High Performance Knees

SUMMARY

nowadays, younger and younger patients are presenting needing knee replacements

patients' expectations for improvement in function are now higher than in previous years

technical advances have been made in knee replacement prostheses

the way that the knee moves ('kinematics') is more complex by far than just a simple hinge joint

new knee replacement prostheses are now available with better geometry which more closely matches that of the normal knee, allowing more normal kinematics and better flexion

newer harder materials are decreasing wear and tear in artificial joints, which should increase how long they last

The functional results after hip replacement surgery are excellent, with an increase in sporting activities post-op. The results for knee replacements are excellent, in terms of pain relief, however, the percentage of patients participating in sports (as reported in the scientific literature) actually goes down after knee replacement surgery.

How a knee really moves

There is a very clear reason for this. The hip joint is a simple ball and socket joint, and it is therefore relatively easy to replicate the joint mechanics with an artificial replacement. The knee joint, however, moves in a far more complex fashion. The knee is not just a simple hinge joint; as the knee bends (flexes), the tibia (shin bone) also rotates, and furthermore the tibia also slides backwards slightly (more so on the outer side than the inner side). The way that the knee moves (it's kinematics) is thus highly complex - far from a simple hinge or a ball and socket joint. This complex movement is guided by a number of factors, including the shapes of the bones, the tension in the various ligaments inside and around the knee, and the pull of the muscles.

Studies using real-time X-rays watching knee replacement patients walking post-operatively demonstrate that the artificial joints display very different kinematics compared to a normal knee joint, with the artificial joint sometimes sliding, rolling and clunking around quite considerably. This excessive irregular movement can contribute to increased wear and tear in the artificial joint, which may be a factor in early failure of the knee. Also, due to the shape as well as the movements of most artificial knees, post-operatively patients often find it difficult to flex (bend) the knee up fully.

Standard knee replacements, to-date, resurface the ends of the femur (thigh bone) and the tibia (shin bone) with metal surfaces, which are stuck onto the ends of the bones with special bone cement. However, in most knee replacements, the shapes of the prostheses do not accurately replicate the contours of the normal knee. In addition, the main ligament in the knee, the anterior cruciate ligament (ACL) gets removed and is not replaced.

The latest developments

New designs of knee replacement are now available, which aim to not only relieve knee arthritis patient of their pain, but also help them regain their active lifestyles with improved joint function.

Possibly the most advanced new type of knee replacement prosthesis now available is the Smith & Nephew Journey Knee. This was developed with input from a team including some of the best and most respected knee surgeons in the world. It was designed with the assistance of large anatomic databases of 3-D computer models from CT/MRI scans, parametric computer aided design (CAD) modelling and computer virtual lower leg simulators.

	CAD computer modelling
	CAD modelled femoral prosthesis
	CAD modelled tibial tray

The Journey Knee is shaped differently from a normal knee replacement, with more natural contours, and it also replaces the anterior cruciate ligament (ACL) with a central ultra-high molecular weight polyethylene central post, which engages stably into the femoral part of the prosthesis.

The Journey Knee prosthesis from Smith & Nephew

Studies have demonstrated that the Journey Knee replacement maintains knee flexion in patients with a normal pre-operative range of knee motion, and significantly increases knee flexion in those patients who had a decreased pre-operative range of motion.

X-ray showing the high range of flexion

achieved with the Journey Knee.

In addition, the femoral prosthesis of the Journey Knee is made of a metal alloy called Oxinium. This is another relatively new development in the design of knee replacement prostheses. With Oxinium implants, the surface of the metal alloy is treated by a process called 'anodisation', which creates a thin but initimately integrated surface layer where the metal is oxidised. This makes the surface of the metal far harder, with properties more like a ceramic, which significantly improves the joint's wear characteristics. In a study, when the Oxinium Journey Knee was compared to a standard prosthesis made from the usual cobalt chrome alloy, the Oxinium Journey Knee demonstrated 75% less wear on the polyethylene spacer that sits in the middle of the knee replacement. It is envisaged that this should hopefully result in an improvement in the longevity of these prostheses, so that the knee replacement can potentially endure more wear cycles and last longer.

The Journey Knee represents another significant leap forwards in the quest for the 'ideal' knee replacement.

Our surgeons at LONDON SPORTS ORTHOPAEDICS are fully trained in the use of the Journey knee prosthesis. For further information on the Journey Knee or any other aspect of knee replacement or partial knee replacement surgery, please contact us directly.

(Please note that specific medical advice for individual patients cannot be given by telephone or e-mail, but appointments can easily be booked with any one of our specialist consultants.)

Article written by

Mr Ian McDermott

Consultant Knee Surgeon and Hip Surgeon, London & Northwood