In vitro and in vivo degradation of lactic acid-based interference screws used in cruciate ligament reconstruction

Abstract

Nowadays, many degradable polymers are being used under the form of interference screws to fix the bone–tendon–bone autograft in anterior cruciate ligament reconstruction. However, little is known about the post-implantation fate of these screws, especially about the formation of crystalline residues which seems to be a critical factor for the success of surgery with temporary implants based on lactic and glycolic acid derived polymers (PLAGA). In an attempt to bring in some new insights, various high molecular weight stereoregular poly(lactide)s (PLAX with X=percentage of l-lactyl units) obtained by ring-opening polymerization of lactides in the presence of zinc-metal (PLA98-Zn), zinc lactate (PLA98-Znlac) or stannous octoate (PLA100-Sn), were processed by injection-molding to make interference screws to be compared. In vivo data were collected from screws implanted in sheep knees with follow ups ranging from 6 months to 5 years. Histology confirmed the heterogeneous degradation mechanism introduced nearly 10 years ago from in vitro investigations of homemade implants having simpler geometry. The effects of the initiator system (zinc- or tin derivatives) used to polymerize the lactide monomer on the properties of injection molded interference screws was also investigated in vitro in a phosphate buffer solution at 37°C. Major differences in terms of hydrophilicity, hydrolysis rate and loss of mechanical properties were observed between PLA-Zinc and PLA-Tin. Discussion of the behavior of interference screws of different compositions was made on the basis of the present understanding of PLAGA morphology and degradation characteristics.