Poly(methyl methacrylate): a biocompatible acrylic

Sean Newham

Sean Newham finished his BA in Natural Sciences at Cambridge University (specializing in Material Sciences and Metallurgy) in 2010.He now works as a software developer at Granta Design, working within the team responsible for Granta's data products.

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medicalDiscovered in 1877 and patented in 1933, PMMA, or acrylic, is often used as a lighter, more shatter-resistant alternative to glass. It is easy to process and make, resulting in a low cost versatile material used for everything from windows in aquariums, to protecting the audience from stray pucks in ice hockey rinks, and even in shoes.

What is interesting about PMMA, though, is its biocompatibility. Despite being formed by polymerizing Methyl Methacrylate, an irritant, and possibly a carcinogen, PMMA is extremely biocompatible, resistant to long exposure to temperatures, chemistry and cell action of human tissue.

In orthopaedic surgery, PMMA is known as bone cement, and is used to fill the space between implant and bone. It is used for this because, in addition to its biocompatibility, PMMA is easy to make and manipulate in a hospital environment.

In an operation, bone cement is made from a powder and a liquid. The powder is pre-polymerized PMMA or MMA co-polymer, and the liquid is made from MMA monomer, an accelerator, and an inhibitor. This forms a putty-like dough, which can be applied between the metal or plastic device and the bone, as you would apply grout between tiles, to ensure anchorage of the material. The dough is not adhesive and does it form a bond with either material.

However, there are a number of reasons why PMMA is not ideal for this job. One is its Fracture Toughness (Ref. 1) (0.7-1.6 MPa.m0.5) compared to bone (3.5 to 6.6 MPa.m0.5), which can cause breaks in this material at stresses where bone, and indeed the implant, would be fine.  Another serious problem with this process is the high temperature during polymerization. Recent research has shown (Ref 2) that the dough can reach temperatures above 80°C outside the body and around 50°C inside the body, damaging the bone. This translates to a longer recovery time for patients. Furthermore, small amounts of un-polymerized MMA can remain in the cement, which can find its way into the body and may be the cause of hypotension in such operations.

But if PMMA is so bad, why are we using it as bone cement? The short answer is we have nothing that is as easy to make in-situ with the same biological and mechanical properties. Research is currently taking place all over the world, looking for a slightly better material for this purpose.

PMMA is also used to make dentures and fillings. Because it is easy to dye and quick to form it is possible to make dentures and fillings that very closely match the size and color of the original teeth. Aside from the mouth and all over the skeletal system, PMMA has also made it into the eye, historically as a hard contact lens, and more recently as an intraocular lens. The use of which started through a surprising discovery.

An ophthalmologist working with Royal Air Force patients, Sir Harold Ridley, noticed that splinters of PMMA (resulting from shattering airplane wind shields) in the eyes of injured pilots did not trigger the same rejection as glass splinters. This led him to try using this material to replace the damaged lens in the eye in cataract. He achieved permanent implantation in 1950 at St. Thomas Hospital (Ref 3).

A number of PMMA-based copolymers and silicone based materials now compete with PMMA in the intraocular lens market due to the advantage of being flexible and therefore less brittle. In the case of contact lenses, soft lenses can be more comfortable to apply and wear, with recent advancements in oxygen transmissibility greatly improving the lives of people who wear them.

Clearly, this acrylic has many uses in medical treatment. Although it becomes less common to use it alone, advances in materials using similar materials will continue to improve the lives of many.

References:

  1. Fracture Toughness taken from the Medical Materials Database
  2. THERMAL NECROSIS AND PMMA – A CAUSE FOR CONCERN? S. McMahon et alm J Bone Joint Surg Br 2012 vol. 94-B no. SUPP XXIII 64
  3. Ridley H. Intra-ocular acrylic lenses. Trans Ophthalmol Soc UK. 1951;71:617–621

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