Abstract
Long-term changes in trabecular bone structure during the 10 years following onlay grafting with simultaneous mandibular implant placement were studied. Extraoral radiographs of both mandibular sides in eight patients were taken regularly. Bone structure was analysed using a custom-written image analysis program. Parameters studied were trabecular area and perimeter and marrow cavity area and perimeter. After skeletonisation of the trabecular network, the number of end points and branching points, skeleton length, and branch angle were determined. The observed structural changes agree with the development of a more complex and more delicate or fine osseous structure. The bone shows more trabecular branching. All changes are most pronounced in the graft spongiosa, but are also found in the graft cortex and in the original mandible. The mean trabecular branch angle becomes more horizontal. The applied technique can be used to analyse long-term changes in the architecture of bone grafts. Changes found in the graft architecture correspond to changes expected after functional adaptation to loading.
Introduction
Onlay grafting combined with simultaneous placement of two permucosal implants is one of the treatment modalities in extreme atrophy of the edentulous mandible. In the first months and years following onlay grafting, changes occur in height and radiographic density of the graft. Little is known on the short- and long-term changes occurring in the osseous structure of the graft.
A large number of parameters can be used to describe the structure of bone as seen on radiographs. Among these parameters are trabecular area and perimeter, marrow cavity area and perimeter, trabecular skeleton length, number of end points and branching points of the trabecular network, and direction of trabecular branches. In a previous study these parameters were used to analyse changes in the bone structure in the first year after placement of an onlay graft and implants. Early cortical changes were seen, suggesting partial resorption and formation of a more complex structure. In the fourth quarter after surgery, progressive resorption was seen in the graft cortex. In the upper spongiosa of the graft, most parameters indicated bone formation during the first postoperative year. Loading-induced structure changes could not yet be discerned. The measurement technique that was used proved suitable for the study of changes in the architecture of bone grafts. The observed changes in graft architecture were in accordance with previously observed changes in bone density.
Prolonged functional loading could result in more pronounced radiographic changes in the architecture of the onlay graft. According to Wolff’s law, a changed loading pattern, for instance through implants, should result in restructuring of both the mandible and graft. In the present study, long-term changes in the osseous structure of onlay grafts that correspond with functional adaptation to loading are described.
Materials and methods
Patients and method of treatment
Eight patients with severe mandibular atrophy were included in this prospective study. All patients were edentulous in both jaws and lacked adequate retention of their lower denture. The group consisted of two male and six female patients, mean age at surgery 56.7 years (standard deviation (SD) 6.0, range 49.9–66.3 years).
A corticocancellous iliac crest block graft was placed cortical side up as an onlay graft on top of the anterior mandible, using an intraoral approach. The host bed was prepared by making perforations in the upper cortex of the anterior mandible. Two titanium implants fixed the graft to the mandible and no additional bone screws were used. After 4–5 months, second stage surgery was performed and healing abutments were placed. Loading of the 16 implants and the bone by means of a bar-retained overdenture commenced an average of 249 days after implant surgery (SD 49, range 185–304 days).
All patients were followed at regular intervals both clinically and radiologically. The observation period was between 3048 and 3847 days after surgery.
Radiographic technique
Standardised extraoral oblique lateral cephalometric radiographs (OLCRs, Fig. 1 ) were taken at regular intervals after augmentation and loading of the superstructure. The first radiographs, serving as baseline measurement, were usually obtained 5–12 weeks postoperatively. All postoperative radiographs were assigned to the nine postoperative radiographic observation periods (shown in Table 1 ). Every patient appeared once in every postoperative period.
