An interpositional knee arthroplasty is disclosed for attachment to a knee condyle. The interpositional knee arthroplasty comprises a spacer and at least one fastener extending from the spacer. The at least one fastener is designed and dimensioned to extend into the condyle without penetrating through the subcondylar plate. The spacer may be kidney shaped to facilitate attachment to the knee condyle. The fastener comprises at least one peg with at least one deformable fin attached to the peg. The deformable fin may be cup shaped. In operation, at least one hole is formed in a patient's condyle. The at least one hole is dimensioned to receive the at least one peg member and deformable fin, but the hole does not extend to the subcondylar bone. When the peg member is forced into the hole in the condyle, the deformable fin collapses and wedges the peg member in the hole.