Hex-nut loosening following compression instrumentation of the spine

Two cases of failure of compression instrumentation using the slotted hook and bushing (SHB) system due to back-off of the hex nut and loosening of the SHB. Laboratory tests on compression assemblies demonstrated that: hex-nut loosening can be produced in the laboratory by the cyclical loading of compression assemblies; crimping of the threads of the rod prevented hex-nut back-off and loosening of the SHB; crimping of the threads of the rod with the described rod crimper applied at a 90 degree angle did not damage the core of the rod or compromise its ability to withstand cyclical or tension loads; and crimping of the threads of the rod at angles other than 90 degrees damaged the core of the rod and significantly altered its ability to withstand cyclical loading. With subsequent clinical application of these principles, failure of compression instrumentation because of loosening of the SHB seems not to have been reported in the literature.