The Zimmer NexGen CR-Flex Knee Replacement is an artificial joint sold by Zimmer Holdings, Inc., a leading global manufacturer of hip and knee implants. The NexGen CR-Flex’s design, modified from two earlier Zimmer models, is marketed toward total knee replacement patients who prefer a greater range of motion than traditional implants can offer.
The Zimmer NexGen knee system encompasses a full line of products with many interchangeable components. Zimmer introduced the NexGen CR-Flex knee in 2003 boasting unprecedented flexion, or range of motion, for patients fitted with the device.
The “CR” in CR-Flex stands for “cruciate retaining,” which means that the surgeon preserves the posterior cruciate ligament (PCL) when implanting the device. During some operations, surgeons sacrifice the PCL, which serves the function of stabilizing the knee. Zimmer also manufactures the NexGen Gender Solutions CR-Flex, a female-specific component modified to address anatomical differences between the male and female knee.
To achieve safe flexion beyond the industry standard of 120 degrees, Zimmer designed the CR-Flex knee system with numerous technical innovations, including:
- Tibial plate with a medial axis of rotation
- Increased tibiofemoral conformity
- Posterior condyle with decreased height and extended radius
- Lateral condyle with slightly extended posterior distal radius
- Condylar asymmetry of the distal femoral condyles
- Deepened and widened patellar relief in articulation surface
According to Zimmer marketing brochures, the company constructs CR-Flex components with advanced materials, such as Prolong Highly Crosslinked Polyethylene plastic. Zimmer states that this material offers improved resistance to wear, pitting, cracking and oxidative degradation. For certain patients, the company offers minus sizing for the CR-Flex knee’s femoral components — a favorable option that allows surgeons to correct common complications without the need for additional bone or ligament removal.
Several design features of the CR-Flex allow for greater range of motion. The articular surface of the implant (the joint at which the leg bones connect) features a deeper cutout than older models, which reduces tension and provides more clearance for the kneecap’s patellar tendon.
Zimmer also extended the flanges on the device’s femoral components to improve flexion. In addition, the CR-Flex’s deep patellar groove relieves pressure on the kneecap to reduce premature wear.
Problems with the Zimmer CR-Flex Knee
After one year of clinical experience with the CR-Flex knee, Dr. Richard Berger, an orthopedic surgeon and paid consultant for Zimmer, had implanted approximately 125 of his patients with the device. But by early 2006, less than one year later, Berger observed patient X-rays that showed complications with the implants, including signs of loosening where the device joins to the thigh bone.
This loosening, an indication that the device did not completely fuse to the bone as intended, caused his patients to experience painful symptoms. He notified Zimmer of the issue, but the company defended its device and blamed the doctor’s surgical technique. By 2007, Berger had stopped offering the knee implant entirely and learned that several other surgeons had also experienced problems with it.
The NexGen CR-Flex knee, like any artificial knee, comes with risks for certain adverse events, including:
- Unsatisfactory range of motion
- Instability or dislocation
- Corrosion of implant components
- Unequal leg length
- Nerve damage
The following complications may also occur:
- Adverse reactions to the prosthetic’s materials, including allergic reactions or even cancer
- Bone disintegration (osteolysis) and loosening from implant debris
- Blood clots
In March 2010, Dr. Berger and Dr. Craig Della Valle, a colleague from the Rush University Medical Center, presented the results of “The High Failure Rate of a High-Flex Total Knee Arthroplasty Design,” a study they conducted on the NexGen CR-Flex Porous Femoral Component. Berger and Della Valle described their findings at a national conference for the American Academy of Orthopedic Surgeons.
The study detailed the experiences of 108 patients fitted with the component. The doctors observed that 8.3 percent of the study subjects fitted with the device experienced looseness and pain that required a revision surgery to correct. It also showed that another 36 percent of patients in the study experienced loosening of the porous femoral component within two years of surgery, and that the loosening and corrective surgeries were not related to the surgeon’s approach or patient type.
Typically, manufacturers design knee implants to last an average of 12 to 15 years, without needing revision. The Rush University Medical Center doctors concluded that the CR-Flex porous femoral component, while still commercially available, ought to be withdrawn from the market and “should not be used by any patient” because it could require revision surgery long before its expected lifespan. Despite these findings, the NexGen CR-Flex Knee was never recalled.
In addition, several research studies have challenged the true benefits of the CR-Flex’s increased range of motion for patients implanted with the device. One five-year study compared the results of 107 total knee replacement surgery patients, 57 fitted with a high-flexion CR-Flex knee and 50 fitted with a traditional knee implant.
After evaluating all of the patients’ results for range of motion, leg and knee alignment, the position of the implant and how well it fused to bone, researchers found that the CR-Flex offered no significant advantages over conventional artificial knees. Because high-flexion devices like the Zimmer NexGen CR-Flex knee are relatively new to the market, long-term studies of patient outcomes are required to confirm the benefits Zimmer claims they have over traditional implants in terms of premature wear and loosening.
Zimmer NexGen CR-Flex Knee Lawsuits
Considering mid-term studies on patient outcomes and the significantly high failure rate of the NexGen CR-Flex knee component identified by Dr. Berger and Dr. Della Valle, problems with the device could be more serious than Zimmer admits.
Patients who experienced complications with the NexGen CR-Flex knee implant have chosen to file lawsuits against Zimmer, alleging that defects in the implant’s design are causing premature failure and unnecessary injury to patients. Plaintiffs also allege that the CR-Flex has no clinical benefit over traditional devices, as Zimmer claims.
As of August 2013, more than 1,000 lawsuits have been consolidated into the Zimmer NexGen Knee Implant Products Liability Litigation, a multidistrict litigation managed by a single judge in an Illinois U.S. District Court. Additional claims are expected to surface as more patients become aware of issues with the device.