Two fossae in the back of the low back are mostly due to the posterior superior iliac spine. The posterior superior iliac spine is located at the intersection of the lumbar region and the sacrococcygeal region, with two symmetrical bony protrusions on the posterior and lateral sides. There are many muscles and tendons attached to this bony prominence, and in some patients, the tendons here are more closely combined with the skin on the surface, so when standing upright, the tendons may pull and cause the skin here to sink, and then two symmetrical fossae appear in the lumbar region, which is a normal physiological phenomenon, and patients do not need to worry excessively, nor do they need to do the corresponding treatment. Some pathological conditions may be due to bony deformities caused by old fractures of the lumbar spine or local depressions caused by soft tissue scar adhesions, and the depressions are asymmetrical. If a patient is unsure of what caused the lumbar fossa, it is recommended that such patients visit an orthopedic surgeon for help in determining this.