Perinatal outcome of pregnancy complicated by twin anemia-polycythemia sequence: systematic review and meta-analysis

OBJECTIVE

To report the perinatal outcome of monochorionic diamniotic (MCDA) twin pregnancies complicated by twin anemia-polycythemia sequence (TAPS), according to the type of TAPS (spontaneous or postlaser) and the management option adopted.

METHODS

MEDLINE, EMBASE and The Cochrane Library databases were searched for studies reporting on the outcome of twin pregnancies complicated by TAPS. Inclusion criteria were non-anomalous MCDA twin pregnancies with a diagnosis of TAPS. The primary outcome was perinatal mortality; secondary outcomes were neonatal morbidity and preterm birth (PTB). The outcomes were stratified according to the type of TAPS (spontaneous or following laser treatment for twin-twin transfusion syndrome) and the management option adopted (expectant, laser surgery, intrauterine transfusion (IUT) or selective reduction (SR)). Random-effects meta-analysis of proportions was used to analyze the data.

RESULTS

Perinatal outcome was assessed according to whether TAPS occurred spontaneously or after laser treatment in 506 pregnancies (38 studies). Intrauterine death (IUD) occurred in 5.2% (95% CI, 3.6-7.1%) of twins with spontaneous TAPS and in 10.2% (95% CI, 7.4-13.3%) of those with postlaser TAPS, while the corresponding rates of neonatal death were 4.0% (95% CI, 2.6-5.7%) and 9.2% (95% CI, 6.6-12.3%), respectively. Severe neonatal morbidity occurred in 29.3% (95% CI, 25.6-33.1%) of twins after spontaneous TAPS and in 33.3% (95% CI, 17.4-51.8%) after postlaser TAPS, while the corresponding rates of severe neurological morbidity were 4.0% (95% CI, 3.5-5.7%) and 11.1% (95% CI, 6.2-17.2%), respectively. PTB complicated 86.3% (95% CI, 77.2-93.3%) of pregnancies with spontaneous TAPS and all cases with postlaser TAPS (100% (95% CI, 84.3-100%)). Iatrogenic PTB was more frequent than spontaneous PTB in both groups. Perinatal outcome was assessed according to the management option adopted in 417 pregnancies (21 studies). IUD occurred in 9.8% (95% CI, 4.3-17.1%) of twins managed expectantly and in 13.1% (95% CI, 9.2-17.6%), 12.1% (95% CI, 7.7-17.3%) and 7.6% (95% CI, 1.3-18.5%) of those treated with laser surgery, IUT and SR, respectively. Severe neonatal morbidity affected 27.3% (95% CI, 13.6-43.6%) of twins in the expectant-management group, 28.7% (95% CI, 22.7-35.1%) of those in the laser-surgery group, 38.2% (95% CI, 18.3-60.5%) of those in the IUT group and 23.3% (95% CI, 10.5-39.2%) of those in the SR group. PTB complicated 80.4% (95% CI, 59.8-94.8%), 73.4% (95% CI, 48.1-92.3%), 100% (95% CI, 76.5-100%) and 100% (95% CI, 39.8-100%) of pregnancies after expectant management, laser surgery, IUT and SR, respectively.

CONCLUSIONS

The present meta-analysis provides pooled estimates of the risks of perinatal mortality, neonatal morbidity and PTB in twin pregnancies complicated by TAPS, stratified by the type of TAPS and the management option adopted. Although a direct comparison could not be performed, the results from this systematic review suggest that spontaneous TAPS may have a better prognosis than postlaser TAPS. No differences in terms of mortality and morbidity were observed when comparing different management options for TAPS, although these findings should be interpreted with caution in view of the limitations of the included studies. Individualized prenatal management, taking into account the severity of TAPS and gestational age, is currently the recommended strategy. © 2021 International Society of Ultrasound in Obstetrics and Gynecology.

Relationship between peripartum maternal cardiac phenotype and maternal outcome in women with hypertensive disorders of pregnancy

Background

Hypertensive disorders of pregnancy (HDP) are a leading cause of maternal and neonatal morbidity worldwide.

Purpose

The aim of this study is to assess maternal cardiac function in women affected by HDP in singleton pregnancy and investigate the relationship between abnormal cardiac findings and maternal outcomes.

Methods

In this single-centre prospective longitudinal study, 190 women with a diagnosis of HDP underwent standard trans-thoracic echocardiography (TTE) in the immediate peripartum period from February 2019 to December 2020. Left ventricle morphology (LVM) and diastolic dysfunction (DD) were evaluated according to according to British Society of Echocardiography guidelines. Patients were classified into three groups according to TTE findings: (1) normal LVM and DD, (2) abnormal LVM or abnormal DD, (3) abnormal LVM and abnormal DD. Maternal indices were compared among these groups.

Results

56 (29.5%) patients affected by HDP were included in group 1, 69 (36.3%) in group 2 and 65 (34.2%) in group 3. Gestational age at delivery and birthweight centile were similar among the groups. Women in group 3 were significantly older than group 2 and group 1 (35.1±5.4 years vs 32.6±6.3 vs 33.1±5.8 years years, respectively, p=0.043). Group 2 and 3 showed a higher blood pressure in the first trimester of pregnancy compared to group 1 (mean arterial pressure: 94.3±7.2 mmHg vs 95.5±8.2 mmHg vs 91.6±8.3 mmHg, p=0.024), while no significant difference was found in body mass index among the three groups (group 1: 26.4±5.4, group 2: 28.0±6.4; group 3: 27.7±5.0, p=0.293). HDP women with LVM and DD (group 3) were more likely to be admitted to high dependency unit (35.4%) than women in group 2 and 1 (14.5% and 23.6%, respectively, p=0.019).

Conclusions

Abnormal echocardiographic findings were associated with a worse maternal cardiovascular phenotype that required a closer maternal cardiovascular monitoring in the peripartum period.

Funding Acknowledgement

Type of funding sources: Public grant(s) – EU funding. Main funding source(s): Veronica Giorgione and Carolina Di Fabrizio have received funding from European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 765274 (iPLACENTA project).

Opthalmic artery doppler measurements and cardiovascular assessment in pregnancies with placental dysfunction

 

Women affected by hypertensive disorders of pregnancy and fetal growth restriction frequently present alterations in the cardiovascular system, even before the clinical onset of the pathology. The objective of this study is to assess the relationship between peripheral vascular system (ophthalmic and uterine arteries), cardiac output and adverse fetomaternal outcomes.

Methods

Women with singleton pregnancies from mid second trimester until delivery were prospectively included in this observational study performed at our Hospital, between December 2020 and July 2021. The inclusion criteria included women with evidence of placental dysfunction (preeclampsia and fetal growth restriction) and normotensive controls.

Vascular systemic resistance (VSR), arterial stiffness and cardiac output were measured by ultrasound (uterine and ophthalmic arteries), plethysmography and continuous Doppler. Study variables were compared using t-test and Chi-squared test according to whether they were normally distributed or not.

Results

The analysis included 32% women with hypertensive disorders of pregnancy and 67% normal pregnancies.

Statistically significant difference was observed in the Second to First Peak Systolic Velocity Ratio (2PSVR) of the ophthalmic artery (p=0.011), heart Rate (p=0.03), stroke volume (p=0.004) and cardiac output (0.094) between the two groups. There were significant correlation between parameters of the central haemodynamics and those of the uteroplacental circulation, as well as ophthalmic artery Doppler.

Conclusion

Ophthalmic artery 2PSVR is a simple and potentially promising measurement to assess women with hypertensive disorders of pregnancy. However, this parameter must be validated against the well described Uterine artery Doppler. A bigger cohort is needed to clarify and determine its effectiveness.

Funding Acknowledgement

Type of funding sources: Public grant(s) – EU funding. Main funding source(s): iPlacenta

Strain analysis by two-dimensional speckle tracking echocardiography for evaluating left ventricular systolic function in women with pre-eclampsia

Background

Previous echocardiographic studies have shown that most women affected by pre-eclampsia present with mild-to moderate diastolic dysfunction and left ventricle remodelling with a preserved systolic function. These alterations appear more severe when pre-eclampsia develops before 34 weeks of gestation.

Purpose

The aim of this study is to compare left ventricular systolic (LV) function by using 2-D speckle tracking echocardiography in women with early-onset and late-onset pre-eclampsia.

Methods

In this single-centre prospective longitudinal study, 119 women with a diagnosis of pre-eclampsia underwent standard trans-thoracic echocardiography (TTE) in the immediate peripartum period from February 2019 to December 2020. LV function was assessed using two-dimensional speckle-tracking strain imaging on 4-chamber views with a frame rate of 60–90 frames/second. Strain analysis quantification was performed using a commercial software.

Results

Pre-eclampsia was diagnosed before and after 34 weeks in 37.3% (44/119) and 62.7% (74/119) of the patients, respectively. Maternal characteristics, such as maternal age, body mass index, pre-existing hypertension and nulliparity, did not show any significant difference between the two groups. Although LV remodelling/hypertrophy and diastolic dysfunction occurred more often in early-onset pre-eclampsia compared to late-onset (65.9% vs 60.8% and 59.1% vs 51.4%, respectively), this difference was not statistically significant (p=0.580 and p=0.414, respectively). Similarly, LV mass index was 80.1±16.3 in pre-eclampsia <34 and 79.1±15 >34 (p=0.715) and E/e' was 7.8±1.9 and 7.6±1.7 (p=0.424). However, global longitudinal strain (GLS) was significantly lower (−16.4±2.4 vs −17.6±2.4, p=0.030) and apical rotation was higher (11.1±5.9 vs 8.7±4.7, p=0.019) in early-onset pre-eclampsia. A positive weak correlation has been found between GLS and difference in days from pre-eclampsia diagnosis to delivery (r=0.2, p=0.002).

Conclusions

Lower GLS in women affected by early-onset pre-eclampsia compared to late-onset pre-eclampsia might be useful to detect sub-clinical LV systolic impairment. Although further studies are needed, this sensitive marker may have a role in identifying women at risk of preterm delivery and/or severe maternal morbidity in the peripartum period.

Funding Acknowledgement

Type of funding sources: Public grant(s) – EU funding. Main funding source(s): Veronica Giorgione and Carolina Di Fabrizio have received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 765274 (iPLACENTA project).

1547 An Alternative Approach to High-Risk Resternotomy

Introduction

Resternotomies are associated with substantial perioperative morbidity and mortality. Strategies described in literature mostly involve peripheral cannulation pre-sternotomy. Disadvantages of this technique relate to prolonged systemic heparinisation and cardiopulmonary bypass (CPB) time and the sequelae of hypothermic circulatory arrest. We describe a two-stage approach that potentially reduces the complications associated with high-risk resternotomy.

Method

3 high-risk patients (from pre-operative CT images) were referred for redo complex aortic surgery. A right mini-thoracotomy incision was first made in the 4th or 5th intercostal space. The right lung was isolated and careful blunt dissection was carried out to mobilise the heart and great vessels attached to the sternum. Once these structures were free, thoracotomy incision was closed. A standard median sternotomy was then performed and central cannulation carried out after systemic heparinisation. Rest of the surgery was performed routinely. In one patient, aortic aneurysm was heavily adherent and attempts to mobilise it fully proved impossible. Resultantly, systemic heparinisation was administered and the patient was cannulated in the right superficial femoral artery and right atrium (via mini-thoracotomy). CPB was instituted and the patient cooled to 28 °C. Right superior pulmonary vein vent was introduced to prevent left ventricular distension from hypothermic ventricular fibrillation. Once the heart and aneurysm were decompressed on full CPB, complete mobilisation was performed safely. All 3 patients survived surgery without major complications.

Conclusions

Meticulous preoperative planning is key to management of high-risk resternotomy. We describe a novel technique which we believe minimises risk of morbidity and mortality in these complex cases.

Defining the decision-to-delivery interval at caesarean section: narrative literature review and proposal for standardisation

The decision-to-delivery interval is a widely used term at non-elective caesarean section. While the definition may appear self-evident, there is no universally agreed consensus about when this period begins and ends. We reviewed the literature for original research utilising the terms 'decision-to-delivery', 'decision-to-incision' or 'incision-to-delivery' and examined definitions used for decision, delivery, incision, as well as any additional time intervals that were assessed. Our analysis demonstrated an inconsistent non-standardised approach to defining these intervals, which might have clinical practice and medicolegal ramifications. We propose that the decision-to-delivery interval should be defined as follows: the interval between the time at which the senior obstetrician makes the decision that a caesarean section is required and the time at which the fetus (or first fetus in the case of multiples) is delivered. The decision time should ideally be recorded contemporaneously in the medical notes or partogram.

Twin chorionicity-specific population birth-weight charts adjusted for estimated fetal weight

OBJECTIVES

To construct chorionicity-specific birth-weight reference charts for dichorionic diamniotic (DCDA) and monochorionic diamniotic (MCDA) twin pregnancies, incorporating estimated-fetal-weight (EFW) data in order to adjust for the relationship between suboptimal growth and preterm delivery. An additional aim was to determine if the inclusion of complicated twin pregnancies impacts on the reference charts produced.

METHODS

The inclusion criteria for this retrospective cohort study were twin pregnancy of known DCDA or MCDA chorionicity, known pregnancy outcome, last ultrasound scan within 14 days before birth and delivery between 25 and 38 weeks' gestation (Analysis A). An analysis was also conducted excluding pregnancies with complications recorded (Analysis B). Previously published twin EFW reference ranges were used in the analysis. A joint statistical model for EFW and observed birth weight for each pregnancy was created in order to estimate population birth-weight reference ranges corresponding to the distribution expected if all pregnancies delivered at any given gestational age. It was not assumed that the median EFW was equal to birth weight for any given gestational age. The models were fitted using a Bayesian approach.

RESULTS

We retrieved data on 1664 twin pregnancies, of which 707 DCDA and 241 MCDA pregnancies met the inclusion criteria. In Analysis A, the estimated population median birth weight was similar to the median EFW at around 27 weeks' gestation but fell below the EFW values with increasing gestation, being 156 g lower in both DCDA and MCDA pregnancies at 35 weeks; this finding was confirmed by direct comparison of the last EFW and birth-weight values in each pregnancy. When the analysis was repeated after excluding complicated twin pregnancies (Analysis B), compared with Analysis A, there was very little difference in the median birth-weight results obtained across gestation. The largest absolute difference between Analyses A and B for DCDA twins was at 31, 32 and 33 weeks, with a 9-g lower median birth weight in Analysis A compared with Analysis B. The largest absolute difference for MCDA twins was greater than that for DCDA twins, with a 21-g lower median birth weight at 25 weeks in Analysis A compared with Analysis B.

CONCLUSIONS

We have established population chorionicity-specific birth-weight reference charts for DCDA and MCDA twin pregnancies, corresponding to the range expected were all pregnancies to deliver at any given gestational age. In this population of twins, the median birth weight was consistently lower than that reported for singletons, and there was variation in the median birth weight at different gestational ages according to chorionicity. © 2021 The Authors. Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology. - Legal Statement: This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

Controversies in the management of twin pregnancy

Despite many advances in antenatal care, twin pregnancies still experience more adverse outcomes, in particular perinatal morbidity and mortality. They also pose a multitude of challenges and controversies, as outlined in this Review. Moreover, they are less likely to be included in clinical trials. Many issues on classification and management remain under debate. Efforts at standardizing diagnostic criteria, monitoring protocols, management and outcome reporting are likely to reduce their perinatal risks. The top 10 most important research uncertainties related to multiple pregnancies have been identified by both clinicians and patients. More robust research in the form of randomized trials and large well-conducted prospective cohort studies is needed to address these controversies. © 2020 The Authors. Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.

57 The Use of a Hemi Glabella Flap for Reconstruction of Medial Canthus Defects

Background

Medial canthal reconstruction is a challenging task due to the complex anatomy. The glabellar flap is a common viable technique; however, this results in narrowing of the eyebrows, bulky nasal dorsum and horizontal scarring which is aesthetically displeasing. The senior author in this paper has developed an intuitive modification to the technique.

Method

A rotational advancement flap involving the upper lateral nasal wall with the hemi glabella was formed and transferred to the medial canthal defect. The donor site was closed in a V-Y manner. Complete closure of defect was achieved in all patients.

Results

Reconstruction using the Hemi-Glabellar technique was performed on 12 patients following resection of BCC in or near the medial canthus area. Superficial cellulitis was noted in 2 patients, they were managed oral antibiotics. There was bruising in 7 patient which resolve spontaneously in 4-7 days. All patients had a good outcome at 2 months and 6 months follow up. There was no flap loss and all patients were satisfied with the aesthetic outcome.

Conclusions

The technique highlighted can be performed easily and is applicable to reconstruction to defects of the medial canthus with excellent aesthetic outcomes with an inconspicuous scar and supple skin with matching colour

Counseling in maternal-fetal medicine: SARS-CoV-2 infection in pregnancy

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a zoonotic coronavirus that crossed species to infect humans, causing coronavirus disease 2019 (COVID-19). Despite a potentially higher risk of pregnant women acquiring SARS-CoV-2 infection compared with the non-pregnant population (particularly in some ethnic minorities), no additional specific recommendations to avoid exposure are needed in pregnancy. The most common clinical symptoms and laboratory signs of SARS-CoV-2 infection in pregnancy are fever, cough, lymphopenia and elevated C-reactive protein levels. Pregnancy is associated with a higher risk of severe SARS-CoV-2 infection compared with the non-pregnant population, including pneumonia, admission to the intensive care unit and death, even after adjusting for potential risk factors for severe outcomes. The risk of miscarriage does not appear to be increased in women with SARS-CoV-2 infection. Evidence with regards to preterm birth and perinatal mortality is conflicting, but these risks are generally higher only in symptomatic, hospitalized women. The risk of vertical transmission, defined as the transmission of SARS-CoV-2 from the mother to the fetus or the newborn, is generally low. Fetal invasive procedures are considered to be generally safe in pregnant women with SARS-CoV-2 infection, although the evidence is still limited. In pregnant women with COVID-19, use of steroids should not be avoided if clinically indicated; the preferred regimen is a 2-day course of dexamethasone followed by an 8-day course of methylprednisolone. Non-steroidal anti-inflammatory drugs may be used if there are no contraindications. Hospitalized pregnant women with severe COVID-19 should undergo thromboprophylaxis throughout the duration of hospitalization and at least until discharge, preferably with low molecular weight heparin. Hospitalized women who have recovered from a period of serious or critical illness with COVID-19 should be offered a fetal growth scan about 14 days after recovery from their illness. In asymptomatic or mildly symptomatic women who have tested positive for SARS-CoV-2 infection at full term (i.e. ≥ 39 weeks of gestation), induction of labor might be reasonable. To date, there is no clear consensus on the optimal timing of delivery for critically ill women. In women with no or few symptoms, management of labor should follow routine evidence-based guidelines. Regardless of COVID-19 status, mothers and their infants should remain together and breastfeeding, skin-to-skin contact, kangaroo mother care and rooming-in throughout the day and night should be practiced, while applying necessary infection prevention and control measures. Many pregnant women have already undergone vaccination, mostly in the USA where the first reports show no significant difference in pregnancy outcomes in pregnant women receiving SARS-CoV-2 vaccination during pregnancy compared with the background risk. Vaccine-generated antibodies were present in the umbilical cord blood and breast milk samples of pregnant and lactating women who received the mRNA COVID-19 vaccine. Based on the available limited data on the safety of the COVID-19 vaccine in pregnancy, it seems reasonable to offer the option of vaccination to pregnant women after accurate counseling on the potential risk of a severe course of the disease and the unknown risk of fetal exposure to the vaccine. © 2021 International Society of Ultrasound in Obstetrics and Gynecology.

Effect of monochorionicity on perinatal outcome and growth discordance in triplet pregnancy: collaborative multicenter study in England, 2000-2013

OBJECTIVES

To compare perinatal outcome and growth discordance between trichorionic triamniotic (TCTA) and dichorionic triamniotic (DCTA) or monochorionic triamniotic (MCTA) triplet pregnancies.

METHODS

This was a multicenter cohort study using population-based data on triplet pregnancies from 11 Northern Survey of Twin and Multiple Pregnancy (NorSTAMP) maternity units and the Southwest Thames Region of London Obstetric Research Collaborative (STORK) multiple pregnancy cohort, for 2000-2013. Perinatal outcomes (from ≥ 24 weeks' gestation to 28 days of age), intertriplet fetal growth and birth-weight (BW) discordance and neonatal morbidity were analyzed in TCTA compared with DCTA/MCTA pregnancies.

RESULTS

Monochorionic placentation of a pair or trio in triplet pregnancy (n = 72) was associated with a significantly increased risk of perinatal mortality (risk ratio, 2.7 (95% CI, 1.3-5.5)) compared with that in TCTA pregnancies (n = 68), due mainly to a much higher risk of stillbirth (risk ratio, 5.4 (95% CI, 1.6-18.2)), with 57% of all stillbirth cases resulting from fetofetal transfusion syndrome, while there was no significant difference in neonatal mortality (P = 0.60). The associations with perinatal mortality and stillbirth persisted when considering only pregnancies not affected by a major congenital anomaly. DCTA/MCTA triplets had lower BW and demonstrated greater BW discordance than did TCTA triplets (P = 0.049). Severe BW discordance of > 35% was 2.5-fold higher in DCTA/MCTA compared with TCTA pregnancies (26.1% vs 10.4%), but this difference did not reach statistical significance (P = 0.06), presumably due to low numbers. Triplets in both groups were delivered by Cesarean section in over 95% of cases, at a similar gestational age (median, 33 weeks' gestation). The rate of respiratory (P = 0.28) or infectious (P = 0.08) neonatal morbidity was similar between the groups.

CONCLUSIONS

Despite close antenatal surveillance, monochorionic placentation of a pair or trio in triamniotic triplet pregnancy was associated with a significantly increased stillbirth risk, mainly due to fetofetal transfusion syndrome, and with greater size discordance. In liveborn triplets, there was no adverse effect of monochorionicity on neonatal outcome. © 2020 International Society of Ultrasound in Obstetrics and Gynecology.

[Lung imaging in severe asthma]

INTRODUCTION

Asthma is a common disease whose diagnosis does not typically rely on the results of imaging. However, chest CT has gained a key place over the last decade to support the management of patients with difficult to treat and severe asthma.

STATE OF THE ART

Bronchial wall thickening and mild dilatation or narrowing of bronchial lumen are frequently observed on chest CT in people with asthma. Bronchial wall thickening is correlated to the degree of obstruction and to bronchial wall remodeling and inflammation. Diverse conditions which can mimic asthma should be recognized on CT, including endobronchial tumours, interstitial pneumonias, bronchiectasis and bronchiolitis. Ground-glass opacities and consolidation may be related to transient eosinophilic infiltrates, infection or an associated disease (vasculitis, chronic eosinophilic pneumonia). Hyperdense mucous plugging is highly specific for allergic bronchopulmonary aspergillosis.

PERSPECTIVES

Airway morphometry, air trapping and quantitative analysis of ventilatory defects, with CT or MRI, can help to identify different morphological subgroups of patients with different functional or inflammatory characteristics. These imaging tools could emerge as new biomarkers for the evaluation of treatment response.

CONCLUSION

Chest CT is indicated in people with severe asthma to search for additional or alternative diagnoses. Quantitative imaging may contribute to phenotyping this patient group.

Biomechanical assessment of orbital fractures using patient-specific models and clinical matching

INTRODUCTION

Orbital wall fractures consider one of the most common fractures in the maxillofacial trauma. These fractures caused by two mechanisms, the buckling mechanism and hydraulic mechanism. This study aims to compare between the two mechanisms in terms of intensity and extension using the finite elements method.

MATERIAL AND METHODS

Three-dimensional model of the skull was generated using computed tomography data of young male patient. Virtual loads were applied on the eyeball and the infra-orbital rim separately. Von Mises stresses were examined in each simulation.

RESULTS

The simulation predicted fractures on the infra-orbital rim and orbital floor when simulating the hydraulic mechanism, and on the orbital floor and mesial wall when simulating the buckling mechanism.

CONCLUSION

Biomechanical studies are essential part in understanding maxillofacial fractures mechanisms. The results confirmed and ascertained what is seen clinically, and explained clearly the two mechanisms of orbital fractures.