Simulation of unilateral equinus using an adjustable orthosis in children: design,feasibility and biomechanical effects

Orthosis design

An orthosis was custom molded for each child on the anterior surface of the leg and the dorsal surface of the right foot (Figure 1). Each orthosis consisted of two parts made of semi-rigid polyethylene SIMONA PE-W®. This material is commonly used to make orthotics in children. It is sufficiently rigid to constrain the joint while being flexible enough to allow comfortable gait. The two parts were linked by two anterolateral Gillette ®-type joints. The orthosis was lined with an 8-mm polyethylene layer for comfort and to avoid injury. A turnbuckle linked the two parts and allowed the degree of limitation of dorsiflexion to be adjusted. Plantarflexion was free because of the sliding mechanism of the turnbuckle. Self-adhesive bands were used to fix the device to the leg and the foot.

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Figure 1.

Photograph and diagram of the orthosis. (A) Orthosis (side view). (B) Detailed view of the turnbuckle composed of 3 parts and attachments. The central part slides into the superior part. The inferior part is a screw. The knurl of the central part allows the degree of equinus to be adjusted. The device used without the turnbuckle is the orthosis free (OF) condition. (C) Diagram of the orthosis and the five tested positions of equinus.

Protocol

Spatiotemporal, kinematic and kinetic gait parameters were recorded using an optoelectronic system (Vicon MX 13, Oxford Metrics, Oxford, UK) with nine infrared cameras and two AMTI force platforms 120 cm × 60 cm (Advanced Mechanical Technology Inc., Watertown, MA, USA). A total of 16 reflective markers were positioned on the lower limbs according to the protocol by Davis et al. ¹⁰ The markers did not interfere with the orthosis.

Ten healthy children (four females, six males) with an average age of 9.7 years ± 1.25, weight 32.22 kg ± 6.59 and height 1.38 m ± 0.08 were included. The research protocol was approved by our institutional regional research ethics committee. Children and parents signed informed consent for participation and publication. Gait was recorded bare foot without the orthosis (normal gait (NG)), while wearing the orthosis with the turnbuckle removed (orthosis free (OF)), with the orthosis in four successive positions of dorsiflexion limitation (+10°, 0°, −10°, −20°) and at the maximum dorsiflexion limitation allowed by the system (maximum plantarflexion (MP)) (Figure 1). The dorsiflexion angle was determined using a handle goniometer using the proximal and distal part of the fibula and the head of the fifth metatarsal as references. ¹¹ We chose to increase the equinus progressively during the session in order to obtain a better tolerance. To avoid fatigue, children could rest during the adjustment and could interrupt the experiments at any time. All the children carried out the entire protocol without the occurrence of any fatigue.

Each child carried out four 10-meter walks for each angle of dorsiflexion limitation. Since gait velocity influences both kinematic and kinetic parameters of gait ¹² a velocity close to 1 m/s was imposed. Using a simple stop watch, trials were discarded if the time differed by more than 10 percent of the imposed velocity (0.09 to 1.10 m/s). There were no statistically significant differences in the spatiotemporal gait parameters between conditions. Each subject walked approximately 50 meters prior to the data collection in order to become accustomed to each new position of the orthosis. No instructions were provided regarding gait strategy.

Data and statistical analysis

Joint kinematics and kinetics were computed using the VICON Plug-in Gait model. ¹⁰ Gait kinematics and external joint moments were extracted based on Benedetti’s recommendations ¹³ and maximal ankle power generation and absorption were calculated with a custom-made program (Table 1). The kinetic data were normalised to subject weight without taking into account the weight of the orthosis (approximately 0.3 kg). Since the sample of children was small and each child was compared to themselves in each condition, a Wilcoxon’s paired rank test (Statistica v 6.0 ®) was used to analyze each parameter. A difference was considered significant when p<0.05.

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Table 1.

Mean (and standard deviation) of ankle kinematics and kinetics for each condition.

Kinematics of the right ankle (degree)	OF		NG		+10°		0°		–10°		–20°		MP
	Mean	SD	Mean	SD	Mean	SD	Mean	SD	Mean	SD	Mean	SD	Mean	SD
A1 Dorsiflexion at foot strike	−0.93	3.30	−2.10	2.85	−0.09*	3.23	−1.73	4.44	−8.47**	6.06	−16.04**	3.90	−21.65**	4.17
A2 Max plantarflexion in loading response	−3.51	2.88	−4.76*	2.46	−2.81	2.82	−3.99	3.24	–		–		–
A3 Max dorsiflexion in stance phase	11.33	1.73	14.86*	2.96	12.13	2.17	9.21	5.39	1.16**	4.28	−6.08**	3.82	−11.61**	4.82
A4 Dorsiflexion at toe off	−2.98	3.74	−6.79*	5.33	−2.02	3.39	−4.49	4.80	−9.36**	3.73	−15.45**	4.05	−22.92**	5.95
A5 Max plantarflexion in swing phase	−9.49	2.59	−15.81**	6.29	−8.34	4.02	−10.22	4.99	−14.99**	5.32	−22.02**	2.85	−28.57**	2.85
A6 Total sagittal plane excursion	20.82	2.70	30.67**	7.24	20.52	3.48	19.43	4.82	16.14**	3.47	15.99*	3.12	16.96**	2.18
A7 Total coronal plane excursion	0.98	0.54	2.01**	1.03	0.90	0.43	1.02	0.61	1.39**	0.74	1.37*	0.72	1.71*	0.88
A8 Max eversion in stance phase	0.14	0.64	0.89	0.58	−0.05	1.24	0.44	1.30	−0.07	1.25	−0.74*	0.96	−1.51*	1.02
A9 Max inversion in swing phase	−0.55	0.60	−0.95	0.85	−0.50	1.04	−0.24	0.91	−0.81	1.05	−1.44*	1.01	−2.04**	0.93
A10 Mean eversion – inversion	−0.23	0.59	0.07	0.6	−0.16	1.09	0.07	1.10	−0.48	1.04	−1.11*	0.98	−1.90**	1.03
A11 Mean angle of progression	−7.29	3.71	−11.88*	4.42	−8.47*	3.39	−7.9	3.51	−5.32	5.17	−4.21	5.23	−2.39*	5.78
Kinetics: ankle joint moments (Nm/kg)
AM1 Max plantarflexion moment	−0.06	0.05	−0.10*	0.06	−0.05	0.14	−0.05	0.06	–		–		–
AM2 Max dorsiflexion moment	1.24	0.10	1.37*	0.12	1.16	0.20	1.21	0.18	1.09**	0.16	0.92**	0.18	0.76*	0.15
Kinetics: ankle joint power (W/kg)
AP1 Max power absorption	−0.57	0.15	−0.68	0.19	−0.55	0.15	−0.55	0.19	−0.50	0.13	−0.63	0.24	−1.66	1.33
AP2 Max power generation	1.32	0.45	2.45**	0.65	1.38	0.44	1.21	0.61	0.81**	0.19	0.69**	0.21	0.65*	0.27

Significant differences between the orthosis free (OF) condition and each condition (normal gait (NG), +10°, 0°, –10°, –20° of dorsiflexion and maximal plantarflexion adjustment (MP)) are shown by * when p<0.05 and ** when p<0.01. MP, maximal plantarflexion; NG, normal gait; OF, orthosis free; adjustment.

Results

The orthosis was well tolerated by the children, there were no complaints of pain but there was slight discomfort at MP. Changes in ankle kinematics and kinetics in the six conditions are shown in Figure 2 and Table 1. Because there were some differences between OF and NG (Table 1), the effects of the different positions of the orthosis were compared with the OF condition.

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Figure 2.

Ankle kinematics (left column), moments (middle column) and power (right column) for each condition. Gray dotted line (mean and standard deviation): orthosis free (OF). Black line (mean and standard deviation): position of the orthosis (normal gait (NG), +10°, 0°, -10°, -20° of dorsiflexion and maximal plantarflexion (MP)).

Kinematic changes were not significant for small limitations of dorsiflexion (+10° and 0°) but became significant from the −10° position. The amount of dorsiflexion at initial contact (A1) was close to the clinical adjustment of the orthosis from the 0° condition (−1.73° at 0°, −8.47° at −10° and −16.04° at −20°).

The amount of plantarflexion measured clinically at the maximum position of the orthosis was −28.22° ± −2.86 and the amount of plantarflexion during gait (A1) was −21.65° ± 4.17. Equinus occurred during stance phase (A3) from the −20° condition. The mean maximum plantarflexion angle obtained during stance phase was −11.61° ± 4.82 at MP. The orthosis allowed plantarflexion to occur during the pre–swing phase of the gait, even at MP. During swing phase, the increased equinus induced a greater maximum plantarflexion angle (A5), from −9.49° (OF) to −28.57° (MP). Changes in the frontal plane were in the order of 1−2° and the mean foot progression (A11) was more internal from −7.29° (OF) to −2.39° (MP).

Similarly to the kinematic parameters, significant kinetic changes occurred from the −10° position. The initial plantarflexion moment (AM1) disappeared and was replaced by an initial dorsiflexion moment. The maximum dorsiflexion moment (AM2) decreased from 1.24 Nm/kg at OF to 0.76 Nm/kg at MP and had a biphasic aspect. Maximum power generation (AP2) decreased from 1.32 W/kg at OF to 0.65 W/kg at MP and, in parallel, a power absorption peak (AP1) occurred from −10° during the loading response.