Longitudinal migration and inducible displacement of the Mobility Total Ankle System

Figures & data

Table 1. Patient demographics^a and outcome score. Values are median (range)

Outcome scores	Preoperative	1 year	2 years	MD PO–2yr ^b	p-value ^c
SF-36, PCS	26 (17–40)	40 (14–51)	39 (14–59)	10 [20]	0.002
SF-36, MCS	54 (18–69)	52 (33–70)	57 (35–66)	1 [20]	0.7
AOS	58 (33–78)	33 (9–94)	34 (13–78)	–23 [16]	0.0006

^a Sex: 12 M, 11 F; Age: 62 (28–75); and BMI: 28.8 (26.0–34.5)

^b MD PO–2yr is the mean difference between preoperatively and 2 years, where the

square brackets show the sample size of the matched pairs

^c Paired 2-tailed t-tests.

Figure 1. Sample double-examination results for one patient, depicting overlaid implant positions from each RSA examination (gray and magenta), which should be identical. The mismatch in the tibial component (A) showed high imprecision of the system due to the symmetry of the implant. Simplification of the implant to its spherical tip (B) greatly improves the precision. The position of the talar component (C) was precisely repeated in this double examination.

Figure 2. MTPM longitudinal migration of the spherical tip of the tibial component (A) and the talar component (B). Thin black lines: individual patients. Dashed line: group median. Red, green, blue, and magenta: patients who underwent surgical revision. The discontinuities and missed data points indicate where the patients missed follow-up examinations or where the results of the examinations were not usable.

Table 2. Longitudinal migration

		Follow-up time
	3 months	6 months	1 year	2 years	2 year mean 95% CI
Tibial spherical tip, EGS
n	19	21	20	18
MTPM (mm)	0.46 (0.21–1.17)	0.70 (0.13–1.33)	0.88 (0.12–1.81)	0.90 (0.17–2.28)
X: posterior–anterior (mm)	0.06 (0.20)	0.08 (0.28)	0.13 (0.31)	0.16 (0.33)	–0.08 to 0.33
Y: inferior–superior (mm)	0.42 (0.27)	0.56 (0.32)	0.66 (0.36)	0.80 (0.46)	0.58 to 1.03
Z: lateral–medial (mm)	–0.02 (0.23)	–0.04 (0.32)	–0.05 (0.42)	–0.16 (0.44)	–0.38 to 0.06
Tibial component, MBRSA
n	19	21	20	18
MTPM (mm)	1.68 (0.28–2.66)	1.67 (0.60–2.71)	1.80 (0.31–3.30)	1.89 (0.41–5.49)
X: posterior–anterior (mm)	0.04 (0.62)	–0.13 (0.52)	–0.06 (0.71)	–0.18 (0.87)	–0.61 to 0.26
Y: inferior–superior (mm)	0.40 (0.27)	0.56 (0.31)	0.65 (0.38)	0.77 (0.47)	0.54 to 1.01
Z: lateral–medial (mm)	0.11 (0.22)	0.13 (0.26)	0.05 (0.23)	0.03 (0.30)	–0.12 to 0.18
Rx: valgus–varus (°)	–0.4 (1.1)	–0.2 (1.2)	–0.3 (1.3)	–0.4 (1.4)	–1.1 to 0.3
Ry: external–internal rotation (°)	–0.6 (2.6)	–0.9 (2.2)	–0.4 (2.3)	–1.8 (2.8)	–3.2 to –0.4
Rx: dorsiflexion–plantarflexion (°)	0.6 (0.8)	0.6 (1.1)	1.0 (1.2)	0.8 (1.4)	0.1 to 1.5
Talar component, MBRSA
n	16	18	17	15
MTPM (mm)	0.50 (0.12–1.26)	0.61 (0.09–2.03)	0.85 (0.17–2.85)	1.19 (0.39–1.95)
X: posterior–anterior (mm)	0.05 (0.22)	0.08 (0.40)	0.11 (0.39)	0.00 (0.18)	–0.10 to 0.10
Y: inferior–superior (mm)	–0.13 (0.14)	–0.25 (0.24)	–0.43 (0.30)	–0.41 (0.33)	–0.59 to –0.23
Z: lateral–medial (mm)	–0.01 (0.13)	–0.01 (0.16)	–0.01 (0.25)	–0.10 (0.19)	–0.21 to 0.00
Rx: valgus–varus (°)	0.1 (0.5)	0.1 (0.5)	0.5 (0.9)	0.3 (0.9)	–0.2 to 0.8
Ry: external–internal rotation (°)	–0.3 (0.9)	–0.3 (0.7)	–0.4 (1.2)	–0.0 (1.1)	–0.6 to 0.6
Rx: dorsiflexion–plantarflexion (°)	0.5 (0.7)	0.6 (1.3)	0.7 (1.3)	0.2 (1.5)	–0.6 to 1.0

The values are given as mean (SD), except for MTPM which is given as median (range)

Table 3. Complications and longitudinal migration

			Patient (Follow-up time, year)
	Pt 1 (2)	Pt 2 (2)	Pt 3 (2)	Pt 4 (2)	Pt 6 (1)	Pt 7 (2)	Pt 11 (2)	Pt 14 (2)	Pt 21 (1)
Tibial spherical tip, EGS
MTPM (mm)	0.74	0.89	0.55	1.52	0.98	0.17	0.59	0.96	0.53
X: posterior–anterior (mm)	0.12	0.33	0.06	0.14	–0.24	–0.09	–0.19	0.33	–0.27
Y: inferior–superior (mm)	0.65	0.62	0.47	1.48	0.74	0.13	0.38	0.80	0.05
Z: lateral–medial (mm)	–0.34	0.55	–0.27	–0.28	–0.60	–0.07	0.40	–0.43	0.45
Tibial component, MBRSA
MTPM (mm)	1.98	1.59	1.65	2.28	1.99	0.41	1.39	2.86	2.16
X: posterior–anterior (mm)	1.51	0.19	0.44	–0.15	–1.24	–0.06	–0.26	1.27	1.19
Y: inferior–superior (mm)	0.35	0.57	0.46	1.52	0.76	0.04	0.31	0.68	0.10
Z: lateral–medial (mm)	0.08	0.51	–0.01	–0.32	0.08	0.12	0.11	0.03	0.20
Rx: valgus–varus (°)	–2.4	0.4	–1.0	–0.4	–1.9	0.6	1.3	–1.2	1.1
Ry: external–internal rotation (°)	1.2	–2.9	3.5	–3.1	–2.3	–0.5	–1.7	–3.4	1.5
Rx: dorsiflexion–plantarflexion (°)	0.5	0.6	0.2	1.7	–0.9	0.3	0.8	1.9	2.8
Talar component, MBRSA
MTPM (mm)	1.53	1.19	0.76	1.95	0.85	n/a	0.80	1.33	2.85
X: posterior–anterior (mm)	0.05	–0.13	–0.08	0.29	–0.02	n/a	–0.34	0.02	1.47
Y: inferior–superior (mm)	–0.76	–0.21	–0.29	–0.76	–0.49	n/a	–0.17	–0.80	–1.05
Z: lateral–medial (mm)	–0.23	0.02	0.11	0.04	0.03	n/a	–0.30	–0.01	0.75
Rx: valgus–varus (°)	–1.1	0.9	0.6	1.9	0.4	n/a	0.9	0.4	3.6
Ry: external–internal rotation (°)	–0.5	–0.1	–0.9	–2.7	1.1	n/a	–0.1	0.7	–2.7
Rx: dorsiflexion–plantarflexion (°)	–2.2	–3.1	–1.3	0.8	0.9	n/a	0.7	1.6	3.3

Patient complications:

Pt 1: medial malleolar fracture.

Pt 2: implant malaligned, wound healing delayed, superficial infection, packed and treated with antibiotics, scoped and debrided at 3.3 years, revised to fusion at 5 years.

Pt 3: heterotopic bone fragment removed.

Pt 4: increased ankle pain at 15 months, stress fracture at 17 months.

Pt 6: aspirated for pain at 6 months, infection consult at 8 months, arthroscopic removal of loose body at 12 months, revised to Hintegra at 28 months, 2 year RSA exam was not completed.

Pt 7: surgical complications (malleolar fracture) and poor outcomes, RSA exams were not usable for talus due to marker overprojection.

Pt 11: loose medial body, pain and edema at 3 years, revised to Hintegra at 5.5 years.

Pt 14: surgical complications, poor outcomes, pain, debridement, heterotopic bone formation.

Pt 21: revised to fusion at 15 months.

Table 4. Inducible displacement

		Follow-up time
	3 months	6 months	1 year	2 years	2 year mean 95% CI
Tibial spherical tip, EGS
n	19	21	22	18
MTPM (mm)	0.11 (0.03–0.52)	0.10 (0.02–0.43)	0.09 (0.03–0.19)	0.07 (0.03–0.68)
X: posterior–anterior (mm)	–0.02 (0.08)	–0.03 (0.09)	–0.01 (0.07)	–0.01 (0.33)	–0.09 to 0.08
Y: inferior–superior (mm)	0.05 (0.11)	0.05 (0.04)	0.04 (0.04)	0.02 (0.46)	–0.02 to 0.06
Z: lateral–medial (mm)	–0.04 (0.12)	–0.01 (0.10)	–0.01 (0.06)	–0.01 (0.44)	–0.01 to 0.01
Tibial component, MBRSA
n	19	21	22	18
MTPM (mm)	1.34 (0.47–3.50)	1.38 (0.20–5.09)	1.36 (0.20–4.02)	1.34 (0.63–3.68)
X: posterior–anterior (mm)	–0.22 (0.52)	–0.41 (0.75)	–0.29 (0.70)	–0.35 (0.44)	–0.57 to –0.13
Y: inferior–superior (mm)	0.05 (0.16)	0.09 (0.11)	0.10 (0.11)	0.09 (0.11)	0.04 to 0.15
Z: lateral–medial (mm)	–0.01 (0.25)	–0.07 (0.20)	–0.03 (0.25)	0.01 (0.32)	–0.15 to 0.17
Rx: valgus–varus (°)	0.1 (0.2)	0.1 (0.1)	0.1 (0.1)	0.1 (0.1)	0.0 to 0.1
Ry: external–internal rotation (°)	–0.6 (3.1)	–1.8 (3.4)	–0.8 (3.0)	1.3 (3.5)	–0.4 to 3.0
Rx: dorsiflexion–plantarflexion (°)	–0.8 (1.2)	–0.3 (1.2)	–0.3 (1.0)	–0.6 (0.7)	–1.0 to –0.3
Talar component, MBRSA
n	18	20	21	18
MTPM (mm)	0.36 (0.12–1.04)	0.38 (0.07–0.99)	0.40 (0.18–1.30)	0.49 (0.27–1.15)
X: posterior–anterior (mm)	0.00 (0.10)	0.01 (0.17)	0.04 (0.11)	0.11 (0.10)	0.06 to 0.16
Y: inferior–superior (mm)	–0.04 (0.06)	–0.05 (0.07)	–0.06 (0.08)	–0.06 (0.05)	–0.09 to –0.03
Z: lateral–medial (mm)	–0.02 (0.06)	–0.01 (0.07)	–0.01 (0.12)	0.01 (0.10)	–0.04 to 0.06
Rx: valgus–varus (°)	–0.1 (0.4)	–0.0 (0.4)	0.0 (0.6)	0.0 (0.4)	–0.2 to 0.2
Ry: external–internal rotation (°)	0.1 (0.8)	0.2 (0.5)	0.2 (0.8)	–0.1 (1.2)	–0.7 to 0.5
Rx: dorsiflexion–plantarflexion (°)	–0.1 (0.6)	0.1 (0.8)	0.1 (1.0)	–0.4 (0.5)	–0.6 to –0.1

The values are given as mean (SD), except for MTPM which is given as median (range)