Do High-Flex Total Knee Replacements Function Better? Are they Safe?

Many patients who are contemplating a total knee replacement operation have questions about whether their range of movement will be improved and if they will be able to resume their active lifestyles. In response to these questions, some makers of knee replacement prosthetics are offering what is terms as a “high-flex system”, marketed as a prosthetic that more closely mimics the full range of natural movement. However, patients being offered these high-flex systems may have reason to be cautious.

A study carried out between March 2003 and September 2004 examined how the Zimmer NexGen LPS-Flex high-flex knee prosthetic functioned in 47 different patients all implanted by a single surgeon. That study found that at around the 32-month mark, 38% of the cases showed aseptic loosening around the femoral components; 21% of the cases had required a revision surgery due to progressing loosening and pain. When the level of flexion was specifically examined, the researchers found that for the group of patients who did not experience loosening, the average flexion was 125°, compared to 136° for those who experienced loosening. In addition, the proportion of patients who were able to squat, kneel or sit cross-legged was greater (85%) among the patients who had experienced loosening, compared to patients who did not experience loosening (44%). Ultimately the study concluded that while the high-flex implant did allow for a greater range of motion, it also showed the above rates of loosening associated with that greater flexibility.

Zimmer, a maker of high-flex knee replacement prosthetics—including the NexGen LPS-Flex system that was included in the study—tells a different story. The company promotes its line of high-flex products as a solution for patients who want to reclaim their active lifestyle after knee replacement. Specifically, Zimmer claims, contrary to what the study found, that its products are able to safely accommodate flexibility up to 155°, allowing patients a higher range of motion that would enable them to climb stairs, sit comfortably in chairs, and engage in hobbies such as gardening or golf, as well as activities like kneeling or sitting cross-legged. Further, it has been suggested that problems in surgical technique could account for the loosening, rather than the product design or manufacture.

Another study compared high-flex prosthetics, like the NexGen System, with normal prosthetics and actually found no significant difference between the two for most patients. Specifically, this study revealed that for the average patient without any special demands, the additional knee flexion provided by the high-flex design made no significant difference on the results of an individual patient’s “knee society score”, a grade on the overall range of movement that the patient experiences.

Together, this information could indicate that for the average patient, there may be little or no added concrete benefit to receiving a high-flex system, but there could be added risks including the possibility of loosening of the prosthetic, pain, and even revision surgery.

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.