What are the different types of implants used in total knee replacement?

Types of Implants in Total Knee Replacement

Primary Implant Categories

Total knee replacement implants are primarily categorized into three main types based on constraint level: posterior cruciate-retaining (CR), posterior cruciate-substituting (PS), and constrained designs including condylar constrained knee (CCK) and rotating hinge (RH) implants. 1

Standard Implant Designs

Posterior Cruciate-Retaining (CR) and Posterior Cruciate-Substituting (PS) Implants:

• These represent the standard implant types used in the vast majority of primary TKAs 1
• CR implants preserve the posterior cruciate ligament, while PS implants substitute for its function 1
• Both designs are appropriate when adequate ligamentous stability exists and bone stock is sufficient 1

Constrained Implants:

• Condylar Constrained Knee (CCK) implants are indicated for primary cases with severe deformity or ligamentous instability and most revision cases 1
• Rotating Hinge (RH) implants provide maximum constraint and are reserved for cases requiring the highest level of stability 1
• The key to long-term success is achieving stability with adequate and stable fixation between components and underlying bone 1

Material Composition Options

Cobalt-Chrome (CoCr) Alloy:

• This remains the standard material for femoral components in conventional TKA 2
• CoCr implants weigh approximately 390 grams 2
• Radiolucent lines appear in approximately 19% of cases with CoCr implants at mid-term follow-up 2

Titanium (Ti) Alloy:

• Ti implants weigh approximately one-third less than CoCr (134g vs 390g) 2
• Despite the lighter weight, approximately 70% of patients do not perceive Ti implants as lighter in clinical practice 2
• Ti implants demonstrate a lower rate of radiolucent lines (9%) compared to CoCr implants (19%), though clinical outcomes remain equivalent 2
• The lower modulus of titanium theoretically provides advantages, but this has not translated to measurable clinical benefits in functional scores, range of motion, or pain levels 2

Metal-Free Ceramic Implants:

• Novel alumina/zirconia ceramic composite systems (BPK-S) represent an emerging alternative to metal implants 3
• These demonstrate significant improvements in Knee Society Score, Oxford Knee Score, and EQ-5D at 3 and 12 months postoperatively 3
• Non-progressive partial radiolucent lines occurred in 6 cases, but no osteolysis or implant loosening was observed 3
• Ceramic implants may be particularly valuable for patients with metal allergies, as they do not induce or exacerbate allergic reactions 3
• Current evidence is limited to short-term follow-up studies 3

Specialized Design Variations

Patient-Specific Implants:

• These are designed and fabricated based on CT data of the individual patient's leg 4
• Patient-specific systems include both personalized single-use instruments and individualized implants that restore the patient's native knee anatomy and kinematics 4
• The surgical plan accounts for anatomical and biomechanical axes without requiring additional navigation aids 4
• Preliminary data are promising, but long-term comparative clinical data are still lacking 4

Unicompartmental and Patellofemoral Implants:

• Seventeen studies have focused on unicompartmental knee replacement for isolated compartment disease 5
• Six studies examined patellofemoral joint resurfacing or replacement 5
• All studies reported improvements in pain and/or function compared to baseline 5

Clinical Considerations for Implant Selection

Primary TKA Selection Algorithm:

• For standard primary cases with intact ligaments and adequate bone stock: CR or PS implants 1
• For primary cases with severe deformity or ligamentous instability: CCK implants 1
• For patients with documented metal allergy: Consider ceramic implants 3

Revision TKA Selection:

• Most revision cases require more constrained implants (CCK or RH) 1
• The choice depends on the degree of bone loss, ligamentous insufficiency, and need for stability 1

Important Caveats:

• The most common failure mechanisms requiring revision are loosening (39.9%), infection (27.4%), instability (7.5%), periprosthetic fracture (4.7%), and arthrofibrosis (4.5%) 5
• Polyethylene wear is no longer a major cause of failure with modern implant designs 5
• Long-term TKA failure rates remain less than 1% per year, with patient satisfaction rates ranging from 75% to 89% 5
• Newer implant designs claiming improved functional outcomes often lack independent scientific documentation and are substantially more expensive 6