Analysis of polyethylene wear in plain radiographs

Figures & data

Figure 1.  Analysis of PE wear with PolyWare Digital Edition demonstrating digital edge detection.

Table 1.  Repeatability of radiographic double wear measurements within the strategies.

Analysis strategy	Median (range)	SDdif-intra ^a (mm)	Bias ^b ± LOA ^c	95% CI ^d (mm)	p-value ^e (mm)
2D measurements of wear
PW6 ^f	4.02 (2.63–6.24)	0.61	–0.08 (± 1.22)	–0.49 to 0.33	0.7
PW2 ^g	2.28 (0.72–4.88)	0.18	0.05 (± 0.37)	–0.08 to 0.17	0.4
PW1 ^h	3.40 (1.55–4.62)	0.28	–0.02 (± 0.56)	–0.21 to 0.17	0.8

^a SD_dif-intra is the random variation within a strategy comparing double measurements.

^b Bias: systematic variation within a strategy.

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

^d 95% confidence interval for the bias.

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

^f PW₆: PolyWare PE wear analysis using 6 follow-up radiographs.

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

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

Figure 2.  Patients 1 through 11 sorted by increasing magnitude of wear for each of the 3 (1-, 2-, and 6-radiograph) strategies.

Table 2.  Comparison of repeatability and concurrent validity between strategies

Analysis strategy	Repeatability		Concurrent validity
	Relative repeatability^a	p-value ^b	SDdif-inter ^c(mm)	Bias ^d ± LOA ^e (mm)	95% CI of true bias ^f (mm)	p-value ^g
Absolute 2D measurements
PW6 ^h vs. PW2 ^i	3.34	< 0.01	0.59	1.81 (± 1.19)	1.41–2.21	< 0.001
PW6 vs. PW1 ^j	2.17	0.03	1.12	1.00 (± 2.24)	0.24–1.75	0.01
PW^1 vs. PW2	1.54	0.21	1.26	–0.81 (± 2.52)	0.05–0.49	0.02

^a Relative repeatability: ratios of variance.

^b p-value: test of variance between strategies (Pitman's test).

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

^d Bias: systematic variation between strategies.

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

^f 95% confidence interval for the bias.

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

^h PW₆: PolyWare PE wear analysis using 6 follow-up radiographs.

ⁱ PW₂: PolyWare PE wear analysis using the postoperative radiograph and the final follow-up radiograph.

^j PW₁: PolyWare PE wear analysis using only the final follow-up radiograph.

Figure 3.  Bland-Altman plots (left) and scatter plots (right) with lines of equality for repeatability measures for each of the three strategies.PW₆: PolyWare using 6 follow-up radiographs; PW₁: PolyWare using only the final follow-up radiographs; PW₂: PolyWare using the postoperative and the final follow-up radiographs. In the Bland-Altman plots (left-hand panels): x-axis, average of 2 measurements; y-axis, difference between 2 measurements (y = measurement 1 – measurement 2); red lines, 95% limits of agreement; dashed line, bias from 0; long solid green line, y = 0 line; dots, individual double measures. In the scatter plots (right-hand panels): x-axis, first measurement; y-axis, second measurement; maroon lines, lines of equality.

Figure 4.  Bland-Altman plots (left) and scatter plots (right) with lines of equality for concurrent validity between the three strategies. PW₆: PolyWare using 6 follow-up radiographs; PW₁: PolyWare using only the final follow-up radiographs; PW₂: PolyWare using the postoperative and the final follow-up radiographs. In the Bland-Altman plots (left-hand panels): x-axis, average of the measurements of 2 strategies; y-axis: difference between measurements of two strategies; red lines, 95% limits of agreement; dashed line, bias from 0; long solid green line, y = 0 line; dots, individual double measures. In the scatter plots (right-hand panels): maroon lines, lines of equality.