Strain gauge biomechanical evaluation of forces in orbital floor fractures

Transcription

1 British Journal of Plastic Surgery (2003), 56, 3 9 q 2003 The British Association of Plastic Surgeons. Published by Elsevier Science Ltd. All rights reserved. doi: /s (02) Strain gauge biomechanical evaluation of forces in orbital floor fractures F. Ahmad, W.N.A. Kirkpatrick*, J. Lyne, M. Urdang, L.J. Garey and N. Waterhouse* Imperial College Faculty of Medicine, London, UK; and *The Craniofacial Unit, Chelsea and Westminster Hospital, London, UK SUMMARY. 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. q 2003 The British Association of Plastic Surgeons. Published by Elsevier Science Ltd. All rights reserved. Keywords: strain, gauge, force, orbital, floor, fractures. 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. Lang 1 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, 2 it was Smith and Regan 3 who coined the term blow-out fracture. Whilst the orbital floor is most commonly involved in a blowout fracture, a similar injury may affect the medial orbital wall, alone or in combination. 4,5 Less frequently, the roof of the orbit may be involved. 6 9 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. Since the buckling theory, proposed by Le Fort, 10 and the subsequent hydraulic theory, proposed by Pfeiffer, 11 were introduced, many studies 3,12 19 and case reports 5,7,8,20 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, 12,13 fixed cadaver skulls, 3,14 16 synthetic models 13,17,18 or even primate models. 19 Where intact unfixed human specimens have been studied, the small numbers of cadaver orbits has severely limited the protocol. 3,12,13,15,16,18 In some cases, the orbital contents were not even in situ. 12,15,16 Many studies failed to quantify the force applied 3,14,15 or to maintain consistent points of impact, 3,16 and some failed to quantify and restore the intraocular pressure. 3 More recently, Waterhouse et al 21 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 3

2 4 British Journal of Plastic Surgery 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 21 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. 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: 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. 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. 21 Between 0.7 and Figure 1 The approach to the orbital floor via a Weber Ferguson incision and a 1.5 cm anterior maxillary antrostomy. 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 (me).

3 Strain gauge biomechanical evaluation of forces in orbital floor fractures 5 Figure 2 The placement of the anterior and posterior strain gauges, which are seen attached to the antral surface of the orbital floor. 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. were minimal compared with those recorded anteriorly. Strain did not exceed 221 me in the posterior gauge, while conversely, the anterior gauge consistently showed readings in excess of 3756 me. 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 Results 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. 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). Quantitative results 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. 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 Figure 3 The calibrated drop tower positioned to deliver isolated trauma accurately to the orbital rim or globe.

4 6 British Journal of Plastic Surgery Figure 4 Templates demonstrating the combined fractures from all cadavers following (A) rim-directed (green) and (B) globe-directed (red) trauma. a fracture by the buckling mechanism was 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 me 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 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. 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. 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 Figure 5 A 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 6 A 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.

5 Strain gauge biomechanical evaluation of forces in orbital floor fractures 7 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 postmortem 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, 22 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. 23 However, Hampson 23 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 Figure 7 Typical strain gauge readings from the orbital floor following trauma to (A) the orbital rim and (B) the globe. In (A) the highamplitude 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. 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, 12 which found that the hydraulic mechanism required a force ten times that required by the buckling mechanism to produce a fracture. Jones and Evans 14 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 Table 1 Height of impactor drop at which fractures first occurred in the orbital floor of each cadaver Cadaver Height of drop for globe trauma (cm) Height of drop for rim trauma (cm) average

6 8 British Journal of Plastic Surgery Figure 8 The 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. convex portion of the floor bulges upward into the back of the orbit, and hence receives most of the force transmitted by the globe. 11 By measuring the distance from the orbital axis to the orbital walls, Jones and Evans 14 confirmed that the anteroposterior axis along which the globe would move when pushed into the orbit is towards the posterior medial convex floor. Our investigations confirm the previous findings of Waterhouse et al 21 ; 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 me 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 me 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. 24 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.

7 Strain gauge biomechanical evaluation of forces in orbital floor fractures 9 References 1. Lang W. Traumatic enophthalmos with retention of perfect acuity of vision. Trans Ophthalmol Soc UK 1889;9: Converse JM, Smith B. Enophthalmos and diplopia in fractures of the orbital floor. Br J Plast Surg 1957;9: Smith B, Regan WF. Blow-out fracture of the orbit: mechanism and correction of internal orbital fracture. Am J Ophthalmol 1957;44: Edwards WC, Ridley RW. Blow-out fracture of the medial orbital wall. Am J Ophthalmol 1968;65: Dodick JM, Galin MA, Littleton JT, Sod LM. Concomitant medial wall fracture and blow-out fracture of the orbit. Arch Opthalmol 1971;85: Curtin HD, Wolfe P, Schramm V. Orbital roof blow-out fractures. Am J Roentgenol 1982;139: Raflo GT. Blow-in and blow-out fractures of the orbit: clinical correlation and proposed mechanisms. Ophthalmic Surg 1984;14: Kulwin DR, Leadbetter MG. Orbital rim trauma causing a blow-out fracture. Plast Reconstr Surg 1984;73: Koltai PJ, Amjad I, Meyer D, Feustel PJ. Orbital fractures in children. Arch Otolaryngol Head Neck Surg 1995;121: Le Fort R. 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: Pfeiffer RL. Traumatic enophthalmos. Arch Ophthalmol 1943;30: Fujino T. Experimental blow-out fracture of the orbit. Plast Reconstr Surg 1974;54: Tajima S, Fujino T, Oshiro T. Mechanism of orbital blow-out fracture I: stress coat test. Keio J Med 1974;23: Jones DEP, Evans JNG. Blow-out fracture of the orbit: an investigation into their anatomical basis. J Laryngol Otol 1967;8: Phalen JJ, Baumel JJ, Kapkin PA. Orbital floor fractures: a reassessment of pathogenesis. Nebr Med J 1990;25: Bessière E, Depaulis J, Verin P, Lauzeral G. Quelques considérations anatomo-pathologiques et physiopathologiques sur les blow-out fractures. J Med Bordeaux 1964;141: Behrendt S, Rochels R. Mechanism of the formation of orbital floor fractures: holographic inferometry studies. Ophthalmologe 1993; 90:31 3. in German. 18. Fujino T, Sugimoto C, Tajima S, Moribe Y, Sato TB. Mechanism of orbital fracture: analysis by high speed camera in a two dimensional eye model. Keio J Med 1974;23: Green RP, Peters DR, Shore JW, Fanton JW, Davis H. Force necessary to fracture the orbital floor. Ophthal Plast Reconstr Surg 1990;6: Kersten RC. Blow-out fracture of the orbital floor with entrapment caused by isolated trauma to the orbital rim. Am J Ophthalmol 1987;103: Waterhouse N, Lyne J, Urdang M, Garey L. An investigation into the mechanism of orbital blow-out fractures. Br J Plast Surg 1999;52: Hoshaw SJ, Fyhrie DP, Takano Y, Burr DB, Milgrom C. A method suitable for in vivo measurement of bone strain in humans. J Biomech 1997;30: Hampson D. Facial injury: a review of biomechanical studies and test procedures for facial injury assessment. J Biomech 1995;28: Fueger GF, Milauskas AT, Britton W. The roentgenologic evaluation of orbital blow-out injuries. Am J Roentgenol Radium Ther Nucl Med 1966;97: The Authors F. Ahmad BSc, Medical Student J. Lyne BSc, Medical Student M. Urdang BSc, Medical Student L. J. Garey MA, DPhil, BM, BCh, Professor of Anatomy Imperial College Faculty of Medicine, London, UK W. N. A. Kirkpatrick BDS, MD, FRCS(Plast), Cleft and Craniofacial Fellow N. Waterhouse MB, ChB, FRCS(Plast), Consultant Plastic and Craniofacial Surgeon The Craniofacial Unit, Chelsea and Westminster Hospital, 369 Fulham Road, London SW10 9NH, UK Correspondence to W. N. A. Kirkpatrick Paper received 26 April Accepted 4 November 2002.