A commercially available cryotherapy system (Erbokryo CS-6-System,
Erbe, Tübingen, Germany) was used for cryoablation. This system
consists of a casing with a control board and up to 6 vacuum-isolated
flexible tubes with a cryoprobe (diameter 3.2 mm) at the end. A
cryoprobe creates a cold zone 3 cm long. The Erbokryo is also fitted
with computer-controlled temperature sensors (diameter 1.2 mm) witch
allow 6 simultaneous measurements.
The cryoprobes were introduced via an access hole 3.6 mm in diameter
drilled perpendicular to the cortical substance. Temperature was
measured inside the cortical substance. Two 15-minute freezing cycles
were done with the probe at full power, with a 6-minute thaw in
between. Prior to starting the cryoablation, the position of the
cryoprobe was checked radiologically and recorded.
24 sheep with a mean weight of 61 kg (range 39–78 kg) were placed
under general anaesthesia and, using a single cryoprobe introduced
through a lateral access hole, one cryoablation was done in the distal
diametaphyseal transitional region of the head of the medial left tibia
and the right femur of each sheep. For the tibial head, a medial access
hole was drilled and the cryoprobe introduced centrally 1.5 cm below
the joint and pushed to the other side of the cortical substance.
For the femoral cryoablation, the cryoprobe was introduced through a
distal, posterolateral access hole in the area of the linea aspera at
the diametaphyseal transition. In addition, 4 temperature sensors were
introduced through access holes drilled radially at 1 cm from the bore
hole. The cryoprobes were only introduced 1 cm into the bone to avoid
freezing the cortical substance. Thus, the necrosis zone (which roughly
corresponds to the -10°C isotherm [24])
only comprised an area of 2.4 × 2.4 in the outer cortical substance.
Control holes and access holes were drilled on the contralateral sides
(left femur, head of the right tibia).
All operations were done under 600 mg clindamycin i.v. and 12 hours
prior to surgery, each animal also received thrombosis prophylaxis (0.3
ml nadroparin calcium [Fraxiparin®] s.c.). There was no
postop thrombosis prophylaxis since all animals were fully mobile after
anaesthesia. The pulmonary-arterial pressure (PAP) and the central
venous pressure (CVP) were measured via a pulmonary catheter inserted
into the jugular vein. Measurements were done before the first
cryoablation (1, Fig. 1, 2, 3), after the first cryoablation on the right femur (2, Fig. 1, 2, 3) and the head of the left tibia (3, Fig. 1, 2, 3),
as well as after drilling the control holes immediately after shifting
the animals from the right back to the left unilateral recumbent
position (4, Fig. 1, 2, 3). Intra- and perioperative monitoring was complemented by measurements of heart rate (Fig. 3) and oxygen saturation via pulse oxymetry, as well as blood counts (Fig. 4), deep body temperature (Fig. 5),
blood gases and electrolytes after each cryoablation. Postoperative
complications were monitored clinically. 8 animals were sacrificed at
8, 16 and 32 weeks postop and tissue samples taken from the lungs and
from blood vessels in the areas of cryoablation were examined for signs
of embolism. Samples were taken from each lobe and from the femoral
vein, fixed in formaline and stained with hematoxylin and eosin (HE).
Furthermore, the ablation sites were examined histologically for
infection of the soft tissue respectively osteomyelitis. X-rays were
taken after the animals were sacrificed.
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