Knee Replacement Implant – Types of Knee Implants

The Knee Joint is one of the most important bone joints in our body as it determines our mobility and hence quality of life. Structurally speaking, it’s a complex one, formed by 4 bones. The femur or thigh bone on the top, tibia or the larger shin bone from the lower leg, the fibula or smaller shin bone and the patella or knee-cap. Tendons, ligaments and cartilage provide the required cushioning, support, flexibility and seamless movement at the joint.

Knee Replacement

Like with any other part of the body, the knee joint is vulnerable to wear-and-tear from age or other conditions such as:

• Osteoarthritis: This is a degenerative joint disease in which the cartilage and bones start breaking down gradually. Can occur anywhere in the body but has a pronounced effect on the knee-joint. Can be hereditary or acquired.
• Hereditary conditions like osteoarthritis, arthritis, rheumatoid arthritis and developmental abnormalities
• Obesity
• Avascular necrosis or bone death
• Injury or trauma – can be one-time impact from an accident or repeated slips and falls
• Bone cancer at the knee joint (quite rare)

All these conditions can cause intense pain in the knee joint. The person may find difficulty in walking, climbing stairs, cycling or sporting activities. If left untreated for long, the person can be relegated to a wheelchair or even bed-ridden. The initial options for treatment include physiotherapy, braces, anti-inflammatory medication and corticosteroid injections. When none of these work out, an artificial, knee-replacement implant is the only solution. This may be required in either or both knees, although the replacement is done one after another.

Read More: Myths and Facts about Total Knee Replacement

What does a Knee Implant look like?

The structure of the implant depends on whether the person is undergoing a partial or total knee replacement.

Partial knee replacement is done when only one of the 3 major bones at the joint is damaged, and the others bones as well as cartilage, tendons and ligaments are all in a fairly good condition. So only one component of the implant is replaced.

Whereas, in total knee replacement, the wear-and-tear is quote pronounced all over the knee joint. It is then preferred to replace all the 4 components of a knee implant.

1. The femoral component: This is a large, curved piece that is attached to the end of the resurfaced femur or thighbone. This part comes in various metal or ceramicoptions. Since this part engages in the most movement, it’s generally made of cobalt-chromium alloys which are most durable.
2. The tibial component: This is a flat piece that is attached to the top of the resurfaced tibia or larger shin-bone. This platform is usually made of a softer metal like titanium alloy along with a polyethylene insert.
3. The patellar component: This is a dome-shaped piece that replaces the existing patella or knee-cap when it starts rubbing against the thighbone. This part is made of a durable plastic like polyethylene.
4. A plastic spacer: This is a polyethylene piece that fits between the tibial and femoral components. It helps provides a smooth, gliding action for the implant, so the person can bend and flex the new knee joint like before.

The fibula is not much affected by knee conditions described earlier, so most implants do not modify the fibula in any way.

Materials used in an implant

• Cobalt-Chromium Alloys: This is the most preferred material for the femoral component as it has to bear a lot of traction. The material is tough, durable and biocompatible, which means, it will not be attacked by the body’s immune system. For people with nickel allergies, an alternate material is used.
• Titanium and Titanium Cobalt: This is the most preferred material for the tibial component as it does not suffer much traction or rubbing during movement. The material is soft, elastic and biocompatible which makes it ideal for thenatural bone surrounding the implant.
• Polyethylene: This is a strong plastic and used in the tibial and patellar components as well as the plastic spacer. The plastic parts allow the metal parts to glide smoothly for seamless movement.

Types of Knee implants

• Fixed bearing: This is the most common type, but not as strong as the others. Its suited for people who are not overweight, who do not have an active lifestyle and the elderly who will not put a lot of wear and tear on the implant. Here, a polyethylene piece sits on the top of the tibial component to act as a cushion. However, this plastic piece starts wearing out gradually which causes the implant to loosen. A loosened implant is painful and will require a revision surgery or a re-implant.
• Mobile bearing – rotating platform: Here the plastic cushion that sits on top of the tibial component is not fixed like the previous type, and can rotate slightly. This gives greater flexibility both on the inside and outside of the knee allowing the person to make deeper knee bends.
• Mobile bearing – medial pivot: This design is more sophisticated and uses a ball-and-socket approach as against most implants which use a hinge joint approach. This intricate design allows for complete rotation, twisting, bending and flexing like a natural knee-joint would do. That is why, this is more expensive.
• PCL retaining: Here the design retains the existing Posterior Cruciate Ligament which is not damaged much.
• PCL substituting: Here, the implant uses a plastic component to substitute for the damaged PCL.
• Gender specific: The anatomy of a woman’s knee-joint is slightly different from that of a man, so this types of implant factors this.

Success Rates

 Knee-joint implants are safe and effective which is why they are becoming popular with each passing day. Theoretically, most implants should last between 15 to 20 years. In reality, 95% of them do, so the number of re-implant or revision surgeries is quite low. The practice is evolving with new materials and implant-designs being experimented with all the time. All these will improve success rates and enhance the quality of life for people with knee-joint conditions.