Histopathological Evaluation of Orthopedic Medical Devices: Definitions and Types of Orthopedic Materials 

Orthopedic biomaterials materials play an essential role in the fixation of fractures. The materials are continuously evolving with new ones coming into the market regularly. All Orthopedic surgeons require a broad range of knowledge of these materials to stay abreast of this dynamic field.

The choice of a material depends on its qualities, which include corrosion resistance, rigidity, tissue receptivity, and biocompatibility.  The choice also depends on the surface structure of the material because this affects its stability within the surrounding cement mantle or the skeleton.

Definitions
Osteoinduction: This is the process of recruiting immature cells, then encouraging them develop into preosteoblasts. It is usually crucial for the bone-healing process.

Osseointegration: This is the functional and structural connection between the surface of a load-bearing implant and the living bone. It also refers to the formation of bone directly opposed to the surface of an artificial implant.

What is the ideal orthopedic implant material?
As many types of implant materials are available, the choice of material to use in any particular situation is typically determined by the orthopedic surgeon. Bone graft substitutes are either osteoinductive or osteoconductive.

As profiled in an article by Nicolette Jackson and others, synthetic graft substitutes are typically osteoconductive when used alone, but may be osteoinductive when used together with other materials (i.e. cellular factors).

The ideal implant material should have the following characteristics:

• Biocompatible with the surrounding tissue
• Chemically inert
• High fatigue resistance
• Great strength
• Corrosion-Proof
• Low elastic modulus
• Good wear resistance

Though it is difficult to find a material that has all of the above properties, the search remains for biomaterials that exhibit as many of these qualities as possible.

Manufactured orthopedic implant materials
Currently, the manufacturers of orthopedic implant materials invest heavily in R&D to explore new materials and improve the existing ones. The research is ongoing and continuously evolving.

The materials used in prosthetic devices mostly fall into three main categories, which are:

• Polymers
• Metals
• Ceramics

Polymers
Over the past few years, the use of biodegradable polymers as orthopedic implant materials has exponentially increased. These may be useful for the fabrication of fracture fixation plates, rods, staples, screws, sutures, clips, suture anchors, and more.

The biodegradable polymers slowly removed from the body, thus eliminating the need for a second surgery for their removal.  Several polymers that are widely used in orthopedics include high-density polyethylene (HDPE) or ultra-high-molecular-weight polyethylene (UHMWP).

Metals
Metals function well as orthopedic implant materials due to their inherent strength. They have shape memory and are resistant to degradation from fatigue.  The metals used in implants include titanium alloys, pure commercial titanium (Ti), cobalt-chromium (Co-Cr) alloys, or stainless steel (surgical grade).  Stainless steel is liable to plastic deformation and has poor fatigue strength, thus, it is mainly used for temporary implants.

Ceramics
In the past three decades, the interest in ceramics as orthopedic implant materials has dramatically increased. The ceramics used in implants include calcium phosphates, aluminum oxide (Alumina), glass-ceramics, magnesia, and partially stabilized zirconia.  Though these materials are weak under shear and tension, they are very resistant to compression. Another disadvantage of ceramics is that they are brittle.  Calcium phosphates are very popular because of their low reactivity and high biocompatibility.

Bringing a New Medical Device to Market
If you are considering testing a new orthopedic device, StageBio offers the expertise and capabilities to perform histopathology evaluation for medical devices.

We have multiple histopathology labs with more than 20 board-certified pathologists. We have fully equipped laboratories in OH, VA, MA, and MD.

In addition to general toxicology work, we also provide neuropathology, Immunohistochemistry (IHC), and archiving services. Please contact us for more information.

References:
Histopathological Evaluation of Orthopedic Medical Devices: The State-of-the-art
in Animal Models, Imaging, and Histomorphometry Techniques

Nicolette Jackson, Michel Assad, Derick Vollmer, James Stanley, and Madeleine Chagnon