Patient, 12 years old, always had low back pain after activity, bending over, and sitting for long periods of time. The child likes to play sports and has no significant history of trauma. A radiograph was taken at an outside hospital, which revealed an L4 isthmus nonunion. What is isthmus? The outline of a vertebra is circled in red in the figure below. The thick black line shows the isthmus, which is anatomically defined as the junction of the upper and lower articular processes. What’s wrong with a cracked isthmus? Let’s start with the stabilizing structure of the spine. The spine is made up of vertebrae that “line up” one after the other. Why don’t the upper vertebrae run forward when our spine bends forward? That is because of the constraints of the intervertebral discs, joint capsules, muscles and ligaments. But actually the most important structure that prevents the vertebrae from sliding forward is the upper and lower articular processes at the back of the vertebrae. Look at the diagram below, let’s take the L3 vertebrae as an example, the lower articular process of the L3 vertebrae hooks onto the upper articular process of the L4 vertebrae like a hook. Therefore, the L3 vertebra will not slip forward when the lumbar spine is flexed anteriorly. However, in this child, the isthmus of the L4 vertebra was broken, which is equivalent to the center of this hook being broken. The force that prevents the lumbar vertebrae from moving forward is focused entirely on the disc between the L4 and L5 vertebrae and the surrounding soft tissues. Over time this causes fatigue in the above structures, which results in pain. By the time these soft tissue structures can no longer pull on this vertebral body, slippage occurs. See the picture below. So how was this child treated? First, he was given an MRI, which revealed that the disc at L4-5 was still very healthy, indicating that the isthmic fissure hadn’t occurred long enough for the slip to be apparent. If the split isthmus can be repaired and fixed, the “hook” can be rebuilt, and future slippage can be prevented. So, I had the isthmus repaired, and the result was very good. (The signal of the L4-5 disc is still very normal, indicating that there is no damage to the disc, and the slippage is not obvious.) (In the X-ray after the repair, the broken isthmus has been tightened by the pressure of the internal fixation, and I implanted crushed bone between the broken ends of the isthmus, so that the broken isthmus will be permanently fused together in about 1 year, and the internal fixation can be removed at that time.)