Knee

The knee joint is a complex hinge joint, which is actually a combination of 3 joints, which allow, not only bending and flexing of the knee, but, also, internal and external rotation. Each joint is dedicated to a specific function, allowing assessment, diagnosis and treatment planning to be targeted at an individual joint.

Targeted Physiotherapy has an important role to play in improving the function of the joint through specific techniques and exercise programmes. Although taking a good history and performing a thorough assessment is usually enough to diagnose the problem, sometimes, it is necessary to have an MRI scan to confirm the exact type and extent of damage within the knee. Arthroscopy (keyhole surgery) is also a great tool in diagnosing knee conditions and, also, treating them, with soft tissue procedures.

It is also possible to experience referred knee pain, when there is a problem in the back or the hip and some knee pains, especially when younger, can be caused by poor muscle control and function of the knee, rather than any actual problem with the knee joint itself. Therefore, thorough assessment and diagnosis is essential.

When the knee becomes arthritic, initially, the pain is intermittent. As the arthritis develops, the pain will worsen and become continuous. Initially, pain should be managed with painkillers and Physiotherapy. However, when this fails to be effective, a knee replacement is indicated.

Despite common belief regarding joint replacements, it is not necessary to wait until a certain age, such as over the age of 65, before having a knee replacement. If assessment and diagnosis shows significant arthritic change in the joint and painkillers and Physiotherapy are no longer effective at dealing with the pain, a knee replacement, such as the ‘mobile bearing knee’ (MBK) can be considered, as it has a superior lifespan and will not wear like other knee replacements.

There are 2 types of knee replacement:

Fixed bearing knee:
Does not restore normal anatomical function and wears more quickly. This is because the load put through the knee is put through a smaller contact area. The stress generated at these contact sites is often higher than the resilience of the polyethelyne that it is made from, causing the replacement to wear and fail after 10 years in an older individual.

Mobile bearing knee:
2 Types:
Fully congruent: Opposing parts of the joint replacement fit exactly, allowing less stress within the joint replacement and less wear.
Partially congruent: This has better congruency and function than the fixed bearing knee, but has more stress within the joint than the fully congruent mobile bearing knee as the opposing parts of the joint replacement do not fit exactly together.

 

Currently, the fixed bearing knee is more popular because, surgically, it is far less technical and demanding for the surgeon. However, the mobile bearing knee should be the replacement of choice, as it more closely repairs normal anatomical function within the joint, lasts longer and can be used in younger, more active, patients.

Polyethylene failure
A major cause of failure in replacement joints such as the fixed bearing knee is polyethelyne wear. Polyethelyne is the white material that can be seen in the previous pictures in the middle of the joint, between the opposing metal parts. A simple way of demonstrating this is by using the equation

Therefore, the larger the contact area when load is put through the knee replacement, the less stress there will be, and the smaller the contact area, the more stress there will be. This is demonstrated in the graph below:

Graph comparing contact stress in MPa between MBK - mobile bearing knee (FULLY CONGRUENT), LCS - low contact stress (PARTIALLY CONGRUENT) and Non-conf (NON CONGRUENT) replacement knees

The stress tolerance of polyethylene is 10 MPa. As can be seen in the graph, even the average contact stress in a fixed bearing knee is 12.5 – 25 MPa, leading to failure of the polyethylene and a shorter lifespan of the prosthesis. For this reason, it is important that the fixed bearing knee is not used in patients aged less than 70 years old. If only one part of the knee has significant arthritis it can be replaced by a unicondylar knee, or patellofemoral replacement.

Mr. Kumar is a specialist knee surgeon and has published 6 articles on knee surgery as follows:
“Replacement arthroplasty of the valgus knee’. A modified lateral collateral capsular approach for replacement of valgus arthritic knee. Published in the Journal of Bone and Joint Surgery [Brj 1998;80-B: 859-86 1

“Proximal realignment during total knee arthroplasty of the valgus knee”. Published in the Journal of Bone and Joint Surgery [BrJ 1998;80-B:Supp I 103-104

"A simple instrument for balancing flexion and extension gaps in total knee arthroplasty”. I designed this instrument and used it on 50 consecutive knees. Published in The Knee Journal. 1998;5:2;292,93-94

A Case Report “Fracture of the medial tibial plateau following Unicompartment Knee Replacement” Published in The Knee Journal. 4 1997;177-178

“Medial Unicornpartment Arthroplasty of the Knee” (10 years retrospective study of 100 cases.) Published in The Knee Journal 1998.

Prospective study of 5 years result of MBK in younger individuals. Accepted for publication.

Case Study of Successful Complicated Knee Surgery

Knee replacement surgery is possible, even when previous knee surgery on the same knee has failed. In this example, the patella was dislocated at the age of 22 leading to gradual subluxation of the tibio-femoral joint. He presented to our team at the age of 52 and his story can be heard in this video clip.