THA and TKA are the most frequently performed joint replacement surgeries. The annual demands for primary THA and TKA are estimated to reach 174% and 673% by the year 2030, respectively.15,16 Patients undergoing total joint arthroplasty (TJA) have a high risk of developing thrombotic complications due to reduced blood flow, thrombophilia, and damage to the venous linings.17–20 Thus, after TJA, patients are recommended to routinely receive prophylactic anticoagulation.19 However, HIT — one of the most dangerous pro-thrombotic disorders — may occur during anti-thrombotic prophylaxis.1,12,21 HIT patients have a higher risk of developing paradoxical thrombotic syndrome.22–24 Hence, it is critical to understand the incidence and risk factors of HIT.
Despite the performance of TJA soaring from 2005 to 2014, the incidence of HIT after TJA has remained at 0.01–0.03%. This is consistent with the results reported by Craik.10 In 2008, the American Academy of Orthopedic Surgeons (AAOS) released a guideline recommending thromboprophylaxis after TJA.17 Thereafter, chemoprophylaxis has been done more routinely.10,12,25 Although it has not been commonly diagnosed, HIT has been recognized as one of the thromboembolic complications of patients after TJA. HIT has resulted to negative effects on surgical outcomes. Interestingly, compared with the incidence of postoperative HIT to other procedures, Dhakal et al.26 reported that the risk of HIT after TJA was lower. A possible explanation might be the application of low molecular weight heparin (LMWH) rather than UFH. Thus, routine laboratory monitoring was not recommended for HIT after TJA.10,25,27 However, according to our study, the cases of inpatient mortalities in post-THA patients were 13.75 times higher (2.2% vs. 0.16%) and 8.3 times higher than in post-TKA patients (0.58% vs. 0.07%), respectively. It is critical to identify the HIT risk factors and keep in mind that HIT may take place during LMWH exposure.
There have been controversial conclusions about the risk factors of HIT. In contrast to our results, Warkentin et al.28 and Chaudhry et al.29 found the female gender as a risk factor to HIT. This may be due to the different immune responses after various surgeries29, or the different thromboprophylactic therapies employed.28 The racial differences30 and polymorphism variabilities31 have also been reported by other researchers. For example, non-Caucasians had higher thrombotic risks of HIT development after undergoing any procedure on their current admission.30 Furthermore, large-scale and teaching hospitals served as protective factors after THA.
The study of preoperative comorbidities may be valuable in preventing HIT. For THA, AIDS, hypertension, psychoses, and pulmonary circulation disorders increased the risk of postoperative HIT. In TKA patients, pulmonary circulation disorders and weight loss were identified as risk factors. To date, there is no established mechanism able to explain the above findings. One possible explanation is the formation of nonspecific and dysregulated antibodies triggered by pathological processes.32 Specially, in the case of psychoses, the combined use of heparin and psychotropic medications may result in adverse hematologic effects.33
The application of novel anticoagulants may provide a good opportunity of decreasing HIT. Argatroban and lepirudin, both direct thrombin inhibitors, have been estimated to have comparative efficacy and safety in the management of HIT.34 Danaparoid has been found to have less influence on platelets compared with other anticoagulants because it inhibits factor Xa selectively.35 However, given the low incidence of HIT after TJA, LMWH is still the preferred chemoprophylaxis after TJA in patients without a history of HIT.17,19,20
This study had several limitations. First, as a retrospective study utilizing the NIS database, coding and data-entry errors may have existed and led to an erroneous estimation of the HIT. Second, the limited elements and absence of follow-up in the NIS database made it impossible to analyze the long-term HIT outcomes. For instance, we were unable to accurately select patients with heparin therapy through the ICD-9 code. This made it inevitable to rule out patients without heparin admission. Finally, the different risk factors of HIT after THA and TKA were identified without established mechanisms. Bias in the medical records, utilization of tourniquet in the TKA, and the different seroconversion rates of anti-PF4/heparin antibodies between the two procedures36 might have contribute to the difference in the results. Further prospective, evidence-based studies are needed to determine the exact causalities between comorbidities and HIT.