Do high-flex knee prostheses improve range of movement? Is there increased risk?

Knee Prostheses
People use their knees to maneuver on a daily basis. When those people experience knee problems, they sometimes look to high knee prostheses to improve their range of movement. A new generation of knee prostheses has been introduced with the intention of improving post-operative knee flexion. The problem is that this new generation might not be effective or at least may not be as safe as older models. In fact, according to the Journal of Bone and Joint Surgery, trials were analyzed that suggested that no clinically relevant or statistically significant improvement was obtained in flexion with the ‘high-flex’ prostheses. Click here for the full report. Is it possible, however, that not only the range of movement fails to improve, but there are increased risks as well?

Risks
As mentioned above, there appears to be little or no new benefit in these new designs, but there does appear to be added risks or problems. You might be wondering how these products could get approved if they aren’t effective. The answer might stem from the fact that the new designs did not go through FDA approval process for a new device. Instead, they went through the much less rigorous 510(k) review process. They were able to go that route because the product was determined to be a modification to an existing approved design. To make matters worse, there is a history of products that have gone the 510(k) route without actually being mere minor modifications to an existing design. In certain products, these changes have resulted in the products being less safe than the modified design.

People’s Responses
Those people who have turned to the new products are not happy and are voicing their complaints on a website.

One person said, “High Flexion TKR is a myth.”

The site administrator stated, “The RPF(J and J) designers suffered the greatest blow with Dr Han’s paper in JBJS b, reporting early femoral component loosening with Nex gen LPS FLex(Zimmer). They reported early femoral component loosening in 38% of knees. J Bone Joint Surg Br. 2007 Nov;89(11):1457-61. High incidence of loosening of the femoral component in legacy posterior stabilised-flex total knee replacement.

The administrator also referred to a paper which listed four basic problems that are far from being solved. Those problems include:

1) Is high flexion really necessary?
2) Is high flexion directly correlated to functional outcome?
3) Are the posterior condylar and extra cuts essential, to sacrifice so much extra bone to achieve higher flexion, or would the new high flex inserts eventually pave the way?
4) There is eventually a role of many confounding factors like preoperative range of motion and patient profiles which may affect eventual range achieved?

Conclusion
Ultimately, the patient’s prior condition, not the device, is determinative with respect to resulting flexibility. The principal predictive factor of postoperative range of flexion, regardless of the degree of soft tissue release or implant design, is the preoperative and intraoperative range of flexion. That begs the question, is the knee prostheses really worth the risk? Each person needs to evaluate his situation individually. When making a decision, however, it is critical to realize that you may not receive any benefit. Worse yet, you could experience additional risks or problems.

Product Identification Notice: In any of our web pages referring to Zimmer NexGen products, these pages are intended to provide information about and are referring to the same Zimmer products that were identified by Dr. Berger in his presentation and study entitled "The High Failure Rate of a High-Flex Total Knee Arthroplasty Design," and were written about in the NY Times article titled "Surgeon vs. Knee Maker: Who’s Rejecting Whom?," and that are reported in the FDA MAUDE database, where reports involving the Zimmer NexGen CR Flex Cementless or Porous Femoral component can be found and also those products associated with aseptic loosening of the Zimmer MIS Tibial Component that led certain doctors to discontinue its use, and to recommend discontinued use of this product in primary TKA until the etiology of the high early failure rate of this implant is determined. See prospective studies comparing the results of the Zimmer MIS Tibial Component using conventional and minimally invasive exposures as well as comparing fixation with different types of bone cement as described in Early Aseptic Loosening With a Precoated Low-Profile Tibial Component A Case Series. Not all Zimmer NexGen products are involved in our investigations and lawsuits.