Knee arthroplasty has made enormous progress in recent decades to enable patients with severe osteoarthritis or joint problems to lead a pain-free and active lifestyle. In particular, the guided or coupled knee prosthesis (knee TEP) has established itself as a solution that is used in cases with advanced instability or complex joint diseases. This modern variant of the artificial knee joint offers a maximum of stability and functionality.

The Coupling Degrees of Knee Prostheses

1. Partially coupled knee prostheses (cruciate-retaining, CR)

These prostheses usually preserve the posterior cruciate ligament, allowing for more natural stability and movement guidance.

• Description: The design allows the posterior cruciate ligament to retain its function, which optimizes power transmission in the knee. The prosthesis supports this without taking over the entire stability.
• Indication: Suitable for patients with mild to moderate knee damage where the posterior cruciate ligament is still functional.
• Example: Total prostheses that specifically preserve the posterior cruciate ligament (CR prostheses).
• Benefits:
• Almost natural movement patterns.
• Preservation of ligament structures leads to better proprioception.
• Disadvantages:
• Not suitable for degenerated or damaged cruciate ligaments.
• Risk of instability

2. Posterior-stabilized knee prostheses (posterior-stabilized, PS)

These prostheses replace the function of the posterior cruciate ligament with a mechanical stabilizer.

• Description: A central guiding cam base and a cam in the prosthesis system take over the function of the posterior cruciate ligament, ensuring stability in flexion and extension. The anterior cruciate ligament remains removed.
• Indication: Patients with a missing or non-functional posterior cruciate ligament.
• Example: Fully coupled PS knee prostheses.
• Benefits:
• Reliable stability with limited ligament apparatus.
• Precise and defined motion guidance.
• Disadvantages:
• Increased wear of the central stabilizer possible.

3. Strongly coupled or axis-guided knee prostheses

These prostheses have a mechanical axis that stabilizes the knee joint and guides movements within a predetermined radius.

• Description: The coupling mechanism connects the femoral and tibial parts of the prosthesis via an axis, actively stabilizing the knee joint regardless of ligament or tissue conditions.
• Indication: Patients with severely damaged or completely unstable ligament apparatus, for example, in revisions, tumor resections, or severely deformed knees.
• Example: Guided knee prostheses.
• Benefits:
• High stability even with complete ligament and bone loss.
• Safe motion sequence in case of serious complications.
• Disadvantages:
• More complex design requires precise implantation.
• High loss of bone material during surgery.

4. Joint-bridging or modular knee prostheses

These are used for extreme cases in which a large part of the knee joint and surrounding structures have been damaged or removed.

• Description: These systems use modular components that can bridge the entire area between the femoral and tibial parts. Metal extensions are partially used to compensate for significant bone loss.
• Indication: Frequently used in tumor patients, after multiple revisions or severe traumas with extensive tissue damage.
• Example: Tumor prostheses or prostheses with bone bridges.
• Benefits:
• Highest stability and adaptation to extremely complicated cases.
• Disadvantages:
• Complex surgery with significant loss of natural structures.

The choice of the appropriate coupling degree depends on the individual initial situation of the patient. While uncoupled or partially coupled prostheses are suitable for less damaged knees, highly coupled or joint-bridging prostheses offer maximum stability in complex diseases or after repeated interventions. The decision should always be made on the basis of careful diagnostics and individual patient counseling.

What is an axis-guided knee prosthesis?

A hinged knee prosthesis or coupled knee prosthesis is a special form of artificial knee joint, primarily developed for patients whose ligament apparatus is damaged or insufficient. Unlike conventional knee prostheses, the hinged variant partially or completely takes over the stabilization of the joint.

The central axis, which is integrated between the femur (thigh bone) and tibia (shinbone), acts as a mechanical guide and ensures:

• Lateral stability during movements
• Precise joint movements, even with significant ligament wear
• High longevity, even in complex conditions

Differences between hinged knee prostheses and conventional models

Hinged knee prostheses or coupled knee prostheses differ fundamentally from conventional models in their design, function, and associated benefits for specific patient groups. While conventional knee prostheses rely primarily on the natural stability of the surrounding tissue and remaining ligaments, hinged models actively take over the stabilization of the knee joint.

1. Mechanical Stability and Axial Guidance
   In an axially guided knee prosthesis, a specially designed joint mechanism – consisting of a central axial guidance element – ensures precise movement guidance. Unlike conventional prostheses, which leave a large part of the movement stability to the cruciate ligaments and collateral ligaments, integrated guidance components take over this task in axially guided models. This ensures a safe movement sequence, especially in patients with severely damaged or missing cruciate ligaments.
2. Indications and Patient Groups
   Conventional knee prostheses are particularly suitable for patients whose ligament apparatus and structures near the knee joint are in good condition. In contrast, guided models are preferably used in patients with unstable knee joints, severe arthrosis damage or malalignments, where natural stability is no longer sufficient. Also in revisions of previous knee prosthesis implantations or in complex orthopedic cases, guided models are often the better choice.
3. Biomechanical differences
   A significant difference lies in the way prostheses absorb and distribute forces. Conventional knee prostheses attempt to replicate the natural anatomy and function of the knee as accurately as possible, which, however, brings limitations in unstable joints. Hinged knee prostheses, on the other hand, guide the movement along a predefined axis, which significantly improves function in cases of severe damage. This stability is particularly advantageous in situations where lateral or rotational stresses occur.
4. Surgical Requirements
   The implantation of a guided knee prosthesis requires specialized surgical knowledge and precise planning, as the mechanical axis of the leg must be taken into account exactly. In contrast, the requirements for conventional prostheses are often less complex, especially for less severely damaged joints. The positioning and fixation of the guide components in guided prostheses also require high precision, as they form the foundation for correct function and stability.
5. Long-term Results and Functionality
   Hinged knee prostheses offer significantly better results for specific patient groups in terms of stability, pain reduction, and mobility. Especially in patients with severely degenerated or traumatically damaged joints, where conventional models reach their limits, hinged models convince with their durability and resilience. Studies show that they often achieve higher satisfaction in these specific indications. At the same time, conventional models are often sufficient for patients without complex damage patterns and also offer excellent long-term results.

In summary, hinged knee prostheses offer a significant advantage in terms of stability and function in difficult cases, while conventional prostheses remain an excellent solution for less complex interventions. The choice of the appropriate model depends heavily on the individual patient's situation and the requirements for stability, mobility, and quality of life.

When is a coupled knee prosthesis necessary?

A hinged knee prosthesis is typically used in complex situations. Indications include:

1. Advanced ligament loss:

• After accidents or injuries to the cruciate ligaments or collateral ligaments.

1. Severe osteoarthritis or rheumatoid arthritis:

• Especially when the joint integrity is severely compromised.

1. Revision Surgeries:

• In cases of failed initial implantations or loosened knee prostheses.

1. Deformities of the Knee Joint:

• For example, due to axial deviations (bowlegged or knock-kneed), which other prosthesis types cannot compensate for.

Benefits of the hinged knee prosthesis

A hinged knee prosthesis offers significant advantages over conventional prostheses, particularly in complex knee problems. Here are the key benefits:

1. Optimal Stability: The mechanical axis ensures stability even with completely insufficient ligaments.
2. High mobility: Patients benefit from natural movement guidance and improved function.
3. Adaptability: Specifically designed for challenging anatomical conditions.
4. Longevity: Modern materials ensure high abrasion resistance and thus long durability.

Materials and Technology

The choice of material is a crucial factor for the function and durability of a knee prosthesis. Hinged knee prostheses are made of:

• High-performance plastics: These minimize wear and ensure smooth movement.
• Titanium and Alloys: Lightweight, stable, and biocompatible.
• Ceramic elements: Reduce friction and increase durability.

Modern manufacturing techniques such as 3D printing also allow for precise adaptation to individual anatomy.

The Operation: Precision and Expertise

The implantation of a hinged knee prosthesis is technically demanding and requires detailed preoperative planning. The central aspects include:

1. Digital planning: Scaled X-rays and CT data help determine the ideal prosthesis size and position.
2. Minimally invasive procedures: These techniques minimize soft tissue trauma and accelerate recovery.
3. Navigation technology: Computer-assisted systems ensure millimeter-accurate implantation.

Risks and complications

As with any surgery, there are also certain risks associated with the implantation of a guided knee prosthesis:

• Risk of infection (now extremely low due to sterile techniques).
• Possible loosening of the prosthesis.
• Postoperative muscle building is necessary to achieve optimal results.

Rehabilitation: Achieving Recovery

A successful rehabilitation is the key to fully exploiting the benefits of a guided knee prosthesis. The rehabilitation process includes:

1. Early Mobilization: Patients are mobilized with walking aids as early as the first day after surgery.
2. Physiotherapy: Strengthening exercises and gait training promote the development of the surrounding musculature.
3. Long-term follow-up care: Regular check-ups ensure long-term function.

Conclusion: The added stability

The hinged knee prosthesis represents a maximal variant of modern knee arthroplasty. Due to its ability to provide a maximum level of stability even in unstable joints, it offers many patients a real chance at quality of life. The combination of innovative technology, precise implantation technique, and comprehensive aftercare makes it an excellent choice for very complex knee problems.

With proper preparation, an experienced surgeon, and an individualized rehabilitation program, the path back to an active life is possible.