The Role of the ACL in a Total Knee Replacement Surgery (Arthroplasty)

By: Gordon Roedel, Orthopedic Surgeon / Hip & Knee Replacement Specialist 

Perhaps no ligament in the human body is as well-recognized as the anterior cruciate ligament (ACL) of the knee, as ACL injuries are common among athletes. In a healthy knee, the ACL plays an important role in stabilizing the knee and guiding motion. This raises important questions for many patients who are discussing total knee replacement (TKR) with OSMS orthopedic surgeons:

What happens to my ACL during surgery? How can my knee function without it?

First, let’s discuss the role of the ACL in a healthy knee.

The ACL is one of the four major ligaments of the knee, connecting the thigh bone (femur) to the shin bone (tibia). It passes through the center of the joint, and its normal function is to prevent the tibia from shifting forward and rotating out from under the femur. This stability is important during sudden changes in direction, such as jumping, pivoting, or coming to a rapid stop. As a result, an ACL tear can make an otherwise healthy knee feel unstable while playing sports.

Why is the ACL removed in total knee replacement?

The landscape of the knee has often changed by the time many patients begin discussing knee replacement surgery with us. As osteoarthritis progresses, cartilage breaks down, the alignment of your knee may change, and in most cases, the ACL is already stretched, scarred, frayed, or altogether absent by the time of surgery. This is the main reason that knee replacements over the last 20 to 30 years have routinely removed the ACL, even if it is still present. Rather than relying on the ligament, the new artificial implants take on the role of the ACL and provide the stability the new knee needs.

What is a total knee replacement?

It’s important to understand that knee replacement surgery involves resurfacing the worn-out bone and cartilage surfaces of the knee joint. A metal cap covers the curved end of the femur bone, while a baseplate replaces the top of the tibia. A medical-grade plastic liner is inserted between the metal components, and its curved, smooth surface matches the curve of the femur, allowing the knee to move. These implants work together with the remaining soft tissues of the knee—muscles, tendons, and ligaments surrounding the joint surface—to provide both stability and mobility for the knee.

Knee replacements change the kinematics (how the bones move relative to one another) of the knee, and standard implants are built to support the new mechanics rather than preserving the ACL and its original role. Keeping the ACL with the new mechanics could cause more issues than removing the ligament altogether.

Are there knee replacements that preserve the ACL?

While most standard knee replacement implants require removal of the ACL, a small subset is designed to retain all four ligaments of the knee. These bicruciate-retaining implants are designed with the idea that preserving the ACL may result in a knee that moves and feels more naturally. However, the operation is technically demanding, requires a healthy and functional ACL, and may not be suitable for patients with substantial deformity, stiffness, ligament damage, or advanced bone loss.

Furthermore, clinical studies have not consistently shown that preserving the ACL during total knee replacement produces better pain relief, function, or patient satisfaction compared to conventional designs. In fact, some studies have shown a higher rate of complications or reoperations with bicruciate-retaining designs compared to standard implants.

What about partial knee replacements?

Another ACL-preserving knee replacement option is a partial knee replacement. This is an option for patients with arthritis isolated to one area of the knee, allowing the surgeon to retain the ACL while resurfacing the worn-out compartment.

Partial knee replacements offer more natural-feeling kinematics, earlier recovery, and improved range of motion because the ligaments stay intact and help maintain stability. However, partial knee replacement is appropriate only when arthritis and other damage are sufficiently limited. Ligament function, alignment, symptoms, and condition of the other knee compartments must all be considered.

What if my ACL has been previously injured or reconstructed?

When the ACL is injured, the knee’s cartilage (the smooth cushioning on the ends of the bones) can be damaged at the same time, and it may wear down unevenly in the years that follow. Over time, that wear can lead to arthritis, which is why many people who have an old ACL injury or reconstruction end up needing a knee replacement. Because most total knee replacements do not depend on the ACL, the ligament does not need to be reconstructed at the time of surgery.

Previous ACL reconstruction can make the operation somewhat more complex. Your surgeon may need to account for scar tissue, bone tunnels, retained screws, or other alterations to your anatomy.  Nevertheless, studies have shown that patients with ACL-deficient knees and those with prior ACL reconstruction can experience substantial pain relief and improved knee function following total knee replacement.

It is important to tell your surgeon about any previous ligament injury or surgery, even if it occurred many years ago.

Determining the Best Choice for You Starts with Expert Advice

The ACL plays an important role in stabilizing and guiding a healthy knee, but a total knee replacement can take over that role and improve patient success. Before deciding whether you need a standard replacement, an ACL-preserving approach, or a partial knee replacement, it’s best to talk with an orthopedic surgeon.

Have questions about your own knee replacement options?

Schedule an appointment with Dr. Gordon Roedel or any of our OSMS orthopedic surgeons today.

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