COMPONENTS OF TOTAL KNEE REPLACEMENT

TOTAL KNEE REPLACEMENT

PARTS OF KNEE REPLACEMENT PROSTHESIS

Introduction:

Total knee replacement is surgical procedure in which the damaged cartilage lining of bone is replaced with artificial components, thereby eliminating the pain caused by diseased joint.

The most common indication is completely worn out knee joint by osteoarthritis. Knee replacement is also considered in certain arthritic conditions like Rheumatoid arthritis.

Parts of Artificial knee implant:

The components of artificial knee replicate the shape of joint surface so as to allow smooth rolling and gliding movement. Structure and shape of knee joint varies from person to person and also across the genders. In order to address this, the implant companies manufacture artificial joints in different sizes so that the operating surgeon can choose the best fit.

There are also several designs of implant based on patient’s age, gender and lifestyle.  It is the surgeon who can choose and suggest the best design and brand that best suits your need.

There are three main parts

-         Femoral (thigh bone) component

-         Tibial (Shin bone) component

-         Plastic (Cushion) component

-         Patellar component

Femoral (Thigh bone) component:

The lower end of femur in knee joint is a curved surface with central groove allowing movement of knee cap. The prosthesis component is made up metal and is curved with central groove mimicking the natural surface of femur. Femoral component is made up Chrome cobalt alloy or Zirconium alloy or Oxinium .

Tibial (Shin bone) component:

The upper end of shin bone is replaced with flat metal platform which can accommodate Plastic (cushion). Tibial component is made of Titanium alloy or Chrome-cobalt alloy.

Plastic (Cushion):

The plastic (Cushion) is also called as insert accommodates into the tibial metal platform. The plastic component has inbuilt curvature to accept the curvature of femoral component. If surgeon prefers to do a ligament sparing surgery the plastic has notch to accommodate the cruciate ligaments. In case cruciate ligaments are removed it has cam to take over the function. The material used in the manufacturing of plastic is High Molecular Weight Polyethylene (HMWP).

Patellar component:

The back side of knee cap is also replaced by removing damaged cartilage with polyethylene button. Some absolute indications warrant’s patella replacement whereas in some cases its surgeons preference.

TYPES OF TOTAL KNEE PROSTHESIS DESIGN

TYPES OF KNEE PROSTHESIS

1.     Fixed bearing :

This is the most common implant design used. As the name implies the Plastic (polyethylene) insert is fixed firmly on the metal platform of tibia. The femoral component of the prosthesis rolls over this plastic (polyethylene) insert. This design provides motion of knee joint in one plane i.e. bending and straightening of knee.

2.     Mobile bearing implants (rotating platform)

In mobile bearing knee the plastic insert (polyethylene) can rotate short distances on the metal platform of tibia. This design allows few degrees of sideways rotation of knee. The mobile bearing implants needs good ligament support as it allows sideways rotation of knee. They are not indicated in cases where the ligaments are lax or loose. These knee designs are slightly costlier than fixed bearing designs.

3.     Medial pivot implants: 

A normal knee has a component of pivoting motion during bending of knee. This design replicates bending, twisting and rotating of your natural knee.

4.     Personalized implants:

The shape and size of bones in knee joint varies from person to person. During routine total knee replacement surgery, knee implants are available in 6 to 10 different sizes. The surgeon then selects the implant that closely matches with patient size and shape of bones and some minor adjustments are done to fix the implant to the bone. This technique sometimes compromises the precession fit of the implant to the bone. In few cases and instances, such minor adjustments are acceptable, but in few gross adjustments are disaster can result in early failure of knee implant.

To overcome such problem the knee implant companies came up with idea and devised

-         Customized implants ( implant design for a given patient)

-          Personalized implants (Gender specific)

Customized implants:

In order to create customized implant for a particular patient, surgeon gets MRI or CT scan and gets 3D images of the knee joint. This data is collected by the implant design companies to create patient specific instrumentation to make cuts in the bone. They also create patient specific knee implant which actually matches the natural knee.

Personalized implants: Gender specific:

Research studies have shown that there is mismatch in shape and size of bones of knee joint in males and females.  Due to such differences surgeons make some adjustments in the routine knee replacement surgery in order to fix the available implants especially in women.

The main difference in male and female knees is the shape of the lower end of thigh bone. In females lower end of thigh bone is smaller from side to side than front to back. To accommodate this change some implant companies have devised extra set of knee designs to fit female knees.

The choice of selecting customized or personalized implant design is a surgeon preference and changes from patient to patient.   

Posterior cruciate ligament (PCL) Retaining or Substituting implants:

Posterior cruciate ligament is one of the major ligaments in the knee. It prevents roll back of femur over the tibia when bending the knee. During knee replacement surgery this ligament can be preserved or removed based on surgeons choice, type of implant used and during surgical balancing of knee.

Posterior cruciate ligament (PCL) retaining:

In this surgical technique ligament is preserved which also gives additional stability. Femoral and tibial components have notches to accommodate the ligament and plastic insert is flat.

Posterior cruciate ligament (PCL) substituting:

In this surgical technique ligament is removed. In order to compensate the function of ligament the plastic insert has raised slope in front and post in the back. The femoral component has notch to accommodate the post which gives additional stability to that of lost ligament.

The choice of using the specific design and brand of implant is upon the surgeon. There are various factors in choosing the implant which could related to patient or surgeon himself.  Surgeon can choose a particular brand and design with which he is familiar and is best suited for the patient.  

Dr.A.Mohan Krishna
Consultant Orthopedic surgeon
Apollo hospital
Hyderabad,
Appointments :9247258989
                       9441184590
bonesandjointsclinic@gmail.com
www.drmohankrishna.com
www.bonesandjointsclinic.com