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 Table of Contents  
EDITORIAL
Year : 2017  |  Volume : 3  |  Issue : 1  |  Page : 1-5

Individualized three-dimensional printed cage for spinal cervical fusion


Department of Neurosurgery, Klinikum Karlsruhe, SKK, Karlsruhe, Germany

Date of Web Publication19-Jun-2017

Correspondence Address:
Uwe Spetzger
Department of Neurosurgery, Klinikum Karlsruhe, SKK, Moltkestrasse 90, D-76131 Karlsruhe
Germany
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/digm.digm_12_17

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  Abstract 

[INLINE:1]
Uwe Spetzger, MD, is the Chairman of Department of Neurosurgery, Klinikum Karlsruhe, and Institute for Anthropomatics, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany. Uwe Spetzger received his medical degree in 1989 at the Medical Faculty, University of Heidelberg, Germany. He passed the US American medical exam (ECFMG) in 1990. He has started his neurosurgical training in 1990 at the Department of Neurosurgery, Technical University (RWTH) Aachen and got his board certification of neurosurgeon in 1996. The Grant of the Wilhelm-Tonnis-Foundation of the German Society of Neurosurgery (DGNC) enables his scientific internship in 1999 at Department of Neurosurgery, University of Illinois at Chicago (UIC). In June 1999, he passed the European Examination in Neurosurgery (EANS). From 1999 - 2002 he was vice-chairman of the Department of Neurosurgery at the University of Freiburg and the director of the interdisciplinary centre for skull base surgery at Freiburg University. Since 2002 he is Chairman of the Department of Neurosurgery, Klinikum Karlsruhe and in 2003 he became an active member of the Faculty of Computer Science, Humanoids and Intelligence Systems Lab - Institute for Anthropomatics at KIT. Prof. Spetzger became the president of the international Society of Medical Innovation and Technology iSMIT in 2013 and the congress president of the 66th Annual Meeting of German Society of Neurosurgery DGNC in 2015 and the Vice-President of the International Society of Digital Medicine in 2016. He is member of several national and international neurosurgical and medical technological societies. His main surgical and research interests are cerebrovascular surgery, skull base surgery, computer-assisted and robotic surgery, neuronavigation and spinal microsurgery.


How to cite this article:
Spetzger U, Koenig AS. Individualized three-dimensional printed cage for spinal cervical fusion. Digit Med 2017;3:1-5

How to cite this URL:
Spetzger U, Koenig AS. Individualized three-dimensional printed cage for spinal cervical fusion. Digit Med [serial online] 2017 [cited 2017 Aug 17];3:1-5. Available from: http://www.digitmedicine.com/text.asp?2017/3/1/1/208445


  Introduction Top


Today, spinal surgery is one of the fastest-growing markets in medical business worldwide, because the concept of minimal-invasive surgical techniques and the development of smart technologies continuously reducing surgical trauma and improving the treatment results. The paper “trends and variations in cervical spine surgery in the United States: Medicare beneficiaries, 1992–2005” confirmed an enormous increase of cervical spine fusions, reaching a rate of 206%.[1] Especially, in industrialized countries, this trend is even quickened, due to the population age pyramid structure. Continuing advances in computer-assisted and image-guided technology, surgical robotics and especially the development of smart implants and biomaterials will promote the continuing revolution in spinal surgery. Used increasingly in medical applications, direct metal three-dimensional (3D) printers create fully dense and highly accurate metal parts using patented laser melting technology. Therefore, future looks bright for the new technology of 3D printed implants in modern spinal surgery.

At present, anterior cervical discectomy and fusion with implantation of cages, made of various biocompatible materials, with or without anterior plating, is the standard surgical treatment of cervical degenerative disk disease with cervical myelopathy and/or radiculopathy.[2],[3],[4],[5] A tremendous amount of cervical cages, offered by the industry, mimic the anatomy of the intervertebral disc space more or less, whereas size and design of the cages are adapted to average shapes and sizes of patient's intervertebral discs. The strategy of stand-alone cages without additional ventral plating reduces the invasiveness of the procedure and gains more and more overall acceptance in Europe and Asia too. However, a perfect and patient individual fit of the implant is a prerequisite for the adequate stabilization.

The philosophy and overall goals of our innovative scientific project are: “Adapt the implant to the patient's individual anatomy - and not as usual - adapt the patient's anatomy to a commercially available implant”. This development aims to create a cervical cage that perfectly fits the individual endplates of the adjacent vertebral bodies, to avoid fusion complications such as secondary dislocation or subsidence of the implant. The 3D printing technology enables the manufacture of a patient-specific and individualized cervical cage for anterior cervical discectomy and cervical spinal fusion.

This paper detailed the planning, manufacturing and the surgical implantation of a patient individualized and 3D printed titanium cage for cervical spinal fusion.

In an interdisciplinary scientific industrial cooperation, we realized this project, with computer-aided planning (virtual reality [VR] interactive simulation), manufacturing by 3D printing (selective laser melting [SLM]) and surgical implantation of an individualized cervical cage. Simulation and planning were performed together with 3D Systems, Rock Hill, SC, USA. The manufacturing and 3D printing of the cage were performed by Emerging Implant Technologies (EIT) GmbH, Tuttlingen, Germany. This innovative surgical procedure was performed by the author and co-author, Spetzger et al., at the Department of Neurosurgery, SKK, Karlsruhe, Germany. The worldwide first spinal cervical surgical procedure implanting such a customized 3D printed cervical cage initially was performed in May 2015, and was published as unique technical innovation in the field of modern spine surgery.[6]


  Virtual Reality Simulation and Computer-Aided Planning Top


A 3D model of the patient's cervical spine is rendered, by using a DICOM computed tomography dataset (1.0 mm slice thickness) [”rendered anatomy of the cervical spine” [Figure 1]. After analyzing the 3D model with emphasis on deformities, the kyphosis is virtually corrected by repositioning of the C 6 and C 7 vertebrae [”repositioned anatomy,” [Figure 2]. By this procedure, the individualized cage obtains the ideal lordotic angle for restoring the sagittal balance of the cervical spine.{Figure 1}{Figure 2}

The next planning step is the computer based and virtual surgical resection of osteophytes [”resected anatomy,” [Figure 3]. The simulated resection of posterior osteophytes is necessary for the adequate decompression of the spinal cord and nerve roots. The simulation of the resection of anterior osteophytes is considered if they obstruct the entrance into the disc space or especially in the case of symptomatic dysphagia.{Figure 3}

Finally, the cage implantation can be simulated on the workstation and is shown in 3D and freely rotatable to check the implant's accuracy of fit [”implant placement,” [Figure 4]. The determination of the optimum height of the implant takes the height and facet joint orientation of adjacent levels into consideration. The planning of the final cage shape and design starts with the existing data of an EIT standard titanium cage, which is modified according to the patient's individual anatomy. After the rendering of the patient's individual shape of the endplates the final height of implant is determined [Figure 5]. The neurosurgeon reviews the entire presurgical planning of the whole procedure and the cage design and modifies this VR simulation interactively. This last step finalizes the definitive dimension and shape of the cage before it goes into production by 3D printing.{Figure 4}{Figure 5}


  Manufacturing and Three-Dimensional Printing of the Implant Top


The porous titanium cage is manufactured slice by slice by EIT (EIT GmbH, Tuttlingen, Germany), using the modern additive production process SLM. During this 3D printing procedure, a very thin powder layer of the titanium alloy TiAl6V4 is applied to a base plate. The titanium alloy powder is completely melted by a laser beam and makes up a tight layer after consolidation. After this process the base plate is lowered by 30–50 μm and the next layer is applied consecutively. This procedure is repeated until all layers are completed and the cage reached its final shape. The guidance of the laser beam is carried out by special 3D CAD software (3D-Systems Corporation, Rock Hill, SC 29730, USA) that divides the device into several layers and calculates the lanes of the laser. On the basis of the 3D data set of the patient, the precalculated 3D form of the cage is highly precise replicated and shows the exact anatomy of the patient's individual disk space [Figure 6].{Figure 6}

Designed in partnership with 3D Systems and produced using its cloud-based manufacturing services, the porous cervical implant imitates the structure and characteristics of natural bone, because it mimics the trabecular structure of human osseous anatomy inside of a titanium fusion cage. The cage is made of EIT Cellular Titanium with an 80% porosity and a pore size of 0.65 mm that provides very high primary stability and good preconditions for secondary bony fusion without an additional synthetic bone graft [Figure 6].


  Surgical Implantation Top


The anterior discectomy and decompression with cervical fusion was performed using the standard anterolateral approach to the level C 6/7 of the cervical spine.

First, resection of anterior osteophytes is performed as virtually planned and computer-based simulated, followed by discectomy and microsurgical decompression of the cervical spinal canal and the foramina. Posterior osteophytes that narrow the spinal canal and the foramina are selectively removed with a 4 mm high-speed diamond drill or a 2 mm Kerrison punch under the operating microscope. During the microsurgical procedure, we avoid damaging of the bony endplates of the adjacent vertebral bodies to allow the perfect fit of the cage.

The final step is the cage implantation under microscopic and X-ray control. The retractor is put under slight over-distraction to increase the disk space for the implantation of the 3D printed cage. Therefore, the cage “found itself” the correct position after suspending distraction due to its unique and perfectly fitting endplate design [Figure 7]. Furthermore, it was impossible to move the implanted cage in any direction with the inserting instrument after suspending distraction for the same reason.{Figure 7}

The pilot project of the first implantation of an individualized 3D printed cervical cage ever resulted in a high accuracy of fit of the implant [Figure 7]. Thus, it can be assumed that an individualized cervical implant provides excellent primary stability. After removal of the distractor and pins, the final position of the cage is checked by direct visualization under the operating microscope and by intraoperative fluoroscopy. Direct postoperatively the perfect result is confirmed by anterior-posterior and lateral X-ray [Figure 8].{Figure 8}


  Discussion Top


Individualized cages for cervical fusion for the surgical treatment of degenerative cervical spinal diseases have the potential to be the next step in the development of up-to-date spinal implants. First attempts were made, to precalculate the exact dimension of the cages to re-calibrate and achieve a physiological spinal curvature and sagittal balance of the spine after the fixation.[7] Especially in modern neurosurgery, there is an analogy to computer-aided design cranial implants for the reconstruction of skull defects.

Nevertheless, the manufacturing process of individualized 3D printed spinal implants is more complex because of the minor dimensions of the disk space and the small contours of the spine. Thus, the production process also takes much more time, effort, and costs compared to a standard titanium cage, especially, when considering the intensive planning and high-end manufacturing procedures.[8] It is likely that the production of a higher number of individualized cages will lower the implant costs. The biggest challenge will be the minimization of the additional cost due to a further process optimization.

Besides these disadvantages of an individualized 3D printed implant, there are some potential advantages in avoiding implant-related complications, like a better load-bearing surface, and a lower rate of implant dislocation and cage subsidence into the bony endplates of adjacent vertebral bodies.[9] Therefore, we expect a lower overall rate of revision surgeries with the customized 3D printed cages.

The future trend in spinal implants, especially for interbody fusion devices like cages, will be individualized forms and shapes adapted to the patient's specific anatomical situation using modern computer-based planning and simulation. These individually molded implants will increase fitting accuracy and will perfectly adapt and restore the balance and profile of the spinal column.


  Perspective Top


We summarize that the technical preconditions for the planning and manufacturing of individualized 3D printed cervical fusion cages using specific patient data are given. The implantation of these cages is as uncomplicated as the implantation of standard cages. If the improved load-bearing surface will be able to reduce the rate of implant dislocation and cage subsidence should be evaluated in the future. The production of individualized 3D printed cages at a reasonable price has to be figured out by further collaboration of spine surgeons and industrial partners. The era of VR in surgery, as well as in 3D printing in surgery is a new milestone in medical history.[10] “The future of patient individualized spinal implants has just begun.”

Acknowledgment

We would like to express our gratitude to Mr. Guntmar Eisen and his team from EIT, Emerging Implant Technologies GmbH, Tuttlingen, Germany for their effort to manufacture the implant, and Mr. Miles Frasca from 3D Systems, Rock Hill, SC, USA, doing the whole computer-assisted planning and the visualization and design of the implant. Without this industrial and scientific cooperation this project would not have been possible.

 
  References Top

1.
Wang MC, Kreuter W, Wolfla CE, Maiman DJ, Deyo RA. Trends and variations in cervical spine surgery in the United States: Medicare beneficiaries, 1992 to 2005. Spine (Phila Pa 1976) 2009;34:955-61.  Back to cited text no. 1
    
2.
Yamagata T, Takami T, Uda T, Ikeda H, Nagata T, Sakamoto S, et al. Outcomes of contemporary use of rectangular titanium stand-alone cages in anterior cervical discectomy and fusion: Cage subsidence and cervical alignment. J Clin Neurosci 2012;19:1673-8.  Back to cited text no. 2
[PUBMED]    
3.
Cabraja M, Oezdemir S, Koeppen D, Kroppenstedt S. Anterior cervical discectomy and fusion: Comparison of titanium and polyetheretherketone cages. BMC Musculoskelet Disord 2012;13:172.  Back to cited text no. 3
[PUBMED]    
4.
Wu WJ, Jiang LS, Liang Y, Dai LY. Cage subsidence does not, but cervical lordosis improvement does affect the long-term results of anterior cervical fusion with stand-alone cage for degenerative cervical disc disease: A retrospective study. Eur Spine J 2012;21:1374-82.  Back to cited text no. 4
[PUBMED]    
5.
Kolstad F, Nygaard ؘP, Andresen H, Leivseth G. Anterior cervical arthrodesis using a “stand alone” cylindrical titanium cage: Prospective analysis of radiographic parameters. Spine (Phila Pa 1976) 2010;35:1545-50.  Back to cited text no. 5
    
6.
Spetzger U, Frasca M, König SA. Surgical planning, manufacturing and implantation of an individualized cervical fusion titanium cage using patient-specific data. Eur Spine J 2016;25:2239-46.  Back to cited text no. 6
    
7.
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. J Spine Surg 2016;2:314-8.  Back to cited text no. 7
    
8.
Spetzger U, Von Schilling A, Winkler G, Wahrburg J, König A. The past, present and future of minimally invasive spine surgery: A review and speculative outlook. Minim Invasive Ther Allied Technol 2013;22:227-41.  Back to cited text no. 8
    
9.
Womack W, Leahy PD, Patel VV, Puttlitz CM. Finite element modeling of kinematic and load transmission alterations due to cervical intervertebral disc replacement. Spine (Phila Pa 1976) 2011;36:E1126-33.  Back to cited text no. 9
    
10.
Hoang D, Perrault D, Stevanovic M, Ghiassi A. Surgical applications of three-dimensional printing: A review of the current literature and how to get started. Ann Transl Med 2016;4:456.  Back to cited text no. 10
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]



 

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