Non-invasive computer-assisted measurement of knee alignment

Figures & data

Figure 1. Leg model with rigid tracker mountings.

Figure 2. External tracker mountings with curved base plates and adjustable elasticated straps. (a) Mounting of trackers on thigh and calf. (b) Foot tracker mounting.

Table I.  The mean and standard deviation (SD) of each set of tests was used to compare the difference in repeatability of the rigid leg model and the non-invasive tracker mounting on the volunteers.

	Single registration		Multiple registrations
	Leg model	Volunteer	Leg model	Volunteer
n	20	20	20	20
Mean (SD) (°)	2.1 (0.07)	1.4 (0.13)	1.6 (0.5)	1.5 (0.7)
Range (°)	2.0–2.3	1.1–1.6	0.9–2.8	0.3–2.5
F-test	p = 0.008		p = 0.34

Table II.  Mean difference and 95% limits of agreement of repeat supine alignment measurements in extension with leg stationary and before and after both standing and collateral stress maneuvers (all measurements in degrees)

	Registration 1		Registration 2
	Mean difference	±1.96 SD	Mean difference	±1.96 SD
Coronal MFT angle consecutive	0.03	1.2	−0.02	1.1
Coronal MFT angle before & after stance	−0.1	1.4	0.07	1.1
Coronal MFT angle before & after stress	0.2	1.7	0.2	1.0
Sagittal MFT angle consecutive	0.2	2.2	−0.1	2.1
Sagittal MFT angle before & after stance	0.5	2.8	0.7	2.6
Sagittal MFT angle before & after stress	−0.3	2.2	−0.9	1.7

Table III.  Inter-registration agreement of supine and standing coronal and sagittal MFT angles, and relative change following varus-valgus stress (measurements in degrees).

MFT angle	Mean difference	±1.96 SD
Supine coronal	−0.2	0.8
Supine sagittal	0.2	1.2
Change with varus stress	−0.3	1.3
Change with valgus stress	−0.2	1.1
Standing coronal	0.2	2.9
Standing sagittal	0.1	5.0

Figure 3. Bland-Altman plots showing mean difference (solid black line) and 95% limits of agreement (dotted grey lines) of MFT angular measurements for two trials: (a) supine coronal; (b) supine sagittal; (c) with varus stress; (d) with valgus stress; (e) standing coronal; and (f) standing sagittal.