Friday, May 3, 2013

How are Hip Implants attached to Bone? Do you use Glue?

The bonding of hip joint replacement parts to bone is very important to the success of the surgery. Patients are often interested in how this is accomplished. This topic has been highly researched in orthopedic journals for many years. Historically, bone cement (poly methyl methacrylate) was used to glue the parts of hip implants to bone.
Methyl-methacrylate By Jynto and Ben Mills [Public domain], via Wikimedia Commons

In general, bone cement is very successful when used as a bonding agent. It needs to be properly prepared and pressurized into the bone and it works well. More recently, researchers have concluded that bone cement can break down over time in active patients. At the same time, orthopedic engineers have designed implants that have a roughened surface that bonds directly to bone without the use of cement. As a result, the orthopedic community has gradually shifted to the use of cement-less,  porous coated implants for hip replacement.
Both the cup and the stem are usually implanted in what we call a "press-fit" manner. That means that the prosthesis is just a little bit larger than the opening in the bone. When the implants are impacted into position, they jam into place and have enough roughness and surface friction that they stay in place and are immediately stable. Usually a patient can put their full weight on the hip immediately after surgery without concern for the implants coming loose. Here is a hip replacement x-ray.

X-ray showing a total hip implant placed without cement or screws. Note that the metallic prosthesis is in direct contact with the bone.
Next I will cover the use of bone cement for knee replacement.

Wednesday, May 1, 2013

What is a Hip Replacement implant made of? Titanium?

Patients are often curious as to what hip replacement implants are made of. This is an important topic since there are certain materials that have proven to result in long-lasting, well functioning hip replacements. Patients often guess that the prosthesis is made of titanium and they are partially correct. Modern hip replacements generally do have at least some titanium parts. Hip replacements are usually made up of four component parts, each of which can be made of a different material.
1. Cup or Acetabular component - usually made of Titanium or Tantalum both of which are porous and integrate well with bone
2. Liner - Polyethylene which is very resistant to wear
3. Head or Ball - Ceramic heads are the best for longevity, Cobalt Chrome can also be used
4. Stem or Femoral prosthesis - Titanium or other porous or roughened metal that allows bone to attach

Here is a diagram:

Next we will discuss the use of bone cement in joint replacement surgery.

What material is a Knee Replacement made of? Titanium?

Another question that I get asked all the time has to do with what the knee replacement implants are made of. Patients often guess that it is made of titanium.
By Pumbaa (original work by Greg Robson) (Application: (generated by script)) [CC-BY-SA-2.0-uk (], via Wikimedia Commons

The answer is yes, some parts of knee replacements are made of titanium, however, it is more complicated than that.
A knee replacement usually contains four separate parts that are combined. From top to bottom the components are:
1. Femoral component - usually made of Cobalt Chrome
2. Patella component - usually made of Polyethylene
3. Tibial insert - Polyethylene
4. Tibial tray - Titanium or Cobalt Chrome

Here is an illustration:

The reason for the different materials is simple. Metal can not rub on metal so there needs to be polyethylene wherever motion is occurring  The patella rubs on the metal femoral prosthesis, so it is made of polyethylene. The Femoral prosthesis rotates on the tibial insert so the insert is also made of polyethylene. The parts that attach to bone are metal. The reason why the femoral prosthesis is made of cobalt chrome is because it is hard and smooth and therefore causes a low rate of wear. The tibial tray component can be made of either titanium or cobalt chrome since it is not part of the joint, there is no movement involving the tibial tray, it supports the liner and sits on the bone. Titanium is often preferred for metals that come in contact with bone because it is less stiff and more bio-compatible.
I will review the components of a hip replacement implant next.