Implant Constructs in TKR — CR, PS, Kinematic Alignment

Part of The Joint Replacement Guide — Phase 4: The Operation · Article 25 of 44 · Knee

Key Points

  • Knee replacements are available in different designs according to how they work with—or substitute for—the knee’s ligaments.
  • Cruciate-retaining (CR) and posterior-stabilised (PS) implants are two established options, but they are not the only designs available.
  • Research has not demonstrated a clinically important overall advantage of CR or PS replacement for every patient.
  • Alignment philosophy describes how the components are positioned relative to the leg and the patient’s individual anatomy.
  • Mechanical alignment remains the traditional reference standard, while kinematic, restricted kinematic and functional alignment are evolving alternatives.
  • Implant design, ligament balance and alignment are connected, but they are not the same decision.

The Cruciate Ligaments

The natural knee contains two cruciate ligaments:

  • the anterior cruciate ligament, or ACL; and
  • the posterior cruciate ligament, or PCL.

These ligaments help control the forward-and-back movement and rotation of the knee.

The ACL is removed during most conventional total knee replacements because it occupies the space required for the implant. However, it is not accurate to say that it is removed in every possible knee-replacement design. Less commonly used bicruciate-retaining implants are designed to preserve both the ACL and PCL in carefully selected patients.

The PCL may be preserved or removed, depending on its condition, the implant design, the knee’s deformity and the surgeon’s preferred method of balancing the joint.

Cruciate-Retaining Implants

A cruciate-retaining, or CR, replacement preserves the PCL. The implant is designed to work with the retained ligament to help control movement between the femur and tibia, particularly as the knee bends.

Potential reasons for choosing a CR implant include:

  • a healthy and functional PCL;
  • preservation of more of the patient’s natural soft-tissue anatomy;
  • avoiding the additional femoral bone cut required by many PS designs; and
  • the surgeon’s experience with CR balancing.

A CR replacement does not reproduce completely normal knee movement, and retaining the PCL does not guarantee that the knee will feel more natural.

The PCL must also be balanced carefully. If it is too tight, movement can be restricted or abnormal forces can pass through the polyethylene insert. If it is too loose or damaged, the knee may be unstable. Occasionally, a surgeon intending to use a CR implant may decide during the operation that the PCL is unsuitable and change to another design.

Posterior-Stabilised Implants

In a posterior-stabilised, or PS, replacement, the PCL is removed. A central post on the polyethylene insert engages with a cam on the femoral component as the knee bends.

This mechanism helps guide movement and provides forward-and-back stability after the PCL has been removed. It does not reproduce every function of the natural ligament.

A PS design may be considered when:

  • the PCL is absent, damaged or unsuitable;
  • substantial deformity or stiffness makes dependable PCL balancing difficult;
  • the surgeon wants more predictable control of femoral movement during bending; or
  • the surgeon’s established technique is based on a PS construct.

Many PS implants require an additional box-shaped cut in the femur to accommodate the cam mechanism. Recognised but uncommon design-specific problems include wear or fracture of the polyethylene post and a condition called patellar clunk, in which scar tissue catches near the cam-and-post mechanism.

Other Implant Designs

CR and PS are not the only options.

A cruciate-substituting or ultracongruent design usually removes the PCL but uses a more deeply shaped polyethylene surface to provide stability rather than a PS cam-and-post.

Medial-stabilised designs aim to provide greater stability on the inner side of the knee while permitting more movement on the outer side, attempting to reproduce aspects of normal knee motion.

If one or both collateral ligaments are significantly deficient, a more constrained implant may be required. These implants provide additional mechanical stability but also transfer greater forces to the implant–bone connection. They are therefore normally reserved for knees that need that additional constraint.

Which Design Is Better?

Comparative studies have generally found no clinically important overall difference between CR and PS replacements in pain, function or patient satisfaction.[1]

PS designs may produce a small average increase in knee bending in some studies, but the difference is generally too small to be important to most patients. Evidence has not shown that preserving the PCL consistently makes a replacement feel more natural.

National registry data demonstrates that several construct types can perform well. However, results depend on the particular implant brand, fixation, bearing, patient group and surgical technique—not simply whether the label says CR or PS.[2]

The decision may take account of:

  • the condition of the PCL;
  • the degree of deformity or stiffness;
  • the stability of the collateral ligaments;
  • previous surgery;
  • bone quality;
  • the implant’s established results; and
  • the surgeon’s experience and balancing technique.

What Is Alignment?

Alignment describes how the femoral and tibial components are positioned relative to the bones, joint line and overall axis of the leg.

This is different from deciding whether to use a CR or PS implant. Either design may potentially be used with more than one alignment philosophy, although particular implants and balancing techniques may be better suited to particular strategies.

The objective is not simply to make an X-ray look straight. The surgeon aims to produce a stable, balanced knee in which the components are appropriately positioned and the forces passing through the implant remain acceptable.

Mechanical Alignment

Mechanical alignment is the traditional reference philosophy for total knee replacement. It generally aims to position the components relative to the mechanical axes of the femur and tibia and to create an overall neutral leg axis.

Because natural anatomy varies, achieving this target may require changes to the patient’s pre-arthritic joint-line orientation and releases of tight ligaments to balance the spaces on the inner and outer sides of the knee.

Mechanical alignment has a long clinical history and extensive implant-survivorship evidence. However, critics argue that applying approximately the same neutral target to knees with different natural anatomy may contribute to an artificial feeling or require greater soft-tissue adjustment in some patients.

Kinematic Alignment

Kinematic alignment aims to recreate aspects of the patient’s estimated pre-arthritic joint surfaces and joint-line orientation. The underlying theory is that restoring more of the individual anatomy may reduce the need for ligament releases and produce a knee that feels more natural.

The surgeon must estimate the anatomy that existed before cartilage and bone were lost through arthritis. Kinematic alignment does not simply mean placing the components wherever the worn joint happens to sit.

Some studies report modest improvements in particular patient-reported scores after kinematic alignment, while others find no meaningful functional difference. A randomised trial reporting ten-year results found no significant difference in patient-reported outcomes or revision-free survival between kinematic and mechanical alignment, although the study involved a relatively small and selected patient group.[3]

Longer-term evidence is growing, but it remains less extensive than the evidence supporting conventional mechanical alignment.

Restricted Kinematic and Functional Alignment

Many surgeons do not use completely unrestricted kinematic alignment.

Restricted kinematic alignment attempts to reproduce the patient’s anatomy while keeping component and overall limb alignment within predetermined safety boundaries. If the natural anatomy falls outside those boundaries, the plan is adjusted.

Functional alignment is a more recent and variably defined philosophy. It usually starts with an individualised alignment plan and then adjusts component position to achieve an intended pattern of ligament balance and knee movement. It is commonly associated with robotic or computer-assisted systems.

These terms are not used identically by every surgeon or in every study. This makes apparently simple comparisons between alignment philosophies more difficult.

The Role of Robotics and Navigation

Robotic and computer-assisted systems can help the surgeon measure anatomy, plan component position and reproduce a chosen alignment target more accurately.

The technology does not decide which philosophy is correct. A robot can be programmed to pursue mechanical, kinematic, restricted kinematic or functional targets, depending on the system and the surgeon’s plan.

More accurate execution of a plan does not automatically mean that the plan produces better long-term patient outcomes. The clinical benefit of robotic assistance and the ideal alignment target remain active areas of research.

What Matters Most?

A successful knee replacement requires several decisions to work together:

  • appropriate implant constraint;
  • accurate component sizing and positioning;
  • acceptable overall alignment;
  • balanced ligaments throughout movement;
  • stable kneecap tracking;
  • secure fixation; and
  • protection of the surrounding soft tissues.

There is no single CR, PS or alignment formula that is right for every knee. Equally, “personalised” alignment should not mean that recognised mechanical limits are ignored without a considered clinical reason.

Your surgeon should be able to explain the design and alignment philosophy they recommend, why it is appropriate for your knee and what alternatives are reasonable.

Questions You May Wish to Ask

  • Will my PCL be preserved or removed?
  • Which implant design are you recommending, and why?
  • Could the implant choice change during the operation?
  • Which alignment philosophy do you use?
  • Is that approach modified for my individual anatomy?
  • Will navigation or robotic assistance be used?
  • What evidence supports the complete implant system being proposed?

This article provides general information and does not replace individual clinical advice. Implant design and alignment should be selected according to your anatomy, ligament function and the judgement and experience of your surgical team.

References

  1. Verra WC, van den Boom LGH, Jacobs W, Clement DJ, Wymenga AAB, Nelissen RGHH. Retention versus sacrifice of the posterior cruciate ligament in total knee arthroplasty for treating osteoarthritis. Cochrane Database Syst Rev. 2013;(10). doi: 10.1002/14651858.CD004803.pub3.
  2. National Joint Registry. Primary knee replacement by fixation, constraint and bearing. In: 22nd Annual Report 2025. NJR knee construct data.
  3. Gibbons JP, Zeng N, Bayan A, et al. No difference in 10-year clinical or radiographic outcomes between kinematic and mechanical alignment in TKA: a randomized trial. Clin Orthop Relat Res. 2025;483(1):140–149. doi: 10.1097/CORR.0000000000003193.
  4. Wang G, Chen L, Luo F, Luo J, Xu J. Superiority of kinematic alignment over mechanical alignment in total knee arthroplasty during medium- to long-term follow-up: a meta-analysis and trial sequential analysis. Knee Surg Sports Traumatol Arthrosc. 2024;32(5):1240–1252. doi: 10.1002/ksa.12093.
  5. Jamali AA, Shekhar A, Dungy D, Stewart SL. Kinematic versus mechanical alignment: a systematic review of systematic reviews and meta-analyses of randomised controlled trials. J Exp Orthop. 2024;11(4). doi: 10.1002/jeo2.70044.

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