To examine the relationship between molar bite force (MBF) and both the upper and lower axis inclination of the incisors.

The sample consisted of 51 participants aged between 18–27years (mean age: 21.04±2.03), with normal occlusion and a skeletal Class I relationship, no sign of temporomandibular disorders, and no previous orthodontic treatment. Lateral cephalograms were taken and traced and the following measurements were identified: upper incisor angle (U1:SN), lower incisor angle (L1:GoMe), and the interincisal angle (U1:L1). Three sets of groups were established according to the three previously mentioned measurements (G1: decreased; G2: normal; and G3: increased). MBF was measured using a specific sensor that is sensitive to the pressure. Patients were asked to bite in the central occlusion, chew, then swallow the saliva. The mean values and standard deviation of MBF were obtained in each position and in every group then compared between related groups using the independent sample t-test.

MBF values were significantly lower in the decreased upper incisor angle group during chewing at each side than in the normal (P=0.02 right, 0.01 left) and increased (P=0.03 right, 0.02 left) groups,while higher in the increased lower incisor group compared with the normal group (P=0.01) during chewing at the right side. In addition, there was a significant positive correlation between the upper incisor axis and MBF values both in central maximum clenching and chewing. Moreover, there was a positive correlation between the MBF and lower incisor axis during chewing, whereas a negative correlation between the MBF and interincisal angle was detected during chewing and swallowing.

There was a linear relationship between the MBF and incisor axis. The MBF was higher when both the upper and lower inclination of incisors increased.