Epiphyseal and physeal cartilage: Normal Gadolinium-enhanced MR imaging

Glycosaminoglycan-mediated loss of cathepsin K collagenolytic activity in MPS I contributes to osteoclast and growth plate abnormalities

Mucopolysaccharidoses are a group of lysosomal storage diseases characterized by the build-up of glycosaminoglycans (GAGs) and severe skeletal abnormalities. As GAGs can regulate the collagenolytic activity of the major osteoclastic protease cathepsin K, we investigated the presence and activity of cathepsin K and its co-localization with GAGs in mucopolysaccharidosis (MPS) type I bone. The most dramatic difference between MPS I and wild-type mice was an increase in the amount of cartilage in the growth plates in MPS I bones. Though the number of cathepsin K-expressing osteoclasts was increased in MPS I mice, these mice revealed a significant reduction in cathepsin K-mediated cartilage degradation. As excess heparan and dermatan sulfates inhibit type II collagen degradation by cathepsin K and the spatial overlap between cathepsin K and heparan sulfate strongly increased in MPS I mice, the build up of subepiphyseal cartilage is speculated to be a direct consequence of cathepsin K inhibition by MPS I-associated GAGs. Moreover, isolated MPS I and Ctsk(-/-) osteoclasts displayed fewer actin rings and formed fewer resorption pits on dentine disks, as compared with wild-type cells. These results suggest that the accumulation of GAGs in murine MPS I bone has an inhibitory effect on cathepsin K activity, resulting in impaired osteoclast activity and decreased cartilage resorption, which may contribute to the bone pathology seen in MPS diseases.

Post-closed reduction perfusion magnetic resonance imaging as a predictor of avascular necrosis in developmental hip dysplasia: a preliminary report

INTRODUCTION

Avascular necrosis (AVN) of the femoral head remains a major complication in the treatment of developmental dysplasia of the hip (DDH) in infants. We performed a retrospective analysis to look at the predictive ability of postclosed reduction contrast-enhanced magnetic resonance imaging (MRI) for AVN after closed reduction in DDH.

METHODS

Twenty-eight hips in 27 infants (aged 1-11 months) with idiopathic hip dislocations who had failed brace treatment underwent closed reduction +/- adductor tenotomy and spica cast application under general anesthesia. Magnetic resonance imaging of the hips after intravenous gadolinium contrast injection for evaluation of epiphyseal perfusion was obtained immediately after cast application. Patients were followed with serial radiographs for a minimum of 1 year after closed reduction. Presence of AVN was determined by the presence of any one of the 5 Salter criteria by 2 readers. Magnetic resonance imaging was graded as normal, asymmetric enhancement, focal decreased enhancement, or global decreased enhancement by 2 radiologists.

RESULTS

Six (21%) of 28 hips showed evidence of clinically significant AVN on follow-up radiographs. Fifty percent of the hips with AVN, but only 2 of 22 hips without AVN, showed a global decreased MRI enhancement (P < 0.05, Fisher exact test). Multivariate logistic regression indicated that a global decreased enhancement was associated with a significantly higher risk of developing AVN (P < 0.01), independently of age at reduction (P = 0.02) and abduction angle.

CONCLUSIONS

In addition to accurate anatomical assessment of a closed reduction in DDH, gadolinium-enhanced MRI provides information about femoral head perfusion that may be predictive for future AVN. At present, it is premature to use the perfusion information for routine clinical use. However, it opens the door to studies looking at repositioning or alternative reduction methods that may reduce the risk of AVN in this higher risk group.