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Cementless fixation for TKA: It is as good as gold
As always, patient selection is key to success.
I agree that cemented fixation is certainly the gold standard. In fact, just last year, we reported our 5- and 10-year results on our own cemented knees, and we had 100% survival of our cemented components with patients at an average of 75 years old. But, the average age of our cementless patients is about 55 years, so it’s a different age population.
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If you’re going to use cementless fixation, you have to know it is at least as good as cement and hopefully, in the long-term, it will be better.
There was a lot bad information about cementless fixation for total knee replacement that came out early on after trials in animal models. You can put just about anything into a dog model that has any kind of roughness or any kind of beaded surface, and within 6 weeks you will see great attachment.
But, that was somewhat deceiving 25 years ago because in humans, it wasn’t as good as we thought it would be. We started to see implants coming back with fibrous tissue attachment, and that discouraged a lot of people. Some of the early results of cementless fixation had to do with poor fixation and poor porous coatings.
360° change
There are many new types of porous coatings available today. I think that we are making a 180° turnaround: Cementless fixation in the knee is making its way back. Orthopedic researchers and industry leaders have performed a lot of basic science work to determine which metals work better. We now know that titanium works better than cobalt chrome. You can get good spot-welding with cobalt chrome, as we proved in a bilateral study.
Part of the problem with cementless fixation is what we’re trying to fix to: 76% of the proximal tibia is marrow and space, and only 24% is actual bone. It is very porous and you have to fill in the gaps with something. You can either use acrylic cement or a slurry of cancellous bone, which is what we have done for 20 years.
I think our cementless fixations have been successful because we place the biologic cement at the interface. We slide it around, filling in the gaps, not making a layer in there but simply filling in the gaps between the trabeculae much like acrylic cement would do.
We’ve published our results from another plug study where we put slurry in one side and nothing into the other. There was 65% more activity at the interface with almost as much increased ingrowth at the end of 6 weeks in the slurry interface. Therefore, the biologic activity of the slurry of bone is there — it’s free bone morphogenetic protein (BMP) — and it makes a huge difference.
Better kinematics
We have also learned to do better kinematics on the knee. We demonstrated a 40% improvement in the weight-bearing capacity at the proximal tibia. We simply match what the patients have for slope. That’s an easy thing to do with the variety of instruments we have now.
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Doing a cementless operation is different — there are different techniques. You should not pulse-lavage all the marrow elements out because that’s where the BMP is. You should not “cook” the bone when you are sawing through it; thermal necrosis occurs at 55°. Normal sawing without irrigation will cause thermal necrosis.
We have many clinical studies that show it doesn’t make a difference if the patients are old or young. We’ve looked at the survivorship of metal-backed patellas and actually had one of the highest survival rates reported: 96% survival at 10 years.
Even with metal-backed patellas, which almost no one uses anymore, the interface remained solid, although some did wear through on the edge.
We used to do 89% of our knees in a cementless fashion; now we do a little bit less. Our clinical study looking at the survival rate 10 to 14 years postoperatively, published in Clinical Orthopaedics and Related Research, showed that 98% of the femurs and tibias were still fixed with stable interfaces.
We published lots of postmortem retrievals out to 16 years showing that once you get ingrowth into the tibial component, it maintains itself.
In conclusion, you can get excellent clinical results with the cementless device, but it’s not for every patient.
It’s certainly not for the 75-year-old patient who has osteoporotic bone, but for our younger, active patients, it’s certainly equal to cemented results, and I think that it’s going to be better over the long term.
We have not had any loosening over time once the implant attaches, and I think cementless knees are on their way back.
For more information:
- Aaron A. Hofmann, MD, can be reached at the Department of Orthopedics, 50 N. Medical Drive, Room 3-B165. Salt Lake City, UT 84132-0001; 801-587-5400; fax: 801-587-5411; e-mail: aaron.hofmann@hsc.utah.edu.