Orthopaedist finds implant stability, sizing and fixation still essential to TKA success

Registries show TKA results are gradually improving, but also report crude 9% revision rates.

Successful total knee arthroplasty outcomes greatly depend on using a proven surgical technique and selecting appropriately designed implants that avoid the most common failure mechanisms, according to a recent lecture on managing primary total knee arthroplasty through better implant selection.

Ignore the “glossary of promises” and claims of long-term primary total knee arthroplasty (TKA) survivorship that some implant manufacturers make in their product literature, Jan Victor, MD, of Brugge, Belgium, said.

“Look at form, fixation, bearing surface, stability and kinematics. Make up your mind and make a choice for the patient,” he said during his presentation at the 25th Annual Current Concepts in Joint Replacement Winter Meeting in Orlando, U.S.A.

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Implant factors

This is an example of significant frontal post wear and deformation caused by indirect impingement of the femoral component in extension. Images: Victor J

Among the implant factors Victor discussed were having the form and size match or fit that of the natural knee, firmly fixing femoral and tibial components, and selecting tibial trays that minimize polyethylene wear and production of osteolysis.

With a multitude of TKA implants on the market, “At the end of the day, it all comes down to function and durability,” he said.

Victor noted that the annual crude revision rates in national joint replacement registries are close to 9%, which is different from the positive spin given to TKA results by the implant companies. Yet, “We know from the national registries that we have been improving” our primary TKA results steadily from the early 1980s through the late 1990s.

Matter of perspective

Victor emphasized how important it is for physicians to have the right perspective when assessing performance data for TKA implants, from simulator study to kinematic testing results. He said they should ensure they are asking the “right” questions about implants that determine their true performance under variable conditions.

For starters, he suggested objectively assessing each TKA implant’s features as good or bad.

The form and size of a prosthesis matters most, according to Victor. “As TKA is a surface replacement within an existing soft tissue envelope, form is of utmost importance. In the natural knee, the femoral sulcus is lateral to the mid-plane and so it should be in the implants. Also, the form of the trochlea should match the patella.”

Choosing properly sized implants can help avoid femoral component overhang, notching and having to change the posterior condylar offset. He recommended that surgeons keep several femoral sizes in their armamentarium.

Smooth hard surfaces

Victor recommended selecting tibial trays for polyethylene inserts that mitigate problems of osteolysis with modular TKA prostheses. He also recommended having a tight locking mechanism and a polished base plate. He even advocated using monoblock components if necessary.

Cement is the best method of fixing TKA implants to bone, he said. “If you want to go cementless, you will have to live with the occasional early failure.”

Concerning femoral bearing surfaces, using hard- and smooth-surfaced components reduces scratching and some third-body wear. This approach can also protect the post in posterior-stabilized designs from wearing, he said.

“On stability and kinematics you need good sagittal stability. You need femoral rollback and a physiologic actual rotation,” Victor said.

For more information:

• Jan Victor, MD,can be reached at St. Lucas Hospital, Department of Orthopaedics, Beukenlaan 23, Brugge, 8310 Belgium; +32-50-37-44-23; e-mail: j.victor@skynet.be. He has intellectual property rights with Smith & Nephew Orthopaedics.

Reference:

• Victor JMK. Which implant do I pick? A glossary of promises. #85. Presented at the 25th Annual Current Concepts in Joint Replacement Winter Meeting. Dec. 10-13, 2008. Orlando, U.S.A.