• Orthopaedic surgery represents a high risk factor for the development of venous thromboembolism (VTE). Historical data estimated that hip fractures and lower limb arthroplasty without prophylaxis acquired deep vein thromboses (DVT) in 40–60% and therefore are at risk of pulmonary embolism (PE).
• Current data suggest VTE is rare, with the majority that are detected being asymptomatic. However, as VTE carries the risk of a potentially fatal outcome, an understanding of how clots form, knowledge of the risk factors for VTE and maintaining an appropriate risk prevention strategy is vital for orthopaedic surgeons.

• A combination of venous stasis, hypercoagulability (or activated coagulation in response to trauma) and endothelial damage (which constitute Virchow’s triad) may be observed in the majority of orthopaedic patients. One or more of these factors will lead to the formation of platelet aggregates at any sites of altered or turbulent blood flow (including valves or sites of compression) with consequent local release of thrombogenic substances, stimulating the formation and further expansion of a fibrin thrombus.
• The deep veins in the calf are the most common site, leading to the formation of deep vein thrombosis (DVT). Venous return relies on local muscle contraction (or muscle pump) and valves to prevent back-flow.

Risk factors for thrombogenesis

• Systemic:
• Malignancy
• Surgery or trauma
• Infection
• Obesity
• ? Smoking
• Patient specific:
• Immobility
• Previous DVT/PE
• Family history
• Known clotting abnormality/thrombophilia
• Pregnancy or hormone therapy
• Lower limb surgery (particularly hip fracture)
• Major surgery
• Varicose veins

Consequences of thrombogenesis

• Many patients will remain asymptomatic while others may develop symptomatic pulmonary embolism without evidence of DVT. Potential outcomes following a deep vein thrombosis include:
• Eventual dissolution of the clot or re-canalisation if total occlusion was present.
• Embolisation to distant locations, often lodging in, and potentially occluding, the pulmonary arteries (pulmonary embolism, PE). PE may lead to VQ (ventilation/perfusion) mismatch, which results in hypoxia and right heart failure, which can be fatal.
• Organisation within the deep veins possibly causing incompetence of local valves. Long-term this can result in venous stasis leading to oedema, ulceration, pain and the possibility of recurrent emboli (termed post-thrombotic syndrome).

• Often venous thromboembolism is asymptomatic and so incidence is difficult to determine, leading to great differences in reported rates.
• Without prophylaxis, rates of objectively confirmed DVT within 7–14 days from surgery after lower limb orthopaedic surgery are between 40 and 60%.¹Fewer than 14% are likely to develop symptomatic venous thromboembolism.^2,3Between 10 and 30% of these symptomatic patients will have a proximal DVT⁴(involving femoral or popliteal veins), which has greater potential for embolisation or development of post-thrombotic syndrome.⁵##
• Without prophylaxis, incidence:
• Total knee replacement

DVT 40–85%

Proximal DVT 5–20%

Symptomatic PE 1–2%

Fatal PE <1%^4,6,7,8

• Total hip replacement

DVT 40–60%

Proximal DVT 18–35%

Symptomatic VTE 2–5%

Fatal PE <1%^4,7,8

• Controversy remains due to multiple modalities and industry involvement. There is no clear consensus as to the best method of prophylaxis for patients.
• Risk assessment: it is vital to assess every patient for BOTH bleeding and thromboembolism risk factors and to determine the need for prophylaxis.
• Modifiable risk factors: simple measures include early mobilisation, adequate hydration, limited use of tourniquets and early fixation of fractures (to allow early mobilisation) in high-risk patients.
• Mechanical prophylaxis: the use of graduated compression stockings (which promote venous blood flow, reducing pooling of blood in the ankles and oedema) and intermittent calf pumps is encouraged when they are not contraindicated.
• Chemical prophylaxis: the use of aspirin, warfarin, low-molecular weight heparins and selective clotting factor inhibitors have all been shown to influence the rate of symptomatic VTE.
• Concern remains with regard to postoperative bleeding and wound healing issues and there is no clear consensus about the most efficacious drug.
• Current National Institute for Health and Care Excellence (NICE) guidelines for elective hip or knee replacement:

• Continue mechanical prophylaxis until mobility is not significantly reduced.
• Hip replacement and hip fracture: continue chemical prophylaxis for 28–35 days.
• Knee replacement: continue chemical prophylaxis for 10–14 days.