Full-thickness Calvarial Defects Research Articles

Is Polymethylmethacrylate Reliable and Practical in Full-Thickness Cranial Defect Reconstructions?

The study aimed to evaluate the success of polymethylmethacrylate cranioplasty combined with various soft tissue coverage techniques in repairing full-thickness cranial defects, to compare our results with similar studies published before, and to describe a simple method of implant premolding. A total of 17 patients who had cranial defects due to various etiologies underwent polymethylmethacrylate cranioplasty. In 10 patients, the implant premolding method was applied. The soft tissue coverage was obtained by primary closure, local flaps, or free flaps. The follow-up period ranged from 36 hours to 5 years. Only 1 implant became exposed among the 17 patients. One patient died 36 hours after the surgery because of myocardial infarction. The remaining 15 patients had no early or late postoperative complications. Good contour restoration and stable reconstruction of the calvarial defects were realized, and a successful combination of various soft tissue coverage techniques was achieved. We concluded that polymethylmethacrylate was a cheap and durable material, useful in full-thickness calvarial defect reconstructions. It can be combined with any soft tissue coverage techniques except skin grafting. In most cases, the simple premolding methods are useful for both defect matching and preventing tissue damage due to exothermic reaction of the material.

Dermal substitutes do well on dura: Comparison of split skin grafting +/− artificial dermis for reconstruction of full-thickness calvarial defects

Large, full-thickness calvarial defects present a series of significant reconstructive challenges involving a range of techniques, including local and free flaps. Occasionally these conventional methods may not be possible due to technical, or patient, factors. Artificial dermis is already widely used in burns surgery and is increasing in oncological reconstruction. We believe that artificial dermis coupled with split-thickness skin grafting provides an excellent option for closure of these defects when other techniques are not appropriate.

An experimental model for the study of craniofacial deformities

To develop an experimental surgical model in rats for the study of craniofacial abnormalities. Full thickness calvarial defects with 10x10-mm and 5x8-mm dimensions were created in 40 male NIS Wistar rats, body weight ranging from 320 to 420 g. The animals were equally divided into two groups. The periosteum was removed and dura mater was left intact. Animals were killed at 8 and 16 weeks postoperatively and cranial tissue samples were taken from the defects for histological analysis. Cranial defects remained open even after 16 weeks postoperatively. The experimental model with 5x8-mm defects in the parietal region with the removal of the periosteum and maintenance of the integrity of the dura mater are critical and might be used for the study of cranial bone defects in craniofacial abnormalities.

Trifocal Biplanar Distraction-Compression Osteosynthesis

Surgical treatment of calvarial defects and deformities has some drawbacks such as donor site morbidity or use of allogenic materials. In this experimental study, we presented a three-dimensional reconstruction of cranial defects by using trifocal biplanar distraction-compression osteosynthesis (TBDCO).Eight sheep with a mean age of 2 years were divided into 2 groups as the control (n = 3) and distraction groups (n = 5). All the animals were operated on under general anesthesia, and 80 x 50-mm full-thickness calvarial defects were created. In the control group, no treatment was performed. In the distraction group, TBDCO was performed with a custom-made distraction device with 2 transport segments. After 5 days of latency period, the anterior transport segment was distracted to a posterior and superior direction (0.7 and 0.3 mm, respectively) and the posterior transport segment was distracted obliquely to an anterior and superior direction (0.7 and 0.3 mm, respectively) with a resulting total vectorial movement of 0.75 mm/d. Active distraction was approximately 29 days, and the consolidation period was 60 days.No major complications were seen during the whole procedure. In the control group, the bone defects remained unhealed. In the distraction group, defects healed and the reconstructed calvarium was dome shaped. Preoperative and postoperative cranial volume measurements of the distraction group animals revealed an increase in cranial volume (P < 0.05). Histologic evaluation showed inductive bone regeneration and mature bone structure development within the distraction zone. Trifocal biplanar distraction-compression osteosynthesis is an effective and safe technique for three-dimensional closure of the cranial defects. In clinical practice, it could provide several advantages over currently available reconstructive procedures.

Regarding “Application-Specific Selection of Biomaterials for Pediatric Craniofacial Reconstruction: Developing a Rational Approach to Guide Clinical Use”

Background Biomaterials provide an invaluable alternative to autogenous bone graft for pediatric craniofacial reconstruction. However, there is no uniform agreement on the choice of biomaterial for different reconstructive needs. Methods Patients who had reconstruction of the craniofacial skeleton with alloplastic materials from 1994 to 2006 by a single surgeon were reviewed. Biomaterials used consisted of three classes: cement pastes, biomaterials designed to be replaced by bone, and prefabricated polymers. The study included 25 patients with a mean age of 5.5 years and a mean follow-up of 3.3 years. Results Cement pastes used for onlay augmentation to the cranial skeleton in eight patients consisted of hydroxyapatite (n = 5) and calcium phosphate (n = 3). One patient had a postoperative infection that resolved with partial implant removal and antibiotics. Biomaterials designed to be replaced by bone consisted of bioactive glass (n = 3) and demineralized bone (n = 8), which were used for inlay reconstruction of full-thickness calvarial defects in 11 patients. Computed tomographic scanning showed adequate bone mineralization in nine patients; two of the three patients with calvarial defects greater than 5 cm in diameter demonstrated variable mineralization. Prefabricated porous polyethylene was used in six patients for either onlay malar augmentation (n = 3) or inlay calvarial reconstruction (n = 3). One patient had a peri-implant infection that resolved with aspiration, irrigation, and intravenous antibiotics. Conclusions The authors developed an algorithm to guide use of biomaterials in craniofacial reconstruction based on whether (1) growth of the underlying craniofacial skeleton is nearly complete (>90 percent); (2) onlay or inlay reconstruction is to be performed; and (3) the reconstruction is performed in a load-bearing or non-load-bearing area.

Transport Distraction Osteogenesis for Closing Full-Thickness Calvarial Defects in Sheep

To prove the hypothesis that transport distraction osteogenesis can be applied to the skull to close critical-size calvarial defects. A sheep model was developed to investigate this hypothesis. In four sheep, bilateral parietal bone windows were created and adjacent osteotomies performed. On the tested side, an adjacent bone segment was transported into the defect. The contralateral side was left untreated as a control. After completion of the distraction and consolidation period, a computed tomography study was performed, and the animals were sacrificed. The newly formed bone was examined macroscopically and histologically. A successful closure of the defect with transport distraction was achieved in all of the animals. The control side healed spontaneously in one (younger) sheep but did not heal in the other three animals. The closure of the bony defect with transport distraction was evident macroscopically as well as on the computerized tomography. Microscopic examination showed new healthy bone formation on the treated side. We conclude that transport distraction is an effective tool in closing full-thickness calvarial defects in adult sheep. Further investigation is needed before applying this promising technique in humans.

Reconstruction of Full-Thickness Calvarial Defect

The reconstruction of scalp defects presents a clinical challenge. Free tissue transfer has played an increasingly important role in the reconstruction of complex scalp defects. In many cases, patient medical comorbidities along with the length of the operative procedures incurs significant patient risk. Artificial dermis, used extensively in burn reconstruction, has emerged as a reconstructive option for the coverage of many complex posttraumatic and postoncologic defects; however, none have described its use for the reconstruction of full-thickness calvarial defects with exposed dura.We report a clinical case of an elderly, medically compromised patient with a full-thickness scalp defect, who underwent successful reconstruction with artificial dermis.The use of artificial dermis and subsequent skin grafting, as was performed in this case, provides a less invasive, less intensive, and satisfactory means of soft tissue reconstruction for full-thickness calvarial defects.

Bioengineering of Calvaria with Adult Stem Cells

Defects of the adult skull do not heal spontaneously, producing challenging problems for the craniofacial surgeon. Reconstruction of such defects requires either the placement of alloplastic material or the harvest of autogenous bone. A technique is described for the reconstruction of critical-sized, full-thickness calvarial defects in the adult rat model using specific adult stem cells, namely, multipotent adult stem cells. The cells were harvested from adult skeletal muscle and cultured in an undifferentiated state within a matrix of polyglycolic acid mesh. An 8-mm critical-sized defect was created in the calvaria of adult rats and either left empty, filled with polyglycolic acid mesh alone, or filled with multipotent adult stem cells seeded into the polyglycolic acid mesh. After 12 weeks, the calvariae were harvested, stained, and blind graded by light microscopy on the presence or absence of reconstituted bone. A total of 22 animals were available for study: seven from the empty defect group, eight from the polymer group, and seven from the polymer plus stem cell group. The mean scores for the three groups were 1.9, 2.3, and 5.3, respectively. Statistical analysis showed statistical significance among the groups as a whole (p < 0.01) and between the polymer plus stem cell group and the empty defect and polymer-alone group. The results demonstrate that regeneration of calvarial bone is possible using stem cells harvested from adult skeletal muscle and seeded into a polyglycolic matrix. The technique may ultimately be used in clinical practice to reconstruct calvarial defects.

Application-Specific Selection of Biomaterials for Pediatric Craniofacial Reconstruction: Developing a Rational Approach to Guide Clinical Use

Biomaterials provide an invaluable alternative to autogenous bone graft for pediatric craniofacial reconstruction. However, there is no uniform agreement on the choice of biomaterial for different reconstructive needs. Patients who had reconstruction of the craniofacial skeleton with alloplastic materials from 1994 to 2006 by a single surgeon were reviewed. Biomaterials used consisted of three classes: cement pastes, biomaterials designed to be replaced by bone, and prefabricated polymers. The study included 25 patients with a mean age of 5.5 years and a mean follow-up of 3.3 years. Cement pastes used for onlay augmentation to the cranial skeleton in eight patients consisted of hydroxyapatite (n = 5) and calcium phosphate (n = 3). One patient had a postoperative infection that resolved with partial implant removal and antibiotics. Biomaterials designed to be replaced by bone consisted of bioactive glass (n = 3) and demineralized bone (n = 8), which were used for inlay reconstruction of full-thickness calvarial defects in 11 patients. Computed tomographic scanning showed adequate bone mineralization in nine patients; two of the three patients with calvarial defects greater than 5 cm in diameter demonstrated variable mineralization. Prefabricated porous polyethylene was used in six patients for either onlay malar augmentation (n = 3) or inlay calvarial reconstruction (n = 3). One patient had a peri-implant infection that resolved with aspiration, irrigation, and intravenous antibiotics. The authors developed an algorithm to guide use of biomaterials in craniofacial reconstruction based on whether (1) growth of the underlying craniofacial skeleton is nearly complete (>90 percent); (2) onlay or inlay reconstruction is to be performed; and (3) the reconstruction is performed in a load-bearing or non-load-bearing area.

Cranioplasty Using Osteoconductive Scaffold and Platelet Glue

An alternative to autogenous bone grafts or to methyl methacrylate in the reconstruction of full-thickness calvarial bone defect is needed. The safety and efficacy of biphasic calcium phosphate osteoconductive scaffold (Triosite) combined with platelet glue for the reconstruction of posttraumatic calvarial bone defect was evaluated in six consecutive patients. Follow-up averaged 30 months. Postoperative evaluations included serial photographs, repeated physical examination, and three-dimensional computed tomography scan. High-fibrin concentration of the platelet glue allowed easy molding and sculpting of the scaffold, providing mechanical stability and avoiding spillage of the granules into the operating field. Neither infection of the surgical site nor extrusion of the scaffold was noted. The contour of the reconstructed calvarium was esthetically acceptable, without any secondary depression. Three-dimensional computed tomography scans 2 years after surgery revealed satisfactory reconstruction of the bone defect. Visual inspection of the reconstructed calvarium 2 years after surgery in one patient evidenced conversion of the scaffold into solid new bone. Section of the biopsy demonstrated new bone formation at the expense of the scaffold. Combining an osteoconductive scaffold with platelet glue offers an interesting alternative to autogenous bone graft or methyl methacrylate for posttraumatic calvarium bone defect reconstruction.

Stimulation of intramembranous bone repair in rats by ghrelin

Researchers in our laboratory have previously shown that ghrelin, a gastric peptide hormone, may regulate mesenchymal cell differentiation into adipocytes and myocytes. Here we show that ghrelin promotes osteogenesis of intramembranous bone and improves the repair of calvarial bone defects in rats. Rats with a 9 mm full-thickness calvarial bone defect received either Bio-Oss (control group) or Bio-Oss mixed with 20 mug ghrelin (treatment group), followed by local administration of saline or ghrelin (10 microg), respectively, on days 5, 10 and 15. After 6 and 12 weeks, new bone formation was assessed. Animals treated with ghrelin showed a significant increase in new bone formation as demonstrated by an increment in bone mineral density and fluorescence labelling of tetracycline relative to the control group. At 6 weeks, bone mineral density increased from 54 +/- 7 (control group) to 78 +/- 9 mg cm(-2) in the treatment group, while the tetracycline fluorescence labelling increased by 61 +/- 15%. A similar increment was observed at 12 weeks. Quantitative reverse transcriptase-polymerase chain reaction showed that expression of alkaline phosphatase (ALP), osteocalcin and collagen type I was elevated. Relative to the control animals, mRNAs for ALP, osteocalcin and collagen type I increased 2.4 +/- 0.4-, 4.7 +/- 1.9- and 4.0 +/- 1.7-fold, respectively, in animals treated with ghrelin for 6 weeks (P < 0.05). At 12 weeks, mRNA levels of ALP, osteocalcin and collagen type I showed a decline relative to levels at 6 weeks but still remained significantly higher than in the control group, with fold changes of 2.4 +/- 0.8, 2.4 +/- 1.2 and 2.1 +/- 0.7, respectively (P < 0.05). This study demonstrated that ghrelin stimulates intramembranous osteogenesis.

Anorganic Bone Mineral Coated with Tetra-Cell Adhesion Molecule Enhances Bone Formation in Rabbit Calvarial Defects

This study was performed to evaluate the effect of anorganic bone mineral (ABM) coated with Tetra-Cell Adhesion Molecule (T-CAM) for bone formation in rabbit calvarial defects and compare the capability of bone formation in ABM coated with T-CAM (ABM/T-CAM) to that in commercially available ABM coated with a synthetic peptide (P-15) which mimics the cell-binding domain of type I collagen, PepGen P-15TM. T-CAM composed of four cell adhesion molecules (RGD, PHSRN, EPDIM, and YH) was synthesized and ABM/T-CAM were prepared by absorbing T-CAM on ABM (OsteoGraf/N-300; Densply Friadent Ceramed Corp., USA). Two 9-mm diameter, full-thickness calvarial defects were made in each rabbit parietal bone and sixteen adult male rabbits were used in this experiment. The defects were reconstructed according to four treatment groups: unfilled, BM-grafted, PepGen P-15TM-grafted, and ABM/T-CAM-grafted. The animals were sacrificed at 2 and 4 weeks after surgery for histologic and histomorphometric evaluation. An active new bone formation were observed in the defects of ABM/T-CAM and PepGen P-15TM grafted groups at 2 and 4 weeks of healing in histologic observation. The results of histomorphometric analysis revealed higher new bone formation in ABM/T-CAM-grafted (14.62±0.6% at 2 weeks, 15.33±2.4% at 4 weeks) and PepGen P-15TM-grafted (12.46±1.0% at 2 weeks, 18.14±1.7% at 4 weeks) groups than in unfilled control (7.03±2.3% at 2 weeks, 8.71±3.4% at 4 weeks) and ABMgrafted (6.59±1.7% at 2 weeks, 9.25±0.8% at 4 weeks) groups at 2 and 4 weeks of healing with statistical significance (P&lt;0.01). The results of this study indicated that the immobilizing T-CAM on ABM enhances the capability of bone substitutes to serve as an effective habitat for bone forming cells in vivo. In conclusion, we suggested that this composite graft material, ABM/T-CAM may be served as an effective tissue-engineered bone graft material in osseous reconstructive surgery.

Management of acute calvarial burns in children.

Although rare, head burns involving the calvarium are a serious complication of burns and electrical injury, and present therapeutic challenges to the surgical burn care team. We evaluated our experiences and compared available strategies to address this challenge. Records of all burned children between January 1986 and December 2000 were reviewed. Twenty-seven children (15 boys and 12 girls) with scalp burns extending at least into the outer table of bone were identified and compared with a matched group of 30 patients admitted for acute thermal burns without skull injury. Flame burn was the injury mechanism in 78% of these patients and electrical injury was the injury mechanism in 22%. The incidence of calvarial burns in our patient population was 1.2% for thermal burns and 5.6% for electrical injuries during the study period. The age distribution was biphasic, with maximums in infancy for thermal burns and in puberty for electrical injuries. Eight of 27 patients (29.6%) developed full-thickness calvarial bone defects. In 23 patients, calvarial burn wound coverage was achieved with bone debridement and immediate or delayed placement of autograft skin. In four patients (all with electrical injury), local scalp flaps were required for closure. The length of hospital stay and overall number of acute operative procedures significantly increased for patients with calvarial burns. Acute calvarial burns are safely managed by bone debridement in combination with staged autografting or early flap coverage. Although flap coverage reduces the number of required procedures, the extensive wound size in thermal burns restricts acute flap procedures primarily to electrical injuries. Compared with patients without skull injury, length of hospital stay and the number of acute operative procedures are tremendously increased in patients with calvarial burns.

Reconstruction of the immature craniofacial skeleton with a carbonated calcium phosphate bone cement: interaction with bioresorbable mesh.

Calcium phosphate cements have been recently introduced for use in craniofacial reconstruction. In the clinical setting, however, pulsations of the underlying brain and dura may interfere with the crystallization of these cements, thereby rendering their use in cranioplasty problematic. To circumvent such problems, many clinicians have interposed synthetic resorbable plates or mesh between the dura and the cement. At the present time, however, little is known about the influence of such materials or their breakdown products on the fate of calcium phosphate cements. The specific aim of this project was to evaluate the biocompatibility, osteoconductivity, and remodeling capacity of a calcium phosphate cement after implantation into experimental calvarial defects when combined with a resorbable mesh underlay. Four 10-mm diameter full-thickness calvarial defects (two frontal, two parietal) were created in each of six 3-week-old Yorkshire pigs. The defects were treated as follows: 1) empty control, 2) macroporous polylactic acid (70/30 L/DL polylactic acid [PLA]) mesh, 3) Norian CRS calcium phosphate cement, and 4) Norian CRS over PLA mesh underlay. Animals were divided into two groups. Half of the animals were killed 30 days after surgery, and half were killed 180 days after surgery, and the graft recipient sites were examined histologically. At 30 days, minimal bone ingrowth was observed in untreated calvarial defects or in those that were treated with PLA plates alone. Defects treated with the cement alone demonstrated a modest amount of new woven bone deposition, primarily at the periphery of the implants. Defects treated with calcium phosphate cement over PLA mesh underlays were characterized by remodeling and woven bone deposition at 30 days, with complete or near-complete osseous bridging of the ectocranial implant surfaces. Progressive bone ingrowth was noted in all defects at 180 days, with near-complete replacement of all Norian CRS implants by host bone. The PLA mesh remained incompletely resorbed at 180 days. No inflammatory response to the implants was observed at either time point. Calcium phosphate cement may be safely used for craniofacial reconstruction in the presence of PLA implants without compromise to its biocompatibility, osteoconductivity, or remodeling capacity.

Effect of a freeze-dried CMC/PLGA microsphere matrix of rhBMP-2 on bone healing.

The hypothesis of this research was that implants of poly(lactide-co-glycolide) (PLGA) microspheres loaded with bone morphogenetic protein-2 (rhBMP-2) and distributed in a freeze-dried carboxymethylcellulose (CMC) matrix would produce more new bone than would matrix implants of non-protein-loaded microspheres or matrix implants of only CMC. To test this hypothesis it was necessary to fashion microsphere-loaded CMC implants that were simple to insert, fit precisely into a defect, and would not elicit swelling. Microspheres were produced via a water-in-oil-in-water double-emulsion system and were loaded with rhBMP-2 by soaking them in a buffered solution of the protein at a concentration of 5.4 mg protein per gram of PLGA. Following recovery of the loaded microspheres by lyophilization, matrices for implantation were prepared by lyophilizing a suspension of the microspheres in 2% CMC in flat-bottom tissue culture plates. Similar matrices were made with 2% CMC and with 2% CMC containing blank microspheres. A full-thickness calvarial defect model in New Zealand white rabbits was used to assess bone growth. Implants fit the defect well, allowing for direct application. Six weeks postsurgery, defects were collected and processed for undecalcified histology. In vitro, 60% of the loaded rhBMP-2 released from devices or microspheres in 5 to 7 days, with the unembedded microspheres releasing faster than those embedded in CMC. In vivo, the rhBMP-2 microspheres greatly enhanced bone healing, whereas nonloaded PLGA microspheres in the CMC implants had little effect. The results showed that a lyophilized device of rhBMP-2/PLGA microspheres in CMC was an effective implantable protein-delivery system for use in bone repair.

Evaluation of a novel malleable, biodegradable osteoconductive composite in a rabbit cranial defect model

The ceramic form of calcium phosphate osteoconductive material such as hydroxyapatite is brittle, non-malleable and non-degradable, and these mechanical properties limit its clinical application in calvarium reconstruction. To improve these properties, we developed a malleable, biodegradable osteoconductive composite composed of tricalcium phosphate particles bound by a gelatin which is set by glutaraldehyde mediated cross-linking. The composite was implanted into a 15 × 15 mm full-thickness, calvarial defect in 20 rabbits for up to 3 months. Twelve rabbits were left unreconstructed as controls. Specimens were retrieved at 2 weeks, 1, 2 and 3 months. Five reconstructed and 3 unreconstructed rabbits were examined for each time period. The assessment included a series of post operative gross examinations, radiographs and histologic evaluations. We are able to demonstrate that this composite is (1) biocompatible, with little tissue reaction; (2) osteoconductive, with progressive growth of new bone into the calvarial defect; (3) biodegradable, with progressive replacement of the composite by new bone, acellular matrix and bone-like material. Replacement of this composite by new bone is postulated to occur by a combination of osteoconduction and biodegradation. These results indicate that further experimental research to combine this malleable, biodegradable, osteoconductive composite with an osteoinductive agent such as bone morphogenetic protein may generate new biomaterial for full-thickness calvarial defect reconstruction.

Importance of the critical-size bone defect in testing bone-regenerating materials.

A substantial interest exists in developing substitute materials and human recombinant bone-inducing factors to enhance bone regeneration both in the craniofacial complex and in other parts of the skeleton. A persistent problem has been to find an animal model that allows for the comparison of different osteopromotive materials. The purpose of this investigation was 1) to determine whether a 5-mm calvarial defect in adult rats fulfilled the requirements of a critical-size bone defect and 2) to discuss the use of this animal model in assessing bone repair in the craniofacial region. Bilateral full-thickness calvarial defects were trephined in the calvarial bone of 56 5- to 6-month-old Wistar rats. Except for minor amounts of bone formation limited to the margins of the defect, none of the defects revealed any signs of spontaneous bone regeneration 6 and 12 months after surgery. The full-thickness 5-mm calvarial defects thus fulfill the criteria for a critical-size bone defect. This animal model allows for an experiment with a paired design, it avoids inclusion of the sagittal suture in the osseous defect, and it thereby minimizes morbidity by reducing the risk of damaging the midsagittal sinus. An adequate experimental model has been developed to evaluate the efficiency of osteopromotive materials in the healing of bone defects in the craniomaxillofacial region.

Combined use of titanium mesh and biocompatible osteoconductive polymer in the treatment of full thickness calvarial defects

Restoration of bony defects is a common problem in craniofacial surgery. This article describes a versatile approach using a combination of titanium micromesh or midimesh and biocompatible osteoconductive polymer to restore calvarial bone loss.

P-6-718 - Severe pediatric head injury. Analyzing the better outcome over a decade

Full thickness calvarial defects present considerable challenges to reconstructive surgeons. In paediatric cases, the use of biomaterials as a substrate for cranioplasty rather than autologous bone is controversial. Alloplastic cranioplasty in adults is supported by several large case series however long term outcome of biomaterial use in paediatric cases is limited. Retrospective seven year analysis of departmental database and clinical records identified 22 patients aged under 18 who had undergone 23 custom made titanium cranioplasties by a single surgeon using the same technique. Data including patient demographics, reason for craniectomy and complications experienced following surgery was obtained. The mean age at operation was 12 years 9 months. The mean defect size was 44.3 cm2. No significant complications related to the cranioplasty were recorded in the early post operative period or during long term review (average follow up 4 years 6 months). No cranioplasty implant required removal. This retrospective case series shows that custom made patient specific titanium cranioplasty is a viable alternative to autologous bone as a reconstructive material in paediatric patients under specific circumstances.

Experimental Hydroxyapatite Cement Cranioplasty

Hydroxyapatite cement is a calcium phosphate-based material that when mixed with water forms a dense paste that sets within 15 minutes and isothermically converts in vivo to a microporous hydroxyapatite implant. This cement was used to reconstruct bilateral 2.5-cm-diameter full-thickness critical-sized parietal skull defects in six cats. One side was reconstructed with 100 percent hydroxyapatite cement, and the other with a mixture of 50 percent hydroxyapatite cement and 50 percent ground autogenous bone by weight. These animals were sacrificed at 6 and 12 months after implantation. Positive and negative controls also were prepared. The anatomic contour of the soft tissue overlying all hydroxyapatite cement implants was well maintained, there were no wound infections or structural failures, and the implants were well tolerated histologically. None of the negative (unreconstructed) control defects was completely filled with repair bone, and all positive (methyl methacrylate) controls demonstrated foreign-body giant-cell formation and fibrous encapsulation of the implants. Examination of decalcified and undecalcified sections revealed progressive but variable replacement of the cement by new bone and soft tissue without a change in the shape or volume of the hydroxyapatite cement-reconstructed areas. New bone comprised 77.3 and 64.7 percent of the tissue replacing the hydroxyapatite cement and hydroxyapatite cement-bone implants, respectively. Replacement of the hydroxyapatite cement implants by new bone is postulated to occur by a combination of osteoconduction and implant resorption. These results indicate that further experimental research leading to the possible application of hydroxyapatite cement for full-thickness calvarial defect reconstruction in humans is warranted.