Kinematic alignment in knee joint replacement
Knee replacement (knee endoprosthesis) is a successful operation for many patients with advanced wear (osteoarthritis) of the knee joint. However, about 15% of patients are not completely satisfied or even dissatisfied with the result. This has many causes, including infection, fracture, or instability that can lead to permanent discomfort. Often, however, in patients with permanent complaints after knee replacement, none of the stated causes can be identified as the trigger of the complaint, and in addition, the X-ray shows no abnormalities and a straight leg.
How does the conventional implantation of a knee prosthesis work?
In the conventional implantation of a knee endoprosthesis, the goal is for every patient to have a straight leg after surgery – regardless of whether a x- or o-leg was previously present. This means that all knees are operated on according to the same scheme, regardless of individual differences (Figures A and B).
However, people with a completely straight leg and a joint line perpendicular to it are rare; the majority have a more or less pronounced x- or o-leg.
X-ray of the whole right leg
This patient has an osteoarthritis of his knee: his joint space between the femur and the tibia (blue arrow) is not present anymore on the medial side. The red line connecting the center of the hip and the knee respectively the center of the knee and the ankle (red lines) is not straight as this patient has an o-leg.
In addition, the joint line between the upper and lower leg bones is not strictly perpendicular to the leg axis, even in the healthy knee, but usually slopes slightly inward (Figure C). This is due to the fact that, when walking, people tend to walk in a narrow-legged rather than a wide-legged position for better weight distribution. Most people manage well in this position throughout their lives.
What can be the consequences of conventional implantation?
By straightening the leg and altering the joint line with the conventional implantation technique of a knee endoprosthesis, the previous anatomy of the knee joint is largely ignored. This alteration of the knee joint geometry can result in a constrained knee joint with unnatural ligament tension, poor range of motion, and pain.
Especially the knee joint is a very complex joint, which reacts sensitively to changes in shape. Even slight changes in the joint line and leg axis can severely disrupt the movement between the thigh and lower leg and, in particular, have a negative effect on the sliding of the patella. In our experience, it is therefore essential that the natural anatomy of the patient is taken into account in knee joint replacement, as in hip joint replacement, and that the same implant position is not aimed at for all patients!
What does kinematic alignment mean?
More than 15 years ago, a technique was developed in the USA with the aim of reconstructing the anatomy of the knee joint individually as it was before the osteoarthritis. This technique is called “kinematic alignment” (from kinematics = theory of movement).
In kinematic alignment, the knee endoprosthesis is placed in each patient individually according to their natural anatomy to restore healthy knee joint motion. For example, if a patient had a o-leg (which usually increases due to wear-related cartilage loss as part of osteoarthritis), the knee will be reconstructed with a mild patient individual o-leg alignment during surgery. Stronger malpositions will of course be corrected! This setting of the knee prosthesis is not only selected in extension, but over the entire course of movement. This also restores the natural tension of the ligaments over the entire course of movement; just as the patient has been used to all his life. This results in a very stable and harmonious movement of the knee joint without instability or overtension. Unlike conventional alignment, the ligaments are not lengthened during surgery, but retain their original tension (Figures C and D).
What is the benefit of kinematic alignment for the patient?
With the kinematic alignment of the knee endoprosthesis, it has been shown in many high-quality scientific studies that the proportion of patients with permanent complaints can be significantly reduced. In particular, patients more often have a so-called “forgotten artificial knee joint”; that is, they can use their knee joint in everyday life like the natural knee joint and do not think about the fact that they have an artificial joint. By sparing the ligaments and muscles, patients can usually be mobilized more quickly during kinematic alignment. Furthermore, biomechanical studies have confirmed that kinematic alignment also reduces joint pressure and thus decreases the load on the implant.
In summary, in kinematic alignment, the artificial knee joint is individually inserted according to the patient’s natural anatomy, resulting in faster mobilization and better outcomes.
At ECOM, we have been performing the implantation of knee prostheses after kinematic alignment for years with great success. Our positive experience with this modern surgical technique is consistent with the good results of clinical studies. Last but not least, our patients benefit from a faster recovery, less surgical trauma and a knee replacement that feels more natural in many cases.
Fig. A: Schematic representation of a left leg: A slight o-leg is shown, as it often occurs. The ligaments are shown in blue. The joint line slopes slightly toward the inside. The sectional planes for the conventionally inserted prosthesis are shown in red, which are not symmetrical.
Fig. B: In conventional implantation of the prosthesis, each leg is straightened. The joint line is also set perpendicular to the leg axis regardless of the individual anatomy. In the example, this leads to overstretching of the inner ligament (in yellow) and a loose outer ligament (in orange). To correct the ligament tension, the inner ligament must then be surgically stretched.
Fig. C: In kinematic alignment, the cuts for the prosthesis are individually adjusted parallel to the patient’s joint line. Taking wear into account, exactly as much of the surface is removed as is replaced by the implant. This is therefore a true surface replacement.
Fig. D: The knee prosthesis is inserted according to the patient’s natural anatomy. As a result, the joint line remains slightly inward sloping. The geometry of the leg remains anatomical, with a mild bow leg. And the ligament tension remains identical to what it was naturally.
Consultation and contact for questions regarding kinematic alignment
Do you have questions about kinematic alignment in knee replacement? Our knee endoprosthetists at ECOM® Excellent Center of Medicine, Prof. Dr. Gollwitzer and Prof. Dr. Weber, will be happy to inform and advise you in detail in a personal consultation. Simply make an appointment. We look forward to seeing you.