MOBILE BEARING TOTAL KNEE
CONTENTS
In the natural knee joint, the joint surfaces between tibia and femur are not conforming. Nature has placed a piece of triangular cartilage between these two joint surface, the meniscus, which takes up and distributes the stresses evenly between the tibial and femoral joint condyles.
The menisci in the natural knee move quite a lot, and the clever surgeons who discovered this fact put a moving piece of polyethylene to replicate the Nature's moving interface system in their total knee joint prostheses.
Such a total knee model is called mobile bearing total knee joint.
Mobile bearing total knee prosthesis
has a polyethylene bearing plate that is very conforming with the femoral condyles. This bearing plate is, however, not affixed to the tibia as in the conventional total knee joint. Instead, the bearing plate moves on the surface of a highly polished metallic tray which is affixed to the tibia.
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.
one disadvantage
1) 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 total knee prostheses
are available in two models:
1) for replacement of only one knee compartment
this model has one semicircular bearing plate moving on a metallic tibial tray
2) for replacement of all three knee compartments (inclusive of patellar surface).
MOBILE BEARING TOTAL KNEE
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The tricompartmental total knee prosthesis has two meniscus-like polyethylene bearing plates. The polyethylene bearing components move on a a highly polished metallic tibial tray. The patellar surface is replaced in the usual way.
The prerequisite for the use of this model is preservation of the knee ligaments, especially of the posterior cruciate ligament.
Some surgeons sacrifice (remove) the posterior cruciate ligament during the total knee replacement. For these operations there are special models of mobile bearing total knees. These models have only one large circular polyethylene plate. The plate is conforming with the condyles of the femoral components and has a post which engages a cam in the femoral component. (See the chapter Posterior stabilized total knee for details of this construction)
The polyethylene plate moves on the surface of a highly polished tibial tray.
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)
Results
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 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).
Radiological reports, however, shoved that up to 30 % of mobile bearing total knees have developed "radiolucent lines" around the total knee components. (Hartford 2001)
References:
Archibeck MJ & White RE J Bone Joint Surg-Am 2002;84-A: 1719-26
Hartford JM et al J Arthroplasty 2001;16; 977-83
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