Knee Prosthesis: How does it work?

The implantation of a knee prosthesis or knee joint prosthesis (synonyms: knee joint endoprosthesis, total knee joint endoprosthesis (KTE, KTEP), total endoprosthesis of the knee (knee TEP), total endoprosthesis (TEP), surface prosthesis, artificial knee joint; artificial knee joint) is a therapeutic surgical procedure in orthopedics, which is used to correct a loss of function or a functional restriction of the knee joint. An artificial knee joint is used in particular in patients diagnosed with osteoarthritis (wear and tear of the joint), which leads to a lack of mobility and is often associated with the presence of a pain stimulus in the knee joint. In addition to osteoarthritis, there are various factors that cause massive damage to the knee joint, so that conservative therapy with the administration of medication to relieve pain or arthroscopy (arthroscopy of the joint with the aid of an endoscope) may not be considered sufficient. Damaging factors of the knee joint may include degenerative osteoarthritis, rheumatoid arthritis (chronic inflammatory multisystem disease that usually manifests itself in the form of synovitis (inflammation of the synovial membrane)), bacterial arthritis, arthritis after an accident, a bone fracture (bone fracture) in the immediate vicinity of the knee joint, a deformation of the knee joint or a malposition of the skeletal apparatus. In addition to the symptoms of pain and loss of mobility, however, a complete stiffening of the knee joint can also occur, depending on the triggering factor. If implantation of a knee joint is indicated as a therapeutic measure, various surgical techniques and prosthesis types can be used. Basically, two types of prosthesis can be distinguished. It is possible to replace the damaged areas of the joint in the form of a partial prosthesis, known as a sled prosthesis, or to replace the entire knee joint by means of a complete prosthesis, known as a “total endoprosthesis” (knee TEP). As a rule, however, the joint surface of the patella (back of the kneecap) is not replaced. Note: In determining the indication for arthroplasty knee replacement, the timing of the surgery is a major factor in the benefit. If surgery is performed too early, the improvements, which may be only mild, must be weighed against the potential complications; if surgery is performed too late, physical mobility may have been limited for some time and additional chronic conditions increase the risk of surgery.

Indications (areas of application)

  • Degenerative osteoarthritis – due to age or stress, damage to the entire knee joint is possible. Osteoarthritis is the most common indication for knee arthroplasty. However, knee prosthesis should not be mentioned as the first therapy of choice, as the therapeutic procedure should only be applied if non-invasive methods are unsuccessful. Furthermore, the patient must have severe invalidating pain (= mobility impairment) with a significant restriction of the quality of life. In principle, at the start of therapy should already have reached the age of 60.
  • Rheumatoid arthritis – this autoimmune disease is based on the lack of self-recognition of the immune system, which results in antibodies attacking and destroying the body’s own structures, so that, for example, inflammation of the joint with cartilage damage can develop.
  • Post-traumatic arthritis – also as a consequence of an accident, it can come due to various factors to a massive joint inflammation (arthritis after accident).
  • Symptomatic knee instability – due to damage to the ligamentous apparatus can significantly increase the risk of injury in the affected patient.
  • Knee stiffness – the stiffening of a joint can have various causes. A few decades ago, the stiffening of a joint represented a common therapeutic method. However, if the stiffening is due to an accident, for example, a reconstruction of the mobility of the joint can be done.
  • Deformities of the knee joint – congenital defects of the position or formation of the knee joint can be corrected by implanting a knee prosthesis.

According to the Sk2 guideline, the following definitions are established for the indication of total knee arthroplasty (knee TEP) [see guidelines below]:

  • Main criteria: these are minimum requirements for indication, which must be fulfilled in normal cases. Possible main criteria for indication for knee TEP are:
    • Gonalgia (knee pain; duration of at least 3 to 6 months; pain occurring intermittently several times a week or continuous pain), evidence of structural damage (osteoarthritis, osteonecrosis; evidence radiologically), failure of conservative therapeutic measures, limitation of quality of life related to the knee joint disease, and subjective distress
  • Ancillary criteria: these may reinforce the recommendation for knee TEP, but are not mandatory for indication. Possible secondary criteria are:
    • Limitation of walking distance and during prolonged standing, instability of the knee joint ECC.
  • Risk factors: these weaken the recommendation for knee TEP because they are associated with an increased complication profile and/or a potentially poor patient-relevant outcome.
  • Absolute contraindications prohibit knee TEP. Absolute contraindications to knee TEP are florid infections in the knee joint.
  • Relative contraindications argue against knee TEP but do not prohibit it in justified cases. Examples of relative contraindications are very high BMI (≥ 40) and significantly reduced life expectancy due to comorbidities (concomitant diseases).

Contraindications

Osteoporosis – the presence of this primarily hormonal condition is a contraindication to knee TEP, as the loss of bone strength increases the risk that the prosthesis could become loose. Moreover, the prosthesis additionally destroys bone tissue.

Before surgery

  • A patient’s need for knee replacement should be determined by the treating physician through both a medical history (doctor-patient discussion) and a precise physical examination. Imaging procedures such as an X-ray examination, sonography, computed tomography (CT; CT knee), or magnetic resonance imaging (MRI; MRI knee) should be performed as necessary to determine the next course of action.
  • With an exact leg measurement including leg balance images, axis corrections are pre-planned and an exact sizing of the prosthesis is made.
  • In suspected cases, an autoimmune disease such as rheumatoid arthritis should be excluded by an antibody determination in the blood or in a biopsy.
  • Before the planned insertion of a knee endoprosthesis, the treating physician should be clear in older patients whether osteoporosis is present. If in doubt, osteodensitometry (bone density measurement) should be performed. The overall risk in patients with osteoporosis for intraoperative and postoperative complications, especially periprosthetic fractures (broken bones), is significantly increased. If necessary, osteoporosis patients with osteoarthritis should receive systemic therapy with bisphosphonates.
  • To reduce a load on the prosthesis after surgery and extend the life and thus the length of stay of the implanted knee prosthesis, the patient should follow a diet before surgery if necessary. However, weight loss is more difficult because the patient is usually no longer capable of physical activity. Due to this, the improvement of fitness status is difficult to achieve.
  • In addition to the preparation and performance of the surgical procedure, according to various scientific studies, the success of the procedure depends on other factors in addition to the patient’s lying time. The better the general condition of the patient, the lower the risk of complications. However, muscular resilience is also an important component in the function of the implanted joint. Targeted muscle building training can minimize the risk of no relevant improvement in joint function. As far as possible, training should be guided by a physiotherapist or a sports medicine specialist.
  • In addition to reducing weight, it is also essential that the attending specialist is informed about both medication and chronic diseases such as diabetes mellitus or cardiovascular disease. The same applies to existing allergies or acute infections.
  • From an infectious disease point of view, it is considered particularly important to minimize the time the patient is lying down before surgery to minimize the risk of infection.
  • In many cases, medications that inhibit blood clotting, such as ASA, must be discontinued before surgery.

Anesthesia (“numbing”) Note: Periarticular (“around the joint”) injection of analgesics (pain medication) has several advantages over peridural analgesia (PDA) in knee TEP surgery (see below): the patient has less postoperative pain, earlier recovery of knee flexion (knee bending), and less nausea. The only malus is relatively frequent transient peroneal paresis/paralysis of the fibular nerve (12% versus 2% with PDA).

The surgical procedure

Implantation of a knee prosthesis is one of the endoprosthetic procedures. As described earlier, different types of prostheses can be distinguished. Knee prostheses are primarily classified according to their degree of coupling. The degree of coupling depends on the loss of functionality of the physiological ligamentous apparatus in the knee joint. The greater the damage to be compensated for, which must be taken over by the implant, the higher the degree of coupling of the prosthesis. For the implantation of a knee joint, regardless of the type of prosthesis, knowledge of the anatomical functional principles is essential, since the implant should maintain physiological functions as naturally as possible. The knee joint itself represents a rolling-sliding joint in which the lower leg rotates around the femur during normal gait. In addition to the rotation, there is also a sliding movement of the bony parts involved. Because of this, the knee kinematics (theory of movement) is complex, which means that the exact preservation of physiological functionality cannot be fully achieved. Classification of the implantation procedures of the different prosthesis types

Partial dentures

  • Medial sled prosthesis – medial sled prosthesis is a relatively gentle procedure, which is characterized by the fact that the intact knee joint components are not removed and replaced. In particular, by preserving the cruciate ligaments, it is possible to almost completely restore physiological knee function. The less invasive implantation of a sled prosthesis leads to at least an equivalent reduction in pain compared to other implantation procedures. Furthermore, the sled prosthesis is characterized by the fact that the range of motion is closer to physiological function than would be possible with implantation of a total surface replacement. In addition, it has been shown in various studies that, among other things, as a result of the lower blood loss, a reduction in the risk of both intraoperative and postoperative complications (during and after surgery) can be achieved. Thus, blood transfusion, which is also associated with risks, must also be performed less frequently. In addition, the rehabilitation phase is relevantly shorter than with knee TEP. However, a decisive disadvantage of the procedure is that the medium- to long-term revision rates are generally higher than for total joint replacement. The correct execution of the surgical technique is of decisive importance for the success of the therapy. Only if absolutely precise work is performed can recovery of physiological joint function be achieved. However, if this is the case, the synergy of preserved joint compartments and newly inserted components can be optimized.
  • Uniknie – in the case of complete preservation of all ligaments of the knee joint especially the cruciate ligaments, this procedure is used. In the presence of damage to one of the two condyles (bony component of the knee joint) and the functionality of the ligaments, this procedure is a less invasive method of maintaining mobility.
  • Bicondylar primary prosthesis – this procedure can be used if the condition is that the anterior cruciate ligament is no longer intact, but the other ligaments are adequately functional. The basic principle of the procedure is the replacement of the articular surfaces of both the femur (thigh bone) and the tibia (shin bone). In addition, the menisci, which are also part of the joint, must be removed. Depending on the implantation system, it is possible to use the procedure even with an intact anterior cruciate ligament without causing damage.
  • Posterior stabilized prosthesis – in the absence of preservation of the function of the posterior and anterior cruciate ligaments, the posterior stabilized prosthesis can be implanted. The principle of the procedure is based on the property of the prosthesis to take over the functions of the cruciate ligaments, causing the tibia to slide forward with increasing flexion or the femur to slide backward.

Full prostheses

  • Knee TEP – in the use of total prosthesis, the removal of the joint surfaces involved is performed by surgically removing the entire joint of the femur and tibia and subsequently renewed. The simplest procedure of total knee arthroplasty is implantation of a surface prosthesis. The procedure involves removal of the damaged cartilaginous surfaces of the tibia and femur, but also involves removal of parts of the surface of the bony component of the knee joint. The resulting exposed bone surfaces can be appropriately shaped to fit the prosthesis to ensure an optimal fit. Only after adaptation is complete is the prosthesis attached to the two bones. Because the prosthesis is anchored in both bones, the risk of loosening of the implant is lower than with the medial sled prosthesis, for example. Nevertheless, complete prevention of loosening is not possible with any prosthesis model.

See also under “Further notes”: “Meta-analysis due todecision to use partial or full knee prosthesis”.

After surgery

After surgery, with the assistance of a physical therapist, the patient should be mobilized immediately with full weight bearing on the operated knee. Postoperative pain and swelling are very common, so pain-relieving therapy is necessary. In addition, light exercise loading of the prosthesis should be started as early as possible. Training can also lower the weight, which can significantly reduce the load on the prosthesis later on and thus prolong the time the prosthesis remains in place. For physical and drug prophylaxis of venous thromboembolism (VTE), see below Pulmonary Embolism/Prevention/Prophylaxis of Venous Thromboembolism (VTE). Note: According to a retrospective cohort study, acetylsalicylic acid (ASA) is equivalent (1.16% versus 1.42%) to anticoagulants (anticoagulants) in thromboembolism prophylaxis: adjusted odds ratio 0.85 with a 95% confidence interval of 0.68 to 1.07. A meta-analysis of more than 6,000 patients confirms that oral administration of acetylsalicylic acid is sufficient to effectively prevent leg vein thrombosis and pulmonary embolism. To reduce postoperative pain, nonpharmacologic treatments such as electrotherapy and acupuncture have been effective in saving the opioid dose. Electrotherapy decreased opioid dose by a mean of 3.50 morphine equivalents in milligrams per kilogram over 48 hours; acupuncture delayed the time to first opioid administration (patient-controlled analgesia) by a mean of 46.17 minutes. Cryotherapy and physical therapy resulted in only modest pain relief. When a passive motion splint (CPM splint; continuous passive motion) is used in addition to physical therapy for passive (motor-driven) motion of the artificial knee joint, it increases the range of motion.

Possible complications

  • Anesthesia – the procedure is performed under general anesthesia or after spinal anesthesia is performed, resulting in various risks. General anesthesia can cause nausea and vomiting, dental damage, and possibly cardiac arrhythmias, among other risks. Circulatory instability is also a feared complication of general anesthesia. Nevertheless, general anesthesia can be considered a procedure with few complications. Spinal anesthesia is also relatively low in complications, but complications can occur with this method as well. Injury to tissue, such as nerve fibers, could lead to a long-lasting impairment of quality of life.
  • Infections – the likelihood of bacterial infections occurring depends on several factors, such as preoperative bed length and age. Infections can cause widespread complications, including sepsis (blood poisoning). Active smokers are more likely to experience wound complications. Deep wound infections occurred twice as often in smokers.
  • Blood loss – despite relatively gentle surgical techniques, there is a risk of having to compensate for relatively severe blood loss.
  • Swelling
  • Pain – approximately 20% of patients complain of persistent discomfort after surgery: possible causes: Instability or periprosthetic infection (Note: knee arthroplasty is always required if periprosthetic infection is suspected).
  • Myocardial infarction (heart attack)-in the first postoperative month after surgery, the risk of infarction was higher by a factor of 8.75; it was increased throughout the first six months after total knee arthroplasty, after which the difference with the control group disappeared
  • Patella fracture (kneecap fracture) – in patients with severe genua vara (bow leg) and knee arthroplasty (knee joint prosthesis); causes: due tocorrection of axial position and/or possible devascularization of the patella (kneecap) during soft tissue mobilization with resection of the fat body.
  • Mortality (mortality rate) 0.25%; with partial prosthesis mortality is 68% lower.

Further notes

  • The knee TEP group was shown to have a significant 7% lower risk of cardiovascular events.
  • 8 out of 10 knee replacements today have a durability of ≥ 25 years.
  • Meta-analysis due todecision to a partial or total knee prosthesis: a partial knee prosthesis is more advantageous with regard to the length of hospital stay, complication rate or mortality (death rate); revision surgeries are significantly less frequent after a total replacement.
  • Patients with isolated medial gonarthrosis showed no difference in clinical outcome (based on the Oxford Knee Score) after 5 years regardless of prosthesis type (partial or total). However, patient satisfaction showed an advantage for the partial arthroplasty.