Influence of copolymer composition of polylactide implants on cranial bone regeneration.

Biodegradable polymers have become useful auxiliary materials for the functional and structural restoration of bone deficiencies. Commercial implants from poly(L/DL-lactide) 70:30 are used clinically for fracture fixation in regions of low load. Implants manufactured from poly(L/DL-lactide) 80:20 are currently being investigated experimentally. The higher degree of crystallinity results in a higher chemical strength and loading capacity which promises advantages for long-term implantation. In this study implants from these two copolymers were applied to promote bone regeneration of bilateral, full thickness, circular cranial defects in 16 adult New Zealand white rabbits. The defects were covered with melt extruded and laser cut polylactide burr hole covers epicranially and endocranially in direct contact to the dura. The defect spaces were kept open with a spacer which created a hollow chamber. Both materials were implanted in each animal. Bone seeking fluorochromes were used to assess the pattern of bone growth. After eight weeks bone regeneration in the defects was assessed radiologically, histologically and by fluorescence microscopy. During the eight weeks observation period the application of a hollow chamber design resulted in almost complete cranial defect healing, whereby the copolymer composition had no effect on the amount or the morphology of the regenerate. The dura mater showed no adverse tissue reactions during these early stages of implantation. Eight weeks is only a short period in the lifetime of the tested polymers and complete bone regeneration can only be expected after complete polymer degradation. Long-term studies or accelerated degradation studies are required to confirm the expected advantages of poly(L/DL-lactide) 80:20.

Cranial defect regeneration in a reserved space.

BACKGROUND: Cranial defects exceeding a certain size do not heal spontaneously and require surgical treatment. The prevention of uncontrolled soft-tissue ingrowth is crucial in the bony healing of such defects. METHODS: Bone regeneration of full-thickness cranial defects was assessed in 16 adult New Zealand White rabbits. A single epicranially placed cover was compared with a hollow chamber with an additional barrier on the dural side. After 8 weeks, bone regeneration in the defects was assessed radiologically, histologically, and by fluorescence microscopy. RESULTS: The hollow chamber design resulted in almost complete cranial defect healing. In contrast, five out of 16 defects covered with a single epicranial burr-hole cover showed hardly any visible bone. Inside the reserved space, there was twice as much bone coverage as compared with burr-hole covers only. CONCLUSIONS: Providing a reserved space for bone regeneration reduces the statistical spread significantly and thus makes the clinical outcome more predictable. Use of a hollow chamber can serve as a useful tool to assess the effect of bone-stimulating factors such as growth factors and bone substitutes. Hollow resorbable implants may offer a new approach in bone regeneration by reducing the need for bone autografting and the associated donor-site morbidity.

Effect of perforations in burr hole covers on cranial bone regeneration in rabbits.

The reconstruction of bone defects is a significant problem in cranio-maxillofacial surgery. In an effort to avoid the known disadvantages of autogenous bone grafting, alternatives have been investigated. Bone substitute materials, degradable or nondegradable, aim at facilitating bone regeneration, while they take over load-bearing functions for a period of time. In this study, the healing of cranial defects in rabbits was assessed using polylactide guiding covers with and without perforations. Bilateral circular cranial defects were produced in 16 New Zealand white rabbits. The defects were covered with extruded and laser cut polylactide burr hole covers, each animal receiving a perforated burr hole cover on one side and a nonperforated one on the contralateral side. Bone seeking fluorochromes were administered at regular intervals. After an observation period of 8 weeks the amount of bone regeneration in the area of the defects was quantified from contact radiographs, and the dynamics of bone formation were assessed by fluorescence microscopy. Stained sections were used to analyze morphologic differences. No signs of adverse tissue reactions or osteolysis were observed. A bone-guiding function was observed for both covers with or without perforations. Intracranial tissue herniation into the defect hindered the regeneration process. Wide intraindividual and interindividual variation became apparent and average defect filling was only 40% within the 8-week observation period. In this model the perforated covers offered no advantage over nonperforated covers. It can be concluded from this study, that the use of external burr hole covers alone does not guarantee a full thickness regeneration of the cranial defect, but it provides a guiding function for promotion of structured bone regeneration.