Should I be worried if my horse has a hunter's bump?

The term hunter's bump generally indicates that the tuber sacrales are more prominent than usual. This is usually accompanied by uneven muscle development.

Hunter's bumps are a relatively common sight in horses, especially in big warmbloods. Most horse owners know what a hunter's bump looks like, but not many know where it comes from. I have heard many explanations, including that it's a conformational fault and that it's an indication of a talented jumper. I don't believe either of those are true. 

When people say their horse has a hunter's bump, what that usually means is that his tuber sacrale (the bony prominences at the top of the croup) are more prominent than usual, creating an appearance of a bump. Sometimes, one of the tuber sacrales might be higher than the other and the horse's hind end muscles might be developed unevenly. 

 The tuber sacrales are the bony prominences of the pelvis.

So, what causes the tuber sacrale to become more prominent or asymmetric? The pelvis attaches to the spine through two small joints, called the sacroiliac (SI) joints. And while we often think of the SI joints and the tuber sacrales as basically the same thing, we need to understand that the actual joints lie much deeper. The SI joints are not meant to move a lot which is why this area is supported by lots of soft tissue, like muscles, fascia and ligaments. This offers extra stabilization and support when the locomotive forces are transferred from the hind end to the front of the horse. When the horse's hind end is overburdened due to extremely rigorous training, a lack of conditioning, fatigue, repetitive motions, or a bad slip in the mud, the soft tissues can get injured. The ligaments stabilizing the pelvic area can tear, causing a loss of stability. This will cause the pelvis to change its position relative to the spine, which makes the tuber sacrales more prominent. 

The actual sacroiliac joints (orange arrows) actually lie significantly lower than the tuber sacrales (green arrows).

Nine times out of ten, when I ask the owners how long their horse has had that bump, they say it's been there ever since they can remember. In most cases a hunter's bump is evidence of a previous injury that could have happened at any point of the horse's life. Often, the horse does not appear to have any big issues and people will then simply write off the bump as a cosmetic fault. The fact that the horse has been working normally with the bump for the last x number of years doesn't necessarily mean that everything is a-ok.  Think about it this way. At some stage the horse has had an injury. A pretty severe one at that – ligament tears are no joke. The body's response to this kind of injury is usually to engage the soft tissue to tighten and create adhesions in an effort to restore stability. This protects the injured structures from further damage, but it also reduces the range of motion of the entire area. When the injured ligament heals, the soft tissue tightness doesn't just go away on its own. The range of motion is still restricted. When this has been going on for years, the nerve endings will adapt to it, causing the horse's »default« movement to be altered. In the long run, a reduced range of motion always means injury. So, the horse with the hunter's bump might be performing well now, but because his pelvis is not moving as it should, sooner or later something will have to give