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Disease Profile

Small patella syndrome

Prevalence estimates on Rare Medical Network websites are calculated based on data available from numerous sources, including US and European government statistics, the NIH, Orphanet, and published epidemiologic studies. Rare disease population data is recognized to be highly variable, and based on a wide variety of source data and methodologies, so the prevalence data on this site should be assumed to be estimated and cannot be considered to be absolutely correct.

<1 / 1 000 000

US Estimated

Europe Estimated

Age of onset






Autosomal dominant A pathogenic variant in only one gene copy in each cell is sufficient to cause an autosomal dominant disease.


Autosomal recessive Pathogenic variants in both copies of each gene of the chromosome are needed to cause an autosomal recessive disease and observe the mutant phenotype.


dominant X-linked dominant inheritance, sometimes referred to as X-linked dominance, is a mode of genetic inheritance by which a dominant gene is carried on the X chromosome.


recessive Pathogenic variants in both copies of a gene on the X chromosome cause an X-linked recessive disorder.


Mitochondrial or multigenic Mitochondrial genetic disorders can be caused by changes (mutations) in either the mitochondrial DNA or nuclear DNA that lead to dysfunction of the mitochondria and inadequate production of energy.

Multigenic or multifactor Inheritance involving many factors, of which at least one is genetic but none is of overwhelming importance, as in the causation of a disease by multiple genetic and environmental factors.


Not applicable


Other names (AKA)

Scott-Taor syndrome; Coxo-podo-patellar syndrome; Ischiopatellar dysplasia;


Congenital and Genetic Diseases; Musculoskeletal Diseases


Small patella syndrome (SPS) is a rare syndrome that mainly affects the way certain bones are formed (developed). A person with SPS usually has very small kneecaps (hypoplastic patella) or may have no kneecaps at all (aplastic). The hip (pelvic) bones may be weak, and the bones of the feet may not have formed correctly. Occasionally, bones in the face may also be affected.[1]

Small patella syndrome is caused by a change (pathogenic variant or mutation) in the TBX4 gene. The syndrome is inherited in an autosomal dominant manner. Diagnosis of the syndrome is suspected based on features that can be seen in a physical exam and on imaging tests such as X-rays. Diagnosis of SPS can be confirmed with genetic testing. Treatment of SPS may include pain management with medications, physical and occupational therapies, and surgical treatments.[1][2]


Small patella syndrome (SPS) may cause changes in the development of the kneecaps and hip bones, as well as changes in the bones of the feet and face. The lungs and spine may also be affected. As the name suggests, most commonly, one or both kneecaps of a person with SPS may be very small (hypoplastic) or completely absent (aplastic). This can cause pain in the knee, wearing down of the soft tissues surrounding the knee (arthrosis), and frequent kneecap dislocations. Changes in the feet may include a large gap between the big toe and second toe (sandal gap deformity) and having flat feet (pes planus).[1]

A person with SPS may have abnormally shaped hip (pelvic) bones which can cause the leg on the affected side to be shorter (coxa vara). The hip bones may also be weak or more likely to break (fracture).[1][3] Rarely, people with SPS may have changes in the bones of the face (distinctive facial features) including having a very small chin (micrognathia), cleft palate, or flattened nose.[1] Other less common features of the syndrome include having a curved spine (scoliosis) or frequent dental cavities. Some people with SPS have pulmonary arterial hypertension. This means that the blood pressure in the arteries of the lungs is higher than expected.[4][5]

This table lists symptoms that people with this disease may have. For most diseases, symptoms will vary from person to person. People with the same disease may not have all the symptoms listed. This information comes from a database called the Human Phenotype Ontology (HPO) . The HPO collects information on symptoms that have been described in medical resources. The HPO is updated regularly. Use the HPO ID to access more in-depth information about a symptom.

Medical Terms Other Names
Learn More:
80%-99% of people have these symptoms
Abnormality of epiphysis morphology
Abnormal shape of end part of bone
Aplasia/Hypoplasia of the patella
Absent/small kneecap
Absent/underdeveloped kneecap

[ more ]

30%-79% of people have these symptoms
Hip dysplasia
Percent of people who have these symptoms is not available through HPO
Autosomal dominant inheritance
Cleft palate
Cleft roof of mouth
Coxa vara
Flat capital femoral epiphysis
Flat end part of innermost thighbone
High palate
Elevated palate
Increased palatal height

[ more ]

Hypoplasia of the lesser trochanter
Little lower jaw
Small jaw
Small lower jaw

[ more ]

Patellar aplasia
Absent kneecap
Patellar dislocation
Dislocated kneecap
Patellar hypoplasia
Small kneecap
Underdeveloped kneecap

[ more ]

Pes planus
Flat feet
Flat foot

[ more ]

Sandal gap
Gap between 1st and 2nd toes
Gap between first and second toe
Increased space between first and second toes
Sandal gap between first and second toes
Wide space between 1st, 2nd toes
Wide space between first and second toes
Wide-spaced big toe
Widely spaced 1st-2nd toes
Widely spaced first and second toes
Widened gap 1st-2nd toes
Widened gap first and second toe

[ more ]

Short femur
Short thighbone
Short stature
Decreased body height
Small stature

[ more ]

Talocalcaneal synostosis
Wide capital femoral epiphyses
Wide end part of innermost thighbone


Small patella syndrome (SPS) is caused by a change (mutation or pathogenic variant) in the TBX4 gene. Almost all genes provide instructions to make proteins that help our bodies develop and function properly. While the exact role of the TBX4 gene in humans is not known, it is known that a very similar gene exists in chickens and mice and helps their hind legs develop properly. Therefore, it is thought that the protein made from the TBX4 gene is involved in the development of the legs, but researchers are still trying to understand how changes in the TBX4 gene cause all of the features seen in a person with SPS, as well as why changes in the gene may affect people differently.[3]


Small patella syndrome (SPS) is suspected when a doctor notices features of the syndrome during a physical exam such as a small or missing kneecap or one leg being shorter than the other. A person may also complain of pain in the knees or frequent dislocations or have a history of broken hip bone.[1] The diagnosis can be confirmed by imaging tests such as an X-ray and by ruling out other syndromes that can cause the kneecap to be very small.[1]

Genetic testing of the TBX4 gene can be used to confirm the diagnosis of SPS. However, not every person with SPS has a change (mutation or pathogenic variant) in the TBX4 gene that can be detected on genetic testing. It is thought that pathogenic variants in other genes may cause SPS as well.[4]

Testing Resources

  • The Genetic Testing Registry (GTR) provides information about the genetic tests for this condition. The intended audience for the GTR is health care providers and researchers. Patients and consumers with specific questions about a genetic test should contact a health care provider or a genetics professional.


The treatment for small patella syndrome (SPS) may depend on the severity of symptoms in each person with the syndrome. Some people with SPS may benefit from pain-relieving medications, occupational therapy, or physical therapy. Other people may require surgery to repair the kneecap.[1][2]

Other features of SPS may be treated as they would in people who do not have SPS. For example, scoliosis may be treated with bracing or surgery. Pulmonary arterial hypertension may be treated with medications or supplemental oxygen.[7]

Learn more

These resources provide more information about this condition or associated symptoms. The in-depth resources contain medical and scientific language that may be hard to understand. You may want to review these resources with a medical professional.

In-Depth Information

  • The Monarch Initiative brings together data about this condition from humans and other species to help physicians and biomedical researchers. Monarch’s tools are designed to make it easier to compare the signs and symptoms (phenotypes) of different diseases and discover common features. This initiative is a collaboration between several academic institutions across the world and is funded by the National Institutes of Health. Visit the website to explore the biology of this condition.
  • Online Mendelian Inheritance in Man (OMIM) is a catalog of human genes and genetic disorders. Each entry has a summary of related medical articles. It is meant for health care professionals and researchers. OMIM is maintained by Johns Hopkins University School of Medicine. 
  • Orphanet is a European reference portal for information on rare diseases and orphan drugs. Access to this database is free of charge.
  • PubMed is a searchable database of medical literature and lists journal articles that discuss Small patella syndrome. Click on the link to view a sample search on this topic.


  1. Lacombe D. Coxopodopatellar syndrome. Orphanet. November 2006; https://www.orpha.net/consor/cgi-bin/OC_Exp.php?lng=en&Expert=1509.
  2. Wang CH, Shu L, Ma LF, Zhou JW, Ji G, Wang F, and Wang J. Medial and lateral retinaculum plasty for congenital patellar dislocation due to small patella syndrome. Orthopedics. November 2013; 36(11):e1418-1423. https://www.ncbi.nlm.nih.gov/pubmed/24200447.
  3. Ischiocoxopodopatellar Syndrome; ICPPS. Online Mendelian Inheritance in Man. January 29, 2016; https://www.omim.org/entry/147891.
  4. Vanlerberghe C, Jourdain AS, Dieux A, Toutain A, Callewaert B, Dupuis-Girod S, Unger S, Wright M, Isidor B, Ghoumid J, Petit F, Boutry N, Escande F, Manouvrier-Hanu S. Small patella syndrome: New clinical and molecular insights into a consistent phenotype. Clinical Genetics. December 2017; 92(6):676-678. https://www.ncbi.nlm.nih.gov/pubmed/29120062.
  5. Penhoat-Gahier M, Chaillous B, Cozic C, Andre V, and Cormier G. Small patella syndrome. Joint Bone Spine. May 2017; 84(3):377-378. https://www.ncbi.nlm.nih.gov/pubmed/27344077.
  6. Kerstjens-Frederikse WS, Bongers EM, Roofthooft MT, Leter EM, Douwes JM, Van Dijk A, Vonk-Noordegraaf A, Dijk-Bos KK, Hoefsloot LH, Hoendermis ES, Gille JJ, Sikkema-Raddatz B, Hofstra RM, and Berger RM. TBX4 mutations (small patella syndrome) are associated with childhood-onset pulmonary arterial hypertension. Journal of Medical Genetics. August 2013; 50(8):500-506. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3717587/.
  7. Diagnosing and Treating Pulmonary Arterial Hypertension. American Lung Association. August 4, 2016; https://www.lung.org/lung-health-and-diseases/lung-disease-lookup/pulmonary-arterial-hypertension/diagnosing-treating-pul-arterial-hypertension.html.
  8. De Heer EW, Gerrits MMJG, Beekman ATF, et al. The Association of Depression and Anxiety with Pain: A Study from NESDA. Sun HS, ed. PLoS ONE. October 15, 2014; 9(10):e106907. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4198088/.

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