Total knee replacement, or arthroplasty, is a surgical procedure that replaces the damaged surfaces of a knee joint with artificial components. This procedure is a common solution for individuals with severe knee pain and functional limitations due to arthritis or injury. Surgeons can select from several types of artificial knee implants, with the choice depending on a patient’s anatomy and the condition of their knee. Among these options, the Cruciate-Retaining, or CR, knee represents a design philosophy aimed at preserving more of the knee’s natural structures.
Defining the Cruciate-Retaining Knee
To understand a cruciate-retaining knee replacement, it is important to recognize the function of the Posterior Cruciate Ligament (PCL). The PCL is a strong band of tissue at the back of the knee, running from the thighbone (femur) to the shinbone (tibia). Its primary role is to prevent the tibia from sliding too far backward in relation to the femur, providing stability during flexion. This ligament contributes to the knee’s normal movement, or kinematics.
A Cruciate-Retaining (CR) implant is engineered to preserve this ligament. During a CR total knee replacement, the surgeon leaves the PCL intact. The artificial components are designed to accommodate the ligament; the tibial component has a flattened or slightly dished surface, and the femoral component has a more open space between its condyles. This design allows the natural PCL to continue its function, guiding the knee’s movement.
The principle of the CR knee is to maintain as much of the patient’s native anatomy as possible. Proponents suggest that by keeping the PCL, the knee may feel more natural and have better proprioception, the body’s sense of its position in space. Retaining the PCL also contributes to a more normal “rollback” of the femur on the tibia as the knee bends, which can improve range of motion and quadriceps efficiency.
CR Knee Versus Other Implant Designs
The most common alternative to a cruciate-retaining knee is the Posterior-Stabilized (PS) design. In a PS knee replacement, the surgeon removes the PCL. This may be done if the ligament is damaged, contracted, or if its removal simplifies the process of balancing the knee’s other ligaments.
To compensate for the absence of the PCL, a PS implant has a built-in mechanical feature. A polyethylene “post” on the tibial component fits into a corresponding “cam” on the femoral component. This post-and-cam mechanism takes over the function of the PCL, guiding the femoral rollback during flexion.
The choice between designs is a key surgical decision. CR designs aim to preserve natural biomechanics by keeping the PCL. In contrast, PS designs offer a more predictable method of stabilizing the knee, especially when the PCL is unhealthy or a significant deformity needs correction.
Another, less common, design is the Ultra-Congruent (UC) or deep-dish implant. It does not have the post-and-cam mechanism of a PS knee. Instead, it features a more deeply curved tibial surface that conforms closely to the femoral component, providing stability without relying on the PCL, which is still removed.
Patient Candidacy for a CR Knee
The selection of a cruciate-retaining knee implant depends on the individual patient’s knee anatomy and condition. The primary factor is the health and functionality of the Posterior Cruciate Ligament (PCL). For a CR knee to be successful, the patient’s PCL must be intact, strong, and capable of performing its stabilizing role.
A good candidate for a CR knee has a knee with minimal deformity, meaning the patient is not severely knock-kneed or bow-legged. Surgeons may not use CR implants in knees with significant deformity, as correcting alignment issues is more challenging while trying to properly tension a retained PCL. Good overall ligamentous stability is another favorable characteristic.
Conversely, a surgeon might opt for a different implant design, such as a posterior-stabilized (PS) knee. If the PCL is damaged, absent, or degenerated due to advanced arthritis, a CR implant is not a suitable choice. Patients with inflammatory arthritis, like rheumatoid arthritis, may be better candidates for a PS knee because the disease can weaken ligaments. A history of previous knee surgeries or significant bone defects might make a PS design a more predictable option.
Surgical and Functional Outcomes
A potential benefit associated with CR knees is a more “natural” feeling of movement. Because the native Posterior Cruciate Ligament (PCL) is preserved, some patients experience better proprioception, or the joint’s ability to sense its position. This can contribute to improved balance and coordination.
Some studies suggest that CR knees may allow for a greater range of motion, particularly in deep flexion. The preservation of the PCL enables more normal knee kinematics, including the natural rollback of the femur on the tibia. This can enhance the efficiency of the quadriceps muscle and may improve performance in activities like climbing stairs.
However, studies comparing CR and posterior-stabilized (PS) knees have found no significant differences in long-term implant survival rates, pain relief, and overall patient satisfaction scores. While some studies show slight advantages for one design in specific functional tasks, these differences are not clinically significant for most patients. The success of a knee replacement depends more on the surgeon’s experience and proper surgical technique than on the implant design.