Compressive strength of calcium carbonate and hydroxyapatite implants after bone-marrow-induced osteogenesis.

Natural coral and structurally similar porous hydroxyapatite (HA) have been used as bone substitutes. They are not osteoinductive but bone formation can be induced by marrow cells, even in extraosseal sites. In our previous study we induced bone formation in porous coral and HA after having implanted the materials in intramuscular pockets in rat. New bone formed only in HA or coral implants soaked with marrow cells; fibrous tissue ingrowth alone was observed in the controls (without marrow). In the present study we examined the effect of tissue ingrowth on the mechanical properties of coral and HA implants obtained in a similar process to that used before. At 12 weeks the compressive strength of HA was higher in the marrow group than in the controls; it was also higher than that of the wet unimplanted material. The HA blocks did not show resorption. Coral resorbed quickly and lost its compressive strength, which was originally higher than in HA. At three weeks the marrow group was stronger than the control specimens. After six weeks only the marrow group, but not the controls, could be tested. Bone ingrowth seemed to maintain the strength of the coral implant even if it was dissolving. The mechanical strength of both materials was comparable to that of cancellous bone.

Shear strength of a distal rabbit femur during consolidation of an osteotomy fixed with a polylactide expansion plug.

The consolidation of a transverse transcondylar osteotomy of the distal rabbit femur, fixed with a self-reinforced polylactide expansion plug, was studied radiographically and mechanically. The peak shear force was assessed by stressing the osteotomy site to failure. The intact contralateral femur of the same rabbit served as a control. Fifteen rabbits were tested in groups of five animals with follow-up times of 6, 12 or 24 wk after fixation of the osteotomy. The absorbable plug, measuring 4.5 mm in diameter and 30 mm in length, had an expandable distal locking blade system. Thirteen osteotomies showed a radiographically solid bony union. The mean shear strength of the specimens was 3.5 MPa at 6 wk, 3.5 MPa at 12 wk and 4.3 MPa at 24 wk. The mean shear strength of the control distal femora was 3.6 MPa. In conclusion, the fixation properties of the newly developed absorbable expansion plug were deemed satisfactory in this experimental fracture model.

Mechanical properties of biodegradable ligament augmentation device of poly(L-lactide) in vitro and in vivo.

To evaluate the mechanical properties of absorbable braided poly(L-lactide) (PLLA) fibre implants, 2.0 and 3.2 mm in diameter, maximum load defined as tensile load carrying capacity, elongation and axial rigidity were investigated after immersion in phosphate-buffered distilled water at 37 degrees C and pH 6.1 and after subcutaneous implantation in rabbit. The results confirm earlier indications that PLLA degrades faster in vivo than in vitro. The non-sterilized 2.0 mm implants lost 69% of initial tensile load carrying capacity in 46 wk in vitro. In vivo the loss of tensile load carrying capacity of the 2.0 and 3.2 mm implants was most marked between 6 to 12 wk. After 48 wk in vivo 2.0 and 3.2 mm implants retained 3 and 4% of initial tensile load carrying capacity respectively. Both in vitro and in vivo, elongation diminished in the same way as the maximum load. In vitro, mean axial rigidity of unsterilized 2.0 mm implants was 64 N during the first 34 wk but fell to 31 N at 46 wk. In vivo the initial mean axial rigidity of the 2.0 and 3.2 mm implants was 29 and 95 N respectively. At 24 wk the mean axial rigidity was 2 N in both implants.

Use of hydroxylapatite/ polymer-composite in facial bone augmentation. An experimental study.

On purpose to achieve better and more controlled alveolar ridge augmentation, biodegradable polyglycolide (PGA) curved implants with porous, particulate hydroxylapatite (HA) were developed to be used in an experimental augmentation in sheep mandible. Prior to surgery curved PGA implants were loaded with particulate HA with help of a rather fast degrading adhesive, polyglycolide/polylactide (PGA/PLA) copolymer. This augmentation device was inserted into the ridge using a slight modification of the tunnelling technique. Macroscopically, a dehiscence with loss of HA particles was observed in 3 of the 16 augmentations. Bone ingrowth was seen in a noticeable degree in one sheep at 24 weeks. Instead, foreign body-type cells were shown at the interface of mandibular bone and hydroxylapatite deposit, as well as abundant connective tissue reaction inside HA deposits. The results of the study give rise to concern about bony integration in the presence of biodegradable polyglycolide substances in HA augmentation. The findings call in question even their use as a carrier for bone forming agents in combination with HA.

Coralline hydroxyapatite reinforced with polylactide fibres in lumbar interbody implantation.

Porous hydroxyapatite HA blocks reinforced with poly-l/dl-lactide fibres were used to maintain the lumbar disc space and to start to create intercorporeal fusion in 23 growing pigs. In four pigs two emptied non adjacent disc spaces were left open. After 3, 6, 12 and 16 weeks the implanted disc blocks were studied radiologically, histologically, histomorphometrically, microradiographically, and with oxytetracycline fluorescence. In plain films slight to moderate ossification of the implanted disc spaces was detected at 12 and 16 weeks. Resorption of the implants was seen radiologically from 3 weeks and fragmentation from 12 weeks onwards. In microradiographs disintegration of the coralline inner structure started at 3 weeks. Histologically, connective tissue ingrowth was seen inside the porous structure from three weeks onwards. Small amounts of new bone were visible and connective tissue inside the implant increased from a mean of 65.6% at 3 weeks to a mean of 79.4% at 16 weeks histomorphometrically. The bone ingrowth varied from 0.7 to 1.7%. A loss of height in the implanted disc spaces was seen (p < 0.05, linear regression analysis). In control pigs the emptied disc spaces lost their height similarly. The implants used were not strong enough to maintain the lumbar disc height.

Polylactide and polyglycolic acid-reinforced coralline hydroxy-apatite for the reconstruction of cranial bone defects in the rabbit.

The possibility of using coralline hydroxy-apatite in combination with polylactide and polyglcycolic acid instead of a bone graft in the skull region is examinated. Coralline hydroxy-apatite blocks strengthened with a membrane made of a combination of polylactide and polyglycolic acid were inserted into bony defects created in 12 rabbit skulls. The blocks were observed during a follow-up of 12 months. They became fixed to the surrounding bone and no adverse effects or harmful reactions in the nearby tissues could be detected.

Holding power of metallic screws in cancellous bone and metallic plates in cortical bone. An experimental study on rabbits using polymethylmethacrylate and bone graft as supporting material.

The holding power of metallic screws in cancellous bone and metallic plates in cortical bone was tested in sixty rabbits (n = 60). These metallic devices were fixed either without support or with the support of plastic material or autogenous bone graft. The fixation firmness was studied radiographically and by a test of the bonding strength. After six weeks, the mean value of the holding power of screws in cancellous bone without support was 440 N, and that of screws supported by polymethylmethacrylate 674 N and by bone graft 540 N. Corresponding values after twelve weeks were 436 N, 664 N, and 616 N. Fixation with polymethylmethacrylate was better than either fixation without support (P < 0.001, t-test) or fixation with bone graft (P < 0.01), whereas fixation with bone graft was better than fixation without support (P < 0.07). The best fixation was achieved by polymethylmethacrylate according to group mean values. Our investigation of the holding power of plates in cortical bone did not reveal any statistically significant differences between the methods used.

Polylactide screws in the fixation of olecranon osteotomies. A mechanical study in sheep.

We studied absorbable self-reinforced poly-L-lactide screws in the fixation of osteotomies in sheep. A left olecranon osteotomy in 10 sheep was fixed with polylactide screws and in an additional 10 sheep with metallic AO cortical screws. Follow-up times were 6 and 12 weeks. 8 polylactide fixations healed and 2 failed. All metal fixations united; one of them had a fracture of the proximal fragment resulting in malposition. After killing the sheep the olecranons were radiographed and the shear strengths of the osteotomies were compared with those of the non-operated contralateral bones. After 6 weeks the mean comparative strength was 74 percent in the polylactide group and 83 percent in the metallic control group. After 12 weeks the corresponding values were 112 and 47 percent (P less than 0.05). Our study demonstrated that the mechanical weakening of fixed bone can be avoided by using absorbable polylactide screws instead of metallic screws. However, polylactide screws are not recommended for use without external support in places of high mechanical strain.