Does the Acetabular Fixation Surface Matter?
In many cases, the issue is with the surgical technique, not the acetabular fixation, says Dr. Rafael Sierra.
Biologic ingrowth acetabular fixation surfaces have been around since the 1970s, and have proven to be effective in supporting fixation of the acetabulum. The three leading coatings in use today are:
- Fiber metal mesh
- Sintered beads
- Porous plasma spray
Studies with follow-up of up to 15 years have shown no demonstrable difference in outcomes among these technologies.
At the recent ICJR East meeting in New York, Rafael J. Sierra, MD, discussed the importance of these acetabular fixation surfaces in total hip arthroplasty. He reviewed the basic requirements for bony ingrowth into the fixation surface:
- The component must be made from biocompatible materials such as cobalt chrome or titanium
- The surface of the implant must have an optimal pore size between 100 and 400µm. The component must be in intimate contact with viable host bone.
- The component must have adequate initial stability.
He believes currently available acetabular components fulfill the above requirements, and combined with improved locking mechanisms and the use of alternative bearing surfaces, have improved acetabular fixation and long-term outcomes.
When problems arise from acetabular fixation, Dr. Sierra believes issues with surgical technique, rather than component design, may be the cause. For example, achieving adequate initial stability is of utmost importance, as more than 40 µm of micromotion between the acetabular component and the host bone can lead to fibrous rather than bony ingrowth. Therefore, appropriate reaming technique and, if necessary, the addition of screws, pegs, or spike, are needed to ensure initial stability.
Dr. Sierra showed a video demonstrating his reaming technique and pointed out that he uses only one or two reamers, and reams only until he obtains bleeding bone, as he finds reaming through the subchondral bone is unnecessary.
It is also important to understand how the choice of cup design will influence whether to under-ream. When using a hemispherical cup, Dr. Sierra will under-ream by 1 mm; however, when using an elliptical cup, he reams “to size.”
Most implant manufacturers have also come out with improved cup materials, such as porous metals, tantalum, and porous titanium, all of which provide a 3D structure very similar to cancellous bone. These new materials have a 67% to 85% overall porosity – two or three times the porosity of conventional porous coatings – with uniform unrestricted interconnected pathways. These materials provide low stiffness and high coefficient of friction, allowing excellent biologic and mechanical characteristics, specifically for initial fixation into compromised bone.
Is this new and improved technology really necessary for routine primary THA? Follow-up outcome studies with traditional implants have shown survivorship of close to 100%. So for an uncomplicated primary hip replacement, the more traditional implants will probably do very well.
However for more difficult primary hip replacements with compromised bone stock, Dr. Sierra finds that these newer designs and materials are worth considering.
Dr. Sierra concluded that with regard to the acetabular components, the question of fixation surfaces is resolved, and that the surgeon should choose an implant that is familiar, as surgical technique is the most important factor as it pertains to initial stability and bony ingrowth. It is, however, worth keeping the newer highly porous designs in mind for more difficult cases, even though these designs are likely not necessary for routine primary THA.
Dr. Sierra’s presentation can be found here.