Presentation of a cone-beam CT scanning protocol for preprosthetic cranial bone grafting of the atrophic maxilla

Abstract

The use of autologous calvarian split thickness bone grafts is a well accepted preprosthetic surgical procedure for reconstruction of the severely atrophied maxilla. Although generally accepted as being a superior bone graft regarding long-term stability, the reported risks in the literature may dissuade the surgeon. A modified cone-beam computed tomography (CBCT) scanning protocol (extended field of view starting from 1 cm below the occlusal plane up to the limit of the cranial vault, 0.4 voxel) is proposed that allows assessment of both the cranial donor site as the maxillary receptor site and the sinus conditions with a single scan. Issues regarding quality of the data, radiation dose and clinical practicability are discussed.

The severely atrophic maxilla (Cawood V and VI), when width and height are insufficient, is a challenge in oral rehabilitation. Several surgical procedures have been described and accepted as useful and reliable techniques including bilateral sinus inlay and onlay and Le Fort I downward fracture interpositioning. A common site for bone grafting is the iliac crest. In 1982, Tessier introduced the cranial bone graft. Although generally accepted as being a superior bone graft regarding long-term stability, the reported risks in the literature (intracerebral, subdural, epidural bleeding) may dissuade the surgeon. Complications are rare and mostly temporary. In the authors’ experience the only complication was a subcutaneous haematoma that occurred in 3.4%.

Although complications are rare and usually not severe, Tessier stated that ‘the use of computed tomographic scanning for evaluation of the thickness of the parietal bones and detection of any abnormalities is necessary’.

The presurgical diagnostic means to assess the receptor site are: orthopantomography; dentascan (cone-beam computed tomography (CBCT) or multi-slice CT); or a combination of the latter imaging techniques. Often no imaging of the donor site is carried out. Sometimes a lateral cephalogram, revealing little information on the morphology of the cranial bone, is taken. Occasionally multi-slice CT scanning is performed.

The purpose of this technical note is to present a new CBCT protocol for simultaneous assessment of the maxillary receptor and cranial donor sites and evaluate its clinical relevance.

Image acquisition technique

The scanning protocol is a modification of the first scan of the ‘triple’ CBCT scan protocol that the authors introduced for routine 3D virtual orthognathic surgery planning. In order to scan the maxillary receptor and cranial donor sites simultaneously the field of view (FOV) was modified (scanning starts 1 cm below the occlusional plane and ends above the cranial vault) and the head position adjusted ( Fig. 1 ). CBCT scanning (i-CAT™, Imaging Sciences International, Inc., Hatfield, Pennsylvania, USA) was performed in ‘Extended Field mode (FOV 17 cm diameter, 22 cm height; scan time 2 × 20 s; voxel size 0.4) at 120 kV (according to DICOM field: 0018,0060 KVP) and 48 mA (according to DICOM field: 0018,1151 XRayTubeCurrent).

Fig. 1
Left: The FOV and positioning in the orthognathic scanning protocol. Right: Modification of the FOV and position of the head in the preprosthetic protocol.

To be able to better orientate the site of cranial bone harvesting, a radio-opaque orientation template was created and placed on the patient’s head before scanning ( Fig. 2 ). The orientation template was custom-made from an alternative alginate impression material (AlgiNot™, Kerr U.S.A., Romulus, USA) and measured 80 mm × 60 mm × 10 mm. Prior to scanning, the orientation template was positioned on the patient’s head in the parietal region, 3 cm dorsal to the hair line and 3 cm lateral to the midline.

Fig. 2
Radio-opaque template on skull.

After scanning, both the anatomy of the cranial donor and maxillary receptor site can be evaluated with the i-CAT Vision™ viewing software. Absence of diploe, venous lacunae and other anatomical variations of the cranial donor site can be screened for ( Fig. 3 ). The maxillary receptor site can be assessed in height and width but additional clinical relevant information on sinus pathology is available (e.g. evaluation of mucosal lining, oro-antral fistulae, fluid levels, drainage problems, corpora aliena). The thickness of the external cranial cortex can be evaluated by comparing the full thickness of the skull to the thickness of the orientation template ( Fig. 4 ).

Jan 26, 2018 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Presentation of a cone-beam CT scanning protocol for preprosthetic cranial bone grafting of the atrophic maxilla
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