Design of Customize Interbody Fusion Cages of Ti64ELI with Gradient Porosity by Selective Laser Melting Process

Intervertebral fusion surgery for spinal trauma, degeneration, and deformity correction is a major vertebral reconstruction operation. For most cages, the stiffness of the cage is high enough to cause stress concentration, leading to a stress shielding effect between the vertebral bones and the cages. The stress shielding effect affects the outcome after the reconstruction surgery, easily causing damage and leading to a higher risk of reoperation. A porous structure for the spinal fusion cage can effectively reduce the stiffness to obtain more comparative strength for the surrounding tissue. In this study, an intervertebral cage with a porous gradation structure was designed for Ti64ELI alloy powders bonded by the selective laser melting (SLM) process. The medical imaging software InVesalius and 3D surface reconstruction software Geomagic Studio 12 (Raindrop Geomagic Inc., Morrisville, NC, USA) were utilized to establish the vertebra model, and ANSYS Workbench 16 (Ansys Inc, Canonsburg, PA, USA) simulation software was used to simulate the stress and strain of the motions including vertical body-weighted compression, flexion, extension, lateral bending, and rotation. The intervertebral cage with a hollow cylinder had porosity values of 80–70–60–70–80% (from center to both top side and bottom side) and had porosity values of 60–70–80 (from outside to inside). In addition, according to the contact areas between the vertebras and cages, the shape of the cages can be custom-designed. The cages underwent fatigue tests by following ASTM F2077-17. Then, mechanical property simulations of the cages were conducted for a comparison with the commercially available cages from three companies: Zimmer (Zimmer Biomet Holdings, Inc., Warsaw, IN, USA), Ulrich (Germany), and B. Braun (Germany). The results show that the stress and strain distribution of the cages are consistent with the ones of human bone, and show a uniform stress distribution, which can reduce stress concentration.

Lumbosacral Fusion Using Instrumented Cage Distraction–Fixation in a Dog with Degenerative Lumbosacral Stenosis

This study aimed to assess the long-term outcome and intervertebral fusion following surgical distraction and stabilization using an intervertebral cage and pedicle screw and rod fixation (PSRF) in a dog with severe degenerative lumbosacral stenosis (DLSS).Degenerative lumbosacral stenosis is a common disorder in large breed dogs and has a multifactorial origin. Surgical treatment by dorsal laminectomy and discectomy results in decompression of neural structures, but when distraction–fixation is applied, the ultimate goal is vertebral fusion. A 4-year-old male neutered Leonberger, presented with DLSS and pre-existent chronic discospondylitis, was treated by dorsal laminectomy, partial discectomy, curettage of the end plates, distraction with an intervertebral spacer (SynCage), and PSRF. At 26 months after surgery, the Helsinki pain score and neurological Griffith score were improved; however, the dog passed away shortly thereafter due to an unrelated disorder. The lumbosacral segment became available for computed tomography (CT), micro-CT and histopathology. On CT, bone volume through the largest hole of the cage was 91.0% and for compact bone 76.1%. Micro-CT and histopathology revealed vertebral fusion. Distraction–fixation using an intervertebral spacer and PSRF was well-accepted in this dog with severe DLSS, and the dog had a good clinical outcome with long-term follow-up. CT, micro-CT and histopathology showed evidence of vertebral fusion.

Development of a lumbosacral intervertebral cage prototype for dogs

ABSTRACT: Several surgical procedures aim to decompress and/or stabilize the lumbosacral (LS) joint of dogs; however, the lumbar interbody fusion technique, by using a cage combined with a bone graft, is the most indicated and used in human medicine. No specific implant is available for application to the canine lumbosacral joint. Thus, this study measured lumbosacral discs in large dogs, determined whether a human cage model could fit the dogs’ L7-S1 intervertebral space, and developed a LS cage prototype for dogs. Ten cadaveric lumbosacral spines from adult dog weighing 20-35kg were used. The dogs had died for reasons unrelated to this study. The vertebral body dimensions and the L7-S1 intervertebral space occupied by the intervertebral disc were measured by lateral and ventrodorsal radiographs and by computed tomography in the dorsal, sagittal, and transverse views. Measurements were also taken of the anatomical specimens in the sagittal and transverse planes. After measuring the intervertebral discs, the following mean measures were obtained for L7-S1 discs: height 12.23mm, dorsal thickness 3.3mm, central thickness 4mm, ventral thickness 5.5mm, and width 24.74mm. The human lumbar cage models from brands LDR, Baumer Orthopedics, Stryker, Synthes, and Vertebral Technologies, Inc. and cervical stabilization cages from the brands B-Braun and Stryker were evaluated and were found to be unsuitable for large dogs. Cervical human cages had measurements similar to those found in this study; however, due to their quadrangular shape, the possibility of being introduced surgically through the surgical accesses available for the articulation between L7-S1 in dogs without injuring the cauda equina or the L7 root is small. A cage model was then developed using 3D modelling software. It was designed for insertion via dorsal laminectomy in the lateral portions of the intervertebral space. To avoid cauda equina lesion, the implant model was developed to be placed laterally to the midline. The cage surface is serrated to prevent using the locking screw to fix it, thus avoiding further injury to nerve structures. The serrated surfaces are also designed to avoid cage migration and promote stability. The prototype allows graft placement in the surrounding intervertebral space, which is fundamental for fusion through integration between the cage and the endplates as well as for bone growth between and around the cage. It was also considered studies on humans showing that the lateral regions of the endplates support a more considerable load. Biomechanical and in vivo studies on the developed model are necessary to evaluate the actual degree of distraction, mobility and the long-term rate of fusion between L7 and S1 and its possible impact on the adjacent motor units, combined or not with dorsal fixation techniques.

Hybrid Strategy of Cervical Artificial Disc and Intervertebral Cage – Biomechanical Effects on the Adjacent Tissues and Implants

Background: Adjacent segment degeneration (ASD) is a concern in multi-level ACDF surgery. Hybrid surgery with C-ADR and ACDF are an alternative treatment to reduce the level of increased rigidity, but biomechanical differences between strategies using one C-ADR and two ACDFs have not been thoroughly investigated.Methods: To evaluate the placement-related effects of using one cervical artificial disc replacement (C-ADR) and two anterior cervical discectomy and fusion (ACDF) on tissue responses and implant behavior. A nonlinear finite element model from the C2 to the T1 vertebrae was developed. Ligament interconnection, follower loads, and weight compression were used to simulate cervical flexion. Within the C4-C7 segments, two placements of one C-ADR and two ACDFs were arranged: PAP (peek cage, artificial disc, and peek cage) and APP.Results: Both PAP and APP consistently induced kinematic and mechanical redistribution to adjacent segments. The C-ADR served as a buffer of the compensated motion and stress from the ACDF segments. The motion and stress of the cranial C2-C3 and C3-C4 segments were greater for the PAP than the APP constructs. However, the caudal C7-T1 segment of the APP construct was more flexed and stressed. Serially stacked cages of the APP placement increased bone-cage stresses, potentially inducing subsidence and loosening. The sandwiched C-ADR of the PAP construct accommodated the compensated motion and stress from the adjacent ACDFs more than the APP construct.Conclusions: The PAP and APP placements cause more severe ASD progression at the cranial and caudal segments, respectively. The PAP placement is preferred for concerns regarding ACDF and postoperative degeneration of caudal segments. The APP placement is recommended when C-ADR failure and ASD progression are considered.

Effect of lordosis on adjacent levels after lumbar interbody fusion, before and after removal of the spinal fixator: a finite element analysis

Background Literature indicates that adjacent-segment diseases after posterior lumbar interbody fusion with pedicle screw fixation accelerate degenerative changes at unfused adjacent segments due to the increased motion and intervertebral stress. Sagittal alignment of the spine is an important consideration as achieving proper lordosis could improve the outcome of spinal fusion and avoid the risk of adjacent segment diseases. Therefore, restoration of adequate lumbar lordosis is considered as a major factor in the long-term success of lumbar fusion. This study hypothesized that the removal of internal fixation devices in segments that have already fused together could reduce stress at the disc at adjacent segments, particularly in patients with inadequate lordosis. The purpose of this study was to analyze the biomechanical characteristics of a single fusion model (posterior lumbar interbody fusion with internal fixation) with different lordosis angles before and after removal of the internal fixation device. Methods Five finite element models were constructed for analysis; 1) Intact lumbar spine without any implants (INT), 2) Lumbar spine implanted with a spinal fixator and lordotic intervertebral cage at L4-L5 (FUS-f-5c), 3) Lumbar spine after removal of the spinal fixator (FUS-5c), 4) Lumbar spine implanted with a spinal fixator and non-lordotic intervertebral cage at L4-L5 (FUS-f-0c), and 5) Lumbar spine after removal of the spinal fixator from the FUS-f-0c model (FUS-0c). Results The ROM of adjacent segments in the FUS-f-0c model was found to be greater than in the FUS-f-5c model. After removing the fixator, the adjacent segments in the FUS-5c and FUS-0c models had a ROM that was similar to the intact spine under all loading conditions. Removing the fixator also reduced the contact forces on adjacent facet joints and reduced the peak stresses on the discs at adjacent levels. The greatest increase in stress on the discs was found in the FUS-f-0c model (at both L2/L3 and L3/L4), with intervertebral stress at L3/L4 increasing by 83% when placed in flexion. Conclusions This study demonstrated how removing the spinal fixation construct after bone fusion could reduce intradiscal pressure and facet contact forces at adjacent segments, while retaining a suitable level of lumbar lordosis.

The association of cervical sagittal alignment with adjacent segment degeneration

Purpose Cervical spine surgery may affect sagittal alignment parameters and induce accelerated degeneration of the cervical spine. Cervical sagittal alignment parameters of surgical patients will be correlated with radiological adjacent segment degeneration (ASD) and with clinical outcome parameters. Methods Patients were analysed from two randomized, double-blinded trials comparing anterior cervical discectomy with arthroplasty (ACDA), with intervertebral cage (ACDF) and without intervertebral cage (ACD). C2–C7 lordosis, T1 slope, C2–C7 sagittal vertical axis (SVA) and the occipito-cervical angle (OCI) were determined as cervical sagittal alignment parameters. Radiological ASD was scored by the combination of decrease in disc height and anterior osteophyte formation. Neck disability index (NDI), SF-36 PCS and MCS were evaluated as clinical outcomes. Results The cervical sagittal alignment parameters were comparable between the three treatment groups, both at baseline and at 2-year follow-up. Irrespective of surgical method, C2–C7 lordosis was found to increase from 11° to 13°, but the other parameters remained stable during follow-up. Only the OCI was demonstrated to be associated with the presence and positive progression of radiological ASD, both at baseline and at 2-year follow-up. NDI, SF-36 PCS and MCS were demonstrated not to be correlated with cervical sagittal alignment. Likewise, a correlation with the value or change of the OCI was absent. Conclusion OCI, an important factor to maintain horizontal gaze, was demonstrated to be associated with radiological ASD, suggesting that the occipito-cervical angle influences accelerated cervical degeneration. Since OCI did not change after surgery, degeneration of the cervical spine may be predicted by the value of OCI. NECK trial Dutch Trial Register Number NTR1289. PROCON trial Trial Register Number ISRCTN41681847. Graphic abstract These slides can be retrieved under Electronic Supplementary Material.