Introduction

Maxillary bone deficiency secondary to tooth loss, trauma, infection, tumour resection, maxillary sinus pneumatization, or congenital disease has been a great challenge for reconstruction and retention. Since Brånemark and his team presented the concept of zygomatic implants as a rescue procedure in 1988, patients with an atrophied maxilla have had a choice beyond onlay bone grafting, short implants, a removable prosthesis, and maxillary sinus lift. A 7-year prospective study of 74 zygomatic implants showed a cumulative survival rate (CSR) of 97.2%. A more encouraging up-to-4-year follow-up study on more than 100 consecutive implants showed a 100% success rate. A fixed prosthesis supported by zygomatic implants guarantees a shorter duration of therapy and better pronunciation, comfort, and aesthetics. Similarly to conventional implants, screw-retained dentures supported by zygomatic implants can accept immediate occlusal loading and be functional 48 h after implantation. The loss of bony tissue is usually accompanied by a loss of soft tissue. The rehabilitation of lost soft tissue is customarily performed with obturators. However, in zygomatic cases, local flaps, such as radial forearm free tissue and superficial inferior epigastric artery flaps, can work well with zygomatic implants.

Modifications to the original protocol have been reported in the literature. Two or three bilateral zygomatic implants have been proven to be compatible. However, in most scholars’ opinions, two zygomatic implants on each side, known as the four-implant protocol, is usually sufficient for reconstruction. In the four-implant protocol, the two inferior posterior implants are the traditional ones, starting from the crest of the alveolar process in the second premolar region and extending to the lateral superior wall of the maxillary sinus. The other two implants are placed superior to the existing ones, starting from the crest of the alveolar process in the canine or lateral incisor region and heading towards the lateral inferior wall of the orbital socket.

A previous study on the anatomic measurements of Mongoloid bone only involved elderly edentulous Japanese patients. There is little research regarding Chinese patients who require zygomatic implantation. Anatomical assessments of the traditional zygomatic implant technique are well reported, but few studies have addressed the four-implant protocol, especially with regard to bone thickness in the insertion areas. The fact that zygomatic implantation has not been developed widely in Mainland China might be due to the above-mentioned reasons.

The facial bony curvature in the orbits, zygomatic arches, and maxillary alveolar process experiences age-related and population/subpopulation-related changes. All of these regions have a close relationship with zygomatic implant insertion.

Since Mozzo and colleagues proposed the idea of using cone beam computed tomography (CBCT) in the maxillofacial region, this technique has performed well in the fields of oral and maxillofacial surgery, cariology, endodontics, implantology, and orthodontics because of its lower radiation doses, shorter scan period, fewer artefacts, and greater image precision. Maxillofacial linear and angular measurements based on CBCT images have been proven to have reliable accuracy.

Will the zygoma become thinner with the atrophy of alveolar bone? Will the zygoma become longer with ageing? Will the zygoma become more curved with ageing? Several questions regarding the zygoma in Chinese patients remain unanswered.

To verify whether Chinese patients of different ages are suitable recipients for zygomatic implantation and to detect the age-related changes in the anatomic bases for the insertion of zygomatic implants, we performed linear and angular measurements of implant insertion areas in different age groups of Chinese patients using maxillofacial CBCT scans.

Materials and methods

All the study cases were selected randomly from maxillofacial CBCT scan images taken between June 2010 and June 2012. The edentulous group included 40 subjects between the ages of 62 and 65 years. A total of 120 dentate cases were divided into three groups based on age: (1) Established occlusion group: 40 subjects between 12 and 15 years old; most of the subjects were orthodontic patients. (2) Adult group: 40 subjects between 37 and 40 years old; most of the patients were willing to undergo implant surgery. (3) Elderly group: 40 subjects between 62 and 65 years old; most of the patients were willing to undergo implant surgery.

The inclusion criteria were as follows: presence of at least one maxillary premolar and two maxillary molars on each side for the dentate groups, toothless maxilla for the edentulous group, and no maxillofacial trauma or history of surgery. The exclusion criteria were as follows: skeletal malocclusion, osteomyelitis of the jaws, and cyst or tumour in the maxilla.

All the scans were conducted with the same NewTom VG10048S scanner (NewTom Cone Beam Imaging, QR Srl, Verona, Italy) protocol, with 0.25-mm-thick slices and 30 s to obtain the raw data. Using three-dimensional (3D) images generated by SimPlant 11.04 (Materialise Dental, Leuven, Belgium), linear and angular measurements were obtained by simulating zygomatic implantation in a four-implant protocol by the same senior radiologist.

Each subject maintained an upright sitting position in front of the scanner with his or her chin on the cephalostat. A red light beam was used to ensure that the Frankfort horizontal plane was parallel and that the midsagittal plane was perpendicular to the floor.