Introduction & Historical Perspective

Total hip arthroplasty (THA) is widely celebrated as the "operation of the 20th century" due to its life-transforming clinical efficacy, reproducible outcomes, and durable implant longevity. Pioneered by Sir John Charnley in the early 1960s with the low-friction arthroplasty concept—combining a stainless steel small femoral head (22.25 mm), an ultra-high-molecular-weight polyethylene (UHMWPE) cemented acetabular socket, and poly(methyl methacrylate) (PMMA) acrylic cement—modern THA has expanded to encompass diverse biologic and mechanical fixation strategies, modularity, advanced bearing materials, and kinematic alignment concepts.

Epidemiology & Indications

Osteoarthritis (OA) accounts for >85% of primary THA procedures performed globally according to major registries (e.g., the National Joint Registry of the UK and the Australian Orthopaedic Association National Joint Replacement Registry). Other established indications include:

• Secondary degenerative disease: Avascular necrosis (AVN) / osteonecrosis (Ficat & Arlet stages III–IV), developmental dysplasia of the hip (DDH / Crowe stages I–IV), Perthes disease, and slipped capital femoral epiphysis (SCFE).
• Inflammatory arthropathies: Rheumatoid arthritis, ankylosing spondylitis, and psoriatic arthritis.
• Post-traumatic arthritis: Sequelae of acetabular fractures or femoral head fractures.
• Acute trauma: Displaced intracapsular femoral neck fractures (Garden III/IV) in elderly, cognitively intact, ambulatory patients (aligned with NICE guidelines CG124).
• Oncologic conditions: Pathologic fractures or primary/metastatic bone disease.

Contraindications include active systemic or local joint infection, absence of functional abductor mechanism (relative to approach and implant choice), severe neuropathic joint (Charcot arthropathy), and severe medical instability precluding anaesthesia.

Relevant Anatomy & Biomechanics

Functional Anatomy

The hip is a multiaxial enarthrodial (ball-and-socket) joint. The native acetabular version averages 15°–20° anteversion with an inclination (abduction) of approximately 40°–45°. The native femoral anteversion averages 10°–15°, yielding a normal combined anteversion of 25°–35°.

Biomechanics and Hip Offset

The hip joint experiences contact forces ranging from 2.5 to 3 times body weight during normal level walking, rising to over 5 times body weight during stumbling or climbing stairs. The centre of rotation (COR) determines the lever arm of body weight (L_W) versus the abductor lever arm (L_A):

• Femoral offset: The perpendicular distance from the centre of the femoral head to the long axis of the femoral shaft. Restoring offset tensions the abductor musculature, decreases joint reaction force (R), prevents soft-tissue impingement, and improves stability.
• Acetabular offset: Medialising the acetabular cup brings the COR closer to the midline, decreasing L_W and abductor demands, provided femoral offset is restored to avoid lateral soft-tissue laxity and bony impingement.
• Leg length: Governed by the vertical distance between the pelvic reference (e.g., teardrop or inter-ischial line) and the lesser trochanter. Leg length and femoral offset can be modulated independently using variable-angle neck tapers, modular heads, and lateralised stem geometries.

Spinopelvic Alignment

Normal physiology involves dynamic coordination between the lumbar spine and pelvis: sitting causes posterior pelvic tilt (sacral slope decreases), which naturally increases acetabular anteversion and opening, clearing anterior impingement in flexion. In patients with spinal fusion or stiff degenerated spines:

• "Stiff spine" phenotype: Inability to tilt the pelvis posteriorly when sitting causes anterior impingement and posterior dislocation; inability to tilt anteriorly on standing leads to posterior impingement and anterior dislocation.
• Safe zones must be individualised beyond Lewinnek's classic targets (40° ± 10° inclination, 15° ± 10° anteversion), often necessitating dynamic standing and sitting lateral lumbar-pelvic radiographs.

Surgical Approaches

Posterior (Moore / Southern)

• Interval: Gluteus maximus split; trans-osseous detachment of short external rotators (piriformis, gemelli, obturator internus) and posterior capsule.
• Pros: Excellent acetabular and femoral exposure, preserves abductor mechanism (no gluteus medius/minimus violation), extensile.
• Cons: Historically higher dislocation rate, mitigated by robust anatomical repair of the capsule and conjoint tendons.
• Nerve at risk: Sciatic nerve (particularly the peroneal division during retracting, hyperflexion, or leg lengthening).

Direct Anterior (Smith-Petersen / Hueter interval)

• Interval: Intermuscular and internervous. Superficial: Tensor fasciae latae (TFL; superior gluteal nerve) and Sartorius (femoral nerve). Deep: Rectus femoris (femoral nerve) and Gluteus medius (superior gluteal nerve).
• Pros: Abductors unviolated, lowest early posterior dislocation rates, fluoroscopy-friendly.
• Cons: Steep learning curve, difficult femoral preparation in muscular/obese patients, risk of femoral perforation or periprosthetic fracture.
• Nerves at risk: Lateral femoral cutaneous nerve (LFCN; sensory numbness/meralgia paraesthetica), ascending branch of lateral femoral circumflex artery (requires ligation).

Direct Lateral (Hardinge)

• Interval: Splits the anterior third of gluteus medius and vastus lateralis in continuity, exposing the anterior capsule.
• Pros: Lowest reported dislocation rates across high-risk groups (e.g., neuromuscular disorders, dementia).
• Cons: Prolonged post-operative abductor limp, risk of superior gluteal nerve denervation.
• Nerve at risk: Superior gluteal nerve (anterior branch travels ~3–5 cm proximal to the tip of the greater trochanter).

Anterolateral (Watson-Jones)

• Interval: Between TFL (superior gluteal) and Gluteus medius (superior gluteal).
• Pros: Low dislocation rate, preserves posterior capsule.
• Cons: Restricted posterior acetabular and femoral exposure; risk of injuring the superior gluteal nerve leash.

Implant Fixation & Design

Femoral Components

• Cemented Stems: Rely on micro-interlock with cancellous bone via third-generation cementing techniques (canal brush, pulsatile lavage, distal cement restrictor, retrograde delivery with a cement gun, pressurized vacuum-mixed PMMA).
  • Taper-slip (e.g., Exeter): Polished, collarless, double-tapered; subsides controlled within the cement mantle to convert shear stress into radial compressive stresses.
  • Composite-beam (e.g., Charnley): Matt finish, collared/uncollared; bonded interface designed to prevent subsidence.
• Uncemented (Cementless) Stems: Rely on initial press-fit mechanical stability followed by secondary osseointegration (porous, hydroxyapatite, or trabecular titanium surfaces).
  • Classification of cementless stems (Khanuja):
    • Type 1: Single-wedge (flat taper)
    • Type 2: Double-wedge (metaphyseal fill)
    • Type 3: Tapered round/splined (e.g., Wagner for revision/dysplasia)
    • Type 4: Cylindrical fully coated
    • Type 5: Modular
    • Type 6: Anatomical (curved s-shape)

Acetabular Components

• Uncemented Hemispherical Cups: Standard of care in modern arthroplasty; fixed by 1–2 mm under-reaming press-fit, supplemented by supplementary dome screws if bone quality or stability demands.
• Cemented Polyethylene Cups: Indicated in osteoporotic bone, radiated fields, or where combined with cemented stems in elderly populations.

Bearing Couples

• Ceramic-on-Highly Crosslinked Polyethylene (CoP) & Metal-on-Highly Crosslinked Polyethylene (MoP): Mainstays of practice. Highly cross-linked polyethylene (HXLPE) demonstrates ultra-low volumetric wear rates (<0.01 mm/year).
• Ceramic-on-Ceramic (CoC): Lowest wear and non-toxic debris; vulnerable to squeaking, brittle stripe wear, and rare catastrophic ceramic fracture.
• Metal-on-Metal (MoM): Largely abandoned due to adverse local tissue reactions (ALTR), pseudotumours, and cobalt/chromium metallosis.

Complications

Dislocation & Instability

• Risk Factors: Patient factors (previous lumbar fusion, neuromuscular disorders, cognitive impairment, alcohol abuse), surgical factors (component malposition, abductor insufficiency, impingement, undersized femoral head).
• Classification of Component Position: Lewinnek safe zone (inclination 40° ± 10°, anteversion 15° ± 10°), now augmented by dynamic functional spinopelvic planning.
• Management: Closed reduction under conscious sedation/muscle relaxation. Recurrent instability requires structural investigation (axial CT to assess cup and stem version; radiographs for offset/leg length) and revision surgery (e.g., dual-mobility constructs, constrained liners, or component repositioning).

Periprosthetic Joint Infection (PJI)

• Follows Musculoskeletal Infection Society (MSIS) / EBJIS diagnostic criteria (elevated ESR/CRP, synovial fluid cell count and differential, alpha-defensin, microbiological cultures, histopathology showing >5 PMNs per HPF).
• Management:
  • Acute (<3–4 weeks post-op or <3 weeks acute haematogenous symptoms): Debridement, Antibiotics, and Implant Retention (DAIR) with modular exchange.
  • Chronic: Two-stage exchange (gold standard in North America/UK for resistant organisms) or single-stage exchange (in selected cohorts with known, sensitive organisms and healthy soft-tissue envelope).

Periprosthetic Femoral Fractures

• Stratified using the Vancouver Classification (Duncan and Masri):
  • Type A: Trochanteric region. A_G (greater trochanter; conservative unless displaced >2 cm or abductor deficit) vs A_L (lesser trochanter; conservative unless calcar continuity compromised).
  • Type B: Around or just distal to the stem tip.
    • B1: Stem well-fixed → Open reduction and internal fixation (locking plate, cerclage cables, strut allografts).
    • B2: Stem loose, adequate bone stock → Revision to a long, diaphyseal-engaging fluted tapered stem bypassing defect by ≥ 2 cortical diameters.
    • B3: Stem loose, severe proximal bone deficiency → Revision with modular fluted tapered stem, proximal femoral replacement, or impaction grafting.
  • Type C: Well distal to the stem tip → Treated independently of the arthroplasty by ORIF (plate osteosynthesis).

Venous Thromboembolism (VTE)

• Regulated by NICE guidelines (NG89): multimodal prophylaxis including mechanical (intermittent pneumatic compression devices) and chemical agents (low-molecular-weight heparin, direct oral anticoagulants such as rivaroxaban/apixaban, or aspirin in risk-stratified cohorts) for 28–35 days.

Nerve Injuries

• Most commonly the sciatic nerve (0.5%–2% in primary THA; up to 5% in revision or complex DDH). Peroneal division is selectively vulnerable due to lateral tethering and fewer protective nutrient vessels. Excessive leg lengthening (>2–3 cm) must be avoided.

Exam Pearls for Fellowship Vivas

• The Dorr Classification of Femoral Morphology:
  • Type A: Thick cortices, narrow canal ("champagne-flute"); optimal for press-fit or cemented stems, but high risk of intraoperative fracture with aggressive press-fit broaching.
  • Type B: Progressive cortical thinning, moderate funnel shape; suitable for both.
  • Type C: Wide ("stovepipe") canal, thin paper cortices, wide endosteal cavity; best treated with a cemented polished taper-slip stem to avoid subsidences.
• Sciatic Nerve vs Superior Gluteal Nerve: In the posterior approach, do not place retractors deep to obturator internus unchecked or place excessive traction on flexed/internally rotated knee. In the lateral approach, stay within 3 cm proximal to the trochanteric tip to protect the transverse branch of the superior gluteal nerve supplying tensor fasciae latae.
• Femoral Anteversion Assessment: Assessed by bringing the tibia vertical (foot to ceiling, knee flexed at 90°) with the femur horizontal; the angle between the femoral neck trial axis and the vertical represents stem version.