- Email: [email protected]
Anterior Lumbar Interbody Fusion Using a Personalized Approach: Is Custom the Future of Implants for Anterior Lumbar Interbody Fusion Surgery?
Accepted Manuscript Anterior Lumbar Interbody Fusion (ALIF) using a personalised approach: Is custom the future of implants for ALIF surgery? Ralph J. Mobbs, William CH. Parr, Wen Jie Choy, Aidan McEvoy, William R. Walsh, Kevin Phan PII:
S1878-8750(19)30003-8
DOI:
https://doi.org/10.1016/j.wneu.2018.12.144
Reference:
WNEU 11090
To appear in:
World Neurosurgery
Received Date: 20 December 2017 Revised Date:
19 December 2018
Accepted Date: 20 December 2018
Please cite this article as: Mobbs RJ, Parr WC, Choy WJ, McEvoy A, Walsh WR, Phan K, Anterior Lumbar Interbody Fusion (ALIF) using a personalised approach: Is custom the future of implants for ALIF surgery?, World Neurosurgery (2019), doi: https://doi.org/10.1016/j.wneu.2018.12.144. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Case Report: Anterior Lumbar Interbody Fusion (ALIF) using a personalised approach: Is custom the future of implants for ALIF surgery?
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Ralph J. Mobbsa, b, c, William CH Parr a,d,e, Wen Jie Choya, Aidan McEvoyf , William R Walsha,d, Kevin Phana,b,c Faculty of Medicine, University of New South Wales (UNSW) Sydney, Australia; bNeuroSpine Surgery Research Group (NSURG), Sydney, Australia; cDepartment of Neurosurgery, Prince of Wales Hospital, Sydney, Australia; d Surgical & Orthopaedic Research Laboratories (S&ORL), UNSW, Sydney, Australia; e3DMorphic, Sydney, Australia; fMatrix Medical Innovations, Sydney, Australia; gFaculty of Medicine, University of Sydney, Sydney, Australia
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Correspondence to: Ralph J. Mobbs. Chair of Neuro Spine Surgery Research Group (NSURG), Prince of Wales Private Hospital, Sydney, Australia. Email: [email protected]
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Key words: 3D printed spine implant; patient specific implant; custom device; additive manufacturing; spine surgery; ALIF
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Abstract Background:
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Spine surgery has the potential to benefit from the use of 3D printing technology (additive manufacturing) particularly in cases of complex anatomical pathologies. Custom devices have the potential to reduce operative times, reduce blood loss, provide immediate stability and potentially improve fusion rates.
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Case Description:
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A 34-year-old male presented with 3-year history of bilateral L5 radiculopathy due to bilateral L5 pars defect, and L5/S1 Degenerative Disc Disease and severe foraminal stenosis. ALIF surgery was determined to be the most efficacious method for distraction of the disc space to increase the foraminal volume and stabilization of the motion segment. Surgical decompression and reconstruction was performed in combination with a 3D printed, custom interbody implant. Custom design features included; corrective angulation to restore lumbar lordosis, pre-planned screw holes in the 3D implant and device endplate interface geometry designed to shape-match with the patient’s endplate anatomy.
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Conclusions:
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The use of Patient Specific Implants has reduced operative time significantly, which may offset costs of increased time spent pre-planning the procedure. Surgical procedures can be preplanned using 3D models reconstructed from patient CT and/or MRI scans. Planning can be aided by 3D printed models of patient anatomy, which surgeons can use to train prior to performing complex procedures. When considering implants and prostheses, the use of 3D printing allows a superior anatomical fit for the patient compared to generic devices, with the potential to improve restoration of none-pathological anatomy.
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Introduction
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Three-dimensional (3D) printing (3DP), also known as Additive Manufacturing (AM) and Rapid Prototyping (RP) originated in the late 20th century 1,2 and has rapidly advanced to allow the AM of a vast range of products for aerospace, construction, vehicular and the medical industry3,4. Utilising Computer Aided Design (CAD) programs, design drafts can be converted to a stereolithographic (.stl) file format suitable for a 3D printer to sequence two-dimensional (2D) cross sections of the object, produced by fabricating layer by layer2. High resolution medical images which include Computerised Tomography (CT) and Magnetic Resonance Imaging (MRI) allows segmentation and application of 3DP to reconstruct a watertight 3D isosurface mesh suitable for 3DP5. When 3D reconstructions of anatomy are combined with other CAD and Computer Aided Engineering (CAE) methods, multiple varieties of 3D printed objects can be produced, such as an anatomical region for study, or medical prosthesis for implantation.6,7
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The current uses of 3DP in medicine can be grouped into four overarching groups: i. anatomical models for information transfer purposes such as teaching or surgical planning; ii. tissue bioengineering; iii. patient-specific external bracing and iv. anatomically-fitting prosthetics and implants.7-9 3DP of anatomical models allow surgeons to gain a better anatomical insight of the target anatomy and pathology involved prior to and during the procedure (see supplementary figure 1).8,10,11
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Craniofacial and spinal surgeries, particularly for tumour and vascular where resection results in non-uniform cavities, have the potential to benefit from the use of patient specific devices, manufactured through 3DP, due to complex anatomical considerations and the delicate nature of surrounding structures.12,13 Successful examples of neurosurgical 3D printed implants include a polyethylethylketone (PEEK) skull implant 7,9 and multiple 3D printed implants for spinal procedures in recent days.14-18
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Another need for patient specific approach, often using a custom medical implant, arises when patients present with very abnormal anatomy, either congenital, traumatic or of pathological origin. In such cases the clinician may decide that there is no available generic device that will fit the patient's anatomy without the need for significant surgical remodelling of the anatomy occurring at the time of implantation. Such surgical remodelling of the anatomy naturally results in increased trauma to the patient's tissue, as well as: increased blood loss; longer surgery time with associated increased infection risk19; destabilisation of anatomical structures; potential loss of strong cortical bone and exposure of underlying less strong cancellous bone during loading of the device (this is particularly true for interbody spinal devices). Fortunately, the current regulatory framework in Australia allows for the use of custom Patient Specific Implants (PSIs) to be produced “specifically in accordance with a request by a health professional specifying the design characteristics or construction of the medical device, to be used only in relation to a particular individual”20. We herein report on a patient specific approach to the planning and surgical procedure that utilised a 3D printed custom anterior lumbar interbody fusion (ALIF) implant. The patient's
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preoperative anatomy was deemed to fit the criteria for a custom device by the lead surgeon of the clinical team. The patient specific approach used here included: segmentation and 3D (virtual) reconstruction of anatomy; 3DP of preoperative pathology anatomical models; 3DP of polymer custom device designs for 'fit' testing with the patient's anatomy; Finite Element Analysis to test device designs and predict stress patterns generated in the adjacent anatomy and compare these to the stress patterns predicted if a generic device were used; manufacture of the device via 3DP; implantation of the custom device; postoperative monitoring of the patient.
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The implant was designed with the custom features of pre-angled screw holes to assist with implantation and device endplate interface geometries to match the patient’s anatomy. The implant was Additively Manufactured (3DP) using Direct Metal Laser Sintering (DMLS) from Ti6Al4V Titanium (biomedical grade 5) alloy, and processed by the hospital Central Sterile Services Services Department by steam sterilization before implantation. Ti6Al4V alloy was chosen as a suitable material for manufacture due to its biocompatibility21,22, mechanical properties (lower stiffness than other biocompatible metal alloys that can be manufactured using 3DP such as stainless steel [316L] and cobalt chrome [Co-Cr-Mo])23,24; and the reduced radiological scatter produced compared to other biocompatible metals25. Another material alternative would be PEEK, although this material reportedly does not encourage bone osseointegration or growth onto the surface of the device26,27.
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Although the present device was designed without porosity, the surface topology of 3DP titanium is highly irregular, at the micro and sub-micro scales, presenting an increased surface area for cell and tissue attachment28. This has been reported to be an important factor in achieving initial stabilisation 29and encouraging long term stabilisation through interbody fusion.
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Case Presentation Presentation
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A 34-year-old male presented with 3-year history of bilateral L5 radiculopathy due to bilateral L5 pars defect, and L5/S1 Degenerative Disc Disease and severe foraminal stenosis. Conservative measures assisted with pain control initially; however, his pain had become resistant to physical therapy and injections. Imaging revealed Degenerative Disc Disease (DDD) of the L5/S1 level with significant loss of disc height and bilateral foraminal stenosis with impingement of the exiting L5 nerves. The anatomy of the L5/S1 segment was very unusual with a gross anatomical abnormality (Figure 1A, 1B). Surgery was recommended by the patient’s pain team and physical therapist following a prolonged period of conservative care.
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ALIF surgery was determined to be the most efficacious method for distraction of the disc space to increase the foraminal volume and stabilization of the motion segment30-32. Surgical decompression and reconstruction was performed in combination with a pre-planned 3D printed, PSI. The surgeon (RJM) worked closely with CAD Engineer (WP), Medical Device Company (3DMorphic, Sydney, Australia) and Implant Company (Matrix Medical Innovations, Sydney, Australia) to design and manufacture a custom implant to achieve the surgical goals of immediate stability of the motion segment, disc height elevation and stabilisation through integral screw fixation.
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Custom design features included: corrective angulation of the implant to restore lumbar lordosis; screw holes in the 3D implant that were designed with pre-planned screw trajectories, accounting for the planned screw lengths; device endplate interface geometry that matched the patient’s unique endplate anatomy, to ensure uniform loading of the endplates and device (see Figures 2 and 3).
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Surgical and Prosthesis Planning
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3D virtual models were made through patient CT segmentation and isosurface reconstruction using Materialise (Leuven, Belgium) MIMICS and 3Matic5,33-36. Acrylonitrile Butadiene Styrene(ABS) 3DP models were initially made to provide the surgical team with life size models of the anatomy (see Supplementary Figure 1). ABS parts were printed using Stratasys Fused Deposition Modelling (FDM) using UPrint and Dimensions Elite printers. As well as a visual aide, this gives a physical part that can be manipulated to predict the bone anatomy that will be encountered during the surgical procedure. A custom intervertebral body implant was planned to facilitate the surgery (Figure 2A), given the complexity of the surgery and lack of a viable “off-theshelf” implant that would fit the patient’s anatomy. The custom 3D printed interbody device was designed and manufactured by 3DMorphic and Matrix Medical Innovations (see Supplementary Video 1). Three cage heights with 2mm differences in size (size 0, size +2mm and size +4mm) and pre-planned angulation (to maintain suitable patient lumbar lordosis) were printed prior to surgery to provide options to ensure an optimal fit (Figure 2). The implant was designed to have 4 screw holes for fixation screws, with screw length determined prior to surgery to assist in workflow
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and minimise operative time. The implant designs were tested using CAE Finite Element (FE) analysis to test the overall strength of the cage and whether the design had any potential weaknesses (Supplementary Figure 2). FE models were constructed using protocols detailed in 13,33,34,36 . FE analysis was also performed to assess the effects of device conformance to endplate anatomy on the predicted stress distributions and magnitudes in the superior and inferior endplates and compared to the predicted stress for a similar sized generic device (Figure 3). Test setup was performed according to ASTM F2267-04 “Standard test method for measuring load induced subsidence of intervertebral body fusion device under static axial compression”37. The yield stress for DMLS 3DP titanium is 972-1096 MPa38. The endplates were modelled as PEEK, in accordance to the ASTM standard. The yield stress of PEEK is ~93 MPa39. These FE analyses showed that the custom cage design would withstand the test loading scenarios and should results in a more even distribution of stress in the patient’s vertebral anatomy than using an equivalent generic cage. This is the first time this custom detail has been performed for an ALIF device to our knowledge.
Surgery
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A linear mini-pfannenstiel incision with retroperitoneal exposure and identification of L5/S1 discwasperformed40. Standard discectomy with careful curetting of the endplates to avoid damage to the unique morphology was completed. Note that it is particularly important that the bone endplate anatomy is left intact and unaltered for two reasons: 1) the endplate bone is stiffer (220GPa)41-43 than the underlying cancellous bone (344±148 MPa)44, which has been shown to reduce the likelihood of subsidence45, 2) the workflow for creation of the PSI customises the generic cage design to fit the patient's anatomy, therefore surgical alteration to the patient bone anatomy will reduce the 'goodness of fit' of the device with the anatomy. The 3DP cage was packed with fibre Allograft (Ausbiotechnologies, Sydney, Australia). Indirect decompression of the foramina and correction of the sagittal deformity was achieved with insertion of the custom 3D prosthesis (Figure 4). Insertion and positioning of the prosthesis was noted to be particularly easy with a firm “press-fit” due to marry of the patient's unique endplate geometry and the fit of the custom prosthesis (Figure 5).
Follow-up
The patient was mobilized Day 1 postop, and discharged on Day 3. There were no intraoperative or postop complications. The patient reported immediate relief of his bilateral L5 radiculopathy following surgery that was maintained at the 3-month follow-up consultation. Radiological followup at 3 months indicated the implant was well-positioned and demonstrated early osseointegration with the adjacent endplates.
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Discussion
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One major advantage of three-dimensional (3D)printing (3DP) in medicine is the ability to manufacture one-off patient specific implants.17 Until the advent and advances in 3DP of biocompatible materials, manufacture of custom devices was very difficult due to production line methods of manufacturing implants. Once a production line or mould is set up, each generic device unit can be produced relatively cheaply. However, setting up a production line is very expensive and time consuming, as is producing a limited number of parts via injection moulding46. These constraints were largely prohibitive to the production of one-off custom implants. 3DP technology changes this as the predominant manufacturing cost is based on the volume of material printed, not the complexity of the design, or whether the design is a one-off or one of a thousand identical generic devices. This means that design and manufacture of custom devices is now a reality, but also means that the cost burden in the production workflow is shifted upstream to the design phases. Skilled labour time currently keeps the cost of 3DP custom devices higher than 3DP generic devices. However, the field is moving rapidly and new design technologies are emerging that should reduce the design phase burden and, thereby, the cost of producing patient specific devices.
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Within the surgical field, 3DP has been able to reduce operative time significantly; in the present case, an off-the-shelf implant would have required significant milling of endplates, multiple trials and x-rays, and may have led to a poor outcome. For the present case, the lead surgeon (RJM) for the clinical team, therefore deemed that there was no suitable generic option. This decision was supported by the results of the Finite Element Analyses (FEAs), which showed endplate stress hot spots would have occurred if a generic device were used (Figure 3). The FEA predicted that for the generic device, both the device and endplates would have experienced greater peak stress magnitudes and less uniform stress distribution than for the custom device.
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3DP allows pre-planning and training for complex surgical procedures which maximises surgical outcome for the patient (Supplementary Figure 1)8,18,47-49.When considering implants and prostheses, the use of 3DP allows a superior anatomical fit for the patient (Figures 1-5)50,51, which has been reported as important for longevity of implants (which our FE results agree with, see Figure 3), as well as bone attachment strength and ingrowth52. As a manufacturing technique, 3DP allows unique features to be added specifically to fit the patient’s condition, as seen here(Figure 4B), in a relatively cost effective manner.17 The trajectory of the screw holes can be pre-planned (Figure 2)and effected through the design of the custom device, which provides an improved accuracy during the procedure and reduces the risk of screws exiting the vertebral body, harming delicate surrounding soft tissue structures.8,16,17 Additionally, 3DP of titanium results in a rougher surface finish to the prosthesis than in a machined or forged device. It has been demonstrated that modification of the surface of implants, such as altering surface roughness, and changing the topography, can aid osseointegration22,28. Although the case presented here was successful in translating the preoperative plan into a surgical reality, and in alleviating the patient’s symptoms, the workflow employed here makes the
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process overly burdensome. The need for highly specialised equipment makes the design and production processes of each patient-specific product relatively expensive compared to generic devices.17 For cases where the patient exhibits less extreme anatomy, an “off-the-shelf’ prosthesis will currently be a more economical solution.53,54 However, some of the additional costs are offset by cost reductions in other areas, for example in theatre time.55 The workflow used here to plan the procedure, design and manufacture the custom implant was time consuming, with the requirement to involve multiple parties, with specialised resources and specific knowledge. The skill set required by the surgical team involves having some understanding of CAD design, and integration with design engineers, who themselves need to have a good understanding of anatomy and some understanding of surgical procedures. These currently are effective barriers to the more widespread use of patient specific implants.56, as is the additional time taken for the preoperative CAD planning. However, it is likely that improvements in process workflow as well as CAD/CAE software will streamline this process considerably, enabling the benefits of a patient specific approach to be viable to a greater number of patients.
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The case presented herein was a good fitted well within the current Australian Therapeutic Goods Administration (TGA) regulatory definition and guidelines for a Custom medical device20. However, the TGA are currently expected to produce new regulatory guidelines covering medical applications of 3D modelling, 3DP of polymer anatomical models and 3DP devices, including custom devices. The authors hope that the new regulations will ensure patient safety without being overly burdensome so as to stifle future research, development and commercial viability for patient specific approaches using 3DP in medicine.
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Conclusion
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We herein reported a case of patient-specific approach that utilised a custom 3D printed titanium ALIF implant. The custom implant had a number of design features that enabled successful execution of the pre-operative plan, with pre-angled screw holes ensuring screw trajectories. The implant endplate interface geometry matching the patient’s endplate anatomy ensured the planned positioning of the device was realised operatively and, as Finite Element modelling indicated, should reduce stress hot spots in the patient’s endplate bone. The patient demonstrated rapid recovery with significant clinical improvement. This case advocates the future of patient specific 3D printed prostheses as a viable technique for complex ALIF cases in spinal surgery.
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Ethics
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Ethics approval for this project was obtained from the South Eastern Sydney Local Health District Human Research Ethics Committee, with the reference number 17/089. Open disclosure and written informed consent was obtained from the patient for publication of this manuscript and accompanying images.
Correspondence
Ralph J. Mobbs, NeuroSpine Clinic, Suite 7, Level 7 Prince of Wales Private Hospital, Randwick, Sydney NSW 2031, Australia. Email: [email protected]
Disclosure
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Dr William CH Parr is a founder and director of 3DMorphic. Aidan McEvoy is CEO of Matrix Medical Innovations None of the other authors have any conflicts to declare with regards to this manuscript.
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Evans S, Parr W, Clausen P, Jones A, Wroe S. Finite element analysis of a micromechanical model of bone and a new 3D approach to validation. Journal of biomechanics. 2012;45(15):2702-2705. 37. ASTM ASfTaM. F2267-04, Standard test method for measuring load induced subsidence of intervertebral body fusion device under static axial compression. 2011. 38. Mower TM, Long MJ. Mechanical behavior of additive manufactured, powder-bed laserfused materials. Materials Science and Engineering: A. 2016/01/10/ 2016;651(Supplement C):198-213. 39. Kurtz SM, Devine JN. PEEK biomaterials in trauma, orthopedic, and spinal implants. Biomaterials. 2007/11/01/ 2007;28(32):4845-4869. 40. Mobbs RJ, Lennox A, Ho Y-T, Phan K, Choy WJ. L5/S1 anterior lumbar interbody fusion technique. Journal of Spine Surgery. 09/08/received 09/11/accepted 2017;3(3):429-432. 41. Grant JP, Oxland TR, Dvorak MF. Mapping the structural properties of the lumbosacral vertebral endplates. Spine. 2001;26(8):889-896. 42. Denoziere G, Ku DN. Biomechanical comparison between fusion of two vertebrae and implantation of an artificial intervertebral disc. Journal of biomechanics. 2006;39(4):766775. 43. Roy ME, Rho JY, Tsui TY, Evans ND, Pharr GM. Mechanical and morphological variation of the human lumbar vertebral cortical and trabecular bone. Journal of biomedical materials research. 1999;44(2):191-197. 44. Morgan EF, Keaveny TM. Dependence of yield strain of human trabecular bone on anatomic site. Journal of biomechanics. 2001;34(5):569-577. 45. Suh PB, Lewis C, Puttlitz CM, McGilvray KC. The Influence of Vertebral Endplate Density, Cage Contact Area and Cage Modulus on the Incidence of Interbody Cage Subsidence. The Spine Journal. 2015;15(10):S178. 46. Ruffo M, Tuck C, Hague R. Cost estimation for rapid manufacturing - laser sintering production for low to medium volumes. Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture. 2006/09/01 2006;220(9):14171427. 47. Malik HH, Darwood ARJ, Shaunak S, et al. Three-dimensional printing in surgery: a review of current surgical applications. Journal of Surgical Research. 12// 2015;199(2):512-522. 48. Ayoub AF, Rehab M, O’Neil M, et al. A novel approach for planning orthognathic surgery: The integration of dental casts into three-dimensional printed mandibular models. International Journal of Oral and Maxillofacial Surgery. 4// 2014;43(4):454-459. 49. Knox K, Kerber CW, Singel SA, Bailey MJ, Imbesi SG. Rapid prototyping to create vascular replicas from CT scan data: Making tools to teach, rehearse, and choose treatment strategies. Catheterization and Cardiovascular Interventions. 2005;65(1):47-53. 50. Dean DP, Min K-JP, Bond AB. Computer Aided Design of Large-Format Prefabricated Cranial Plates. Journal of Craniofacial Surgery. 2003;14(6):819-832. 51. Kim B-J, Hong K-S, Park K-J, Park D-H, Chung Y-G, Kang S-H. Customized Cranioplasty Implants Using Three-Dimensional Printers and Polymethyl-Methacrylate Casting. Journal of Korean Neurosurgical Society. 12/31 06/18/received 09/15/revised 12/18/accepted 2012;52(6):541-546. 52. Dalton JE, Cook SD, Thomas KA, Kay JF. The effect of operative fit and hydroxyapatite coating on the mechanical and biological response to porous implants. J Bone Joint Surg Am. 1995;77(1):97-110.
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Birnbaum KMD, Schkommodau EEG, Decker NMD, Prescher AMD, Klapper UMD, Radermacher KEG. Computer-Assisted Orthopedic Surgery With Individual Templates and Comparison to Conventional Operation Method. Spine. 2001;26(4):365-370. Turney BW. A New Model with an Anatomically Accurate Human Renal Collecting System for Training in Fluoroscopy-Guided Percutaneous Nephrolithotomy Access. Journal of Endourology. 2014;28(3):360-363. D'Urso PS, Barker TM, Earwaker WJ, et al. Stereolithographic biomodelling in craniomaxillofacial surgery: a prospective trial. Journal of Cranio-Maxillofacial Surgery. 1999/02/01 1999;27(1):30-37. Martelli N, Serrano C, van den Brink H, et al. Advantages and disadvantages of 3dimensional printing in surgery: A systematic review. Surgery. 6// 2016;159(6):1485-1500.
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Figure Legends
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Figure 1: L5/S1 Degenerative Disc Disease with unusual endplate anatomy not suitable for an “off-the-shelf” implant. A. Unique S1 endplate anatomy. B. 3D Modelling of endplate reveals a complex anatomical geometry. C. Implant planning with endplate matching, D. Generic implant does not fit the interbody space as well as the custom device.
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Figure 2 3D virtual planning of the device. A the size +2mm device with width and depth dimensions shown (compare depth dimension to surgeon design dimension specification in Figure 1A),B 3D isosurface models of anatomy (yellow), +2mm device (grey) and planned integral fixation screw trajectories (green),C Faux(false / virtual) x-ray simulated from the 3D isosurface models in the planned postoperative position with the +2mm device and screws, D the same image as C (virtual x-ray) inverted so as to be comparable to intraoperative (actual/real) xray radiographs (compare to Figure 4 D below).
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Figure 3. Von Mises (VM) stress results of the FE analyses of a generic device (top) compared to the custom device (bottom) with test setup according to ASTM F2267-04 “Standard test method for measuring load induced subsidence of intervertebral body fusion device under static axial compression”. A 500N axial compression load was applied to the FE models. A inferior endplate (S1 superior endplate) VM stress, B generic device (of same width and depth as the custom device) VM stress C superior endplate (L5 inferior endplate) VM stress, generic device D same as A except for custom device E same as B except for custom device F same as C except for custom device. Hotter colours (reds) indicate areas of higher stress, whilst cooler colours (blues) indicate regions of lower stress. The colour scale is set to show a maximum of 5MPa VM stress as purple (see colour scale bar in the centre of the figure). No areas of the custom device were predicted to reach 5MPa VM stress under these loading scenarios, but there were areas in the generic device that showed VM stress magnitudes greater than 5MPa. The yield stress for DMLS 3DP titanium is 972-1096 MPa. The endplates were modelled as PEEK, in accordance to the ASTM standard. The yield stress of PEEK is ~93 MPa. Neither device is predicted to yield or subside under this loading scenario. However, the stress distribution through the custom device is notably more even than the stress distribution through the generic device. Similarly, there are notable stress ‘hot spots’ in the superior and inferior endplates of the generic device model, which are not observed in the custom device model. The lack of stress hotspots in the opposing endplates of the custom device model is due to more even distribution of load between the custom device and the endplates due to the conformity of shape between the custom device geometry and the endplate anatomy. Figure 4: Intra-operative work flow. A. Approach and discectomy with disc space distraction and size0 implant trial. B. Preparation of Implant with Allograft. C. Insertion and final position of size+2mm implant with screw fixation – expandable screws to avoid backout. D. Final on-table Xray to confirm position (compare to planned positioning virtual x-ray Figure 2 D above).
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Figure 5: Postop Imaging. A&B. Excellent positioning of implant equivalent to preoperative modelling. C&D. Midline and Parasagittal images reveal a close match between implant and patient anatomy.
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ACCEPTED MANUSCRIPT Abbreviations Three-dimensional (3D) printing (3DP) Additive Manufacturing (AM) Rapid Prototyping (RP) Computer Aided Design (CAD) programs
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two-dimensional (2D) Computerised Tomography (CT) Magnetic Resonance Imaging (MRI) Computer Aided Engineering (CAE)
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Patient Specific Implants (PSIs)
anterior lumbar interbody fusion (ALIF) Direct Metal Laser Sintering (DMLS) Degenerative Disc Disease (DDD) Acrylonitrile Butadiene Styrene(ABS) Finite Element Analyses (FEAs)
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Therapeutic Goods Administration (TGA)
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polyethylethylketone (PEEK)
ACCEPTED MANUSCRIPT Ralph Mobbs – None William Parr - founder and director of 3DMorphic Wen Jie Choy – None Aidan McEvoy - CEO of Matrix Medical Innovations William R Walsh - None
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Kevin Phan – None
S1878-8750(19)30003-8
DOI:
https://doi.org/10.1016/j.wneu.2018.12.144
Reference:
WNEU 11090
To appear in:
World Neurosurgery
Received Date: 20 December 2017 Revised Date:
19 December 2018
Accepted Date: 20 December 2018
Please cite this article as: Mobbs RJ, Parr WC, Choy WJ, McEvoy A, Walsh WR, Phan K, Anterior Lumbar Interbody Fusion (ALIF) using a personalised approach: Is custom the future of implants for ALIF surgery?, World Neurosurgery (2019), doi: https://doi.org/10.1016/j.wneu.2018.12.144. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Case Report: Anterior Lumbar Interbody Fusion (ALIF) using a personalised approach: Is custom the future of implants for ALIF surgery?
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Ralph J. Mobbsa, b, c, William CH Parr a,d,e, Wen Jie Choya, Aidan McEvoyf , William R Walsha,d, Kevin Phana,b,c Faculty of Medicine, University of New South Wales (UNSW) Sydney, Australia; bNeuroSpine Surgery Research Group (NSURG), Sydney, Australia; cDepartment of Neurosurgery, Prince of Wales Hospital, Sydney, Australia; d Surgical & Orthopaedic Research Laboratories (S&ORL), UNSW, Sydney, Australia; e3DMorphic, Sydney, Australia; fMatrix Medical Innovations, Sydney, Australia; gFaculty of Medicine, University of Sydney, Sydney, Australia
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Correspondence to: Ralph J. Mobbs. Chair of Neuro Spine Surgery Research Group (NSURG), Prince of Wales Private Hospital, Sydney, Australia. Email: [email protected]
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Key words: 3D printed spine implant; patient specific implant; custom device; additive manufacturing; spine surgery; ALIF
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Abstract Background:
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Spine surgery has the potential to benefit from the use of 3D printing technology (additive manufacturing) particularly in cases of complex anatomical pathologies. Custom devices have the potential to reduce operative times, reduce blood loss, provide immediate stability and potentially improve fusion rates.
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Case Description:
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A 34-year-old male presented with 3-year history of bilateral L5 radiculopathy due to bilateral L5 pars defect, and L5/S1 Degenerative Disc Disease and severe foraminal stenosis. ALIF surgery was determined to be the most efficacious method for distraction of the disc space to increase the foraminal volume and stabilization of the motion segment. Surgical decompression and reconstruction was performed in combination with a 3D printed, custom interbody implant. Custom design features included; corrective angulation to restore lumbar lordosis, pre-planned screw holes in the 3D implant and device endplate interface geometry designed to shape-match with the patient’s endplate anatomy.
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Conclusions:
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The use of Patient Specific Implants has reduced operative time significantly, which may offset costs of increased time spent pre-planning the procedure. Surgical procedures can be preplanned using 3D models reconstructed from patient CT and/or MRI scans. Planning can be aided by 3D printed models of patient anatomy, which surgeons can use to train prior to performing complex procedures. When considering implants and prostheses, the use of 3D printing allows a superior anatomical fit for the patient compared to generic devices, with the potential to improve restoration of none-pathological anatomy.
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Introduction
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Three-dimensional (3D) printing (3DP), also known as Additive Manufacturing (AM) and Rapid Prototyping (RP) originated in the late 20th century 1,2 and has rapidly advanced to allow the AM of a vast range of products for aerospace, construction, vehicular and the medical industry3,4. Utilising Computer Aided Design (CAD) programs, design drafts can be converted to a stereolithographic (.stl) file format suitable for a 3D printer to sequence two-dimensional (2D) cross sections of the object, produced by fabricating layer by layer2. High resolution medical images which include Computerised Tomography (CT) and Magnetic Resonance Imaging (MRI) allows segmentation and application of 3DP to reconstruct a watertight 3D isosurface mesh suitable for 3DP5. When 3D reconstructions of anatomy are combined with other CAD and Computer Aided Engineering (CAE) methods, multiple varieties of 3D printed objects can be produced, such as an anatomical region for study, or medical prosthesis for implantation.6,7
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The current uses of 3DP in medicine can be grouped into four overarching groups: i. anatomical models for information transfer purposes such as teaching or surgical planning; ii. tissue bioengineering; iii. patient-specific external bracing and iv. anatomically-fitting prosthetics and implants.7-9 3DP of anatomical models allow surgeons to gain a better anatomical insight of the target anatomy and pathology involved prior to and during the procedure (see supplementary figure 1).8,10,11
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Craniofacial and spinal surgeries, particularly for tumour and vascular where resection results in non-uniform cavities, have the potential to benefit from the use of patient specific devices, manufactured through 3DP, due to complex anatomical considerations and the delicate nature of surrounding structures.12,13 Successful examples of neurosurgical 3D printed implants include a polyethylethylketone (PEEK) skull implant 7,9 and multiple 3D printed implants for spinal procedures in recent days.14-18
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Another need for patient specific approach, often using a custom medical implant, arises when patients present with very abnormal anatomy, either congenital, traumatic or of pathological origin. In such cases the clinician may decide that there is no available generic device that will fit the patient's anatomy without the need for significant surgical remodelling of the anatomy occurring at the time of implantation. Such surgical remodelling of the anatomy naturally results in increased trauma to the patient's tissue, as well as: increased blood loss; longer surgery time with associated increased infection risk19; destabilisation of anatomical structures; potential loss of strong cortical bone and exposure of underlying less strong cancellous bone during loading of the device (this is particularly true for interbody spinal devices). Fortunately, the current regulatory framework in Australia allows for the use of custom Patient Specific Implants (PSIs) to be produced “specifically in accordance with a request by a health professional specifying the design characteristics or construction of the medical device, to be used only in relation to a particular individual”20. We herein report on a patient specific approach to the planning and surgical procedure that utilised a 3D printed custom anterior lumbar interbody fusion (ALIF) implant. The patient's
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preoperative anatomy was deemed to fit the criteria for a custom device by the lead surgeon of the clinical team. The patient specific approach used here included: segmentation and 3D (virtual) reconstruction of anatomy; 3DP of preoperative pathology anatomical models; 3DP of polymer custom device designs for 'fit' testing with the patient's anatomy; Finite Element Analysis to test device designs and predict stress patterns generated in the adjacent anatomy and compare these to the stress patterns predicted if a generic device were used; manufacture of the device via 3DP; implantation of the custom device; postoperative monitoring of the patient.
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The implant was designed with the custom features of pre-angled screw holes to assist with implantation and device endplate interface geometries to match the patient’s anatomy. The implant was Additively Manufactured (3DP) using Direct Metal Laser Sintering (DMLS) from Ti6Al4V Titanium (biomedical grade 5) alloy, and processed by the hospital Central Sterile Services Services Department by steam sterilization before implantation. Ti6Al4V alloy was chosen as a suitable material for manufacture due to its biocompatibility21,22, mechanical properties (lower stiffness than other biocompatible metal alloys that can be manufactured using 3DP such as stainless steel [316L] and cobalt chrome [Co-Cr-Mo])23,24; and the reduced radiological scatter produced compared to other biocompatible metals25. Another material alternative would be PEEK, although this material reportedly does not encourage bone osseointegration or growth onto the surface of the device26,27.
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Although the present device was designed without porosity, the surface topology of 3DP titanium is highly irregular, at the micro and sub-micro scales, presenting an increased surface area for cell and tissue attachment28. This has been reported to be an important factor in achieving initial stabilisation 29and encouraging long term stabilisation through interbody fusion.
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Case Presentation Presentation
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A 34-year-old male presented with 3-year history of bilateral L5 radiculopathy due to bilateral L5 pars defect, and L5/S1 Degenerative Disc Disease and severe foraminal stenosis. Conservative measures assisted with pain control initially; however, his pain had become resistant to physical therapy and injections. Imaging revealed Degenerative Disc Disease (DDD) of the L5/S1 level with significant loss of disc height and bilateral foraminal stenosis with impingement of the exiting L5 nerves. The anatomy of the L5/S1 segment was very unusual with a gross anatomical abnormality (Figure 1A, 1B). Surgery was recommended by the patient’s pain team and physical therapist following a prolonged period of conservative care.
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ALIF surgery was determined to be the most efficacious method for distraction of the disc space to increase the foraminal volume and stabilization of the motion segment30-32. Surgical decompression and reconstruction was performed in combination with a pre-planned 3D printed, PSI. The surgeon (RJM) worked closely with CAD Engineer (WP), Medical Device Company (3DMorphic, Sydney, Australia) and Implant Company (Matrix Medical Innovations, Sydney, Australia) to design and manufacture a custom implant to achieve the surgical goals of immediate stability of the motion segment, disc height elevation and stabilisation through integral screw fixation.
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Custom design features included: corrective angulation of the implant to restore lumbar lordosis; screw holes in the 3D implant that were designed with pre-planned screw trajectories, accounting for the planned screw lengths; device endplate interface geometry that matched the patient’s unique endplate anatomy, to ensure uniform loading of the endplates and device (see Figures 2 and 3).
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Surgical and Prosthesis Planning
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3D virtual models were made through patient CT segmentation and isosurface reconstruction using Materialise (Leuven, Belgium) MIMICS and 3Matic5,33-36. Acrylonitrile Butadiene Styrene(ABS) 3DP models were initially made to provide the surgical team with life size models of the anatomy (see Supplementary Figure 1). ABS parts were printed using Stratasys Fused Deposition Modelling (FDM) using UPrint and Dimensions Elite printers. As well as a visual aide, this gives a physical part that can be manipulated to predict the bone anatomy that will be encountered during the surgical procedure. A custom intervertebral body implant was planned to facilitate the surgery (Figure 2A), given the complexity of the surgery and lack of a viable “off-theshelf” implant that would fit the patient’s anatomy. The custom 3D printed interbody device was designed and manufactured by 3DMorphic and Matrix Medical Innovations (see Supplementary Video 1). Three cage heights with 2mm differences in size (size 0, size +2mm and size +4mm) and pre-planned angulation (to maintain suitable patient lumbar lordosis) were printed prior to surgery to provide options to ensure an optimal fit (Figure 2). The implant was designed to have 4 screw holes for fixation screws, with screw length determined prior to surgery to assist in workflow
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and minimise operative time. The implant designs were tested using CAE Finite Element (FE) analysis to test the overall strength of the cage and whether the design had any potential weaknesses (Supplementary Figure 2). FE models were constructed using protocols detailed in 13,33,34,36 . FE analysis was also performed to assess the effects of device conformance to endplate anatomy on the predicted stress distributions and magnitudes in the superior and inferior endplates and compared to the predicted stress for a similar sized generic device (Figure 3). Test setup was performed according to ASTM F2267-04 “Standard test method for measuring load induced subsidence of intervertebral body fusion device under static axial compression”37. The yield stress for DMLS 3DP titanium is 972-1096 MPa38. The endplates were modelled as PEEK, in accordance to the ASTM standard. The yield stress of PEEK is ~93 MPa39. These FE analyses showed that the custom cage design would withstand the test loading scenarios and should results in a more even distribution of stress in the patient’s vertebral anatomy than using an equivalent generic cage. This is the first time this custom detail has been performed for an ALIF device to our knowledge.
Surgery
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A linear mini-pfannenstiel incision with retroperitoneal exposure and identification of L5/S1 discwasperformed40. Standard discectomy with careful curetting of the endplates to avoid damage to the unique morphology was completed. Note that it is particularly important that the bone endplate anatomy is left intact and unaltered for two reasons: 1) the endplate bone is stiffer (220GPa)41-43 than the underlying cancellous bone (344±148 MPa)44, which has been shown to reduce the likelihood of subsidence45, 2) the workflow for creation of the PSI customises the generic cage design to fit the patient's anatomy, therefore surgical alteration to the patient bone anatomy will reduce the 'goodness of fit' of the device with the anatomy. The 3DP cage was packed with fibre Allograft (Ausbiotechnologies, Sydney, Australia). Indirect decompression of the foramina and correction of the sagittal deformity was achieved with insertion of the custom 3D prosthesis (Figure 4). Insertion and positioning of the prosthesis was noted to be particularly easy with a firm “press-fit” due to marry of the patient's unique endplate geometry and the fit of the custom prosthesis (Figure 5).
Follow-up
The patient was mobilized Day 1 postop, and discharged on Day 3. There were no intraoperative or postop complications. The patient reported immediate relief of his bilateral L5 radiculopathy following surgery that was maintained at the 3-month follow-up consultation. Radiological followup at 3 months indicated the implant was well-positioned and demonstrated early osseointegration with the adjacent endplates.
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Discussion
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One major advantage of three-dimensional (3D)printing (3DP) in medicine is the ability to manufacture one-off patient specific implants.17 Until the advent and advances in 3DP of biocompatible materials, manufacture of custom devices was very difficult due to production line methods of manufacturing implants. Once a production line or mould is set up, each generic device unit can be produced relatively cheaply. However, setting up a production line is very expensive and time consuming, as is producing a limited number of parts via injection moulding46. These constraints were largely prohibitive to the production of one-off custom implants. 3DP technology changes this as the predominant manufacturing cost is based on the volume of material printed, not the complexity of the design, or whether the design is a one-off or one of a thousand identical generic devices. This means that design and manufacture of custom devices is now a reality, but also means that the cost burden in the production workflow is shifted upstream to the design phases. Skilled labour time currently keeps the cost of 3DP custom devices higher than 3DP generic devices. However, the field is moving rapidly and new design technologies are emerging that should reduce the design phase burden and, thereby, the cost of producing patient specific devices.
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Within the surgical field, 3DP has been able to reduce operative time significantly; in the present case, an off-the-shelf implant would have required significant milling of endplates, multiple trials and x-rays, and may have led to a poor outcome. For the present case, the lead surgeon (RJM) for the clinical team, therefore deemed that there was no suitable generic option. This decision was supported by the results of the Finite Element Analyses (FEAs), which showed endplate stress hot spots would have occurred if a generic device were used (Figure 3). The FEA predicted that for the generic device, both the device and endplates would have experienced greater peak stress magnitudes and less uniform stress distribution than for the custom device.
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3DP allows pre-planning and training for complex surgical procedures which maximises surgical outcome for the patient (Supplementary Figure 1)8,18,47-49.When considering implants and prostheses, the use of 3DP allows a superior anatomical fit for the patient (Figures 1-5)50,51, which has been reported as important for longevity of implants (which our FE results agree with, see Figure 3), as well as bone attachment strength and ingrowth52. As a manufacturing technique, 3DP allows unique features to be added specifically to fit the patient’s condition, as seen here(Figure 4B), in a relatively cost effective manner.17 The trajectory of the screw holes can be pre-planned (Figure 2)and effected through the design of the custom device, which provides an improved accuracy during the procedure and reduces the risk of screws exiting the vertebral body, harming delicate surrounding soft tissue structures.8,16,17 Additionally, 3DP of titanium results in a rougher surface finish to the prosthesis than in a machined or forged device. It has been demonstrated that modification of the surface of implants, such as altering surface roughness, and changing the topography, can aid osseointegration22,28. Although the case presented here was successful in translating the preoperative plan into a surgical reality, and in alleviating the patient’s symptoms, the workflow employed here makes the
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process overly burdensome. The need for highly specialised equipment makes the design and production processes of each patient-specific product relatively expensive compared to generic devices.17 For cases where the patient exhibits less extreme anatomy, an “off-the-shelf’ prosthesis will currently be a more economical solution.53,54 However, some of the additional costs are offset by cost reductions in other areas, for example in theatre time.55 The workflow used here to plan the procedure, design and manufacture the custom implant was time consuming, with the requirement to involve multiple parties, with specialised resources and specific knowledge. The skill set required by the surgical team involves having some understanding of CAD design, and integration with design engineers, who themselves need to have a good understanding of anatomy and some understanding of surgical procedures. These currently are effective barriers to the more widespread use of patient specific implants.56, as is the additional time taken for the preoperative CAD planning. However, it is likely that improvements in process workflow as well as CAD/CAE software will streamline this process considerably, enabling the benefits of a patient specific approach to be viable to a greater number of patients.
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The case presented herein was a good fitted well within the current Australian Therapeutic Goods Administration (TGA) regulatory definition and guidelines for a Custom medical device20. However, the TGA are currently expected to produce new regulatory guidelines covering medical applications of 3D modelling, 3DP of polymer anatomical models and 3DP devices, including custom devices. The authors hope that the new regulations will ensure patient safety without being overly burdensome so as to stifle future research, development and commercial viability for patient specific approaches using 3DP in medicine.
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Conclusion
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We herein reported a case of patient-specific approach that utilised a custom 3D printed titanium ALIF implant. The custom implant had a number of design features that enabled successful execution of the pre-operative plan, with pre-angled screw holes ensuring screw trajectories. The implant endplate interface geometry matching the patient’s endplate anatomy ensured the planned positioning of the device was realised operatively and, as Finite Element modelling indicated, should reduce stress hot spots in the patient’s endplate bone. The patient demonstrated rapid recovery with significant clinical improvement. This case advocates the future of patient specific 3D printed prostheses as a viable technique for complex ALIF cases in spinal surgery.
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Ethics
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Ethics approval for this project was obtained from the South Eastern Sydney Local Health District Human Research Ethics Committee, with the reference number 17/089. Open disclosure and written informed consent was obtained from the patient for publication of this manuscript and accompanying images.
Correspondence
Ralph J. Mobbs, NeuroSpine Clinic, Suite 7, Level 7 Prince of Wales Private Hospital, Randwick, Sydney NSW 2031, Australia. Email: [email protected]
Disclosure
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Dr William CH Parr is a founder and director of 3DMorphic. Aidan McEvoy is CEO of Matrix Medical Innovations None of the other authors have any conflicts to declare with regards to this manuscript.
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References Schubert C, van Langeveld MC, Donoso LA. Innovations in 3D printing: a 3D overview from optics to organs. British Journal of Ophthalmology. 2014;98(2):159-161. 2. Hull CW. Apparatus for production of three-dimensional objects by stereolithography. Google Patents; 1986. 3. 3DSystems. Solutions. http://www.3dsystems.com/solutions/overview. Accessed 24th March, 2017. 4. Mironov V, Boland T, Trusk T, Forgacs G, Markwald RR. Organ printing: computer-aided jet-based 3D tissue engineering. Trends in Biotechnology.21(4):157-161. 5. Tan CJ, Parr WCH, Walsh WR, Makara M, Johnson KA. Influence of Scan Resolution, Thresholding, and Reconstruction Algorithm on Computed Tomography-Based Kinematic Measurements. Journal of Biomechanical Engineering. 2017;139(10):104503-104503104505. 6. Gross BC, Erkal JL, Lockwood SY, Chen C, Spence DM. Evaluation of 3D Printing and Its Potential Impact on Biotechnology and the Chemical Sciences. Analytical Chemistry. 2014/04/01 2014;86(7):3240-3253. 7. Ventola CL. Medical applications for 3D printing: current and projected uses. PT. 2014;39(10):704-711. 8. Izatt MT, Thorpe PLPJ, Thompson RG, et al. The use of physical biomodelling in complex spinal surgery. European Spine Journal. 02/14 09/07/received 12/08/revised 12/11/accepted 2007;16(9):1507-1518. 9. Klein GT, Lu Y, Wang MY. 3D Printing and Neurosurgery—Ready for Prime Time? World Neurosurgery. 9// 2013;80(3–4):233-235. 10. D'Urso PSMPF, Williamson ODMBBSF, Thompson RGB. Biomodeling as an Aid to Spinal Instrumentation. Spine. 2005;30(24):2841-2845. 11. D'Urso PSP, Askin GF, Earwaker JSF, et al. Spinal Biomodeling. Spine. 1999;24(12):1247-1251. 12. Tack P, Victor J, Gemmel P, Annemans L. 3D-printing techniques in a medical setting: a systematic literature review. Biomedical engineering online. Oct 21 2016;15(1):115. 13. Aquilina P, Parr WCH, Chamoli U, Wroe S. Finite Element Analysis of Patient-Specific Condyle Fracture Plates: A Preliminary Study. Craniomaxillofacial Trauma & Reconstruction. 2015;8(2):111-116. 14. Xu N, Wei F, Liu X, et al. Reconstruction of the upper cervical spine using a personalized 3D-printed vertebral body in an adolescent with Ewing sarcoma. Spine. 2016;41(1):E50E54. 15. Choy WJ, Mobbs RJ, Wilcox B, Phan S, Phan K, Sutterlin CE. Reconstruction of Thoracic Spine Using a Personalized 3D-Printed Vertebral Body in Adolescent with T9 Primary Bone Tumor. World Neurosurgery. 2017/09/01/ 2017;105(Supplement C):1032.e10131032.e1017. 16. Mobbs RJ, Coughlan M, Thompson R, Sutterlin CE, Phan K. The utility of 3D printing for surgical planning and patient-specific implant design for complex spinal pathologies: case report. Journal of Neurosurgery: Spine. 2017/04/01 2017;26(4):513-518. 17. Phan K, Sgro A, Maharaj MM, D’Urso P, Mobbs RJ. Application of a 3D custom printed patient specific spinal implant for C1/2 arthrodesis. Journal of Spine Surgery. 12/06/received 12/07/accepted 2016;2(4):314-318. 18. Wilcox B, Mobbs RJ, Wu A-M, Phan K. Systematic review of 3D printing in spinal surgery: the current state of play. Journal of Spine Surgery. 09/04/received
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09/08/accepted 2017;3(3):433-443. 19. Pull ter Gunne AF, Cohen DB. Incidence, Prevalence, and Analysis of Risk Factors for Surgical Site Infection Following Adult Spinal Surgery. Spine. 2009;34(13):1422-1428. 20. TGA TGA. Therapeutic Goods (Medical Devices) Regulations 2002. 21. Sidambe AT. Biocompatibility of Advanced Manufactured Titanium Implants—A Review. Materials. 2014;7(12):8168-8188. 22. Takatsuka K, Yamamuro T, Nakamura T, Kokubo T. Bone‐bonding behavior of titanium alloy evaluated mechanically with detaching failure load. Journal of Biomedical Materials Research Part A. 1995;29(2):157-163. 23. Niinomi M. Mechanical properties of biomedical titanium alloys. Materials Science and Engineering: A. 1998/03/15/ 1998;243(1):231-236. 24. Long M, Rack HJ. Titanium alloys in total joint replacement—a materials science perspective. Biomaterials. 1998/09/01/ 1998;19(18):1621-1639. 25. Ernstberger T, Heidrich G, Bruening T, Krefft S, Buchhorn G, Klinger HM. The interobserver-validated relevance of intervertebral spacer materials in MRI artifacting. European Spine Journal. 2007/02/01 2007;16(2):179-185. 26. Olivares-Navarrete R, Gittens RA, Schneider JM, et al. Osteoblasts exhibit a more differentiated phenotype and increased bone morphogenetic protein production on titanium alloy substrates than on poly-ether-ether-ketone. The Spine Journal. 2012/03/01/ 2012;12(3):265-272. 27. Phan K, Hogan JA, Assem Y, Mobbs RJ. PEEK-Halo effect in interbody fusion. Journal of clinical neuroscience : official journal of the Neurosurgical Society of Australasia. Feb 2016;24:138-140. 28. Simmons CA, Valiquette N, Pilliar RM. Osseointegration of sintered porous-surfaced and plasma spray-coated implants: an animal model study of early postimplantation healing response and mechanical stability. Journal of biomedical materials research. 1999;47(2):127-138. 29. Brentel AS, Vasconcellos LMRd, Oliveira MV, et al. Histomorphometric analysis of pure titanium implants with porous surface versus rough surface. Journal of Applied Oral Science. 2006;14(3):213-218. 30. Giang G, Mobbs R, Phan S, Tran TM, Phan K. Evaluating Outcomes of Stand-Alone Anterior Lumbar Interbody Fusion: A Systematic Review. World Neurosurg. Aug 2017;104:259-271. 31. Teng I, Han J, Phan K, Mobbs R. A meta-analysis comparing ALIF, PLIF, TLIF and LLIF. Journal of clinical neuroscience : official journal of the Neurosurgical Society of Australasia. Oct 2017;44:11-17. 32. Rao PJ, Phan K, Giang G, Maharaj MM, Phan S, Mobbs RJ. Subsidence following anterior lumbar interbody fusion (ALIF): a prospective study. Journal of Spine Surgery. 04/01/received 04/12/accepted 2017;3(2):168-175. 33. Parr W, Chamoli U, Jones A, Walsh W, Wroe S. Finite element micro-modelling of a human ankle bone reveals the importance of the trabecular network to mechanical performance: new methods for the generation and comparison of 3D models. Journal of biomechanics. 2013;46(1):200-205. 34. Parr W, Wroe S, Chamoli U, et al. Toward integration of geometric morphometrics and computational biomechanics: new methods for 3D virtual reconstruction and quantitative analysis of Finite Element Models. Journal of Theoretical Biology. 2012;301:1-14. 35. Parr WCH, Wilson LAB, Wroe S, Colman NJ, Crowther MS, Letnic M. Cranial Shape and the Modularity of Hybridization in Dingoes and Dogs; Hybridization Does Not Spell the End for Native Morphology. Evolutionary Biology. 2016;43(2):171-187.
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Evans S, Parr W, Clausen P, Jones A, Wroe S. Finite element analysis of a micromechanical model of bone and a new 3D approach to validation. Journal of biomechanics. 2012;45(15):2702-2705. 37. ASTM ASfTaM. F2267-04, Standard test method for measuring load induced subsidence of intervertebral body fusion device under static axial compression. 2011. 38. Mower TM, Long MJ. Mechanical behavior of additive manufactured, powder-bed laserfused materials. Materials Science and Engineering: A. 2016/01/10/ 2016;651(Supplement C):198-213. 39. Kurtz SM, Devine JN. PEEK biomaterials in trauma, orthopedic, and spinal implants. Biomaterials. 2007/11/01/ 2007;28(32):4845-4869. 40. Mobbs RJ, Lennox A, Ho Y-T, Phan K, Choy WJ. L5/S1 anterior lumbar interbody fusion technique. Journal of Spine Surgery. 09/08/received 09/11/accepted 2017;3(3):429-432. 41. Grant JP, Oxland TR, Dvorak MF. Mapping the structural properties of the lumbosacral vertebral endplates. Spine. 2001;26(8):889-896. 42. Denoziere G, Ku DN. Biomechanical comparison between fusion of two vertebrae and implantation of an artificial intervertebral disc. Journal of biomechanics. 2006;39(4):766775. 43. Roy ME, Rho JY, Tsui TY, Evans ND, Pharr GM. Mechanical and morphological variation of the human lumbar vertebral cortical and trabecular bone. Journal of biomedical materials research. 1999;44(2):191-197. 44. Morgan EF, Keaveny TM. Dependence of yield strain of human trabecular bone on anatomic site. Journal of biomechanics. 2001;34(5):569-577. 45. Suh PB, Lewis C, Puttlitz CM, McGilvray KC. The Influence of Vertebral Endplate Density, Cage Contact Area and Cage Modulus on the Incidence of Interbody Cage Subsidence. The Spine Journal. 2015;15(10):S178. 46. Ruffo M, Tuck C, Hague R. Cost estimation for rapid manufacturing - laser sintering production for low to medium volumes. Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture. 2006/09/01 2006;220(9):14171427. 47. Malik HH, Darwood ARJ, Shaunak S, et al. Three-dimensional printing in surgery: a review of current surgical applications. Journal of Surgical Research. 12// 2015;199(2):512-522. 48. Ayoub AF, Rehab M, O’Neil M, et al. A novel approach for planning orthognathic surgery: The integration of dental casts into three-dimensional printed mandibular models. International Journal of Oral and Maxillofacial Surgery. 4// 2014;43(4):454-459. 49. Knox K, Kerber CW, Singel SA, Bailey MJ, Imbesi SG. Rapid prototyping to create vascular replicas from CT scan data: Making tools to teach, rehearse, and choose treatment strategies. Catheterization and Cardiovascular Interventions. 2005;65(1):47-53. 50. Dean DP, Min K-JP, Bond AB. Computer Aided Design of Large-Format Prefabricated Cranial Plates. Journal of Craniofacial Surgery. 2003;14(6):819-832. 51. Kim B-J, Hong K-S, Park K-J, Park D-H, Chung Y-G, Kang S-H. Customized Cranioplasty Implants Using Three-Dimensional Printers and Polymethyl-Methacrylate Casting. Journal of Korean Neurosurgical Society. 12/31 06/18/received 09/15/revised 12/18/accepted 2012;52(6):541-546. 52. Dalton JE, Cook SD, Thomas KA, Kay JF. The effect of operative fit and hydroxyapatite coating on the mechanical and biological response to porous implants. J Bone Joint Surg Am. 1995;77(1):97-110.
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Birnbaum KMD, Schkommodau EEG, Decker NMD, Prescher AMD, Klapper UMD, Radermacher KEG. Computer-Assisted Orthopedic Surgery With Individual Templates and Comparison to Conventional Operation Method. Spine. 2001;26(4):365-370. Turney BW. A New Model with an Anatomically Accurate Human Renal Collecting System for Training in Fluoroscopy-Guided Percutaneous Nephrolithotomy Access. Journal of Endourology. 2014;28(3):360-363. D'Urso PS, Barker TM, Earwaker WJ, et al. Stereolithographic biomodelling in craniomaxillofacial surgery: a prospective trial. Journal of Cranio-Maxillofacial Surgery. 1999/02/01 1999;27(1):30-37. Martelli N, Serrano C, van den Brink H, et al. Advantages and disadvantages of 3dimensional printing in surgery: A systematic review. Surgery. 6// 2016;159(6):1485-1500.
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Figure Legends
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Figure 1: L5/S1 Degenerative Disc Disease with unusual endplate anatomy not suitable for an “off-the-shelf” implant. A. Unique S1 endplate anatomy. B. 3D Modelling of endplate reveals a complex anatomical geometry. C. Implant planning with endplate matching, D. Generic implant does not fit the interbody space as well as the custom device.
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Figure 2 3D virtual planning of the device. A the size +2mm device with width and depth dimensions shown (compare depth dimension to surgeon design dimension specification in Figure 1A),B 3D isosurface models of anatomy (yellow), +2mm device (grey) and planned integral fixation screw trajectories (green),C Faux(false / virtual) x-ray simulated from the 3D isosurface models in the planned postoperative position with the +2mm device and screws, D the same image as C (virtual x-ray) inverted so as to be comparable to intraoperative (actual/real) xray radiographs (compare to Figure 4 D below).
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Figure 3. Von Mises (VM) stress results of the FE analyses of a generic device (top) compared to the custom device (bottom) with test setup according to ASTM F2267-04 “Standard test method for measuring load induced subsidence of intervertebral body fusion device under static axial compression”. A 500N axial compression load was applied to the FE models. A inferior endplate (S1 superior endplate) VM stress, B generic device (of same width and depth as the custom device) VM stress C superior endplate (L5 inferior endplate) VM stress, generic device D same as A except for custom device E same as B except for custom device F same as C except for custom device. Hotter colours (reds) indicate areas of higher stress, whilst cooler colours (blues) indicate regions of lower stress. The colour scale is set to show a maximum of 5MPa VM stress as purple (see colour scale bar in the centre of the figure). No areas of the custom device were predicted to reach 5MPa VM stress under these loading scenarios, but there were areas in the generic device that showed VM stress magnitudes greater than 5MPa. The yield stress for DMLS 3DP titanium is 972-1096 MPa. The endplates were modelled as PEEK, in accordance to the ASTM standard. The yield stress of PEEK is ~93 MPa. Neither device is predicted to yield or subside under this loading scenario. However, the stress distribution through the custom device is notably more even than the stress distribution through the generic device. Similarly, there are notable stress ‘hot spots’ in the superior and inferior endplates of the generic device model, which are not observed in the custom device model. The lack of stress hotspots in the opposing endplates of the custom device model is due to more even distribution of load between the custom device and the endplates due to the conformity of shape between the custom device geometry and the endplate anatomy. Figure 4: Intra-operative work flow. A. Approach and discectomy with disc space distraction and size0 implant trial. B. Preparation of Implant with Allograft. C. Insertion and final position of size+2mm implant with screw fixation – expandable screws to avoid backout. D. Final on-table Xray to confirm position (compare to planned positioning virtual x-ray Figure 2 D above).
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Figure 5: Postop Imaging. A&B. Excellent positioning of implant equivalent to preoperative modelling. C&D. Midline and Parasagittal images reveal a close match between implant and patient anatomy.
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ACCEPTED MANUSCRIPT Abbreviations Three-dimensional (3D) printing (3DP) Additive Manufacturing (AM) Rapid Prototyping (RP) Computer Aided Design (CAD) programs
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two-dimensional (2D) Computerised Tomography (CT) Magnetic Resonance Imaging (MRI) Computer Aided Engineering (CAE)
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Patient Specific Implants (PSIs)
anterior lumbar interbody fusion (ALIF) Direct Metal Laser Sintering (DMLS) Degenerative Disc Disease (DDD) Acrylonitrile Butadiene Styrene(ABS) Finite Element Analyses (FEAs)
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Therapeutic Goods Administration (TGA)
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polyethylethylketone (PEEK)
ACCEPTED MANUSCRIPT Ralph Mobbs – None William Parr - founder and director of 3DMorphic Wen Jie Choy – None Aidan McEvoy - CEO of Matrix Medical Innovations William R Walsh - None
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Kevin Phan – None