Do You Still Have an ACL After Knee Replacement?

In a standard total knee replacement, the ACL is removed during surgery. This is true even when the ligament is still functional beforehand. The procedure reshapes the ends of the femur and tibia so extensively that the ACL’s attachment points are cut away, making removal a practical necessity for implant fitting. However, certain newer implant designs and partial knee replacements do preserve the ACL, and the question of whether that ligament should be saved has become one of the more active debates in joint replacement surgery.

Why Standard Total Knee Replacement Removes the ACL

Total knee arthroplasty, the formal name for a full knee replacement, has been performed since the 1970s and remains one of the most common elective surgeries worldwide. The standard procedure involves cutting the damaged surfaces from the bottom of the thighbone and the top of the shinbone, then capping both with metal and plastic components. The bone cuts needed to seat these implants run directly through the areas where the ACL attaches, so the ligament gets sacrificed as part of the process.

What surprises many people is that the ACL is often still intact when surgeons go in to replace an arthritic knee. A study examining ligament status in arthroplasty patients found that the ACL was still intact in about 78% of cases, and the posterior cruciate ligament (PCL) was intact in 98%.¹ Separate research looking at osteoarthritic knees headed for surgery found that roughly 23% had a completely ruptured ACL, meaning the rest still had some or all of the ligament working.² So a functioning ligament is being routinely discarded in the majority of total knee replacements, which raises an obvious question about whether that is the best approach.

What Happens to the PCL

While the ACL is almost universally removed in conventional total knee replacement, the PCL gets more nuanced treatment. Surgeons choose between two broad implant categories: cruciate-retaining designs that keep the PCL, and posterior-stabilized designs that remove it and replace its function with a plastic post and cam mechanism built into the implant.

Whether retaining or sacrificing the PCL makes a meaningful difference has been studied extensively, and the honest answer is that neither approach is clearly superior overall. A Cochrane systematic review found that sacrificing the PCL gave patients a slightly greater range of motion (about two to three degrees more bending) but no difference in pain or most other outcomes.³ A separate meta-analysis of 14 randomized trials similarly found that posterior-stabilized implants produced better range of motion, with no significant difference in pain scores, complication rates, or most functional measures.⁴

A large registry-based study added some long-term perspective: implant survival at 15 years was slightly higher in knees where the PCL was retained (around 96-97%) compared with those where it was excised (about 95%), though the clinical significance of that small gap is debatable.⁵ That same study noted that if the PCL happens to get damaged during surgery, converting to a posterior-stabilized design is not strictly necessary as long as the knee remains stable. In practice, many surgeons develop a preference for one approach and use it for most of their patients, which means the implant your surgeon is most experienced with often matters more than the theoretical advantages of either design.

How Implants Compensate for the Missing ACL

In a natural knee, the ACL prevents the shinbone from sliding forward under the thighbone and helps guide the complex rolling and gliding motion of the joint through its arc of bending. When the ACL is removed during standard total knee replacement, the implant’s shape takes over those responsibilities. The plastic liner that sits between the metal components is contoured to create stability, and in posterior-stabilized designs, the post-and-cam mechanism physically blocks abnormal movement.

One problem with losing the ACL is that it changes how the knee moves in ways the patient can sometimes feel. In a natural knee, the femur rolls backward on the tibia as you bend deeper. Without the ACL, some implants allow what’s called paradoxical anterior sliding, where the femur slips forward instead of rolling back during bending. A biomechanical study found that implants lacking any ACL substitute showed 7 to 8 millimeters of abnormal posterior shift in extension, while designs that either retained or mechanically substituted for the ACL tracked much closer to the native knee’s motion pattern.⁶

Newer implant geometries have tried to address this. Medial pivot designs, for instance, use a highly conforming shape on the inner side of the knee to create a stable pivot point that mimics the natural knee’s tendency to rotate around the medial compartment.⁷ These design refinements have narrowed the gap between artificial and natural knee motion, but they haven’t closed it entirely, which is part of why interest in preserving the ACL has grown.

Bicruciate-Retaining Knee Replacement

Bicruciate-retaining total knee arthroplasty, or BCR TKA, is the approach that keeps both the ACL and the PCL intact. It’s an appealing idea: if the ligaments are still working, why not preserve them and let the knee move more naturally? The concept is not new; first-generation BCR implants existed decades ago. But those early designs struggled with difficult surgical technique and poor long-term survival, and the approach fell out of favor.⁸ Second-generation designs with improved implant geometry and better surgical instruments have revived interest.

The biomechanical argument for BCR is solid. An in vivo study comparing knee motion found that patients with a bicruciate-retaining implant experienced more normal-like movement patterns during early bending, which the researchers attributed to the presence of the ACL.⁹ There is also reason to think that keeping the ACL helps with proprioception, the body’s sense of where the joint is in space, because the ligament is packed with nerve endings that feed position information to the brain.¹⁰

Clinical results have been encouraging at medium-term follow-up. A prospective cohort study found that at five years, BCR TKA showed significantly better patient-reported outcomes than standard cruciate-retaining designs across multiple measures, including joint awareness, stiffness, function, and pain.¹¹ At three years, patients reported improved quality of life, though the difference in how “aware” they were of the artificial joint did not quite reach statistical significance.¹² The concept of joint awareness matters here: one of the most common complaints after knee replacement is that the knee never feels fully natural. Reducing that awareness, making the knee feel more like it’s “your own,” is a goal that BCR designs are specifically targeting.

The Catch With Bicruciate-Retaining Designs

If keeping both ligaments sounds like the obvious choice, the complication data explain why most surgeons have not switched. Early revision rates for BCR TKA have been higher than the field is comfortable with. One study reported three-year survival of only 88%, with revisions for tibial loosening, ACL impingement, and pain being the most common reasons for failure.¹³ For context, standard total knee replacements routinely achieve survival rates above 95% at 15 years, so 88% at just three years is a red flag.

A more recent study of a newer BCR design showed better numbers: 97.3% aseptic revision survival at two years, with only a few reoperations including one for ACL rupture, one for infection, and several for stiffness.¹⁴ These are early results, though, and the procedure clearly demands more of the surgeon. The bone cuts must be made more carefully to avoid damaging the ACL, the implant has to be positioned precisely to accommodate the ligament, and the surgeon needs to verify intraoperatively that the ACL is actually functional, not just physically present.

Patient selection is also more restrictive. A survey of 346 knee surgeons found that most would not perform BCR TKA in patients over 80, those with a BMI above 35, or those with more than 10 degrees of angular deformity or flexion contracture.¹⁵ A systematic review confirmed that ACL intactness was verified either clinically before surgery or by direct inspection during the operation in most published studies.¹⁶ In other words, you need the right patient, the right ligament, and the right surgeon for this to work.

Partial Knee Replacement Preserves the ACL by Default

There is another way to keep your ACL after a knee replacement, and it is already well established: a partial knee replacement, formally called unicompartmental knee arthroplasty (UKA). This procedure resurfaces only the worn-out compartment of the knee, typically the inner (medial) side, while leaving the rest of the joint, including both cruciate ligaments, untouched.

Traditionally, having a functional ACL has been considered a requirement for partial knee replacement. Classic selection criteria list an intact ACL alongside other factors such as correctable alignment and not being significantly overweight.¹⁷ The thinking is that the ACL helps control the joint’s motion and protects the implant from abnormal forces.

That strict requirement has loosened somewhat. A systematic review and meta-analysis found no significant difference in outcomes between partial knee replacements done in ACL-intact versus ACL-deficient knees at three to ten years of follow-up.¹⁸ This is still considered somewhat controversial, and most surgeons continue to prefer patients with a working ACL for partial replacement. But it does open the door for people who might have been told they needed a full replacement simply because their ACL was torn.

Combining ACL Reconstruction With Partial Replacement

For patients who have both a worn-out medial compartment and a torn ACL, some surgeons have started doing both procedures at once: a partial knee replacement combined with ACL reconstruction. This avoids the need for a full knee replacement in patients who would otherwise be pushed toward one solely because of their ligament status.

A systematic review of studies combining these two procedures found that it was safe, with significant improvements in functional and clinical outcomes.¹⁹ A more recent comparative study went further, directly comparing this combined approach against total knee replacement. The results showed comparable functional scores between the two groups at latest follow-up, with no increased risk of major complications in the combined group.²⁰ This is a niche procedure performed at relatively few centers, but it represents an option worth knowing about if you are younger, active, and facing both problems simultaneously.

The Condition of the ACL Before Surgery

An important piece of this puzzle is that arthritis itself damages the ACL over time, even without a specific injury. A study comparing osteoarthritic knees to non-arthritic controls found that complete ACL rupture was present in about 23% of arthritic knees versus under 3% of controls.² Among those with a ruptured ACL, fewer than half actually remembered a knee injury, suggesting the ligament had degenerated gradually rather than tearing in a single event. People with ACL rupture also had more severe arthritis on imaging and more narrowing of the inner joint space.

Even in knees where the ACL looks physically intact, the tissue itself is often compromised. A histological study examining ACL specimens from arthroplasty patients found that 85% had microscopic changes in the ligament tissue, including calcium deposits and tiny cysts, with more severe changes in older and heavier patients.²¹ This means that even when a surgeon looks at the ACL during surgery and thinks it appears intact, the ligament may already be weakened at the cellular level. It partly explains the difficulty of bicruciate-retaining surgery: the ACL you are trying to save may not be as healthy as it looks.

Does Keeping the ACL Help You Get Back to Sports?

One motivation for preserving the ACL is the hope that it will let people return to higher levels of activity after surgery. The data on this are mixed. In conventional designs where the ACL is already gone, a prospective study of 227 procedures found no difference in return-to-sport rates, activity levels, or weekly hours spent exercising between posterior-stabilized and cruciate-retaining (PCL-retaining) implants.²² In other words, whether the PCL was kept or removed did not change how active patients were.

For bicruciate-retaining designs, the five-year data showing better function and pain scores is promising, but long-term activity data and head-to-head comparisons with standard designs in athletic populations are still thin. A network meta-analysis comparing all four major implant philosophies (posterior-stabilized, cruciate-retaining, bicruciate-substituting, and bicruciate-retaining) found that early differences in range of motion between designs tended to disappear at longer follow-up, with no significant differences in patient-reported outcomes or complication rates at final assessment.²³ The early advantages that newer designs show may be real, or they may simply reflect the fact that the patients selected for these procedures tend to be younger and healthier to begin with.

When Preservation Actually Matters Most

The strongest practical case for ACL preservation comes down to proprioception and the subjective experience of the knee. Many patients after standard total knee replacement say the joint works fine but does not feel like their own knee. It’s a common enough phenomenon that orthopedic researchers have developed specific questionnaires to measure “forgotten joint” status, meaning how often a patient forgets they even have an artificial knee. The bicruciate-retaining approach aims squarely at this problem, and the early evidence suggests it makes a difference for the right candidates.

For most people getting a knee replacement today, though, the ACL will be removed. Standard implants compensate for the loss effectively, and the procedure has an enormous track record of success. The question of whether the ACL should be preserved is less about whether it can be done and more about whether the added surgical complexity and stricter patient selection are worth the incremental gains. That calculation is different for a 55-year-old recreational tennis player with mild arthritis and an intact ACL than it is for a 75-year-old with severe deformity. Talking to your surgeon about which design is being considered and why is the single most useful thing you can do before scheduling surgery.