The final follow-up plain radiograph is sufficient for clinical evaluation of polyethylene wear in total hip arthroplasty

Figures & data

Table 1. Patient demographics

Input variable	Mean	(range)
Age (years)	53	(44–65)
Height (cm)	172	(158–182)
Weight (kg)	84	(61–114)
Cup size (mm)	55	(50–62)
Polyethylene thickness (mm)	9.2	(6.8–11.8)
Follow-up (years)	6.1	(5.3–7.1)
Harris hip score at 5 years (points)	96	(84–100)
Gender ratio (males:females)	4:8
Hip side (right:left)	7:5

Figure 1. Polyethylene wear analysis with the PolyWare software. A. Circles are fitted to the peripheries of the cup and femoral head shadows B. At the end of the analysis a solid cup model is added to the output. Right hips are flipped about the vertical axis to look like left hips prior to analysis.

Figure 2. Polyethylene wear analysis in a right hip with model-based RSA, using elementary geometric shape models (EGS-RSA). Spheres are matched by an EGS algorithm to the peripheries of the femoral head and cup, and femoral head penetration is calculated as change in distance between their centers.

Table 2. Repeatability of radiographic double-examination wear measurements for the 3 methods

Analysis method	Mean (range)(mm)	SDdif-intr a	Bias ^b (±LOA) ^c(mm)	95% CI ^d(mm)	p-value ^e
2D measurements
PW2 ^f	0.80 (0.28–1.78)	0.26	-0.09 (±0.53)	-0.26–0.08	0.25
PW1 ^g	1.07 (0.69–1.47)	0.11	0.04 (±0.22)	-0.04–0.11	0.29
EGS-RSA ^h	0.60 (0.13–1.09)	0.11	0.06 (±0.23)	-0.02–0.13	0.11
3D measurements
PW2	1.12 (0.27–2.20)	0.31	-0.05 (±0.62)	-0.25–0.15	0.61
PW1	1.48 (0.86–2.31)	0.44	-0.03 (±0.87)	-0.31–0.25	0.82
EGS-RSA	0.75 (0.26–1.47)	0.16	0.05 (±0.31)	-0.05–0.15	0.33

^a SD_dif-intra is the random variation within a method comparing double examinations.

^b Bias: systematic variation within a method.

^c LOA: limits of agreement around the bias (95% prediction interval = SD_dif-intra x 1.96).

^d 95% confidence interval for the bias.

^e p value (paired t-test) bias between methods.

^f PW2: PolyWare PE wear analysis using the postoperative and final follow-up radiographs.

^g PW1: PolyWare PE wear analysis using only the final radiographic follow-up radiographs.

^h EGS-RSA: radiostereometric analysis of PE wear using sphere models (the “gold standard”).

Figure 3. Bland-Altman plots and scatter plots, with lines of equality for repeatability measures for each of the 3 methods. Bland-Altman plots (columns 1 and 3); x-axis: average of two measurements; y-axis: the difference between 2 measurements (y = measurement 1 – measurement 2); red lines: 95% limits of agreement; dashed line: bias from 0; solid green line: y = 0 line; dots: individual double measures. Scatter plots (columns 2 and 4); x-axis: first measurement; y-axis: second measurement; maroon lines: lines of equality; EGS-RSA: radiostereometric analysis using sphere models; PW1: PolyWare using only the final follow-up radiographs; PW2: PolyWare using the postoperative and final follow-up radiographs.

Table 3. Comparison of repeatability and concurrent validity between methods

Analysis method	Repeatability		Concurrent validity
	Repeatability ratio^a	p-value^b	SDdif-intr ^c (mm)	Bias^c (±LOA)^e (mm)	CI 95% of true bias ^f (mm)	p-value^g
2D measurements
PW1 ^h relative to EGS-RSA ^i	^i1.02	0.95	0.27	0.48 (±0.55)	0.30–0.65	< 0.001
PW2 ^j relative to. EGS-RSA	2.32	< 0.001	0.44	0.21 (±0.89)	-0.08–0.49	0.14
PW1 relative to PW2	2.36	< 0.01	0.34	0.27 (±0.68)	0.05–0.49	0.02
3D measurements
PW1 relative to EGS-RSA	2.80	0.002	0.56	0.73 (±1.13)	0.37–1.09	< 0.001
PW2 relative t. EGS-RSA	2.00	0.03	0.53	0.36 (±1.06)	0.03–0.70	0.04
PW1 relative to PW2	1.40	0.28	0.45	0.36 (±0.90)	0.08–0.65	0.02

^a Repeatability ratio: ratios of variance.

^b p-value: test of variance between methods (F-test).

^c SD_dif-inter: random variation from the 2 different methods.

^d Bias: systematic variation between methods.

^e LOA: limits of agreement around the bias (95% prediction interval = SD_dif-inter x 1.96).

^f 95% confidence interval for the bias.

^g p-value (paired t-test) bias between methods.

^h PW1: PolyWare PE wear analysis using only the final follow-up radiographs.

ⁱ EGS-RSA: radiostereometric analysis of PE wear using sphere models (the “gold standard”).

^j PW2: PolyWare PE wear analysis using the postoperative and final follow-up radiographs.

Figure 4. Bland-Altman plots and scatter plots with lines of equality for concurrent validity between the 3 methods. Bland-Altman plots (columns 1 and 3); x-axis: average of the measurements of 2 methods; y-axis: difference between measurements of 2 methods; red lines: 95% limits of agreement; dashed line: bias from 0; solid green line: y = 0 line; dots: individual double measures. Scatter plots (columns 2 and 4); maroon lines: lines of equality; EGS-RSA: radiostereometric analysis using sphere models; PW1: PolyWare using only the final follow-up radiographs; PW2: PolyWare using the postoperative and final follow-up radiographs.

Table 4. Marginal cost analysis in € (euros) for polyethylene wear analysis by EGS-RSA and with PolyWare as used in this study. For easy comparison, the staff costs are listed per patient

Cost area	EGS-RSA ^a	PW2 ^b	PW1 ^c
Investment costs (1 time)
Additional X-ray equipment	80,604	0	0
Calibration box	10,075	0	0
A3 transparency scanner	0	5,165	0
Software	34,595	510	510
Education of radiographer	1,542	0	0
Education of research assistant	6,166	1,542	1,542
Total investment costs	132,982	7,217	2,052
Staff costs per patient
Radiographer	35	12	12
Research assistant	102	77	39
Total staff costs per patient	137	89	51

^a EGS-RSA: radiostereometric analysis of PE wear using sphere models (2 stereo radiographs were used).

^b PW2: PolyWare PE wear analysis using the postoperative and final follow-up plain radiographs.

^c PW1: PolyWare PE wear analysis using only the final radiographic follow-up plain radiographs.