Can a Knee Replacement Become Dislocated?

A knee replacement can become dislocated, though it is one of the rarer complications of the procedure. Reported incidence for full tibiofemoral dislocation after total knee arthroplasty falls between roughly 0.15% and 0.5% of cases, making it far less common than other complications like infection or stiffness.¹ The numbers climb higher for certain implant types, and related problems like patellar instability add another dimension. Despite its rarity, dislocation demands urgent medical attention and often requires additional surgery.

Why Knee Replacements Dislocate Less Often Than Hip Replacements

People familiar with joint replacement surgery tend to associate dislocation with hip replacements, where the ball-and-socket design creates a well-known risk of the femoral head popping out of the acetabular cup. Knee replacements work differently. Instead of a ball in a socket, the implant mimics the knee’s natural hinge-like motion with metal components covering the ends of the thighbone and shinbone and a plastic spacer sitting between them. Ligaments and the surrounding soft tissue envelope hold everything in place. Because the geometry of the knee does not depend on one piece sitting inside another the way a hip does, gross dislocation is inherently less likely.

That said, the knee’s stability after replacement still depends on a balance between implant design, soft tissue tension, and correct alignment. When any of those elements fails, dislocation becomes possible. The relative rarity of knee dislocation has also meant it receives less attention in pre-surgery counseling, so patients who experience it are sometimes caught off guard.

Types of Dislocation After Knee Replacement

Not all knee replacement dislocations look the same. The type depends on the implant design and on which part of the joint complex gives way.

Tibiofemoral Dislocation

This is the classic version that people picture: the femoral (thighbone) component separates from the tibial (shinbone) component, causing the joint to shift out of alignment entirely. It can happen anteriorly (the tibia slides forward relative to the femur) or posteriorly (the tibia slides backward). Posterior dislocation is more commonly reported. The knee looks visibly deformed and is immediately painful and non-functional. This is the type captured in that 0.15–0.5% incidence figure, and it typically constitutes a surgical emergency.

Patellar Dislocation

The kneecap (patella) tracks in a groove on the front of the femoral component. If the groove alignment is off or the surrounding muscles and tendons are imbalanced, the kneecap can slip laterally out of its track. Patellar instability after total knee arthroplasty is described as a known but serious complication, and bilateral patellar dislocation after the procedure is exceedingly rare.² Patients with patellar dislocation often notice a catching or giving-way sensation when bending or straightening the knee, sometimes accompanied by visible lateral shift of the kneecap.

Mobile Bearing Dislocation

Some knee replacement designs use a mobile-bearing insert, a plastic spacer that is designed to rotate or glide slightly rather than being locked in place. This can improve natural-feeling movement, but it also introduces the possibility that the bearing itself can spin out of position. In lateral unicompartmental (partial) knee replacements, bearing dislocation occurs in roughly 1–6% of cases, most often in a medial direction and typically when the knee is in a flexed position.³ This is a meaningfully higher rate than full tibiofemoral dislocation after a total knee replacement, which makes sense: the bearing is a separate moving piece, and the partial replacement leaves more of the knee’s native anatomy in play, introducing variability in soft tissue tension.

When a mobile bearing dislocates, it can sometimes be treated by replacing just the bearing rather than redoing the whole implant. However, research on medial unicompartmental knees found that half of patients who had bearing exchange alone experienced a repeat dislocation, and most of those eventually required conversion to a full knee replacement.⁴ That high redislocation rate suggests a simple bearing swap is only appropriate in selected patients where the underlying cause of the dislocation can be clearly identified and corrected.

What Causes a Knee Replacement to Dislocate

Dislocation generally results from a mismatch between the forces acting on the joint and the structures holding it together. The causes fall into two broad categories: surgical factors and patient factors.

Surgical and Implant Factors

The surgeon’s choices during the operation play a large role. In a case series of six dislocated total knee replacements, the identified causes included imbalance of the flexion gap (in four patients), inadequate implant selection (one patient), malrotation of the components (one patient), and rupture of the medial collateral ligament.¹ The flexion gap is the space between the femur and tibia when the knee is bent. If the surgeon creates too much laxity in that gap, the joint can separate under load. Similarly, if the implant components are rotated slightly off-axis during installation, the extensor mechanism (the quadriceps tendon and patellar tendon working together to straighten the knee) becomes unbalanced, predisposing the joint to instability.⁵

Implant design matters, too. Cruciate-retaining designs, which preserve the posterior cruciate ligament, provide less built-in constraint than posterior-stabilized designs, which replace the ligament’s function with a post-and-cam mechanism inside the implant. In the case series mentioned above, five of the six patients who dislocated had cruciate-retaining implants. That does not mean cruciate-retaining implants are bad; they are extremely successful for the right patient. But in cases where soft tissue balance is borderline, they offer less of a safety net against dislocation.

Patient-Related Factors

Certain characteristics raise a patient’s vulnerability regardless of how well the surgery goes. Obesity, neuropsychiatric disorders, and severe pre-existing deformity of the knee (significant inward or outward angulation) are all recognized patient-related risk factors for dislocation.⁶ Obesity increases the mechanical load on the implant and can make it harder for the surgeon to achieve precise soft tissue balancing. Severe deformity means the ligaments around the knee may already be stretched or damaged before surgery, reducing their ability to stabilize the new joint.

Neuropsychiatric conditions deserve particular attention. A case report documented a patient with an underlying neuropsychiatric illness and hypertonia (abnormally increased muscle tone) who experienced bilateral posterior knee dislocation after bilateral robotic-assisted knee replacement.⁷ Conditions that affect muscle control, coordination, or awareness of limb position can overwhelm even a well-placed implant. People with poorly controlled seizure disorders, certain movement disorders, or cognitive impairments that prevent them from following post-operative activity restrictions carry elevated risk.

What a Dislocated Knee Replacement Feels and Looks Like

Unlike subtle loosening or slow-developing wear, a tibiofemoral dislocation is hard to miss. The knee suddenly becomes visibly deformed, often with an obvious angular shift. Weight-bearing is impossible, and the pain is severe. In posterior dislocation, the shinbone slides backward, leaving a noticeable step between the front of the thigh and the lower leg. In anterior dislocation, the tibia juts forward.

Patellar dislocations are less dramatic in appearance but still distressing. The kneecap moves visibly to the outside of the knee, and the patient loses the ability to actively straighten the leg. Some people experience recurrent subluxation, a partial slip that reduces on its own, before a full dislocation occurs. Pain during stair climbing or rising from a chair, a sense of the knee giving out, and visible lateral tracking of the kneecap during movement are warning signs that should be reported to the surgeon.

Neurovascular Dangers

One reason dislocation of a knee replacement is treated as an emergency is the risk to blood vessels and nerves near the joint. The popliteal artery runs directly behind the knee, and the peroneal nerve wraps around the upper fibula just below it. A significant posterior dislocation can stretch or compress these structures. Case reports have documented artery occlusion (blockage of blood flow) and peroneal nerve palsy (foot drop and loss of sensation along the outer lower leg) following dislocation of total knee replacements.⁸ Vascular compromise is a limb-threatening situation. If blood flow is not restored quickly, the consequences can range from chronic pain and nerve damage to, in the worst scenario, amputation. This is why anyone who suspects their knee replacement has dislocated should seek emergency care immediately rather than waiting for a scheduled appointment.

How a Dislocated Knee Replacement Is Treated

The first step for a tibiofemoral dislocation is typically a closed reduction: the surgeon manipulates the joint back into position under anesthesia without opening the knee surgically. In the six-patient case series described earlier, all six patients underwent urgent closed reduction under general anesthesia as the initial treatment.¹ After reduction, the knee is typically braced and the patient begins a period of conservative management to let things stabilize.

However, closed reduction alone rarely solves the underlying problem. The same structural mismatch that allowed the dislocation in the first place remains, which means residual instability usually persists. All six patients in that series eventually required revision arthroplasty after a period of conservative treatment. Four were revised from a cruciate-retaining design to a posterior-stabilized design, giving the knee more built-in constraint. One patient with a ruptured medial collateral ligament needed conversion to a rotating hinge design, which provides the highest level of intrinsic stability.

The choice of how much constraint to build into the revision implant is made during the operation itself, based on what the surgeon finds when the knee is open. If there is significant ligament damage or bone loss, a rotating-hinge prosthesis with an anti-dislocation system or a long axial cone may be chosen to maximize the “jumping distance,” the amount of force needed to dislocate the new implant.⁹ The degree of constraint is essentially stepped up until the surgeon is confident the knee will stay put. Long-term management frequently requires this kind of revision approach.¹⁰

Long-Term Outcomes After Revision for Dislocation

Revision surgery for dislocation generally improves function, but it does not always return the knee to the level people hope for. A study of 39 patients who underwent revision surgery for patellar dislocation after total knee arthroplasty found that functional scores improved significantly after the procedure. However, two thirds of those patients still had residual disabilities and pain at an average follow-up of just over three years.¹¹ That is a sobering statistic. Revision surgery is more complex than the original procedure, involves a longer recovery, and works with tissue that has already been operated on at least once. Scar tissue, weakened bone, and stretched-out ligaments all reduce what the surgeon has to work with.

This is not to say revision is futile. A knee that is functionally dislocated or chronically unstable is far worse than a revised knee with some residual limitations. The point is that prevention and prompt treatment of instability before it progresses to full dislocation are far preferable to dealing with the aftermath.

Reducing Your Risk

You cannot eliminate the risk of dislocation entirely, but several factors are within your control or your surgeon’s. Before surgery, managing your weight reduces the mechanical stress on the new joint and makes it easier for the surgeon to achieve precise soft tissue balance. If you have any neurological condition that affects muscle control or coordination, discuss it thoroughly with your surgical team; it may influence the implant type chosen or the post-operative precautions recommended.

During surgery, the decisions about implant design, component rotation, and soft tissue balancing are the surgeon’s domain, but you can ask questions. Patients with significant deformity or ligament laxity may benefit from a more constrained implant design from the outset rather than a minimally constrained one that relies heavily on native ligaments for stability. Experienced high-volume knee replacement surgeons tend to encounter and correct alignment issues more readily, so the choice of surgeon matters.

After surgery, following activity restrictions during the early recovery period is important. Avoid deep squatting, heavy lifting, and sudden twisting in the weeks after the procedure while the soft tissues heal around the implant. Report any sensation of the knee giving way, catching, or feeling unstable to your surgeon promptly. Early subluxation episodes, where the joint shifts partially and then corrects itself, are often a warning sign that something is off with alignment or soft tissue tension. Catching this early gives the surgical team a chance to intervene with bracing, physical therapy adjustments, or minor revision before a full dislocation occurs.

Dislocation Risk With Partial Versus Total Knee Replacement

Partial (unicompartmental) knee replacements deserve separate attention because their dislocation profile differs from that of total knee replacements. A partial replacement resurfaces only the damaged portion of the knee, usually the inner (medial) or outer (lateral) compartment, leaving the rest of the native joint intact. Mobile-bearing partial replacements, in particular, trade the friction-reducing benefit of a rotating insert against the risk of that insert dislocating. As noted earlier, the dislocation rate for mobile-bearing lateral partial replacements runs between 1% and 6%, considerably higher than the rate for total knee replacements.³

Most bearing dislocations in partial replacements happen when the knee is bent and the joint surfaces are slightly distracted, or pulled apart. The amount of distraction needed to cause dislocation varies from patient to patient, which means some people are inherently more vulnerable based on the looseness of their particular joint. Fixed-bearing partial replacements, where the plastic insert is locked to the tibial component, do not carry this specific risk, though they have their own trade-offs in wear patterns and constraint.

If you are considering a partial knee replacement and your surgeon recommends a mobile-bearing design, it is worth asking about the dislocation rate for your specific anatomy and what the plan would be if a bearing dislocation occurred. Given the high redislocation rate after simple bearing exchange, some surgeons prefer to convert directly to a total knee replacement rather than attempt a bearing swap, especially if the initial dislocation occurred without an obvious correctable cause.⁴

How Rotating-Hinge Implants Address Recurrent Instability

For patients with the most severe instability, whether from massive ligament loss, failed prior revisions, or progressive neurological conditions, rotating-hinge knee replacements represent the last line of defense. These implants have a mechanical axis that connects the femoral and tibial components, functioning like a true hinge while also allowing some internal and external rotation. They provide the greatest intrinsic constraint of any knee replacement design, which means they rely the least on surrounding soft tissue for stability.

Even rotating-hinge implants can dislocate, though it takes substantially more force. When they do, the consequences are serious because there are few remaining options for further revision. Surgeons selecting a rotating-hinge design for a patient with severe compromise are advised to use a long rotational post and to consider adding an anti-dislocation mechanism if there is significant intraoperative laxity.⁹ The decision about which specific hinge design to use depends on the individual patient’s bone quality, remaining ligaments, and the reason the previous implant failed. Getting this choice right is critical because there may not be a viable next step if this implant also fails.

1
PubMed Central. Dislocation following total knee arthroplasty: A report of six cases
2
PubMed Central. Bilateral Patella Dislocation after Total Knee Arthroplasty: A Report of Two Cases and a Review of the Literature
3
PubMed Central. Mechanisms of mobile bearing dislocation in lateral unicompartmental knee replacement
4
Knee Surgery & Related Research. Redislocation after Bearing Exchange for the Treatment of Mobile Bearing Dislocation in Medial Unicompartmental Knee Arthroplasty
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PubMed. Revision total knee arthroplasty for patellar dislocation in patients with malrotated TKA components
6
PubMed Central. Dislocation after Posterior Stabilized Primary Total Knee Replacement: A Rare Complication in Four Cases
7
PubMed Central. A Rare Complication of Sequential Bilateral Posterior Knee Dislocation Following Bilateral Robotic Assisted Primary Knee Replacement: Case Report
8
International Journal of Surgery Case Reports. Catastrophic case of the total knee arthroplasty dislocation: A case report
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PubMed Central. Dislocation of rotating-hinge total knee arthroplasty
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PubMed. Revision surgery for a dislocated constrained total knee arthroplasty
11
PubMed. Revision surgery for patellar dislocation after primary total knee arthroplasty