Porous hydroxyapatite granules for alloplastic enhancement of the facial region

Hydroxyapatite is a biocompatible alloplast with the same chemical composition as bone. It is readily incorporated into host bone, does not undergo appreciable resorption, does not incite a clinically significant foreign body reaction, and resists infection. This article describes forms of hydroxyapatite, procedures for use, and clinical examples.     

Chin augmentation with porous hydroxyapatite blocks

The results of chin augmentation by porous hydroxyapatite blocks were evaluated clinically and radiographically in nine patients. Recovery of a satisfactory facial profile was achieved by the procedure, and all patients but one, with removal of the implant because of infection, were satisfied with the results of the surgery. Radiographically, the implants were incorporated into the bony structure of the mandible by deposition of bone at and around the bone-implant interface and ingrowth of bone into pores. Cephalometrically, the procedure was found to provide very stable results, with little change in the position of the implants and no appreciable resorption of the implants and bone. Despite heavy chemotherapy, histologic examination of the removed hydroxyapatite showed the presence of areas with numerous inflammatory cell infiltration and no bone formation. Thus, the procedure is quite useful for chin augmentation, but utmost care should be taken to avoid infection.     

The use of coralline hydroxyapatite with bone marrow, autogenous bone graft, or osteoinductive bone protein extract for posterolateral lumbar spine fusion

STUDY DESIGN: A posterolateral lumbar arthrodesis animal model using coralline hydroxyapatite as a bone graft substitute. OBJECTIVE: To determine the effectiveness of coralline hydroxyapatite as a bone graft substitute for lumbar spine fusion when used with bone marrow, autogenous bone graft, or an osteoinductive bone protein extract. SUMMARY OF BACKGROUND DATA: Coralline hydroxyapatite is commonly used as a bone graft substitute in metaphysial defects but its use in a more challenging healing environment such as the posterolateral spine remains controversial. There are no published animal studies in which the use of coralline hydroxyapatite has been evaluated in a posterolateral lumbar arthrodesis model. METHODS: Single-level posterolateral lumbar arthrodesis was performed at L5-L6 in 48 adult New Zealand White rabbits. Rabbits were assigned to one of three groups based on the graft material they received: 3.0 mL coralline hydroxyapatite 1.5 mL plus bone marrow; 1.5 mL coralline hydroxyapatite plus 1.5 mL autogenous iliac crest bone; and, 3.0 mL coralline hydroxyapatite plus 500 micrograms bovine-derived osteoinductive bone protein extract on each side. Rabbits were killed after 2, 5, or 10 weeks, and the spines were excised and evaluated by manual palpation, radiographs, tensile biomechanical testing, and nondecalcified histology. RESULTS: Fusions were assessed by manual palpation at 5 weeks for comparisons among the three groups of graft materials. The coralline hydroxyapatite used with bone marrow produced no solid fusions (0/14). When combined with an equal amount of autogenous iliac crest bone, coralline hydroxyapatite resulted in solid fusion in 50% (7/14) of the rabbits (P < 0.05). When combined with the osteoinductive growth factor extract, the coralline hydroxyapatite resulted in solid fusion in 100% (11/11) of the rabbits (P < 0.05). The fusion masses in the growth factor group were significantly stronger (1.8 +/- 0.2 vs. 1.3 +/- 0.1; P = 0.02) and stiffer (1.5 +/- 0.2 vs. 1.2 +/- 0.1, P = 0.04) based on tensile testing to failure when normalized to the adjacent unfused level. CONCLUSION: These data indicate that coralline hydroxyapatite with bone marrow was not an acceptable bone graft substitute for posterolateral spine fusion. When combined with autogenous iliac crest bone graft-coralline hydroxyapatite served as a graft extender yielding results comparable to those obtained with autograft alone. Coralline hydroxyapatite served as an excellent carrier for the bovine osteoinductive bone protein extract yielding superior results to those obtained with autograft or bone marrow.     

The efficacy of interconnected porous hydroxyapatite in achieving posterolateral lumbar fusion in sheep

STUDY DESIGN: An animal study was performed to evaluate lumbar spinal fusion radiologically and mechanically. OBJECTIVES: To assess the efficacy of interconnected porous hydroxyapatite in achieving posterolateral lumbar arthrodesis in sheep. SUMMARY OF BACKGROUND DATA: Posterolateral spinal arthrodesis with autologous bone graft is the gold standard procedure for lumbar fusion. The procedure for harvesting bone from the iliac crest increases morbidity. Interconnected porous hydroxyapatite has been used effectively as an alternative to cancellous bone graft material in metaphyseal bone defects. Little is known about the efficacy of interconnected porous hydroxyapatite in achieving lumbar spinal fusion. METHODS: Four groups of seven sheep underwent bisegmental posterolateral lumbar fusion with instrumentation using different intertransverse graft material. In group 1, no graft material was used. In group 2, autologous bone was used. Group 3 had interconnected porous hydroxyapatite. Group 4 had an equip of interconnected porous hydroxyapatite and autologous bone. The animals were killed at 20 weeks after surgery. Radiographs and computed tomography images were obtained. The fusion masses were graded for bone resorption and trabecular connectivity on the computed tomography images. Mechanical testing of the specimens was performed, and the three-dimensional segmental motion was measured in flexion/extension, axial rotation, and lateral bending. RESULTS: The radiographic images were difficult to interpret because of the radiodense interconnected porous hydroxyapatite granules. According to mechanical stability criteria, the fusion rate for the different groups was as follows: 100% (14/14) for the autologous bone group, 72% (10/14) for the bone/interconnected porous hydroxyapatite group, 50% (7/14) for the pure interconnected porous hydroxyapatite group, and 15% (2/14) for the sham group. CONCLUSIONS: Spinal arthrodesis using interconnected porous hydroxyapatite alone or mixed with bone as graft material reduced segmental motion. It was not, however, as effective as autologous bone graft material in achieving spinal arthrodesis. The sheep model using autologous bone achieved a 100% fusion rate. Because the nonunion rate for a single level in humans may be as high as 40%, the fusion rate with bone/interconnected porous hydroxyapatite in humans may be lower than the 72% found in the sheep model. The little resorption of the radiodense interconnected porous hydroxyapatite granules made the radiologic evaluation of the fusion masses difficult.     

Osseous integration of bovine hydroxyapatite ceramic in metaphyseal bone defects of the distal radius

Hydroxyapatite ceramic made of bovine spongiosa was used as structural support material in a prospective study to correct bone defects experienced after reduction in distal radius fractures. The study took place over a 3-year period (1992-1999) and comprised 14 patients. Osseous integration was analyzed via biopsies and magnetic resonance imaging. Long-term follow-up monitoring involving magnetic resonance imaging in 13 of the 14 patients showed fibrovascular growth within incorporated hydroxyapatite material. Osseous integration was demonstrated in magnetic resonance images by gadolinium uptake and by the presence of osteoid layers and endothelialized vessels. Hydroxyapatite ceramic offers a biologically acceptable alternative to autologous bone when augmenting distal radius fracture fixation.     

The morphogenesis of bone in replicas of porous hydroxyapatite obtained from conversion of calcium carbonate exoskeletons of coral.

The morphogenesis of bone in a porous hydroxyapatite substratum was studied after intramuscular implantation in adult primates. Replicas of porous hydroxyapatite that had been obtained after hydrothermal conversion of the calcium carbonate exoskeleton of coral (genus Goniopora) were implanted intramuscularly in twenty-four adult male baboons (Papio ursinus). Serial sections from specimens that had been harvested at three, six, and nine months showed that initially the formation of fibrous connective tissue was characterized by a prominent vascular component and by condensations of collagen fibers assembled at the interface of the hydroxyapatite. The morphogenesis of bone was intimately associated with the differentiation of the connective-tissue condensations. Bone formed without an intervening endochondral phase. Although the amount of bone varied considerably, in several specimens extensive bone developed, filling large portions of the porous spaces and culminating in total penetration by bone within the implants. The mean volume fraction composition of the specimens was 20.8 +/- 1.0 per cent (mean and standard error) for bone, 17.3 +/- 1.7 per cent for connective-tissue condensation, 31.9 +/- 1.0 per cent for fibrovascular tissue, 6.4 +/- 0.6 per cent for bone marrow, and 34.6 +/- 0.5 per cent for the hydroxyapatite framework. The amount of bone and marrow increased at each time-period, and the hydroxyapatite framework was significantly reduced between six and nine months. This indicated a moderate biodegradation over time, which was possibly a result of incomplete conversion of carbonate to hydroxyapatite. Linear regression analysis showed a negative correlation between the hydroxyapatite framework and the magnitude of bone formation within the porosities of the hydroxyapatite (p = 0.0001). Biochemical coating of the hydroxyapatite substratum with an allogeneic fibrin-fibronectin protein concentrate prepared from baboon plasma did not significantly increase the amount of bone formation within the porous spaces. The hydroxyapatite substratum may have functioned as a solid-phase domain for anchorage of bone morphogenetic proteins.     

Osteogenesis by recombinant human bone morphogenetic protein-2 at skeletal sites

Osteogenesis was evaluated in the mandibular bone by combinations of various dosages of recombinant human bone morphogenetic protein-2, atelopeptide Type I collagen, and porous hydroxyapatite (four groups: Group I, 2 micrograms recombinant human bone morphogenetic protein-2, atelopeptide Type I collagen, and porous hydroxyapatite; Group II, 10 micrograms recombinant human bone morphogenetic protein-2, atelopeptide Type I collagen, and porous hydroxyapatite; Group III, 50 micrograms recombinant human bone morphogenetic protein-2, atelopeptide Type I collagen, and porous hydroxyapatite; Control Group, only atelopeptide Type I collagen and porous hydroxyapatite). The prepared materials were implanted in the mandibular bone hole (7 mm in diameter, 2 mm deep). Three weeks later, the alkaline phosphatase activity in the implanted region was determined, and the histologic features of the excised tissue were examined. There were significant differences in histologic and biochemical findings among the four groups. In the recombinant human bone morphogenetic protein-2 implanted groups, osteogenesis increased with the dosage of recombinant human bone morphogenetic protein-2, as assessed by alkaline phosphatase activity and histologic findings. The results suggest that atelopeptide Type I collagen is an effective carrier for recombinant human bone morphogenetic protein-2 and that porous hydroxyapatite would be advantageous for clinical application as a material to maintain its original form after implantation.     

Anterior interbody fusion of the cervical spine with coralline hydroxyapatite.

STUDY DESIGN: A nonrandomized, retrospective human study of patients requiring anterior discectomy and reconstruction from C3 to T1. The pattern of incorporation, presence or absence of disc space collapse, maintenance of correction, and clinical outcomes were considered. OBJECTIVE: To determine the efficacy of coralline hydroxyapatite as a bone replacement in anterior interbody fusions of the cervical spine used in conjunction with rigid plate fixation. SUMMARY OF THE BACKGROUND DATA: Autograft is the gold standard for anterior interbody fusion of the cervical spine. Reported complication and morbidity rates with the use of autograft are as high as 21%. Using allograft instead of autograft presents numerous problems including lower rates of fusion. Other bone substitutes such as ceramics and polymethylmethacrylate are ineffective for fusion. METHODS: Twenty-six skeletally mature patients underwent anterior decompression, stabilization, microdiscectomy, and reconstruction with Pro Osteon 200 (Interpore Cross International, Irvine, CA) coralline hydroxyapatite and AO anterior cervical locking plates. Iliac crest autograft, local bone, and allograft were not used. RESULTS: The minimum follow-up period was 2 years (average, 30 months). There was no evidence of plate breakage, screw breakage, resorption of the implant, or pseudarthrosis. Two patterns of incorporation were identified. The implant incorporated totally in 100% of the disc spaces. Average hospital stay was 1.6 days. The average decrease in pain was 75.8%. There was no evidence of nonunion. CONCLUSIONS: The use of Pro Osteon 200 with rigid anterior plating seems promising as a bone replacement in the cervical spine. The incorporation rate is exceedingly high, and the complication rate nonexistent.     

The use of hydroxyapatite blocks for innominate osteotomy: a report of three cases

We evaluated the use of hydroxyapatite blocks as spacers in pelvic osteotomy. The hydroxyapatite blocks fused with autologous bone tissue within 10 months and were highly biocompatible even after 10 years. They prevent pelvic deformities and are as effective as autogenous bone grafts.     

Osseointegration of hydroxyapatite porous-coated femoral implants in a canine model.

This study evaluated the quality and quantity of osseointegration of two thicknesses of hydroxyapatite coating on press-fit, porous-coated titanium implants in a canine hip model. In 12 dogs, titanium press-fit porous-coated prostheses were implanted. The stems had a 50-microm thickness hydroxyapatite coating, 100-microm thickness hydroxyapatite coating, or no hydroxyapatite coating. The animals were randomized into one of three groups and received one of the three implants. The implants were retrieved and examined 4 months after implantation. Direct juxtaposition of bone to the surface of the hydroxyapatite-coated implants with no intervening fibrous tissue layer was observed. There was no histologic evidence that hydroxyapatite initiated any foreign body reaction, nor was there any irregularity or resorption of the hydroxyapatite coating. There was a statistically significant greater degree of total bone apposition and bone ingrowth in the implants coated with hydroxyapatite at the level of the isthmus and the calcar. No statistical difference was found between the two groups with hydroxyapatite coatings in the degree of bone ingrowth or bone apposition.     

Use of hydroxyapatite to fill cavities after excision of benign bone tumours. Clinical results

We treated 75 patients with benign bone tumours by curettage and filling the defect with calcium hydroxyapatite (HA). There were 28 women and 47 men with a mean age of 27.7 years (3 to 80). The mean follow-up was for 41.3 months. Postoperative radiological assessment revealed that the implanted HA was well incorporated into the surrounding host bone in all patients. Two patients suffered fractures in the postoperative period. Two patients complained of pain associated with HA in the soft tissues, but this diminished within six months. No patient had local pain at the final follow-up. Recurrence of the tumour was seen in three cases. Histopathological study of the implanted area showed removal of the HA by histiocytes and multinucleated giant cells, and the formation of much appositional bone. We conclude that HA is an excellent bone-graft substitute in surgery for benign bone tumours.     

Healing of segmental bone defects with granular porous hydroxyapatite augmented with recombinant human osteogenic protein-1 or autologous bone marrow.

Hydroxyapatite is a synthetic bone graft, which is used for the treatment of bone defects and nonunions. However, it is a rather inert material with no or little intrinsic osteoinductive activity. Recombinant human osteogenic protein-1 (rhOP-1) is a very potent biological agent, that enhances osteogenesis during bone repair. Bone marrow contains mesenchymal stem cells, which are capable of new bone formation. Biosynthetic bone grafts were created by the addition of rhOP-1 or bone marrow to granular porous hydroxyapatite. The performance of these grafts was tested in a sheep model and compared to the results of autograft, which is clinically the standard treatment of bone defects and nonunions. A 3 cm segmental bone defect was made in the tibia and fixed with an interlocking intramedullary nail. There were five treatment groups: no implant (n=6), autograft (n=8), hydroxyapatite alone (n=8), hydroxyapatite loaded with rhOP-1 (n=8), and hydroxyapatite loaded with autologous bone marrow (n=8). At 12 weeks, healing of the defect was evaluated with radiographs, a torsional test to failure, and histological examination of longitudinal sections through the defect. Torsional strength and stiffness of the healing tibiae were about two to three times higher for autograft and hydroxyapatite plus rhOP-1 or bone marrow compared to hydroxyapatite alone and empty defects. The mean values of both combination groups were comparable to those of autograft. There were more unions in defects with hydroxyapatite plus rhOP-1 than in defects with hydroxyapatite alone. Although the differences were not significant, histological examination revealed that there was more often bony bridging of the defect in both combination groups and the autograft group than in the group with hydroxyapatite alone. Healing of bone defects, treated with porous hydroxyapatite, can be enhanced by the addition of rhOP-1 or autologous bone marrow. The results of these composite biosynthetic grafts are equivalent to those of autograft.     

In vivo behaviour of low-temperature calcium-deficient hydroxyapatite: comparison with deproteinised bovine bone

This study aims to evaluate in detail the biological osteoconductive properties of the low-temperature synthetic porous calcium-deficient hydroxyapatite and to compare it with the biological apatite. Bone reactions to granules of similar sizes of the low-temperature hydroxyapatite and commercially available non-sintered deproteinized bovine bone were compared. Two different temperatures were used to fabricate two batches of newly developed porous hydroxyapatite with different carbonate groups content and specific surface area. The histological analysis of specimens with histomorphometry was performed at different time after in vivo implantation. Based on histological analysis, the level of bone formation in the spaces between the implanted granules and through the interconnected pores of all implanted materials within a cortical region (bone area ingrowth 72–85 %) was several-fold higher than within a cancellous bone site (bone area ingrowth 16–28 %) at three and six months after implantation. Within the cancellous bone site, bone coverage of the implanted material at six months was significantly higher in hydroxyapatite material fabricated using low-temperature synthesis and subsequent processing at 150°C than in hydroxyapatite scaffold developed using low-temperature synthesis with subsequent processing at 700°C or deproteinized bovine bone. According to our study, the bioactive properties of the low-temperature calcium-deficient hydroxyapatite are comparable with the biological apatite. The favourable influence of a high specific surface area of a low-temperature calcium-deficient hydroxyapatite on in vivo bone formation was emphasized.     

Strontium Localization in Bone Tissue Studied by X-Ray Absorption Spectroscopy

Strontium has recently been introduced as a pharmacological agent for the treatment and prevention of osteoporosis. We determined the localization of strontium incorporated into bone matrix from dogs treated with Sr malonate by X-ray absorption spectroscopy. A new approach for analyzing the X-ray absorption spectra resulted in a compositional model and allowed the relative distribution of strontium in the different bone components to be estimated. Approximately 35–45 % of the strontium present is incorporated into calcium hydroxyapatite (CaHA) by substitution of some of the calcium ions occupying highly ordered sites, and at least 30 % is located at less ordered sites where only the first solvation shell is resolved, suggesting that strontium is surrounded by only oxygen atoms similar to Sr²⁺ in solution. Strontium was furthermore shown to be absorbed in collagen in which it obtains a higher structural order than when present in serum but less order than when it is incorporated into CaHA. The total amount of strontium in the samples was determined by inductively coupled plasma mass spectrometry, and the amount of Sr was found to increase with increasing dose levels and treatment periods, whereas the relative distribution of strontium among the different components appears to be independent of treatment period and dose level.     

羟基磷灰石微粒人工骨修复皮样囊肿切除后额骨凹陷畸形

目的探讨羟基磷灰石微粒人工骨修复额部皮样囊肿切除后骨骼凹陷畸形的疗效及并发症。方法2000年2月-2005年5月，采用羟基磷灰石微粒人工骨修复13例额部皮样囊肿切除后的额骨凹陷畸形患者。男9例，女4例，年龄17～41岁。囊肿均在婴、幼儿期出现，囊肿大小6cm×4cm～10cm×8cm。设计发际缘或囊肿周缘的切口，完整切除皮样囊肿，并去除额骨凹陷处表面的骨膜等软组织后，以适量的羟基磷灰石微粒人工骨充填修复额骨凹陷。于术后1周，1、6个月进行临床和X线片检查。结果术后患者切口均Ⅰ期愈合，无血肿、血清肿、感染及羟基磷灰石颗粒人工骨移位等并发症发生。均获随访1～20个月，额骨凹陷完全修复，局部皮肤平滑，外观无明显凹陷；局部检查和X线片示，植入的羟基磷灰石颗粒人工骨与周围骨质结合紧密，无明显缝隙，人工骨未见移位。结论羟基磷灰石微粒人工骨充填修复皮样囊肿切除后的额骨凹陷是一种简单易行和较理想的方法。     

Porous ceramics as bone graft substitutes in long bone defects: A biomechanical,histological, and radiographic analysis

Three porous ceramic bone graft materials were compared with regard to their ability to heal a 2.5 cm defect created surgically in a bilateral canine radius model. The ceramic materials were analyzed at 12 and 24 weeks after surgery and included tricalcium phosphate, hydroxyapatite, and collagen hydroxyapatite, which contained a mixture of 35% tricalcium phosphate and 65% hydroxyapatite with added collagen. Each material was evaluated alone and with added bone marrow aspirate. All the implants were compared with a graft of autogenous cancellous bone in the contralateral radius. Biomechanical testing and radiographic evauation revealed that the addition of bone marrow aspirate was essential for tricalcium phosphate and hydroxyapatite to achieve results comparable with those of cancellous bone. Collagen hydroxyapatite performed well without the addition of bone marrow, although the addition of marrow did have a positive effect. Further qualitative radiographic and histological analysis demonstrated that tricalcium phosphate was the only ceramic that showed any sign of degradation at 24 weeks. This observed degradation proved to be an important factor in evaluating radiographs because the radiodensity of collagen hydroxyapatite and hydroxyapatite interfered with the determination of radiographic union. At 24 weeks, tricalcium phosphate with bone marrow was the material that performed most like cancellous bone. In this study, the biomechanical and radiographic parameters of tricalcium phosphate with bone marrow were roughly comparable with those of cancellous bone at 12 and 24 weeks. Tricalcium phosphate was the only implant that showed significant evidence of degradation at 24 weeks by both histological and radiographic evaluations, and this degradation took place only after a degree of mechanical competence necessary for weight-bearing was achieved.     

羟基磷灰石晶体纤维增强磷酸钙骨水泥力学性能及生物相容性研究

目的将高长径比的羟基磷灰石(HA)晶体纤维添加入磷酸钙骨水泥(CPC)中,研究HA晶体纤维对CPC抗压力学性能增强的最佳配方及在体内的生物相容性。方法采用水热合成法制备高长径比HA晶体纤维,并验证其细胞毒安全性。将2.5%、5%及10%(wt%)HA晶体纤维添加入CPC中,分析并获取HA晶体纤维对CPC力学性能增强的最佳配方。使用最佳配方制备CPC+HA晶体纤维复合材料,植入大鼠胫骨近端骨缺损模型,4、8周组织学观察HA晶体纤维添加入CPC的成骨性能及生物相容性。对照组为单纯CPC实验组。结果成功制备高长径比HA晶体纤维,其细胞毒安全性为1级(RGR79%)。与对照组相比,2.5%及5%HA晶体纤维添加入CPC可以增强材料的抗压力学性能(P<0.05),其中5%HA晶体纤维对CPC抗压性能的增强效果最佳,10%HA晶体纤维略降低CPC的抗压性能,但无统计学差异(P>0.05)。大鼠体内研究结果显示:与对照组相比,5%HA晶体纤维添加入CPC复合材料植入后材料周围骨体积分数(BV/TV)无显著性差异(P>0.05),HA晶体纤维添加入CPC后材料周围成骨与材料接触好,生物相容性良好。结论水热合成法制备的HA晶体纤维作为添加剂,具有增强CPC抗压力学性能,减低CPC脆性,生物相容性良好的特点,可用作骨植入生物材料的添加剂。     

Comparison of Hydroxyapatite Granules to Autogenous Bone Graft in Fusion Cages in a Goat Model

Background

With the increased use of fusion cages to achieve lumbar intervertebral fusion, the question arises as to the potential for bone ingrowth from the host bone through the entire cage. Is it even necessary to have an autogenous graft to achieve total bone incorporation?

Methods

Nine adult male goats had fusion cages implanted into three vertebral bodies. The design was Surgical Dynamics/Ray Fusion Cage, measuring 21 mm × 14 mm. In each animal, one fusion cage was filled with autogenous graft, one with hydroxyapatite, porous granules, and the other with nonporous granules. Amount of new bone formation was determined by backscatter electron microscopy at 3 months post implantation in all animals.

Results

The histologic section shows that there was total incorporation in all specimens at 3 months. There was slightly more new bone (43%) with the nonporous granules compared with the porous granules (35%). The amount of residual void space was about the same in all specimens, indicating that the amount of new bone formation was similar and not statistically different in cages filled with hydroxyapatite granules versus granules of autogenous bone.

Conclusion

This study confirms that total incorporation by ingrowth of new bone can be expected in fusion cages. The amount of ingrowth is about the same for autogenous graft versus hydroxyapatite granules. Apparently, it is not necessary to use bone graft to achieve successful bone incorporation if an acceptable biocompatable lattice, such as hydroxyapatite granules, is used.     

In vivo histological changes occurring in hydroxyapatite cranial reconstruction--case report

Histological changes were observed in a hydroxyapatite plate and hydroxyapatite granules used to repair a craniotomy defect and removed after 2 years and 9 months of use. The hydroxyapatite plates and granules had completely fused to the cranium, with new bone formation on the dural side extending in a three-dimensional matrix along the pores with the Haversian system in the center. New bone formation was less extensive under the artificial dura than under normal dura. This finding suggests that the dura has the ability to promote bone formation. A new vessel was found along the interconnecting pores. The interconnecting pores allow osteoconduction in the hydroxyapatite plate, so new bone formation can progress. Hydroxyapatite has osteoconduction properties and is biocompatible, so gains strength in vivo through new bone formation, and is the ideal material for artificial bones. Factors important to achieving good bone formation after cranial reconstruction surgery include presence of the dura, and pore size approximate to the Haversian system (100-500 microns) and interconnecting pores in the hydroxyapatite plate.