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Since the first description of orbital blow-out fractures, there has been much confusion as to their aetiology. Two principal mechanisms have been proposed to explain these fractures, the buckling and hydraulic mechanisms, caused by trauma to the orbital rim and the globe of the eye, respectively. Previous experimental and clinical studies have aimed to support one or other of these two theories. However, these studies have failed to provide quantifiable data to objectively support their conclusions. We present the results of a study of these two proposed mechanisms under identical conditions, using quantifiable intraocular pressure, variable and quantifiable force, and quantifiable bone strain distribution with strain gauge analysis in fresh intact human post-mortem cadavers. Both qualitative and quantitative findings suggest that efforts to establish one theory over the other as the primary mechanism have been misplaced. Both mechanisms produce orbital floor fractures, although these fractures differ fundamentally in their size and location. We have objectively demonstrated that it is easier to fracture the orbital floor by the hydraulic mechanism than by the buckling mechanism, and provided quantitative data for the average force required to displace the orbital floor.
The term ‘blow-out fracture of the orbit’ is widely used, and refers to a specific syndrome in which a fracture of the orbital floor occurs without involvement of the orbital rim, usually occurring as a result of a force impacting on the soft tissues of the orbit. This condition must be distinguished from an injury to the orbital floor associated with other fractures of the zygoma, maxilla or nasoethmoid bones.
provided one of the earliest descriptions of the signs and symptoms of such a blow-out fracture. Although this fracture pattern was first formally described by Converse and Smith,
who coined the term ‘blow-out fracture’. Whilst the orbital floor is most commonly involved in a blow-out fracture, a similar injury may affect the medial orbital wall, alone or in combination.
The orbital blow-out fracture and symptoms complex are readily recognisable clinically, but the mechanisms involved are not easily apparent. Current opinion is divided as to the exact aetiology mechanism underlying this fracture pathology. Experimental and clinical studies have aimed to support one or other of two proposed mechanisms: the ‘buckling’ and the ‘hydraulic’ theory.
The buckling theory contends that direct trauma to the infraorbital rim may cause transient of the rim deformation. This is transmitted to the thinner orbital floor, causing disruption of the bone without fracture of the rim. The hydraulic theory, in contrast, proposes that hydraulic pressure from the globe is transmitted to the walls of the orbit, resulting in fracture of the thin orbital floor.
Etude experimentale sur les fractures de la machoire superieure.
Rev Chir de Paris.1901; 23 (Translated by: Tessier P. The classic reprints: experimental study of fractures of the upper jaw: I and II. Plast Reconstr Surg, 1972; 50: 497–506): 208-227
have attempted to validate one or other of these two proposed mechanisms. However, most of these studies have been flawed in their design protocols. Many studies used dry skulls,
using fresh, intact unfixed human post-mortem cadavers, directed quantifiable forces onto the same area on the infraorbital rim. In their study, the majority of fractures were located in the anterior and anteromedial aspects of the orbital floor, and no medial wall involvement was observed. Significantly, no herniation of the orbital contents into the maxillary antrum was reported. The same study investigated the hydraulic mechanism, using quantifiable forces confined to the globe. Fresh cadavers were again studied, and the intraocular pressure was quantified. The authors concluded that, in contrast to fractures following isolated trauma to the rim, fractures following trauma to the globe were located more posteriorly and posteromedially, with invariable medial wall involvement. Herniation of periorbital contents into the maxillary antrum was common.
There are no previous studies directly comparing the two proposed injury mechanisms under identical conditions. In particular, no studies have provided quantitative data on bone strain distribution across the orbital floor, and the forces required to produce these fractures. This study, in reproducing the experimental protocols of Waterhouse et al
describes both qualitatively and quantitatively, the consequences of direct injuries to either the globe, or the orbital rim, using biomechanical data obtained from strain gauges placed on the orbital floor.
1. Materials and methods
Nine fresh, unfixed human post-mortem cadavers, providing a total of 18 orbits, were used for this study. None of the causes of death was believed to influence the strength of the orbital floor. Six female and three male cadavers were studied. The average age was 83.5 years (range: 68–94 years).
The experiments were completed between 1 and 7 days post mortem (mean: 3.7 days). Cold or frozen bodies were allowed to reach room temperature to avoid the possibility that ice crystals might increase bone brittleness.
Strain gauges were placed on the inferior surface of the orbital floor on the maxillary antral roof, without damaging the orbital tissues. Strain gauges were not applied to the medial orbital wall, to avoid interfering with the anatomical support of the orbit.
A Weber–Ferguson incision permitted reflection of a laterally based flap, exposing the anterior maxillary wall. An anterior antrostomy, using an osteotome, created a 1.5 cm diameter opening in the maxillary sinus (Fig. 1) . The sinus mucosal lining was meticulously removed using a Tessier periosteal elevator. Care was taken to avoid creating artefactual fractures of the orbital floor. The antral roof was cleaned with acetone on a cotton bud, and two single-foil uniaxial strain gauges (FLA511, Techni Measure Limited, Warwickshire, UK) were fixed to the cleansed surface using cyanoacrylate gel. These gauges were positioned to record strain in the anterior and posterior components of the orbital floor. Consistent gauge placement was achieved by aligning the gauge with the medial border of the infraorbital canal (Fig. 2) . Leads from the gauge were soldered onto pilot leads from a two-channel Fylde amplifier, which was connected to a Gould 20 MHz transient oscilloscope. This recorded changes in the electrical resistance of the wire in the strain gauge.
Figure 1The approach to the orbital floor via a Weber–Ferguson incision and a 1.5 cm anterior maxillary antrostomy.
As intraocular pressure is known to fall rapidly after death, it was necessary to reinflate the globes to within the normal range to reproduce in vivo conditions, as described by Waterhouse et al.
Between 0.7 and 1.2 ml of a solution of 70% glycerol and 30% saline, injected into both the anterior and the posterior chambers of the globe, was found to maintain intraocular pressure at normotensive levels (10–20 mmHg) for about 15 minutes. Intraocular pressure was monitored using a Perkins tenometer. Reinflation of the globe was occasionally necessary, prior to successive impacts.
The left orbit of each cadaver was used to study the hydraulic mechanism; the focal impact being directed onto the reinflated left globe. The right orbit was used to study the buckling mechanism, with the impacts being directed onto the mid-point of the infraorbital rim.
The impactor used was a solid cylindrical mass of 232 g with bevelled edges that minimised any soft-tissue injury following impact. The force of the impact was determined by the height from which the weight was released from a calibrated drop tower. This height was varied between 30 and 130 cm (Fig. 3) . After successive impacts, the circuitry was checked for electrical integrity, and the orbital floor was inspected for fractures transantrally using a powerful light source. In addition, the floor was carefully explored with a periosteal elevator to palpate any fractures. Photographs documented the fractures produced, which were also drawn onto an orbital template. The oscilloscope reading (in mV) was recorded on a graph and these recordings were converted to strain units (μϵ).
Figure 3The calibrated drop tower positioned to deliver isolated trauma accurately to the orbital rim or globe.
To exclude the possibility that the Weber–Ferguson incision, antrostomy and extirpation of the sinus mucosa might affect the structural integrity of the orbital complex, a separate control study was performed in one of the cadavers. In this control experiment, impacts were performed following reinflation of the eye but prior to the Weber–Ferguson incision and removal of the sinus mucosa. The floor was inspected transantrally following the usual incision and antrostomy.
2. Results
2.1 Qualitative results
Fractures were produced in both orbits of each of the nine cadavers. One orbit sustained an associated rim fracture. This occurred as a result of the maximum impact force onto the rim. Globe rupture also occurred in one orbit following the maximum impact force onto the globe. The fracture pattern templates obtained following impacts to the orbits are collectively shown in Figure 4.
Figure 4Templates demonstrating the combined fractures from all cadavers following (A) rim-directed (green) and (B) globe-directed (red) trauma.
Rim trauma clearly produced fractures that were localised to the anterior and anteromedial parts of the orbital floor (Fig. 5) . Only one orbit demonstrated extension of the fracture more posteriorly. In contrast, all orbits with impact forces focused on the globe, sustained fractures of the posterior and posteromedial aspects of the floor. In seven out of nine orbits these fractures also extended anteriorly to the anteromedial part of the floor (Fig. 6) .
Figure 5A typical linear anterior-floor fracture (arrows) is seen following rim-directed trauma. The anterior (A) and posterior (P) strain gauges can also be seen.
Figure 6A large segment of the anterior and posterior floor, with the attached anterior (A) and posterior (P) strain gauges, has fractured (arrows) following trauma to the globe.
Figure 7 shows typical examples of the strain gauge tracings obtained from the orbital floor following rim and globe trauma. The height of impactor drop at which fractures were first detected by the strain gauges, as demonstrated by a permanent shift in the baseline reading from the oscilloscope tracings, is recorded in Table 1.
Figure 7Typical strain gauge readings from the orbital floor following trauma to (A) the orbital rim and (B) the globe. In (A) the high-amplitude anterior (lower tracing) and low-amplitude posterior (upper tracing) readings indicate that the strain is limited to the anterior aspect of the orbital floor. In (B) the high-amplitude readings from both the anterior (lower tracing) and the posterior (upper tracing) strain gauges indicate that strain is distributed across the entire orbital floor.
Rim trauma. In seven of the eight experimental orbits where trauma was directed onto the rim, the strain gauge readings on the posterior part of the orbital floor were minimal compared with those recorded anteriorly. Strain did not exceed 221 μϵ in the posterior gauge, while conversely, the anterior gauge consistently showed readings in excess of 3756 μϵ. These low readings were in marked contrast to the high strain readings obtained from the posterior part of the orbital floor when the impact was directed onto the globe. The mean height from which the weight was dropped in order to produce a fracture by the buckling mechanism was 67.50 cm, equivalent to a force of 1.54 J.
Globe trauma. In five of the eight experimental orbits, posterior strain exceeded anterior strain; in the remainder, strain in excess of 3756 μϵ was demonstrated in both the anterior and the posterior parts of the orbital floor. Nonetheless this antero-posterior difference was not as marked as that between the anterior and posterior parts of the orbital floor during rim-directed trauma. In contrast to the strain gauge readings on the floor of orbits receiving rim-directed trauma, strain was much more evenly distributed across the orbital floor following trauma to the globe. Figure 8 shows the mean strain gauge readings for all eight subjects when the weight was dropped from different heights. The mean height from which the weight was dropped in order to produce a fracture was 53.75 cm, equivalent to a force of 1.22 J. Thus, it is evident that less force is required to produce a fracture from globe-directed trauma than from rim-directed trauma.
Figure 8The mean readings from the anterior and posterior strain gauges for both rim and globe trauma in all eight experimental cadavers. Black diamonds: anterior strain gauge for globe impact; pink squares: posterior strain gauge for globe impact; yellow triangles: anterior strain gauge for rim impact; green crosses: posterior strain gauge for rim impact.
Control study. In the control study, the orbit in which the trauma was directed onto the rim showed an anterior fracture only. In contrast, the orbit with globe-directed trauma produced typical fractures in both the anterior and the posterior parts of the orbital floor. These observations were consistent with those in the orbits of the experimental cadavers.
3. Discussion
The mechanism of blow-out fractures of the orbital floor has long been a subject of controversy in the scientific literature. To date, most studies have been flawed by inconsistencies in the experimental protocols. No previous studies have objectively investigated the strain distribution across the floor of the orbit following simulation of either proposed mechanism.
A wide range of poorly quantified forces has been used in previous studies, and the true forces required to fracture the orbital floor were unknown until this study. We believe that only fresh, intact, unfixed human post-mortem cadavers can provide a satisfactory in vitro model for in vivo conditions in the study of orbital floor fractures.
Furthermore, only the use of accurate strain gauges, regarded as the ‘gold standard’ for the measurement of bone strain in vivo,
can provide information on the forces exerted across the orbital floor during trauma.
This study is the first to simulate both the hydraulic and the buckling mechanisms under identical conditions in fresh unfixed intact human post-mortem cadavers. It is also the first time that strain gauges have been used to quantify the forces transmitted to the orbital floor to the point of fracture. Isolated, variable and quantifiable forces have been applied to both the globe and the infraorbital rim to investigate the pathogenesis of orbital floor fractures.
A possible criticism of this model is that cadavers may not be representative of the general population, being generally older. The average age of our cadavers was 83.5 years, as opposed to the average age of 36 years for patients with blow-out fractures.
states that age has relatively little effect on the strength of the facial bones. Weakening of the bony walls occurs mainly in the roof of the orbit, and rarely in the orbital floor, and hence does not fundamentally affect the occurrence of fractures in this area.
It is important to ensure that the invasive aspects of the experimental procedure do not affect the production or the location of fractures. The control study demonstrated that the anterior antrostomy and the removal of maxillary sinus mucosa did not affect the location of the fractures produced by either mechanism.
A further consideration was the placement of the strain gauges. We were concerned that subperiosteal placement of strain gauge on the orbital aspect of the orbital floor itself, may have interfered with the anatomical integrity of the orbit, thus producing a source of error. Since the thin and relatively weak anterior wall of the maxillary sinus plays little or no role in the support of the orbital floor, the anterior maxillary sinus antrostomy permitted ideal examination of the orbital floor and provided sufficient access to apply the strain gauges.
The strain gauges reliably and accurately recorded strain on the antral surface of the orbital floor, and were reliable indicators of the presence of a fracture experimentally, as demonstrated by permanent displacements in the oscilloscope readings. In some cases, the strain gauges detected fractures that were not immediately apparent visually, supporting criticism of previous studies where simple visualisation or palpation of the orbital floor in exenterated orbits was used to detect fractures.
Our results have demonstrated that the mean force required to produce a fracture with the buckling mechanism (1.54 J) is more than that with the hydraulic mechanism (1.22 J). This is in sharp contrast to the study of Fujino,
have simulated the hydraulic mechanism on fresh human post-mortem cadavers. The authors subdivided the orbital floor into six zones of relative thickness. In their experimentally produced fractures, 79% occurred in the posterior medial floor. Their findings suggested that thinness of the bone alone is not the only decisive factor leading in the increased incidence of fractures in the orbital floor. The posterior convex portion of the floor bulges upward into the back of the orbit, and hence receives most of the force transmitted by the globe.
; namely, that the buckling mechanism produces fractures in the anterior and anteromedial aspects of the orbital floor without medial wall involvement, and furthermore, confirms that the hydraulic mechanism produces fractures distributed throughout the anterior and posterior parts of the orbital floor. Additionally, in contrast to the buckling mechanism, medial-wall involvement is frequent and herniation is a feature of the hydraulic mechanism.
The strain gauge data obtained in this study provides further support for these observations. In all cases, hydraulic fractures produced strains in excess of 3756 μϵ in both the anterior and the posterior orbital floor. For the first time, this provides quantitative evidence for the involvement of the entire orbital floor in stresses produced by the hydraulic mechanism. Conversely, fractures produced by the buckling mechanism had strain gauge readings of greater than 3756 μϵ only in the anterior part of the orbital floor. Strain gauges in rim-traumatised orbits recorded minimal strain in the posterior aspect of the orbital floor. For the first time, in the study of orbital floor fractures, we have developed a reliable experimental protocol using biomechanical strain gauges, that we believe to have applications in future investigations of the forces involved in fractures of the craniofacial skeleton.
We did not observe herniation of orbital contents into the maxillary antrum following trauma to the infraorbital margin. In contrast, hydraulic fractures frequently resulted in the herniation of orbital contents, with the infraorbital nerve commonly prolapsing into the maxillary antrum. Herniation could be more accurately observed in this study than in previous studies, since it was observed directly from the maxillary antrum rather than following the reduction of herniated contents from the orbit. We believe that this herniation into the maxillary sinus is primarily due to the hydraulic pressure and not the effect of gravity, since herniation was still observed in supine cadavers.
In clinical situations, isolated trauma to either the rim or the globe may be uncommon, and it seems likely that impacts to both the rim and the globe may occur simultaneously, such that in many cases of orbital floor fracture, the clinical picture is the result of a combination of both mechanisms. None the less, this study demonstrates that there are indeed two separate mechanisms responsible for the fracture pattern observed. We propose that the buckling mechanism produces a fracture by the transmission of forces to the anterior aspect of the orbital floor. This mechanism produces linear fractures without the herniation of orbital contents into the maxillary antrum. The hydraulic mechanism produces fractures that include the posterior floor, with frequent herniation.
To date, no clinically useful classification of orbital floor fractures exists. Fueger et al proposed a classification with nine subtypes of fracture based on a review of just 38 clinical cases.
Indeed three of their categories were based on only a single case. From our studies, we believe that orbital floor fractures can be divided into two categories, allowing to the following simple classification.
•
Type 1: a small fracture confined to the anterior to mid-medial floor of the orbit in which herniation of orbital contents is unusual.
•
Type 2: a large fracture, usually involving the entire orbital floor and medial wall, in which herniation of orbital contents is frequent.
In conclusion, this study supports the hypothesis that both the hydraulic and the buckling mechanisms produce fractures of the orbital floor. However, these fractures appear to be characteristically different entities. The buckling mechanism produces fractures of the orbital floor that are limited to the anterior and anteromedial floor. In contrast, the hydraulic mechanism produces larger fractures that frequently involve the anterior and posterior orbital floor as well as the medial wall.
We have, for the first time, accurately recorded that the mean force exerted on the orbital floor required to produce a fracture by the buckling mechanism in human post-mortem cadavers is 1.54 J. Similarly, we have demonstrated that the mean force required for the hydraulic mechanism is 1.22 J. These figures clearly demonstrate that it is relatively easier to fracture the orbital floor by the hydraulic mechanism than by the buckling mechanism.
We propose that the use of strain gauges offers a quantitative method of investigating the forces involved in other fractures of the craniofacial skeleton.
References
Lang W
Traumatic enophthalmos with retention of perfect acuity of vision.
Etude experimentale sur les fractures de la machoire superieure.
Rev Chir de Paris.1901; 23 (Translated by: Tessier P. The classic reprints: experimental study of fractures of the upper jaw: I and II. Plast Reconstr Surg, 1972; 50: 497–506): 208-227
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