Before in vivo
measurements were performed, the system was tested in vitro
as described for a similar system [18
]. To measure axial forces and bending moments in the left tibia of New Zealand White rabbits, a ring fixateur system was designed based on a Smith & Nephew 3/4 ring fixation system (Figure ). The rings at the proximal and distal positions were commercially available 3/4 rings (50 mm in diameter; Fa Smith & Nephew, Marl, Germany). The two rings in the middle, specially designed to fix the force sensors, were aluminium to reduce mass. The two proximal and the two distal rings were consistently joined by two sleeves, threaded rods, and four screw nuts. The proximal and the distal parts were connected by four force sensors (KD24S, ± 100 N, Me-Meßsysteme GmbH, Hennigsdorf, Germany) (total weight approximately 250 g). The small s-shaped force sensors were oriented parallel to the bone and connected with a measurement amplifier via an associated board (GSV-4BT, Me-Meßsysteme GmbH, Hennigsdorf, Germany). The strain gauge measurement amplifier had a LiPo-battery, four ports, and wireless transmission. A standard Bluetooth dongle was used as a receiver and data were acquired, recorded, and processed using LabView©
(National instruments Germany GmbH, München, Germany).
Figure 1 Schematic diagram of the instrumented ring fixateur system. Force sensors were placed between the middle 3/4-rings, enabling measurement of all axial forces between the proximal and distal sections of the ring fixator system (a); schematic diagram for (more ...)
Concurrent with force recordings, movement was recorded with a webcam, enabling calculation of the association between measured loadings and movement. Based on Newton's law [19
] the axial force N was calculated as follows:
where F1,2,3,4 were the forces measured with the four different force sensors (Figure ).
To calculate the bending moment Mb, distances from each sensor to the medial-lateral axis and to the anterior-posterior axis were determined from radiographs. Mx, the moment in the direction of the medial-lateral axis, was calculated as follows:
where a and b were the distances of the sensors to the lateral-medial axis. My was the moment in the direction of the anterior-posterior axis and was calculated similar to Mx.
where c and d were distances between the sensors and the posterior-anterior axis.
The value of the maximum bending moment Mb was calculated as follows:
The direction α of the bending moment was determined as:
The angle was zero for the medial direction and it is mathematical positive defined (Figure ).
The experiment was conducted under a protocol approved by an ethics committee in accordance with German federal welfare legislation (AZ 509.6-42502/3-07/1304).
Adult female New Zealand White rabbits (n = 5, Charles River, Sulzfeld, Germany) were used. Anaesthesia was induced with S-ketamin hydrochloride (20 mg/kg, im) and medetomidin (25 mg/kg, im) and maintained with isoflurane (2-3 vol.%) in oxygen mixture (1 l/min 100% O2). After clipping and routine disinfection of the surgical field, the ring fixateur system was attached to the rabbit tibia. Holes were drilled in the bone and wires (1.0 mm in diameter; (Fa Smith & Nephew, Marl, Germany) were applied (tightened with 30 N tensile force). Two crossed wires were used in the middle half rings, whereas one wire was used for the proximal and the distal half ring to fix the threaded rods of the fixateur system parallel to the tibia. Before recovery from anaesthesia, radiographs were taken in four planes (0°, 45°, 90°, 315°) and distances between each force sensor and the central axis of the bone were measured.
After an adaption period of 2 weeks, four of the rabbits were anaesthetised and an ostectomy (3-4 mm) of the tibia was performed (Figure ). Following the ostectomy and radiographic verification (Figure ), the threaded rods were replaced by force sensors (Figure ). Consequently, all developing forces were transferred via the sensors [20
]. The force sensors were connected (40 cm cables) to a board. Calibration of the force sensors without weight bearing was performed under general anaesthesia. Thereafter, the cables and board were securely wrapped in a bandage to protect them from damage between measurements.
Figure 2 Operative procedure of the rabbit tibia and preparation for measurement. The rabbit tibia was ostectomised to transfer all developing forces via the force sensors: intraoperative picture (a) and post operative radiographic verification (b), ostectomy (more ...)
In one rabbit, measurements without ostectomy were performed as control. The calibration procedure and measurement were done as described for the other rabbits. Three measurements were collected daily on 2 days. Mean maximum values and maximum single values of axial forces and bending moments were calculated.
Rabbits were given enrofloxacin (10 mg/kg, po) daily for 10d after surgery. A clinical examination was conducted daily to assess wound healing, swelling, pain, and lameness. For analgesia, meloxicam (0.15 mg/kg, po) was throughout the entire post operative measurement period, whereas buprenorphine (0.15 mg/animal) was given for 2d post operatively and prior to each measurement to prevent pain-induced reduced weight bearing.
Measurements with weight bearing in motion were conducted 6, 8, 10, 13, 15, 20, and 27d post operatively (three minutes per measurement, repeated three times each measurement day). In addition, one rabbit was measured at 34 and 41d post operatively. During measurements, the measurement amplifier and the LiPo-battery were fixed to the board of the force sensors and attached to a backpack to avoid adding an additional load to the leg. Data recordings were done as described above. During the measurement period, video recording was done to enable association of force and motion data. To calculate the average maximum bending moment for each measurement day, the 10 highest measured values were used. Student's t-test was used to detect differences between measurement days.