The effect of unilateral lingual nerve injury on the kinematics of mastication in pigs.

OBJECTIVE: This study evaluates the effect of unilateral lingual sensory loss on the spatial and temporal dynamics of jaw movements during pig chewing. DESIGN: X-ray Reconstruction of Moving Morphology (XROMM) was used to reconstruct the 3-dimensional jaw movements of 6 pigs during chewing before and after complete unilateral lingual nerve transection. The effect of the transection were evaluated at the temporal and spatial level using Multiple Analysis of Variance. Temporal variables include gape cycle and phase durations, and the corresponding relative phase durations. Spatial variables include the amplitude of jaw opening, jaw yaw, and mandibular retraction-protraction. RESULTS: The temporal and spatial dynamics of jaw movements did not differ when chewing ipsilateral versus contralateral to the transection. When compared to pre-transection data, 4 of the 6 animals showed significant changes in temporal characteristics of the gape cycle following the transection, irrespective of chewing side, but the specific response to the lesion was highly dependent on the animal. On the other hand, in affected individuals the amplitude of jaw movements was altered similarly in all 3 dimensions: jaw opening and protraction-retraction increased whereas jaw yaw decreased. CONCLUSION: The variable impact of this injury in this animal model suggests that individuals use different compensatory strategies to adjust or maintain the temporal dynamics of the gape cycle. Because the amplitude of jaw movements are more adversely affected than their timing, results suggest that maintaining the tongue-jaw coordination is critical and this can come at the expense of bolus handling and masticatory performance.

Flexibility of feeding movements in pigs: effects of changes in food toughness and stiffness on the timing of jaw movements.

In mammals, chewing movements can be modified, or flexible, in response to changes in food properties. Variability between and within food in the temporal characteristics of chewing movements can impact chewing frequency and rhythmicity, which in turn may affect food breakdown, energy expenditure and tooth wear. Here, we compared total chewing cycle duration and intra-cycle phase durations in pigs chewing on three foods varying in toughness and stiffness: apples (low toughness, low stiffness), carrots (high toughness, low stiffness), and almonds (high toughness, high stiffness). We also determined whether within-food variability in timing parameters is modified in response to changes in food properties. X-ray Reconstruction Of Moving Morphology (XROMM) demonstrates that the timing of jaw movements are flexible in response to changes in food properties. Within each food, pigs also exhibited flexibility in their ability to vary cycle parameters. The timing of jaw movements during processing of high-toughness foods is more variable, potentially decreasing chewing rhythmicity. In contrast, low-toughness foods result in jaw movements that are more stereotyped in their timing parameters. In addition, the duration of tooth-food-tooth contact is more variable during the processing of low-stiffness foods compared with tough or stiff foods. Increased toughness is suggested to alter the timing of the movements impacting food fracture whereas increased stiffness may require a more cautious control of jaw movements. This study emphasizes that flexibility in biological movements in response to changes in conditions may not only be observed in timing but also in the variability of their timing within each condition.

An ontogenetic perspective on symphyseal fusion, occlusion and mandibular loading in alpacas (Vicugna pacos).

A primary hypothesis for the evolution of mandibular symphyseal fusion in some mammals is that it functions to resist loads incurred during routine mastication. Anecdotal support for this hypothesis is based on the fact that when the symphysis fuses, it typically does so early during postnatal ontogeny prior to or around the time of weaning. However, little is known about the process of fusion, particularly relative to feeding behaviors and the dynamics of mastication, including occlusion and masticatory loading. In the present study, we investigate the timing and process of symphyseal fusion in alpacas (Vicugna pacos) in the context of maturation of the oral apparatus and oral behavior. We also report on in vivo strains from the symphysis and corpus in young alpacas prior to and following full fusion and M1 occlusion. Results show that fusion begins rostrally by 1 month and is complete by 6-7 months whereas all deciduous premolars and M1 come into occlusion by 6 months. Although symphyseal loading patterns are maintained throughout ontogeny, in young alpacas symphyseal strain magnitudes are low compared with adults but corpus strain magnitudes are comparable to those found in adults. Reduced symphyseal loading in young individuals is contrary to what might be predicted given that the symphysis is still fusing. When considered in light of the development of occlusion and rumination, strain magnitudes may be necessarily low and reflect an overall delay in the maturation of masticatory dynamics.

Occlusal development and masseter activity in Alpacas (Lama pacos).

Tooth eruption and the development of occlusion are significant ontogenetic changes in the masticatory apparatus of mammals. Here, we test the hypothesis that changes in masseter activity are correlated with increased occlusal contacts at major stages of dental development in the alpaca, Lama pacos. We compare electromyographic data from the superficial and deep masseter in infant and juvenile alpacas prior to and following m1 occlusion and from adults with full permanent dentitions. The pre-m1 and post-m1 occlusion groups exhibit similar masseter activity durations, chewing cycle durations, and with the exception of the balancing-side deep masseter, similar timing differences between the jaw muscles. On average, the balancing-side deep masseter fires significantly later in the post-m1 occlusion group. The m2-m3 group exhibits significantly longer chewing cycle length and an even later firing balancing-side deep masseter. Increased occlusion is also associated with an increase in the relative amount of working-side superficial and deep masseter muscle activity when compared with the balancing side muscles. Although the development of occlusal relations in infant and juvenile alpacas are associated with minor changes in masseter activation patterns, additional molar occlusal contacts increase chewing cycle duration resulting in concomitant changes in masseter recruitment patterns. Currently, we cannot rule out that musculoskeletal development influences masseter activity as demonstrated in other mammals. However, the data presented here indicate that alpacas have a relatively delayed onset of the adult motor pattern that may be correlated with changes in occlusal relations due to tooth eruption.