Historically, hip replacement surgery has been an operation prone to clot formation in the veins of the legs (thrombosis). If these clots dislodge, they migrate through the veins and right heart (right atrium and right ventricle) to the lungs, a complication known as pulmonary embolism. The symptoms the patient may notice depend on the size of the clots: small clots may produce no symptoms or a transient cough, while larger clots are usually associated with chest pain and shortness of breath. If the clots are very large they may compromise the pulmonary circulation and can be life threatening. However, the risk of serious pulmonary embolism has declined substantially in the past three decades, reflecting advances in the understanding of the formation of clots and its prevention, and numerous improvements in anesthesia, surgical techniques, and patient care.
In this review we will describe the critical factors, which in our experience have reduced the risk of thromboembolism. They include preoperative assessment of patient's predisposing factors to clot formation, preoperative blood donation, epidural anesthesia, expeditious surgery minimizing femoral vein occlusion and blood loss, intermittent pneumatic compression of the legs, active foot and ankle exercises immediately after surgery and early ambulation.
Patient's Predisposing Factors
Patients with a history of previous clot formation are at increased risk. Other risk factors include old age, obesity, cancer, and certain types of cardiac disease. Some medications, which increase the risk of clotting, such as estrogen and contraceptive pills, should be discontinued prior to surgery.
Recent advances in molecular genetics can identify patients with a genetic predisposition to clot formation. At the Hospital for Special Surgery, we studied patients who developed postoperative pulmonary embolus and found that they had a higher chance of having a genetic predisposition, compared with patients who did not develop clots after surgery. In the future, rapid-screen genotype tests may become part of the preoperative patient evaluation, identifying those patients at highest risk for clot formation.
Preoperative Blood Donation
We have observed that patients who donate their own blood approximately one week before surgery tend to develop fewer clots in their legs, and have less chances of developing a pulmonary embolus. This beneficial effect is most likely related to the blood becoming less viscous following the blood donation.
There is ample literature, which demonstrates that the risk of thromboembolism is lower with regional anesthesia than with general anesthesia. During the last two decades our anesthesiologists have refined epidural anesthesia at our institution. In addition, the blood pressure is reduced during the operation, minimizing blood loss, and allowing the surgeons to proceed with the surgery expeditiously, with perfect visualization of the anatomical structures.
Hip replacement surgeons at our institution work with a well trained, dependable and consistent surgical team. They follow an efficient sequence of surgical steps to expedite the operation, thus improving outcome.
Certain steps of the operation require that the lower extremity be placed in an extreme position, which interrupts the venous blood flow, favoring clot formation. Extending the leg to a neutral position, whenever possible, restores venous blood flow.
As the femoral canal is prepared to receive the prosthetic femoral component, the intramedullary contents are carefully aspirated, to prevent them from flowing to the local veins, which also favors clot formation.
Preheating the prosthetic femoral component and the powder of the cement to approximately 40 degrees centigrade, reduces the time of cement hardening, with a concomitant reduction in surgical time, cement porosity and without an adverse affect on the biomechanical properties of the cement.
Patients who are operated on with regional anesthesia will not regain active motion of the lower extremities for a variable period of time after surgery. Intermittent pneumatic compression devices provide a mechanical means of increasing venous flow, by the intermittent inflation of air-filled cuffs placed around the legs. Once the patient recovers motor activity, he or she should be strongly encouraged to "pump" ankles, feet, and toes to augment the venous blood flow. However, this activity may or may not be performed by the patient due to pain, sleep, etc. and under these circumstances, pneumatic compression of the calves is most beneficial.
Multiple studies have shown the importance of early ambulation to minimize postoperative complications, including clot formation.
At the Hospital for Special Surgery, enteric-coated aspirin is the preferred postoperative prophylaxis for patients with no additional risk factors for clot formation and who mobilize promptly.
Aspirin has a long-term record of safety. It is inexpensive, easy-to-administer, and well tolerated, with analgesic and antifebrile effects. Aspirin is also efficacious in reducing arterial complications (heart attacks, strokes, etc.). Thus, aspirin remains our prophylaxis of choice for patients with no additional risk factors for clot formation.
At our hospital we indicate warfarin (Coumadin) for patients with predisposing factors to clot formation or who are already on such a drug for preexisting conditions. Warfarin prophylaxis becomes effective only after a few days of administration, and provides no protection during the immediate postoperative period, when clot formation is maximally activated. The dose response is variable and with the current short hospital stay, it is difficult to achieve an adequate prophylactic level prior to discharge. Warfarin daily dose and response has to be monitored with periodic blood tests by the physician after discharge.
While the reported experience with warfarin is favorable in reducing the risk of clots and embolism, there is an associated increased risk of bleeding complications.
LOW-MOLECULAR WEIGHT HEPARINS (LMWH)
During the last decade, LMWH have been advocated for prophylaxis against clot formation. An aggressive marketing program, including sponsored publications and instructional courses, has contributed to their initial popularity. No monitoring is required, as the dose is adjusted according to body weight, but the prophylaxis is costly and patients need to be taught subcutaneous self-injection to continue the administration in the outpatient setting. As with all anticoagulant drugs, LMWH also present an increased risk of bleeding complications.
The risk of pulmonary embolus after total hip replacement has decreased markedly during the past three decades, as a result of the numerous improvements and advancements cited above. This is supported by our collective experience during the last decades, encompassing over 50,000 hip replacements, in which the current prevalence of pulmonary embolus is less than 1%.
Future investigations should concentrate on establishing reliable means of identifying patients who are predisposed to postoperative clot formation. If such patients could be routinely identified prior to surgery, they could be successfully managed, with the appropriate medical prophylaxis and means of surveillance selectively chosen to match their individual predisposition.
This review was partially funded by the generous donation of Mr. Alberto Foglia and his wife, Maria Pia.