A new approach for assessment of wear in metal-backed acetabular cups using computed tomography: A phantom study with retrievals

Figures & data

Figure 1. Acetabular cup, polyethylene liner, a titanium shell (52–58 mm outer diameter), and a femoral head (28 mm).

Figure 2. This figure from a CMM measurement shows the wear vector as an arrow and its angle to the normal axis of the acetabular cup opening.

Figure 3. Landmarks on CT images of cup (left) and head (central), and landmarks and normal vector of horizontal reference plane (right) for determination of the wear vector angle. The normal axis is marked as a line in the right panel.

Figure 4. Difference against mean for CT and CMM measurements of linear wear (A) and of wear vector angle (B). (Notice that two of the points coincide in each case). Difference against mean for CT and CMM measurements of head diameter (C).

Table 1.  Repeatability, accuracy, and interval estimate of bias at 95% confidence level for the proposed CT method

Repeatability	Accuracy	Interval estimate of bias
Linear wear (mm)	0.40	0.55	−0.15 ±0.33
Wear vector angle to normal axis	26°	27°	−0.66 ± 2.06
Head diameter (mm)	0.27	0.80	−0.53 ± 0.25

Table 2.  Results for linear wear from CT and CMM measurements on acetabular cups

Cup no.	Mean wear (SD) from CMM, mm	Mean wear (range) from CT, mm
1	4.2 (0.05)	4.2 (4.3–4.2)
2	3.4 (0.05)	2.9 (2.7–3.0)
3	2.7 (0.03)	3.3 (3.2–3.4)
4	3.2(0.01)	3.0 (3.0–3.0)
5	2.9 (0.05)	2.3 (2.3–2.3)
6	3.5(0.01)	3.4 (3.4–3.5)
7	3.1 (0.05)	2.3 (2.1–2.6)
8	0.2(0.01)	0.4 (0.3–0.5)

Table 3.  Wear vector angle against normal of acetabula cup opening plane measured with CMM and CT using dedicated software. Values are given in degrees

Cup no.	CMM (SD) (range)	CT (range)	CT minus CMM
1	106 (0.8) (106–107)	94 (94–94)	−12
2	127(1.4) (125–128)	144 (135–154)	18
3	127 (0.0) (127–127)	143 (141–145)	16
4	86 (1.5) (84–87)	57 (48–67)	−28
5	92 (8.3) (82–98)	92 (84–100)	−0.2
6	91 (0.0) (91–91)	101 (99–103)	10
7	127 (0.0) (127–127)	110(100–120)	−17
8	94 (0.2) (94–94)	103(99–107)	9