BY-NC-ND 4.0 license Open Access Published by De Gruyter September 30, 2016

Patient-specific hip prostheses designed by surgeons

Florian Coigny, Adi Todor, Horatiu Rotaru, Ralf Schumacher and Erik Schkommodau

Abstract

Patient-specific bone and joint replacement implants lead to better functional and aesthetic results than conventional methods [1], [2], [3]. But extracting 3D shape information from CT Data and designing individual implants is demanding and requires multiple surgeon-to-engineer interactions. For manufacturing purposes, Additive Manufacturing offers various advantages, especially for low volume manufacturing parts, such as patient specific implants. To ease these new approaches and to avoid surgeon-to-engineer interactions a new design software approach is needed which offers highly automated and user friendly planning steps.

1 Introduction

Patient-specific shaped implants for bone support or bone replacement are an emerging trend in surgical interventions [1]. Implants in the facial area for example need to match the shape of the face in order to create an acceptable functional and aesthetic result [2]. For other indications, patient specific implants have a better surgical outcome or faster recovery times. Revisions of joint implants, for example, need a good interface between the implant and the patient’s remaining specific bone structure. Shape matching surfaces of patient bone and implant improve stability and preserve most of the patient’s remaining bone [3]. Today, two main methods are used to customize bone implants. One is that the surgeon intra-operatively bends and cuts implants in order to match the desired shape. This procedure prolongs the operation time and results in high expenses for the time in the operating theatre. Further, the bending adaptations weaken the metal structures and implants could break under biomechanical loads. Additionally, this surgical approach is seldom as accurate as a patient specific preoperatively produced implant. Another approach to implant customization starts by designing the implant pre-operatively based on the patient’s own computer tomography (CT) image data. This is realized by collaborating surgeons and design engineers and needs several interactions and iteration steps which take time and lead to a risk of errors. The implant is then produced according to this design with conventional machining methods such as milling or turning. This is cost and time intensive because such production methods are used for mass production.

The aim is to develop a novel production chain where the engineer is no longer the originator of the implant design. The surgeon becomes the designer of patient specific solutions while the classic design phase is eliminated and shorter lead-times can be achieved. Additionally 3D-printing is used, which is suitable for low volume production. Patient-specific stem parts of hip joint prostheses served as an example indication to demonstrate the feasibility of the whole new process chain. The design process starts by importing a patient’s CT scan dataset and ends with the design of a patient-specific hip stem. This design data then gets sent out to a manufacturer for production and post-processing.

2 Methods

The process starts with the surgeon using highly automated and user friendly software. It enables surgeons to plan and create 3D implant designs based on patient CT data with few interaction steps and without needing any engineering or computer aided design (CAD) know-how. The software implements a design workflow which has been developed in close collaboration with surgeons where users are guided step by step. The surgeon starts the workflow by importing patient CT images from a USB-Stick or other data media. Uni-lateral 3D slice images that contain the hip cup and stem area with maximum 2 millimetres slice thickness and a gantry angle of 0 degrees are required.

After checking and complementing missing patient information, the surgeon marks anatomical properties such as the patient’s leg axis or the hip center in the patient images (Figure 1). This information is acquired on virtually generated 2D x-ray images that surgeons are already used to working with. 3D coordinates of the properties are gained by letting the surgeon work on two orthogonally oriented images.

Figure 1: User interface for defining anatomical properties.

Figure 1:

User interface for defining anatomical properties.

In the next steps, the surgeon defines size and position of the implant based on the previously acquired anatomical properties. The last planning step requires the surgeon to define the area of the hip stem that will have a patient specific shape. A new algorithm then modifies standard implant geometry in the marked areas in order to match the automatically recognized cortical bone from the images. For the final check, the surgeon sees a 3D model of the patient specific implant, which can be inspected from all directions and in detail by rotating and zooming. A slice by slice overlay of implant contours on patient images allows verification of the surface match and the correct positioning. If the surgeon agrees to the design, it can be exported and uploaded to a manufacturer (Figure 2).

Figure 2: Left: Screenshot of finished patient-specific implant design. Right: 3D-printed from titanium.

Figure 2:

Left: Screenshot of finished patient-specific implant design. Right: 3D-printed from titanium.

For the validation, twelve individual hip stem implants have been designed by surgeons using this new planning and design process. Four different surgeons that where using the software, filled out a questionnaire that was based on the usability measurement method of Lewis [4]. The questionnaire contained 22 goals about usability and functionality that should be reached. Each goal could be rated with “very positive” “positive”, “medium”, “negative” and “very negative”.

3 Results

With this method, an individual hip implant stem is typically planned and designed within 10 min. The feedback revealed that 19 out of 22 goals were rated “positive” or “very positive” by the subjects. The questions 9, 13 and 21 received “medium” rating from some of the participants (Figure 3). They concerned clarity of error messages, user instructions, and the performance of one particular workflow step. A working web platform for uploading the design file, tracking of implant production and delivery status was also developed.

Figure 3: Results of the software evaluation.

Figure 3:

Results of the software evaluation.

4 Conclusion and outlook

The successful test series of surgeon-designed implants showed that the approach is feasible. Complex shaped individual implants could be designed by surgeons themselves in only minutes. No surgeon-to-engineer interactions were needed anymore. For the future, the customised design of the stem needs further work to ensure that the proximal region of the stem is suitably supported by the surround bone. The customised region of the stem needs to be extended through the proximal body to the reference line for the incision depth.

Taking manufacturing guidelines of 3D printing processes into account during the design phase could further automate the process and thereby reduce production costs.

Acknowledgement

We would like to thank the whole project team consisting of Daniel Xander1, John Truong1, Markus Degen1, Dominique Brodbeck1, Sozon Tsopanos2, Andy Payne2, Edward Draper3, Sarrawat Rehman3, Ruben Wauthle4, Thomas Kosche5, Manfred Boretius6 and Matthias Fockele7 for their contributions.

1University of Applied Sciences and Arts Northwestern Switzerland 2The Welding Institute (United Kingdom) 3JRI Orthopaedics (United Kingdom) 4Layerwise NV (Belgium) 5BCT GmbH (Germany) 6Listemann AG (Liechtenstein) 7ReaLizer GmbH (Germany)

Author’s Statement

Research funding: The research leading to these results has received funding from the European Union’s Seventh Framework Programme managed by REA Research Executive Agency [FP7/2007–2013] under grant agreement no 2623859. Conflict of interest: Authors state no conflict of interest. Informed consent: Informed consent has been obtained from all individuals included in this study. Ethical approval: The research related to human use complies with all the relevant national regulations, institutional policies and was performed in accordance to the tenets of the Helsinki Declaration, and has been approved by the authors’ institutional review board or equivalent committee.

References

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Published Online: 2016-9-30
Published in Print: 2016-9-1

©2016 Florian Coigny et al., licensee De Gruyter.

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License.