What Causes Knee Replacement Implants to Fail?

The main causes of knee implant failure are wear and loosening, infection, instability, leg fracture or stiffness.

Five reasons why knee replacements fail

Wear and loosening

A properly functioning implant depends on their proper fixation to the bone. Fixation is usually achieved by gluing the implant to the bone. Some surgeons prefer to use biological fixation, which means that no cement is placed between the implant and the bone.

Although the knee implants are held firmly in place during the initial knee replacement surgery, they may become loose over time. The cause of loosening may not always be clear, but high impact activity, excess weight and wear and tear of the polyethylene components may all be contributing factors.

Friction caused by the joint surfaces rubbing against each other wears away the surface of the implant, creating a build-up of tiny particles around the joint. In a process known as aseptic (non-infected) loosening, the bond between the implant and the bone is broken as the body tries to digest these wear particles. During this process, normal, healthy bone is also digested (a condition known as osteolysis), which can weaken or even fracture the bone.

When the knee prosthesis loosens, the patient may experience pain, alignment changes or instability.

Four Bar Pneumatic Knee Joint

Infection

Using current surgical techniques and antibiotic regimens, the risk of infection in total knee arthroplasty is less than 1%. However, when it does occur, infection is a devastating complication of any surgical procedure. In total knee arthroplasty, large metal and plastic implants can act as surfaces for bacteria to attach - a process known as biofilm formation. The location of these bacteria makes them unavailable for antibiotics. Even if the implant remains well fixed, revision surgery is required to stop the pain, swelling and drainage caused by infection.

Revision surgery of an infected knee can take many forms. Depending on the extent of the infection and damage, the surgeon will determine one of two basic courses of action.

Simple irrigation of the knee joint while leaving the original prosthetic component in place

Complete replacement of the new implant during total knee revision surgery

Depending on the severity of the infection and the virulence of the bacteria, a complete revision of the infection will involve either a one-stage or two-stage procedure. In a one-stage procedure, the previously infected component is removed as part of a complete debridement, the knee is thoroughly cleaned and the new revision component is placed in one surgical setting (i.e. on the same day).

In the two-stage surgery, two separate procedures are performed several weeks apart: the first procedure involves the removal of the old prosthesis and the insertion of a piece of bone cement containing antibiotics (called an antibiotic-impregnated bone cement spacer). The second procedure involves the removal of the spacer and the insertion of a new prosthesis. Intravenous antibiotics are often given as one of the two options for eradicating the infection.

Single Axis Prosthetic Knee Joint

Instability

Instability occurs when the soft tissue structures surrounding the knee are unable to provide the stability required for adequate function during standing or walking. Instability may be caused by increased soft tissue laxity (loosening) or improper positioning or alignment of the prosthesis. Soft tissues, including the ligaments around the knee, may stretch after surgery and no longer provide the support needed for the knee. Pain and/or the feeling of the knee "disappearing" may affect knee function. If these symptoms cannot be successfully treated by non-surgical means (such as the use of braces or physiotherapy), revision surgery may be required.

Leg fractures

The type and extent of the fracture will determine whether revision surgery is required. Peri-prosthetic fractures that destabilise fixation or implants (fractures around knee implants) may require revision surgery.

Rigidity

In some cases, a total knee replacement may not allow you to achieve the range of motion required to perform daily activities. This can happen if too much scar tissue builds up around the knee, preventing the joint from moving fully. When this happens, some patients may benefit from knee manipulation under anaesthesia. During this procedure, the patient returns to the operating room and the surgeon uses his hands to bend and straighten the knee over the body to help restore flexibility.

If excessive scar tissue has formed around the knee and movement is significantly impaired and you have not had much success with the operation, revision surgery may be considered to allow you to regain movement for daily activities.