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 Table of Contents  
ORIGINAL ARTICLE
Year : 2021  |  Volume : 7  |  Issue : 1  |  Page : 45-51

Orthopedic implant design from concept to final tested product: A design surgeon's experience


Department of Surgical Sciences, Division of Orthopaedic Surgery, Faculty of Medicine and Health Sciences, Stellenbosch University, Cape Town, South Africa

Date of Submission09-Feb-2021
Date of Decision27-May-2021
Date of Acceptance28-May-2021
Date of Web Publication30-Jun-2021

Correspondence Address:
Dr. Henry Sean Pretorius
Department of Surgical Sciences, Division of Orthopaedic Surgery, Faculty of Medicine and Health Sciences, Tygerberg Hospital, Stellenbosch University, Cape Town, 7505
South Africa
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jllr.jllr_2_21

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  Abstract 


Background: Orthopedic surgeons are invariably faced with situations where contemporary surgical techniques and implants are not satisfactory for a specific clinical scenario. One such scenario frequently arises around the lack of implant choice for intramedullary fixation of radius and ulna fractures. We will describe our experience designing and testing a new radius and ulna nail, inclusive of protecting the underlying intellectual property. Design Process: Phase 1: Identifying a need: Current intramedullary forearm devices have abandoned the ability to place interlocking screws at the nondriving end and are therefore not length stable. As current nails only come in 20 mm length increments, this may pose challenges in attaining accurate anatomical length restoration. Phase 2: Concept: (1) Anatomically designed in terms of length, diameter, and radius of curvature. (2) Nail inventory that has the optimal choice of implants to manage the majority of forearm injuries. (3) Locking system at the nondriving end that is easily targeted and has an acceptable radiation exposure for freehand locking. Phase 3: Anatomical study: Multiplanar reconstruction of upper limb computer tomography angiography scans were used to analyze the forearm osteology of 98 individuals. Primary measurements included the lengths of both radius and ulna shafts, the minimum canal diameter size, the proximal shaft angle of the ulna, and the radius of curvature of the radius. The size of the proximal ulna and distal radius were also measured for design parameters of the nondriving end of the nail. Phase 4A: Prototype design: To improve the usability of these nails, the design priorities were set as: (1) Locking hole design. (2) Jig and instrument design for insertion and removal. (3) Pressure release during insertion. Phase 4B: Prototype testing: Prototype testing consisted of nail insertion into human cadaver forearm bones using the initial prototype and instruments. The design aspects of the implant such as the locking holes with X-ray-assisted screw insertion or the radius of curvature were also evaluated. Mechanical testing will also be done.

Keywords: Comminuted forearm fracture, design, forearm nail, medical device conception, prototype


How to cite this article:
Pretorius HS, Ferreira N. Orthopedic implant design from concept to final tested product: A design surgeon's experience. J Limb Lengthen Reconstr 2021;7:45-51

How to cite this URL:
Pretorius HS, Ferreira N. Orthopedic implant design from concept to final tested product: A design surgeon's experience. J Limb Lengthen Reconstr [serial online] 2021 [cited 2021 Dec 8];7:45-51. Available from: https://www.jlimblengthrecon.org/text.asp?2021/7/1/45/320050




  Introduction Top


Orthopedic surgeons are invariably faced with situations where contemporary surgical techniques and implants are not satisfactory for a specific clinical scenario. One such scenario frequently arises around the lack of implant choice for intramedullary fixation of radius and ulna fractures. Comminuted forearm fractures, as seen following high energy injuries such as gunshots or motor vehicle accidents, pose specific treatment challenges. The ability to restore length and normal anatomy is paramount to obtain union and normal forearm function.[1],[2] Current treatment options include plates and screws, unlocked intramedullary devices, and external fixation, with plate fixation considered the gold standard for stabilizing radius and ulna fractures.[3]

Plate fixation of forearm fractures is, however, not always feasible. Segmental fractures or fractures with segmental comminution may pose a specific treatment challenge. Gunshot injuries to the radius may lead to extensive bony comminution and loss of radial length that is difficult to address with contemporary means.[4] Although a relatively straight bone, the ulna is particularly susceptible to injuries due to its subcutaneous location.[4] With the high incidence of interpersonal violence in South Africa,[5] ulna fractures resulting from defense injuries are frequently seen, and the accompanying soft tissues envelope compromise may pose concerns for plating. High energy trauma that causes segmental bony injuries combined with extensive soft-tissue injuries is particularly difficult to manage. Stabilizing these injuries with conventional plate and screw constructs requires additional iatrogenic soft-tissue dissection that may be deleterious.

To overcome these issues, locked intramedullary fixation of the radius was introduced in 2000, and the functional outcome of nail fixation has been comparable to that of plate fixation.[6] Intramedullary fixation with interlocking systems can maintain bone length while requiring less iatrogenic soft-tissue trauma during insertion. This concept has been previously marketed by Smith and Nephew Inc. (Memphis, TN, United States), with union rates reported to be equivalent to those of plates.[7],[8] The difficulty of locking and the learning curve associated with the procedure is shown with freehand locking times for the Foresight nail between 4 and 14 min, as reported by Weckbach.[6] The restoration of the radius bow was also not consistently restored as this required the surgeon to do this manually, with the only guide being the other arm.[9] This invariably led to anatomically straight radii and increased nonunion rates.[1],[10] The product was subsequently withdrawn from the market due to a new product line that was introduced, known as Trigen (Smith and Nephew Inc., Memphis, TN), the Trigen line did not include forearm nails, and the old nail was discontinued.

The current lack of length and rotationally stable nails for the forearm makes the effective treatment of complicated fractures a challenge. This paper aims to describe the design process of a novel anatomical interlocking intramedullary nailing system for forearm fractures up to the final prototype tested before clinical use.


  Design Process Top


Phase 1: Identifying a need

The ability to provide skeletal stabilization to comminuted forearm fractures that is length stable, recreate normal anatomy, and facilitate union without the need for additional iatrogenic soft-tissue damage can be considered the panacea for these difficult to manage injuries. In our busy trauma unit, we have seen an average of three forearm gunshots per month for the past 4 years.[11]

The ideal solution for this clinical problem was the use of length stable, anatomically contoured intramedullary devices that would allow percutaneous insertion, as is frequently used for femur, tibia, and humerus fractures. Previously, available devices such as the Foresight Nail (Smith and Nephew, Memphis)[12] were discontinued due to a change in their product line. Similar products do not provide locking options in the nondriving end of the nail, for example, Acumed Forearm Nail[13] (Acumed, Hillsboro, Oregon, USA). The difficulty with locking and iatrogenic radial nerve injury is often cited as a complication of locking nails. To provide length stability, these nails need to be placed onto the subchondral bone, and choosing the optimal nail length is of paramount importance. As current nails only come in 20 mm length increments, this may pose challenges in attaining accurate anatomical length restoration.

Current jig design (Acumed Forearm Nail Hillsboro, Oregon, USA)[13] for locking the driving end of the nail is poorly placed when locking in the radius, resulting in the arm having to be placed in an awkward position which can lead to malrotation of the fracture in some instances. This may also rotate the nail in the intramedullary canal, which can lead to a loss of the rotational stability of the nail.

The natural curve of the radius is termed the bow of the radius and refers to the external cortical morphology of the bone. In contrast, the radius of curvature refers to the internal morphology of the radius medullary canal. Current intramedullary fixation relies on cortical interference to stabilize the radius. When a small diameter nail is used, it may not exert enough force on the cortex to restore the curvature of the bone. This is true even if the nail has been bent to an adequate radius of curvature and can leave the surgeon with a radius that is virtually straight. If the nail is placed as previously described, it allows minimal rotational stability, although an interlocking design would achieve this in all cases.

Therefore, a complete redesign of forearm interlocking nails that addressed the shortcomings of current and previous nailing systems was required.

Phase 2: Concept

Having identified the need for a length stable intramedullary nail that will allow interlocking at both ends and restore the native forearm anatomy, the ideal product would have the following characteristics:

  1. Anatomically designed in terms of length, diameter, and radius of curvature
  2. Nail inventory that has the optimal choice of implants to manage the majority of forearm injuries
  3. Locking system at the nondriving end that is easily targeted and has an acceptable radiation exposure for freehand locking
  4. Locking of the driving end in a comfortable position that does not require the arm to be rotated for jig locking.


To better understand the anatomical requirements of the ideal forearm nailing system, a detailed anatomical study needed to be undertaken. This was shown to be useful in using computer tomography (CT) scans and shape modeling in product design by Kozic et al., Manić et al., and Swieszkowski et al.[14],[15],[16]

Phase 3: Anatomical study

Modern implant design is more frequently based on extensive anatomical studies to produce implants contoured and sized to fit the average person undergoing the procedure.[15] Anatomically contoured distal radius plates, for examples, were designed using CT scans of the wrist.[17] Anatomical studies describing the anatomy of the entire length of the radius and ulna and relationships between the bones are lacking.

Multi-planar reconstruction (MPR) of upper limb CT angiography scans were used to analyse the forearm osteology of 98 individuals and is currently and has been accepted for publication.[18] Scans of forearms with no bony abnormalities and performed for unrelated pathologies were assessed. All computed tomography scans were performed with a Siemens SOMATOM Emotion 6 with minimum slice thicknesses of 0.23 mm. The image files were stored as Digital Imaging and Communications in Medicine format (DICOM) files. All measurements were made using RadiAnt 4.2.1 (Medixant, Poland) DICOM viewing software. Primary measurements included the lengths of both radius and ulna shafts, the minimum canal diameter size, the proximal shaft angle of the ulna, and the radius of curvature of the radius. The size of the proximal ulna and distal radius were also measured for design parameters of the driving end of the nail. Multiple additional measurements were taken for statistical analysis of anatomical correlations. The size of the proximal ulna and distal radius were also measured for design parameters of the nondriving end of the nail.

Data were analyzed using STATISTICA (v13, TIBCO Software). Considering the anatomical nature of the measurements taken, all data were normally distributed as expected. Data are described as means ± standard deviations with 95% confidence intervals indicated in parentheses. Categorical data are described as frequencies with the count indicated in parentheses.

The 95% confidence interval of radius and ulna lengths was calculated from the collected data to establish the optimal nail length range. The minimum size of the intramedullary canal was considered during the design of the diameter of the nails. The radius of curvature of the radius and the proximal ulna was measured to bend the nail to an appropriate curve that would fit the internal geometry of these bones[18] [Table 1].
Table 1: Pertinent computerized tomography scan measurements with design-related implications

Click here to view


Once the size parameters for these nails were understood, attention was paid to specific design features to improve nail insertion and interlocking ergonomics.

Phase 4: Prototype design

To improve the usability of these nails, three design priorities were defined.

Locking hole design

The concept of having a wide access to an opening while still being able to have as small an opening as possible is illustrated in the Oillet design of archery holes in medieval castles. The angled walls, embrasures terminate in a small round opening that allows a wide angle of approach. This gave the maximum firing angle for the archer while giving the attacking force the smallest hole to shoot through. With the incorporation of this concept in nail interlocking hole design [Figure 1], the approach angle and surface of the interlocking hole are much larger, allowing a locking screw to be inserted at an angle of up to 35° greatly improving ease of drill and screw insertion.
Figure 1: Screw hole design showing the 35° approach angle

Click here to view


Jig and instrument design for insertion and removal

The main aspect of the jig design was to be a tapered block with flanges that fit into complementary slots on the driving end of the nail to control rotation. The block needed to be attached to the nail with a set screw. This would lock the nail to the jig and maintain stability in all directions. The block needed to be designed with a U-shaped radial projection that will allow a drill guide to lock the driving end of the nail in the metaphysis.

Pressure release during insertion

During insertion of an intramedullary implant, a buildup of medullary pressure may lead to iatrogenic fractures or propagation of comminuted areas of the fracture. As the nail is advanced in the medullary cavity, longitudinal vents in the nail will allow pressure to be released. This will result in a cone-shaped design on the nail cross-section.

The design material was chosen as titanium as this provided the closest modulus to normal bones and was selected for its strength and machinability. These concepts of mechano-regulation theory and the biomechanics of bone healing the choice of implant materials have been described well by Mehboob and Chang and Olson et al.[19],[20]

Two-dimensional nail design drawings were rendered based on the analyzed anatomical data [Figure 2]. Modern computer-aided design (CAD) (Solidworks-2018: Dassault Systèmes, Vélizy-Villacoublay, France) programs and additive manufacturing methods meant direct digital design, and manufacture of a sample was relatively easy and inexpensive.
Figure 2: Design drawings showing technical aspects of the nail

Click here to view


Once the final design was approved, a prototype device was manufactured, and all the entire instrumentation needed for insertion. This instrumentation was “unfinished” to keep manufacturing costs as low as possible, as changes to the instrument designs were anticipated.

Phase 5: Prototype testing

Prototype testing consisted of nail insertion into human cadaver forearm bones using the initial prototype and instruments. The initial cadaver study was used to test all aspects of the nail design and then to use that information to improve the implant, implant attachment to the jig, and the jig itself. The design aspects of the implant, such as the locking holes or the radius of curvature, also needed to be evaluated.

Having completed the first cadaver study, the prototypes were modified to incorporate the identified design improvements. Any failures and design flaws, including screw thread failure, nail failure, or jig malalignment, were identified and reported.

A second cadaver study represented the final testing of the implant through all aspects of insertion and removal. The cadaver studies in total comprised forty forearm bones, the reamer broke off in one canal, and one canal was too thin to ream. This made 38 successfully inserted nails available for locking. With two holes per nail, 76 locking attempts were made. Locking times averaged 44.5 s, with an average of 5.5 exposures per locking attempt. The maximum exposure time for locking was 13 s. Once the designer and engineers were satisfied with the outcomes of the prototype testing, a “Design Freeze” was initiated. This stopped any further changes from being made to the design so that it could undergo mechanical testing. If design changes were to be made after mechanical testing was conducted, the testing phase would have to be repeated.


  Mechanical Testing Top


The literature refers to these initial processes as knowledge acquisition and knowledge representation, with the testing phase referred to as finite element analysis (FEM).[21] Following design modifications, the nail will need to undergo mechanical testing (FEM) to establish if the product meets safety standards (ASTM F1264-16). With testing, the nail had a 4-point bending force yield mean of 566 N and stiffness mean of 67.1 N/mm. The torsional stiffness mean of 0.088 Nm/o. In the bending fatigue test, the results show a fatigue strength of 523 Nm with a 50% survival rate at 1 million cycles, the value increases to 90% survival with a fatigue strength of 440Nm. Saka et al. had similar results with mechanical testing of forearm nails.[22]

On completion of testing, the product must follow strict regulatory processes including clinical testing and ethical approval, before it can be registered for a CE mark which allows it to be sold in the European Union and other countries. These regulations can be accessed on the official EU website (https://ec.europa.eu/health/md_sector/overview_en). A similar registration with the United States Food and Drug Administration (FDA) will be needed before the product can be sold commercially.


  Intellectual Property Top


When embarking on a design project, maintaining confidentiality and protection of intellectual property (IP) is essential. If the design is disclosed in a public forum before IP protection is secured, for some forms of IP, the opportunity of protection will be lost. It is essential to conclude the appropriate agreements between relevant parties for the various phases of development, and eventually, commercialization is advised. Where a design and IP is created, questions surrounding IP ownership must be addressed to avoid future legal complications; this includes specifically copyright assignment to the designs, patent, and design protection.

IP developed at or designers affiliated with an academic institution are often subject to university IP Policies and national legislation that impacts ownership and commercialization. See, for example, the US Bayh-Dole Act.

These acts require that IP developed at the university or public funds belongs to the university, and its protection and commercialization are managed through the technology transfer office. The commercialization model is generally to file patent, trademark, or design applications and identify and negotiate with potential companies for royalties. Royalty fees will be net income as the institution may offset monies paid for legal costs and design registration and then distributed as per agreement. If not affiliated with an academic institution, if IP protection is to be obtained, all legal and patent registration costs are borne by the designer or his/her employer.

IP law is a specialist area, and patent attorneys are appointed to assist with the process. First, a prior art search is conducted on preexisting designs to ascertain the novelty of the invention or design. The designer is requested to define the appropriate keywords to assist with these searches and review the results to establish whether any similarities with the proposed new product exist. If similarities are found, a written rebuttal as to why the design should be considered unique (novel and inventive) is made to the patent lawyer. If the design is unique and no similar design is registered, a patent specification, including design drawings, are prepared and filed at the patent office that would examine the patent (or design in the case of a design application), and if all requirements are met, after a few years will grant the patent or design.


  Ethical Considerations Top


Before clinical use, a cadaver study needs to be performed for setting the surgical technique, the implant design, its use, and security. Not only it proves the implant can be used in Humans, but it secures the specific surgical technique and modalities of use. Then, a clinical trial on patients can be initiated once all approvals are secured according to regulations. Please note that, before both the cadaveric and the clinical trials, approval from the Ethics Committee is required. It is generally performed in the Institution where the development team works. Risk evaluation is a crucial element not only for the Ethics Committee but also the FDA, CE Notified Bodies and the implant company (Quality Management System). Postimplantation tracing is required for evaluating complications and risks linked to the implants.


  Discussion Top


This paper aims to describe the design process of a novel anatomical interlocking intramedullary nailing system for forearm fractures. Following a CT-based anthropomorphic study, CAD was used to manufacture a novel forearm nailing system. Proof of concept was attained following the successful insertion, interlocking, and removal of the manufactured nails in a cadaver model. This paper aims to describe the design process of this novel anatomical interlocking intramedullary nailing system for forearm fractures.

From a surgeon's point of view, the design process relates to the knowledge of the relevant anatomy and the implants that are available to the orthopedic community. With the continued use of available products, it often becomes evident that those implants do not always cater for injuries seen in practice. A compromise is often sought and frequently involves using available products in innovative, sometimes “off-label,” ways. When these scenarios occur with sufficient frequency, new solutions need to be found in the design of new implants or modifications of existing implants.

A specific aspect of this design process was to address the problems with existing interlocking forearm nails, which included interlocking that placed the radial nerve at iatrogenic risk and the shape mismatch between patient anatomy and available implants. With specific reference to radius nailing, the radial nerve is at risk with proximal locking, and the nail must either have modifications to prevent injury or remove this option as proposed by Köse et al.[23] Bansal avoided the problem by electing to omit proximal locking of the nail in the radial neck if the locking hole was more than 30 mm from the joint surface.[9],[23]

The inevitable problem that arises is the lack of rotational stability when proximal locking is omitted. An additional concern with contemporary implants is the loss of radius curvature after implant insertion. To prevent this, surgeons need to create an appropriate bend in the nail manually; this will be done with templates, so the surgeon can choose an appropriate radius of curvature that fits in with the patient's anatomy. Our philosophy was to address these shortcomings and add unique ergonomic features during the design of a new forearm intramedullary nailing system.

We experienced a lot of difficulties to succeed in our project. It took us 4 years from conception to the final design of the clinical medical device. Bureaucracy and legal implications can be daunting to a first-time designer, and it is essential to engage in a project with relevant professionals to limit time and financial wastage. The financial implications should not be underestimated. Some support can be provided by medical device companies, while the surgical team and university departments can often support the design and testing of the medical device. Some additional funding may be provided by academic or government institutions (loans for product development, design, manufacturing, and legal costs).

Designing a medical product may be frustrating, but with the correct motivation and perseverance can be extremely rewarding when the surgeon sees the results of his work in patients.


  Summary Top


The design of orthopedic implants requires a structured approach initiated with the identification of specific needs, setting technical requirements, and finding possible solutions. Best solutions are integrated into the design of the implant and prototypes in collaboration with a medical device company, which masters manufacturing and quality processes (ISO13485, CE and FDA certifications). The prototype evaluation includes testing (mechanical testing and cadaveric test), redesign, and repeat testing to improve any identified design faults.

Before a product is ready to use clinically, thorough validation is secured at each stage of the development (design, software mechanical testing, prototyping, cadaver study, and testing on modifications or improvements). Further steps that we are entering are required before branding including comparing our product to other previous ones to prove the underlying concept and the final device (required in 510[k] FDA and CE approvals), then clinical trial after Ethical Committee approval.

Thanks to a dedicated design pathway, ideas can be brought to fruition in the form of a novel orthopedic product despite the process being labor intensive and time consuming that can be protected by IP law and commercially exploited through licensing if interest exists in industry and a partner can be identified.

Designs are protected by Copyright© and pending European Community, USA, and South African design registration.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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