Understanding knee replacement surgery: why, how and outcomes

Knee replacement surgery is the last course of treatment for severe and/or constant knee pain. If you’re at the point of considering knee replacement surgery, you will likely have already tried using painkillers, physiotherapy and walking aids to ease your symptoms but have reached a point where despite these treatments, your knee pain is severe or worsening. 

You may find that your knee pain prevents you from walking more than a few hundred metres at a time, wakes you up at night and even resting offers no relief. 

At this point, your doctor may refer you to an orthopaedic knee surgeon to determine whether knee replacement surgery is suitable and if so, which type. 

Partial versus total knee replacement surgery

To understand what is replaced during a total or partial knee replacement, it helps to first understand the anatomy of your knee joint.

Anatomy of your knee

Your knee is made of three compartments:

1. Medial compartment or inner side of your knee — this is where your thigh bone (femur) and shinbone (tibia) meet in the area closest to your opposite knee
2. Lateral compartment or outer side of your knee — this is where your femur and tibia meet in the area furthest away from your opposite knee
3. Patellofemoral joint — this is where your femur meets the back of your kneecap (patella) in the front part of your knee 

During a knee replacement surgery, the damaged bony surfaces inside your knee are resurfaced ie damaged bone is replaced with plastic and metal implants. 

Total knee replacement

This refers to resurfacing of the medial and lateral compartments of your knee and sometimes also the patella. It requires cutting through at least one of the four major ligaments in your knee, usually the anterior cruciate ligament (ACL), in order to insert the implants.

This is a major operation and you will need to use crutches for at least six weeks. Your new artificial knee will feel different to your natural knee and may not achieve a full range of movement.

Partial knee replacement

This usually refers to resurfacing of only one compartment of the knee and in most cases, this is the medial compartment. This is because knee arthritis, the leading cause for needing knee replacement surgery, most commonly affects the medial compartment. 

All of your knee ligaments will be left untouched and consequently, partial knee replacement surgery has a faster recovery time than total knee replacement surgery. In most cases, you can expect to come off crutches in six weeks. 

As more of your own tissue is retained and your ligaments are not cut, the movement of your artificial knee may feel more like your natural knee. 

As a partial knee replacement operation is not as major as a total knee replacement operation, the risks of surgery are slightly lower. However, not everyone is suitable for partial knee replacement surgery — your cartilage, ligaments and other knee tissues need to be in good condition. Your orthopaedic surgeon also needs to be trained in the procedure to be able to offer it to you.

Your knee implants

Knee implants are made of metal and plastic. In a total knee replacement, the femur and tibia are resurfaced using metal; usually cobalt chrome for the femur and titanium for the tibia. A piece of plastic (polyethylene cross-linked with gamma radiation to make it very hard-wearing) then joins the resurfaced femur and tibia together, acting as the bearing surface of the joint. 

All implants used in the UK are recorded on the National Joint Registry, which shows how long they’ve been used for and how long they lasted ie when they were removed and replaced with new implants (revision rate). The different types of implants are also independently rated based on their performance against national clinical guidelines.  

On average, the revision rate for knee replacements after 10 years is around 3%, rising to 6–7% after 15 years. 

How long your knee replacement lasts will depend on your individual circumstances, including how much use your knee gets and whether any fractures develop around it. 

Outcomes of knee replacement surgery

Knee replacement surgery, combined with extensive post-surgery physiotherapy, muscle strengthening and rehabilitation, is designed to relieve knee pain and restore function. However, as with any surgery, there are always risks, such as infection, poor healing and tissue damage.

Knee replacement surgery, specifically, comes with a high risk of numbness around your scar and difficulty kneeling on your scar. Fractures can also develop around the implants, which may lead to revision surgery.

One of the most common risks of total knee replacement surgery is that the knee audibly clicks and clunks — this occurs in about one in five cases. Your knee replacement may also not feel normal or become stiff, which will need further rehabilitation to restore full range of movement. In some cases, these symptoms never go away completely.

Advances in knee replacement surgery

During a partial knee replacement, your ligaments are left intact and consequently, the positioning of the implants is bound by the existing properties of your knee. However, in a total knee replacement, the ligaments are cut, allowing your surgeon greater scope in aligning the placement of your implants. Advances have been made in how this alignment is carried out.

Traditionally, the implants are positioned using mechanical alignment, where the tibia is cut perpendicular to the alignment of the knee in a neutral position. The femur is then cut to match it. The neutral position refers to the position of the knee joint for the population average, disregarding variations in anatomy.

However, only 15% of the population have the standard neutral position, with most people being slightly bow-legged or knock-kneed. Mechanical alignment of knee implants doesn’t account for this. This may lead to a poorly balanced knee, which clicks or does not ‘feel right’. However, the development of kinematic or individualised alignment does account for individual anatomy variations.

This refers to checking an individual’s knee alignment before the implants are placed. Using CT scanning technology and robotics, the implants can be better aligned to the surrounding anatomy of the knee. This leads to a perfectly balanced knee, with better functional outcomes for each individual patient.