Diagnosis: Patellar hypoplasia with patellofemoral compartment dysplasia causing altered biomechanics with a patella tracking disorder resulting in hypertrophic tendinopathy of the quadriceps tendon and patellar tendonitis.
Discussion: Hypoplasia of the patella is associated with a number of syndromes including Familial patella aplasia-hypoplasia, Small patella syndrome, Scott-Taor syndrome, Coxopodo patellar syndrome, Ischiopubic patellar syndrome, and Nail-patella syndrome. These entities are all autosomal dominant conditions that have been linked with either patellar aplasia or hypoplasia with the underlying genetic defect found on chromosome 17 that encodes for a transcription factor for lower extremity development.
Familial patella aplasia-hypoplasia (PTLAH) is considered to represent an autosomal dominant inheritable disorder of isolated congenital aplasia or hypoplasia of the patella as the only clinical feature. These patients do not exhibit any other associated skeletal anomalies. Patients with this entity typically complain of only minor knee discomfort with walking but do report difficulties with running, riding a bike, stopping abruptly, and climbing stairs.
Scott and Taor first described patellar hypoplasia and aplasia associated with abnormalities of the pelvic girdle. They are the first to term “small patella syndrome.” In contrast to PTLAH, Small patella syndrome (SPS) is a skeletal dysplasia with the dominant feature as hypoplasia or aplasia of the patella, but in association with additional skeletal deformities of the lower extremity. This includes interrupted ossification centers of the ischium and inferior pubic ramus, an infra-acetabular axe-cut notch, drum stick appearance of the proximal femoral epiphysis, elongated femoral necks, hypoplasia of the lesser trochanter, ball and socket ankle joint, pes planus deformity, broad-based plantar clacaneal exostosis, tarsal coalition, widening of the interspace between the first and second digits as well as short fourth and fifth digits of the foot. An individual with SPS is unlikely to have all of the features described as the phenotypic expressions vary.
The patient in this case has not yet been evaluated for additional features associated with small patellar syndrome. His presentation to the referring orthopedic surgeon likely resulted from the patellar tendonitis causing acute symptoms of anterior knee pain.
References:
1. Bongers, Ernie, et al. "Mutations in the Human TBX4 Gene Cause Small Patella Syndrome." American Journal of Human Genetics 2004; 74: 1239-1248.
2. Dellestable F, et al. "The 'Small Patella' Syndrome, Hereditary Osteodysplasia of the Knee, Pelvis, and Foot." Journal of Bone and Joint Surgery 1996. 78-B: 63-65.
3. Mangino, Massimo, et al. "Localization of a Gene for Familial Patella Aplasia-Hypoplasia (PTLAH) to Chromosome 17q21-22." American Journal of Human Genetics 1999; 65: 441-447.
4. Scott, JE, Taor, WS. "The 'Small Patella' Syndrome." Journal of Bone and Joint Surgery 1979; 61-B: 172-175.
