Hilkka Peltoniemi, Nureddin Ashammakhi, Eero Waris, Pertti Törmälä, Timo Waris

Department of Surgery, Division of Plastic Surgery, Oulu University Central Hospital, Finland Institute of Biomaterials,Tampere University of Technology, Tampere, Finland


Our previous experimental studies on sheep have been focused on the basic healing process of a craniotomy line (I) and unstable craniotomies (II), fixed with resorbable polylactide or titanium miniplates. In the first study (I), plating of a 2.5-mm wide craniotomy line resulted in faster and more even consolidation of the line when plated with a SR-PLLA plate compared with narrow titanium plating. By 20 weeks, all SR-PLLA plated sides had healed, whereas none of the titanium plated sides consolidated during the follow-up of one year. Histomorphometric analysis revealed superior healing immediately under the SR-PLLA plate (p<0.001); no statistically significant difference could be shown on the dural side. The difference can be explained by the principle of GBR (guided bone regeneration) - a wide, slowly resorbable implant protects the osseous defect from invasion of connective tissue.

In the second study (II), symmetrical frontal bone craniotomies in 20 lambs were fixed with a flexible, punched 96/4 PDLLA plate and four slowly degrading SR-PLLA (n=10) or rapidly degrading SR-PGA (n=10) miniscrews and, on the contralateral side, with a titanium miniplate. During the follow-up time of 4-104 weeks, no loss of fixation, dislocation, instability, clinical foreign body reactions or infections were observed in MRI, radiography or histology. Clinically, no difference in consolidation was observed between the fixation methods, and the osteotomies were stable on palpation. In histologic cross sections, 60-70% of the original 2.35-mm wide craniotomy defect had consolidated from the bone margins by 4 weeks. Thereafter, consolidation remarkably slowed down against vertically oriented connective tissue filling the bone defect. Defects were observed in 13 of 28 lines at 26-52 weeks, and most were connected to the holes existing in the plate. Histomorphometry showed no difference in consolidation with SR-PLLA miniscrew versus titanium fixation or between the two resorbable fixation methods. Fixation with rapidly degrading SR-PGA miniscrews resulted in wider connective tissue filled bone defects than on the contralateral titanium side (p<0.05) (unrestricted skeletal growth), and thicker bone segments (p<0.005), (no stress shielding). The PDLLA plate disappeared completely by 104 weeks. The SR-PGA screws had hydrolyzed and fragmented at 4-6 weeks and were resorbed by 12 weeks, whereas the SR-PLLA miniscrews retained their integrity and holding power for 26 weeks and were mostly resorbed at two years. Thus, rapidly degrading (SR-PGA) miniscrews may be used to avoid restriction of growth in infant neurocranium. Degradation of PGA was accompanied by a typical non-specific foreign-body reaction and initial transient osteolysis with decreased osteoid formation around the screw channel, but compensatory intense osteoid formation followed after resorption of the polymer. The foreign body reaction to PLLA was considerably milder. Slow resorption of bone tissue around the screw heads was noted during degradation and resorption of the polymer, but a reparative phase with new bone formation followed and the bone surface was restored.

In a cadaveric sheep skull model (III), 9 methods of fixation were mechanically tested for compression strength of rectangular osteotomies. The tested fixation systems were variable plating methods with flexible 0.5-mm thick PDLLA plates and SR-PLLA miniscrews, SR-PDLLA wire fixation, and metallic fixation (microplates, wires). Resorbable plate systems tolerated approximately 46-76 N, SR-PDLLA wire 143 N, metallic wire 239 and metallic microplating 305 N. The results were slightly in favor of large plates and melting of the screw heads against the plate.

Future experiments:
1) study on intracranial plating (sheep model) and cranial screws (rabbit)
2) memory of the new PDLLA mesh plate: osteotomy in sheep orbita: Lactosorb heat-shaped plate versus Bionx® mesh plate
3) compression strength testing of the new PDLLA mesh plate in human cadaveric model, compared to Lactosorb® plates.