Magnetic resonance imaging features of allografts

Objective. To investigate the magnetic resonance imaging (MRI) features of allografts at various time intervals after surgery in patients with osteoar ticular allografts. Design and patients. Sixteen patients who were treated with osteoarticular allografts and who were followed over time with MRI studies as part of thei r long-term follow-up were retrospectively selected for this study. T1-weig hted images were obtained both before and after gadolinium administration a long with T2-weighted images. All images were reviewed by an experienced mu sculoskeletal radiologist, with two other experienced radiologists used for consultation. Imaging studies were organized into three groups for ease of discussion: early postoperative period (2 days to 2 months), intermediate postoperative period (3 months to 2 years), and late postoperative period ( greater than 2 years). Results. In the early postoperative period, no gadolinium enhancement of th e allograft was visible in any of the MR images. A linear, thin layer of pe riosteal and endosteal tissue enhancement along the margin of the allograft was visible in images obtained at 3-4 months. This enhancement appeared gr adually to increase in images from later periods, and appears to have stabi lized in the images obtained approximately 2-3 years after allograft placem ent. The endosteal enhancement diminished after several years, with examina tions conducted between 6 and 8 years following surgery showing minimal end osteal enhancement. However, focal enhancement was noted adjacent to areas of pressure erosion or degenerative cysts. All the cases showed inhomogenei ty in the marrow signal (scattered low signal foci on T1 with corresponding bright signal on T2), and a diffuse, inhomogeneous marrow enhancement late r on. Conclusion. We have characterized the basic MRI features of osteoarticular allografts in 16 patients who underwent imaging studies at various time poi nts as part of routine followup. We believe that the endosteal and perioste al enhancement observed on MRI during the first few months to 2 years follo wing surgery represents vascular ingrowth and early skeletal repair. The zo ne of periosteal enhancement could also include the new bone laid on the su rface of the allograft through which the soft tissues bind to the cortex. T he exact reason for the inhomogeneity in the marrow signal, and the diffuse , inhomogeneous marrow enhancement is not clear. This may represent saponif ied and/or necrotic marrow fat interspersed with the fibrovascular tissue. The features noted here should provide radiologists with useful information regarding imaging characteristics they can expect to see in other allograf t replacement patients.