Patient Specific Knee Options

Total knee replacement is often the last option when non-surgical treatment options fail to relieve osteoarthritis of the knee. Osteoarthritis, the most common type of arthritis is a degenerative joint disease in which the joint cartilage gradually wears away. As a result, bones rub together resulting in extreme pain. The knee can be divided into three compartments:

• Patellofemoral – the compartment behind the kneecap
• Medial compartment – the compartment on the inside of the knee
• Lateral compartment – the area on the outside of the knee joint

Traditionally total knee replacement surgery involves replacing the damaged cartilage and bone from the surface of the knee joint with a metal implant. This surgery requires substantial amount of bone removal that may have certain limitations. In total knee replacement all the compartments are replaced where as in partial knee replacement only the affected compartment is replaced.

ConforMIS® has developed the personalized knee resurfacing implants with unique advantages against traditional knee replacement options. To design a personalized implant for every individual, surgeon will perform the CT scan of the affected knee of the patient. A 3D image of a patient’s knee is created from CT scans and an implant is designed to conform exactly to individual size and shape.

The advantages of ConforMIS® knee implant include:

• Offers superior fit to individual size and shape
• Requires fewer bone cuts and preserves most of the existing bone and thereby have better prognosis
• Restores your knee’s normal range of motion
• Less traumatic procedure
• Result in faster recovery
• Decreased postoperative pain

Following surgery, you will be able to bear the weight and walk with assistance. You can return to your daily activities within 6 weeks.

Personalized implants by ConforMIS® are of various types including iTotal® CR, iUni® G2, iDuo® G2.

Custom fitted total knee arthroplasty is a newer technology in total knee replacement surgery. It is an advanced procedure using an individualized patient-specific knee implant for replacement of all three components of the knee. The difference with custom knee replacement from other knee replacement surgeries is the use of an MRI scan prior to the surgery that provides a clear view of the shape and structure of the different components of the joint. The implants are then designed specifically to match the natural shape of the articulating surfaces of the patient’s knee based upon the MRI. Unlike other knee replacement surgeries, custom fitted total knee replacement can also be used for severe knee arthritis.

Procedure

Prior to the surgery several MRI scans are taken to understand the specific anatomy of the affected joint. Along with the MRI images a special tool called a jig is also used to provide a clear view of implantation. The patient specific jigs have a built-in image guidance which simplifies the surgical procedure and improves the outcome. Based on the MRI images the surgeon plans the surgery and calculates the cutting measurements that help the surgeon to remove the necessary amount of bone, at specific angles. This customized cut provides better implantation, more reliable alignment and removal of less amount of bone.

Benefits

There are several advantages to customized total knee replacement arthroplasty over the regular traditional implants, which include:

• No compromise on the size of the implant as it is patient specific
• Provides better fit of the implant as the implant conforms to the patient’s unique shape of the articulating surfaces of the knee joint
• Maximizes the implants contact area decreasing the polyethylene contact stress thereby prolonging the life of the implant
• Preserves more natural bone

Maximizes proximal tibial coverage and thus reduces the risk of tibial implant caving in and loosening

The ConforMIS iTotal CR is an individualized patient specific implant for replacement of all the three compartments of the knee. It is thus most appropriate for patients with knee arthritis and knee damage requiring implant for not one or two but all the three compartments of the knee. It is designed specifically to match the natural shape of the articulating surfaces of the patient’s knee. This is done by mapping the articulating surfaces of the femur and tibia using the data from the CT scan of the patient’s knee. It also comes with disposable patient specific iJig instrumentation with built-in image guidance which simplifies the surgical procedure and improves the outcome.

The several advantages of this patient specific or iTotal customized total knee replacement implants over the regular traditional implants are:

• No compromise on the size of the implant as is patient specific
• Provides better fit of the implant as the implant conforms to the patient’s unique shape of the articulating surfaces of the knee joint
• Maximizes the implants contact area decreasing the polyethylene contact stress thereby prolonging the life of the implant
• Preserves more natural bone
• Maximizes proximal tibial coverage and thus reduces the risk of tibial implant caving in and loosening

Indications for use:

• Chronic joint disease due to trauma, osteoarthritis, rheumatoid arthritis or osteonecrosis of the knee
• Moderate varus, valgus or flexion deformity with not much damage to the ligaments
• Revision surgery for failed osteotomy, hemiarthroplasty and unicondylar, patella-femoral or bi-compartmental implants

Note: The iTotal CR implant require the use of bone cement for fixation.

Although iTotal Customized total knee replacement systems are recommended in various conditions, they are not appropriate in patients with:

• local or systemic infection
• insufficient bone stock on the femoral or tibial surfaces
• young patient with skeletal immaturity
• severe fixed valgus or varus deformity of greater than 15 degrees
• sensitivity to metals such as nickel
• conditions of the knee joint such as osteoporosis, joint neuropathy and others in which the bone, muscle or nerves of the knee to be operated are compromised
• severe instability due to major loss of osteochondral structure or absence of collateral ligament integrity

Knee implants are artificial devices that form the essential parts of the knee during a knee replacement surgery. The knee implants vary by size, shape, and material. Implants are made of biocompatible materials that are accepted by the body without producing any rejection response. Implants can be made of metal alloys, ceramics, or plastics, and can be joined to the bone. The metals used include stainless steel, titanium, and cobalt chrome; whereas, the plastic used is polyethylene.

Components of a Knee Implant

The components of a knee implant replicate the natural shape and structure of the knee joint. The components used may depend on the size of the body and vary from patient to patient. There are 3 components in a knee implant:

Femoral component: The femoral component is generally made of metal and curves around the thighbone. It has a channel that allows the kneecap to move up and down as the knee is in motion.

Tibial component: The tibial component is usually a flat metal platform with a plastic (polyethylene) cushion. The metal portion attaches to the tibia and has a stem that goes into the tibia for stability. The plastic portion, also known as a spacer, acts as a cushion between the femoral and tibial metallic components.

Patellar Component: The patellar component is a dome-shaped device made of polyethylene that replicates the patella.

Types of Knee Implants

The type implant used may depend on the patient’s bone quality and the health of the ligaments surrounding the knee. Based on the patient’s age and activity level, there are different types of knee implants used in knee replacement surgery.

Fixed bearing implants: The polyethylene part of the tibial component is attached firmly to the metal platform. The femoral component moves on the cushion provided by the polyethylene. Fixed bearing implants are generally used in less active and elderly patients.

Mobile bearing implants: The polyethylene part of the tibial component can rotate inside the metal part, and therefore provides greater mobility to patients. Mobile bearing implants are designed to bear greater stress compared to fixed bearing implants, and require greater support from the ligaments surrounding the knee. Mobile bearing implants are preferred in generally young and active patients. Medial pivot implants are a type of mobile bearing implants that can twist, bend, rotate, flex, and perform all the functions of a natural knee.

Posterior cruciate ligament (PCL) substituting implants: The posterior cruciate ligament stabilizes the knee against forces that push the tibia towards the femur. PCL substituting implants are used in patients with an injured PCL. The PCL substituting implants are provided with a raised surface on the cushion of the tibial component, which acts as a center post and fits into a cam in the femoral component and duplicates the actions of the injured PCL.

Posterior cruciate ligament (PCL) retaining implants: PCL retaining implants are used in patients who have a healthy posterior cruciate ligament. Unlike PCL substituting designs, PCL retaining implants do not have the center post and cam design. The femoral and tibial components have grooves to hold the posterior cruciate ligament.

Custom knee implants: For patients with smaller bone structure, the sizes of the implants required may vary. Gender specific implants and narrow femur implants, can provide better alignment of the bones, and may be used depending on the preference of the orthopedic surgeon.

Types of Fixation

Depending on the age and activity level of the patient undergoing knee replacement surgery, an orthopedic surgeon may recommend any of the available three types of implant fixation.

Cemented Fixation: The components of the artificial joint are held together with special bone cement. The bone cement is made from a special polymer called polymethylmethacrylate (PMMA). Cemented fixation is an option for less active patients. Too much stress on cemented fixation can lead to fatigue fractures.

Cementless Fixation: Cementless implants are coated with a porous material. They attach to the new bone that grows to the surface of the implant via bone ingrowth. The implant may be fixed using screws or pegs until bone ingrowth. Cementless fixation is an option for more active patients with good bone quality.

Hybrid Fixation: Hybrid fixation uses a combination of cemented and cementless fixation. The tibial and patellar components are fixed with cement, and the femoral component is fixed without cement.