Knee Replacement – Advances

Advances in Knee Replacement

• Total knee replacement is a very successful operation. More than 80 percent of patients get good or excellent results. A small subset of patients (5-20 percent) are not fully satisfied with the results of the surgery. This may be due to various factors.
• Patient factors: These are pre-operative stiffness, inadequate exercises, co-existing medical conditions like degenerative spinal stenosis, diabetic neuropathy, etc. These are not under the control of the surgeon.
• Surgeon factors: Some patients have more pain in the post-operative period. This is again multi-factorial. One of the reasons could be the surgical approach. In an effort to minimize the surgical trauma, less invasive approaches have been advocated.
• Implant design: Implants for knee replacement are manufactured in a range of sizes. There is 3-4 mm difference between one size and the next. Sometimes, the patient’s bone may fall in between two sizes. Usually the nearest size is chosen and gives a good result. In some patients, this may cause some discomfort or altered mechanics. To avoid this mis-match the difference between successive sizes must be reduced. In addition, the shape of the implants and the design factors may influence the result of the operation. This has led to creation of various designs and use of different materials.
• Surgical technique: The instruments used for performing knee replacement are very precise. This ensures accuracy while doing the operation. However they rely on certain bony landmarks to guide the operation. If the landmarks are distorted by disease or the bone quality is poor, errors can creep during the operation. Sometimes there is pre-existing deformity in the bone which interferes with the manual instruments. To address these issues surgical navigation systems have been developed.
• In spite of all these advances, the standard operation using regular implants is very successful. To put things in perspective, more than 90 percent of knee replacements worldwide are done in this way.
• Read on to know more about these advances.

Design variations and Advanced Bio-materials:

• Different manufacturers have introduced implants of varying designs and composition.
• Design changes:
  • Some are incremental changes like multiple sizing options, high flexion tibial inserts, varying degrees of constraint (stability), etc.
  • Others are based on different design philosophy. These include gender-specific implants, mobile tibial tray, medial pivot knee, anatomical knee, ligament-sparing knee, ligament-replacing knee, etc.
• Advanced Bio-materials: Improvements in material sciences have led to implants with different alloys, different surface finish (titanium niobium nitride, or ceramic finish), improved polyethylene (Vitamin-E treated), etc.
• These changes are made with a view to
  • improving the fit of the implant to differing body size and shape
  • improving the stability, the bio-mechanics (kinematics) and function of the knee
  • improving the longevity of the implants
• Before considering these it is important to realize that the standard bi-condylar design works well for most patients – 80 percent or more. It has stood the test of time unlike some designs which are recent and do not have long-term follow-up. These newer implants confer certain additional advantages but are not absolutely essential.
• The choice of the implant depends upon the surgeon’s preference and training, the patient factors (age, activity level, expectations, etc) and cost considerations.

Unicompartmental knee replacement:

In some patients only one half of the knee is affected by arthritis (usually inner half). Replacing only the diseased half of the knee can treat this. This is done through a smaller incision and the recovery is faster. The function is also better. However, not all patients are suitable for unicompartmental replacement. The arthritis should be limited to only one side of the joint. Any arthritis affecting the other side of the knee will give rise to pain. The patient should not have severe deformity, contractures or ligament insufficiency. The procedure is not applicable for inflammatory types of arthritis.

Minimally invasive knee replacement:

The usual approach to knee replacement involves splitting the quadriceps muscle in the middle and reflecting the kneecap to one side. Minimally invasive knee replacement is done using an incision that splits a smaller portion (or none) of the muscle. This should lead to a faster recovery. This is generally done for slim patients with good flexibility in the knee. It is difficult in obese patients and those who have stiff knees. The procedure requires special instruments and is best done with computer navigation as there a possibility of component malpositioning. In practice, it is seen that the final result with a standard approach is the same as with minimally invasive approach, although a bit slower in the initial post-operative period.

Custom fit knee replacement:

During knee replacement 8-10 mm of bone is removed from the surface of femur and tibia to accommodate the implants. This is done manually with the help of sizing instruments, alignment guides, cutting jigs (guides) and tensioning devices. The surgeon relies on X-rays, scanograms and manual assessment for executing the procedure.

In custom fit knee replacement, cutting guides are made for the patient before the operation. A 3-D model of the knee is generated using an MRI or CT scan. This is then used to create patient-specific cutting guides for bone resection. Advantages of this technique are: improved accuracy, simplification of the procedure, shorter surgical times and decreased blood loss. The disadvantage is the cost and the time required to generate the cutting guides (about 10 days).

Computer navigated knee replacement and robotic surgery:

The instruments and techniques employed in a joint replacement are very precise and allow a very accurate procedure to be performed. However certain landmarks like the centre of the ankle, and especially the centre of the hip, cannot be determined precisely with these instruments. During minimally invasive surgery even the landmarks in the surgical field itself are less visible.

Surgical navigation systems utilize an array of stereoscopic infra-red cameras, infra-red reflectors (tracking devices) which are placed on the bones and on specially calibrated instruments, and a computer. The computer generates a virtual 3-dimensional model of the bones. The position of the cutting guides and the path of the instruments is displayed. It also gives numerical values of the bony dimensions, the amount and angulation of bone resection and implant sizes. This information is used to guide the placement of implants. It also allows balancing of the ligaments. Simplified navigation systems, which are less complex, are also available. 

The advantages are: improved accuracy, better surgeon feedback during the surgery and decreased blood loss. It is especially useful when there is a pre-existing deformity in the femur or tibia. Computer navigation allows the surgeon to perform minimally invasive surgery. The disadvantages are: increased cost, increase in surgical time and a small extra incision. 

Robotic devices have also been developed to execute the bony part of the procedure with improved accuracy and reduction of blood loss.

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