Mannosidosis is an extremely rare genetic disease characterized by a deficiency of the lysosomal enzyme, α-mannosidase. This enzyme is necessary for cleavage of mannose from many glycoproteins. In the absence of this enzyme, mannose accumulates in cells throughout the body, including the joints and the synovium. This disease causes many skeletal changes including dysostosis multiplex, synovial hypertrophy, and Charcot-type joints. We report the case of a girl, aged 9 years and 6 months, who developed bilateral patellar dislocation and severe synovial hypertrophy secondary to α-mannosidase deficiency. Her disease was further complicated by Charcot elbow and bilateral hip and elbow avascular necrosis.