Austria's Christian Parzer, 31, was also taken by helicopter to hospital with suspected spinal injuries but was later released with little more than cuts and bruises.
The crash happened when Briton's Steve Abbott nudged Klaus Klaffenboeck as he tried to overtake him, sending the Austrian's machine swerving and somersaulting across the track and flinging Parzer off the sidecar. Two more sidecars swerved to avoid the accident, collided and rolled over. Prior, a passenger, was thrown back first into a safety barrier.
The race was won by the Swiss pair Rolf Biland and Kurt Waltisperg, with Britain's Steve Webster and the Swiss Adolf Haenni second.
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