Table 4. Mixed linear regression model results.

		Total hip arthroplasty		Total knee arthroplasty
Independent variable ⧩		Value at WOMAC score of 50 (intercept)	Difference for 10 points of WOMAC score (slope)	Value at WOMAC score of 50 (intercept)	Difference for 10 points of WOMAC score (slope)
	Dependent variable ➢	SF12 Mental Component Score		SF12 Mental Component Score
WOMAC Pain score	Preoperative	46.5 (45.8 to 47.2)	1.9 (1.5 to 2.2)	47.3 (46.5 to 48.1)	2.0 (1.7 to 2.4)
	Change at 1 year	-4.0 (-5.3 to -2.8)	-0.4 (-0.7 to 0.0)	-5.0 (-6.0 to -3.9)	-0.0 (-0.4 to 0.4)
	P-value	<0.001	0.074	<0.001	0.89
WOMAC Function score	Preoperative	46.3 (45.7 to 47.0)	1.9 (1.6 to 2.2)	46.5 (45.8 to 47.2)	2.1 (1.8 to 2.5)
	Change at 1 year	-3.5 (-4.5 to -2.4)	-0.2 (-0.6 to 0.2)	-3.8 (-4.7 to -2.8)	0.0 (-0.3 to 0.4)
	P-value	<0.001	0.29	<0.001	0.82
	Dependent variable ➢	SF12 Physical Component Score		SF12 Physical Component Score
WOMAC Pain score	Preoperative	35.6 (35.0 to 36.1)	1.9 (1.6 to 2.2)	36.5 (36.0 to 37.1)	1.8 (1.5 to 2.0)
	Change at 1 year	0.1 (-0.9 to 1.2)	0.6 (0.2 to 0.9)	-0.9 (-1.7 to -0.1)	0.4 (0.1 to 0.7)
	P-value	0.79	0.001	0.035	0.007
WOMAC Function score	Preoperative	35.5 (35.0 to 36.0)	2.0 (1.8 to 2.2)	35.7 (35.2 to 36.1)	1.7 (1.5 to 1.9)
	Change at 1 year	0.2 (-0.6 to 1.0)	1.0 (0.7 to 1.3)	0.1 (-0.6 to 0.8)	0.8 (0.6 to 1.1)
	P-value	0.45	<0.001	0.87	<0.001

Mixed linear regression models, with a random intercept for each patient, and for a contralateral joint nested within patient (when required), of the SF12 Physical or Mental component score as dependent variables, and the WOMAC Pain or Function scores as independent variables, in patients with arthroplasty of the hip or knee. 95% confidence intervals are in parentheses. SF12: Short-Form 12-item questionnaire. WOMAC: Western Ontario and McMaster Universities Osteoarthritis Index 12-item questionnaire.