Ischiopatellar dysplasia

Rare autosomal dominant skeletal dysplasia affecting the patellae, pelvis, and feet

Ischiopatellar dysplasia
Synonyms	Small patella syndrome, ischiocoxopodopatellar syndrome, Scott-Taor syndrome, coxo-podo-patellar syndrome
Pronounce	N/A
Specialty	Medical genetics, Orthopedics, Pediatrics
Symptoms	Patellar hypoplasia, absent or small patellae, knee pain, patellar dislocation, pelvic bone anomalies, pes planus, wide gap between the first and second toes
Complications	Recurrent patellar instability, knee osteoarthritis, gait difficulty, foot pain, hip abnormalities, rarely pulmonary arterial hypertension
Onset	Congenital
Duration	Lifelong
Types	N/A
Causes	Usually heterozygous pathogenic variants in the TBX4 gene
Risks	Family history of autosomal dominant skeletal dysplasia
Diagnosis	Clinical examination, radiography, skeletal survey, MRI, and genetic testing
Differential diagnosis	Nail-patella syndrome, familial patella aplasia-hypoplasia syndrome, patellar dislocation, skeletal dysplasia
Prevention	N/A
Treatment	Supportive care, orthopedic management, physical therapy, pain control, treatment of patellar instability, genetic counseling
Medication	N/A
Prognosis	Variable; many individuals have normal life expectancy with orthopedic follow-up
Frequency	Rare
Deaths	N/A

Ischiopatellar dysplasia is a rare inherited skeletal dysplasia characterized mainly by absent, small, hypoplastic, laterally displaced, or dislocated patellae, together with characteristic abnormalities of the pelvis and feet. It is most often inherited in an autosomal dominant manner and is commonly associated with pathogenic variants in the TBX4 gene, a transcription factor important for lower-limb development."Mutations in the Human TBX4 Gene Cause Small Patella Syndrome".American Journal of Human Genetics.2004;74(6)

1239-1248.doi:10.1086/421331.PMID:15106123.PMC:1182087.

The disorder is also called small patella syndrome, ischiocoxopodopatellar syndrome, or Scott-Taor syndrome. The name reflects the typical involvement of the ischium, hip, foot, and patella. Some modern descriptions use the term ischiocoxopodopatellar syndrome with or without pulmonary arterial hypertension because TBX4 variants may occasionally be associated with pulmonary arterial hypertension.TBX4 variant associated with coxopodopatellar syndrome / ischiopatellar dysplasia(link). ClinVar, National Center for Biotechnology Information.Coxopodopatellar syndrome(link). Orphanet.

Overview

Ischiopatellar dysplasia primarily affects the lower skeleton. The most recognizable feature is hypoplasia or aplasia of the patella, but the condition also affects the pelvis, hip joint, femur, tarsal bones, and toes. Patients may present in childhood or adulthood with knee instability, difficulty squatting, recurrent patellar dislocation, foot deformities, or abnormal findings on radiographs.

The clinical severity is variable. Some people have mild findings and are diagnosed only after another family member is evaluated, while others have significant orthopedic symptoms. Intrafamilial variability has been reported, meaning that affected members of the same family can have different degrees of patellar, pelvic, and foot involvement."Scott-Taor Syndrome: A Clinical Case Report".Cureus.2024;16(1)

e51437.doi:10.7759/cureus.51437.PMC:10758582.

Signs and symptoms

The features of ischiopatellar dysplasia are mainly skeletal and orthopedic.

Patellar and knee findings

• Patellar hypoplasia - Small or underdeveloped patellae are a central feature of the disorder.

• Patellar aplasia - Complete absence of one or both patellae may occur.

• Patellar dislocation - The patella may be laterally displaced or recurrently dislocated.

• Patellar instability - Affected people may have a sense that the kneecap is unstable, especially during walking, running, squatting, or climbing stairs.

• Knee pain - Pain may occur because of abnormal tracking of the patella or associated joint stress.

• Knee osteoarthritis - Degenerative joint disease may develop in adulthood, especially when there is chronic patellar malalignment.

• Genu recurvatum - Hyperextension of the knee has been described in some affected individuals.

• Quadriceps weakness - Weakness around the knee may worsen instability and functional limitation.

Pelvic and hip findings

• Pelvic bone abnormality - Characteristic pelvic radiographic findings may help establish the diagnosis.

• Ischium hypoplasia - The ischial region may show absent or delayed ossification.

• Ischiopubic junction abnormality - Delayed or absent ossification of the ischiopubic junction is a classic radiographic feature.

• Infra-acetabular axe-cut notch - A distinctive notch below the acetabulum may be seen on pelvic radiographs.

• Hip dysplasia - Developmental hip abnormalities may occur in some patients.

• Coxa vara - Abnormal angulation of the femoral neck has been reported in some historical descriptions.

• Femoral neck elongation - Elongated femoral necks may be present in some affected individuals.

• Lesser trochanter hypoplasia - Hypoplasia of the lesser trochanter has been described.

Foot and toe findings

• Pes planus - Flat feet are common.

• Sandal gap - A wide gap between the first and second toes may be present.

• Syndactyly - Fusion or webbing of toes may occur.

• Short fourth toe - Shortening of the fourth toe or fourth ray may be present.

• Short fifth toe - Shortening of the fifth toe or fifth ray may be present.

• Clinodactyly - Curving of the toes may occur.

• Talocalcaneal coalition - Abnormal connection between tarsal bones may be seen on imaging.

• Tarsal coalition - Fusion or coalition of bones in the foot may contribute to stiffness or pain.

• Clubfoot - Clubfoot deformity has been described in some patients.

• Ball-and-socket ankle joint - This radiographic foot and ankle feature has been reported in coxopodopatellar syndrome.

Other possible findings

• Cleft palate - Cleft palate has been reported in some affected individuals, although it is not universal.

• Craniofacial dysmorphism - Mild craniofacial differences may occur in some patients.

• Dental anomaly - Dental abnormalities have been reported rarely.

• Spine deformity - Spinal abnormalities have been described in some cases.

• Pulmonary arterial hypertension - Some TBX4-related disease presentations include pulmonary arterial hypertension, so cardiopulmonary symptoms should be evaluated when present.

• Developmental delay - Developmental or speech delay has been reported in some case descriptions, but it is not considered a defining feature of classic isolated small patella syndrome.

Causes

Ischiopatellar dysplasia is usually caused by heterozygous pathogenic variants in the TBX4 gene.

• TBX4 - A gene encoding T-box transcription factor 4, which is important in lower-limb and hindlimb development.

• Transcription factor - A protein that helps regulate gene expression during embryonic development.

• Lower limb development - The developmental process most strongly affected in classic ischiopatellar dysplasia.

• Chromosome 17 - The TBX4 gene is located on chromosome 17q23.2.

• Pathogenic variant - A disease-causing genetic change that disrupts normal gene function.

The disorder was originally recognized by its skeletal pattern before the responsible gene was identified. Studies later showed that mutations in TBX4 cause small patella syndrome and related ischiocoxopodopatellar phenotypes."Mutations in the Human TBX4 Gene Cause Small Patella Syndrome".American Journal of Human Genetics.2004;74(6)

1239-1248.doi:10.1086/421331.PMID:15106123.PMC:1182087.

Inheritance

Ischiopatellar dysplasia is generally inherited in an autosomal dominant pattern.

• Autosomal dominant inheritance - One altered copy of the disease-associated gene is sufficient to cause the disorder.

• Familial condition - The disorder may affect multiple members of the same family.

• Variable expressivity - Different affected relatives may have different signs and severity.

• Reduced penetrance - Some people with a pathogenic variant may have subtle or incomplete clinical features.

• De novo mutation - A new genetic variant may occur in an affected person with no known family history.

Each child of an affected individual has a 50 percent chance of inheriting the familial pathogenic variant, although the severity of symptoms cannot be predicted reliably from inheritance alone.

Pathophysiology

• TBX4 is a developmental gene involved in limb patterning. When one copy of the gene is altered, normal development of the lower limb, patella, pelvis, and foot may be disrupted.

The most affected structures include:

• Patella - A sesamoid bone within the tendon of the quadriceps muscle.

• Ischium - A lower part of the pelvic bone involved in the characteristic pelvic findings.

• Acetabulum - The hip socket, below which infra-acetabular notches may be visible.

• Femur - The thigh bone, which may show femoral neck or trochanteric abnormalities.

• Tarsal bones - Foot bones that may show coalition or abnormal morphology.

• Toes - The fourth and fifth toes may be shortened, and the first and second toes may be widely spaced.

Diagnosis

Diagnosis is based on a combination of clinical findings, family history, imaging, and genetic testing.

Clinical examination

Clinical evaluation may identify the characteristic pattern of knee, pelvis, and foot findings.

• Knee examination - Assessment of patellar size, patellar tracking, instability, dislocation, pain, and range of motion.

• Foot examination - Assessment of pes planus, sandal gap, toe shortening, toe syndactyly, and gait.

• Hip examination - Assessment for hip pain, reduced motion, or developmental hip abnormalities.

• Gait analysis - Evaluation of walking pattern, balance, foot posture, and knee mechanics.

• Family history - Review of relatives with small kneecaps, recurrent knee dislocation, flat feet, hip abnormalities, or similar radiographic findings.

Radiography

Radiographic evaluation is central to diagnosis.

• Knee radiography - May show absent, hypoplastic, small, laterally placed, or dislocated patellae.

• Pelvic radiography - May show delayed or absent ossification of the ischiopubic junction.

• Infra-acetabular axe-cut notch - A characteristic pelvic radiographic sign.

• Foot radiography - May show tarsal coalition, short fourth and fifth rays, pes planus, or other foot bone abnormalities.

• Skeletal survey - May be helpful when patellar aplasia or hypoplasia is found, especially in children or patients with multiple skeletal findings.

Magnetic resonance imaging

• Magnetic resonance imaging may be used when soft-tissue detail is needed.

• Knee MRI - Useful for evaluating patellar cartilage, extensor mechanism, soft tissues, traumatic injury, and patellar instability.

• Pelvic MRI - May help evaluate complex hip or pelvic anatomy when radiographs are unclear.

• Ligament injury - MRI may be useful after knee trauma in patients with abnormal patellar anatomy.

Genetic testing

Genetic testing can confirm the diagnosis.

• TBX4 genetic testing - Sequencing or deletion-duplication testing may identify a pathogenic variant.

• Multigene panel - A skeletal dysplasia or patellar aplasia panel may be used when the diagnosis is uncertain.

• Exome sequencing - May be considered when targeted testing is negative and the phenotype is complex.

• Genetic counseling - Recommended for affected individuals and families because of autosomal dominant inheritance.

Differential diagnosis

The differential diagnosis includes other causes of absent or hypoplastic patellae, knee instability, pelvic anomalies, and skeletal dysplasia.

• Nail-patella syndrome - An autosomal dominant disorder caused by LMX1B variants, classically involving nail dysplasia, patellar aplasia or hypoplasia, iliac horns, elbow abnormalities, and kidney disease.

• Familial patella aplasia-hypoplasia syndrome - A hereditary disorder with absent or small patellae but a different clinical pattern.

• Patellar dislocation - A common orthopedic condition that may occur without a genetic skeletal dysplasia.

• Developmental dysplasia of the hip - A hip disorder that may overlap with some pelvic or femoral findings.

• Tarsal coalition - A foot condition that may be isolated or part of a broader syndrome.

• Multiple epiphyseal dysplasia - A skeletal dysplasia that may cause joint pain and epiphyseal abnormalities.

• Chondrodysplasia - A group of skeletal disorders affecting cartilage and bone development.

• Arthrogryposis - Congenital joint contracture disorders that may involve the lower limbs.

• Skeletal dysplasia - A broad category of inherited disorders affecting bone development.

Management

There is no cure that reverses the underlying developmental bone differences. Management is supportive and individualized.

Orthopedic care

• Orthopedic evaluation - Assessment by an orthopedic specialist is useful for patellar instability, knee pain, hip symptoms, and foot deformities.

• Patellar instability treatment - May include bracing, physical therapy, activity modification, or surgery in selected cases.

• Knee brace - May help some patients with instability or pain.

• Physical therapy - Strengthening of the quadriceps, hip abductors, and core muscles may improve knee function.

• Gait training - May help patients with foot deformity, pain, or instability.

• Orthotic device - Shoe inserts or custom orthoses may help pes planus and foot pain.

• Pain management - May include activity adjustment, physical therapy, non-opioid pain relievers, or referral to pain specialists.

• Surgery - Considered for severe recurrent patellar dislocation, disabling malalignment, severe tarsal coalition, or progressive joint disease.

Foot and ankle management

• Pes planus - Managed with supportive shoes, orthotics, strengthening, and monitoring.

• Tarsal coalition - Managed with activity modification, orthotics, immobilization, or surgery in severe cases.

• Toe syndactyly - Usually treated only if it causes functional or footwear problems.

• Foot pain - Managed with footwear modification, orthotics, physical therapy, and orthopedic follow-up.

Pulmonary and cardiovascular assessment

Because some TBX4 variants are associated with pulmonary vascular disease, patients with suggestive symptoms should be evaluated.

• Pulmonary arterial hypertension - Consider evaluation when there is unexplained shortness of breath, exercise intolerance, chest pain, syncope, or abnormal cardiopulmonary findings.

• Echocardiography - A noninvasive test that may screen for pulmonary hypertension when clinically indicated.

• Pulmonology - Specialist evaluation may be appropriate when respiratory symptoms are present.

• Cardiology - Specialist evaluation may be appropriate when pulmonary hypertension or cardiac symptoms are suspected.

Genetic counseling

• Genetic counseling - Helps families understand inheritance, recurrence risk, testing options, and reproductive choices.

• Cascade testing - Testing relatives may identify mildly affected family members.

• Prenatal testing - May be possible when a familial pathogenic variant is known.

• Preimplantation genetic testing - May be considered by some families using assisted reproduction.

Prognosis

The prognosis is variable and depends on the severity of orthopedic problems and whether additional features such as pulmonary arterial hypertension are present.

• Normal life expectancy - Many affected individuals with isolated skeletal findings have normal life expectancy.

• Chronic knee symptoms - Patellar instability and knee pain may persist or recur.

• Osteoarthritis - Degenerative knee changes may develop earlier when patellar malalignment is severe.

• Functional limitation - Some individuals may have difficulty with running, squatting, stairs, or prolonged walking.

• Quality of life - Orthopedic support, physical therapy, and appropriate footwear may improve daily function.

Epidemiology

Ischiopatellar dysplasia is rare. Fewer than 100 affected individuals have been described in some reviews and case reports, although mild cases may be underdiagnosed."A novel frameshift TBX4 variant in a family with ischio-coxo-podo-patellar syndrome".Human Genome Variation.2025;doi:10.1007/s13258-024-01589-5.

Because the condition is inherited in an autosomal dominant manner and can be mild, some families may remain undiagnosed until a child or adult is evaluated for knee instability, abnormal patellae, or characteristic pelvic imaging.

History

The condition has been described under several names, reflecting different parts of the skeletal pattern.

• Scott-Taor syndrome - Historical name used in older case descriptions.

• Small patella syndrome - Name emphasizing the hallmark finding of absent or hypoplastic patellae.

• Ischiopatellar dysplasia - Name emphasizing involvement of the ischium and patella.

• Ischiocoxopodopatellar syndrome - Name emphasizing involvement of the ischium, hip, foot, and patella.

The identification of TBX4 as the causative gene helped connect these overlapping clinical descriptions into a genetically defined disorder.

Patient education

Patients and families should understand that ischiopatellar dysplasia is a rare genetic skeletal condition that mainly affects the lower limbs.

• Family history - Affected relatives may have mild findings, such as small kneecaps, flat feet, or a wide gap between the first and second toes.

• Knee protection - Activities that repeatedly trigger patellar dislocation or severe pain may need modification.

• Physical therapy - Strengthening and conditioning can improve knee stability and function.

• Orthotics - Shoe inserts may help flat feet and foot pain.

• Genetic counseling - Important for family planning and understanding autosomal dominant inheritance.

• Medical evaluation - New shortness of breath, fainting, chest pain, or exercise intolerance should be evaluated because of the possible association of some TBX4 variants with pulmonary arterial hypertension.

When to seek medical care

Medical evaluation is appropriate when a person has congenital or familial patellar abnormalities, knee instability, or characteristic foot and pelvic findings.

• Recurrent patellar dislocation - Repeated kneecap dislocation should be assessed by an orthopedic specialist.

• Persistent knee pain - Chronic pain may indicate maltracking, instability, or early degenerative disease.

• Difficulty walking - Gait difficulty may reflect knee, foot, or hip involvement.

• Flat feet - Symptomatic pes planus may benefit from orthotic or orthopedic care.

• Hip pain - Hip symptoms may require imaging to evaluate associated pelvic or femoral abnormalities.

• Family history - Relatives of an affected person may need evaluation when symptoms or skeletal features are present.

• Shortness of breath - Unexplained dyspnea should be evaluated, especially in a person with a known TBX4 variant.

See also

• Skeletal dysplasia

• Patella

• Patellar hypoplasia

• Patellar dislocation

• Nail-patella syndrome

• Autosomal dominant

• TBX4

• T-box transcription factor

• Pelvic bone

• Pes planus

• Tarsal coalition

• Pulmonary arterial hypertension

• Medical genetics

• Orthopedics

External links

• ClinVar - TBX4 and coxopodopatellar syndrome
• Orphanet - Coxopodopatellar syndrome
• Mutations in the Human TBX4 Gene Cause Small Patella Syndrome
• Scott-Taor Syndrome: A Clinical Case Report