Total Hip Replacement

Total hip replacement (THR), also called total hip arthroplasty (THA), is the surgical resurfacing of both articulating surfaces of the hip joint — the acetabulum and the femoral head — with prosthetic components. It is among the most successful and cost-effective operations in all of surgery, reliably abolishing pain and restoring mobility in end-stage hip disease. For NEET PG, the high-yield clusters are: indications, fixation (cemented vs cementless), bearing surfaces, the safe zone of cup placement, and the spectrum of complications (with dislocation being the commonest early and aseptic loosening the commonest long-term).

Definition & Components

A THR replaces the diseased femoral head and acetabular cartilage with an artificial ball-and-socket. It is distinguished from hemiarthroplasty (only the femoral head is replaced, e.g. Austin–Moore or bipolar prosthesis) and from hip resurfacing (femoral head is capped, neck preserved; e.g. Birmingham hip resurfacing, now largely abandoned due to metal-on-metal wear).

The four components of a modern modular THR:

Component	Material options	Notes
Acetabular shell (cup)	Titanium / Co-Cr	Press-fit (cementless) or cemented PE cup
Acetabular liner	Polyethylene / ceramic	The "socket" bearing surface
Femoral head	Co-Cr / ceramic / oxinium	The "ball"; varies in diameter
Femoral stem	Co-Cr / Ti alloy	Cemented or cementless

High-yield: A standard THR is modular — head, liner, shell and stem are separate pieces assembled intra-operatively, allowing the surgeon to fine-tune leg length, offset and bearing combination.

Indications

The single most common indication worldwide is primary osteoarthritis of the hip. Indications can be grouped:

1. Osteoarthritis (primary/secondary) — commonest overall.
2. Rheumatoid arthritis and other inflammatory arthropathies (ankylosing spondylitis, psoriatic arthritis).
3. Avascular necrosis (AVN) of the femoral head — late stages (Ficat III–IV) after the head has collapsed.
4. Displaced fracture neck of femur in the elderly (Garden III–IV) — THR favoured over hemiarthroplasty in the active, physiologically fit, independently mobile elderly; hemiarthroplasty for the frail, low-demand patient.
5. Failed internal fixation / non-union of femoral neck fractures.
6. Tumours around the hip (limb salvage).
7. Sequelae of childhood disease — old DDH, Perthes, slipped capital femoral epiphysis.

High-yield: In a fit, active elderly patient with a displaced intracapsular neck-of-femur fracture, THR > hemiarthroplasty (better functional outcome, lower revision/reoperation, less acetabular erosion). Frail/low-demand → hemiarthroplasty.

Fixation: Cemented vs Cementless

This comparison is a perennial favourite.

Feature	Cemented	Cementless
Bonding agent	PMMA (polymethyl-methacrylate) bone cement	Bony ingrowth/ongrowth (porous/HA-coated)
Stability	Immediate	Initial press-fit, then biological
Ideal patient	Elderly, osteoporotic bone	Young, good bone stock
Initial weight-bearing	Immediate full	Often protected initially
Revision	Cement removal difficult	Easier in young
Specific risk	Bone cement implantation syndrome (BCIS)	Thigh pain, stress shielding

PMMA acts as a grouting/space-filling agent, not a glue — it does not adhesively bond, but interlocks mechanically with bone trabeculae. It is exothermic on setting.

High-yield: Bone cement implantation syndrome — hypotension, hypoxia, arrhythmia, even cardiac arrest during cementation/prosthesis insertion. Caused by embolisation of marrow fat/cement monomer and the vasodilatory effect of methyl-methacrylate monomer. Anaesthetist must be warned before cementing.

A hybrid THR = cemented stem + cementless (press-fit) cup. A reverse hybrid = cementless stem + cemented cup.

Bearing Surfaces

The articulation between head and liner determines wear and longevity.

Bearing	Pros	Cons / NEET point
Metal-on-polyethylene (MoP)	Cheap, time-tested	PE wear debris → osteolysis
Ceramic-on-polyethylene (CoP)	Low wear, common today	Cost
Ceramic-on-ceramic (CoC)	Lowest wear, best for young	Squeaking, brittle fracture
Metal-on-metal (MoM)	Large heads, low volumetric wear	Metallosis, ARMD, raised serum Co/Cr — largely abandoned

High-yield: Polyethylene wear debris → macrophage activation → osteolysis → aseptic loosening. This particle-disease pathway is the dominant cause of late failure and the reason highly cross-linked PE and ceramic bearings were developed. Metal-on-metal causes pseudotumours / adverse reaction to metal debris (ARMD) and elevated cobalt-chromium ions.

Mnemonic for bearing wear (least → most): Ceramic-on-ceramic beats all; remember "Ceramic is king for the young."

Surgical Approaches & the Safe Zone

Common approaches: posterior (Moore/southern) — most popular, but highest dislocation rate; direct lateral (Hardinge) — lower dislocation but risk to superior gluteal nerve → abductor weakness/Trendelenburg gait; anterior (Smith–Petersen/DAA) — muscle-sparing, risk to lateral femoral cutaneous nerve (meralgia paraesthetica).

The acetabular cup must be positioned within Lewinnek's safe zone to minimise dislocation and impingement:

High-yield — Lewinnek safe zone: Acetabular inclination (abduction) 40° ± 10° (i.e. 30–50°) and anteversion 15° ± 10° (i.e. 5–25°). Cups outside this zone have markedly higher dislocation rates.

Femoral stem anteversion is typically 10–15°; the combined anteversion (cup + stem) concept guides stability.

Stepwise stability principle: Correct cup version → correct stem version → adequate offset & leg length → restored soft-tissue tension → stable hip.

Complications

This is the most heavily tested area. Organise by timing.

Immediate / intra-operative: bone cement implantation syndrome, fracture of femur/acetabulum during impaction, nerve injury, vascular injury.

Early (days–weeks):

• Dislocation — the commonest early complication. Posterior approach + posterior dislocation predominates. Risk factors: malpositioned cup (outside safe zone), neuromuscular disease, non-compliance with precautions, prior surgery.
• Venous thromboembolism (DVT/PE) — THR is one of the highest-risk operations for VTE; routine prophylaxis (LMWH / DOAC / aspirin + mechanical) is mandatory. Untreated, DVT incidence is very high; PE is a leading cause of early post-operative mortality.
• Infection (early prosthetic joint infection, PJI) — Staphylococcus aureus and coagulase-negative staph (S. epidermidis, biofilm) predominate.
• Nerve injury — sciatic nerve is the most commonly injured (especially its common peroneal division, presenting as foot drop). Leg lengthening and posterior approach increase risk.

Late (months–years):

• Aseptic loosening — the commonest long-term complication and commonest overall cause of revision THR. Driven by particle (wear) disease → osteolysis.
• Periprosthetic fracture — classified by the Vancouver classification.
• Late/chronic PJI — often low-virulence organisms, biofilm-forming.
• Heterotopic ossification — graded by Brooker classification; prophylaxis with indomethacin or low-dose radiotherapy in high-risk patients.
• Leg-length discrepancy, squeaking (CoC), trunnionosis, implant wear.

High-yield two-liner: Commonest early complication = dislocation. Commonest late complication / commonest cause of revision = aseptic loosening.

High-yield — nerve at risk: Sciatic nerve (peroneal division) is the most commonly injured nerve in primary THR. In revision surgery, the common peroneal nerve lies in close proximity posteriorly and is especially vulnerable; foot drop is the classic deficit. Always document distal neurovascular status pre- and post-op.

Vancouver classification (periprosthetic femoral fracture)

Type	Site	Management principle
A	Trochanteric (AG greater, AL lesser)	Often conservative / fixation
B1	Around/just below stem, stem stable	ORIF (plate/cables)
B2	Around stem, stem loose, good bone	Revise stem (long stem)
B3	Around stem, stem loose, poor bone stock	Revision + bone graft / proximal femoral replacement
C	Well below the stem tip	ORIF, ignore prosthesis

High-yield: The single most important question in periprosthetic fracture is "Is the stem loose?" — B1 (stable) is fixed, B2/B3 (loose) need revision.

Diagnosis & Investigation of Choice

Diagnosis of the underlying disease is largely clinical + plain radiograph (AP pelvis + lateral hip). For specific scenarios:

• AVN femoral head, early (X-ray normal): MRI is the investigation of choice — most sensitive, detects the marrow oedema and "double-line sign" before collapse.
• Loosening: serial plain radiographs showing progressive radiolucent lines (>2 mm), migration, cement-mantle fracture.
• Periprosthetic joint infection (PJI): raised ESR & CRP screen; definitive = joint aspiration with synovial fluid WBC count, differential, and culture. Synovial alpha-defensin and leucocyte esterase are useful adjuncts. The 2018 ICM / MSIS criteria combine serum markers, synovial counts, culture and histology.

PJI vs aseptic loosening — the crucial differential:

Feature	Septic loosening (PJI)	Aseptic loosening
Pain pattern	Rest + night pain, constant	Mechanical, start-up pain
ESR / CRP	Raised	Normal
Aspirate	High WBC, neutrophils, +culture	Sterile, low WBC
Onset	Any time, early or late	Usually late

Management / Treatment of Choice

THR is offered when conservative measures fail (analgesia, paracetamol-first per WHO ladder, NSAIDs, physiotherapy, weight reduction, walking aids) and the patient has disabling pain limiting daily life, supported by radiographic end-stage disease.

Peri-operative essentials:

• Antibiotic prophylaxis: single-dose IV cephalosporin within 60 min of incision.
• VTE prophylaxis: LMWH / DOAC + mechanical methods.
• Blood conservation, tranexamic acid to reduce transfusion.

Managing established complications:

• Dislocation: first episode → closed reduction under sedation/GA + abduction bracing + precautions. Recurrent → revision (correct component malposition, constrained liner, dual-mobility cup).
• PJI management depends on timing:
  • Acute (<3–4 weeks): DAIR — Debridement, Antibiotics, Implant Retention (with exchange of modular liner/head).
  • Chronic: two-stage revision (gold standard) — remove implant, insert antibiotic-loaded cement spacer, IV antibiotics, then re-implant.
• Aseptic loosening: revision arthroplasty.

High-yield: DAIR (debridement + antibiotics + implant retention) is reserved for acute PJI with a well-fixed implant; two-stage revision is the gold standard for chronic PJI.

Hip Precautions (Posterior Approach)

To prevent posterior dislocation, the classic instructions:

Avoid: flexion >90°, adduction past midline, and internal rotation. Practically — don't sit on low chairs, don't cross legs, use a raised toilet seat, sleep with an abduction pillow.

Mnemonic — posterior dislocation position = "sitting low and crossing legs": flexion + adduction + internal rotation. (Anterior dislocation, less common, occurs with extension + external rotation.)

Key Differentials (Causes of a Painful THR)

When a patient returns with a painful prosthesis, work through:

1. Infection (PJI) — must always be excluded first (ESR/CRP → aspiration).
2. Aseptic loosening — radiolucent lines, migration.
3. Periprosthetic fracture — history of trauma, X-ray.
4. Dislocation/instability.
5. Adverse reaction to metal debris (ARMD) in MoM bearings — raised serum cobalt/chromium, pseudotumour on MARS-MRI.
6. Iliopsoas impingement / tendinitis, trunnionosis.
7. Referred pain — lumbar spine, vascular claudication, periprosthetic stress fracture.

High-yield: In any painful THR, exclude infection first with ESR, CRP and joint aspiration before attributing pain to mechanical loosening.

Recently asked / exam angle

• The commonest early complication of THR is dislocation; the posterior approach has the highest dislocation rate — repeatedly tested single-best-answer.
• Lewinnek safe zone numbers (inclination 40±10°, anteversion 15±10°) are direct-recall favourites.
• Sciatic nerve (peroneal part) as the most commonly injured nerve, and common peroneal nerve proximity in revision surgery → foot drop — image/clinical vignette.
• Bone cement implantation syndrome — sudden intra-operative hypotension/hypoxia during cementing — classic anaesthesia–ortho overlap MCQ.
• Aseptic loosening = commonest cause of revision; mechanism = polyethylene wear-particle osteolysis.
• Vancouver B2 (loose stem) → revise the stem, while B1 (stable) → ORIF — assertion-reason style.
• THR vs hemiarthroplasty in displaced femoral neck fracture: active fit elderly → THR.
• Two-stage revision = gold standard for chronic PJI; DAIR for acute PJI.
• MoM bearings → metallosis, raised Co/Cr ions, pseudotumours (ARMD).
• Posterior hip precautions = avoid flexion >90°, adduction, internal rotation.

Rapid revision

1. Commonest indication for THR worldwide = primary osteoarthritis.
2. Cemented THR suits elderly osteoporotic bone; cementless suits young patients with good bone stock.
3. PMMA cement is a grouting/interlocking agent, not a glue, and is exothermic on setting.
4. Bone cement implantation syndrome = intra-op hypotension, hypoxia, arrhythmia — warn the anaesthetist before cementing.
5. Lewinnek safe zone: inclination 40 ± 10°, anteversion 15 ± 10°.
6. Posterior approach = most popular but highest dislocation rate; precautions: no flexion >90°, no adduction, no internal rotation.
7. Commonest early complication = dislocation; commonest late complication / cause of revision = aseptic loosening (PE wear-particle osteolysis).
8. Sciatic nerve (peroneal division) = most commonly injured nerve; common peroneal nerve is at risk in revision surgery → foot drop.
9. MRI is the investigation of choice for early AVN of the femoral head.
10. Painful THR → exclude infection first (ESR, CRP, aspiration); synovial alpha-defensin is a useful adjunct.
11. Vancouver classification of periprosthetic fracture: B1 stable → ORIF, B2/B3 loose → revise stem.
12. Acute PJI → DAIR; chronic PJI → two-stage revision (gold standard); MoM bearings → ARMD with raised serum cobalt/chromium.