Knee prosthesis
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Which surgical methods are there for a knee prosthesis? Which type of prosthesis are recommended and how high are the chances of healing? Here you will find information on the implantation of a knee prosthesis.

A knee joint supports a human for their whole life. It connects the thigh and the lower leg with a small round bone plate, the kneecap. Thereby, the cartilage layer reduces the friction between the bones like a bumper.

When the cartilaginous mass wears down over the years, joint wear (gonarthrosis) occurs. According to the World Health Organization (WHO), it occurs in 3,6% of the population and is thus among the most frequent signs of wear and tear of the musculoskeletal system. Women suffer almost twice as often from gonarthrosis than men. Pain-free movement is not possible anymore due to the cartilage damage. In this case, a knee prosthesis can make sense.

When is a knee prosthesis necessary?

If the pain is persistent, at first conservative therapy approaches will be taken. These include physiotherapy, medication for pain, the injection of a lubricating substance into the joint and weight reduction.

However, in the case of advanced arthrosis, the pressure on the bones increases. The bones form densifications and hardenings, which has a negative impact on the mobility of the knee joint. If these changes are also visible on the x-rays, an artificial knee joint as a replacement for the worn-down joint surface is worth considering.

Which knee prostheses are the best?

In Germany, more than 160,000 artificial knee joints are implanted every year. In most cases, the prostheses are made from the metal alloy chrome-cobalt-molybdenum and a gliding surface made from the synthetic material polyethylene. For the possibilities of surgery, a distinction is made between joint-preserving surgery and joint replacement.

For a partial joint-preserving endoprosthesis, only the damaged joint surfaces of the thighbone and the shin bone facing each other are replaced. These prostheses are also called unicompartmental knee replacement or unicondylar prosthesis.

The joint-replacing total endoprosthesis (also called total knee replacement) is a replacement for the whole joint surface of the thighbone and the tibial plateau.

Depending on the intact parts of the knee joint, a non-hinged, partially hinged or hinged prosthesis can be implanted. Thus, the intervention can be tailored individually to the patient. This is important because the extent of the arthritic disease differs from patient to patient.

Preparation for surgery

If a knee prosthesis is recommended to you, the first step is a consultation and preliminary examinations with your doctor. Questions about your general health, tolerance of anesthesia, allergies to metal compounds or medicine and other medicinal products are clarified. In addition, the conduct of several imaging processes are ordered (X-ray or MRI). On the basis of recent images of your knee joint, the choice of the implant is made.

Doctors also recommend to stop smoking several weeks before the surgery, which allows the wound to heal better. Physical activity which is easy on the joints can also strengthen your circulation and thus the flow of blood in the tissue.

You should  pack your bags for the hospital accordingly: comfortable track pants, flat shoes (without laces) as well as a backpack as you will need underarm crutches or a wheelchair from the hospital after surgery.

Also try to adapt your living environment to your situation: a small supply of food, a toilet seat lifter and removing potential risks of slipping and stumbling can make your everyday life after surgery easier.

Conventional vs. minimally-invasive surgical method

An artificial knee joint can be implemented with two different surgical methods.

The conventional or classic surgical method implies the complete opening of the knee joint through an incision on the front of the knee. Then, the surgeons open the muscles over the knee in order to access the kneecap. The kneecap can be folded away. The injured soft parts such as menisci and the surface of bones or cartilage can be removed. The remaining bones are smoothed out with a surgical file and prepared for the implantation of the artificial knee joint. The doctors determine the individual size and form of the final knee prosthesis with a trial prosthesis. Depending on the type of implant, it is fixed with or without cement. After testing the functionality and stability of the implant, the open tissue is closed with a surgical suture.

The minimally-invasive surgical method has a lower impact on the tissue, because the kneecap and the surrounding muscles are not affected. The doctors implant the prosthesis through an already existing gap within the muscle layers. Thus, in comparison to the conventional surgical method, less muscle weaknesses occur after surgery and the mobility also maintained after the intervention. Moreover, the small incisions in the skin heal quickly and leave considerably smaller scars.

However, this surgical method is not suitable for every patient. For example, obese patients have a thicker layer of fat under the skin. Thus, it is complicated for the surgeon to precisely place the implant. Furthermore, not every knee prosthesis can be implanted with the minimally-invasive method. It is important to choose a clinic with experienced doctors who have already implanted a lot of knee prostheses.

Risks of surgery

Nowadays, the implantation of a knee joint is a routine surgery which can entail the same complications as any other surgical procedure. Possible risks are:

  • bacterial infection of the surgical wound
  • injured nerves and blood vessels (which can lead to more severe loss of blood)
  • pain, swelling or reddening
  • circulation problems, cardiac arrhythmia, nausea, vomiting after general anesthetic

Specific complications of a knee prosthesis surgery are:

  • bacterial infection due to the implant, in the worst case leading to blood poisoning. You get antibiotics before and after surgery for prevention.
  • if there is a lack of exercise after the operation, adhesion of the implant can occur
  • deep vein thrombosis or pulmonary embolism, you receive anticoagulants for two weeks (pills or injections)
  • pain in the surrounding muscles and restricted mobility
  • feeling of instability in the joint due to loosening of the prosthesis

What happens after surgery?

On the day of the operation, early mobilization training starts so you can get back to your daily life as quickly as possible.

The physiotherapeutic treatment plan will give you more freedom to move every week. At first, you will do minimal bending and stretching movements. To prevent you from putting too much weight on the knee, you will get instructions on how to get into an upright position from a lying position without stressing the knee or how to dress yourself with the help of underarm crutches.

Patients with a partial endoprosthesis can already regain their mobility with the help of outpatient physiotherapy. After the implantation of a full endoprosthesis, the patient stays in an orthopedic rehabilitation clinic for up to three weeks. The same applies to both cases: Take your time to get used to the implant.

The stitches are usually removed 15 days after surgery. By taking part in physiotherapy, you can already put weight on your muscles after six weeks.

Living with a knee prosthesis

The mobility of the knee joint after the implantation depends on different factors. The anatomical range of motion starts at 0 degrees when the knee is straight to a maximum of 130 degrees when it is bent. After the implantation of a full endoprosthesis, the average range of motion is between 0 and 100 or 110 degrees. 100 degrees is perfectly sufficient for most activities.

The aftercare-checkups are very important. Post-operative complications such as the loss of functionality or specific pain should be documented by doctors.  In the case of an early loosening of the implant, a change of prosthesis (knee revision) has to be done.

However, clinical studies show that the revision rate for total knee prostheses are below 5% within 10 years and an improvement of functions of 30-51% is achieved. This means that 95 % of all patients with a knee prosthesis use this for up to ten years. Around 80% are satisfied with their knee prosthesis in the long term.

Sources: 

Aldinger P., Clarius M., Herre J., Martin J. (2015) Welche Knieprothese ist für mich die richtige?. In: Künstliches Kniegelenk. Urban und Vogel, Munich. https://doi.org/10.1007/978-3-89935-284-9_5 . Zuletzt abgerufen am 29.04.2021.

Institut für Qualität und Wirtschaftlichkeit im Gesundheitswesen (IQWiG). Kniearthrose (Gonarthrose). Gelenkersatz bei Kniearthrose. https://www.gesundheitsinformation.de/gelenkersatz-bei-kniearthrose.html#Wie-gut-hilft-ein-künstliches-Kniegelenk? . Zuletzt abgerufen am 29.04.2021.

Institut für Qualität und Wirtschaftlichkeit im Gesundheitswesen (IQWiG). Kniearthrose (Gonarthrose). Was erwartet mich vor und nach der Operation?. https://www.gesundheitsinformation.de/was-erwartet-mich-vor-und-nach-der-operation.html . Zuletzt abgerufen am 29.04.2021.

Institut für Qualität und Wirtschaftlichkeit im Gesundheitswesen (IQWiG). Kniearthrose (Gonarthrose). Worin unterscheiden sich Knieprothesen?. https://www.gesundheitsinformation.de/worin-unterscheiden-sich-knieprothesen.html . Zuletzt abgerufen am 29.04.2021.

Jansen, K.; Jensen, J. Steen. Operative technique in knee disarticulation, Prosthetics and Orthotics International: August 1983 – Volume 7 – Issue 2 – p 72-74. https://www.researchgate.net/publication/305367262_Operative_technique_in_knee_disarticulation . Zuletzt abgerufen am 29.04.2021

Lüring, C. Welche Aspekte sprechen für die Knieprothese bei der Gonarthrose?. Trauma Berufskrankh 18, 226–230 (2016). https://doi.org/10.1007/s10039-015-0074-9 . Zuletzt abgerufen am 29.04.2021.