January 2008 - Proscan Imaging

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January 2008 Case of the Month

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Compiled by: Daniel B. Maloney, D.O.

History: 14 year old male with anterior left knee pain.

Exam: Multisequence MRI of the knee

Findings: The petalla is markedly reduced in size compared to the femur and tibia. The patella is displaced laterally articulating with the lateral half of the femoral trochlea. Flattening of the trochlear groove is evident. Inflammation is noted along the superior aspect of the Hoffa fat pad with focal tiny fluid collections indicating a patellar tracking disorder. The distal quadriceps tendon is enlarged with hazy abnormal internal signal intensity compatible with chronic tendinosis or hypertrophic tendinopathy. The proximal patellar tendon demonstrates bright internal signal alteration on water weighted sequences without enlargement radiating from the origin on the patella in an appearance typical of patellar tendonitis or jumper�s knee.

Images: Sagittal proton density (figure 1) and T2 (figure 2) weighted images show the relative small size of the patella in relation to the femur and tibia, the hypertrophic tendinopathy of the distal quadriceps tendon, the tendonitis involving the proximal patellar tendon, and the inflammation and fluid within the superior aspect of the Hoffa fat pad. The axial 3D T1 (figure 3) weighted image demonstrates the lateral position of the patella along the lateral half of the femoral trochlea with dysplasia and flattening of the trochlear groove.

Figure 1                                                                                                                           Figure 2


Figure 3

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: Bongers, Ernie, et al., Mutations in the Human TBX4 Gene Cause Small Patella Syndrome. American Journal of Human Genetics, 74:1239-1248, 2004.
Dellestable, F, et al., The �Small Patella� Syndrome, Hereditary Osteodysplasia of the Knee, Pelvis, and Foot. Journal of Bone and Joint Surgery, 78-B: 63-65, 1996.
Mangino, Massimo, et al., Localization of a Gene for Familial Patella Aplasia-Hypoplasia (PTLAH) to Chromosome 17q21-22. American Journal of Human Genetics, 65: 441-447, 1999.
Scott, JE, Taor, WS. The �Small Patella� Syndrome. Journal of Bone and Joint Surgery, 61-B: 172-175, 1979.