Wear is the progressive loss of material from the articulating surfaces. It remains the primary limiting factor in the survivorship of joint replacements, largely due to biological reactions to particulate debris.

• Abrasive Wear: Occurs when a harder surface cuts or ploughs into a softer surface (e.g., a scratched CoCr femoral head articulating against polyethylene).
• Adhesive Wear: Results from microscopic local welding between two surfaces under load; as motion continues, particles are torn away.
• Third-Body Wear: Triggered when loose, unattached particles (such as PMMA cement fragments, bone chips, or metal debris) become trapped between the bearing surfaces, rapidly accelerating damage.
• Linear vs. Volumetric Wear: Linear wear is the one-dimensional penetration of the femoral head into the liner. Volumetric wear is the total volume of material lost; this is the primary driver of macrophage-mediated osteolysis.

Titanium Alloys (Ti-6Al-4V)

• Properties: Exceptional biocompatibility, excellent corrosion resistance, and high fatigue strength.
• Modulus of Elasticity: ~110 GPa. This is closer to cortical bone (~15-30 GPa) than other metals, which significantly minimizes stress shielding and subsequent proximal bone resorption.
• Applications: Uncemented femoral stems, acetabular shells (promotes osteointegration).
• Critical Drawback: Notch sensitive with very poor wear characteristics. It must never be used as an articulating bearing surface.

Cobalt-Chromium Alloys (CoCrMo)

• Properties: High stiffness, excellent tensile strength, and highly polishable to a smooth finish.
• Modulus of Elasticity: ~210 GPa (very stiff). While this increases stress shielding in uncemented stems, the stiffness is advantageous in cemented stems to protect the cement mantle from fragmentation.
• Applications: Femoral heads, TKR femoral components.
• Complications: Release of cobalt and chromium ions can lead to Adverse Local Tissue Reactions (ALTR), pseudotumors, and metallosis, particularly noted in taper tribocorrosion (trunnionosis).

Ultra-High Molecular Weight Polyethylene (UHMWPE)

• Cross-linking: Gamma irradiation is used to cross-link polymer chains, drastically improving wear resistance (HXLPE – Highly Cross-Linked Polyethylene).
• The Oxidation Problem: Irradiation creates free radicals. If exposed to oxygen in vivo, these free radicals cause oxidation, leading to embrittlement, delamination, and catastrophic failure.
• Thermal Treatments: Historically treated by melting (eliminates all free radicals but reduces mechanical strength) or annealing (preserves strength but leaves some free radicals).
• Vitamin E Doping: Modern HXLPE is often doped with Vitamin E (Alpha-tocopherol). As a powerful antioxidant, it acts as a free radical scavenger, eliminating the need for post-irradiation melting and preserving the mechanical properties of the plastic.

Metal-on-Polyethylene (MoP)

• The historical gold standard with excellent long-term survivorship data.
• Highly forgiving regarding component positioning compared to hard-on-hard bearings.
• Failure Mode: Macrophage-mediated osteolysis. Polyethylene particles between 0.1 and 1.0 µm are the most biologically active, triggering an inflammatory cascade.

Ceramic-on-Polyethylene (CoP)

• Ceramic (Alumina or Zirconia-toughened Alumina) heads are harder, smoother, and more scratch-resistant than CoCr.
• Creates better lubrication fluid films, significantly reducing the volumetric wear of the polyethylene liner.
• Often the preferred choice in younger, higher-demand patients.

Ceramic-on-Ceramic (CoC)

• Exhibits the lowest wear rates of all combinations; wear debris is biologically inert, virtually eliminating osteolysis.
• Complications: Risk of component fracture (though minimized with modern Biolox Delta ceramics) and audible “squeaking” caused by stripe wear, micro-separation, or edge loading.
• Highly unforgiving to malpositioning (e.g., steep cup inclination).

Successful THA requires restoring the hip’s rotational center while respecting the kinetic chain linking the spine, pelvis, and femur.

The Offset Paradigm

• Femoral Offset: The perpendicular distance from the center of the femoral head to the anatomical axis of the femur. Increasing offset moves the femur laterally, which increases the abductor lever arm (reducing the joint reaction force and minimizing limp) and increases soft tissue tension (reducing dislocation risk).
• Global Offset: Acetabular Offset + Femoral Offset. Over-medializing the cup without restoring femoral offset leads to abductor weakness and impingement.
• Center of Rotation (COR): Must be restored to the native tear drop. A high hip center (uncorrected dysplasia) critically decreases the abductor moment arm and exponentially increases the joint reaction force, accelerating wear.

Spinopelvic Mechanics (Crucial for Instability Prevention)

Cup positioning must dynamically accommodate changes in pelvic tilt during postural shifts (standing to sitting).

• Pelvic Tilt (PT): The angle between a vertical line and a line from the center of the S1 endplate to the center of the bifemoral heads. Pelvic retroversion (posterior tilt) increases functional acetabular anteversion.
• Pelvic Incidence (PI): A fundamental rule in orthopedics is that Pelvic Incidence is a morphological constant; it is fixed after skeletal maturity and does not change with position. It is the algebraic sum of Pelvic Tilt (PT) and Sacral Slope (SS). PI = PT + SS.
• The Stiff Spine (e.g., Ankylosing Spondylitis, Lumbar Fusion): A spine that cannot flex forces the pelvis to compensate. If a patient with a fused lumbar spine sits, their pelvis cannot retrovert to provide anterior clearance. This causes anterior bony impingement and posterior dislocation. Surgical Adjustment: Place the cup in more anteversion to prevent posterior dislocation during sitting.

Direct Anterior Approach (Smith-Petersen)

• Position: Supine, often on a specialized traction table.
• Superficial Plane: Sartorius (Femoral n.) and Tensor Fasciae Latae (Superior Gluteal n.).
• Deep Plane: Rectus Femoris (Femoral n.) and Gluteus Medius (Superior Gluteal n.).
• Vascular Danger: Ascending branch of the Lateral Femoral Circumflex Artery (must be ligated during deep dissection).
• Neurological Danger: Lateral Femoral Cutaneous Nerve (LFCN) injury causes thigh numbness/meralgia paresthetica. Avoid by staying lateral to the TFL fascia. Femoral nerve injury is rare but catastrophic, occurring from retractor malposition over the anterior acetabular wall.

Anterolateral Approach (Watson-Jones)

• Position: Lateral decubitus or supine.
• Plane: Between TFL and Gluteus Medius. (Note: Not a true internervous plane as both are supplied by the Superior Gluteal Nerve).
• Pitfall: The Superior Gluteal Nerve enters the deep surface of the gluteus medius approximately 3 to 5 cm proximal to the tip of the greater trochanter. Proximal splitting of the muscle beyond this point causes devastating denervation and permanent Trendelenburg gait.

Posterior Approach (Moore/Southern)

• Position: Lateral decubitus.
• Plane: No true internervous plane. Muscle splitting through Gluteus Maximus (Inferior Gluteal n.).
• Technique: Detachment of the short external rotators (Piriformis, Superior Gemellus, Obturator Internus, Inferior Gemellus) near their insertion on the greater trochanter. Must protect the Quadratus Femoris to avoid bleeding from the ascending branch of the medial femoral circumflex artery (MFCA).
• Neurological Danger: Sciatic Nerve. At highest risk during posterior retractor placement or extreme limb lengthening.

Dorr Classification of Femoral Bone

Dictates stem selection based on the cortico-medullary ratio.

• Type A (Champagne Glass): Thick cortices, narrow canal. Excellent for proximally coated uncemented stems.
• Type B (Normal): Moderate cortices. Accommodates most uncemented and cemented stems.
• Type C (Stovepipe): Thin cortices, wide capacious canal (osteoporotic). High risk of intraoperative fracture with press-fit stems. Cemented fixation is heavily preferred.

Cemented Stem Design

• Force-Closed (Taper-Slip, e.g., Exeter): Highly polished, collarless stems. Designed to subside slightly into the cement mantle, putting the cement under constant hoop compression (where cement is strongest). Must NEVER be used without a distal cement plug.
• Shape-Closed (Composite Beam, e.g., Charnley): Matte or roughened surface. Designed to bond with the cement. Relies on the stem-cement interface remaining intact.

Uncemented Stem Design

• Proximally Coated (Metaphyseal Fit): Flat-tapered or wedge-shaped. Allows proximal load transfer, reducing distal stress shielding.
• Fully Coated (Diaphyseal Fit): Cylindrical, fully porous coated. Bypasses poor proximal bone. Commonly used in revision settings (Paprosky Type II/III bone loss).

The Head Size Dilemma

• Dislocation Resistance: Increasing head size (e.g., 28mm to 32mm or 36mm) increases the Jump Distance (the translation required for the head to escape the liner) and increases the impingement-free range of motion.
• Wear Mechanics: In standard Metal-on-Polyethylene (MoP), larger heads increase the sliding distance, thereby increasing volumetric wear and accelerating macrophage-mediated osteolysis. Highly Cross-Linked Polyethylene (HXLPE) or Ceramic bearings are required to safely utilize large heads (>32mm).

Trunnionosis (MACC)

• Pathophysiology: Mechanically Assisted Crevice Corrosion (MACC) occurs at the modular head-neck taper junction. Micro-motion disrupts the passive oxide layer of the metals, leading to galvanic corrosion and the release of metal ions (Cobalt and Chromium).
• Risk Factors: Large CoCr heads on Titanium stems (mixed-metal junctions), increased offset (longer lever arms create higher bending moments on the trunnion), and tissue fluid ingress into the taper.
• Clinical Presentation: Pain, swelling, and the formation of Adverse Local Tissue Reactions (ALTR) or pseudotumors. Characterized by elevated serum Cobalt levels.

Vancouver Classification of Periprosthetic Femur Fractures

Instability & Dislocation

• Early (<3 months): Usually due to component malposition, soft tissue laxity, or patient non-compliance. Management: Closed reduction and bracing (abduction brace). If recurrent, evaluate component position via CT.
• Late (>5 years): Often secondary to polyethylene wear (liner eccentric wear alters the COR and creates instability) or late-onset abductor insufficiency. Management: Revision of worn components; utilize dual-mobility cups or constrained liners as salvage.
• Directional Impingement:
  • Posterior Dislocation: Hip in flexion, adduction, internal rotation. Associated with anterior impingement (e.g., retroverted cup).
  • Anterior Dislocation: Hip in extension, external rotation. Associated with posterior impingement (e.g., excessively anteverted cup).

Periprosthetic Joint Infection (PJI)

• Diagnosis: Relies on MSIS Criteria. Sinus tract communicating with the joint or two positive cultures with the same organism are definitive. Elevated synovial WBC (>3,000 cells/µL for knees, similar threshold for hips) and PMN% (>80%) are highly suggestive.
• Surgical Strategies:
  • DAIR (Debridement, Antibiotics, Implant Retention): Only viable for acute post-op infections (<4 weeks) or acute hematogenous spread in a previously well-functioning joint. Must exchange modular components (head and liner).
  • Two-Stage Revision: The gold standard for chronic PJI. Stage 1: Complete explantation, radical debridement, placement of antibiotic-loaded PMMA spacer. IV antibiotics for 6 weeks. Stage 2: Re-implantation once inflammatory markers normalize and joint aspiration is negative.

Precise templating dictates implant sizing, alignment goals, and anticipates the need for augments or stems.

Radiographic Assessment (Standing Full-Length Views)

• Mechanical Axis (MA): A line drawn from the center of the femoral head to the center of the ankle talar dome. In a neutral knee, this passes precisely through the center of the knee joint.
• Anatomical Axis (AA): The diaphyseal lines of the femur and tibia.
• Valgus Cut Angle (VCA): The angle between the femoral anatomical axis and the femoral mechanical axis. This dictates the distal femoral cut angle, typically ranging from 5° to 7° valgus.
• Tibial Slope: The native posterior slope of the proximal tibia is generally 7° to 10°. Recreating or altering this slope during surgery directly impacts the flexion gap and PCL tension.

Deformity Analysis

• Varus Deformity: Often associated with medial bone loss and medial collateral ligament (MCL) contracture. Requires medial release (deep MCL, semimembranosus, pes anserinus) for correction.
• Valgus Deformity: Characterized by lateral bone wear, lateral compartment contracture (IT band, LCL, popliteus), and medial ligamentous attenuation. Carries a high risk of peroneal nerve stretch injury during correction.

Medial Parapatellar (Classic)

• The gold standard. Incision runs from the proximal pole of the patella, coursing medially over the patella, and ending medial to the tibial tubercle.
• Pros: Highly extensile; provides excellent exposure, especially for complex deformities or revisions.
• Cons: Disrupts the extensor mechanism blood supply; higher risk of patellar maltracking if not closed properly.

Subvastus Approach

• Avoids splitting the vastus medialis obliquus (VMO). The VMO is elevated from the intermuscular septum and retracted laterally.
• Pros: Preserves extensor mechanism completely; potentially faster return of quadriceps control and straight leg raise. Maintains intact superior genicular artery.
• Cons: Technically demanding; poor exposure in obese patients or those with stiff knees (<90° preoperative flexion).

Midvastus Approach

• Splits the VMO muscle fibers bluntly, starting at the superomedial border of the patella and extending proximally and medially.
• Pros: A compromise between standard parapatellar and subvastus; better early quad function than parapatellar, better exposure than subvastus.

The paradigm of TKA alignment is actively evolving, moving from strict mechanical rules toward individualized kinematic restoration.

• Mechanical Alignment (MA): The traditional dogma. Aims to cut the femur and tibia perpendicular (90°) to their respective mechanical axes. Creates a neutral overall limb axis (0° Hip-Knee-Ankle angle) but alters the native joint line, frequently requiring extensive soft tissue releases to balance the gaps.
• Kinematic Alignment (KA): Aims to restore the patient’s pre-arthritic, native joint lines (the three kinematic axes of the knee). The implants are positioned as resurfacing tools rather than reshaping tools. Bone cuts are modified to match native anatomy (often leaving slight residual varus), drastically reducing the need for ligament releases.
• Restricted Kinematic Alignment (rKA): A hybrid approach. Restores native kinematics provided the patient’s anatomy falls within a “safe zone” (e.g., overall limb alignment within ±3° of neutral). If the native anatomy is extreme, the cuts are adjusted to fall within this safe boundary to prevent catastrophic implant failure.
• Anatomic Alignment (AA): Specifically reconstructs the joint line to a 3° varus tibial cut and a 3° valgus femoral cut. This keeps the joint line parallel to the ground during the single-leg stance phase of gait.

Measured Resection

Bone cuts are made based strictly on anatomical landmarks, completely independent of soft tissue tension. Soft tissues are balanced *after* the components are trialed.

• Distal Femoral Cut: Dictates the extension gap. Based on the Valgus Cut Angle derived from templating.
• Anterior Femoral Cut: Must avoid notching the anterior femoral cortex, which creates a stress riser leading to periprosthetic fractures. Correct rotation ensures a flush cut resulting in the classic “Grand Piano Sign” on the anterior femur.

Femoral Rotational Axes

Correct femoral rotation is critical for patellofemoral tracking and achieving a balanced, rectangular flexion gap.

• Surgical Transepicondylar Axis (sTEA): A line connecting the lateral epicondylar prominence to the medial epicondylar sulcus. This is the functional axis of knee flexion. The femoral component should be placed parallel to this axis.
• Whiteside’s Line (Anteroposterior Axis): A line drawn from the deepest part of the trochlear groove to the center of the intercondylar notch. The femoral component should be placed perpendicular to this line.
• Posterior Condylar Axis (PCA): A line connecting the posterior aspects of the medial and lateral femoral condyles. In a normal knee, the medial condyle is larger and extends further posteriorly. Therefore, to align the implant with the sTEA, the femoral cutting block must be externally rotated (typically 3°) relative to the PCA.

Gap Balancing Technique

Prioritizes equal soft tissue tension. The proximal tibia is cut first. Ligaments are released until the extension gap is rectangular. The knee is flexed to 90°, placed under tension, and the femoral rotation is set parallel to the cut tibia, guaranteeing a rectangular flexion gap.

Cruciate Retaining (CR)

• Relies on a competent PCL to control femoral rollback during deep flexion.
• Paradoxical Anterior Translation: If the flexion gap is tight or the PCL is overly tight, the femur may translate anteriorly on the tibia during flexion instead of rolling back, leading to limited flexion and accelerated poly wear.

Posterior Stabilized (PS)

• The PCL is intentionally resected. The implant uses a central tibial post and a femoral cam to force femoral rollback.
• Provides more reliable kinematics and is easier to balance in the presence of severe deformities or previous patellectomy.
• Patellar Clunk Syndrome: A classic PS complication. A fibrosynovial nodule forms at the superior pole of the patella. As the knee extends from deep flexion, the nodule catches in the intercondylar box, causing a painful, audible “clunk”. Requires arthroscopic debridement.

Constrained Condylar Knee (CCK) & Hinges

• CCK: Features a tall, thick tibial post. Restricts varus/valgus and rotational instability. Indicated when MCL/LCL are attenuated. Requires intramedullary stem extensions to share the mechanical stress.
• Rotating Hinge: Mechanically links the femur and tibia. Indicated for global instability, polio, severe bone loss, or salvage revisions.

• Periprosthetic Joint Infection (PJI): The most devastating complication. Diagnosis utilizes MSIS (Musculoskeletal Infection Society) criteria.
  • Acute (<4 weeks post-op or acute hematogenous): Treated with DAIR (Debridement, Antibiotics, and Implant Retention) with polyethylene exchange.
  • Chronic (>4 weeks): Requires a 2-stage revision. Stage 1: Explantation, thorough debridement, and placement of an antibiotic-eluting cement spacer. Stage 2: Re-implantation after eradication of infection (normal ESR/CRP, negative aspirations).
• Aseptic Loosening: The primary cause of late failure. Driven by macrophage-mediated osteolysis in response to particulate polyethylene debris.
• Mid-Flexion Instability: Knee is stable in full extension and 90° flexion, but unstable at 30°-60° of flexion. Often caused by joint line elevation (using a thicker poly to balance a loose extension gap) or mismatch between the coronal and sagittal femoral radii of curvature.
• Peroneal Nerve Palsy: Highest risk during the correction of severe, fixed valgus deformities or flexion contractures. Presents post-operatively as foot drop. Immediate Management: Remove all compressive dressings and flex the knee to 20-30° to relieve tension on the nerve.
• Arthrofibrosis (Stiffness): Defined as a functionally limiting restriction in ROM. If conservative measures fail, Manipulation Under Anesthesia (MUA) is most effective when performed between 6 to 12 weeks post-operatively.

Accurate diagnosis relies on the Musculoskeletal Infection Society (MSIS) / International Consensus Meeting (ICM) Criteria. Diagnosis requires meeting Major Criteria OR a specific score from Minor Criteria.

Major Criteria (Definitive PJI if ONE is present):

• Two positive periprosthetic cultures with phenotypically identical organisms.
• A sinus tract communicating with the joint.

Minor Criteria (Scoring System):

• Elevated Serum CRP and ESR: (Thresholds vary by chronicity, e.g., CRP > 10 mg/L, ESR > 30 mm/hr in chronic knees).
• Elevated Synovial Fluid WBC: > 3,000 cells/µL (Note: native septic arthritis is typically > 50,000).
• Elevated Synovial Fluid PMN%: > 80%.
• Positive Histology: > 5 neutrophils per high-power field in 5 high-power fields observed from periprosthetic tissue at high magnification (x400).
• Single Positive Culture: One positive periprosthetic tissue or fluid culture.
• Next-Gen Biomarkers: Synovial Alpha-defensin, Leukocyte Esterase (LE) strip test (highly specific point-of-care test), and Synovial CRP.

DAIR (Debridement, Antibiotics, and Implant Retention)

• Indications: Strictly limited to acute postoperative infections (< 4 weeks from index surgery) or acute hematogenous infections (< 3 weeks of symptom onset) in a well-fixed, previously functioning prosthesis.
• Technique: Radical open synovectomy, copious pulsatile lavage, and mandatory exchange of all modular components (polyethylene liner, femoral head). Followed by prolonged IV and oral biofilm-active antibiotics (e.g., Rifampin for staph).

Two-Stage Revision (The Gold Standard for Chronic PJI)

• Stage 1 (Resection): Complete removal of all hardware and cement. Radical debridement of all non-viable bone and soft tissue. Placement of an antibiotic-loaded PMMA spacer (Tobramycin + Vancomycin typically added).
  • Static Spacers: Used when there is massive bone loss or compromised soft tissue envelope (e.g., deficient extensor mechanism).
  • Articulating Spacers: Preserves soft tissue tension, prevents collateral ligament contracture, and maintains joint space, facilitating easier Stage 2 reimplantation.
• Interim: Minimum 6 weeks of pathogen-specific IV antibiotics. Must monitor ESR/CRP trend. Joint aspiration performed prior to Stage 2 (must be culture negative).
• Stage 2 (Reimplantation): Re-entry, removal of spacer, repeat frozen section histology (< 5 PMNs/HPF confirms clearance), and implantation of revision components using complex bone-loss reconstruction techniques.

Determines the degree of column support and guides the choice of reconstructive implant.

Dictates how and where a revision femoral stem can achieve rigid biological fixation.

Safe extraction of well-fixed implants or cement mantles is critical to prevent iatrogenic Type B3 or Type C periprosthetic fractures.

Extended Trochanteric Osteotomy (ETO)

• Concept: A controlled, laterally based osteotomy of the proximal femur that hinges anteriorly on the vastus lateralis and preserves the gluteus medius/minimus attachments.
• Indications:
  • Removal of a well-fixed, fully porous-coated stem.
  • Removal of a well-fixed cement mantle in a varus-remodeled femur.
  • Correction of severe proximal femoral deformity to allow straight reaming for a revision stem.
  • Enhanced exposure for complex acetabular revisions (Paprosky III).
• Technique Pearls:
  • The osteotomy must extend distal to the tip of the porous coating or the distal cement plug (typically 12-15 cm long).
  • A prophylactic cerclage cable is placed just distal to the planned osteotomy to prevent downward propagation of the fracture line.
  • Closure is achieved utilizing cerclage cables looped around the revision stem and the osteotomized fragment. The fragment heals rapidly due to the preserved vascular pedicle of the vastus lateralis.