Rotating platform flex knee replacement : New innovation for complete knee bending

Dr Sanjeev Jain

Arthritis of knee is the world’s leading cause of disability. Arthritis causes pain, swelling, inability to walk, reduced bending of knee, inability to sit cross legged, deformity and so on.

Conventional total knee replacement is a well-known treatment for advanced knee arthritis. Knee replacement surgery provides excellent results to the patient, relieving him from pain. The surgery also improves walking ability and allows pain-free knee bending. Most importantly, it improves the quality of life of the patients.

Knee replacement surgery is a highly technical operation. In this surgery, lower end of femur, upper end of tibia and patella are reshaped with varieties of jig and instruments and final implant has been put along with bone cement over reshaped bone. Femoral and tibial components of prosthesis are made of high grade metal – cobalt chrome molybdenum and tibial insert and patella button is made of a very special plastic – ultra high molecular weight polyethylene. This plastic insert is fixed to the tibial base plate. During surgery these components are fixed with polymethamethylacrelate bone cement to the parent bone. There are varieties of knee joint prosthesis available for surgeons.

There are two kinds of knee designs available. One is cruciate substituting and other is cruciate retaining design. It is the design and surgical technique responsible for posterior femoral roll back which facilitates flexion. Biomechanically, after conventional knee replacement, generally one gets 90 to 100 degrees of knee flexion or bending as conventional knee replacement have designed to accommodate flexibility up to 100 degrees.

Recent innovation of knee design is a breakthrough where complete flexion could be achieved. Desire for high flexion may be dictated by patient’s favourite activities or cultural background. Many daily activities require the ability to flex the knee beyond 125 degrees.

Rotating Platform Flex (RPF) knee system is designed to provide high knee flexion. This is posterior cruiciate substituting design where action of posterior cruciate ligament is duplicated by cam of femoral component and post of tibial plastic insert. In RPF design in deep knee flexion, the refined cam and post mechanism becomes congruent and offers a third weight bearing surface. This design also assures low contact stress in deep flexion and it’s unique cam and post mechanism maintains joint stability throughout the range of motion. Since plastic insert or bearing rotates between two components of the joint, it provides many advantages. This bearing mobility helps to maintain optimum contact area in rotation. Redefined posterior curve of the femoral condyle and predictable posterior femoral rollback is the key issue to provide more flexion.

The fixed bearing conventional knee provides 100 degrees of flexion. For most patients, this is not enough. The rotating platform knee is specially designed to safely accommodate up to 155 degrees of flexion in patients who has good pre operative range of motion. This means that with appropriate rehabilitation a patient can resume an active life style after total knee replacement – deeply bending the knee for recreationa. Day to day activities, which may include prolonged kneeling, squatting and cross-legged sitting can also be done.

Apart from the design, surgical technique also play an important role in getting more flexion. Total knee replacement is not only to put prosthesis on the bone, rather it is a well-defined technically demanding surgery, where balancing of the ligaments and soft tissue are of importance to get excellent results. To get more flexion, posterior tissue release and balancing has to be perfect. In this RPF design, tibial bearing rotates or it’s mobile, so to keep it stable inside, ligament and soft tissue balancing plays a major role.

This RPF design and surgical technique are going to change the scenario of total knee replacement completely. Complete knee bending is a pre requisite for the patients of Asia, Japan and more so in Indian continent. This will also raise high level of confidence in patients to get surgery done as they will also get good flexion. So now, days have gone when patients would hesitate to get knee replacement done due to reduced knee bending after knee replacement.

Complete bending knee prosthesis – RPF knee and special technique has made a breakthrough in the field of knee replacement surgery.

The writer is a Consultant Joint Replacement and Hip Resurfacing Surgeon with Dr L H Hiranadani Hospital, Mumbai.