Knee flexion influences periprosthetic BMD measurement in the tibia

Figures & data

Figure 1. The Maxim I-beam stem (A) and cruxiate (finned) stem (B) tibia stem components used (Biomet).

Figure 2. The human phantom bone fixed in neutral flexion on a retrograde nail in a clamp, allowing adjustment of flexion and rotation. Because the scans were made with a “spine program”, we used nylon boards and 2 long rice bags to imitate the soft tissues of the abdomen and loin.

Figure 3. The constructed leg positioner of soft foam for clinical AP scans allowed a reproducible leg position of approximately 25° knee flexion and neutral rotation. The sides of the positioner were filled with rice.

Figure 4. AP densitometry analysis of a right tibia (implant) with software-automated metal removal (blue) and bone-edge detection (yellow line), and manual positioning of the 3-ROI BMD cruxiate stem template. The bone of the fibula was excluded from the analysis.

Figure 5. LA densitometry analysis of a right tibia (cruxiate stem implant) with software-automated metal removal (blue) and bone-edge detection (yellow line), and manual positioning of the 3-ROI BMD analysis template. The bone of the fibula was included in the analysis.

Table 1. Measured periprosthetic BMD (g/cm²)) of the phantom bones with increasing degrees of knee flexion. 5 repeat measurements were obtained for each position. A template of 3 ROIs was used. Values are mean (range) and standard deviation (SD)

	Cruxiate stem tibial implant			I-beam stem tibial implant
	Mean BMD (range)	SD	p-value ^a	Mean BMD (range)	SD	p-value ^a
Neutral	8.465 (8.441–8.515)	0.028		4.735 (4.722–4.730)	0.013
5° flexion	8.366 (8.321–8.405)	0.032	< 0.01	4.719 (4.706–4.750)	0.019	0.3
10° flexion	8.499 (8.486–8.524)	0.015	0.1	4.711 (4.701–4.749)	0.022	0.2
15° flexion	8.240 (8.119–8.362)	0.096	< 0.01	4.423 (4.374–4.456)	0.035	< 0.01
20° flexion	8.219 (8.166–8.245)	0.031	< 0.01	4.429 (4.299–4.493)	0.075	< 0.01

^a Statistical testing from neutral to increasing position of flexion (unpaired t-test).

Table 2. Repeatability of clinical BMD measurements (double examination)

Analysis method	Average BMD ^a (range)	Bias ^b (95% CI)	Sr ^c	Repeatability limits ^d
AP measurements
ROI-1 (g/cm^2)	0.912 (0.592–1.266)	0.003 (-0.012–0.019)	0.032	0.089
ROI-2 (g/cm^2)	0.793 (0.503–1.080)	0.002 (-0.010–0.014)	0.025	0.070
ROI-3 (g/cm^2)	1.010 (0.617–1.446)	-0.002 (-0.010–0.006)	0.017	0.048
LA measurements
ROI-1 (g/cm^2)	0.659 (0.383–1.056)	-0.007 (-0.016–0.002)	0.020	0.055
ROI-2 (g/cm^2)	0.882 (0.538–1.408)	0.007 (-0.015–0.030)	0.050	0.140
ROI-3 (g/cm^2)	0.757 (0.492–1.165)	-0.002 (-0.016–0.011)	0.030	0.084

^a Average of the mean of double BMD measurements with the range in parentheses.

^b Bias: mean difference between the first and the second BMD measurement (systematic variation of repeatability within the ROI).

^c S_r: repeatability standard deviation for a single BMD measurement (ASTM 2008).

^d 95% limit repeatability of 2 test results (1.96 × √2 × S_r).

Figure 6. Bland-Altman plot for repeatability (double examinations). X-axis: average of the double measurements; y-axis: difference between double measurements; red lines: 95% limits of agreement; dashed line: bias from 0; long, solid green line: y = 0; dots: individual double values.

ASTM: Standard Practice for Use of the Terms Precision and Bias in ASTM Test Methods. E177. 2008. Subcommittee E11.20 on Test Method Evaluation and Quality Control, ASTM International, West Conshohocken, PA, USA.

 Google Scholar