Evaluate the effectiveness of Collagenase Santyl ointment when used in conjunction with NPWT for the treatment of chronic pressure ulcers and determine whether the addition of Collagenase Santyl ointment has a positive effect on wound healing. Through volumetric changes and histological evaluation, we wish to test the hypothesis that the application of Collagenase Santyl ointment in conjunction with NPWT facilitates improved wound healing and mitigates some potentially harmful effects of NPWT alone (e.g. pain with dressing removal and tissue ingrowth