Small patella syndrome (SPS) is a rare genetic disorder. It is a type of skeletal dysplasia or a condition which affects the growth of bone and cartilage. SPS is rare, with under 50 people known to be affected worldwide.
The main symptoms of SPS include abnormal hip (pelvic) bone and knee development. These bones may harden more slowly than normal or may not harden at all. The kneecap may be absent. Other symptoms include changes in the feet, such as a wide gap between the first and second toes, and changes in the femur or thigh bone. Some babies with SPS are born with an opening in the roof of the mouth (cleft palate), a small jaw, a flattened nose, or a prominent forehead. Complications of SPS may include knee or hip dislocations beginning in infancy, knee pain and difficulty running or riding a bicycle. Some individuals with SPS have no symptoms. Symptoms including facial features may vary within the same family.
SPS is caused by a mutation or error in the TBX4 gene. TBX4 gene makes a protein which is a transcription factor. Transcription factors are needed to convert the information in the gene (DNA) to mRNA (messenger RNA). mRNA is then used by our cells to make proteins. If not enough TBX4 transcription factor is made, a number of proteins will be affected causing the bones of the knee, feet, femur and pelvis to form abnormally. SPS is autosomal dominant which means only one copy of the TBX4 gene needs to have a mutation to cause SPS.
Diagnosis by medical imaging and genetic testing is important to rule out similar conditions that may need different treatments. Early surgery may decrease the risk of further complications. Other treatments may include pain therapy. If you or your child has been diagnosed with SPS, talk to your doctor about the most current treatment options. A genetic counselor can explain the genetics. Support groups will help connect you with others living with skeletal dysplasia.