Introduction: A posterior stabilized knee implant takes over the job of the posterior cruciate ligament, and that single design decision shapes how the whole primary TKA system fits together.
For a resident learning primary total knee arthroplasty, the letters PS on a femoral component label are easy to read and harder to place. The label refers to more than a shape. It tells you which ligament the implant is prepared to substitute, which tibial insert belongs with it, and how the surgeon will balance the knee once the bone cuts are made. The walkthrough below follows a PS femoral component through a primary TKA system, explains what the cam and post actually do during flexion, and sets the PS idea next to the cruciate-retaining alternative so the difference is clear both in the operating room and in a design review.
The femoral component is the metal half of the joint. It caps the prepared distal femur, restores a smooth articulating surface, and defines the geometry that the tibial insert glides against. In a primary TKA system, three parts do most of the visible work: the femoral component, the polyethylene tibial insert, and the tibial baseplate that carries the insert. Follow them in order and the logic becomes easy to hold in your head. The femur carries the curved bearing surface and, in a PS design, the intercondylar box and cam. The insert carries the post. The baseplate anchors the construct to the proximal tibia. Each part constrains what the others can do, which is why a component is never chosen in isolation. Reading a PS component also means reading its preparation references. The XU Knee System femoral component is a CoCrMo posterior stabilized design supplied in left and right versions, with 11 size steps and a 9 mm distal and posterior resection reference. Those numbers matter to trainees because they connect an implant label to concrete surgical moves: how much bone the cuts remove, how many sizes the instrument tray has to cover, and why a left femur and a right femur are not interchangeable parts on the back table. CoCrMo is the established metal for this bearing surface, and the left/right split follows the asymmetric geometry of the distal femur rather than a manufacturing preference.
The posterior cruciate ligament normally limits how far the tibia can slide backward and helps the femur roll back on the tibia as the knee bends. A PS implant replaces that function with a mechanical pair: a cam on the femoral component and a post rising from the tibial insert. The two are not in contact all the time. In extension and light flexion they sit apart, and the knee behaves much like one with a retained ligament. As flexion increases, the cam swings toward the post, meets it, and from that point the pair begins to guide motion instead of simply allowing it.
The cam sits inside the intercondylar box of the femoral component, and the post stands up from the middle of the polyethylene insert. Engagement is a timing question as much as a geometry question. If the cam meets the post too early, the knee feels constrained through the range a patient uses for walking. If it meets too late, the femur can translate before the mechanism takes over. That is why the post position, the cam shape, the insert thickness, and the femoral size are set as a group rather than one at a time. In practice, a resident can watch the trial components come together in flexion and see engagement happening instead of memorizing a number.
Once the cam contacts the post, further flexion pushes the femur backward relative to the tibia, which is the rollback the intact ligament used to provide. Rollback matters because it keeps the extensor mechanism working efficiently and gives the joint room to bend without impingement at the back of the knee. In a PS knee, that motion comes from the implant rather than from soft tissue tension, so the surgeon is no longer dependent on a ligament that may be stretched, scarred, or removed. The trade-off is that the mechanism has to be shaped and positioned well, since a poorly timed cam-post interaction can produce a stiff or unstable feel that soft tissue alone will not correct.
A cruciate-retaining implant keeps the patient's posterior cruciate ligament and leaves the femoral component without a cam and the insert without a post. The ligament continues to do its normal job, so rollback and posterior restraint come from living tissue. The PS design takes the opposite path: it removes the ligament from the equation and builds a substitute into the implant. Neither approach is newer or better in general; they answer different surgical situations. What matters for a trainee is that the choice appears early in the case, because it decides which femoral component, which insert, and often which tibial preparation the operation will use. That early decision is where a lot of confusion starts. A surgeon may plan a CR knee, then find the ligament tight, attenuated, or mechanically unreliable once the knee is exposed, and switch to a PS system mid-procedure. The XU Knee System handles that switch at the component level: the PS femoral component is built to match XU PS tibial inserts and tibial baseplates, so the substitution is a system decision rather than a one-off part pulled from a shelf. The engineering also supports flexion up to 155° in this design, which shows how the developers framed the flexion envelope. The angle any individual patient reaches after surgery still depends on soft tissue, rehabilitation, and the knee they started with.
The PS femoral component exists because some knees need the posterior cruciate ligament's work done by the implant instead of by tissue. Once you can trace the cam, the post, and the matching insert as one working set, the label on the box stops being a code and becomes a description of how the knee will be guided. For residents, the practical habit is to read a primary TKA system by following that chain from femoral preparation to insert design to the balance the surgeon can still control. That chain, more than any single specification, is what the operation actually depends on.
A: It carries the curved bearing surface for the distal femur and adds a cam inside the intercondylar box. That cam meets a post on the tibial insert as the knee flexes, so the implant supplies the posterior restraint and rollback that the posterior cruciate ligament would normally provide. Within a primary TKA system, the component therefore sets both the articulating geometry and the substitution mechanism, which is why it has to be matched to the right insert.
A: A cruciate-retaining design keeps the patient's posterior cruciate ligament and uses a femoral component without a cam and an insert without a post. A posterior stabilized design removes that ligament from the picture and builds a cam-and-post substitute into the components. The practical difference is where posterior stability comes from: living soft tissue in the cruciate-retaining case, the implant mechanism in the PS case. Component matching follows from that choice, since each design uses its own insert.
A: A PS design is often considered when the posterior cruciate ligament is damaged, stretched, or unreliable, or when the surgeon expects that retaining it would make balancing difficult. Severe deformity and stiffness also push the decision toward substitution. Many surgeons therefore use a PS design as their routine primary choice for those situations, while others keep a cruciate-retaining knee for patients with a healthy, well-functioning ligament.
Total Knee Arthroplasty — NCBI Bookshelf NBK560580
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