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Massive endoprostheses rely on extra-cortical bone bridging (ECBB) to enhance fixation. The aim of this study was to investigate the role of selective laser sintered (SLS) porous collars in augmenting the osseointegration of these prostheses.The two novel designs of porous SLS collars, one with small pores (Ø700 μm, SP) and one with large pores (Ø1500 μm, LP), were compared in an ovine tibial diaphyseal model. Osseointegration of these collars was compared with that of a clinically used solid, grooved design (G). At six months post-operatively, the ovine tibias were retrieved and underwent radiological and histological analysis.Porous collars provided a significantly greater surface (p < 0.001) for the ingrowth of bone than the standard grooved design. Significantly greater extracortical pedicle formation was seen radiologically around the grooved design (length p = 0.002, thickness p < 0.001, surface area p = 0.002) than around the porous collars. However, the ingrowth of bone occurred from the transection site into the porous structure of both types of collar. A fivefold increase in integration was seen with the SP and a threefold increase in the LP design when compared with G (p < 0.001).SLS porous collars allow the direct ingrowth of more bone and are better than current designs which rely on surface ongrowth and ECBB.