instrumented level, the decrease in the facet loads was less than at the adjacent levels; the corresponding decrease being 26% at L3-L4, 25% at L4-L5, and 13.4% at L5-S1 when compared to the intact. Intradiscal pressure changes in the L4-L5 and L3-L4 segments were minimal. Shear stresses at the Charité artificial disc-L5 endplate interface were higher than those at S1 interface. However, in the load control mode, the increase in facet loads in extension was approximately 14%, as compared to the intact case. The hybrid testing protocol is advocated because it better reproduces clinical observations in terms of motion following surgery, using pure moments. Using this approach, we found that the Charité artificial disc placement slightly increases motion at the implanted level, with a resultant increase in facet loading when compared to the adjacent segments, while the motions and loads decrease at the adjacent levels. However, in the load control mode that we believe is not that clinically relevant, there was a large increase in motion and a corresponding increase in facet loads, as compared to the intact.

A total of 115 patients were randomized in a 1:1 ratio to a Bryan artificial disc replacement (56) or an anterior cervical fusion with allograft and a plate (59). The purpose of this study is to examine the functional outcome and radiographic results of this prospective, randomized trial to determine the role of the Bryan artificial cervical disc replacement for patients with 1-level cervical disc disease. Artificial cervical disc replacement has become an option for cervical radiculopathy. Previous studies have evaluated the efficacy of this alternative without the scientific rigor of a concurrent control population. This study is a pooled data set from 3 centers involved in the U.S. FDA Investigational Device Exemption trial evaluating the Bryan artificial cervical disc. The purpose of this study is to examine the functional outcome and radiographic results of this prospective, randomized trial to determine the role of the Bryan artificial cervical disc replacement for patients with 1-level cervical disc disease; 12-month follow-up is available for 110 patients and 24 month follow-up complete for 99 patients. There are 30 males and 26 females in the Bryan group and 32 males and 27 females in the fusion group. The average age was 43 years (Bryan) and 46 years (fusion). Disability and pain were assessed using the Neck Disability Index (NDI) and the Visual Analog Scale (VAS) of the neck and of the arm pain. SF-36 outcome measures were obtained including the physical component as well as the mental component scores. Range of motion was determined by independent radiologic assessment of flexion-extension radiographs. We report a prospective, randomized study comparing the functional outcome of cervical disc replacement to an anterior cervical fusion with results of 99 patients at 2 years. Prospective data were collected before surgery and at 6 weeks, 3, 6, 12, and 24 months after surgery. The average operative time for the control group was 1.1 hours and the Bryan Group 1.7 hours. Average blood loss was 49 mL (control) and 64 mL (Bryan). Average hospital stay was 0.6 days (control) and 0.9 days (Bryan). The mean NDI before surgery was not statistically different between groups: 47 (Bryan) and 49 (control). Twelve-month follow-up NDI is 10 (Bryan) and 18 (control) (P = 0.013). At 2-year follow-up, NDI for the Bryan group is 11 and the control group is 20 (P = 0.005). The mean arm pain VAS before surgery was 70 (Bryan) and 71 (control). At 1-year follow-up, Bryan arm pain VAS was 12 and control 23 (P = 0.031). At 2-year follow-up, the average arm pain VAS for the Bryan group was 14 and control 28 (P = 0.014). The mean neck pain VAS before surgery was 72 (Bryan) and 73 (control). One-year follow-up scores were 17 (Bryan) and 28 (control) (P = 0.05). At 2 years: 16 (Bryan) and 32 (control) (P = 0.005). SF-36 scores: Physical component--Before surgery Bryan 34 and control 32. At 24 months: Bryan 51 and control 46 (P = 0.009). More motion was retained after surgery in the disc replacement group than the plated group at the index level (P < 0.006 at 3, 6, 12, and 24 months). The disc replacement group retained an average of 7.9 degrees of flexion-extension at 24 months. In contrast, the average range of motion in the fusion group was 0.6 degrees at 24 months. There were 6 additional operations in this series: 4 in the control group and 2 in the investigational group. There were no intraoperative complications, no vascular or neurologic complications, no spontaneous fusions, and no device failures or explantations in the Bryan cohort. The Bryan artificial disc replacement compares favorably to anterior cervical discectomy and fusion for the treatment of patients with 1-level cervical disc disease. At the 2-year follow-up, there are statistically significant differences between the groups with improvements in the NDI, the neck pain and arm pain VAS scores, and the SF-36 physical component score in the Bryan disc population.

A new artificial intervertebral disc was developed, and its intrinsic biomechanical properties, bioactivity, and the effectiveness as a total disc replacement were evaluated in vitro and in vivo. To introduce a new artificial intervertebral disc and to evaluate the in vitro mechanical properties, fusion capacity to bone, and segmental biomechanics in the total intervertebral disc replacement using a sheep lumbar spine. The loss of biologic fusion at the bone-implant interface and prosthetic failures have been reported in previous artificial discs. There have been no clinically applicable discs with detailed experimental testing of in vivo mechanics and interface fusion capacity. The artificial intervertebral disc consists of a triaxial three-dimensional fabric (3-DF) woven with an ultra-high molecular weight polyethylene fiber, and spray-coated bioactive ceramics on the disc surface. The arrangement of weave properties was designed to produce mechanical behavior nearly equivalent to the natural intervertebral disc. Total intervertebral disc replacement at L2-L3 and L4-L5 was performed using 3-DF disc with or without internal fixation in a sheep lumbar spine model. The segmental biomechanics and interface histology were evaluated after surgery at 4 and 6 months. The tensile-compressive and torsional properties of prototype 3-DF were nearly equivalent to those of human lumbar disc. The lumbar segments replaced with 3-DF disc alone showed a significant decrease of flexion-extension range of motion to 28% of control values as well as partial bony fusion at 6 months. However, the use of temporary fixation provided a nearly physiologic mobility of the spinal segment after implant removal as well as excellent bone-disc fusion at 6 months. An artificial intervertebral disc using a three-dimensional fabric demonstrated excellent in vitro and in vivo performance in both biomechanics and interface histology. There is a potential for future clinical application.

BackgroundUntil quite recently spinal disorder problems in the U.S. have been operated by fusing cervical vertebrae instead of replacement of the cervical disc with an artificial disc. Cervical disc replacement is a recently approved procedure in the U.S. It is one of the most challenging surgical procedures in the medical field due to the deficiencies in available diagnostic tools and insufficient number of surgical practices For physicians and surgical instrument developers, it is critical to understand how to successfully deploy the new artificial disc replacement systems. Without proper understanding and practice of the deployment procedure, it is possible to injure the vertebral body. Mixed reality (MR) and virtual reality (VR) surgical simulators are becoming an indispensable part of physicians’ training, since they offer a risk free training environment. In this study, MR simulation framework and intricacies involved in the development of a MR simulator for the rasping procedure in artificial cervical disc replacement (ACDR) surgery are investigated. The major components that make up the MR surgical simulator with motion tracking system are addressed. FindingsA mixed reality surgical simulator that targets rasping procedure in the artificial cervical disc replacement surgery with a VICON motion tracking system was developed. There were several challenges in the development of MR surgical simulator. First, the assembly of different hardware components for surgical simulation development that involves knowledge and application of interdisciplinary fields such as signal processing, computer vision and graphics, along with the design and placements of sensors etc . Second challenge was the creation of a physically correct model of the rasping procedure in order to attain critical forces. This challenge was handled with finite element modeling. The third challenge was minimization of error in mapping movements of an actor in real model to a virtual model in a process called registration. This issue was overcome by a two-way (virtual object to real domain and real domain to virtual object) semi-automatic registration method.ConclusionsThe applicability of the VICON MR setting for the ACDR surgical simulator is demonstrated. The main stream problems encountered in MR surgical simulator development are addressed. First, an effective environment for MR surgical development is constructed. Second, the strain and the stress intensities and critical forces are simulated under the various rasp instrument loadings with impacts that are applied on intervertebral surfaces of the anterior vertebrae throughout the rasping procedure. Third, two approaches are introduced to solve the registration problem in MR setting. Results show that our system creates an effective environment for surgical simulation development and solves tedious and time-consuming registration problems caused by misalignments. Further, the MR ACDR surgery simulator was tested by 5 different physicians who found that the MR simulator is effective enough to teach the anatomical details of cervical discs and to grasp the basics of the ACDR surgery and rasping procedure

A prospective, randomized, multicenter, Food and Drug Administration-regulated, investigational device exemption clinical trial. To compare the safety and effectiveness of lumbar total disc replacement (TDR) with the CHARITE artificial disc (DePuy Spine, Raynham, MA) to anterior lumbar interbody fusion for the treatment of single-level degenerative disc disease from L4-S1 unresponsive to nonoperative treatment. In addition, to evaluate the radiographic outcomes of lumbar artificial disc replacement at either L4-L5 or L5-S1 with the CHARITE artificial disc as compared to anterior lumbar interbody fusion with cylindrical cages and iliac crest bone graft; and to determine if a correlation exists between clinical outcomes and surgical accuracy of TDR placement within the disc space. Prior investigators have reported excellent radiographic results with the CHARITE artificial disc for the treatment of lumbar degenerative disc disease. These encouraging results are the product of retrospective reviews without a control. Very few studies have reported on the segmental motion of an intervertebral level implanted with an artificial disc, and no studies have reported a correlation of radiographic and clinical outcomes. A prospective, randomized, multicenter, US Food and Drug Administration, investigational device exemption study with 24-month follow-up was performed at 14 centers throughout the United States. A total of 304 subjects were randomized in a 2:1 ratio, with 205 in the investigational group (TDR with the CHARITE artificial disc) and 99 in the control group (anterior lumbar interbody fusion with BAK cages and iliac crest bone graft). A total of 71 TDR training cases were performed (up to 5 at each site) before randomization beginning at each site. Plain radiographs were analyzed for each subject in both groups regarding range of motion (ROM) in flexion/extension, restoration of disc space height, and subsidence. Prosthesis placement in the coronal and midsagittal planes was analyzed for the 276 patients with TDR. Correlations were performed between prosthesis placement and clinical outcomes. Patients in the investigational group had a 13.6% mean increase, and those in the control group an 82.5% decrease in mean flexion/extension ROM at 24 months postoperatively compared to baseline. Patients in the investigational group had significantly better restoration of disc height than the control group (P < 0.05). There was significantly less subsidence in the investigational group compared to the control group (P < 0.05). The surgical technical accuracy of CHARITE artificial disc placement was divided into 3 groups: I, ideal (83%); II, suboptimal (11%); and III, poor (6%), and correlated with clinical outcomes. The flexion/extension ROM and prosthesis function improved with the surgical technical accuracy of radiographic placement (P = 0.003). Preoperative ROM in flexion/extension was restored and maintained in patients receiving a TDR. TDR with the CHARITE artificial disc resulted in significantly better restoration of disc space height, and significantly less subsidence than anterior interbody fusion with BAK cages. Clinical outcomes and flexion/extension ROM correlated with surgical technical accuracy of CHARITE artificial disc placement. In the majority of cases, placement of the CHARITE artificial disc was ideal.

Adjacent level effects of bi level disc replacement, bi level fusion and disc replacement plus fusion in cervical spine- a finite element based study

Spinal fusion for degenerative disc disease has been associated with a variety of side effects, including increased morbidity, infection, failed back syndrome, pseudoarthrosis, and acceleration of degenerative changes in adjacent intervertebral discs and facet joints. Based on the experience of arthroplasty of hip and other joints, there has been an escalating in research emphasizing the design and development of an artificial disc prosthesis. However, these artificial discs have not been very successful when compared with hip or knee replacements. Based on clinical and anatomical data, multiple authors have postulated that the degenerative process of intervertebral disc evolves through three stages; dysfunction, instability and stabilization, with relatively distinct clinical and radiological findings. Even though magnetic resonance imaging is considered as the primary diagnostic tool for degenerative disease, it is unable to reliably ascertain which disc level is responsible for generating axial pain symptoms. Consequently, discography is the most precise test to localize the level of pathology. Multiple design criteria have been proposed for an ideal intervertebral prosthesis which included endurance, materials behavior, geometry, kinematics, dynamics, motion constraints, fixation to bones and safety. The development of artificial disc technology has culminated into two types of disc replacements, namely total disc replacement and a nucleus pulposus replacement. In addition, four prosthetic models have been proposed which include hydraulic, elastic, composite, and mechanical. Clinical outcomes of total disc replacement and nucleus replacement have been variable. This review describes natural history of disc disease and the diagnostic process, anatomical and biomechanical considerations, design criteria for an ideal intervertebral prosthesis, evolution of artificial disc prosthesis, clinical outcomes of the total disc and nucleus replacement, and prospects and meridians for future research.

BackgroundThe ideal surgical approach for cervical disk disease remains controversial, especially for multilevel cervical disease. The purpose of this study was to investigate the biomechanics of the cervical spine after 3-level hybrid surgery compared with 3-level anterior cervical discectomy and fusion (ACDF).Material/MethodsEighteen human cadaveric spines (C2-T1) were evaluated under displacement-input protocol. After intact testing, a simulated hybrid construct or fusion construct was created between C3 to C6 and tested in the following 3 conditions: 3-level disc plate disc (3DPD), 3-level plate disc plate (3PDP), and 3-level plate (3P).ResultsCompared to intact, almost 65~80% of motion was successfully restricted at C3-C6 fusion levels (p<0.05). 3DPD construct resulted in slight increase at the 3 instrumented levels (p>0.05). 3PDP construct resulted in significant decrease of ROM at C3-C6 levels less than 3P (p<0.05). Both 3DPD and 3PDP caused significant reduction of ROM at the arthrodesis level and produced motion increase at the arthroplasty level. For adjacent levels, 3P resulted in markedly increased contribution of both upper and lower adjacent levels (p<0.05). Significant motion increases lower than 3P were only noted at partly adjacent levels in some conditions for 3DPD and 3PDP (p<0.05).ConclusionsACDF eliminated motion within the construct and greatly increased adjacent motion. Artificial cervical disc replacement normalized motion of its segment and adjacent segments. While hybrid conditions failed to restore normal motion within the construct, they significantly normalized motion in adjacent segments compared with the 3-level ACDF condition. The artificial disc in 3-level constructs has biomechanical advantages compared to fusion in normalizing motion.

In degenerative disc disease (DDD), increased loading in the posterior column increases facet joint subchondral bone density and may lead to facet joint degeneration. While spinal fusion is commonly used to treat patients with symptomatic DDD, increased stress at the levels adjacent to fusion may accelerate facet joint and adjacent segment degeneration. Artificial disc replacements have been developed as an alternative to fusion. In this prospective study, the effects of disc replacement with the CHARITE Artificial Disc on facet joint loading and integrity were evaluated. Thirteen patients aged <50 years with symptomatic DDD were recruited. Computed tomography (CT) osteoabsorptiometry was performed prior to the implantation of the CHARITE Artificial Disc and six months after. With this technique, increases or decreases in facet joint loading and integrity are indicated by corresponding changes in subchondral bone density. Changes in the distribution of load alter the distribution of the areas of maximum bone density. Clinical outcome was also assessed at pre-operative and 6 and 12 month post-operative visits using the Visual Analogue Scale back and leg pain scores, the Oswestry Disability Index and the Short Form-36 (SF-36) questionnaire. The height of the intervertebral space at the operated level was monitored by lateral X-ray. Subchondral bone density was evaluated in the facet joints of all 13 patients at the operated level, 12 patients at the level above the operated segment, and five patients at the level below the operated segment. Quantitative measurements revealed no significant increases (> or =3%) in subchondral bone density of the facet joints at any level in any patient. Significant decreases (> or =3%) in subchondral bone density were measured at the operated level in 10/13 patients, at the level above the operated segment in 6/12 patients, and at the level below the operated segment in 3/5 patients. There were no changes in the distribution of the areas of maximum bone density in any of the studied facet joints at 6 months compared with pre-operative measurements. Clinical outcome scores were improved at 6 and 12 months compared with baseline. The mean intervertebral space height at the operated level was increased following implantation of the CHARITE Artificial Disc and was 1.8 times greater than the pre-operative height at both 6 and 12 months. In this study, replacement of degenerated intervertebral discs with the CHARITE Artificial Disc was not associated with increased loading of the facet joints at the operated or adjacent levels. Decreases in subchondral bone density may indicate reduced loading in the posterior column following disc replacement compared with loading in the pre-operative degenerated spine. Further study is required to establish the baseline for healthy subchondral bone density and to compare this baseline with long-term measurements in patients undergoing disc replacement.