Knee arthroplasty has advanced enormously over the past few decades. Patients with severe osteoarthritis or other degenerative diseases of the knee joint benefit from modern knee prostheses (total knee replacement), which alleviate pain and improve mobility. A central aspect of implanting an artificial knee joint is the correction of misalignments.

Traditionally, a neutral mechanical axis was aimed for to ensure even loading of the knee prosthesis. However, newer approaches increasingly question this practice and favor concepts such as kinematic alignment, where existing axis deviations are left within a certain range. This article provides a comprehensive overview of various resection techniques, correction of malalignments, and current developments in knee arthroplasty.

Anatomy and Biomechanics of the Knee Joint

The knee joint is the largest joint in the human body and connects the femur (thigh bone) to the tibia (shin bone). It is a complex hinge joint that enables both flexion and extension movements as well as slight rotational movements. The stability of the knee joint is ensured by various structures such as ligaments, menisci, and muscles.

The natural axis of the leg often has a slight O- (varus) or X-position (valgus). These natural deviations can vary individually and influence the joint load as well as the tension of the soft tissues. A correction that is not optimally considered can lead to problems in the long term, which is why alternative alignment strategies are gaining importance.

Malalignment of the Knee Joint: Causes and Effects

Misalignments of the knee joint can be congenital or develop over the course of life. Common causes include:

• Degenerative Diseases: Osteoarthritis leads to cartilage degradation and can alter the joint geometry.
• Trauma: Injuries can lead to misalignments if left untreated or inadequately treated.
• Inflammatory diseases: Rheumatoid arthritis and other systemic diseases can permanently damage the joint structure.

These malalignments significantly affect the joint mechanics, lead to uneven loading and can accelerate wear. A balanced correction approach is therefore essential.

How does a knee prosthesis work?

A knee prosthesis is an artificial joint replacement designed to restore the function of a knee joint damaged by osteoarthritis, injuries or other diseases. Modern knee prostheses consist of several components that are precisely coordinated to enable natural mobility and stability.

1. Structure of a Knee Prosthesis

A knee prosthesis typically consists of three main components:

• Femoral component (thigh bone portion): This metal component replaces the damaged joint surfaces of the femur and glides over the polyethylene surface of the tibial component.
• Tibial component: This metal platform is placed on the upper end of the tibia and serves as a stable base for the artificial gliding core made of polyethylene.
• Patellar component: Optionally, the patella can also be replaced with a polyethylene component to ensure optimal gliding ability.

Additionally, there are different coupling degrees, which vary depending on how much the prosthesis needs to replace the stability of the natural knee ligaments.

2. Functionality of a knee prosthesis

The knee prosthesis functions by replicating the mobility of the natural knee joint. Modern knee TEPs (total endoprostheses) are designed such that they:

• Enable a low-friction gliding movement between the artificial joint surfaces.
• Preserve or reconstruct the physiological axis of the leg as much as possible.
• Consider the natural ligament tension , especially in kinematic alignment.
• Ensure high stability in standing and fluid mobility when walking

3. Range of motion and stability after knee replacement

After a successful implantation, a knee prosthesis can enable flexion of 120° or more , depending on the patient and surgical method. Modern implants are designed for a lifespan of over 20–30 years and are made of low-wear materials such as highly cross-linked polyethylene and cobalt-chromium alloys.

4. Different alignment types of knee replacement

Depending on anatomical conditions and individual malalignment, knee prostheses are today aligned according to different principles:

• Measured Resection (Mechanical Alignment):
  → Here, the knee joint is aligned so that the leg receives a straight axis , regardless of the original position.
• Kinematical Alignment:
  → The natural ligament tension is largely maintained by allowing slight O- or X-leg deformities to remain. This often results in a more natural feeling of movement.

5. How does a knee prosthesis feel?

Patients often report a significant reduction in pain and an improved quality of life after a total knee replacement. In the first few months, the implant may still feel foreign, but with targeted physiotherapy, a nearly normal range of motion can often be achieved.

Measured Resection: The traditional technique for knee replacement

The Measured Resection technique is one of the oldest methods for implanting a knee prosthesis. Here, the knee is aligned so that a mechanically neutral axis is achieved, regardless of the patient's original anatomy.

Procedure

• Bone cuts: The bones are resected based on predefined angles and dimensions.
• Alignment: Special instruments ensure that the bone cuts are performed in an optimal axis.
• Soft tissue balancing: After bone cuts, the soft tissues, especially the ligaments, are adjusted to achieve even tension.

Benefits

• Standardized, proven technique with good long-term results.
• Good reproducible results.
• Uniform loading of the prosthesis components.

Disadvantages

• Potentially increased soft tissue stress due to forced adaptation to a standardized axis.
• Can lead to unphysiological joint stress.
• Not every patient benefits from a complete axis correction.

Kinematic Alignment: An individual approach to knee replacement

In Kinematic Alignment , the knee prosthesis design is adapted to replicate the patient's natural anatomy as accurately as possible. This technique allows for a slight varus or valgus position to be maintained in order to preserve the natural ligament tension and soft tissue balance.

Procedure

• Patient-specific planning: Advanced imaging techniques help to determine the natural axis.
• Minimally invasive soft tissue adaptation: Soft tissues are not unnecessarily stretched or tightened to ensure even loading.
• Individual bone cuts: Resection is performed along the natural knee axis.

Benefits

• Reduces tension on ligaments and muscles.
• Can lead to a shorter rehabilitation period.
• Enables a more natural joint movement.

Disadvantages

• Requires more precise preoperative planning.
• May not be suitable for certain patients.
• The long-term results are not yet as comprehensively documented as with the traditional method.

The trend towards partial correction of malalignments in knee arthroplasty

In recent years, the understanding has increasingly prevailed that a complete correction is not always the best solution. Especially with kinematic alignment, a moderate malalignment is maintained in order not to impair the natural joint dynamics.

• Varus correction: From a severe bowleg to a mild bowleg.
• Valgus correction: A severe knock-knee is corrected to a mild knock-knee.

These moderate corrections can help to ensure that the soft tissues are not unnaturally tense and that mobility is maintained.

Future prospects in knee arthroplasty

The future of knee arthroplasty will be strongly influenced by personalized implant designs, robot-assisted surgery, and advanced biomechanical insights. Individualized solutions such as kinematic alignment will be further optimized and increasingly integrated into clinical practice.

Conclusion

The complete correction of malalignments in connection with a knee prosthesis (knee TEP) is not always the best solution. Modern techniques such as kinematic alignment allow for a more individualized approach to artificial knee joints, which offers advantages for many patients. The choice of the right technique should be made individually to achieve the best possible long-term results.