Mechanics of resorbable magnesium: Overview among implant materials

Mechanics of materials are characterized in certain parameters such as utimate tensile strength, elongation and more. This a very engineering focused way of describing features. In trauma care other features are highlighted:

• Bending resistance until breakage of the fracture plate,
• Easy and controllable insertion of screws,
• Torque resistance of the screw drive before the screw drive is rounded,
• Safe and stable bone fragment repostioning and many more.

As magnesium is a light metal and the use as biomaterial is relatively new, we will think through some implant design features and compare it to existing implant materials.

A lot of the material behaviour and the derived implant behaviour can be read in the Tension-Elongation Diagram. We have brought an actual exemplary test result from our lab:

The x-axis describes the elongation of the material, the resulting tensile strength is displayed on the y-axis. The tensile strength is displayed in measured Force divided by the cross section of the rod. The unit of the resulting tension is Newton (Force) by Section (mm²) or N/mm².

To visualize this: A force of 500N (Meaning the gravity force of appr. 50kg) applied on a rod with a diameter of 2mm would result in a tension of 500N/ (1mm²*π) = 166 N/mm²
In this example, a 2mm magnesium rod can withstand the gravity load of 100kg or 1000N before plastic deformation starts and it begins to deform. Up to this point it will always return into its original geometry (like a spring). Loads above this limit result in a plastic, permanent deformation.

Using fracture plates, it is of essential importance to allow small bending operations to precisely match the anatomical shape of the fractured bone. The design of the alloy behaviour must therefore leave room for a plastic deformation in a controlled manner, this can be found in the diagram in the plastic deformation after the elastic phase.

Prominent polymer materials such as PEEK display a value of 100 N/mm², translated to a 2mm rod allow a load of 30kg. Resorbable materials like PLLA or PLDLA normaly reach their limit at around 50 N/mm². Biocomposite materials, such as used by most sports medicine companies, even reach lower limits as the mixed ceramic material such as ß-TCP further weakens the structure.

In comparison to resorbable materials, modern surgical permanent titanium materials reach out to 800-1000 N/mm² with the known usual possibilities of bending manipulation., which allows a downsizing of trauma implants. In comparison cortical bone reaches up to 250 N/mm².

The mechanical resistance of magnesium is in the middle in between polymers and titanium materials. In comparison to polymers, which are favorable to break instantly (ductile behaviour) , magnesium implants allow anatomical bending for a safe configuration.

A trauma implant always has, in most cases, two or three, interfaces: The is the implant-Instrument interface, the so-called drive, an optional screw-plate interface and the screw bone interface.

The implant-instrument interface is incorporated in the screw head. The torque to protrude the screw into the bone is transmitted through a small surface. This results in a lot of load on this section. Polymer designs have trouble to transmit the required torque and establish a resulting compression force due to their low mechanical resistance. In comparison with titanium, magnesium implants require special attention concerning the drive design. The required insertion torques, around 1-2Nm for 2.7mm cortical bone screws, represent values closer to the mechanical limits.

As bioabsorbable magnesium screws are inserted into hard cortical bone, we only apply the more stable drive shape designs. Most of our products will feature a star shaped drive interface. In internal testing we have measured the maximum torques before screw-driver cams out and the drive is finally “stripped” or “rounded”, meaning a total failure of the interface.

In a total perspective, a mechanical chain has always one component which represents the weakest link. In screw design, this should always be the drive. The instrument (driver) shall not break, as it is expensive and possibly harmful to the user and the patient (because of possible sharp pieces). If the screw head shears off completely, a headless threaded pin is hardly removable. In comparison to titanium implants, in an “emergency” magnesium implants can be overdrilled using standard surgical drills.

As a result, the magnesium designs, require a larger drive (meaning transmitting surface) than comparable titanium implants to ensure the same usability. The maximum torque which can be applied by manual insertion is very much influenced by the handle of the screwdriver. After these lessons, we put a lot of stress on the right sizing for the screwdriver handle and have applied torque limiter whenever useful.

The world of resorption adds very interesting new aspects into the equation. We will look more closely into this in another blog post.

Author: KR

Contact: research@medical-magnesium.com