Thromboprophylaxis

Why it matters. Venous thromboembolism (VTE) covers deep vein thrombosis (DVT) and pulmonary embolism (PE), and it is the most common preventable cause of in-hospital death. After THA and TKA without prophylaxis, venographic DVT rates are 40-60% and symptomatic PE 1-2%.

What the 40-60% counts. It is the most commonly misquoted number in VTE prophylaxis. It comes from the pre-prophylaxis era, when trials screened every patient with routine ascending venography, and it therefore counts overwhelmingly distal, asymptomatic calf thrombi that would never have declared themselves clinically. It is emphatically not a 40-60% risk of a symptomatic DVT.

How to quote it. Present it as the venographic denominator that established why prophylaxis became mandatory, and pair it with the modern figure: symptomatic VTE after elective THA/TKA is now roughly 1% or less at 90 days with prophylaxis and early mobilisation. Making that distinction is what separates a candidate who has memorised a number from one who understands it.

Risk without prophylaxis, by procedure. These figures are why prophylaxis is mandatory:

• THA - DVT 42-57%, PE 0.9-28%
• TKA - DVT 41-85%, PE 1.5-10%
• Hip fracture - DVT 46-60%, fatal PE 3-12%
• Major trauma - DVT 40-80%

Virchow's triad. All three components are present in orthopaedic surgery:

• Stasis - immobility, surgery, prolonged recumbency
• Endothelial injury - surgical trauma, inflammation
• Hypercoagulability - surgery-induced, thrombophilia

Where the thrombus forms. Most clinically significant DVTs originate in the deep calf veins, the posterior tibial and peroneal veins and the soleal sinuses, and propagate proximally. Proximal DVT, in the popliteal, femoral and iliac veins, carries the highest PE risk. Upper limb DVT, in the subclavian and axillary veins, is uncommon in orthopaedics.

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Natural history. Thrombus forms in the calf veins during or after surgery and may propagate proximally, in 20-30% if untreated. Proximal DVT can embolise to the pulmonary circulation, and the mortality of the resulting PE depends on clot burden and cardiopulmonary reserve.

Procedure sets the baseline. Risk stratification by procedure guides the intensity and duration of prophylaxis.

Patient factors add to procedural risk. Each carries its own relative risk and implication for prophylaxis:

The model. The Caprini score is the most widely used weighted, cumulative VTE risk-assessment model. Each risk factor is assigned 1, 2, 3 or 5 points by severity, and the points are summed.

Score bands. The total maps to a prophylaxis recommendation:

• 0 - very low risk: early ambulation
• 1-2 - low: mechanical
• 3-4 - moderate: chemical or mechanical
• 5 or more - high: chemical plus mechanical, extended duration

The ceiling in orthopaedics. An elective arthroplasty and a hip, pelvis or leg fracture each score 5 on their own, so essentially every major orthopaedic patient is already "high risk" before a single individual factor is added. That ceiling is why the score has only modest discrimination within the arthroplasty population: it cannot separate the high-risk arthroplasty patient from the very-high-risk one.

Where it earns its place. Its value here runs the other way. Use it to surface the extra factors (prior VTE, active cancer, thrombophilia) that justify longer or higher-intensity prophylaxis rather than to decide whether to give it, and to risk-assess the lower-risk and mixed surgical patients for whom it was derived.

Signs are unreliable. Half of DVTs are asymptomatic and clinical signs miss 50%. Screening is not recommended: the focus is prevention with appropriate prophylaxis rather than detection.

DVT. The signs, when present:

• Calf pain, worse with dorsiflexion (Homans' sign)
• Asymmetric leg swelling
• Warmth of the affected limb
• Subtle erythema
• A palpable cord, the thrombosed vein

PE. A clinical emergency: act immediately if it is suspected.

• Dyspnoea, the most common symptom
• Pleuritic chest pain, sharp and worse on inspiration
• Tachycardia, heart rate over 100
• Hypoxia, SpO2 under 92%
• Syncope in massive PE

Pre-test probability. The Wells score estimates the probability of DVT or PE and guides further investigation.

D-dimer. Sensitive but not specific, so a negative result is useful to rule VTE out. It is always elevated after surgery, which makes it useless post-operatively: rely on clinical suspicion and imaging instead.

Compression ultrasound is the gold standard for DVT. A non-compressible vein is thrombus, and sensitivity exceeds 95% for proximal DVT.

CTPA (CT pulmonary angiography) is the gold standard for PE. Use a V/Q scan if contrast is contraindicated.

Mimics. The swollen, painful post-operative limb and the breathless post-operative patient both have important mimics. Anchoring on VTE risks missing a treatable alternative, and treating a mimic as VTE exposes a fresh surgical wound to unnecessary anticoagulation.

The principle. For high-risk procedures, combine mechanical and chemical prophylaxis and extend it beyond discharge. Mechanical prophylaxis reduces VTE on its own and adding a chemical agent reduces it further. For high-risk procedures mechanical prophylaxis is always indicated, even when a drug is given. The AAOS guideline is less prescriptive for elective arthroplasty, asking for pharmacological and/or mechanical prophylaxis without mandating an agent.

Mechanical methods. Three devices are used:

• Graduated compression (TED) stockings - below-knee or thigh-high; reduce DVT by 50% alone; proper fit is essential
• Intermittent pneumatic compression (IPC) - sequential calf compression, applied in theatre and continued post-operatively; more effective than stockings alone, and best combined with them
• Venous foot pumps - an alternative to IPC, which may be used if calf access is limited

Drugs. Enoxaparin is the gold standard, DOACs are convenient oral alternatives, and aspirin is acceptable to complete an extended course. Edoxaban and dabigatran are further DOAC options.

Timing is a trade. A first dose given too early increases bleeding; one given too late may allow thrombus to form.

Gold standard against the trials. Calling LMWH the gold standard sits alongside RECORD1 and RECORD4, in which rivaroxaban was more effective than enoxaparin on a composite driven by asymptomatic venographic DVT (see the Evidence Base). Aspirin's place rests on EPCAT II: it was non-inferior when used to complete extended prophylaxis after an initial anticoagulant, a five-day rivaroxaban lead-in in that trial, so it can finish a course begun with LMWH or a DOAC.

Duration. VTE risk persists beyond discharge, so extended prophylaxis is mandatory after arthroplasty and hip fracture and in-hospital prophylaxis alone is inadequate. The durations below follow the ACCP; the AAOS leaves duration to surgeon discretion.

In theatre. These measures complement mechanical and chemical prophylaxis:

• Minimise tourniquet time: the longer the tourniquet, the higher the risk
• Handle tissue gently to reduce endothelial injury
• Regional anaesthesia may reduce VTE compared with general
• Start IPC before induction
• Keep the patient adequately hydrated and avoid hypovolaemia
• Avoid hypotension and maintain perfusion

Anticoagulation has its own price. Every regimen balances VTE risk against bleeding risk.

Why it is examined. HIT is a serious complication of heparin products and the one prophylaxis complication where the intuitive response is wrong. Check platelets on days 5-10 when using LMWH.

The paradox. Immune (type II) HIT is caused by IgG antibodies against complexes of heparin and platelet factor 4 (PF4). The antibody-PF4-heparin complex activates platelets, so despite a falling platelet count the patient becomes intensely prothrombotic, with venous and arterial "white clots" rather than bleeding. Type I is a mild, early, non-immune, self-limiting dip and is benign.

Who and when. HIT is commoner with unfractionated heparin than LMWH, and commoner after surgery. The platelet fall is classically on days 5-14, or within a day if heparin was given in the preceding 100 days or so (rapid-onset HIT).

The 4T score estimates pre-test probability, each item scoring 0, 1 or 2:

Reading the score. A total of 0-3 is low probability and effectively excludes HIT clinically; 4-5 is intermediate and 6-8 high. An intermediate or high score triggers treatment and laboratory confirmation.

Management runs against instinct. The patient is clotting, not bleeding:

1. Stop all heparin immediately, including LMWH, heparin flushes and heparin-bonded lines
2. Start a non-heparin anticoagulant at therapeutic dose: a direct thrombin inhibitor such as argatroban, or danaparoid or fondaparinux
3. Do not transfuse platelets prophylactically, which fuels the thrombosis
4. Do not start warfarin until the platelet count recovers (over 150): early warfarin precipitates venous limb gangrene and skin necrosis through protein C depletion, so overlap and bridge instead
5. Confirm the diagnosis with an anti-PF4/heparin immunoassay (ELISA), which is sensitive but not specific, plus the serotonin-release assay (SRA), the specific functional gold standard
6. Document a heparin allergy and avoid heparin thereafter

On the ward. Of these measures, early mobilisation is one of the most effective:

• Mobilise early, weight-bearing on day 0-1 when possible
• Continue IPC until ambulatory
• Begin bilateral TED stockings immediately
• Encourage oral fluids
• Elevate the legs when in bed
• Active ankle exercises hourly when awake

After discharge. The course continues at home:

1. At discharge - educate on the symptoms of VTE (calf pain, swelling, shortness of breath) and to seek immediate help if they occur
2. Days 1-14 - continue LMWH or DOAC as prescribed, make sure the patient understands the regimen, and teach self-injection for LMWH
3. Days 14-35 - the patient may switch to aspirin to complete the course, on the EPCAT II evidence, continuing until 35 days in total
4. Follow-up - stop prophylaxis at 35 days if fully mobile; continue longer if high-risk or immobile

Compliance. Patient compliance with home prophylaxis is essential, so simplify regimens where possible.

With prophylaxis. DVT falls to 2-5%, symptomatic PE to less than 0.5% and fatal PE to less than 0.2%. Appropriate prophylaxis makes VTE a rare event after modern arthroplasty. The patient factors that raise risk are tabulated under Classification.

Global epidemiology: Without prophylaxis, venographic DVT occurs in roughly 40–60% after major lower-limb arthroplasty or hip fracture. Modern prophylaxis plus early mobilisation has driven symptomatic VTE after elective THA/TKA to roughly 1% or less at 90 days, with fatal PE now well under 0.5%. The greatest residual burden of fatal PE is in hip-fracture and polytrauma patients.

Side-by-side major guidelines:

Aspirin versus anticoagulants. EPCAT II and PREVENT CLOT show aspirin is non-inferior for clinically important outcomes, yet PREVENT CLOT found a small excess of mostly distal, often asymptomatic DVT with aspirin. Whether this matters clinically, and in which patients, remains debated.

How long after TKA. Guidelines diverge from 10-14 days (NICE for TKA, and some ACCP options) up to 35 days. The thrombotic burden after TKA is lower than after THA, so the marginal benefit of prolonged prophylaxis after knee replacement is uncertain.

Mechanical-only prophylaxis. For low-risk arthroplasty with rapid mobilisation, some advocate mechanical methods plus aspirin alone. Robust trials isolating the independent contribution of stockings, IPC and chemical agents are lacking.

Risk tools. Caprini and other scores are widely used but were not derived for arthroplasty and have modest discrimination. No universally validated tool exists to individualise agent and duration in elective joint replacement.

Screening. AAOS recommends against duplex screening of asymptomatic patients, yet some units still screen high-risk groups. Treating asymptomatic distal DVT of uncertain significance risks over-anticoagulation.

Distal versus proximal DVT. Isolated distal (calf) DVT detected on surveillance has a low embolic risk. Whether to anticoagulate it, surveil it or ignore it is unresolved and varies by guideline and bleeding risk.