A knee assembly refers to the complete set of parts that make up either a prosthetic knee (worn by someone with an above-knee amputation) or a total knee replacement implant (surgically placed inside the joint). Both types share the same goal: replicating the stability, movement, and weight-bearing function of a natural knee. The specific components differ significantly between the two, so understanding which type you’re dealing with determines what’s “in the box.”
Prosthetic Knee Assembly Components
A prosthetic knee assembly is the external device that replaces the knee joint for people with above-knee limb loss. It sits between the socket (which fits over the residual limb) and the lower pylon or shin tube that connects to the prosthetic foot. Every prosthetic knee breaks down into two main categories of parts: functional components and functional mechanisms.
Functional components are the elements that store, release, or absorb energy during walking. These include springs (which store energy and help the knee return to a straight position), dampers (which resist motion to slow the knee down at controlled speeds), and in powered models, small motors that actively drive the joint. Functional mechanisms are the mechanical frame elements that provide structural stability, allow the knee to bend and straighten, and let a prosthetist adjust how the knee behaves.
Single-Axis vs. Multi-Axis Frames
The simplest prosthetic knees use a monocentric (single-axis) design, meaning the knee pivots around one fixed point, much like a door hinge. These knees rely on their internal damping structures to keep the joint stable when you’re standing on it. Polycentric knees, by contrast, use a four-bar linkage, a set of interconnected bars that shift the knee’s center of rotation as it bends. This changing pivot point makes the knee inherently more stable during standing and shortens the overall length of the prosthesis when it’s bent, which helps with ground clearance while walking.
Hydraulic and Pneumatic Controls
Inside many prosthetic knee units, a small cylinder filled with fluid (hydraulic) or air (pneumatic) acts as the damper. Hydraulic cylinders use oil to provide strong, smooth resistance, which is better suited for people who walk at varying speeds. Pneumatic cylinders use compressed air and tend to be lighter, though they offer less resistance at higher walking speeds. These cylinders connect to valves that a prosthetist can adjust to change how quickly or slowly the knee swings and how much resistance it provides when you put weight on it.
Microprocessor Knee Components
Advanced prosthetic knees add a layer of electronic components on top of the mechanical frame. A microprocessor knee like the College Park Icon includes angle sensors that track the knee’s position, an inertial measurement unit (IMU) for spatial awareness, and a linear valve arrangement that adjusts hydraulic resistance in real time. The onboard sensors sample data 100 times per second, feeding information to a processor that decides how much resistance to apply at each moment of your stride. These units also include a rechargeable battery and are often sealed against water and dust. The battery is typically replaceable when it eventually wears out.
Total Knee Replacement Implant Components
A total knee replacement assembly is a surgical implant designed to resurface a damaged knee joint from the inside. The FDA classifies a standard total knee as having three main sections, each with its own parts.
The femoral component is a curved metal cap that fits over the reshaped end of the thighbone. It recreates the rounded surface that the original bone provided. The tibial component has two pieces: a metal baseplate that sits on top of the shinbone, and a plastic (polyethylene) bearing surface that snaps or sits on top of that baseplate. The patellar component, when used, is a small polyethylene button with a metal backing that resurfaces the underside of the kneecap. Together, these pieces recreate the three contact surfaces of a natural knee: thighbone on shinbone, and kneecap on thighbone.
Materials Used in Each Part
The metal components are most commonly made from cobalt-chromium alloy or titanium alloy (often with a titanium nitride coating). Cobalt-chromium is stiffer and more wear-resistant, making it a popular choice for the femoral component where smooth articulation matters most. Titanium is closer in stiffness to natural bone, which helps preserve bone density around the implant over time. Research comparing the two found that cobalt-chromium implants led to greater bone density loss in the surrounding shinbone than titanium-coated implants, particularly in the areas just below the tibial baseplate. For patients with weaker bones, titanium-based implants may offer an advantage in maintaining bone stock long term.
The plastic bearing surface is made from ultra-high-molecular-weight polyethylene, a durable medical-grade plastic engineered to withstand millions of bending cycles. This insert is the part most likely to wear out over the life of the implant.
Fixed-Bearing vs. Mobile-Bearing Designs
The polyethylene insert can be locked in place (fixed-bearing) or allowed to move slightly on the tibial baseplate (mobile-bearing). In a fixed-bearing design, the plastic snaps firmly onto a titanium alloy baseplate. Because the insert doesn’t move, all the complex twisting and sliding motions of walking happen at a single surface, which can concentrate wear.
Mobile-bearing designs decouple that motion into two separate interfaces. Depending on the specific design, the insert may rotate freely around the shin’s central axis, glide front-to-back, or both. This spreads out the contact stresses and can reduce wear. To allow smooth movement, mobile-bearing baseplates are typically made from highly polished cobalt-chromium alloy rather than titanium, since the insert needs to glide against the metal surface underneath it.
Polyethylene Insert Sizing
Getting the right tension in the knee’s ligaments depends heavily on the thickness of the polyethylene insert. Traditionally, inserts start at 9 or 10 mm thick and increase in 2 mm steps. Newer systems now offer 1 mm increments, at least through the most commonly used range. For example, one system provides 1 mm steps from 9 to 14 mm, then switches to 2 mm steps from 16 to 18 mm. That finer resolution gives surgeons a better chance of achieving balanced ligament tension without having to release or tighten soft tissues to compensate for a spacer that’s slightly too thick or thin.
How the Implant Attaches to Bone
The knee assembly doesn’t function unless it’s firmly anchored. There are three fixation approaches: cemented, cementless, and hybrid.
Cemented fixation uses bone cement (polymethylmethacrylate, or PMMA), a fast-setting polymer that has been used in joint replacement for over 60 years. The cement fills the gap between the implant and the prepared bone surface, transferring load evenly and providing immediate stability. Cementless fixation skips the cement entirely. Instead, the implant surfaces are roughened, porous-coated, or 3D-printed to encourage bone to grow directly into the metal. Some cementless implants add a hydroxyapatite coating, a calcium-based mineral layer that promotes faster and stronger bone attachment compared to bare metal. Modern cementless designs use highly porous metals like porous tantalum or porous titanium to maximize bone ingrowth potential.
Hybrid fixation splits the difference: the tibial side is cemented for immediate stability in the softer shinbone, while the femoral side is left cementless, relying on bone ingrowth into the harder thighbone.
Surgical Instrumentation Included With the Assembly
A knee assembly kit shipped to a hospital doesn’t just contain the implant. It includes a set of temporary surgical instruments used during the procedure. A typical single-use instrumentation set comes in multiple sterilized trays and includes femoral trial components (temporary stand-ins for the final implant, used to check fit and alignment), tibial baseplate trials, insert trials and spacer blocks for gauging ligament tension, and a tibial fin punch for preparing the bone to accept the implant’s anchoring features. These instruments are used during surgery and discarded afterward. They’re not part of the permanent implant but are essential to getting the assembly positioned correctly.
How Long a Knee Assembly Lasts
For total knee replacements, the survival rate across all causes of failure is 90 to 95% at 10 years and 80 to 90% at 15 years. The most common reason an assembly eventually fails is wear of the polyethylene insert, followed by loosening of the metal components from the bone. Infection, instability, and fracture around the implant account for a smaller share of revisions. For prosthetic knee assemblies worn externally, lifespan depends on the design and activity level, but most mechanical components are serviceable or replaceable without discarding the entire unit.