Implant Constructs in TKR — CR, PS, Kinematic Alignment

Part of The Joint Replacement Guide — Phase 4: The Operation · Article 24 of 42 · Knee

Key Points

• Knee replacement implants come in different designs depending on whether one of your original knee ligaments (the posterior cruciate ligament) is kept or replaced.
• Cruciate-retaining (CR) and posterior-stabilised (PS) are the two most common construct types, with broadly comparable outcomes.
• Alignment philosophy — how the implant is positioned relative to your natural anatomy — is a genuinely evolving and debated area of modern knee surgery.

Cruciate-Retaining (CR) Implants

Your knee has two cruciate ligaments running through its centre — the anterior cruciate ligament (ACL), which is always removed during knee replacement, and the posterior cruciate ligament (PCL). A cruciate-retaining (CR) implant is designed to preserve and rely on your own PCL for some of the joint’s stability, aiming to maintain a more natural feeling knee motion, provided the PCL is healthy and functioning well at the time of surgery.

Posterior-Stabilised (PS) Implants

A posterior-stabilised (PS) implant removes the PCL and uses a built-in mechanical post-and-cam mechanism within the implant itself to provide the stability the PCL would otherwise have contributed. This is often preferred where the PCL is damaged, absent, or of poor quality, or where a surgeon judges it will give more reliable, consistent stability for a particular knee.

Which Is Better?

This has been debated in orthopaedic research for decades, and the honest answer is that large studies and registry data consistently show broadly comparable long-term outcomes between well-performed CR and PS knee replacements. The choice is generally guided by the condition of your own PCL found at surgery, your surgeon’s usual practice and training, and specific features of your knee anatomy, rather than one design being definitively superior for everyone.

Kinematic Alignment

Traditionally, knee replacements have been positioned using “mechanical alignment,” aiming for a neutral, mechanically straight leg axis regardless of your knee’s original individual shape. A newer philosophy, kinematic alignment, instead aims to recreate your own knee’s natural, pre-arthritic alignment and joint line — the idea being that a more individualised fit may feel more natural and improve function. This remains a genuinely active area of research and debate; some studies show promising early results for kinematic alignment, particularly in patient-reported comfort, while longer-term implant survivorship data comparing the two philosophies is still accumulating.

In More Depth

The rise of robotic and computer-assisted surgery, covered in a dedicated article shortly, has renewed interest in alignment philosophy generally, because these technologies make it technically easier to reliably achieve more individualised alignment targets than was practical with traditional manual instrumentation. This is a genuinely evolving field — it would not be accurate to say the debate between mechanical and kinematic alignment is fully settled, and different surgeons, reasonably, take different views based on their reading of the current evidence. What matters practically is that your surgeon has a clear, considered rationale for the approach they use, rather than there being one single universally “correct” answer at this stage.

This article is general information and does not replace individual clinical advice.

Sources

• National Joint Registry — Annual Report, knee implant construct data
• Cochrane Database of Systematic Reviews — cruciate-retaining versus posterior-stabilised knee replacement
• British Association for Surgery of the Knee (BASK) — alignment philosophy resources

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