Fracture of the central polyethylene tibial spine in posterior stabilized total knee arthroplasty. 
Between 1993 and 1998, 332 I-BII (Insall Burstein II) prostheses were implanted in our department.  We studied 5 cases of tibial polyethylene spine fractures.  The fractures occurred 3 to 5 years following the surgery.  All cases with fractures had mild medial laxity that was noted on follow-up of 2 to 3 years following arthroplasty.  One also developed mild hyperextension.  All the patients had a revision surgery.  In 2 cases, the polyethylene was changed to a thicker one, together with medial reinforcement.  Three cases were revised to a rotating hinge prosthesis.