Joint wear and tear(osteoarthritis) caused by permanent incorrect and excessive strain destroys the physiological (healthy) joint function. Joint wear and tear is the progressive abrasion of the cartilage surfaces on the bony joint partners over time. This leads to the breakdown of cartilage cells and inflammatory reactions, resulting in pain.
The video shows the progression of osteoarthritis and its effects on the joint:
If the disease is at an advanced stage and can no longer be treated satisfactorily with conservative therapy measures, the only thing that usually helps is the use of an artificial knee joint - a knee endoprosthesis. In this way, doctors can effectively counteract pain and limited mobility.
The type of endoprosthesis used in a knee joint affected by osteoarthritis depends on the degree of knee osteoarthritis and the areas of the knee joint affected. A total knee replacement is not always necessary. It is often sufficient to replace only parts of the joint.
One of the following prosthesis systems is used as a knee prosthesis:
- Sled prosthesis: For isolated damage to the inner (common) or outer (rare) area of the knee joint.
- Posterior patellar replacement: For isolated retropatellar arthrosis (wear under the kneecap)
- Knee TEP (total surface replacement )
- Unlinked knee TEP (surface replacement): Complete replacement of all parts of the joint. If several areas of the knee joint are worn but the knee ligaments are intact.
- Partially coupled knee TEP: Total endoprosthesis in cases of malalignment with moderate imbalance of the ligamentous apparatus or cruciate ligament damage The endoprosthesis simultaneously ensures part of the ligamentous function. Only partial stabilization of the joint and support of the existing collateral ligament structures.
- Coupled knee TEP: total endoprosthesis in cases of massive ligament instability and significant axial misalignment. In this type of prosthesis, the tibial plateau (shin bone) and femoral part (thigh bone) are firmly connected to each other via a coupling mechanism.
The individual knee TEP models also differ in a number of other parameters, such as
- Material (titanium and its alloys, cobalt-based alloys, steel)
- Coating (titanium/aluminum and ceramic oxide)
- Surface structure (micro-/macro-structuring)
- Cemented vs. cement-free
- Shafted vs. non-shafted
The principle of the endoprosthesis of the knee joint is shown in the video:
A knee TEP can be implanted using either the cementless or cemented technique. However, the vast majority of knee TEP implants are cemented. Cementation is the use of an additional anchoring material in the form of a synthetic material, the bone cement, which firmly anchors the new joint components in the bone.
Cementless knee TEP
In a cementless knee TEP, the prosthesis bearing is fixed in the bone using a press-fit or form-fit. This "clamping" of the knee TEP leads to primary stability. Secondary ingrowth of cancellous bone structures is then possible. Immediate loading of the knee TEP is usually still possible.
With this type of anchoring, a healthy bone matrix is a prerequisite for good long-term results. For this reason, osteoporosis and metabolic bone disorders of any kind are considered contraindications. This means that the knee TEP cannot be implanted without cement in these cases.
One advantage of cementless knee TEP can be the easier removal of the endoprosthesis if this is necessary for a prosthesis replacement. Less bone substance is lost during removal.
Nevertheless, the altered loading conditions can lead to the breakdown of bone matrix (lyses). This particularly affects the area of the femur that is located underneath the implant.
Cemented knee TEP
Here, a plastic (polymethylmetacrylate) serves as an interface between the bone matrix and the knee prosthesis. This implantation technique is widely used. It can also be used in patients for whom a cementless endoprosthesis is not possible due to reduced bone quality/density.
After implantation, the total knee endoprosthesis can always be fully loaded.
Hybrid endoprosthetics
This combines the advantages of both implantation techniques. In most cases, the tibial part (shin bone) is cemented and the femoral part (thigh bone) is implanted without cement.
Postoperative full weight-bearing is also possible here.
X-ray image of a knee TEP © LittleSteven65 / Fotolia
The knee joint can be accessed in several ways as standard. The patient is in the supine position during the procedure.
The incision is usually made anteriomedially (i.e. from the front towards the inside of the leg) over the kneecap (patella). The knee joint capsule can be opened in various standard ways.
The parapatellar approach (next to the kneecap) offers an excellent surgical overview of all three compartments.
One disadvantage of this approach is that the vastus medialis muscle is disrupted by the incision. The procedure can therefore be associated with a weakening of the muscle.
The so-called midvastus approach offers an almost equally good overview. However, bleeding with hematomas can occur due to the incision in the area of the vastus medialis.
With the subvastus approach, the medial vastus is not or only slightly traumatized. However, this restricts the view of the surgical site during the procedure.
Minimally invasive implantation procedures, similar to hip endoprosthetics, are also possible on the knee joint. They mainly spare soft tissue structures such as the vastus medialis muscle and the tendon of the quadriceps femoris muscle. These techniques are regularly used for partial knee joint replacements in particular.
They therefore make a decisive contribution to mobilizing patients more quickly after surgery and preventing muscular insufficiencies.
Around 7 percent of all patients undergo another operation on their knee in the first ten years after their artificial joint has been fitted.
The causes of revision operations for a knee TEP are varied. Sometimes infections and malfunctions of the artificial joint become noticeable. If the joint continues to hurt after the implantation of a knee TEP, the causes must be carefully investigated.
It is not always possible to find a cause. This includes, for example, "arthrofibrosis" - a painful thickening and shrinkage of the joint capsule. It is thought to be caused by excessive scarring within the joint capsule. It leads to a permanent functional impairment of the joint. The reasons for this complication are still largely unknown.
Signs of wear on the artificial joint and loosening of the prosthesis usually only occur much later after the initial operation. They can usually be diagnosed.
Planning a knee prosthesis replacement operation
The first step is a detailed clinical examination to assess
- Joint function,
- ligament stability,
- muscle function,
- scar situation and the
- tendency to swell.
X-ray checks check the fit of the knee TEP. The position of the prosthesis can also be checked using special images.
It is very important to rule out an infection in the knee. In addition to blood tests , punctures of the joint are essential for diagnosis. Sometimes it is also necessary to take tissue samples.
Revision surgery should only be performed once the causes have been clarified. Furthermore, a revision strategy should be determined. Here are three typical examples of the replacement of a knee TEP:
- Replacement of the polyethylene inlay (running surface of the joint) due to wear/abrasion or slight instability
- Replacement of the posterior surface of the patella as a second procedure due to persistent anterior knee pain
- Replacement of the artificial joint due to infection or uncontrollable malfunction/ligament instability, loosening or malpositioning (one or two-stage)
Prosthesis models for revision knee surgery
Revision endoprostheses are usually modular in design. They consist of a large number of individual parts that can be combined in a variety of ways. This allows them to be adapted very well to the patient's individual situation.
The experience of the clinic and the surgeon is of great importance for a successful revision. Preoperative planning is essential, and possible complications must also be considered.
Specialist orthopaedic clinics or endoprosthetics centers have the necessary experience and the appropriate special instruments.
Follow-up treatment after knee prosthesis replacement surgery
The aim of revision surgery is for the patient to be able to bear full weight and move their joint again quickly. Only in exceptional situations does the patient have to adhere to restrictions in this regard for a certain period of time.
Individually tailored pain therapy enables gentle and rapid mobilization.
Close, interdisciplinary cooperation between physiotherapists, anesthetists and surgeons is crucial to the success of the operation. Overall, a similar aftercare concept is followed as for the initial operation.
Results of revision knee surgery
Before revision surgery, the specific problem should be discussed in detail with each patient and the objectives and expectations should be compared. The risk of complications increases with each revision operation. The indication for revision knee surgery should therefore be made with great care.
It is advisable to consult an experienced arthroplasty specialist.
