Knee Replacement Loosening: Information & Specialists in Knee Replacement Loosening

In Germany, approximately 190,000 artificial knee joints (also known as knee endoprostheses or total knee replacements) were implanted in 2019. Most artificial knee joints last at least ten to 15 years.

However, it can happen that the anchoring of individual components of a knee prosthesis in the bone weakens. This leads to what is known as knee prosthesis loosening.

Early loosening of a knee endoprosthesis is defined as loosening that occurs within the first two years after knee TEP surgery.

Twenty years ago, about eight percent of knee replacements had to be replaced (revision surgery or replacement surgery). In 2017, that figure was about six percent.

Knee prosthesis loosening can generally be divided into two types.

• Septic knee prosthesis loosening: A bacterial infection (= septic, sepsis) in the area of the implant leads to loosening of the prosthesis.
• Aseptic knee prosthesis loosening: All forms of loosening not caused by a bacterial infection (= aseptic).

Septic knee prosthesis loosening primarily occurs in the weeks and months following total knee replacement (TKR) surgery. However, it can also occur years later. Aseptic loosening is particularly common in knee prostheses that have been in place for more than two years.

Typically, patients experience no or only mild knee discomfort at the onset of knee prosthesis loosening. Occasionally, however, knee pain may also begin suddenly.

Over time, the pain—which is usually activity-related—increases, and the maximum distance the patient can walk decreases. The pain can also be triggered by force applied to the knee, such as

• a twisting motion in the knee or
• a blow or fall to the knee.

Some patients also experience localized pain at the site where the implant is attached to the bone. This is often on the shinbone below the knee, for example.

A swollen knee or fluid buildup in the knee joint may also occur.

If the cause is septic, the knee may be red and warm to the touch. In some cases, pus may ooze from the scar, which may be splitting open. A fever with chills may also develop.

As loosening and symptoms progress, mobility in the knee joint is reduced. In some cases, the patient may limp, and climbing stairs becomes significantly more difficult.

In later stages, pain occurs even at rest. The risk of bone fractures in the area of the prosthesis increases with severe loosening.

In addition, knee deformities (bowlegs, knock-knees) may develop.

Possible causes of knee prosthesis loosening include, for example:

• Bacterial infections that develop due to the introduction of bacteria.
• Knee prostheses are made of metal alloys and polyethylene. The wear and tear of polyethylene can cause the bone in the area of the prosthesis to erode, both directly and indirectly. This is also known as particle-related disease or wear-induced osteolysis.
• Arthrofibrosis, which is the excessive formation of connective tissue cells (scarring) in the knee joint following total knee replacement surgery
• Misalignment or improper positioning of individual components of the prosthesis
• Loosening of the prosthesis fixation
• Knee joint instability due to insufficient function of the extensor mechanism
• Periprosthetic fractures, i.e., bone fractures in the area of the prosthesis
• Damage or injury to the patella or the patellar prosthesis

Risk factors for septic loosening of a knee prosthesis

There are various circumstances in which the immune system either no longer functions properly or is suppressed. An immune system that is not functioning adequately can no longer fight off invading bacteria as effectively. This makes it easier for pathogens to multiply and cause an infection in the prosthesis.

Risk factors for septic knee prosthesis loosening include, for example:

• Immune deficiencies
• Diabetes mellitus—people with diabetes have a 10-fold increased risk of prosthesis infection compared to those without diabetes; overweight people with diabetes have an even higher risk—20 times greater
• Rheumatoid arthritis
• Chronic inflammatory skin disease
• Cancer
• Coagulation disorders
• Use of corticosteroids or anticoagulants
• Advanced age
• Severe obesity
• Tobacco, alcohol, and drug abuse

Risk factors for aseptic loosening of a knee prosthesis

• Advanced age
• Severe obesity
• Problems with the prosthesis material, the bone cement, or the technique used to implant and secure the prosthesis. This can occur with both metal implants and those made of polyethylene
• Limited experience of the surgeon in implanting the artificial knee joint, such as selecting an implant that is too large or too small, failing to properly adjust the stability, or cementing it incorrectly
• Bone already compromised by osteoporosis or osteonecrosis
• Pre-existing conditions (diabetes, gout, rheumatism, osteoarthritis, etc.)
• Traumatic events, such as an accident or a fall

Osteoporosis (insufficient bone density, as shown on the right) can be a reason for knee revision © crevis | AdobeStock

Knee pain eventually leads those affected to see a doctor. Based on the symptoms, gait pattern, and medical history, it is often suspected that the symptoms are attributable to the knee prosthesis. The doctor asks the patient, for example,

• when the total knee replacement (TKR) surgery took place,
• what pre-existing conditions the patient has, and
• whether any traumatic events have occurred.

To rule out a septic cause, a complete blood count is performed and the CRP level in the blood is measured. Fluid is extracted from the joint via joint aspiration and examined in the laboratory. If necessary, a tissue sample from the area around the prosthesis can provide further insights.

The most important imaging method is an X-ray of the knee. When evaluating the X-ray, the doctor looks for typical signs of loosening. To better identify changes, X-rays taken at different times are compared with one another.

Signs of prosthesis loosening include, for example,

• cracks in the cement,
• bone fractures,
• implant fractures,
• changes in the prosthesis’s position,
• bone changes,
• wear of the sliding surfaces, and
• changes in the axis.

Computed tomography (CT) and bone scintigraphy can also provide important clues as to the cause of the symptoms.

If a knee prosthesis has become loose, a knee revision (surgery to replace the knee prosthesis) must be performed. This means that the prosthesis is removed and replaced with a new one (known as a revision prosthesis).

The removal (called explantation) of the old prosthesis and the insertion of the new one usually take place during a single surgical procedure. This is referred to as a one-stage revision surgery.

This is a complex surgical procedure. Therefore, you should only have the procedure performed by experts in orthopedics and trauma surgery with extensive experience in knee replacement surgery.

The goal of knee revision surgery is to

• restore stability,
• relieve symptoms, and
• regain full range of motion in the knee joint.

However, if the cause of the loosening is a bacterial infection, treatment becomes significantly more complex. In such cases, a single surgery is often insufficient; at least two surgical procedures are required. This approach is therefore referred to as a two-stage revision surgery.

During the first surgery,

• the prosthesis is removed,
• the infected tissue is cleaned and, if necessary, removed, and
• antibiotic therapy is initiated.

Only once the infection has completely healed can the new knee prosthesis be implanted. The initial goal of treatment is to eliminate the infection. Stability, mobility, and freedom from symptoms are achieved only in the second step.

The implantation of a revision knee prosthesis following aseptic loosening is more complex than the initial total knee replacement (TKR) procedure. In principle, however, the procedures are comparable, including with regard to the subsequent risk factors for renewed loosening (see above).

Thanks to advances in prosthetic technology and material properties, however, the artificial knee joint can now be expected to last longer than was previously the case.

If the infection associated with septic prosthesis loosening can be completely eradicated, the prognosis is also good.

If, on the other hand, the infection keeps flaring up, this may result in further surgical procedures. In some cases, an amputation may even be necessary to prevent the infection from spreading.

Above all, it is important that you have knee surgery performed only by proven experts in knee arthroplasty. As the surgeon and the hospital gain more experience, the risk of subsequent knee prosthesis loosening decreases.

Otherwise, you should avoid activities that put strain on your knees or only engage in them after consulting your doctor.

Since being severely overweight is also a risk factor for prosthesis loosening, you should avoid being overweight.