Question

Describe a biomedical application of poly-methyl-methacrylate (PMMA) or PMMA containing polymers, copolymers or blends.

Answer 1

SOLN:

Poly-methyl-methacrylate is commercially, the most important member of a range of acrylic polymers which may be considered structurally as derivatives of acrylic acid (II).

Properties: Poly-methyl-methacrylate and the related alkyl methacrylates have been the subject of extensive investigation.The glass transition process in PMMA in the atactic polymer occurs as a loss in the acoustic spectrum at 390 K. The ester group motion occurs at approximately 291 K and the relaxation processes associated with the methoxy and methyl group have been assigned respectively to transitions at 233 and 166 K.

Biomedical Application of PMMA:

Implant fixation : Poly-methyl-methacrylate (PMMA) is one of the most frequently injected biomaterials in orthopaedic surgery.Entrapped air, blood and debris in the interface, and poor penetration into cancellous interstices, all associated with the finger packing technique used initially to fill intramedullary canals, were addressed by injecting the cement into the canal using a device similar to a caulking gun. Pressurizing cement in the medullary canal achieves penetration into the interstices of the endosteal and metaphyseal cancellous bone. Current cement technique for intramedullary stems uses a cement gun to retrograde fill a plugged, clean intramedullary canal with cement prepared under vacuum to minimize air bubbles. The stem is then pushed into the cement-filled canal achieving a uniform mantel 2–4 mm thick. However, many contemporary stemmed implants are fixed biologically through bone apposition or bone in-growth not using cement. Secure non-cemented fixation requires correct fit and immediate stability. When initial stability is not possible, PMMA remains the best fixation option. Non-stemmed implants are cemented by pressurizing the PMMA into the broad surfaces of epiphyseal/ metaphyseal cancellous bone rather than injection into the intramedullary canal. Pressurization involves injection of the cement into the exposed cancellous bone surfaces with the nozzle of a cement gun and by capturing cement under the implant as it is impacted into place.

PMMA is also injected into vertebral bodies for fixation of fragility fractures of the spine, injected into screw holes to augment internal fixation in osteopenic bone, and injected into structural voids following resection of benign tumors to control dead space and support the surrounding bone. In many of these applications antimicrobial powder can be added to the PMMA for drug delivery when local delivery of antimicrobials is needed.

Other materials used to achieve implant fixation such as porous metal and calcium ceramics are commercially bonded to implant surfaces, not injected. With better understanding of bioactive peptides, the future may bring the use of signaling molecules to augment biologic implant fixation. Bioactive peptides could be commercially bonded to the substrate surface or injected in liquid or gel form at the time of implantation.