Multiscale biomechanical responses of adapted bone–periodontal ligament–tooth fibrous joints

Abstract

Reduced functional loads cause adaptations in organs. In this study, temporal adaptations of bone-ligament-tooth fibrous joints to reduced functional loads were mapped using a holistic approach. Systematic studies were performed to evaluate organ-level and tissue-level adaptations in specimens harvested periodically from rats given powder food for 6 months (N = 60 over 8,12,16,20, and 24 weeks). Bone-periodontal ligament (PDL)-tooth fibrous joint adaptation was evaluated by comparing changes in joint stiffness with changes in functional space between the tooth and alveolar bony socket. Adaptations in tissues included mapping changes in the PDL and bone architecture as observed from collagen birefringence, bone hardness and volume fraction in rats fed soft foods (soft diet, SD) compared to those fed hard pellets as a routine diet (hard diet, HD). In situ biomechanical testing on harvested fibrous joints revealed increased stiffness in SD groups (SD:239-605 N/mm) (p<0.05) at 8 and 12 weeks. Increased joint stiffness in early development phase was due to decreased functional space (at 8wks change in functional space was −33 µm, at 12wks change in functional space was −30 µm) and shifts in tissue quality as highlighted by birefringence, architecture and hardness. These physical changes were not observed in joints that were well into function, that is, in rodents older than 12 weeks of age. Significant adaptations in older groups were highlighted by shifts in bone growth (bone volume fraction 24wks: Δ-0.06) and bone hardness (8wks: Δ−0.04 GPa, 16 wks: Δ−0.07 GPa, 24wks: Δ−0.06 GPa). The response rate (N/s) of joints to mechanical loads decreased in SD groups. Results from the study showed that joint adaptation depended on age. The initial form-related adaptation (observed change in functional space) can challenge strain-adaptive nature of tissues to meet functional demands with increasing age into adulthood. The coupled effect between functional space in the bone-PDLtooth complex and strain-adaptive nature of tissues is necessary to accommodate functional demands, and is temporally sensitive despite joint malfunction. From an applied science perspective, we propose that adaptations are registered as functional history in tissues and joints.

Tensile testing of the mechanical behavior of the human periodontal ligament

“…The teeth used for biomechanical testing were imaged [ 15 ] by microcomputed tomography (Viva CT 80, Scanco Medical), as shown in Fig. 1 .…”

Association of feeding behavior with jaw bone metabolism and tongue pressure

“…Another study suggested that the alveolar trabecular bone loss induced by occlusal hypofunction is correlated with a decrease in the number of osteoclasts, and this bone resorption causes a reduction in bone volume to the minimum necessary in the absence of forces stimulating the alveolar bone, such as occlusal force, in growing rats [39] . Also, a soft diet affected the turnover of periodontal ligament collagen and altered the bone dynamics of the tooth furcation area in young rats [40] . In a histological study, a 4-week powdered diet resulted in trabecular bone loss and enlargement of the bone marrow cavity; however, trabeculae attached to the periodontal ligament remained in the maxilla and mandible of the rats [30] .…”

Periodontal ligament entheses and their adaptive role in the context of dentoalveolar joint function

“…In mammalian species, function-related thresholds are most often modulated by food hardness (7, 34, 91–94). Previous studies have shown that the width of the PDL-space changes in response to magnitude and direction of occlusal loads (7).…”

“…Previous studies have shown that the width of the PDL-space changes in response to magnitude and direction of occlusal loads (7). Hyperfunction during which force exceeds physiologic loading (increased magnitude and decreased frequency) has shown to increase PDL-space through an upregulated osteoclastic response, while conditions of hypofunction (decreased magnitude and increased frequency) illustrated modeling of the PDL width to a narrower space than under control conditions (17, 34, 78, 83, 95).…”