The NexGen LPS-Flex knee, manufactured by Zimmer Holdings, Inc., is one of many interchangeable components that are compatible with the company’s NexGen knee system — one of the most frequently implanted total knee replacements.
Zimmer developed the LPS-Flex knee to provide patients with post-operative range of motion beyond the capabilities of other devices on the market. Patients with more active or demanding lifestyles are specifically interested in artificial knees that offer more than 120 degrees of flexion, the motion used to bend the knee. Numerous technical innovations in the LPS-Flex’s design allow it to achieve deeper flexion for select patients.
For the knee to bend, the lower part of the thigh bone (femur) rolls and slides on top of the upper shinbone (tibia). The patella, or kneecap, protects the knee joint and increases leverage for the tendons that control the knee. The LPS-Flex knee features a femoral component that attaches to the femur and a tibial component that attaches to the tibia.
By increasing the thickness of the LPS-Flex’s posterior condyles (which mimic projections on the femur that roll and slide against the tibia), Zimmer improved the contact area of the joint’s articulation surface. As a result, the LPS-Flex can potentially achieve up to 155 degrees of flexion.
The full name of the LPS-Flex is the Zimmer NexGen Legacy Posterior Stabilized Flex knee. In posterior stabilized knee implants like the LPS-Flex, the polyethylene plastic cushion of the tibial component features a raised surface with an internal post. This post fits into a special bar, called a cam, which is fixed to the femoral component.
These pieces serve the same function as the posterior cruciate ligament (PCL), a major ligament of the knee that surgeons must remove when implanting the LPS-Flex. While both prevent the femur from sliding too far forward on the tibia as the knee bends, a modified cam on the LPS-Flex increases range of motion, helps improve the joint’s stability and assists with rollback during extension of the knee.
Zimmer also significantly modified the tibial component’s cushion with a larger and deeper cutout. This helps relieve the high level of stress placed on soft tissues as they are stretched and pulled during deep flexion.
Several variations of the NexGen LPS-Flex knee exist. The LPS-Flex Fixed Bearing knee resembles most knee implants in that the tibial component’s cushion is firmly fixed to the metal implant it rests on. The LPS-Flex Mobile knee, whose cushion can rotate slightly, gives patients a few more degrees of knee rotation. Zimmer also offers Gender Solutions components for the LPS-Flex, which have modified dimensions to more accurately recreate the female knee.
Complications with the Zimmer NexGen LPS-Flex Knee
While Zimmer designed the NexGen LPS-Flex knee implant to provide exceptional range of motion, follow-up studies on patients fitted with the device have returned mixed results regarding their ability to achieve deep flexion. Numerous investigators have found that the LPS-Flex’s design offers relatively minor gains in range of motion. In addition, these features may weaken the stability of the implant and increase stresses on the device, which can result in loosening and other complications that cause premature failure.
Patients face a wide range of potential complications during and after total knee replacement surgery, including:
- Loosening of knee components
- Soft tissue damage
- Poor range of motion
- Leg length discrepancies
- Bone fractures
- Blood clots and other cardiovascular disorders
- Metal sensitivity
- Excessive wear
Zimmer describes several adverse effects unique to the NexGen LPS-Flex mobile and fixed bearing knee systems in the package inserts for these devices, including:
- Knee stiffness
- Excessive wear that leads to bone damage (osteolysis) and loosening of the implant
- Joint instability or dislocation
- Partial dislocation (subluxation) of the tibia and femur
- Bearing disassembly of the tibia and femur
In a study published in 2007, researchers evaluated the results of 47 NexGen LPS-Flex total knee replacement patients, including 25 who were fitted with double replacements and 22 who had one knee replaced. A single surgeon performed the knee replacements between March 2003 and September 2004.
After an average of 23 months, 15 knees required a revision surgery to correct pain, loosening and other complications. These devices showed loosening of the femoral component where surgeons cemented the device to bone. By an average of 32 months, researchers found that the device’s femoral component loosened in 27 patients.
Once researchers began investigating what factors could cause such high rates of loosening, they found that weight-bearing, high-flexion activities played a major role in the problem. The complications and ensuing revision surgeries occurred despite the fact that Zimmer designed the NexGen LPS-Flex to allow patients to perform activities that require deep flexion.
Similar results emerged from a 2011 study evaluating the outcomes of 166 patients fitted with 218 NexGen LPS-Flex knees. While the implant significantly improved the patients’ range of motion, radiographic imaging revealed loosening of the femoral component in 30 knees. Seven LPS-Flex knees required revision surgery at an average of about four years after implantation. Researchers concluded that all of these outcomes may be associated with high flexion activities, such as squatting and kneeling.
Lawsuits Related to NexGen LPS-Flex Knee Implants
As an increasing number of issues with the Zimmer NexGen suite of knee replacement devices became known, the U.S. Judicial Panel on Multidistrict Litigation began transferring Zimmer NexGen knee replacement lawsuits to the U.S. District Court for the Northern District of Illinois for pretrial proceedings. Among these claims were complaints filed by patients who experienced loosening and other complications with NexGen LPS-Flex and LPS-Flex Gender components.
Patients injured by the LPS-Flex and other NexGen high flexion components allege that Zimmer failed to properly warn patients of the dangers associated with their use. Claiming that multiple NexGen device components are unfit for sale in the United States, plaintiffs are seeking compensation from Zimmer, as well as punitive damages.
As of August 2013, more than 1,000 lawsuits have been added to MDL 2272, the Zimmer NexGen Knee Implant Products Liability Litigation. Experts predict more claims will be added to the multidistrict litigation before the cases come to trial.