Introduction: CR and PS knee implant concepts solve the same motion problem in two different ways, because one keeps the posterior cruciate ligament and the other replaces it.
Every total knee design has to decide what happens to the posterior cruciate ligament. A cruciate-retaining, or CR, concept leaves that ligament in place and expects it to keep working. A posterior stabilized, or PS, concept substitutes for it with a mechanical cam-post structure built into the implant itself. That single decision shapes the femoral component, the tibial insert, and the way posterior stability is produced during knee motion. Those two paths also explain why CR and PS are better understood as answers to different questions than as two versions of one answer, and why the structures sitting on a surgical tray look so different once you know what each design is trying to do.
In a CR knee implant, the femoral component carries an open intercondylar notch so the posterior cruciate ligament can pass through and stay attached to both the femur and the tibia. The ligament keeps guiding the knee, and posterior restraint still comes from biology: the PCL, the joint capsule, the surrounding soft tissues, and the shape of the articular surfaces working together. Because the implant depends on that living tissue, a CR tibial insert is usually flatter and less conforming than its PS counterpart, leaving room for the rollback and rotation that a retained ligament produces. Balancing this kind of knee is therefore a soft-tissue task as much as an implant task, and the quality of the ligament becomes part of the design's daily performance. A PS femoral component handles the same region very differently. The open notch gives way to a box, and a cam sits across the back of that box. The posterior cruciate ligament is resected, so its stabilizing role moves into the implant, which is exactly where the term PCL-substituting comes from. This is also why the parts have to travel as a set: a PS femoral component works with the PS tibial insert and the tibial baseplate of the same system, not with a CR insert. ShinvaUnited's PS femoral component, part of the XU Knee System, is designed for PCL-substituting procedures and uses a cam-post structure for posterior stabilization. The contrast is easy to see in a device-education session, where two femoral components sit side by side on a table: one with an open notch for the ligament, one with a closed box and cam that will do the ligament's work.
PCL substitution is really a decision about where posterior stability comes from. Keeping the ligament means stability stays biological. Replacing it means stability becomes mechanical, produced by the geometry of the components rather than by how well a ligament happens to hold tension. Three ideas are worth keeping separate whenever that trade-off comes into view.
Once substitution is part of the design, posterior restraint becomes a property of the component set, something that can be specified, checked, and reproduced from one case to the next.
The two concepts exist because the posterior cruciate ligament and the surrounding soft tissues are not the same in every knee. A CR design addresses a knee where the PCL is expected to be preserved and balanced, so the question it answers is whether that ligament can be kept and made to work inside the implant's envelope. A PS design answers a different question: what happens when the ligament cannot be counted on? When the PCL is damaged, when deformity has stretched or contracted it, or when predictable posterior restraint is preferred without depending on ligament tensioning, a PCL-substituting design supplies that restraint mechanically. Neither concept wins in general. A CR design placed in a knee with a compromised ligament leaves posterior restraint to tissue that may not deliver it well, while a PS design brings its own considerations, such as the intercondylar bone preparation needed for the box and the way the post interacts with the cam over time. The difference also shows up in motion. In a ligament-guided knee, the pattern of femoral rollback varies with the patient's own anatomy and with how the soft tissues were balanced. In a substituted knee, rollback is guided by contact between two manufactured surfaces, which makes the motion more consistent from case to case and easier to reason about when studying kinematics. That is the practical value of separating the two concepts: a CR femoral component tells you the ligament is part of the stability plan, while a PS femoral component tells you the implant is. Reading a tray becomes faster that way, too, because an open notch with a flat insert and a closed box with a post insert each announce which stability strategy is in front of you.
CR and PS knee implants differ first and most fundamentally in what they do with the posterior cruciate ligament. CR retains it, keeps an open notch for it, and lets biological tissue share the work of posterior restraint. PS substitutes it, replaces the notch with a box and cam, and places that restraint inside the component set, which is why a PS femoral component must be matched with its own PS tibial insert and tibial baseplate. Understanding the cam-post structure in those basic terms makes the two concepts easier to tell apart and easier to explain, whether the goal is reading a surgical tray correctly or following how posterior stability is produced during flexion. The design intent behind a PCL-substituting femoral component is a useful place to see those mechanics in a single piece of hardware.
A：A CR concept retains the posterior cruciate ligament, leaves an open intercondylar notch in the femoral component so the ligament can pass through, and pairs with a flatter tibial insert that allows the ligament to guide motion. A PS concept substitutes for that ligament with a cam-post mechanism, uses a boxed femoral component, and pairs with a tibial insert that carries a central post. Both approaches manage posterior translation, but one relies on retained tissue and the other relies on component geometry.
A：The cam on the femoral component engages the post on the PS tibial insert as the knee flexes. That contact limits how far the tibia can move backward and guides the femur through a controlled rollback, taking over the restraining role the resected ligament used to play. Because the restraint comes from the implant, the femoral component has to be matched with the PS tibial insert and tibial baseplate of the same system so the cam and post interact as designed.
A：Substitution becomes relevant when the posterior cruciate ligament is not expected to be preserved or reliably balanced, such as a damaged ligament or a knee where deformity has affected its function. It is also used when a surgical team prefers posterior restraint that comes from the implant rather than from ligament tensioning. That choice belongs to the operating team and depends on the individual knee, so the concepts are best understood as different design answers rather than as a ranking.
Knee Replacement Implants - OrthoInfo - AAOS
Popliteal Aneurysm Repair (Archived) - StatPearls - NCBI Bookshelf
