Serious facial fractures in New Zealand from 1979 to 1998

We present data on the incidence, aetiology, age, sex and ethnic distribution of facial fractures in New Zealand for the 20-year period from 1979-1998. Most facial fractures (78.9%) occurred in males with a rate of 65.5/100,000, person-years compared with 21% in females with an incidence of 17/100,000. While the injury rate peaked in males between the ages of 20-24 years (200/100,000), it peaked between 15-19 years (34.7/100,000) in females. The most common causes of facial fracture in both genders were assault (14/100,000) and being unintentionally struck by an object or person (9.5/100,000) which is consistent with similar data from South Africa and the USA. The rates of fracture in Maori (68.1/100,000) were approximately twice those of Pacific Islanders (37/100,000) or other ethnic groups (34.2/100,000).

Tongue pressure patterns during water swallowing

Bolus propulsion during the normal oral phase of swallowing is thought to be characterised by the sequential elevation of the front, middle, and posterior regions of the dorsum of the tongue. However, the coordinated orchestration of lingual movement is still poorly understood. This study examined how pressures generated by the tongue against the hard palate differed between three points along the midline of the tongue. Specifically, we tested three hypotheses: (1) that there are defined individual patterns of pressure change within the mouth during liquid swallowing; (2) that there are significant negative pressures generated at defined moments during normal swallowing; and, (3) that liquid swallowing is governed by the interplay of pressures generated in an anteroposterior direction in the mouth. Using a metal appliance described previously, we measured absolute pressures during water swallows in six healthy volunteers (4 male, 2 female) with an age range of 25-35 years. Participants performed three 10-ml water swallows from a small cup on five separate days, thus providing data for a total of 15 separate water swallows. There was a distinct pattern to the each of the pressure signals, and this pattern was preserved in the mean obtained when the data were pooled. Furthermore, raw signals from the same subjects presented consistent patterns at each of the five testing sessions. In all subjects, pressure at the anterior and hind palate tended to be negative relative to the preswallow value; at mid-palate, however, pressure changes were less consistent between individuals. When the pressure differences between the sites were calculated, we found that during the swallow a net negative pressure difference developed between anterior and mid-palate and a net positive pressure difference developed between mid-palate and hind palate. Large, rapid fluctuations in pressure occurred at all sites and these varied several-fold between subjects. When the brief sharp reduction in pressure that occurred early in each swallow was used to determine the sequence of events, we found that activity occurred first at the anterior of the palate followed by the mid-palate and then the hind palate. There was a considerably longer and more variable delay between the start of activity at the front of the palate than at the rear of the palate. To obtain an index of the "effort" involved in generating the pressures at each site regardless of direction (positive or negative), we obtained the product of the root mean square (RMS) pressure change during each swallow (kPa) and its duration (s). Overall, the most effort appears to have occurred at the front of the palate and the least at mid-palate. Our results also showed that some participants exerted a small amount of midline pressure when swallowing, while others used a relatively large amount of tongue pressure. We conclude that while tongue behaviour during swallowing follows a classical sequence of rapid shape changes intended to contain and then propel the bolus from the oral cavity to the pharynx, there is a large range of individual variability in how this process is accomplished.

Patterns of emergence of the human mental nerve

This study investigated the path of emergence of the mental nerve in a number of human population groups. Skeletal material comprised 117 Negro skulls (53 males), 114 caucasoid skulls (62 males) and 100 pre-contact Maori skulls (70 males). In each case, the path of emergence was classified into posterior, anterior, right-angled or multiple. Those cases with severely resorbed alveolar ridges that made classification difficult were excluded from the study. Additionally, 56 cadaveric mandibles were examined, in which an osteotomy of 1cm was made on either side of the mental foramen to expose the nerve. The most common pattern of emergence in caucasoids and Maoris was a posterior direction (86.7% of caucasoid males, 90.2% of caucasoid females; 85.5% of Maori males, 93.1% of Maori females). In Negroes the most common pattern was a right-angled path of emergence (45.8% of males, 45.0% of females), with this difference between population groups being statistically significant (Pearson's chi(2): males=23.4, females=45-97; P<0.01). Multiple foramina were rare, with the highest incidence being in Maori and Negro males. Cadaveric data supported the findings of the skeletal investigation, with the dominant emergence recorded as posteriorly directed (80.7% of males, 86.6% of females). It was concluded that while the traditionally accepted ontogenetic explanation for the inclination of the mental nerve might be applicable to caucasoids and Maoris, it fails to explain the observed right-angled emergence pattern in Negroes. Hence, the nerve's emergence might be genetically, rather than functionally, determined. The study did not show a measurable anterior loop in the emergence of the mental nerve that would have any significant impact on treatment planning for implants in the anterior mandible.

Restoration of non-carious cervical lesionsPart I. Modelling of restorative fracture

OBJECTIVE

As a typical non-carious cervical lesion, abfraction is a common clinical occurrence which requires restorative treatment in most patients. Nonetheless, the relatively poor clinical longevity of cervical dental used for restoring abfraction lesions has been a major concern of dentists and patients. The continuing loss of hard tissue and, in turn, the low retention of the restorative materials in situ motivates an in-depth exploration of the failure mechanism of the biomaterials involved. Despite considerable biomechanical relevance, conventional application of linear static finite element analysis (FEA) does not consider the fracture failure process, nor does it provide a quantitative predictive analysis for restorative design. This paper adopts a novel Rankine and rotating crack model to trace the fracture failure process of the cervical restorations.

METHODS

In contrast to the existing linear FEA, this study presents a nonlinear fracture analysis in an explicit finite element framework, which involves an automatic insertion of initial crack, mesh updating for crack propagation and self contact at the cracked interface.

RESULTS

The results are in good agreement with published clinical data, in terms of the location of the fracture failure of the simulated restoration and the inadequacy of the dental restoratives for abfraction lesions. The success of the proposed model also demonstrates the potential for the monitoring and prediction of mechanical failure in other brittle biomaterials in a clinical situation.

Modelling of fracture behaviour in biomaterials☆

One of the most frequent causes of degradation and failure of quasi-brittle biomaterials is fracture. Mechanical breakdown, even when not catastrophic, is of particular importance in the area of biomaterials, as there are many clinical situations where it opens the path for biologically mediated failures. Over the past few decades the materials/biomaterials community has developed a number of numerical models, but only with limited incorporation of brittle failure phenomena. This article investigates the ability of a non-linear elastic fracture mechanics (NLEFM) model to reliably predict failure of biomaterials with a specific focus on the clinical settings of restorative dentistry. The approach enables one to predict fracture initiation and propagation in a complex biomechanical status based on the intrinsic material properties of the components. In this paper, we consider five examples illustrating the versatility of the present approach, which range from the failure of natural biomaterials, namely dentine and enamel, to a restored tooth, a three unit all ceramic bridge structure and contact-induced damage in the restorative layered materials systems. It is anticipated that this approach will have ramifications not only to model fracture events but also for the design and optimisation of the mechanical properties of biomaterials for specific clinically determined requirements.

The course and distribution of the inferior alveolar nerve in the edentulous mandible

The authors undertook the study to determine the pattern of intrabony distribution of the branches of the inferior alveolar nerve (IAN) in the edentulous mandible. One hundred seven human cadaveric mandibles were microdissected from their buccal aspect. In each case the height and distribution of the IAN was classified. The authors found that the IAN was located in the lower half of the mandible in 73% of males and 70% of females. The most common branching pattern observed was a single nerve trunk with a series of simple branches directed at the superior border of the mandible (59.6% males, 52% females). The second most common pattern was that of a small nerve plexus in the molar region (21.1% males, 26% females). The results showed that the pattern of distribution does not significantly differ between the sexes, between sides of the jaw, or with age.

The biomechanical modelling of non-ballistic skin wounding: blunt-force injury

Knowledge of the biomechanical dynamics of blunt force trauma is indispensable for forensic reconstruction of a wounding event. In this study, we describe and interpret wound features on a synthetic skin model under defined laboratory conditions. To simulate skin and the sub-dermal tissues we used open-celled polyurethane sponge (foam), covered by a silicone layer. A drop tube device with three tube lengths (300, 400, and 500 mm), each secured to a weighted steel scaffold and into which a round, 5-kg Federal dumbbell of length 180 mm and diameter 8 cm was placed delivered blows of known impact. To calculate energy and velocity at impact the experimental set-up was replicated using rigid-body dynamics and motion simulation software. We soaked each foam square in 500 mL water, until fully saturated, immediately before placing it beneath the drop tube. We then recorded and classified both external and internal lacerations. The association between external wounding rates and the explanatory variables sponge type, sponge thickness, and height were investigated using Poisson regression. Tears (lacerations) of the silicone skin layer resembled linear lacerations seen in the clinical literature and resulted from only 48.6% of impacts. Poisson regression showed there was no significant difference between the rate of external wounding for different sponge types (P = 0.294) or different drop heights (P = 0.276). Most impacts produced "internal wounds" or subsurface cavitation (96%). There were four internal "wound" types; Y-shape (53%), linear (25%), stellate (16%), and double crescent (6%). The two-way interaction height by sponge type was statistically significant in the analysis of variance model (P = 0.035). The other two-way interactions; height by thickness and sponge type by thickness, were also bordering on statistical significance (P = 0.061 and P = 0.071, respectively). The observation that external wounds were present for less than half of impacts only, but that nearly all impacts resulted in internal wounds, might explain the observed haematoma formation and contusions so often associated with blunt-force injuries. Our study also confirms the key role of hydrodynamic pressure changes in the actual tearing of subcutaneous tissue. At the moment and site of impact, transferred kinetic energy creates a region of high pressure on the fluid inside the tissue. As a result of the incompressibility of the fluid, this will be displaced away from the impact at a rate that depends on the velocity (or kinetic energy) of impact and the permeability and stiffness of the polymeric foam and skin layer.

Tongue contractions during speech may have led to the development of the bony geometry of the chin following the evolution of human language: A mechanobiological hypothesis for the development of the human chin

One of the most fundamental yet unanswered questions of human evolution is that of the development of the chin. Whereas it is known that the chin, or mentum osseum, is an unique anatomical feature of modern humans that emerged during the Middle and Late Pleistocene, its origin and biomechanical significance are the subjects of intense controversy. Theories range from the suggestion that the chin evolved as a result of progressive reduction of the dental arch, which left it as a protrusion, to the hypothesis that it provided resistance to mandibular bending during mastication. Until now however, no accepted functional explanation of the human chin has emerged. Here, we develop the hypothesis that the actions of the tongue and non-masticatory orofacial muscles may have played a significant role on the development of the human chin. We report numerical simulations of the forces and resultant stresses developed in hypothetical chinned and non-chinned mandibles. Using empirical data and estimates of the forces generated by the human tongue during speech, our hypothesis suggests that the chin might in fact have developed as a result of the actions of the tongue and perioral muscles, rather than as a buttress to withstand masticatory induced stress. This provides a new perspective on the generation of the chin and importantly, suggests that its appearance may be causally related to the development of the human language.

Development of a skull/brain model for military wound ballistics studies

Reports on penetrating ballistic head injuries in the literature are dominated by case studies of suicides; the penetrating ammunition usually being .22 rimfire or shotgun. The dominating cause of injuries in modern warfare is fragmentation and hence, this is the primary threat that military helmets protect the brain from. When helmets are perforated, this is usually by bullets. In combat, 20% of penetrating injuries occur to the head and its wounding accounts for 50% of combat deaths. A number of head simulants are described in the academic literature, in ballistic test methods for helmets (including measurement of behind helmet blunt trauma, BHBT) and in the 'open' and 'closed' government literature of several nations. The majority of these models are not anatomically correct and are not assessed with high-velocity rifle ammunition. In this article, an anatomically correct 'skull' (manufactured from polyurethane) and 'brain' (manufactured from 10%, by mass, gelatine) model for use in military wound ballistic studies is described. Filling the cranium completely with gelatine resulted in a similar 'skull' fracture pattern as an anatomically correct 'brain' combined with a representation of cerebrospinal fluid. In particular, posterior cranial fossa and occipital fractures and brain ejection were observed. This pattern of injury compared favourably to reported case studies of actual incidents in the literature.

Damage to apparel layers and underlying tissue due to hand-gun bullets

Ballistic damage to the clothing of victims of gunshot wounds to the chest can provide useful forensic evidence. Anyone shot in the torso will usually be wearing clothing which will be damaged by the penetrating impact event and can reportedly be the source of some of the debris in the wound. Minimal research has previously been reported regarding the effect of bullets on apparel fabrics and underlying tissue. This paper examines the effect of ammunition (9 mm full metal jacket [FMJ] DM11 A1B2, 8.0 g; and soft point flat nose Remington R357M3, 10.2 g) on clothing layers that cover the torso (T-shirt, T-shirt plus hoodie, T-shirt plus denim jacket) and underlying structures represented by porcine thoracic wall (skin, underlying tissue, ribs). Impacts were recorded using a Phantom V12 high speed camera. Ejected bone debris was collected before wound tracts were dissected and measured; any debris found was recovered for further analysis. Size and mass of bony debris was recorded; fibre debris recovered from the wound and impact damage to fabrics were imaged using scanning electron microscopy (SEM). Remington R357M3 ammunition was characteristically associated with stellate fabric damage; individual fibres were less likely to show mushrooming. In contrast, 9 mm FMJ ammunition resulted in punch-out damage to fabric layers, with mushrooming of individual fibres being more common. Entry wound sizes were similar for both types of ammunition and smaller than the diameter of the bullet that caused them. In this work, the Remington R357M3 ammunition resulted in larger exit wounds due to the bullet construction which mushroomed. That fabric coverings did not affect the amount of bony debris produced is interesting, particularly given there was some evidence that apparel layers affected the size of the wound. Recent work has suggested that denim (representative of jeans) can exacerbate wounding caused by high-velocity bullet impacts to the thigh when the bullet does not impact the femur. That more bony debris was caused by Remington R357M3 rather than 9 mm FMJ ammunition was not surprising given the relative constructions of these two bullets, and is of interest to medical practitioners.

Axial-wall inclination angle and vertical height interactions in molar full crown preparations

OBJECTIVE

This study has been designed to evaluate the interaction of axial wall heights with inclination angles in full crown tooth preparations. The interaction of these parameters was related to the resulting preparation surface area.

MATERIALS AND METHODS

A right regular pyramid was used to simulate a single mandibular molar preparation with known convergence angles and vertical heights. Various combinations of these two variables allowed the calculation of surface areas with a formula for the area of a pyramid and right triangles through trigonometric manipulations. The pyramidal model system had a 9-mm square base with vertical heights from 3- to 5-mm and single-side inclination angles from 2 to 25 degrees. The occlusal surface was a flat, square or rectangular surface and was included in the total area.

RESULTS

A percentage of surface area lost or gained served as the dependent variables. The significance levels were set at 10.0% or greater magnitude of loss/gain in a surface area compared to the ideal 2 degree-level. Significant area loss was demonstrated in all alpha-level comparisons. The largest change was found in the 5-mm height grouping compared to the 3-mm height grouping, -36.6% difference between groups at the 2 degree-level.

CONCLUSIONS

Axial single-side inclination angles greater than 10 degrees in 3- and 4-mm height-molars are detrimental to maximum surface area in full crown restorations. The 5-mm axial wall height with < or = 10 degree single wall has been shown to maximize the luting agent surface area between restoration and tooth structure.

Analysis of experimental cranial skin wounding from screwdriver trauma

As part of a more extensive investigation of skin wounding mechanisms, we studied wounds created by five common screwdrivers (straight, star, square or Robertson, Posidriv and Phillips) on the shaven foreheads of 12 freshly slaughtered pigs. We fixed the different screwdriver heads to a 5-kg metal cylinder which was directed vertically onto each pig head by a droptube of 700 mm length. We examined skin lesions by photography and also by scanning electron microscopy (SEM). Our evaluation of differences in wound shape and size was based on geometric morphometric methods. Our results show that there are obvious morphological differences between the straight head and the other types. The straight-headed screwdriver penetrates the skin by a mode II crack which results in a compressed skin plug with bundles of collagen fibres forming skin tabs within the actual wound. The sharper-tipped screwdrivers wedge open the skin (mode I), with a clearly defined edge with no skin plugs. Geometric morphometric analysis indicates that shapes of skin wounds created by the five screwdriver types could be classified into three different groups. The straight head results in the most differentiated wound profile, with the Robertson or square and some specimens of star, and also the Posidriv and Phillips giving similar wound outlines. SEM evaluation of wounds created by a new and worn straight-head screwdrivers shows that the outline of the worn screwdriver head is reflected in the shape of the wound it created.

Effect of Down syndrome on the dimensions of dental crowns and tissues

Abnormal growth in Down syndrome (DS) is reflected by variable reduction in size and simplification in form of many physical traits. This study aimed to compare the thickness of enamel and dentine in deciduous and permanent mandibular incisor teeth between DS and non-DS individuals and to clarify how these tissues contribute to altered tooth size in DS. Sample groups comprised 61 mandibular incisors (29 permanent and 32 deciduous) from DS individuals and 55 mandibular incisors (29 permanent and 26 deciduous) from non-DS individuals. Maximum mesiodistal and labiolingual crown dimensions were measured initially, then the crowns were sectioned midsagittally and photographed using a stereomicroscope. Linear measurements of enamel and dentine thickness were obtained on the labial and lingual surfaces of the crowns, together with enamel and dentine-pulp areas and lengths of the dentino-enamel junction. Reduced permanent crown size in DS was associated with a reduction in both enamel and dentine thickness. After adjustments were made for tooth size, DS permanent incisors had significantly thinner enamel than non-DS permanent teeth. The DS permanent teeth also exhibited significant differences in shape and greater variability in dimensions than the non-DS permanent teeth. Crown dimensions of deciduous incisors were similar in size or larger in DS compared with non-DS deciduous teeth. Enamel and dentine thicknesses of the deciduous teeth were similar in DS and non-DS individuals. The findings indicate that growth retardation in DS reduces both enamel and dentine deposition in the permanent incisors but not in the earlier-forming deciduous predecessors. The results are also consistent with the concept of amplified developmental instability for dental traits in DS.

Three-dimensional computed tomography reconstruction in treatment planning for large ameloblastoma

A case of an unusually large mandibular ameloblastoma is reported in which standard radiologic examination failed to provide adequate visualization of the tumor's limits because of overlapping structures and the presence of compound shadows. Computed tomography imaging and three-dimensional reconstruction proved invaluable in the treatment planning of this large lesion, because it allowed the surgeon to visualize and comprehend the geography of the tumor and its relationship to surrounding structures and soft tissues and to formulate a surgical plan. Use of this technique helped ensure complete excision of the tumor without excessive removal of uninvolved tissue.

Neck and shoulder muscle activity during standardized work-related postural tasks

The aim of the present study was to assess the activity levels of the sternocleidomastoid muscle and upper trapezius muscle during static postures under controlled and standardized conditions, and to determine whether the muscle activity differed between sexes. Electromyographic (EMG) activity was recorded unilaterally from the sternocleidomastoid and upper trapezius muscle in 17 participants whilst they were performing various postural tasks. EMG amplitude was measured by the root mean square values of the raw signals and normalized to peak maximum contractile values for each muscle (%MVC). The intensity of muscle activity was ranked as light (<3%MVC), moderate (3%MVC ≤ EMG ≤ 8%MVC), and substantial (>8%MVC). During most tasks the two muscles contracted light to moderately. Head leaning and shoulder shrugging postures yielded substantial muscle activity in both muscles. Muscle activity did not differ significantly between male and female participants (F = 3.1; p = 0.078). Our findings provided normative values, which will enhance future studies of muscle activity during work in a natural, unrestrained environment.

Effect of occlusal vertical dimension on swallowing patterns and perioral electromyographic activity

Abnormal swallow patterns have been associated with specific dentofacial traits, such as an anterior open bite, but the cause-effect relationship between swallowing and malocclusion remains highly controversial. The aim of this research was to determine the effects of acute change in occlusal vertical dimension (OVD) on intraoral pressure swallow patterns and perioral electromyographic activity (EMG) during swallowing. Ten volunteers (five female, five male; 27-32 years) repeated standardised swallowing tasks as the OVD was progressively increased using mandibular trays of different heights. Standardised swallowing tasks were performed repetitively with each tray in place. Individual swallowing waveforms were quantitatively and qualitatively analysed. Peak pressure, swallow duration, time to peak pressure and lip EMG peak activity were assessed for each swallow. Data were analysed using mixed-model analysis. As OVD increased, lip peak pressure during swallowing increased almost threefold (+2·1 kPa; P ≤ 0·001), whereas swallow duration increased by 12·7 per cent (+160 ms; P = 0·01) at lip level and by 26·4 per cent (+270 ms; P < 0·001) at tongue level. Perioral muscle activity during swallows increased by 43·7 per cent (P ≤ 0·01) up to the OVD where resting lip seal was not attainable. Swallowing waveforms varied markedly between individuals, but interindividual waveforms were only minimally affected. The adaptive response and the waveform similarities associated with OVD variation supports the existence of a central control mechanism for swallowing, which may be modified by peripheral inputs.

Wisdom Tooth Formation as a Method of Estimating Age in a New Zealand Population

Dental ageing relies on assumptions aboutthe progression of tooth development from the middletrimester to adulthood and relative stability of thisprocess in the face of adverse dietary, hormonal, diseaseor nutritional factors. Most studies of dental ageingemploy the method of Demirjian et al., (1973), which isbased upon an assessment of crown and root formationstages from dental radiographs. Unfortunately, thismethod has a ceiling effect at age 16, when the secondmolar attains full maturity. The aim of our study was toextend the window of ageing by using the developmentof the third molar teeth. Panoramic radiographs of 207(105 males) children aged between 7 years, 6 months and18 years formed the basis of this study. Upper and lowerleft wisdom teeth were scored according to Demirjian et al. (1973) by a single examiner. Intra-examiner reliabilitywas evaluated by repeat scoring of a randomly selected(10%) sample one week after the initial staging. Theseshowed a consistency of 76% for the mandibular data and95% for the maxillary data, giving an overall percentageof 85%. When the re-scored teeth were not consistentwith their original score, this differed only by one stage.In this population males were advanced in their thirdmolar development and this trend was more markedfor maxillary than mandibular wisdom teeth. Hence, theNew Zealand population examined, males were moreadvanced in their third molar development and thistrend was more marked for maxillary than mandibularteeth.