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Predictors of Postoperative Drain Output and Hemoglobin Decline After Primary Total Knee Arthroplasty: A Retrospective Cohort Study of 486 Cases

In a retrospective study of 486 primary total knee arthroplasties, female sex and cruciate-retaining implant design were identified as independent predictors of lower postoperative drain output and smaller hemoglobin decline, though the models explained only a modest portion of the overall variability.

Original authors: shakib bader, khalil nasrallah, timaa shker, hilal mbarky, shadi karayanni, haim shtarker, einal bathish

Published 2026-09-18

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CC BY 4.0

Original authors: shakib bader, khalil nasrallah, timaa shker, hilal mbarky, shadi karayanni, haim shtarker, einal bathish

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

When a knee joint wears out from years of use, the most reliable way to restore movement and stop pain is to replace it with a new artificial joint. This surgery, known as a total knee replacement, is one of the most common operations performed worldwide. However, the procedure is not without its costs to the body. Cutting into the knee and reshaping the bone inevitably causes bleeding. Some of this blood is visible and drains out through a tube placed in the wound, while a significant amount stays hidden inside the tissues or is reabsorbed by the body, never seen by the surgeon. Managing this blood loss is critical; losing too much can leave a patient weak and anemic, requiring a blood transfusion, while losing too little might suggest the surgery was less invasive than it truly was. For decades, doctors have debated what factors truly drive this blood loss: is it the surgeon's technique, the type of anesthesia, the patient's age, or the specific design of the metal and plastic parts used to rebuild the knee?

A team of researchers at Galilee Medical Center in Israel set out to answer these questions by looking closely at nearly five hundred knee replacement surgeries. They focused on a specific group of patients who all underwent the same type of primary, or first-time, knee replacement performed by a single senior surgeon. By keeping the surgeon and the hospital protocols identical, the team could strip away the noise of different operating styles and see which patient characteristics or surgical choices actually mattered. They tracked two main things: how much fluid came out of the drain tube after surgery and how much the patients' hemoglobin levels dropped. Hemoglobin is the protein in red blood cells that carries oxygen, and a sharp drop in this number is a clear sign that the body has lost a significant amount of blood.

The study involved 486 patients who had their knee replaced between 2021 and 2026. Every patient received the same standard care, including a dose of a medication called tranexamic acid before the cut was made to help the blood clot, and a blood thinner after surgery to prevent clots. The researchers recorded details about the patient, such as their age, weight, and whether they had diabetes, as well as details about the surgery, like how long a tourniquet was used to stop blood flow during the operation and whether the surgeon used a robotic arm to help with the bone cuts. They also noted the specific design of the knee implant used, which generally comes in two main varieties: one that keeps the knee's natural ligament intact and one that replaces it with a mechanical post-and-cam mechanism.

When the researchers analyzed the data, they found that the amount of blood lost was not random. Two factors stood out as having a clear, independent influence on the outcome. First, the sex of the patient played a major role. On average, male patients lost significantly more blood through the drain and experienced a larger drop in their hemoglobin levels compared to female patients. The data showed that men had a total drain output that was roughly 137 milliliters higher than women, and their hemoglobin dropped by about half a gram per deciliter more than women's. This difference held true even after the researchers accounted for other factors like age and body weight.

The second major factor was the type of knee implant used. The vast majority of patients in the study received a cruciate-retaining implant, which preserves the patient's original ligament. A very small number of patients, only sixteen, received a posterior-stabilized implant, which replaces that ligament with a mechanical hinge. The study found that the patients with the posterior-stabilized implants lost more blood. Their drain output was, on average, about 139 milliliters higher than those with the other type of implant, and their hemoglobin levels fell by an additional 0.6 grams per deciliter. The researchers noted that because so few people received this specific implant, the numbers should be viewed with caution, but the trend was clear in their data.

Surprisingly, many other factors that doctors often worry about did not show a connection to blood loss in this study. The length of time the tourniquet was used, the patient's age, their body mass index, and whether they had diabetes did not independently predict how much blood was lost. Even the use of robotic assistance to guide the surgery did not show a statistically significant difference in blood loss, though the study included very few robotic cases to draw a firm conclusion. The researchers also found that routine blood clotting tests taken before surgery did not help predict who would bleed more.

The study concluded that while sex and the specific design of the knee implant are the strongest predictors of blood loss in this setting, they only explain a small part of the story. The mathematical models used by the researchers could only account for about 11.5% of the variation in drain output and 4.3% of the variation in hemoglobin drops. This means that for every patient, there is a large amount of individual variability that science has not yet pinned down. Perhaps it is the unique way a person's tissues heal, or subtle differences in how the body manages fluids that are not captured by standard tests. The findings suggest that while doctors can expect men and patients with certain implant types to lose slightly more blood, the overall picture of blood loss remains complex and highly individual, reminding us that even in a standardized surgical world, the human body retains its own unpredictable nature.

Technical Summary: Predictors of Postoperative Drain Output and Hemoglobin Decline After Primary Total Knee Arthroplasty

Problem Statement
Postoperative blood loss following Total Knee Arthroplasty (TKA) remains a clinically significant concern, associated with increased healthcare utilization and anemia. While blood loss is complex to measure—comprising visible loss (intraoperative and drain output) and hidden loss (retained in tissues or calculated via hemoglobin changes)—evidence regarding specific patient and operative factors that predict drain output specifically is limited. Existing literature is often heterogeneous in surgical technique, blood management protocols, and study design, frequently combining multi-surgeon data or focusing on calculated total blood loss rather than objectively measured early drain output. This study addresses the need for a direct, multivariable evaluation of early drain output within a highly standardized, single-surgeon pathway to isolate independent predictors of drainage and hemoglobin (Hb) decline.

Methodology

Study Design: A retrospective, single-center, observational cohort study conducted at Galilee Medical Center, Israel.
Population: The study included 486 consecutive primary unilateral TKAs performed between January 2021 and July 2026 by a single senior orthopedic surgeon.
Exclusion Criteria: Revision TKAs, bilateral procedures, known bleeding disorders, and insufficient medical record data. Patients on chronic antiplatelet/anticoagulant therapy were included only if discontinued per protocol.
Standardization: To minimize variability, all procedures utilized a standardized midline anterior incision, medial parapatellar arthrotomy, and specific hemostatic techniques.
- Pharmacology: All patients received 1 g intravenous tranexamic acid (TXA) pre-incision and postoperative enoxaparin thromboprophylaxis.
- Drainage: An intra-articular drain was placed in 97.5% of cases (474/486). Drain output was recorded daily by nursing staff until removal.
Variables: Data extracted included demographics, comorbidities, preoperative coagulation parameters, surgical method (conventional vs. robotic-assisted), implant type (Cruciate-Retaining [CR] vs. Posterior-Stabilized [PS]), tourniquet duration, and operative time.
Outcomes:
- Primary: Cumulative postoperative drain output (mL).
- Secondary: Hemoglobin decline ( $\Delta$ Hb), calculated as preoperative Hb minus the lowest postoperative Hb during the index admission.
Statistical Analysis: Two multivariable ordinary least-squares linear regression models were constructed (one for drain output, one for $\Delta$ Hb) adjusting for age, sex, BMI, diabetes, implant type, surgical method, and tourniquet duration. Non-normally distributed variables were analyzed using Wilcoxon rank-sum tests.

Key Results

Drain Output: The mean cumulative drain output was $408.0 \pm 216.9$ $408.0 \pm 216.9$ mL (median 400 mL).
- Sex: Female sex was independently associated with lower total drain output ( $B = -136.9$ mL; $p < 0.001$ ).
- Implant Type: Posterior-stabilized (PS) implants were independently associated with higher total drain output compared to cruciate-retaining (CR) implants ( $B = +138.7$ mL; $p = 0.010$ ).
- Other Factors: Surgical method (robotic vs. conventional), tourniquet duration, age, BMI, and diabetes showed no independent association with total drain output in the adjusted model.
- Model Performance: The regression model explained 11.5% of the variance in drain output ( $R^2 = 0.115$ ).
Hemoglobin Decline: The mean $\Delta$ $Δ$ Hb was $2.4 \pm 1.2$ $2.4 \pm 1.2$ g/dL.
- Sex: Female sex was independently associated with a smaller Hb decline ( $B = -0.5$ g/dL; $p = 0.001$ ).
- Implant Type: PS implants were independently associated with a greater Hb decline ( $B = +0.6$ g/dL; $p = 0.038$ ).
- Other Factors: Similar to drain output, tourniquet duration, age, BMI, and diabetes were not independently associated with Hb decline.
- Model Performance: The model explained only 4.3% of the variance in Hb decline ( $R^2 = 0.043$ ).
Correlations: Weak positive correlations were found between tourniquet duration and POD1 drain output, and between Hb decline and cumulative drain output.

Significance and Claims
The authors claim that within a standardized single-surgeon pathway, sex and implant design are the primary independent predictors of both cumulative postoperative drain output and hemoglobin decline.

Clinical Implications: The findings suggest that male patients may be expected to experience greater drainage and Hb decline under similar operative conditions, and that the use of PS implants may be associated with increased bleeding metrics compared to CR implants.
Limitations and Modesty: The authors explicitly state that the models have modest explanatory capacity (11.5% and 4.3% variance explained), indicating that substantial interpatient variability remains unexplained by the measured variables.
Caveats: The study highlights that the finding regarding PS implants is based on a small subgroup (only 16 cases), creating statistical instability and wide confidence intervals. Consequently, the authors assert these results do not justify altering implant selection solely to limit drainage and require confirmation in larger, more balanced cohorts.
Context: The study reinforces that drain output is not equivalent to total blood loss and that routine preoperative coagulation parameters have limited discriminatory value for predicting bleeding in this elective population. The authors conclude that larger, prospective multicenter studies are necessary to validate these specific associations.

Technical Summary: Predictors of Postoperative Drain Output and Hemoglobin Decline After Primary Total Knee Arthroplasty

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