The use of light/chemically hardened polymethylmethacrylate, polyhydroxyethylmethacrylate, and calcium hydroxide graft material in combination with polyanhydride around implants in minipigs: part I: immediate stability and function

Abstract

Background: The present study is designed as a proof-of-concept study to evaluate light/chemical hardening technology and a newly formulated polymethylmethacrylate, polyhydroxyethylmethacrylate, and calcium hydroxide (PPCH) plus polyanhydride (PA) (PPCH-PA) composite graft material as a bone substitute compared to positive and negative controls in a minipig model.

Methods: PPCH-PA (composite graft); PPCH alone (positive control), PA alone (positive control), and no graft (negative control) were compared. Four mandibular premolar teeth per quadrant were extracted; a total of 48 implants were placed into sockets in three minipigs. Abutments were placed protruding into the oral cavity 4 mm in height for immediate loading. Crestal areas and intrabony spaces were filled with PPCH-PA, PPCH, or PA using a three-phase delivery system in which all graft materials were hardened by a light cure. In the negative control group, implant sites were left untreated. At 12 weeks, block sections containing implants were obtained. Evaluations included periodontal probing, pullout-force load, and stability measurements to determine implant stability, radiographs to examine bone levels, and scanning electron microscopy (SEM)-energy-dispersed spectroscopy to determine bone-to-implant contact.

Results: Probing measurements did not reveal any pathologic pocket formation or bone loss. Radiographs revealed that immediate implant placement and loading resulted in bone at or slightly apical to the first thread of the implant in all groups at 12 weeks. Stability test values showed a relative clinical stability for all implants (range: -7 to +1); however, implants augmented with PPCH-PA exhibited a statistically significantly greater stability compared to all other groups (P <0.05). The newly formed bone in PPCH-PA-treated sites was well organized with less marrow spaces and well-distributed osteocytes. SEM revealed a tighter implant-socket interface in the PPCH-PA group compared to other groups with reduced microfissures and implant-bone interface fractures during pullout testing, whereas implants treated with PA or no graft showed ≈ 10-μm microfissures between the implant and bone with fractures of the intrathread bone.

Conclusions: The newly formulated chemically hardened graft material PPCH-PA was useful in immediate implant placement after tooth extraction and resulted in greater stability and a well-organized implant-bone interface with immediate loading, especially in those areas where cancellous bone was present. The results of this proof-of-concept study warranted further research investigating different healing times and longer durations.

Figure 1

Study timeline. *All procedures were performed under general anesthesia; ^†oral hygiene, implant check, and stability test measurements; ^‡stability test measurements and tissue harvesting; ^§animals were euthanized by an overdose of pentobarbital injections (120 mg/kg).

Figure 2

Soft tissue around implants. A) Soft tissue surrounding the neck of the implants showed normal characteristics. B) Soft tissue probing was made at six points around each implant. Means ± SDs were compared among groups. No statistically significant difference was found among groups (P >0.05).

Figure 3

A) Exposed-thread numbers at the crestal level were calculated as previously described to calculate the radiological bone loss at both mesial and distal sites of each implant. As shown in the respective radiographs for each group, zero indicates no exposure, while 2 and 3 refer to “two and three” threads exposed, respectively. Arrows depict the area evaluated for exposed implant threads. B) Based on counting of the threads exposed mesially and distally for each implant, the PA-alone group showed the highest number of threads exposed (mean ± SD: 2.6 ± 0.8; P < 0.05),^† while PPCH-PA group exhibited the least number of threads exposed (0.8 ± 0.9; P < 0.001).*

Figure 4

Implant stability. Stability test measurements were recorded at 2, 6, and 12 weeks. A) All implants B) Maxillary implants only C) Mandibular only.

Figure 5

Biomechanical (pullout) test. Peak loads during the pullout test were recorded by the load transducer and automatically transferred to a computer software program attached to the machine. A) Comparison of the displacement distance at the maximum load (10,000 kN) among groups. B) Force load at the time of the displacement. No statistically significant difference was found among groups (P >0.05).

Figure 6

The implant–bone interface and composition of newly formed bone around implants were evaluated using field-emission SEM in each group. A) The micrographs represent the implant–bone interface in PPCH-PA in lower and higher magnifications. There were no obvious marrow spaces for the PPCH-PA group. Peri-implant bone of the PA- (B) and PPCH(C)-treated sites are shown. D) Micrographs of bone-to-implant contact area in sites without augmentation. Original magnifications A through D×20.

Figure 7

EDX analysis. A) Composition of implant–bone surfaces in all groups shown in intensity. B) Ca/P ratios were calculated at five areas for each sample, and the mean ± SD was used for statistical comparisons. a.u. = arbitrary units.