Why Total Knee Replacements Treat Cruciate Ligaments Differently
When most people picture a total knee replacement (TKA), they imagine a simple swap of “old parts for new.” In reality, implant designs and surgical choices vary, especially in how the cruciate ligaments (ACL and PCL) are handled. These choices affect implant stability, surgical complexity, and—at least theoretically—how “natural” the knee feels after surgery.
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Two Main Design Families: CR vs. PS
Most modern TKAs fall into one of two categories:
Cruciate-Retaining (CR): The posterior cruciate ligament (PCL) is left intact to provide posterior stability.
Posterior-Stabilized (PS): The PCL is removed, and the implant uses a cam-and-post mechanism built into the plastic insert to mimic its function.
In both designs the anterior cruciate ligament (ACL) is almost always sacrificed.¹,²
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Why the ACL Is Nearly Always Sacrificed
By the time someone is a candidate for TKA, the ACL is often frayed, scarred, or already nonfunctional from osteoarthritis. Preparing the tibia for a metal tray also removes its insertion. Keeping a degenerated ACL adds unpredictable tension and complicates alignment.³
In contrast, the PCL usually remains healthy and anatomically preservable, so CR implants can use it reliably.²,⁴
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How Stability Is Achieved Without Cruciate Ligaments
If one or both cruciate ligaments are resected, stability comes from:
Implant geometry: Cam-and-post designs or highly congruent (deep-dish) inserts restrain translation.
Soft tissue balancing: Surgeons adjust collateral ligaments to achieve symmetrical tension.
This combination reliably reproduces stability even without the native ACL.²,⁵
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The Rare ACL-Preserving Designs
A handful of newer prostheses are built to keep both the ACL and PCL (“bicruciate-retaining” designs). They use modified tibial trays and separate inserts to preserve the tibial spine and ACL insertion.
The idea is to restore more natural knee kinematics and proprioception. However, these procedures are technically demanding and still uncommon—less than 5% of TKAs worldwide.⁶,⁷
We’ll take a deeper dive into these “bicruciate-retaining” approaches and what the research really says in the next article.
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What This Means for Patients
Having your ACL removed during TKA is standard and does not mean your knee will be unstable.
Whether your surgeon preserves the PCL or uses a PS design depends on ligament quality, deformity, and their training/preferences.
Long-term outcomes between CR and PS designs are generally similar when performed well.¹,²,⁵
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Bottom line: In a standard TKA, the ACL is removed and the PCL may be preserved or substituted. Both approaches are time-tested and yield comparable results. Only a small fraction of TKAs try to keep both cruciates, and we’ll explore the promise and pitfalls of those in our next post.
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References
1. Lombardi AV, Berend KR, Adams JB. Why knee replacements fail in 2013: patient, surgeon, or implant? Bone Joint J. 2014;96-B(11 Supple A):101-104.
2. Han HS, Kang SB. Current concepts of posterior-stabilized total knee arthroplasty. Knee Surg Relat Res. 2013;25(1):1-6.
3. Berger RA, Meneghini RM, Jacobs JJ, et al. Results of unicompartmental knee arthroplasty at a minimum of ten years of follow-up. J Bone Joint Surg Am. 2005;87(5):999-1006.
4. Diduch DR, Insall JN, Scott WN, et al. Total knee replacement in young, active patients: long-term follow-up and functional outcome. J Bone Joint Surg Am. 1997;79(4):575-582.
5. Pagnano MW, Trousdale RT. Tibial polyethylene insert options for total knee arthroplasty. Clin Orthop Relat Res. 2000;(380):73-87.
6. Banks SA, Hodge WA. Implant design and patient kinematics after bicruciate-retaining total knee arthroplasty. Clin Orthop Relat Res. 2004;(428):214-221.
7. Howell SM, Shelton TJ, Hull ML. Implant design considerations for bicruciate-retaining total knee arthroplasty. J Knee Surg. 2018;31(6):479-486.
