OBJECTIVETo calculate the volume of occult blood loss after unicompartment knee arthroplasty(UKA), and analyze its influential factors by comparing with total knee arthroplasty.

METHODSA retrospective study of 130 cases from July 2012 to July 2015 were enrolled, of which and 65 cases were UKA, and 65 cases were TKA. In UKA group there were 27 males and 38 females, and the mean age was (62.3±4.3) years old (ranged, 50 to 82 years old). There were 14 cases older than 70 years old, while 51 cases among 50 to 70 years old. The mean Body mass index (BMI) was (23.9±2.6) kg/m²(ranged, 18.3 to 30.1 kg/m²). In TKA group there were 23 males and 42 females, and the mean age was (67.4±4.9) years old (ranged, 57 to 81 years old). There were 34 cases older than 70 years old, while 31 cases among 50 to 70 years old. The mean BMI was (25.6±2.3) kg/m²(ranged, 20.6 to 33.1 kg/m²). Hidden blood loss was calculated according to Gross equation, and the differences between the two groups including different ages and genders were observed subsequently. The differences of red blood cell change between two groups were observed dynamically postoperatively.

RESULTSPostoperative hidden blood loss was (375.25±168.09) ml, HSS score was 87.11±5.39 in UKA group. Hidden blood loss was (898.81±221.47) ml, HSS score was 82.23±3.08 in TKA group. The differences between the two groups were significant. There were no significant differences in ages and genders. There were no significant relationship between hidden blood loss and HSS score or BMI, while the differences of red blood cell change were significant on the second day, the 4th day and the 5th day. There was no allogenic blood transfusion in UKA group, while 3 patients with transfusion in TKA group.

CONCLUSIONSHidden blood loss is part of total blood loss after UKA, and the volume in UKA is less than that in TKA. It could be compensated by the body and rarely affect the knee function. The hidden blood is not a risk factor of transfusion.