Hypophosphatemic (HPP) rickets are disorders related to low levels of phosphate in the blood (hypophosphatemia) as a result of a resorption defect in the kidney. Phosphate is a mineral that is essential for the normal formation of bones and teeth. The bones become painfully soft and bend easily, due to low levels of phosphate in the blood.  In most cases, the signs and symptoms of hereditary hypophosphatemic rickets begin in early childhood. The features of the disorder vary widely, even among affected members of the same family. Mildly affected individuals may have hypophosphatemia without other signs and symptoms. More severely affected children experience slow growth and are shorter than their peers. They develop bone abnormalities that can interfere with movement and cause bone pain. The most noticeable of these abnormalities are bowed legs or knock knees (a condition in which the lower legs are positioned at an outward angle). These abnormalities become apparent with weight-bearing activities such as walking. If untreated, this tends to worsen with time.

Hypophosphatemic rickets is almost always inherited and may be caused by changes (mutations) in any of several genes. There are several forms which are distinguished by their genetic cause and pattern of inheritance. Most commonly hypophosphatemic rickets is due to changes in the PHEX gene and inherited in an X-linked dominant manner (X-linked hypophosphatemia). Less commonly it is inherited in an X-linked recessive manner (often called Dent disease); autosomal dominant manner; or autosomal recessive manner. Treatment involves taking phosphate and calcitriol in order to raise phosphate levels in the blood and promote normal bone formation.

Key radiographic features of HPP in pediatric patients based on specified radiographs of affected newborns, infants, and children, a Radiographic Global Impression of Change (RGI‐C) scale was developed as a tool for scoring changes over time.  RGI‐C scale means to address the unmet need to quantify changes in key skeletal manifestations of pediatric HPP. https://onlinelibrary.wiley.com/doi/full/10.1002/jbmr.3377