Influence of residual alveolar bone height on osseointegration of implants in the maxilla: a pilot study

For sinus floor augmentation and simultaneous implant placement, a minimum of 5 mm of residual bone height has been recommended empirically. This study was designed to test this assumption in an experimental animal trial.

In eight mini pigs, three premolars and two molars were removed on one side of the maxilla. Three months later, the animals were assigned to four groups of two animals each. A cavity was created at the base of the alveolar process so that the residual bone height was reduced to 2, 4, 6 and 8 mm, respectively. Six implants were installed and an inlay augmentation procedure was carried out using a particulated iliac bone graft. Implants were loaded with fixed provisional restorations after a healing period of 6 months. The animals were sacrificed after 6 months of functional loading. Histologic specimens were prepared and histomorphometric analysis was performed [bone-to-implant contact (BIC) ratio, interthread bone area, peri-implant bone area, crestal bone resorption (CBR)].

Two implants were lost during follow-up and fibrous encapsulation was detected in one additional implant. All failures occurred in one animal with a residual alveolar height of 2 mm. On the buccal aspect, BIC turned out to be significantly higher for 6 mm when compared with 2/4 mm (75.8 +/- 26.1 vs. 58 +/- 23.2/53.9 +/- 22.8; P<0.05), while on the palatal aspect, BIC was significantly higher for 6/8 mm when compared with 2/4 mm (80 +/- 17.8/78.9 +/- 10.3 vs. 55.8 +/- 26.5/55.6 +/- 21.3; P<0.05). For an alveolar height of 8 mm, CBR tended to be significantly lower than for bone heights of 2/4 mm (3.8 +/- 2.3 vs. 5.3 +/- 2.6/5.8 +/- 3.9; P<0.05). Correlation analysis revealed a significant association of BIC and interthread bone area as well as a negative association to CBR on the palatal aspect.

The results of the present study show that the combination of maxillary inlay grafting and simultaneous implant placement does not hinder osseous integration even though the alveolar crest has been reduced to a residual height of 4 mm and below. However, according to histomorphometry, the highest predictability is gained in sites with residual bone heights of 6 and 8 mm.