Bone cement – saving lives since the 1940s (Part 1)

It is commonly known that many people typically outlive their joints (namely knees, hips, shoulders and elbows) and therefore need replacements. Damage to joints, especially those in the legs, causes a dramatic loss in quality of life.

Prosthetic joints are successful at alleviating most, if not all of the symptoms associated with injured joints, and by increasing mobility can generally increase both physical and mental health. Chances are, you know somebody who has benefitted from an arthroplasty – there were over 180,000 such operations in England, Wales and Northern Ireland in 2012. While arthroplasties restore quality of life, the operation is fairly invasive (see the images below!)

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(Image on left: a knee arthoplasty; image on the right: an x-ray of a prosthetic knee joint)

As you can see from these images, it is necessary to cut deep into soft tissue to expose the bone. Then in order to fit the prosthetic, the bone has to be cut through, exposing bone marrow. Because of all this exposure, the risk of getting a surgical site infection (SSI) should be high.  But the risk of infection is actually currently low. For revision surgeries, which account for 10% of the total UK cases (secondary, tertiary surgeries etc. after the initial joint replacement operation), only 12% of hip, and 22% of knee revisions are due to an infection.

Orthopaedic surgeons have been combatting infections in advance (medically referred to as prophylactically) for decades. A simple way to do this is by giving patients antibiotics that target the most common causes of surgical site infections, in advance of their surgery. Out of interest, the most common cause of SSIs is Staphylococci species (think MRSA), followed by Enterobacteriaceae (think E. coli). However the issue with just giving patients antibiotics, usually orally or intravenously, is that the antibiotics are then present throughout the body (systemically). While this does mean that the antibiotics can combat infections throughout the body, the dose is reduced, and so is less effective. It is less targeted. Additionally, systemic antibiotics can be harmful. For example, the commonly prescribed antibiotic gentamicin can cause serious damage to the kidneys, and can also cause deafness – in high enough doses.

A preferred method of giving prophylactic antibiotics is to deliver them during surgery, directly to the surgical site, which has two major benefits. The first being an increased concentration of antibiotics (compared to systemic antibiotics) at the surgical site, which is better for preventing and tackling infections. The second benefit is that the drugs will take time to enter the bloodstream, and even when they do, will be in very small amounts, so the risk of toxicity to the patient is reduced. One option would be to apply the antibiotic directly into the surgical site, but this lacks finesse. All of the antibiotics would be present immediately – which would be good at initially fighting an infection, but would be lacking in longer term protection. Plus they would be less fixed, and prone to shifting due to gravity and movement. For context, intravenous antibiotics can be given post-surgery for weeks if not months in some cases. To enable a controlled release over time, incorporating the antibiotic into a material is the answer.

Packing materials are used to fill deep wounds, and aid recovery by draining away excess fluid from a wound. Due to their current use and proximity to the surgical site, packing materials are a good candidate to be loaded with antibiotics. However: not every patient will require packing materials, they are sometimes only used to help treat, not prevent an infection; and packing materials have to be removed – essentially re-exposing the soft tissue, which would not be ideal for somebody making a good recovery. If packing materials are to be used, then they are a good candidate for antibiotic loading, but they are not suitable for every patient.

Part 2, where I explain the importance of bone cement, will follow shortly!

Thanks for reading,

Microbe Stew

All orthopaedic statistics are taken from the National Joint Registry 2013 report

Image of knee arthroplasty

Image of knee x-ray