In a first clinical protocol 19 ankle fractures have been fixed by plates and screws made of biodegradable polylactic acid. Fracture confirmation was achieved within 6 weeks, 52 % of our patients demonstrated an aseptic soft tissue problem caused by delayed clearance of the degrading polylactide particles. In a 2nd protocol with 7 patients volume reduced plates and screws with flat heads were applied, None of these patients had any soft tissue reaction. These results emphasize that the application of plates and screws of polylactic acid is acceptable for fixation of ankle fractures. Soft tissue inflammatory reactions can be avoided by using volume reduced implants.

In a first clinical protocol 19 ankle fractures have been fixed by plates and screws made of biodegradable polylactic acid. Fracture confirmation was achieved within 6 weeks. 52% of our patients demonstrated an aseptic soft tissue problem caused by delayed clearance of the degrading polylactide particles. In a 2nd protocol with 7 patients volume reduced plates and screws with flat heads were applied. None of these patients had any soft tissue reaction. These results emphasize that the application of plates and screws of polylactic acid is acceptable for fixation of ankle fractures. Soft tissue inflammatory reactions can be avoided by using volume reduced implants.

The blockpolymerization and injection-moulding procedure are two main techniques used to produce PLLA for medical devices. Until now there has been no comparison of the well known data of blockpolymerized material and data of injection-moulded material. Test rods (2×3×25 mm) were made from block polymerized balls by the injection-moulding procedure. The rods were incubated in tris-buffer-saline for different periods (0, 2, 4, 6, and 8 weeks) at a pH of 7.4 and a temperature of 37°C. The test methods we used were the three-point-bending, the fatigue behaviour and the calculation of the decrease of molecular weight. Both materials under investigation are chemically similar but show very different mechanical properties, in particular the injection moulded material has a very decreased behaviour and a very low loss of molecular weight. Blockpolymerized PLLA rods show a very fast lost bending strength and after 6 weeks a low resistance against cyclic loads in the fatigue behaviour test. Injection moulded PLLA will be used in future for internal fixation of fractures.

Cylindric titanium rods with different surfaces were axially implanted into the femora of sheep. The three surfaces were grit-blasted titanium, plasma-sprayed titanium and plasma-sprayed hydroxyapatite (HA). After 2 months, a 2-cm segment of the femoral shaft was completely resected to load the implant, and the animals were allowed full weight-bearing for 9 months. Biomechanical and histological evaluation of the implants was undertaken 2 months after implantation and 9 months after the segmental resection. The mechanical testings of well-fixed implants were performed 9 months after segmental resection. Loosening of 45% of the titanium-coated implants was observed in the first 3 weeks, but thereafter, no further loosening occurred. The HA-coated implants remained entirely fixed for 3 weeks, but thereafter, a progressively increasing incidence of loosening up to 55% after 9 months of loading was detected as subsidence on X-radiographs. The maximum push-out strength of the titanium-coated implants was 4.9 MPa compared with 2.3 MPa for HA-coated ones. No significant mechanical interlock between the grit-blasted surface and bone was observed. The HA coating was found to be delaminated in all unstable implants, whereas the titanium coating remained completely intact. Morphometric analyses of well-fixed rods showed complete bony ingrowth onto the HA surface, whereas the contact area between the bone and the two titanium surfaces was less than 40%. Concerning clinical significance bony ingrowth with long-term mechanical interlock between the implant surface and the bone cannot be achieved by grit-blasting or HA-coating. The titanium plasma-coating, however, can induce a bone-implant interface which resists the mechanical stress resulting from continuous cyclic loading in vivo.

The removal torque and the loosening and infection rates of AO/ASIF 4.5 mm Schanz screws (MS) have been compared with those of hydroxyapatite-coated (HA) and hydroxyapatite/silver phosphate-coated (HA/P) AO/ASIF Schanz screws in vivo. In sheep, the left tibia was fixed after a midshaft osteotomy by means of unilateral fixator with 6 Schanz screws in an antero-lateral position. The osteotomy gap was filled with bone wax to avoid bony overgrowth. After operation, the animals were allowed to walk around immediately. The loosening rate was 2.8% at the site of the two HA-coated screws and 36% at the site of the metal screws. The removal torque after 2, 4 and 6 months was 6.2 +/- 2.8 MPa for HA/P, 4.7 +/- 2.7 MPa for HA screws and 1.2 +/- 1.1 MPa for metal screws. Histological and fluoroscopic evaluation revealed intimate bone-HA contact at the cortex near to the fixator, whereas at the site of the metal screws a fibrous tissue layer was constantly found. Only in the cortex opposite to the external fixator a direct bone-metal interface was observed. Fluoroscopic analyses confirmed new bone formation next to the HA surface especially in the cortex adjacent to the fixator, whereas such phenomena were not detected adjacent to the surface of the metal screws. Furthermore, bony resorption around the metal screws (21%) was significantly greater than around the two coated screws (2%). These experimental data suggest that pin loosening can be reduced in external fixators by coating the screws with hydroxyapatite, especially if a long-term fixation is indicated.

The removal torque and the loosening and infection rates of AO/ASIF 4.5 mm Schanz screws (MS) have been compared with those of hydroxyapatite-coated (HA) and hydroxyapatite/silver phosphate-coated (HA/P) AO/ASIF Schanz screws in vivo. In sheep, the left tibia was fixed after a midshaft osteotomy by means of unilateral fixator with 6 Schanz screws in an antero-lateral position. The osteotomy gap was filled with bone wax to avoid bony overgrowth. After operation, the animals were allowed to walk around immediately. The loosening rate was 2.8% at the site of the two HA-coated screws and 36% at the site of the metal screws. The removal torque after 2, 4 and 6 months was 6.2 +/- 2.8 MPa for HA/P, 4.7 +/- 2.7 MPa for HA screws and 1.2 +/- 1.1 MPa for metal screws. Histological and fluoroscopic evaluation revealed intimate bone-HA contact at the cortex near to the fixator, whereas at the site of the metal screws a fibrous tissue layer was constantly found. Only in the cortex opposite to the external fixator a direct bone-metal interface was observed. Fluoroscopic analyses confirmed new bone formation next to the HA surface especially in the cortex adjacent to the fixator, whereas such phenomena were not detected adjacent to the surface of the metal screws. Furthermore, bony resorption around the metal screws (21%) was significantly greater than around the two coated screws (2%). These experimental data suggest that pin loosening can be reduced in external fixators by coating the screws with hydroxyapatite, especially if a long-term fixation is indicated.

Biodegradable poly-L-lactic acid rods made of block-polymerized material (BP; molecular weight 550,000) and injection-moulded rods with high (SGI; molecular weight 121,000) and low (SGA, molecular weight 118,000) molecular orientation were compared 2, 4 and 6 weeks after incubation in enzyme solutions with high hydrolytic activity (esterase, alpha chymotrypsin and peptidase) and in buffer solution (TRIS buffer). The molecular weight, modulus of elasticity, bending strength (three-point bending test), and cyclic bending load to failure applied to the rods in a newly developed testing machine (1 Hz, maximum 100,000 cycles) were compared. The molecular weight of BP material decreased to 36% after 2 weeks, in contrast to the injection-moulded materials, in which it decreased only to 66% even after 6 weeks. The bending strength of all specimen decreased significantly faster in alpha chymotrypsin than in the other media (MANOVA, P < 0.001). SGI had a significantly higher bending strength than SGA, and SGA a higher strength than BP. There was no difference after incubation in the other two enzymatic solutions. BP lost 80% of its initial bending strength (140 N/mm2) after 6 weeks, and SGI and SGA (120 N/mm2) only 20%. Under permanent cyclic loading BP initially resisted 100,000 cycles with an applied cyclic load of 12.5 N/mm2, decreasing after 6 weeks to only 9,500 cycles, in contrast to SGI and SGA, which resisted to 46,000 cycles. There was, however, no relevant difference in the mechanical characteristics of the two injection-moulded rods. These results confirm that BP is degraded significantly faster than SGA and SGI.(ABSTRACT TRUNCATED AT 250 WORDS)

Bonding strength of hydroxyapatite-coated (HA) 4.5 mm AO/ASIF screws and of conventional AO/ASIF screws to bone has been compared in an animal experiment. A 5-hole AO/ASIF compression plate was fixed to the tibia in ten beagle dogs. The cortex beyond the central hole of the plate was resected up to the half of the diameter of the tibial shaft and over a length of 10 mm. In the holes immediately proximal and distal to the central hole either HA-coated or conventional screws were inserted with standardized force. Two conventional screws were inserted into the two holes at the ends of the plate. These screws were removed after 2 weeks. Four animals were sacrificed after 4 and 8 weeks and one animal after 16 and 26 weeks. The removal torque of HA-coated screws was 385.50 +/- 49.1 Ncm, compared with 29.50 +/- 7.62 Ncm for the conventional screws (P < 0.0001; T-test). The fluorescence and morphometric data confirmed new bone formation immediately under the HA surface without any bone resorption or intervening fibrous tissue. At the bone-implant interface of conventional screws, however, extensive bone resorption was observed and a thick intervening layer of fibrous tissue. Bone resorption of 13.6% was observed in the cortical areas under the thread of the AO/ASIF screws, in contrast to 1.9% under the surface of the HA-coated screws. These histological and biochemical data support the clinical use of HA-coated screws if early loosening of the screws is otherwise expected or removal of implants is not intended.

Bonding strength of hydroxyapatite-coated (HA) 4.5 mm AO/ASIF screws and of conventional AO/ASIF screws to bone has been compared in an animal experiment. A 5-hole AO/ASIF compression plate was fixed to the tibia in ten beagle dogs. The cortex beyond the central hole of the plate was resected up to the half of the diameter of the tibial shaft and over a length of 10 mm. In the holes immediately proximal and distal to the central hole either HA-coated or conventional screws were inserted with standardized force. Two conventional screws were inserted into the two holes at the ends of the plate. These screws were removed after 2 weeks. Four animals were sacrificed after 4 and 8 weeks and one animal after 16 and 26 weeks. The removal torque of HA-coated screws was 385.50+/-49.1 Ncm, compared with 29.50+/-7.62 Ncm for the conventional screws (P < 0.0001; T-test). The fluorescence and morphometric data confirmed new bone formation immediately under the HA surface without any bone resorption or intervening fibrous tissue. At the bone-implant interface of conventional screws, however, extensive bone resorption was observed and a thick intervening layer of fibrous tissue. Bone resorption of 13.6% was observed in the cortical areas under the thread of the AO/ASIF screws, in contrast to 1.9% under the surface of the HA-coated screws. These histological and biochemical data support the clinical use of HA-coated screws if early loosening of the screws is otherwise expected or removal of implants is not intended.

Each of 36 mongrel dogs received implants of the following six materials: methyl methacrylate, carbon fibre composite, aluminium oxide-ceramic, titanium alloy, pure titanium and hydroxylapatite (HA). Test rods 7 mm in diameter were used to perform a push-out test, and discs with a diameter of 25 mm and a thickness of 5 mm were used for a plane traction test. The animals got a fluorochrome labelling and were harvested 8 weeks after operation. The results of the push-out test were a stability of 3.6 N/mm2 for HA and very low values for all the other materials, while the plane tension test showed a stability of 1.6 N/mm2 for the HA discs and values that were not even measurable for all the other materials. The interface of HA and the bone could not be separated by mechanical forces; usually the bone or the ceramic material fractured beneath the interface, so that the real stability of the interface must be rated very high. Light and fluorescence microscopy revealed a close bone connection for HA and only partial connections for the other materials. HA-coated materials may have a future in orthopaedics surgery.