Overview
Patella aplasia or hypoplasia (unilateral) refers to a condition where one kneecap (patella) is either completely absent (aplasia) or underdeveloped and smaller than normal (hypoplasia) on only one side of the body. The patella is the small, flat bone that sits in front of the knee joint and plays an important role in protecting the knee and helping with leg movement, especially straightening the leg. When the kneecap is missing or too small on one side, it can affect how the knee works, potentially causing instability, difficulty with certain movements, or discomfort. It is important to note that this specific Orphanet entry is marked as 'OBSOLETE,' meaning it has been retired or merged into a broader classification of patella abnormalities. Patients who were previously classified under this term may now fall under more general categories of patellar aplasia or hypoplasia, or their condition may be recognized as part of a broader genetic syndrome such as nail-patella syndrome or isolated patellar aplasia. Because the kneecap problem occurs on only one side, some individuals may have mild symptoms and adapt well, while others may experience knee pain, instability, or difficulty with activities like running, climbing stairs, or kneeling. Treatment depends on the severity and may range from physical therapy and bracing to surgical options. A thorough evaluation by an orthopedic specialist and possibly a geneticist is recommended to determine whether the patellar abnormality is isolated or part of a larger condition.
Key symptoms:
Missing or very small kneecap on one sideKnee instability or feeling that the knee gives wayDifficulty straightening the leg fullyKnee pain, especially during physical activityTrouble running, jumping, or climbing stairsSwelling around the knee jointLimping or uneven gaitDifficulty kneelingReduced range of motion in the affected kneeMuscle weakness in the thigh on the affected side
Variable
Can be inherited in different ways depending on the underlying gene
Neonatal
Begins at or shortly after birth (first 4 weeks)
Treatments
No FDA-approved treatments are currently listed for OBSOLETE: Patella aplasia/hypoplasia, unilateral.
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Specialists
View all specialists →No specialists are currently listed for OBSOLETE: Patella aplasia/hypoplasia, unilateral.
Treatment Centers
8 centersBaylor College of Medicine Rare Disease Center ↗
Baylor College of Medicine
📍 Houston, TX
🏥 NORDStanford Medicine Rare Disease Center ↗
Stanford Medicine
📍 Stanford, CA
🔬 UDNNIH Clinical Center Undiagnosed Diseases Program ↗
National Institutes of Health
📍 Bethesda, MD
🔬 UDNUCLA UDN Clinical Site ↗
UCLA Health
📍 Los Angeles, CA
🔬 UDNBaylor College of Medicine UDN Clinical Site ↗
Baylor College of Medicine
📍 Houston, TX
🔬 UDNHarvard/MGH UDN Clinical Site ↗
Massachusetts General Hospital
📍 Boston, MA
🏥 NORDMayo Clinic Center for Individualized Medicine ↗
Mayo Clinic
📍 Rochester, MN
👤 Mayo Clinic Center for Individualized Medicine
🏥 NORDUCLA Rare Disease Day Program ↗
UCLA Health
📍 Los Angeles, CA
Travel Grants
No travel grants are currently matched to OBSOLETE: Patella aplasia/hypoplasia, unilateral.
Community
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Caregiver Resources
NORD Caregiver Resources
Support, advocacy, and financial assistance for caregivers of rare disease patients.
Mental Health Support
Rare disease caregiving can be isolating. Connect with counseling and peer support.
Family & Caregiver Grants
Financial assistance programs specifically for caregivers of rare disease patients.
Social Security Disability
Learn how rare disease patients may qualify for SSDI/SSI benefits.
Questions for your doctor
Bring these to your next appointment
- Q1.Is my child's missing or small kneecap an isolated finding, or could it be part of a genetic syndrome?,Should we pursue genetic testing to look for an underlying cause?,What physical therapy exercises are best for strengthening the knee?,Are there activities or sports my child should avoid to protect the knee?,At what point should we consider surgery, and what surgical options are available?,How often should we have follow-up imaging to monitor the knee joint?,What are the long-term risks for arthritis or other knee problems?
Common questions about OBSOLETE: Patella aplasia/hypoplasia, unilateral
What is OBSOLETE: Patella aplasia/hypoplasia, unilateral?
Patella aplasia or hypoplasia (unilateral) refers to a condition where one kneecap (patella) is either completely absent (aplasia) or underdeveloped and smaller than normal (hypoplasia) on only one side of the body. The patella is the small, flat bone that sits in front of the knee joint and plays an important role in protecting the knee and helping with leg movement, especially straightening the leg. When the kneecap is missing or too small on one side, it can affect how the knee works, potentially causing instability, difficulty with certain movements, or discomfort. It is important to note
At what age does OBSOLETE: Patella aplasia/hypoplasia, unilateral typically begin?
Typical onset of OBSOLETE: Patella aplasia/hypoplasia, unilateral is neonatal. Age of onset can vary across affected individuals.