Densitometric Evaluation of Calcium Sulfate Hemihydrate as a Bone Graft Substitute around CAD / CAM Machined Zirconium Dioxide Implants

Aims: This experimental study was conducted to evaluate the effect of β–calcium sulfate hemihydrate (CSH) as a bone graft substitute on the bone response around CAD/CAM machined zirconium dioxide (ZrO2) implants. Materials and methods: Forty ZrO2 implants were digitally designed and manufactured using computer–aided design/ computer aided–manufacturing CAD/CAM machine. Twenty New–Zealand rabbits were included in the experiment and a bed was made for implantation in each head of the left femur. Each animal received a ZrO2 implant in the mesial femoral head and this group of implants was considered as a control group. Then the calcium sulfate hemihydrate was placed in the implant bed at the distal femoral head followed by fixation of an implant, and this group was considered as an experimental group. The twenty rabbits were randomly allocated into four groups, to represent the study periods i.e. 3 days, 7days, 14 days, and 21 days. Bone response was assessed around each of the forty implants by measuring the bone mineral density using densitometric analysis of the digital periapical radiological image which was taken after the euthanization of the animals according to study intervals. Results: The results showed a statistically significant difference between the control group (ZrO2 implants) and the experimental group (ZrO2 implants with CSH) in bone mineral density. Conclusions: the use of CSH as an artificial bone graft around ZrO2 implant is beneficial for increasing bone formation around the implant as it increases bone density.


INTRODUCTION
Ceramics and carbon are biologically inert materials that were used as dental implants and tested for their physical, mechanical, chemical, and electrical differences. Focusing on inertness, thermal conductivity, modulus of elasticity, brittleness, and surface reactions to bond with the bone (1) . Presently, the yttriumstabilized tetragonal zirconia, which showed excellent mechanical and tribological properties altogether with biocompatibility is considered as the ceramic material of choice (2) .
Computer-Aided Design / Computer-Aided Manufacture (CAD/CAM) technology attains different dental restorations by designing and milling processes of two or three-dimensional models using different ready blocks by numerical controlled machines (3) . It is the technique of choice for producing durable tooth-colored and metal-free components from high-strength ceramics, providing the option of chair-side fabrication of indirect restorations (4) .
Bone regeneration procedures are critical for the success of implant treatments in cases where there's a deficiency in bone width and/or height. The cornerstone in these treatments is the use of bone substitutes to make a bone mantle that covers the screw to reinforce implant stability and treatment outcome (5) . The βcalcium sulfate hemihydrate has been used as bone graft material due to its excellent biocompatibility together with its ability for bone regeneration (6) . This is due to its Osteoconduction and Osteoinduction properties (7) .

Study design
Forty zirconia implants were used and divided into two groups each containing twenty implants.
• Group of (ZrO2): consisted of 20 ZrO2 implants, each was implanted in the mesial head of the left femoral bone.

The Calcium Sulfate Hemihydrate bone graft substitute
The Calcium Sulfate Hemihydrate powder was prefabricated and sterilized by Suleiman, M. S., by using gypsum rocks as a raw material taken from Mosul city (9) .

The surgical procedure
The surgical procedure was done under an aseptic environment, the animals were generally anesthetized with intramuscular injections of ketamine hydrochloride 5mg/kg and Xylazine hydrochloride 50mg/kg (10) . The hair over

Statistical analysis
The statistical analysis was performed using IBM SPSS Statistic 19 software. The differences between groups were statistically analyzed using paired-samples T-test and considered to be statistically significant at a P ≤ 0.05 and highly significant at a P ≤ 0.01.      (11) .

After 3 days from implantation,
The measured bone density around the implants' serrations showed an early elevation of bone density with a highly significant difference was observed in the group of (ZrO2+CSH) than the group of (ZrO2). This was explained by the presence of high concentrations of calcium ions which gave more radio-opacity and enhance early recruitment of osteoprogenitor cells and their early differentiation to osteoblasts with early deposition of ground substance especially inorganic material which is also a radioopaque material (12) .

After 7 days from implantation,
A statistically highly significant difference in bone density between both groups was noted with the higher value for the group of (ZrO2+CSH) indicating more bone formation in this group than that of the (ZrO2) group. Meanwhile, the bone density decreased in this period for the group of (ZrO2+CSH) than the previous period this can be explained by the absorption of the calcium sulfate in the tissues due to its fast biodegradation property (7,13) . during the process of bone formation (14) .