Summary TKA fixation is achieved via two primary methods: Cemented (Polymethylmethacrylate/PMMA) or Cementless (Biologic fixation/Press-fit). Goal is to provide stable mechanical interface between implant and bone to transfer loads and prevent loosening. Cemented fixation remains the gold standard with the longest track record of survivorship. Cementless fixation is increasing in popularity, particularly for younger, active patients, driven by advances in porous metal technology (e.g., highly porous titanium, tantalum). Biomechanics & Basic Science Cemented Fixation (PMMA) Mechanism: Mechanical interlock. Cement does not adhere/bond to bone; it interdigitates into the trabecular bone lattice. Biomechanics: Acts as a grout to distribute loads evenly. Young’s Modulus is between cortical and cancellous bone (allows load transfer). Key Technical Factors for Success: Cement Penetration: Ideal interdigitation depth is 3–5 mm into cancellous bone. Pulse Lavage: Crucial to remove blood/fat and open trabecular pores for cement penetration. Cementless Fixation (Biologic) Mechanism: Osseointegration (Bone ingrowth or ongrowth). Requirements for Ingrowth: Micromotion: Must be minimized. Micromotion > 150 microns leads to fibrous tissue formation instead of bone ingrowth. Pore Size: Optimal pore size is 100–400 microns to facilitate vascularization and osteoblast migration. Bone Quality: Requires active host bone biology (osteoblasts) and strong trabecular bone for initial press-fit stability. Surface Technologies: Porous Plasma Spray: Titanium particles sprayed onto base. Sintered Beads: Cobalt-chrome or titanium beads. Trabecular Metal (Tantalum): High porosity (~80%) and low modulus of elasticity (closer to bone), potentially reducing stress shielding. Hydroxyapatite (HA) Coating: Osteoconductive; enhances early gap filling and bone apposition. Indications Cemented TKA Elderly patients (> 70 years). Poor bone quality (Osteoporosis/Osteopenia). Inflammatory arthritis (Rheumatoid Arthritis) due to compromised bone stock. Irradiated bone. Revision TKA (often required when metaphyseal bone is compromised). Cementless TKA Younger patients (< 65 years). High demand/Active patients. Good bone stock (allows for tight press-fit). Morbid Obesity: Some studies suggest cementless may reduce aseptic loosening rates in high BMI patients compared to cemented (controversial, but gaining evidence). Contraindications for Cementless Active infection. Severe osteoporosis (risk of subsidence/fracture). Renal osteodystrophy. Technique: Cemented Fixation Preparation Pulsatile Lavage: Mandatory to remove fat/marrow; increases cement shear strength. Drying: Bone bed must be dry (dry lap sponges, suction) to prevent blood lamination at the interface. Cementing Technique Vacuum Mixing: Reduces porosity of cement, increasing fatigue strength. Pressurization: Applying cement early (doughy phase) and pressurizing into the tibia leads to deeper penetration. Pros Immediate full weight-bearing stability. Antibiotic delivery (via antibiotic-loaded cement). Forgiving technique (can fill small bone defects/gaps). Cons Cement Disease: Risk of thermal necrosis if cement mantle is too thick. Third-body wear: Loose cement particles can accelerate polyethylene wear. Difficult Revision: Removal of well-fixed cement risks massive bone loss. Technique: Cementless Fixation Preparation Precision Cuts: Requires exact bone cuts; gaps > 1–2 mm may prevent ingrowth. Press-Fit: Components are typically oversized (line-to-line or +0.5mm) to generate friction ("scratch fit") for initial stability. Fixation Augmentation Screws: Often used in tibial baseplates to increase initial stability and reduce lift-off/micromotion. Pegs/Keels: Tibial keel design critical for resisting rotational forces. Pros Biological Fixation: Potential for permanent fixation (bone is living, cement is dead). Bone Stock Preservation: Easier to remove implant during revision (often pops off at the interface). OR Efficiency: No waiting for cement to cure (saves ~10–15 mins). Cons Cost: Implants are typically more expensive than cemented (though OR time savings may offset this). Thigh Pain: Reported but less common in knees than hips. Incomplete Ingrowth: Risk of fibrous stability leading to late loosening. Hybrid Fixation Definition Cementless Femur + Cemented Tibia. Rationale Combines the reliable track record of femoral bone ingrowth (high surface area, good bone quality) with the security of cemented tibial fixation. The Tibia is the weak link in cementless TKA; historically, the tibial component has the highest rate of aseptic loosening in all-cementless constructs. Outcomes & Evidence Survivorship Modern Designs: Recent JAAOS/JBJS literature shows no significant difference in 10-year survivorship between modern highly porous cementless TKA and cemented TKA. Historical Data: Older cementless designs (1980s/90s) had higher failure rates due to poor locking mechanisms (backside wear) and poor patchy ingrowth. Complications Aseptic Loosening: Historically higher in cementless tibial components. Newer "highly porous" metals (e.g., 3D printed Titanium) show loosening rates comparable to cement. Radiolucent Lines: More common in cementless tibias initially; non-progressive lines may be normal stress shielding or stable fibrous ingrowth. Economic Analysis Cementless is cost-effective IF operative time savings > 13 minutes or if revision rate is lower long-term. Complications Surgical Site Infection (SSI) Cemented TKA allows for local antibiotic delivery (Antibiotic Bone Cement), which may lower SSI risk in high-risk patients. Periprosthetic Fracture Risk is slightly higher in cementless stems/keels due to the hoop stresses required for press-fit insertion. Polyethylene Wear Backside wear (wear between the poly liner and the metal tray) was a historical failure mode of cementless trays (screw holes allowed debris passage). Modern trays effectively seal screw holes or use monoblock designs to prevent this. Commonly Used Classification System (Fixation Zones) Zone 1: Anterior Flange (Femur) / Anterior (Tibia) Zone 2: Anterior Distal (Femur) / Central (Tibia) Zone 3: Posterior Distal (Femur) / Posterior (Tibia) Zone 4: Posterior Flange (Femur) Note: Radiolucencies in continuous zones (e.g., 1 through 4) suggest loosening.