Development of a congruent tibilal component
In the natural knee joint (as in the artificial total knee prosthesis), the joint surfaces between tibia and femur are not conforming. Nature has placed a piece of cartilage – meniscus – between these two joint surface to provide for conformity. The meniscus, which takes up and distributes the stresses evenly between the non conforming surfaces of the tibial and femoral joint condyles moves a little during knee joint bending and stretching. This small movement helps the meniscus to conform better to the joint surfaces.
The surgeons have identical problem with the lacking congruence between the femoral and tibial components of a total knee. They tried to improve this lacking congruence between artificial knee joint’s surfaces congruence in three steps:
First step: The first tibial components were manufactured of polyethylene only – all-polyethylene components and were fixed by cement directly to the bone tissue of the tibia. They are still in use today, although their form may be changed a little. Soon, the surgeons & material scientists became anxious that the component made of polyethylene alone was too soft. It needed support from something harder.
Second step: Comes metal backed tibial component. The polyethylene plate articulating with femoral component got a metal backing. This hard backing should prohibit the deformation of the soft polyethylene.
The polyethylene inlay was rigidly fixed to the metal backing plate and thus immobile. Soon, this rigid fixation was considered a disadvantage and developement continued to a mobile polyethylene plate.
Third step: The peak of the development is at present the mobile bearing tibial component. The polyethylene plate that articulates with the femoral component lost its fixation to the metallic back up plate and became mobile.
Mobile bearing total knee prosthesis
Has a polyethylene bearing plate that is very conforming with the femoral condyles. The increased conformity is possible because the polyethylene bearing plate moves on the surface of a highly polished metallic tray during extension and flexion of the knee joint. The metallic tray is itself affixed to the tibia.
Advantages and disadvantages
This design has two advantages:
First: the polyethylene bearing plate has a large contact area with the femoral component during the whole range of knee joint movement. Thanks to this large contact area, the stresses on the polyethylene bearing plate are substantially lower than the stresses on the polyethylene plate in the stabile bearing total knee prostheses.
|Type of bearing||Contact area (square mm)|
|stabile bearing||200 – 300|
|mobile bearing||1000 – 1500|
(Orthopaedics Today, July/August 1998)
Second: The bearing plate moves free and does not restricts the natural movements of the femoral component. Thanks to the “unrestricted” movements of the plate, the stresses transmitted on the area where the total knee prosthesis is fixed to the skeleton are low and the risk for loosening of the total knee components is low too.
An additional advantage is a more natural gait pattern and a larger range of movement achieved with these total knee prostheses.
The polyethylene mobile plate has two wearing surfaces: one is the surface opposed to the femoral component, the other one is the surface opposed to the polished tibial tray. It is as yet uncertain, how much polyethylene particles this doubling of wearing surface produces.
The mobile bearing knee prostheses
are available in two models:
1) for replacement of only one knee compartment
UNICONDYLAR MOBILE BEARING KNEE PROSTESIS
|this model has one polyethylene bearing plate moving on a polished metallic tibial tray. To keep this tibial polyethylene plate in place the patient’s knee should have a relatively well preserved ligaments, both the collateral and the cruciate. Picture Oxford Unicondylar (Biomet)|
2) for replacement of two or all three knee compartments (inclusive of patellar surface)
MOBILE EARING TOTAL KNEE
Such total knee prosthesis replaces the whole knee joint’s surfaces. See the picture of Rotaglide mobile total knee above. The polyethylene plate moves on the surface of a highly polished tibial tray. It is kept in place by the conforming metallic femoral component’s joint surfaces.
Who is the candidate for mobile bearing knee joint prostheses
The alleged advantages of mobile bearing knee joint prostheses are increased range of motion in the total knee and low wear of the polyethylene plate with lower risk for loosening. These advantages make this knee prosthesis suitable for young active patients.
The disadvantages of mobile bearing knee joint prostheses
The stability of the mobile polyethylene plate depends on well balanced ligaments and soft tissues around the new knee joint. The precise operation technique is essential for a good result.
The most frequent cause of failure in these prostheses is the accelerated wear, destruction, or dislocation of the mobile polyethylene plate in knees with ligament and soft tissues instability.
The discussion is still ongoing whether the “mobile” polyethylene plate really moves during knee movements, and if it moves, whether it copies the natural movements of the healthy knee parts.
Some studies also claim that the range of movement of mobile bearing total knees is not better than the movement in the conventional stabile bearing total knees. (Archibeck 2002)
As yet there are only reports on relatively small number of operations with mobile bearing knees (unicompartmental and tricompartmental) and their results.
These reports show that between 93 and 96 % of tricompartmental mobile bearing total knees were still in function after 10 years.
Satisfaction with the operation was also high, > 90% of the patients were satisfied with the operation.
The same figure of survival applies to unicompartmental knee prostheses.
These reports also show that the postoperative range pf motion in the mobile bearing total knees improved for patients who have less than 90 degrees of flexion in their knee before the operation (these patients gained 28 degrees of flexion capacity), whereas the patients who had more than 105 degrees of motion before the operation lost 1 degree of motion (on average).
Older radiological reports, however, shoved that up to 30 % of mobile bearing total knees have developed “radiolucent lines” around the total knee components. (Hartford 2001)
Archibeck MJ & White RE J Bone Joint Surg-Am 2002;84-A: 1719-26
Hartford JM et al J Arthroplasty 2001;16; 977-83
Gioe T et al: J Bone Joint Surg-Am, 2009; 91-A: 2104- 2112