The Austin Health Research and Ethics Committee approved a retrospective analysis of historical data. The original data was collected using TEG as routine care (approval number: LNR/19/Austin/21). The study was registered with the Australian New Zealand Clinical Trials Registry (ACTRN12619000315112).

Original participants were recruited from perioperative anesthesia and orthopedic clinics at the Austin Hospital, Melbourne, Australia. Inclusion criteria included adult patients (> 18 years) undergoing primary, elective THA with American Society of Anesthesiologists (ASA) Grade 3 or lower. Exclusion criteria included patients with significant atherosclerotic disease, known coagulopathy or thrombophilia, abnormal liver function tests, impaired renal function (estimated glomerular filtration rate < 60 mL/min/1.73m², KDIGO stage 3A or greater) and use (< 10 d pre-operatively) of any of the following medications: Warfarin, heparin (high or low molecular weight), adenosine diphosphate receptor inhibitors, glycoprotein IIB/IIIA inhibitors, adenosine re-uptake inhibitors or thromboxane inhibitors. Patients were assessed for baseline comorbidities including ischemic heart disease, cerebrovascular disease, diabetes mellitus, smoking status and hypertension.

Anesthesia was managed by a group of anesthesiologists using a standardized care protocol. All patients scheduled for surgery underwent routine pre-operative investigations in a dedicated pre-operative anesthesiology clinic. This included a multidisciplinary review by an anesthesiologist, pharmacist, peri-operative nurse and orthopedic medical officer. Investigations included electrocardiogram (EKG), Chest X-ray and pathology testing including full blood count, urea and electrolytes and coagulation studies. Comorbidities were optimized which included smoking cessation counselling, optimization of cardiovascular risk factors and perioperative anemia and glycemic control. These were managed in accordance with National Australian Guidelines. Patients were provided with full informed consent in the pre-operative anesthesiology clinic and counselled regarding anesthetic technique. Anesthesia preference was for SA unless contraindicated (previous spinal surgery, aortic stenosis, patient refusal). Routine intraoperative anesthesia monitoring included continuous EKG and pulse oximetry as well as non-invasive blood pressure monitoring, with continuous blood pressure monitoring via an arterial line used in select patients (i.e. those with cardiorespiratory comorbidities). The threshold for blood transfusion was a hemoglobin less than 8 g/dL. No patient received intraoperative fibrinolytic therapy (e.g. tranexamic acid). All patients received non-pharmacologic methods of VTE prophylaxis. VTE chemoprophylaxis included enoxaparin (40 mg) administered immediately prior to skin closure, continued daily until hospital discharge. Patient charts were reviewed to ascertain the incidence of postoperative VTE.

Blood samples for TEG measurement were taken at 6 intervals: Immediately pre-operatively (baseline), midpoint intraoperatively (mid-point of surgery), immediately prior to skin closure (end of surgery), and on postoperative days 1, 2 and 5. Blood samples for conventional laboratory coagulation tests were pre-operatively (baseline) and postoperatively on day 5. All samples were collected using a single peripheral aseptic venepuncture. Viscoelasticity was measured on a TEG5000 system (Haemonetics^®, United States) performed by a technician, expert in TEG, blinded to the choice of anesthesia technique. TEG was performed within 4 min of sampling, complying with our institution’s standard technique at time of collection, which has been previously reported[15,16]. TEG variables measured were reaction time (R, min), coagulation time (K, min), clot formation angle (degree), MA (mm) and fibrinolytic index (LY60%).

Postoperatively, blood for TEG and conventional laboratory coagulation assays were sampled approximately 1 h before administration of daily VTE chemoprophylaxis using standard techniques (Supplementary Appendix 1).

No patients were eliminated from this study secondary to a lack of availability of TEG.

Non-parametric statistical analysis (Friedman’s test) followed by the Wilcoxon signed rank test with Bonferroni correction was used to evaluate changes in TEG over time in each group.

The Mann-Whitney U test was used to compare data between patients having GA and SA. Data are presented as medians with quartiles. A P < 0.05 was considered statistically significant.

We observed that TEG findings in the THA population demonstrated a hypercoagulable state postoperatively, characterized by a steady increase in MA regardless of anesthetic technique used. Other TEG parameters demonstrated a transient, intraoperative hypercoagulability using SA, with these parameters returning to baseline by day 5 postoperatively. Fibrinogen levels were significantly elevated postoperatively, in both anesthetic groups. No difference was demonstrated in any other conventional coagulation tests compared to baseline.

Our findings agree with and add to previously published research. A 2013 study examining 61 patients undergoing primary, elective THA found MA increased throughout postoperative day 1, peaked by day 7, and remained elevated until day 14[17]. Like our study, this population all received routine enoxaparin prophylaxis. This study demonstrated a decline in platelet level postoperative day 1, increase after day 3, and peak between days 7-14, and again demonstrated elevated fibrinogen level postoperative day 3 to day 14. This study examined coagulation tests on differing days to our study, measuring TEG and conventional tests immediately postoperatively, day 1, 3, 7 and 14 postoperatively[17]. Similarly, a 2014 study examining 42 patients undergoing THA, also found an increasingly hypercoagulable picture postoperatively[10]. Unlike our data, which demonstrated no difference in any TEG parameter except MA vs baseline, this study demonstrated the hypercoagulable picture of this patient group was mostly attributed to a mixed enzymatic and platelet contribution. Importantly however, this study examined patients on days 1, 4 and 9 postoperatively, compared to days 1, 2 and 5 postoperatively for our study. This study also used a different form of low molecular weight heparin (fraxiparine) and did not report any conventional coagulation tests, as well as enrolling fewer patients. Our findings support other small, observational study data, wherein TEG demonstrates increase in MA post THA[18]. Intraoperative hypercoagulable states detected by TEG have been reported in previous studies and attributed to either surgical trauma or acute blood loss and hemodilution[2,3,19]. The location of hip arthroplasty surgery makes it appropriate for neuraxial anesthesia techniques (epidural and SA). It has been demonstrated neuraxial techniques may attenuate the stress response and improve local blood flow, and have been associated with direct and indirect effects on the hemostatic system[11]. A previous study reported that epidural anesthesia and analgesia attenuated postoperative hypercoagulability as measured by TEG and reduced thromboembolic sequelae post major vascular surgery[20]. However, these findings have not been reproduced[21,22]. We failed to detect an association between coagulation state and anesthetic technique used for THA. Our findings are congruent with existing evidence, which demonstrate perioperative activation of the coagulation and fibrinolysis systems are similar regardless of anesthetic technique used, resulting in SA having no benefit in reducing hypercoagulability in this group[23-28]. The utility of TEG at predicting thromboembolic events in this patient population remains unknown. A comprehensive 2018 meta-analysis of 41 studies found the sensitivity and specificity of TEG at predicting thromboembolic events was 56% (95%CI: 44-67) and 76% (95%CI: 67-83), respectively[29]. This meta-analysis included data from over 10000 individuals from a heterogenous population. A prospective 2016 study focusing on the diagnostic predictive value of TEG in orthopedic patients revealed a sensitivity of 14% and specificity of 62%[30]. However, this study only performed pre-operative TEG, with several variables changing between pre-operative TEG sampling and the development of post-operative VTE limiting the interpretation of these results. To our knowledge, no evidence is currently available which examines the diagnostic predictive value of post-operative TEG at predicting VTE events in orthopedic patients. We believe our unique, novel findings add to the available evidence in this area and could help guide future outcome-based studies in this group.

This study carried several significant limitations. It used data collected several years ago, however we feel that as anesthesia, chemoprophylaxis and surgical principles have not varied significantly vs when data was collected that our study results remain valid, and an important contribution to existing evidence. TEG was collected days 1, 2 and 5, compared to a single day 5 postoperative laboratory test. This was in keeping with the practice at our institution at the time of data collection. Importantly, no platelet counts were recorded for any patient during original data collection. We acknowledge that this is a major weakness in the original study design, given platelet contribution to thrombus formation, and subsequently the observed MA, and possibly the overall hypercoagulable state we observed. This is a single center, small observational study, which may limit the external validity of our findings. However, our hospital is representative of many tertiary institutions with patient outcomes equivalent to those of other tertiary hospitals in Australia[31]. The study was also observational and descriptive only and inadequately powered to assess clinical outcomes, including incidence of VTE or VTE associated complications, such as pulmonary embolism and overall morbidity/mortality. Patients were not actively investigated for venous thromboembolic complications including either the routine use of lower limb ultrasound or CTPA. However, this was never the intention of this research, and we seek to make valid data available to the broader scientific community. Ideally, further research conducted in this area would involve paired TEG and conventional coagulation tests sampling at dedicated, simultaneous time-points perioperatively, including platelet count (and ideally, platelet function).

Our findings imply that despite routine VTE chemoprophylaxis, patients undergoing THA remain in a hypercoagulable state as measured by both TEG and conventional tests. Going further, as TEG MA is a measure of thrombus size, it is influenced by both fibrinogen and platelet number and function[32]. These are in turn both influenced by postoperative inflammatory mediators[33]. This leads us to question whether this patient group optimally anticoagulated. Existing evidence suggests solely targeting thrombin production (through administering low molecular weight heparin) may not provide sufficient protection against platelet activation, hence a hypercoagulable state may persist[34,35]. Increased TEG MA suggests that routine VTE chemoprophylaxis cannot prevent the development of platelet-dependent hypercoagulability after THA and that additional antiplatelet drugs may have a role[36,37]. A recent major review of thromboprophylaxis in major orthopedic surgery concluded that aspirin alone for VTE chemoprophylaxis is not recommended, however it’s value as an adjunct remains unknown[9].

Patients undergoing total hip arthroplasty (THA) are known to be at high risk of developing venous thromboembolism (VTE), causing significant morbidity and mortality. Thromboelastography (TEG) offers real-time information regarding the global coagulation state of a patient. This technology may be useful in investigating the coagulation of this high-risk population.

Available evidence surrounding the use of TEG in this patient cohort is limited, including both observational data, describing the coagulation status in these patients, and interventional data, guiding anticoagulant therapy. Our motivation for this study was to investigate the coagulation state observed in this patient group as assessed by TEG, and examine how these observations change according to time course post-operatively, and anesthetic technique, in order to ultimately improve perioperative care of these high-risk patients.

We aim primarily to demonstrate the coagulation profile of patients undergoing elective THA, using TEG. We secondarily aim to describe how this coagulation pattern varies according to anesthetic technique chosen [spinal neuraxial vs general anesthesia (GA)] and how TEG findings compare to traditional coagulation tests.

We performed a retrospective, observational study, examining archived data of elective THA patients. Patients were selected from a dedicated orthopedic preadmission clinic, meeting strict inclusion criteria, and all received enoxaparin as routine post-operatively. We analyzed baseline TEG maximum amplitude (MA), compared to intraoperative and postoperative days 1, 2 and 5. We then compared observations based on anesthetic technique received (GA vs spinal) and those described by conventional coagulation tests.

We studied a total of 52 patients. We found that MA remained within normal limits, without significant difference from baseline, throughout surgery. We observed elevated MA postoperatively on days 1 and 2, before resolving day 5. This was consistent regardless of anesthetic technique used. All patients had elevated fibrinogen levels day 5 post-operatively, with no other abnormalities detected by conventional coagulation tests.

Patients undergoing elective THA demonstrate postoperative hypercoagulability when assessed by TEG (characterized by elevated TEG MA), despite routine VTE prophylaxis. Anesthetic technique (spinal vs GA) had no influence on the postoperative coagulation profile observed in these patients, as assessed by TEG.

Our study findings imply that routine VTE prophylaxis in patients undergoing elective THA does not ablate the postoperative hypercoagulable state, according to TEG. These findings suggest that further research comparing TEG with both conventional coagulation tests, (including platelet count) and platelet function testing may be useful.