Optimized cervical plate design Based on biodegradable natural polymer material Project

Team 12 Andrew Carney, Casey McDermott, Kyle Ward

Project 23 For

Dr. Krystyna Gielo-Perczak Dr. Sangamesh Kumbar

Dr. Cato Laurencin Ms. Gloria Kolbe

Sangamesh G. Kumbar Ph.D. University of Connecticut Health Center

263 Farmington Avenue Farmington, CT 06030-3711

Phone: 860-679-3955

Executive Summary The proposed device is a cervical plate that is inserted during surgery on the spine and is used to hold a graft in place. Along with a set of screws, the system will work to hold the vertebrae in place while fusion occurs. The cervical plate, and the screws, will be made of a biodegradable polymer that will break down and dissolve over time. In doing this, the device will not only help to heal the degenerated disks in a patient’s spine, but will also negate the need for a second surgery to remove the plate and screws. The polymer being used degrades into biocompatible products so that the patient’s body does not react negatively to the degradation of the plate and screws in any way. The device will be designed using ANSYS, a CAD program. In doing this, the device will be able to be designed to match or exceed the mechanical strength of competing devices, which is critical as a main problem with these cervical plates is being able to bear the loads that the body places on the device. This makes the device quite unique, as all other cervical plates with this mechanical strength either must be removed or degrade into harmful products. Also, by almost exclusively modeling the device using software, the device becomes extremely reasonably priced to fund.

1.0 Introduction

1.1 Background

Dr. Sangamesh Kumbar works for the University of Connecticut Medical Center. A part of his research focuses around surgery, specifically to repair vertebrae in the spine. Degradation of the vertebrae in the spine is inevitable, as it will automatically occur over the course of a person’s life. Degenerative disc disease, or DDD, is a disorder that can cause severe and chronic back pain for many people at some point in their lives. While all people will undergo some form of disc degeneration, not all people have symptoms. The most common treatment for degenerative disc disease is surgery, where a graft is held in place by a plate and screws, while fusion of the vertebrae is allowed to occur. Dr. Kumbar’s research has involved finding and using a new material to create the cervical plate and screws with, as the current materials being used are inadequate. The research has led to the use of a biodegradable polymer, which combines cellulose and hydroxyapatite. The mixture consists of twenty percent hydroxyapatite in weight, with the rest of the eighty percent being made up of cellulose. This combination was chosen

because the research showed that this caused the strongest mechanical properties possible for the polymer. This type of material was researched with the intention of creating a plate and set of screws that would degrade away over time so that a second surgery would not be necessary to remove the system from the patient. This specific biodegradable polymer was chosen because it did not degrade into products that would harm a person after they had broken down. This research is intended to culminate into not only a device that can treat degenerative disc disease, but also a device that will save a vast amount of time and money because the cervical plate will safely degrade and not require a second surgery for the patient.

1.2 Purpose

The device will be a polymeric plate that will hold the loads placed on the vertebrae in the spine until fusion occurs, when the cervical plate will then degrade away into the body safely. Such a device will be very useful to surgeons, as this material contains many benefits that are not available in competing solutions. This device could also become valuable for future projects, involving more than just degenerative disc disease. Because plates and screws are used in many different surgeries, along with the fact that they are made using this new material, it is very viable that they could be used for other surgeries in the future as well.

1.3 Previous Work Done by Others

Anterior cervical plates have long been in use to stabilize the spine of patients who have undergone single and multi-level cervical surgery. Products since have been designed and patented to improve on previous designs, based most importantly on size and biocompatibility.

1.3.1 Products Zephir anterior cervical plate system Zephir cervical plate system was one of the earlier designs patented with variable screw trajectory and orientation. The design was also impressive for its time due to the size which measured only 1.6mm thickness and 15mm wide while still maintain necessary mechanical properties. This is important because many patients often complain about the implant causing discomfort in the throat as well as difficulty swallowing. Another important aspect of the design is a locking system for the screws which keeps them from backing out and therefore keeping the plate in place.

Inion Cervical Spine system: Inion Cervical spine system is a modern novel system that uses a degradable polymer to form the cervical plate and screws. As opposed to the normal titanium based designs, the polymer will degrade through the body once the bone graft has fused to support the spine. Inions current polymer design metabolizes into carbon dioxide and water once hydrolyzed.

1.3.2 Patent Search Results

Search results for anterior cervical plate system result in hundreds of results dating back a couple decades.

Patent: D603962 One such patent result gives us a two-level plate filed in 2009. This design claims an original design for a titanium plate more importantly specifying an original locking system.

Patent: 0195085 Another patent filled in 205 claims various intricacies regarding the orientation and trajectory of the screw holes. These details also claim the system with a biodegradable polymer.

2.0 Project Description 2.1 Objective

The objective of this project is to design a cervical plate that can withstand the body’s environment, as well as to hold up to the loads and forces that the body will place on the plate while it is holding the graft in place on the spine. To do this, the cervical plate must be quite strong mechanically. Different geometry and designs will be analyzed in order to maximize the mechanical strength of the device. Other than withstanding the forces the body places on the cervical plate, the main purpose of the plate is to hold the vertebrae and the graft in place while fusion occurs. While this seems simple, because the fusion takes up to a year to complete, there is a lot of time for things to fall out of place, making it crucial that the plate and screws stay in place and intact during this time period. To do this, a two level fusion system will be used. The cervical plate spans three different vertebrae with two grafts. The plate also has six holes to allow for screws to hold the vertebrae in place. The screws must also be kept from backing out of the vertebrae. To do this, a locking system will be implemented to keep this from

happening. The locking system will secure the screws and not allow them to move backwards out of the vertebrae. Once fusion has completed, the plate and screws must then degrade safely into the body. When dealing with the body’s environment, many factors come into play to ensure that the device will be feasible. Firstly, the device must not harm the patient at all while it is inside the body. The material itself is safe and will not cause injury to the body, but it also must be placed so that while it stays in place, it does not cause harm or discomfort to the patient. To do this, geometric constraints have been placed on the design of the device so that no discomfort or unintended contact with body parts will occur. The cervical plate must also stay intact during the fusion process, before safely degrading into the body. The biodegradable polymer being used has only biocompatible products, so degradation is not an issue. Because the polymer degrades over time though, it is imperative to the objective of the device that it does not degrade until after the fusion has been completed. The device not undergoing bulk degradation, which can cause the material to degrade as a whole and break before the vertebrae are fully fused, achieves this.

2.2 Methods

The Material: The most important aspect of the project is the material which with the cervical plate will be made of. Our group will be using the newly developed cellulose acetate. Table 1 shows a list of all the known physical, mechanical, electrical, thermal, optical and processing properties for cellulose acetate.

Table 1: Shows all the calculated value properties for cellulose acetate.

Our job is to design a structure which will meet the strength requirements of other plates currently on the market. Table 1 shows that the material has a base ultimate tensile strength of 26-52 MPa. Looking at tables 2 and 3, which shows the mechanical properties of two plates currently on the market, our plate must reach a minimum ultimate tensile strength of 45 MPa. However, even this number is misleading. The plate that currently is made out of PLLA/PDLA is rarely used and often sighted for not being strong enough to support the patient’s cervical vertebra. In reality, our plate must ideally have an ultimate tensile strength of at least 100 MPA. This category is only one example. Current ultimate tensile yield and elastic modulus must also be met by the design.

Ti-6Al-4V

Ultimate Tensile Strength 950 MPa 183k psi Ultimate Tensile Yield 880 MPa 128k psi Elastic Modulus 114 GPa 16510k psi Table 2: Shows the mechanical values for Biomet’s Ma-An plate (titanium).

PLLA/PDLA Ultimate Tensile Strength 45 MPa 6530 psi Ultimate Yield Strength 5% 5% Elastic Modulus 3 GPa 435k psi Table 3: Shows the mechanical values for Inion’s Plate which is biodegradable (polymer).

Matching the mechanical properties of current cervical plates must be done without exceeding the size restrictions of the plate. This is one of the most important aspects of the design. Many patients who have used plates that stray outside the norm of designs have experienced constant discomfort.

The Design:

The plate must function as a two level cervical spinal fusion plate, must be less than 18mm wide, must be less than 2.7mm thick and must fit the patient’s own spine distance (the average for a two level fusion being 35mm tall).

Figure 1: Shows an x-ray image of an implanted two level cervical spine plate.

In addition to the above parameters, the plate must follow the natural curve of the spine to and sit flush once implanted. It must have a 5° DGA (degree of alignment) in order to fit on the normal human spine. As with all cervical plates, one of the most important elements of the design are the screws which hold the plate in place. With respect to the design and mechanical properties of the plate, where those holes are placed can have a great effect on the overall performance.

Figure 2: A detailed design layout for New Ortho Polymers Cervical Plate.

Figure 2 provides the basic starting block for our design giving the

project as solid and well accepted foundation upon which to build. This design contains many of the widespread used characteristics for the market’s cervical plates. Modeling: Majority of the design and modeling work will be done in the finite element analysis program Ansys Workbench. Ansys provides a powerful engineering design engine which will allow for not only designing of the plate but also for customizing the specific parameters of cellulose acetate and virtual stress/strain testing to determine if our design will meet with the required mechanical properties.

3.0 Budget

Because the implanted device only consists of the cervical plate, six screws, and the locking system for the screws to stay in place, the cost of the device is very minimal. There are no electrical components that are necessary, nor are there any motors or mechanical items needed for the device. Most of the production of the device will be simulated on the CAD program ANSYS so that numerous prototypes and excess material will not be necessary, allowing for a more cost efficient design. The ANSYS program is also available to the design team free of charge. In creating the cervical plate, the only expenditure to consider is the cost of the raw material. The plate will be designed through the CAD program and the design team will use the machine shop to create the part themselves, free of charge. The same method would be applied to the screws. In regards to the locking system, an exact cost cannot be determined yet, as the material being used for the locking system has not been chosen. The locking system could be designed using something as simple as a polymeric clip, or it could use a metal such as titanium instead. Either way, the price will again be reduced to only the cost of the raw material because the design team will machine the part. Due to the fact that our material is not currently available for sale on the open market, we are unable to estimate a proper budget at this time. A budget will be fully accounted for once the true value of our material and its shaping cost is known.

4.0 Conclusion

Our product will use a natural based polymer plate to be used in two-level anterior cervical surgery. Our polymer will degrade safely while still providing the mechanical strength necessary for fusion. A locking system will be designed to prevent the screws from backing out of the vertebrae. The criteria put forward by FDA and ASTM will be met through FEA testing then through mechanical testing. The budget of our product will be optimized as we use Ansys to be sure our product will meet our needs. Additionally we will use the most efficient method of product creation utilizing injection molding.