Occlusal effects on longitudinal bone alterations of the temporomandibular joint

Abstract

The pathological changes of subchondral bone during osteoarthritis (OA) development in the temporomandibular joint (TMJ) are poorly understood. In the present study, we investigated the longitudinal alterations of subchondral bone using a rat TMJ-OA model developed in our laboratory. Changes in bone mass were examined by micro-CT, and changes in osteoblast and osteoclast activities were analyzed by real-time PCR, immunohistochemistry, and TRAP staining. Subchondral bone loss was detected from 8 weeks after dental occlusion alteration and reached the maximum at 12 weeks, followed by a repair phase until 32 weeks. Although bone mass increased at late stages, poor mechanical structure and lower bone mineral density (BMD) were found in these rats. The numbers of TRAP-positive cells were increased at 12 weeks, while the numbers of osteocalcin-expressing cells were increased at both 12 and 32 weeks. Levels of mRNA expression of TRAP and cathepsin K were increased at 12 weeks, while levels of ALP and osteocalcin were increased at both 12 and 32 weeks. These findings demonstrated that there is an active bone remodeling in subchondral bone in TMJs in response to alteration in occlusion, although new bone was formed with lower BMD and poor mechanical properties.

Figure 1.

In vivo micro-CT images of the TMJ condylar head from the control and experimental (EXP) groups. (A) The images were arranged from left to right according to the time sequence. Local subchondral bone lesions were observed in the experimental group. In general, the lesion regions (indicated by arrows) were larger at 12 wks after surgery than those at the other time-points. (B) Illustration of the methods for measuring the width (a–b) and length (c–d) of the condylar head. (a) The outermost point of the ventral contour of the condyle, (b) the outermost point of the dorsal contour of the condyle, (c) the most anterior point of the condyle, and (d) the most posterior point of the condyle. (C) Illustration of the method for locating the region of interest (ROI) for micro-CT analysis. The surface of the condylar head was equally divided into 3 parts, and the ROIs were located at the center of the middle and posterior parts.

Figure 2.

Subchondral bone loss in the TMJ condyle. (A) Micro-CT analysis showed that significant bone loss started from the end of 8 wks and reached the peak at around 12 wks. (B) Hematoxylin-eosin (HE) staining of sections showed larger trabecular spacing in both the 12- and 32-week experimental groups. (C) Histomorphological parameters confirmed the significant subchondral bone loss in both the 12- and 32-week experimental groups. The black areas show the selected regions used for analysis of the histomorphological parameters (*p < 0.05, **p < 0.01).

Figure 3.

Analysis of osteoclast activity in TMJ condylar subchondral bone. (A) TRAP staining of TMJ subchondral bone from 12- and 32-week control and experimental groups. (B) TRAP-positive cells with more than 3 nuclei were counted as the number of osteoclasts (Oc.N). (C) Real-time PCR analysis of expression of RANKL, OPG, TRAP, and cathepsin K genes from the subchondral bone of mandibular condylar heads in 12- and 32-week control and experimental groups (*p < 0.05, **p < 0.01).

Figure 4.

Analysis of osteoblast activity in TMJ condylar subchondral bone. (A) Osteocalcin immunohistochemical (IHC) staining of TMJ subchondral bone from 12- and 32-week control and experimental groups. (B) The number of osteocalcin-positive cells within the selected area was counted. (C) Real-time PCR analysis of expression of ALP, Col1a1, Col1a2, the ratio of Col1a1/Col1a2, osteocalcin, DMP-1, and VEGF genes from the subchondral bone of the mandibular condylar head in 12- and 32-week control and experimental groups (*p < 0.05, **p < 0.01).