As one of the most successful orthopaedic procedure for treating most end-stage knee diseases, total knee arthroplasty (TKA) is associated with pain relief, restoration of function, and recovery of mobility [1]. Since most candidates for TKA are elderly patients, the co-morbid medical problems often lead to decreased tolerance to surgery. Postsurgical complications, including substantial blood loss are common after TKA. Optimal perioperative management has long been a major focus of medical practice and research for better blood management and rapid rehabilitation. The commonly used interventions include tranexamic acid (TXA) [2], use of tourniquet ing [3], cryotherapy [4], drainage protocols [5], cruciate-retaining prosthesis [6], patient-specific instrumentation (PSI) [7], minimally invasive approach, computer-aided surgical techniques [8], continuous passive motion (CPM) and postoperative knee position regimens [9].

Positioning the knee in flexion in the early postoperative stage (compared with extension) have been reported to be simple and cost-effective way to improve patient outcomes and satisfaction, whilst decreasing the strain on hospital resources, such as requirement for staff, beds, blood transfusion and expensive equipment [10]. However, recent studies about the effects of these two different knee position regimens (flexion versus extension) have reported conflicting results [11].We conducted a meta-analysis of RCTs only to assess the effect of knee position after TKA on patient outcomes, as an aid to inform optimal postoperative management strategy.

Substantial blood loss during TKA often requires allogeneic blood transfusion. However, the potential risk [25] and higher costs associated with allogeneic blood transfusion propel surgeons to find ways to reduce the blood loss during surgery. The first RCT on this topic was published by Speck et al. [9], which found that 70° of knee flexion for 6 h after the cemented TKA significantly reduced blood loss (by 30%) as determined by wound drainage. A few studies [9, 17] have proposed that the knee position after TKA is a simple and cost-effective way to maximize patient outcomes without increasing the side effects, however, the results of these studies are debatable. To clarify this issue, we performed the present meta-analysis, which included recent RCTs comparing postsurgical flexed knee position with extended knee position. It is possible that the different hip and knee angles in different trials might impact the results. When the angles of hip and knee are too large or over-extended, the resulting strain on the vein might hinder the blood backflow [11], so we only included studies in which the hip or knee angles were no more than 90°.

We chose the CBL, HBL, drainage volume, fall in hemoglobin, and requirement for blood transfusion as the parameters indicative of postoperative blood loss. The flexion group was found to have significantly less CBL (MD = −208.08 mL, 95% CI −299.80, −116.37, P < 0.00001), and HBL (MD = −132.15 mL, 95% CI −183.29, −81.00, P < 0.00001), as compared to that in the extension group. It appears that the flexion group had less hidden blood loss which is partially presents as limb swelling and ecchymosis [26]. Although no significant inter-group differences were observed in terms of fall in hemoglobin (MD = −1.79 g/L, 95% CI −5.10, 1.51, P = 0.29) at 24 h after operation, the flexion group appeared to be at an advantage from 48 h to 6 days after operation as the decline of hemoglobin was less in this group (MD = −6.59 g/L, 95% CI −10.95, −2.23, P = 0.003). One reason for this observation could be that only two articles mentioned the fall in hemoglobin at 24 h. Hence, a larger sample size and additional studies are needed to verify the result. A similar result was observed with regard to the drain volumes (MD = −53.32 mL, 95% CI −116.77, 10.13, P = 0.10). This is also reflected in the lesser number of patients requiring blood transfusion (OR = 0.43, 95% CI 0.22, 0.82, P = 0.01) in our study. The need for transfusion was an important indicator not only for the blood loss but also for the impact that the surgery had on the general status.

The subgroup analysis showed that the flexion group had significant superiority in terms of ROM at 5–7 days after surgery (MD = 4.97°, 95% CI 1.85, 8.09, P = 0.002). However, the follow-up results at 6 weeks showed no significant difference in ROM (MD = −0.29°, 95% CI −3.93, 3.34, P = 0.87) between the two groups. The extension group might be associated with increased extravasation of blood within the joint and in the tissues surrounding the knees, which may have lead to increased swelling and poorer recovery.

Compared with a previous systematic review by Faldini [29], there are several improvements in our study. First, this is the first meta-analysis to have quantified the results for the topic. Our methodology is more robust with more stringent inclusion criteria and exclusion of quasi-RCT and non-RCTs from the purview of this meta-analysis for higher validity of the results. Second, six new RCTs comparing these two position after TKA were included in our study. Third, we performed subgroup analyses for fall in hemoglobin and ROM to reduce heterogeneity.

However, there are some limitations in the present meta-analysis. First, the number of studies included in the meta-analysis was relatively small, making it difficult to conduct funnel plots to assess publication bias. Second, there are a lot of variables that could have affected blood loss and ROM after operation, however, a few studies failed to give the details of the confounding factors in their trials. Further studies with more rigorous protocol will be helpful to reduce the bias. Third, the data extracted from the trials had high heterogeneity, with different trials using different angles for knee and hip flexion, which could have affected the results. Finally, most of the included studies failed to give the details about the type of wound dressing used for controlling blood loss, which could have lead to a bias in our findings.

Our results suggest that flexion of the knee is associated with significantly lesser CBL, lesser HBL, decreased requirement for blood transfusion and better ROM at least in the early postoperative period without increasing in complication rate. However, no significant difference was found in wound drainage at 24 h and ROM at 6 weeks, multicenter randomized controlled trials with large sample size are required to validate our findings.