What is a L5S1 pseudo disc bulging?

This is a REALLY rare phenomenon, and is an XRay finding in people who have something called "Spondylolisthesis" and occurs in about 2% of the population... I have to gradually work toward answering your question, so bear with me... If you will copy and paste into Google this URL (the http:... address at the top of the screen): http://images.google.com/images?hl=en&q=Lumbar+vertebra&um=1&ie=UTF-8 Note the facet joints in the pictures. These are essentially the 2 legs of a tripod, the body of the vertebra being the 3rd leg... The way these facet joints join, or 'hook together' is like if you hooked two fingers of each hand together, pulling in opposite directions. They are tied together by ligaments to keep them from slipping appart, just like the ligaments in your other joints (fingers, knees, etc) are tied together. Between each vertebra are the spinal disks, which act as "shock absorbers", and the vertebra are tied together by 7 layers of ligaments... That's how the spine is tied together to keep the vertebra from slipping forward (pulled by the weight of the front of the body) and crushing the spinal nerves as the travel down the spinal canal, located in the back-section of the vertebral spine. In the womb, the skeleton forms as cartilage and calcium begins to be deposited; after we are born it continues to be deposited so the bones all become hardened and able to bear our weight as we mature (all except the joint surfaces that have cartilage there to promote joint movement). In a very few folks, at the L5 level, calcium isn't deposited into one or both facet joint areas as they arise from the body of the vertebra... If one side (facet-area) of this spinal area doesn't "calcify" and become hard, it is called Spondylolysis, and the spine will stay in alignment unless there is severe trauma that fractures the other calcified (normal) part. If neither side in the facet area calcify --and remains soft cartilage-- eventually the areas will separate and allow the spine to slide forward a little, L5 on S1. If it's just a little (grade I) their might be just a small amount of pain since the multiple layers of ligaments that wrap around the vertebra hold the spine together [only 20% of the strength of the spinal column is produced by the bones and ligaments, the other 80% is the produced by the muscles that support the spine]. Depending on the severity, there are 5 grades --or degrees-- of forward slippage of the spine allowed by the fractured L5 area. Now to your question... IF the spine slides forward, it will pull on the spinal disc between L5 & S1 (picture squashing a marshmallow, then sliding it a little). THAT is the way the squashed / pulled forward disc will show up on the Xray... it will appear as though it is bulging, but it really isn't... Sorry it took so long...!