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Small patella syndrome (SPS) is a very rare benign bone dysplasia affecting skeletal structures of the lower limb and the pelvis.
Less that 50 patients have been reported worldwide.
The main clinical features include patellar aplasia or hypoplasia, associated with absent, delayed or irregular ossification of the ischiopubic junctions and/or the infra-acetabular axe-cut notches. Additional features found in the majority of reported patients include femur and foot anomalies (a wide gap between the first and second toes, short fourth and fifth rays of the feet, and pes planus). Craniofacial anomalies (micrognathia, cleft palate, flattened nose and prominent forehead) have been reported occasionally. Intrafamilial variability of the patellar, pelvic and foot anomalies has been described. Signs and symptoms vary from pain resulting from gonarthrosis in elderly subjects to recurrent luxations from infancy, knee pain, and inability to run and ride a bicycle. However, some cases are asymptomatic.
SPS is caused by mutations in the human TBX4 gene (chromosome 17q22). TBX4 mutations account for familial cases with a distinctive facial appearance and those without facial features. At present, there is no evidence for a genotype-phenotype correlation.
Diagnosis is clinical and radiographical.
SPS should be recognized and differentiated from disorders with aplastic or hypoplastic patellae, such as isolated familial patella aplasia-hypoplasia (PTLAH) syndrome and the more severe nail-patella syndrome (NPS).
SPS is inherited in an autosomal dominant manner.
Management and treatment
Early surgical treatment, pain relief therapy and supportive measures should be offered.