In the majority of cases, gram-positive germs(Staphylococcus aureus and epidermidis) are the cause of prosthesis infection. However, germs from the gram-negative spectrum(E. coli and Pseudomonas aeruginosa) can also cause prosthesis infections in rare cases.
There is an increased risk of developing an infection, especially in patients with
previous hip operations on the same side
in their medical history.
There can be various reasons for changing a knee prosthesis that has already been implanted © RFBSIP | AdobeStock
A prosthesis infection is monitored via certain inflammatory parameters in the blood. These include the laboratory values for
- Leukocytes,
- C-reactive protein and
- the erythrocyte sedimentation rate.
If patients already have an additional inflammatory systemic disease, the inflammatory parameters in the blood must be evaluated accordingly by the doctor.
Further diagnostic clarification is carried out in the form of
with subsequent microbiological examination.
The treatment strategy for knee prosthesis replacement depends on
- type,
- duration and
- and severity of the prosthesis infection and the
- the physical constitution of the patient.
Treatment options are
- lavage of the joint with head and inlay replacement (up to 2 weeks for early infections),
- a unilateral prosthesis change (for mono-infections with a simple germ spectrum and a good bony bed),
- or a two-sided change (temporary insertion of an antibiotic-containing placeholder made of bone cement).
Regardless of the implantation procedure (cemented/uncemented), the knee prosthesis is usually replaced on one side in the event of aseptic loosening of the prosthesis.
Instability with pain that has existed since the operation is usually due to inadequate soft tissue balancing. In a large number of cases, it is combined with malrotation of the knee prosthesis.
If pain occurs due to instability in the knee joint, conservative treatment can initially help. This includes
- Physiotherapy,
- phonophoresis and
- infiltrations.
A large number of pain patients with mild instability can be successfully treated conservatively.
Patients whose symptoms are not alleviated usually require surgical intervention. The aim of the operation is to restore the soft tissue balance.
This can be achieved using various release techniques or by switching to prosthesis systems that either partially (partially coupled) or completely (coupled) take over the function of the collateral ligaments.
Ultrasound is one of the diagnostic tools used to assess the soft tissue structures in the knee © New Africa | AdobeStock
Periprosthetic fractures can be caused by
- external force,
- in the case of aseptic loosening, but also
- directly during the operation
occur during surgery. The aim here is to restore the anatomical conditions and bony continuity. In some cases, this can be achieved by preserving the prosthesis using osteosynthesis procedures (locking plate, intramedullary nail).
In most cases, however, it is necessary to change the knee prosthesis to a stemmed (steeled) system. This must bridge the fracture and allow the force to be applied distally (tibial) or proximally (femoral) to the fracture gap.
The use of existing access points and the restoration of the individual layers are also decisive measures when replacing a knee prosthesis.
After removal of the prosthesis components, there are usually extensive tibial and femoral bone defects. These are divided into non-circumferential (small-medium) and circumferential (large) defects. It is therefore possible with most revision prostheses to fit additional augmentation blocks individually depending on the defect.
Larger defects can also be filled with allogenic bone grafts . Due to larger bone defects with a lack of anchoring options for the prosthesis, a stemmed prosthesis can be used. This stem ensures that the load is distributed to the femoral or tibial bone diaphysis.
In the case of pronounced defects with massive loss of substance in the area of the distal femur or proximal tibia, so-called tumor prostheses are also used. These replace the deficient bone parts prosthetically.
When reconstructing knee joints using a revision prosthesis, the following should be taken into account:
- Restoration of the original joint line
- Balanced ratio between flexion and extension gap
- Avoidance of femoral and tibial malrotation
- Soft tissue balancing/alignment
