TKA Cruciate Retention vs Substitution

The cruciate retention versus substitution debate has run for more than 40 years. Early TKAs sacrificed both the ACL and the PCL, and were unstable. Cruciate-retaining designs emerged in the 1980s to preserve "normal" kinematics, and posterior-stabilised designs followed to deal with the difficulty of balancing the PCL and correcting deformity.

Modern evidence shows equivalent long-term survivorship and patient-reported outcomes, so the choice rests on surgeon preference and patient-specific factors. The decision is about the knee in front of you rather than about which design is better.

What the PCL does. In the normal knee the PCL is the primary restraint to posterior tibial subluxation and guides femoral rollback during flexion. It also influences contact patterns and stress distribution, and maintains the moment arm of the quadriceps, which keeps the extensor mechanism efficient.

Cruciate-retaining (CR). The aim is to preserve the native PCL and with it normal knee kinematics. The implant depends on the ligament for posterior stability:

• Flat or slightly conforming tibial bearing
• No femoral box cut, so bone stock is preserved
• Smaller intercondylar notch clearance

The retained PCL has to guide rollback and prevent posterior tibial translation, so it must be balanced to neutral tension through the flexion arc. Too loose, and posterior translation and wear increase; too tight, and the knee is painful with limited motion and difficult to flex to 90° (the balancing section below gives the detail).

Posterior-stabilised (PS). The aim is to substitute for the PCL mechanically with a cam-post articulation:

• Central tibial polyethylene post
• Femoral box cut with a cam surface
• Higher-conformity tibial bearing
• Cam-post engagement at 60-70° of flexion

Nothing depends on the quality of the PCL. The cam-post gives predictable femoral rollback and prevents posterior tibial subluxation, and the greater constraint allows severe deformity to be managed.

CR and PS are not the only choices. The condylar-stabilised (CS) insert, also called ultracongruent (UC), deep-dish or anterior-stabilised (AS), is the middle path in the cruciate debate. The broader implant taxonomy is covered in the tka-implant-design topic; this is the cruciate-strategy angle.

What it is. A polyethylene bearing with a raised anterior lip and a highly conforming, deep-dished articular surface, used on a standard CR femoral component with no box and no post. Anteroposterior stability comes from articular conformity rather than from a mechanical cam-post.

Where it sits. The PCL is usually sacrificed or already deficient. The deep dish and anterior build-up resist anterior femoral translation (paradoxical slide) and provide posterior restraint, which makes it a PCL substitute without a post. It is the option for the surgeon who wants to sacrifice the PCL for easier balancing but avoid a box cut.

What it gains. With no box, posterior femoral bone is preserved and box-related fracture is avoided. With no post, there is no patellar clunk, post fracture, post dissociation or post wear. The insert is typically interchangeable with the CR femur, so CR versus UC can be decided at trialling.

What it costs.

• Stability depends on good flexion-extension gap balance and conformity, not on a hard mechanical stop
• There is no guaranteed cam-post rollback; rollback relies on geometry and the soft tissues
• The high conformity transmits more constraint to the fixation interface, a theoretical loosening concern
• A grossly unbalanced flexion gap can still let the femur jump the anterior lip (spin-out or dislocation), because there is no post to capture it

Examination. Implant selection starts with the examination:

• PCL integrity: posterior drawer test at 90° of flexion, which should have a firm endpoint; excessive laxity suggests an incompetent PCL
• Deformity: varus or valgus alignment and its degree
• Range of motion: flexion contracture and maximum flexion
• Collateral ligament competence

Patient factors. Age and activity level pull in different directions. Younger patients may benefit from CR because it preserves bone, and younger, higher-demand patients are listed in its favour to save bone stock for a future revision; yet high-demand patients may prefer PS for its predictable kinematics, and elderly, lower-demand patients suit PS because the greater constraint improves stability.

Deep flexion. Cultural practices that need deep flexion (kneeling, squatting) are sometimes cited in favour of CR, but this is controversial and PS may actually be better.

Comorbidities. Inflammatory arthropathy is a strong indication for PS. Neurological conditions may benefit from the constraint of PS, and previous trauma or surgery may compromise the quality of the PCL.

Normal kinematics. The PCL matters little early in flexion, when the ACL is the primary stabiliser, and increasingly as the femur rolls back. CR and PS behave alike in the first 30°.

CR against PS. A retained PCL produces rollback that varies with its tension; the cam-post produces it consistently. PS has the greater flexion potential, but there is no functional difference in daily activities.

Imaging. Each study is read for what it says about the PCL and the deformity. A skyline view evaluates the patellofemoral joint.

Laboratory tests. Most primary OA needs no extensive laboratory work-up. Specific scenarios warrant it:

• Inflammatory arthropathy: ESR, CRP, RF and anti-CCP antibodies (a strong PS indication)
• Infection screening: ESR, CRP, and synovial fluid analysis if there is an effusion
• Metabolic bone disease: vitamin D, calcium and parathyroid hormone if suspected
• Coagulopathy screening: INR and PT/PTT if on anticoagulation

When CR is the right knee. CR asks for a PCL that works and a knee that can be balanced around it. The absolute requirements:

• An intact PCL, with no significant attenuation, calcification or dysfunction
• Adequate bone stock: posterior femoral condyles intact, no large defects
• Mild-to-moderate deformity, under 15° of varus or valgus
• Primary OA, a non-inflammatory aetiology with preserved soft tissue quality

What CR offers in return is bone preservation (no femoral box cut or tibial post), a theoretical proprioceptive benefit from PCL mechanoreceptors, lower polyethylene conformity with greater rotational freedom, and freedom from cam-post complications such as clunk, wear and dissociation. It also suits patients whose preoperative MRI shows intact PCL fibres.

When PS is the right knee. PS is chosen when the PCL cannot be trusted or the knee will need more than the PCL can give. The strong indications:

• PCL deficiency from attenuation, previous injury or iatrogenic damage
• Severe deformity, over 15° of varus or valgus, needing extensive releases that will compromise the PCL
• Inflammatory arthropathy (RA, psoriatic arthritis, ankylosing spondylitis): PCL quality is unreliable, and the ligament attenuates over time even if it looks intact today
• Previous patellectomy: the extensor mechanism no longer contributes to anteroposterior control, so the cam-post supplies stability the quadriceps cannot
• Revision TKA: the PCL is often sacrificed or non-functional after primary failure
• Flexion contracture: a fixed flexion deformity over 15-20°

PS brings predictable kinematics independent of PCL quality, easier soft tissue balancing with no PCL tension to manage, greater maximal flexion (130-140°, and 140° or more with high-flex designs), better correction of severe deformity, and more consistent femoral rollback. It suits patients with poor soft tissue quality or previous trauma, and those who want maximum flexion. Surgeon experience matters as well, since PS is generally easier to balance than CR.

The cost of the box. Removing bone for cam clearance creates a posterior femoral defect. This is not a significant concern in primary TKA, where bone stock is ample, and the box cut does not compromise primary outcomes; it becomes relevant in revision, where bone preservation is critical.

The neutral zone. In mild-to-moderate primary OA with an intact PCL, either design is acceptable and surgeon experience and patient factors decide:

• Primary OA with mild deformity (under 10°): both CR and PS achieve excellent results
• Moderate deformity (10-15°): surgeon experience and preference dictate the choice
• Young patients with an intact PCL: CR preserves bone stock, PS gives predictable kinematics
• Obesity: no difference in outcomes between CR and PS

The surgeon. Surgeons comfortable with PCL balancing may prefer CR. Many centres have standardised on PS for consistency, and a single design platform reduces inventory costs and complexity.

The final decision between CR and PS is often made in theatre, once the PCL can be felt and tested under trial components. Always have PS instruments available for every primary TKA, including a planned CR knee, so that conversion can be seamless.

PCL balancing in CR

Assessing the ligament. The preoperative drawer test and imaging come first. In theatre:

1. After arthrotomy and synovectomy, or after exposure and initial bone preparation, palpate the PCL. It should feel firm, taut and continuous; a soft, attenuated or calcified PCL suggests poor function, so plan for PS.
2. With the trial components in, flex the knee to 90° and apply a posterior drawer force. A normal PCL allows under 5mm of translation with a firm endpoint. A tight PCL opens the flexion gap posteriorly and makes it difficult to flex to 90°. A loose PCL allows over 5mm with a soft endpoint.

Balancing it. Each problem has an escalation that ends in conversion:

• Too tight. Release the posterior capsule first, the most common cause. If it is still tight, consider pie-crusting the PCL with an 11 blade (controversial). If it stays tight, convert to PS.
• Too loose. Increase the tibial insert thickness in 1-2mm increments and make sure the femoral component is not too small. If it stays loose, convert to PS to avoid posterior tibial translation and wear.

Posterior capsule release. A blunt Hohmann retractor elevates the capsule off the posterior femoral condyles, starting medially and working laterally. The release should be subperiosteal to avoid injury to the popliteal vessels, and the goal is 1-2mm of extra flexion gap without overstretching the PCL.

Protecting the tibial attachment. The PCL inserts just posterior to the posterior edge of the tibial cut. Mark the insertion before cutting; an overly posterior resection can detach it and force conversion to PS (slope is discussed in the next section).

PS technique

The PS-specific steps:

1. PCL excision. Before or after the bone cuts, as the surgeon prefers. Excise it sharply from the tibial insertion with a curved osteotome or scalpel and remove the entire substance, including the femoral attachment; this makes room for the post and box and removes a potential source of impingement.
2. Femoral box cut. Use a box-cutting guide that references the anterior and posterior femoral cuts. The box is typically 18-20mm wide and 12-15mm deep, leaving 5-7mm of posterior femoral bone loss. Accuracy is critical, because an asymmetric box causes cam-post maltracking and accelerated wear; verify it more than once.
3. Cam-post relationship. Cam-post contact should begin at 60-70° (design-specific). The jump height, the vertical distance from the tibial polyethylene surface to the top of the post, is typically 8-10mm. Leave 2-3mm of clearance between post and cam at full extension to avoid hyperextension impingement.
4. Balancing verification. Tension should be equal through the full range, with no lift-off in extension and no excessive tightness in flexion. At 90°, a gentle posterior force should engage the cam-post with a solid feel, confirming that the mechanism works.

Converting CR to PS in theatre

When to convert.

• The PCL is found to be attenuated, calcified or torn
• The PCL cannot be balanced despite appropriate releases
• The PCL is avulsed during tibial or femoral preparation
• Posterior tibial translation on trial reduction is over 5mm
• There is severe flexion instability after the CR trial reduction

How to convert. Decide early, before the final implants, and tell the team so that PS implants and instruments are ready. Then:

1. Remove the PCL completely from its tibial and femoral attachments with a curved osteotome or rongeur, leaving no remnant that could impinge.
2. Cut the box with a PS guide that references the femoral cuts already made for CR, keeping it symmetrical, and confirm clearance with the trial PS femoral component.
3. Check the tibia: the tibial cut is already made but PS needs a central post hole. Most modern baseplates accept both CR and PS inserts, so verify this one does; if the tibial cut is inadequate, the tibia may need recutting with 1-2mm more resection.
4. Trial the PS components and assess balance through the range: cam-post engagement, no hyperextension impingement, equal flexion and extension gaps, no lift-off and appropriate cam-post clearance.

Why posterior slope matters differently for CR and PS is a classic viva point, because it is about the cruciate strategy rather than alignment in general.

What slope does. Posterior tibial slope opens the flexion gap: more slope gives a larger flexion gap and easier deep flexion, and shifts the femoral contact point posteriorly. It must be matched to the design intent of the specific implant.

In CR, protect the PCL. Because the PCL is retained, the cut is kept near neutral and slope modest and implant-specific, commonly around 0 to 7°. Excessive posterior slope over-opens the flexion gap and alters PCL tension, worsening paradoxical anterior femoral slide and risking flexion instability. A very deep or posterior tibial cut can detach the PCL footprint and force conversion to PS, so do not over-resect posteriorly.

In PS, more latitude but new limits. With the PCL excised, slope can be used more liberally to aid flexion. Too much risks anterior femoral lift-off, edge-loading or impingement of the cam-post in flexion, and recurvatum or hyperextension instability, and it increases posterior loading of the fixation. Slope is still set to the manufacturer's recommendation for that PS design.

In both. Avoid excessive slope, which over-opens the flexion gap, and avoid anterior (reverse) slope, which tightens the flexion gap and can cause flexion stiffness or limited flexion. Match the cut to the implant, not to a single universal number.

Each design fails in its own way. CR knees fail through PCL imbalance, with flexion instability or PCL avulsion. PS knees have the box and the post to answer for: patellar clunk, cam-post wear and dissociation, anterior tibial post fracture, and posterior femoral bone loss from the box cut.

Patellar clunk. A fibrous nodule catches on the femoral box. Prevention is adequate patellar resection without overstuffing the patellofemoral joint. The table gives 5-10%, but the rate is design-dependent: in the series cited below (Snir 2014) it was 1.8% with fixed bearings against 11.7% with mobile high-flex knees.

Post-cam impingement. In hyperextension the post jams against the femoral cam. Prevention is the 2-3mm of clearance at full extension checked during trialling; once present, it may require downsizing the tibial insert or adjusting the bone cuts.

Cam-post dissociation. Excessive stress fractures the tibial post. Correct post height, avoiding excessive constraint and good bone cuts are the prevention.

The outcome data come from national joint registries, the Cochrane review and meta-analyses.

Survivorship. Revision rates at 10 years are equivalent, with over 95% survivorship for both designs and no significant difference.

Function. WOMAC and Knee Society scores are equivalent at 5-10 years, and patient satisfaction is 85-90% for both.

Range of motion. The quoted averages are 120-130° for CR and 130-140° for PS, but randomised trials find a smaller gap: the meta-analyses cited below put the advantage to PS at between 2.4° (Cochrane) and about 7° (Jiang), of uncertain clinical relevance.

Complications. PS has more patellofemoral and clunk problems and CR more instability, but the absolute differences are small.

Global Epidemiology and Practice Variation

TKA is one of the highest-volume elective orthopaedic procedures worldwide, with over 2 million implanted annually and demand rising with ageing populations and increasing obesity. The CR-versus-PS choice varies markedly by region: PS designs dominate in much of North America and parts of Asia, while CR (and minimally-stabilised) designs retain a larger share in Scandinavia and parts of Europe. These patterns reflect training era, implant availability, and institutional standardisation rather than any proven outcome superiority.

Society Guidance and Registry Evidence

Resource-Setting Variation