Stress and displacement pattern evaluation using two different palatal expanders in unilateral cleft lip and palate: a three-dimensional finite element analysis

RPE has been advocated for correcting maxillary transverse deficiency in cleft lip and palate followed by bone grafting [11]. Owing to the difficulty to achieve orthopaedic expansion with RPE alone, it is often combined with a surgically assisted procedure [12]. Recently, microimplant-anchored skeletal expansion can be a viable option which eliminates the need for surgical procedure [14, 15, 16, 17].

Vyas et al. reported that the use of microimplant-assisted palatal distraction as an adjunct to SARPE provided adequate stable skeletal expansion [28]. Even though clinically they could achieve the skeletal expansion, the complex loading pattern and the biomechanical effects of this bone-borne palatal expander on the maxillary bone and the circum-maxillary sutures have not been evaluated. Hence, in the present study, we have used the bone-borne palatal expander and evaluated the stress distribution and displacement pattern using a finite element method.

The implants were placed on the palatal slopes to achieve efficient palatal expansion. Our results showed that with the BBPE, the greatest stress was seen at the implant insertion site which was gradually distributed along the palatal slopes on the cleft and non-cleft sides which is similar to the study reported by Lee et al. [29].

However, the stress distribution in the mid-palatal suture area was not clearly interpreted since there is absence of fusion of two palatal shelves in cleft palate. For this reason, we assessed the stress around the mid-palatal suture area in the pre-maxilla, canine, pre-molar and molar regions. With the BBPE, the greatest stress was observed around the pre-maxillary region as well as at the secondary palatal area when compared to the conventional HYRAX expander.

Holberg et al. in their finite element study has reported that RPE can produce up to 120 N of force and suggested that slow expanders with forces of about 5 N will suffice to bring about the necessary skeletal expansion in cleft patients [24, 30]. However, in the present study, we applied about 5 N of force in the FE model and evaluated the stress distribution around the cleft palate area and the circum-maxillary sutures.

Lee et al. reported that the maximum amount of orthodontic force that can be tolerated by the microimplant is about 400 g and with implants that are 1.8 mm in diameter, 400 g of orthodontic loading produces 30 MPa of force [31]. Furthermore, recent studies have reported that the mini-screws with an optimal 9-mm length had the ability to withstand about 2 N of force without breakage [32, 33]. However, in order to achieve orthopaedic maxillary expansion, it is necessary to apply 5 N of force which has been already reported by Holberg et al. [24], and hence, we applied 5 N of force in our study.

Moreover, due to certain limitations, we confined the force level to only 5 N and future studies might provide further information regarding the stress distribution produced by different force levels with this type of BBPE appliance and can predict ideal force levels and activation protocols.

Furthermore, the unilateral cleft maxilla FE model created by Holberg et al. [24] included about 30,138 tetrahedral elements and 55,064 nodes. In the present study, we have used 255,140 tetrahedral elements and 255,270 nodes to create a refined 3D unilateral cleft maxillary FE model. Studies have reported that the periodontal ligament of the teeth and the viscoelastic property when incorporated in a FE model will greatly influence the outcome of the stress distribution in a traditional tooth-borne rigid palatal expansion appliance [20, 34, 35]. Nonetheless, in the present study, the BBPE used is directly anchored to the bone but not to the teeth.

In the BBPE, transverse displacement was greater at the dentoalveolar region without buccal tipping of the teeth in the contrary conventional expander displaced at the dentition level. Hence, true skeletal expansion can be achieved in cleft palate with the BBPE since the forces are concentrated directly at the alveolar bone level. Nevertheless the bone available, the site of implant placement has to be considered as a pre-requisite, and the appliance design might vary in various cleft palate conditions before using the BBPE.

Among the circum-maxillary sutures, the zygomaticomaxillary suture on the cleft side experienced the highest stress with the BBPE when compared to the conventional HYRAX expander than on the non-cleft side. This agrees with the previous study reported by Gautam et al. [36, 37]. Several studies have also shown that the zygomatic buttress offers the primary resistance to different expansive forces in the circum-maxillary area [36, 37, 38, 39]. Melsen and Melsen have reported that disarticulation is difficult in adolescents and adults due to heavy inter-digitations between the maxilla, palatine and pterygoid process of the sphenoid bone [40].

Stress at the zygomaticotemporal and nasomaxillary sutures on both the cleft and non-cleft sides in the BBPE was greater than that of the conventional HYRAX expander. However, the amount of stress experienced in these sutures was less than that of at the zygomaticomaxillary suture.

Recently, Ngan et al. had reported that in non-cleft class III individuals, the hybrid HYRAX bone-borne expansion device along with maxillary protraction yielded desirable sagittal skeletal change with minimal dental side effects [41]. Also Lin et al had concluded that in non-cleft individuals bone-borne maxillary expansion produced greater transverse orthopedic effects [42]. In such a case, if alternate expansion and constriction protocol is combined with BBPE, it might provide adequate disarticulation and future studies will define a refined expansion protocol in cleft palate patients.

Like many other finite element studies, this study without exception also has limitation due to the mathematical model as well as premises and assumptions used to generate the FE from a single patient which might not completely resemble the general population with individual variability as well as various clinical situations such as mid-palatal sutural viscoelastic property.