A knee spacer is a temporary device placed inside the knee joint after an infected knee replacement is removed. It serves two purposes at once: it releases antibiotics directly into the joint to fight infection, and it holds the space where the permanent implant will eventually go, preventing the surrounding tissues from shrinking and stiffening. Knee spacers are the centerpiece of what surgeons call a “two-stage revision,” where the infected implant comes out first, the spacer does its work for several months, and then a new permanent knee replacement goes in.
Why a Knee Spacer Is Needed
When a knee replacement becomes infected, antibiotics alone often can’t reach the bacteria hiding on the surface of the implant. The solution is to remove the infected hardware entirely, clean out the joint, and start fresh. But you can’t just leave the knee empty for months while the infection clears. Without something holding the joint open, the muscles shorten, scar tissue forms, the joint capsule thickens, and the leg can lose significant length. All of that makes putting in a new knee replacement far more difficult.
The spacer solves this by maintaining joint stability and keeping the soft tissues stretched to a functional length. At the same time, it’s made from bone cement loaded with antibiotics, which leach out gradually into the surrounding tissue. This delivers antibiotic concentrations directly at the infection site that would be impossible to achieve with pills or IV drugs alone. The cement is intentionally mixed to be slightly porous, which helps the antibiotics release more effectively over time. The most common antibiotic combination is gentamicin and vancomycin, chosen because together they cover most of the bacteria responsible for joint infections.
Static vs. Articulating Spacers
There are two main types: static spacers, which lock the knee in place and prevent any movement, and articulating (also called dynamic) spacers, which are shaped more like a real knee joint and allow bending.
The choice matters for recovery. In a study with at least five years of follow-up, patients with articulating spacers had significantly better range of motion after their final surgery: about 111 degrees of bending compared to 82 degrees for static spacers. They also scored higher on measures of physical function and overall knee performance. The reason is straightforward. Allowing the knee to move during the waiting period keeps the quadriceps muscle from shortening, prevents scar tissue from locking up the joint, and preserves the elasticity of the structures that let the knee bend and straighten.
Infection clearance rates, however, are nearly identical between the two types, around 84 to 86 percent. So the decision between static and articulating usually comes down to the severity of bone loss, how stable the joint is, and the surgeon’s judgment about whether the knee can safely handle motion during the interim period.
What the Spacer Looks Like
Most knee spacers are hand-molded in the operating room from antibiotic-loaded bone cement. The surgeon shapes the cement to roughly fit the patient’s joint. Articulating versions have a rounded femoral (thighbone) component and a flat tibial (shinbone) component, mimicking the basic geometry of a knee replacement.
Newer approaches use 3D printing to design patient-specific molds before surgery. These custom spacers fit the joint anatomy more precisely, create less dead space inside the knee, and have smoother surfaces that allow better movement. Studies show they lead to less bone loss during the waiting period, shorter operating times during the second surgery, and higher patient satisfaction compared to hand-shaped versions. Because the surface area is larger relative to volume, they also release antibiotics more efficiently.
How Long You Live With a Spacer
The spacer typically stays in place for 12 to 16 weeks. During this window, your surgical team monitors blood work and clinical symptoms to confirm the infection is clearing. Research suggests that this 12 to 16 week range hits the sweet spot: long enough for thorough infection treatment, but not so long that complications increase. Spacers left in longer than 16 weeks are associated with lower success rates for the second surgery.
Before the permanent implant goes in, surgeons typically take tissue samples from inside the joint to verify the infection is gone. The decision to proceed with reimplantation combines the lab results with whether your clinical symptoms (pain, swelling, redness, warmth) have fully resolved.
Daily Life With a Knee Spacer
Weight bearing is typically restricted while you have a spacer. Most patients use a walker or crutches and are told to limit how much force they put through the leg. Some newer articulating spacer designs are being tested with unrestricted weight bearing, and early results show patients can walk fully on them, though there is a small risk of the spacer shifting out of position.
If you have an articulating spacer, you’ll likely do gentle range-of-motion exercises to keep the joint from stiffening. With a static spacer, the knee stays immobilized, sometimes in a brace. Either way, this interim period is a significant commitment. You’ll have limited mobility for several months and will need help with daily tasks, transportation, and household activities.
Risks and Complications
Knee spacers carry a meaningful complication rate. In one review, mechanical complications occurred in about 18 percent of cases. The most common problems differ by spacer type. Static spacers tend to have cement pieces shift out of position, particularly on the front of the tibia or femur, or hardware cutting into weakened bone. Articulating spacers are more prone to dislocation, where the components slide apart.
Other risks include the infection persisting or coming back despite treatment, progressive bone loss that makes the second surgery more complex, joint stiffness (especially with static spacers), and wound healing problems. Higher body weight and pre-existing bone loss both increase the chance of mechanical failure.
Success Rates for the Full Process
The two-stage revision process, with a spacer in between, successfully eradicates infection in roughly 77 to 87 percent of cases, depending on the study and follow-up length. That means a meaningful minority of patients face persistent or recurrent infection and may need additional surgeries, long-term antibiotic suppression, or in rare cases, permanent removal of the implant without replacement.
Knee-specific outcomes tend to be at the higher end of that range. One long-term observational study found successful infection clearance in 14 out of 16 knee cases. When the process works as planned, patients go on to receive their new permanent knee replacement and begin standard rehabilitation, with the goal of returning to pain-free daily activities.
A Different Kind of Knee Spacer
There is one other device that has been called a “knee spacer,” though it serves a completely different purpose. The UniSpacer was a small metal implant shaped like a meniscus, designed to slip into the knee joint without any bone cuts to treat arthritis on the inner side of the knee. It was marketed as a simpler, less invasive alternative to partial knee replacement. However, its track record was poor: only about 53 percent of implants survived to ten years, with more than a third requiring early revision to a full knee replacement. The device was pulled from the market in 2011. If you come across the term “knee spacer” in the context of arthritis treatment rather than infection, this is likely what’s being referenced, but it is no longer available.