Migration of the uncemented Echo Bi-Metric and Bi-Metric THA stems: a randomized controlled RSA study involving 62 patients with 24-month follow-up

Figures & data

Figure 1. The stems: on the left the Bi-Metric stem and on the right the Echo Bi-Metric stem.

Figure 2. (A) The RSA coordinate system with movements; (B) model-based RSA screenshot while using the RSAcore software.

Figure 3. Flowchart

Table 1. Baseline demographics. Values are mean (range) unless otherwise specified

	Bi-Metric	Echo Bi-Metric
Age, years	66 (49–74)	63 (50–74)
Sex, male/female, n	17/14	17/14
Height, m	1.77 (1.60–1.96)	1.76 (1.60–1.91)
Weight, kg	84 (50–124)	83 (54–122)
BMI	27 (18–38)	27 (20–36)
Operated side, R/L, n	18/13	15/16
Cup size	56 (50–62)	56 (50–62)
Stem size	12 (9–16)	11 (7–16)

Figure 4. Mean Y-translation at each follow-up for the 2 stems (bars are standard error of mean).

Table 2. Mean segment motion (95% CI) of the Bi-Metric (BM) and the Echo Bi-Metric (EBM) stems at follow-up until 24 months. Outliers are included in the analysis

	3 months	6 months	12 months	24 months
BM/EBM, n	29/31	29/31	28/31	27/30
Stem translation, mm
Y–translation: proximal (+), distal (–)
BM	–2.2 (–2.3 to –1.7)	–2.0 (–2.5 to –1.6)	–2.1 (–2.6 to –1.6)	–2.3 (–3.0 to –1.7)
EBM	–2.5 (–3.2 to –1.9)	–2.4 (–3.0 to –1.7)	–2.4 (–3.0 to –1.7)	–2.7 (–3.5 to –2.0)
X–translation
BM	–1.1 (–0.5 to –0.1)	–0.4 (–0.7 to –0.1)	0.1 (–0.3 to 0.0)	–0.4 (–0.6 to –0.1)
EBM	–0.5 (–0.7 to –0.3)	–0.6 (–0.9 to –0.4)	–0.3 (–0.4 to –0.1)	–0.5 (–0.8 to –0.3)
Z–translation
BM	–0.8 (–1.0 to –0.5)	–0.5 (–0.7 to –0.3)	–0.9 (–1.2 to –0.6)	–0.7 (–1.1 to –0.4)
EBM	–0.8 (–1.0 to –0.6)	–0.4 (–0.6 to –0.2)	–0.9 (–1.2 to –0.7)	–0.9 (–1.1 to –0.6)
Stem rotation, degrees
Y–rotation: anteversion (–), retroversion (+)
BM	2.2 (1.5 to 5.0)	2.4 (1.7 to 3.2)	2.5 (1.7 to 3.2)	2.5 (1.6 to 3.3)
EBM	2.5 (1.8 to 3.5)	2.6 (1.9 to 3.5)	2.7 (1.9 to 3.7)	2.7 (1.9 to 3.8)
X–rotation
BM	–0.6 (–0.9 to –0.3)	–0.2 (–0.5 to 0.1)	–0.6 (–1.0 to –0.2)	1.2 (0.7 to 1.6)
EBM	–0.4 (–0.8 to –0.03)	0.0 (–0.2 to 0.3)	–0.8 (–1.2 to –0.5)	1.1 (0.7 to 1.6)
Z–rotation: varus (–), valgus (+)
BM	1.7 (1.0 to 2.5)	1.7 (0.9 to 2.5)	1.8 (1.0 to 2.7)	2.0 (1.1 to 2.8)
EBM	1.7 (0.9 to 2.4)	1.8 (1.0 to 2.5)	1.8 (1.1 to 2.6)	2.0 (1.2 to 2.8)

Figure 5. Mean Y-rotation at each follow-up for the 2 stems (bars are standard error of mean).

Table 3. Y-rotation (°) outliers and and their 24-month clinical score

ID (group)	3 months	6 months	12 months	24 months	OHS	HHS
6 (EBM)	12.8	13.4	14.0	14.3	48	100
8 (BM)	6.9	7.0	7.1	8.0	43	100
23 (BM)	–3.3	–4.2	–4.8	–5.2	48	100

Table 4. Clinical outcome. Values are mean (range)

Score	BM	EBM	p-value
HSS
Preoperative	61 (38–82)	67 (31–85)	0.1
6 months	92 (61–100)	91 (70–100)	0.2
12 months	97 (81–100)	95 (73–100)	1.0
24 months	99 (91–11)	98 (49–100)	0.6
OHS
Preoperative	24 (10–35)	23 (9–40)	0.7
6 months	43 (29–48)	44 (27–48)	0.5
12 months	46 (31–48)	44 (26–48)	0.3
24 months	47 (43–48)	46 (21–48)	0.9