TKA is a highly successful operation performed to treat severe knee arthritis, replacing damaged joint surfaces with artificial components. The procedure involves resurfacing the ends of the thigh bone (femur) and the shin bone (tibia) with metal and plastic implants. The status of the Anterior Cruciate Ligament (ACL) after Total Knee Arthroplasty (TKA) is a common concern for patients. Since the ACL is a major structure, its management during reconstruction directly relates to how the new joint will function.
The Role of the ACL in Natural Knee Mechanics
The ACL is one of the four main ligaments in the knee, positioned deep within the joint and connecting the femur to the tibia. Its primary mechanical function is to act as the principal restraint against the tibia sliding too far forward relative to the femur. This ligament provides approximately 85% of the restraining force needed to prevent this excessive forward movement, particularly when the knee is bent between 30 and 90 degrees. Beyond its mechanical strength, the ligament contains specialized sensory receptors called mechanoreceptors. These receptors contribute to proprioception, which is the body’s sense of the knee joint’s position and movement.
The Direct Answer: ACL Status During Total Knee Replacement
In the majority of standard Total Knee Arthroplasty procedures, the Anterior Cruciate Ligament is intentionally removed. This removal is required by the design of most knee replacement systems.
The decision to excise the ligament is primarily dictated by the need to prepare the bone surfaces and properly seat the artificial components. The surgical cuts made to the tibia and femur to accommodate the metal implants often intersect with the attachment points of the ACL, making its preservation technically challenging or impossible.
Furthermore, the presence of the native ACL can interfere with the fit and alignment of the prosthetic components, potentially leading to issues with range of motion or early implant wear. The vast majority of knee implant designs have been created with the expectation that the ACL will be absent.
How Implant Design Determines ACL Management
The management of the ligaments during TKA is largely determined by the type of artificial knee system selected by the surgeon. The two most common categories of implants are Cruciate-Retaining (CR) and Posterior-Stabilized (PS), and both generally require the ACL to be removed.
Cruciate-Retaining (CR) Implants
CR implants are designed to preserve the Posterior Cruciate Ligament (PCL), which is the other major stabilizing ligament in the knee. Even in this design, the ACL is typically removed to allow for the proper placement of the tibial insert and to ensure a balanced joint. The PCL, if retained, helps to guide the motion of the femur on the tibia and contributes to stability.
Posterior-Stabilized (PS) Implants
PS implants, also known as Cruciate-Substituting designs, are the most frequently used type of knee replacement. In this system, both the ACL and the PCL are removed. The implant compensates for the absence of the PCL through a specialized mechanical feature called the post-and-cam mechanism.
This mechanism consists of a vertical polyethylene post extending from the tibial insert and a corresponding curved metal cam located on the femoral component. As the knee flexes past a certain point, the femoral cam engages the tibial post. This engagement acts as an internal check, substituting for the removed PCL by preventing the femur from moving too far forward.
Functional Stability After ACL Removal
The removal of the ACL does not result in an unstable joint because the artificial knee system is engineered to provide the necessary stability. The stability of the prosthetic knee relies on a combination of factors that effectively replace the functions of the resected ligament.
The geometry of the artificial components themselves is the first source of stability. The shape and contour of the polyethylene tibial insert are designed to cradle the rounded metal femoral component, which inherently limits excessive front-to-back movement.
In cases where the PCL is retained (CR implants), this ligament continues to function as a stabilizer, guiding the femur’s natural rolling and gliding motion. In PS implants, the post-and-cam mechanism reliably substitutes for the PCL’s function, providing a mechanical block against instability.
Furthermore, the surgeon meticulously balances the collateral ligaments—the Medial Collateral Ligament (MCL) on the inside and the Lateral Collateral Ligament (LCL) on the outside—during the operation. This soft-tissue balancing ensures that the knee is snug and stable throughout its full range of motion. The goal of TKA is to create a stable, durable, and functional joint.