tyrosine derived polycarbonate polymers polyanhydride polymers polyorthoester polymers polyphosphazenes calcium phosphate hydroxyapatite bioactive glass plla plda and combinations as noted elsewhere the implants may be introduced to a treatment site using a number of different approaches u2014 anterior posterior lateral posterior lateral etc because of the low profile upon insertion the implant is especially useful in lateral and posterior approaches e g plif and tlif approaches fig1 shows the implantation of a variation of the implant into a site having both a collapsed disc and spondylolisthesis i e a condition where a vertebra is displaced anteriorly in relation to the vertebra below fig1 is a lateral view of an upper vertebra 500 and a lower vertebra 502 separated by an implant 504 having a distal locator arm 506 that is longer than the proximal locator arm 508 step a in the fig1 shows the site having both a collapsed disc and spondylolisthesis with the properly placed but unexpanded implant 504 in step b the implant 504 has been expanded expansion of the noted variation of the depicted implant 504 causes distraction of the vertebrae 500 502 posterior displacement of the upper vertebra 502 and the restoration of a normal lordotic angle fig1 step a shows a lateral view of an intervertebral site between an upper vertebra 500 and a lower vertebra 502 that requires only height preservation step a shows the collapsed implant 510 properly situated for implantation in step b the implant 510 has been expanded to contact the surfaces of the vertebral bones however the vertebrae 500 502 have not been distracted nor translated fig1 provides a procedure for inserting an implant into an intervertebral space that requires distraction.