MRI characteristics of radiographically occult femoral neck fractures in trauma patients with ipsilateral femoral shaft fractures.

PURPOSE: Ipsilateral femoral neck fractures can be seen alongside femoral shaft fractures in high-velocity trauma patients. These neck fractures are often occult on radiographs and CT, and can have a significant impact on patient outcomes if not treated promptly. Limited protocol pelvic MRI has been used to increase sensitivity for these occult fractures. Detailed characterization of these fractures on MRI is lacking. METHODS: 427 consecutive trauma patients presenting to our emergency department who had known femoral diaphyseal fractures but no ipsilateral femoral neck fracture on radiographs or CT were included in this study. These patients were scanned using a limited protocol MRI with coronal T1 and coronal STIR sequences. Presence of an ipsilateral femoral neck fracture and imaging characteristics of the fracture were obtained. RESULTS: 31 radiographically occult ipsilateral femoral neck fractures were found, representing 7% of all cases. All neck fractures were incomplete. All fractures originated along the lateral cortex of the femoral neck and extended medially towards the junction of the medial femoral neck and the lesser trochanter. 58% (18/31) were vertical in orientation. 61% (19/31) did not demonstrate any appreciate edema on STIR images. CONCLUSION: Implementation of limited protocol MRI protocol increases sensitivity for detection of femoral neck fractures in the setting of ipsilateral femoral shaft fractures not seen on radiograph or CT imaging. We describe the characteristic MR imaging features of these fractures.

Artificial intelligence CAD tools in trauma imaging: a scoping review from the American Society of Emergency Radiology (ASER) AI/ML Expert Panel.

BACKGROUND: AI/ML CAD tools can potentially improve outcomes in the high-stakes, high-volume model of trauma radiology. No prior scoping review has been undertaken to comprehensively assess tools in this subspecialty. PURPOSE: To map the evolution and current state of trauma radiology CAD tools along key dimensions of technology readiness. METHODS: Following a search of databases, abstract screening, and full-text document review, CAD tool maturity was charted using elements of data curation, performance validation, outcomes research, explainability, user acceptance, and funding patterns. Descriptive statistics were used to illustrate key trends. RESULTS: A total of 4052 records were screened, and 233 full-text articles were selected for content analysis. Twenty-one papers described FDA-approved commercial tools, and 212 reported algorithm prototypes. Works ranged from foundational research to multi-reader multi-case trials with heterogeneous external data. Scalable convolutional neural network-based implementations increased steeply after 2016 and were used in all commercial products; however, options for explainability were narrow. Of FDA-approved tools, 9/10 performed detection tasks. Dataset sizes ranged from < 100 to > 500,000 patients, and commercialization coincided with public dataset availability. Cross-sectional torso datasets were uniformly small. Data curation methods with ground truth labeling by independent readers were uncommon. No papers assessed user acceptance, and no method included human-computer interaction. The USA and China had the highest research output and frequency of research funding. CONCLUSIONS: Trauma imaging CAD tools are likely to improve patient care but are currently in an early stage of maturity, with few FDA-approved products for a limited number of uses. The scarcity of high-quality annotated data remains a major barrier.

Interventional Therapies for Osteoarthritis: An Update.

Osteoarthritis (OA) is one of the most common diseases worldwide and is expected to increase in incidence as the age of the general population rises. Both oral medications, such as NSAIDs, and surgical treatments used for OA management have limitations. Demand is rising for minimally invasive techniques such as intraarticular injections and percutaneous interventions for use in place of or in conjunction with oral medications and surgical therapies, and the past 2 decades have seen a rapid expanse in both pharmacologic and nonpharmacologic minimally invasive OA treatments. Image guidance with fluoroscopy, CT, or ultrasound is often used in conjunction with these procedures to achieve precise treatment localization to achieve maximal therapeutic effect. The choice of modality used for image guidance is often influenced by clinician experience, patient characteristics, and equipment availability. This article reviews the mechanisms of action, contraindications, complications, and efficacy of conventional and developing minimally invasive OA treatments. The minimally invasive treatment options described in this Review include therapeutic injections such as antiinflammatory agents, viscosupplements, and biologics, as well as nonpharmacologic treatments of subchondroplasty, nerve ablation, genicular artery embolization, intraarticular pulsed radiofrequency therapy, and MRI-guided focused ultrasound therapy.

Accuracy of craniocervical measurements on CT for identifying partial or complete craniocervical ligament injuries in pediatric patients.

PURPOSE: To assess the accuracy of craniocervical measurements for identifying craniocervical injuries and the frequency of subjective findings of craniocervical injuries on CT in pediatric patients. METHODS: Case-controlled retrospective review of patients ≤ 16 years old with craniocervical junction injuries. Receiver operator curves were created for common craniocervical measurements on CT comparing patients with complete and partial craniocervical injuries to uninjured cohort. Frequency of subjective CT findings of craniocervical injury was assessed in the injured cohort. RESULTS: For complete disruption injuries (CD) (n = 27), C1-C2 distance (AUC = 0.90, 95%CI = 0.83-0.97), atlanto-occipital distance (AUC = 0.95-0.98, 95%CI = 0.90-1.00), and basion-dens distance (AUC = 0.90, 95%CI = 0.82-0.98) had excellent accuracy diagnosing injury. Powers ratio (AUC = 0.85, 95%CI = 0.76-0.94) had good, basion-posterior axial line (AUC = 0.74, 95%CI = 0.61-0.86) fair, and atlanto-dental distance (AUC = 0.69, 95%CI = 0.57-0.82) poor accuracy. For partial disruption injuries (PD) (n = 21), basion-dens distance (AUC = 0.75, 95%CI = 0.62-0.88) had fair accuracy diagnosing injury. Powers ratio (AUC = 0.63, 95%CI = 0.47-0.79), C1-C2 distance (AUC = 0.60, 95%CI = 0.45-0.75), atlanto-dental distance (AUC = 0.55, 95%CI = 0.39 = 0.71), atlanto-occipital distance (AUC = 0.63-0.65, 95%CI = 0.47-0.81), and basion-posterior axial line (AUC = 0.60, 95%CI = 0.44-0.76) all had poor accuracy. Eighty-one percent (n = 22) of CD and 38% (n = 8) of PD patients had non-concentric atlanto-occipital joints. One hundred percent of CD patients had ≥ 1 soft tissue finding and eighty-one percent (n = 22) had ≥ 2 findings. Seventy-three percent (n = 16) of PD patients had ≥ 1 soft tissue finding. Eighty-six percent (n = 18) of PD patients had non-concentric atlanto-occipital joints and/or soft tissue findings. CONCLUSION: Craniocervical measurements have poor accuracy for identifying craniocervical injuries in pediatric patients with incomplete craniocervical ligament disruption. Subjective findings of craniocervical injury are frequently present on CT in pediatric patients and can help increase sensitivity for identifying injury.

Imaging evaluation of traumatic carpal instability.

PURPOSE: To review the epidemiology, pertinent wrist anatomy, classification system, and emergent imaging evaluation of carpal instability with a focus on radiographic assessment of instability. METHODS: A review of current literature on carpal instability was performed with summary presentation of carpal instability epidemiology, carpal anatomy, imaging evaluation, and classification with imaging evaluation focused on diagnosis in the emergency setting. RESULTS: Carpal instability is a common pathology in falls on outstretched hand and is likely underdiagnosed due to instability being occult or demonstrating subtle malalignment on static imaging of the wrist. While there is a complex network of intrinsic and extrinsic ligaments contributing to carpal instability, a detailed knowledge of these ligaments is not necessary for radiologists to make an accurate diagnosis in the emergency setting, as identification and classification of carpal instability is based on identification of carpal malalignment patterns on radiography as opposed to identification of specific ligament injuries on advanced imaging. The Mayo classification is the most widely used classification system, which divides carpal instability into four categories: dissociative, non-dissociative, complex, and adaptive. Understanding this classification system allows radiologists to successfully classify almost all carpal instability injuries they will encounter, even in the setting of unusual or rare instability patterns. CONCLUSION: In working with the treating clinician, it is essential that the emergency radiologist is comfortable with identifying and classifying carpal instability. This will ensure prompt treatment of seemingly benign injuries and those that require intervention, surgical or otherwise, improving the likelihood of a good outcome.

Imaging of pediatric cervical spine trauma.

While pediatric cervical spine injuries (CSI) are rare, they are associated with high morbidity and mortality and sometimes require expeditious surgical management. In this article, we aim to improve the diagnostic accuracy of pediatric CSI by reviewing normal pediatric cervical anatomy, typical pediatric CSI patterns, and common mimics of pediatric CSI. A literature review was conducted on pediatric CSI, its epidemiology, and the various imaging manifestations and mimics. The most common pediatric CSI occur in the upper cervical spine owing to the higher fulcrum and larger head at a young age, namely prior to age 9 years, while lower CSI occur more frequently in patients older than 9 years. While various craniocervical measurements may be utilized to identify craniocervical disruption, soft tissue injuries may be the only manifestation, thus making pediatric CSI difficult to diagnose on initial imaging. In the acute setting, CT cervical spine is an appropriate initial imaging modality for pediatric CSI evaluation. MRI serves as an additional tool to exclude or identify injuries when initial findings are equivocal. It is essential to recognize the unique anatomy and biomechanics of the pediatric spine and thus discern common pediatric CSI patterns and their mimics.

Epidemiology and Imaging Classification of Pediatric Cervical Spine Injuries: 12-Year Experience at a Level 1 Trauma Center.

OBJECTIVE. The purpose of this study was to describe the epidemiology and imaging characteristics of cervical spine injuries in children with blunt trauma. MATERIALS AND METHODS. We conducted a retrospective review of the records of all patients 16 years old or younger with a diagnosis of cervical spine injury in the trauma registry of our level 1 trauma center between July 2006 and June 2018. RESULTS. Two hundred thirty-five patients were included in the study: 125 with subaxial cervical spine injuries, 87 with upper cervical spine injuries, and 23 with both subaxial and upper cervical spine injuries. The frequency of isolated upper cervical spine injuries was 73% in patients younger than 3 years old, 48% in patients 3-8 years old, and 29% in patients older than 8 years old. Seventy-one percent of occipital condyle fractures were avulsions, and 26% of dens fractures were avulsions. Type II dens fractures were found only in patients older than 8 years old. Type I and III dens fractures were almost exclusively in patients 8 years old or younger. Injuries classified as AOSpine types A, B, and C comprised 65.6%, 17.2%, and 17.2% of subaxial injuries, respectively. Despite similar mechanism of injury distribution across age groups, the frequency of AOSpine type A injuries in patients older than 8 years old (70.6%) was significantly higher compared with patients younger than 3 years old (40.0%) and those who were 3-8 years old (45.0%). The frequency of AOSpine type B injuries in patients younger than 3 years old (40.0%) was slightly higher than patients who were 3-8 years old (30.0%) and almost three times higher than in patients older than 8 years old (13.8%). CONCLUSION. Pediatric patients have high rates of upper cervical spine injuries, which tend to be distraction injuries that are frequently associated with avulsion fractures. Injury patterns in pediatric patients vary significantly by age, with patients younger than 3 years old being particularly prone to distraction type injuries.

Editorial Commentary: Is Magnetic Resonance Imaging of the Shoulder Ever Appropriate in Evaluating Patients With Calcific Tendinopathy of the Rotator Cuff?

Calcific tendinopathy of the rotator cuff is a common pathology that often presents with clinical symptoms simulating a rotator cuff tear. The reported incidence of rotator cuff tear in the setting of calcific tendinopathy varies widely; however, the reported incidence of full-thickness rotator cuff tear on imaging in calcific tendinopathy is consistently low (<5%). In patients with symptomatic calcific tendinopathy, initial conservative management followed by minimally invasive treatments should be employed prior to performing shoulder magnetic resonance imaging to assess for a rotator cuff tear. A shoulder magnetic resonance imaging may be performed for preoperative planning prior to surgical removal of calcium deposits, but even in this patient population, the incidence of full-thickness rotator cuff tear is low.

Characterization of all-terrain vehicle-related thoracolumbar spine injury patterns in children using the AOSpine classification system.

PURPOSE: To evaluate thoracolumbar spine injury patterns, demographics, and clinical characteristics in pediatric patients following all-terrain vehicle-related trauma. METHODS: A retrospective review of patients 0-17 years old admitted to a level I trauma center following an ATV-related incident from 2004 to 2013 was performed. Thoracolumbar spine injury patterns, accident mechanism, driver/passenger status, and demographic and clinical data were compared between patients with and without a spine injury. RESULTS: Of 456 pediatric patients involved in ATV-related trauma, 36 sustained one or more thoracolumbar spine injuries (7.9%). These patients tended to be older, taller, heavier, and had a higher BMI. ATV rollover was the major statistically significant mechanism of injury to cause spine fractures (61%). Patients with spine injuries had twice the hospital length of stay compared with those without (4 days vs. 2 days, P = 0.003). Nonstructural spine injuries (A0) were the most common type of injury (49.1%), followed by wedge-compression fractures (A1) (41.1%). In patients with a thoracolumbar spine injury, there was a mean of 3.11 spine injuries per child. Four (10%) patients with thoracolumbar spine fractures also sustained a cervical spine fracture. CONCLUSION: Once a thoracolumbar spine injury has been detected in a patient, the entire spinal column should be scrutinized because there is a high likelihood for additional injuries throughout the spine. Younger pediatric patients (≤ 8 years old) exhibit a spine fracture pattern distinct from that of older children who have a mature osseous-ligamentous complex.

Nomenclature of Subchondral Nonneoplastic Bone Lesions.

OBJECTIVE. The purpose of this article is to summarize the nomenclature of nonneoplastic conditions affecting subchondral bone through a review of the medical literature and expert opinion of the Society of Skeletal Radiology Subchondral Bone Nomenclature Committee. CONCLUSION. This consensus statement summarizes current understanding of the pathophysiologic characteristics and imaging findings of subchondral nonneoplastic bone lesions and proposes nomenclature to improve effective communication across clinical specialties and help avoid diagnostic errors that could affect patient care.

Incidence of rotator cuff tears in the setting of calcific tendinopathy on MRI: a case controlled comparison.

OBJECTIVE: To compare the incidence of rotator cuff tears on shoulder MRI in patients who have rotator cuff calcific tendinopathy with that in patients without calcific tendinopathy in a frequency-matched case-control study. MATERIALS AND METHODS: Retrospective review shoulder MRIs of 86 patients with rotator cuff calcific tendinopathy and an 86-patient age-, gender-, and laterality-matched control group using frequency matching. RESULTS: No statistically significant difference (odds ratio: 0.72, 95% confidence interval: 0.38-1.38, p = 0.32) was found in the incidence of rotator cuff tear in the calcific tendinopathy (27.9%) and control groups (34.9%). A significant (p < 0.001) difference in the size of rotator cuff tear was seen between the two groups, with 12.5% of tears being full-thickness in the calcific tendinopathy group and 63.3% of tears being full-thickness in the control group. Only 3 of the 24 (12.5%) rotator cuff tears present in the calcific tendinopathy group occurred at the site of tendon calcification. CONCLUSION: Patients presenting with indeterminate shoulder pain and rotator cuff calcific tendinopathy are not at increased risk for having a rotator cuff tear compared with similar demographic patients without calcific tendinopathy presenting with shoulder pain. Calcific tendinopathy and rotator cuff tears likely arise from different pathological processes.

There has to be an easier way: facet fracture characteristics that reliably differentiate AOSpine F1 and F2 injuries.

PURPOSE: To identify morphologic features of isolated cervical spine facet fractures that can reliably differentiate AOSpine F1 and F2 injuries. MATERIALS AND METHODS: Retrospective review of cervical spine CTs on all patients who sustained isolated cervical fractures of the facets presenting to our level 1 trauma center from August 2012 through December 2015. CTs were reviewed for facet fracture characteristics and AOSpine facet fracture classification. Association between facet fracture characteristics and AOSpine classification was assessed through multivariable logistic regression models. RESULTS: Fifty-six patients with cervical spine fractures isolated to the facets were included in the study. The mean age was 36 (range 9-90) years with 55.4% (n = 31) males. A significant correlation was found between subtype F1 and subtype F2 in laterality (left- or right-sided) (p = 0.004), interfacetal fracture involvement (p < 0.0001), transverse process involvement (p < 0.001), displacement of fracture fragment (p < 0.001), comminution of fracture (p < 0.0001), and vertebral arch disruption (p = 0.001). After multivariable analysis, left side laterality (p = 0.03), transverse process involvement (p = 0.01), and fracture comminution (p = 0.003) were associated with F2 fractures. CONCLUSION: Facet fractures with transverse process involvement or comminution have a higher probability of being an F2 fracture. These characteristics may be helpful when categorizing facet fractures using the AOSpine classification.

Characterization of all-terrain vehicle-related chest injury patterns in children.

PURPOSE: To evaluate chest injury patterns in pediatric patients involved in all-terrain vehicle (ATV) accidents. METHODS: Retrospective review of patients 0-18 years old admitted to a level I trauma institute following an ATV-related incident from 2004 to 2013 was performed. Chest injury type, accident mechanism, driver/passenger status, and demographic and clinical data were compared between patients with and without chest injury. RESULTS: Chest injury was present in 102 (22%) of total 455 patients. The most common chest injuries were pulmonary contusion (61%), pneumothorax (45%), and rib fracture (34%). There were no cardiac, esophageal, or tracheobronchial injuries. Pulmonary lacerations, hemothoraces, and scapular fractures were predominantly not detected on radiography and recognized on CT only (86-92%). Patients with chest injury had longer median hospital stays (3 days vs 2 days, p = 0.0054). There was no significant difference in admission to intensive care after controlling for injury severity scores in patients with chest injury compared to those without (17 vs 9). Eight patients with chest injury died (8%) compared to 2 patients without chest injury (0.6%) (p = 0.0002). CONCLUSIONS: Chest injuries are common in children following ATV accidents and may be a marker of more severe trauma. Increased public awareness of these potentially devastating injuries and continuing safety efforts are needed.

Is elevated body mass index protective against cervical spine injury in adults?

PURPOSE: Correlate body mass index (BMI) with incidence and type of cervical spine injury seen on CT in adult patients presenting with blunt trauma. MATERIALS AND METHODS: Retrospective chart review of all adult blunt trauma patients who had a cervical spine CT performed at our level 1 trauma center during an approximately 3-year period. RESULTS: A statistically significant (p = 0.01) difference in cervical spine injury incidence was present between different BMI groups. Cervical spine injury incidence was 7.7% for underweight (BMI ≤ 18) patients, 7.1% for normal weight (BMI 18-25) patients, 6.2% for overweight/obese (BMI 25-35) patients, and 4.7% for morbidly obese (BMI > 35) patients. Using BMI > 18-25 as a reference group, females with BMI > 25-35 had an adjusted odds ratio (aOR) of 0.56 (CI 0.41-0.75) and females with BMI > 35 had an aOR of 0.42 (CI 0.26-0.70). Males with a BMI ≤ 18 had an aOR of 2.20 (CI 1.12-4.32) and males with BMI > 35 had an aOR of 0.66 (CI 0.46-0.95). A particularly low incidence of cervical spine injury was observed in patients older than 65 in the obese group with a cervical spine injury rate of only 1.4% in this patient population. No statistical significant difference was seen in injury morphology across the BMI groups. CONCLUSION: An inverse relationship exists between BMI and the overall incidence of cervical spine injury. This protective effect appears to be influenced by gender with elevated BMI having lower relative odds of cervical spine injury in women than in men. A particularly low rate of cervical spine injury was identified in obese patients over the age of 65. Routine imaging of all elderly, obese trauma patients with low energy mechanism of injury may not be warranted.

Incidence and Association of CT Findings of Ankle Tendon Injuries in Patients Presenting With Ankle and Hindfoot Fractures.

OBJECTIVE: Tendon injuries are a commonly encountered finding in ankle CT examinations performed for fractures. This study was designed to identify the incidence and associations of tendon injuries in ankle CT examinations performed for fractures. MATERIALS AND METHODS: A retrospective review was performed of 410 patients who underwent ankle CT during a 6-year period. RESULTS: Tendon injuries were common, seen in 25% of all ankle CT examinations. Tendon subluxation-dislocation accounted for most of the tendon injuries (77 of 196 total injuries). Pilon fractures carried 2.2 times increased risk of tibialis posterior tendon injury (p = 0.0094). Calcaneus fractures carried 11.86 times increased risk of peroneus brevis tendon and 10.71 times increased risk of peroneus longus tendon injury (p < 0.0001). Calcaneus fractures also carried 5.21 times increased risk of flexor hallucis longus tendon injury (p = 0.0024). Talus fracture was associated with injury to all flexor compartment tendons. Talus fractures carried 3.43 times increased risk of tibialis posterior tendon injury (p < 0.0001), 4.51 times increased risk of flexor digitorum longus tendon injury (p = 0.0005), and 6.97 times increased risk of flexor hallucis longus tendon injury (p < 0.0001). CONCLUSION: Calcaneal fractures are prone to peroneal tendon injury, and talus fractures are prone to flexor tendon injury. In patients with pilon fractures, it is important to look for tibialis posterior tendon injury, specifically for entrapment. Overall, the most common type of injury is tendon malalignment, so it is imperative to know the normal tendon paths and associated bony landmarks to identify tendon injury.

PTEN hamartoma of the soft tissue: the initial manifestation of an underlying PTEN hamartoma tumor syndrome in a 4-year-old female.

A 4-year-old female was referred to pediatric orthopedic surgery for left leg pain and limping for 3 months following a motor vehicle collision. Recently, the patient's mother had noted left knee swelling and dragging of the left leg when walking. Past medical history was significant for hip dysplasia with slight leg length discrepancy. The patient was otherwise healthy. Physical examination was remarkable for left pre-patellar soft tissue fullness with normal range of motion. There was no warmth or tenderness. Subsequent ultrasound revealed a heterogeneous soft tissue mass superior and medial to the patella with a moderate degree of internal vascularity. MR exhibited a heterogeneous soft tissue mass with heterogeneous signal on both T1- and T2-weighted images centered within the vastus medialis obliquus muscle infiltrating the quadriceps tendon. Excisional biopsy was performed with a histopathologic diagnosis of fibroadipose tissue with anomalous vessels, suggestive of phosphatase and tensin homolog (PTEN) hamartoma of the soft tissue (PHOST). The patient was found to be positive for the PTEN gene mutation on genetic testing. The child was also determined to be macrocephalic, a major criterion for PTEN hamartoma tumor syndrome (PHTS). The geneticist advised the patient to undergo yearly physical examinations and early, routine surveillance for several malignancies occurring with PHTS. This case report presents the ultrasound and MRI appearance of a rare benign tumor typically appearing in pediatric patients. The strong association between PHOST and other soft tissue malignancies and the resulting need for life-long surveillance make PHOST an important pathology to recognize.

Sacral fractures: classification and management.

Sacral fractures are a common component of pelvic fracture patterns and are an increasingly diagnosed injury both due to increased utilization of CT in trauma evaluation as well as an increasing rate of sacral fragility fractures as a result of an increase in general population age. Innovations in minimally invasive surgical techniques have also resulted in an increasing number of sacral fractures undergoing surgical management. It is vital that physicians practicing in an emergency setting are aware of the injury patterns and management of this increasingly injured and treated component of the bony pelvis. This article reviews the sacral anatomy as well as discusses the role of imaging and imaging appearance of sacral fractures. Sacral fracture patterns are described along with both historic and newer classification systems for sacral fractures and current management of sacral fracture.

Association of femoral head and acetabular fractures on computerized tomography: correlation with the Judet-Letournel classification.

PURPOSE: Tha aims of this study were to identify the incidence of femoral head fractures in the setting of acetabular fractures and to determine the relationship between acetabular fracture pattern, the degree of acetabular fracture displacement, and the incidence of femoral head fractures. MATERIALS AND METHODS: This is a retrospective cross-sectional study of 274 patients with 300 acetabular fractures diagnosed on pelvic CT. Acetabular fractures were categorized using the Judet-Letournel classification system. RESULTS: Femoral head fractures were present in 18.0% of acetabular fractures. Fracture patterns with a posterior wall component had a very high (56.3%) incidence of femoral head fracture. Anterior column and anterior column with posterior hemitransverse fractures have a very low (3.4%) incidence of femoral head fracture. Anterior hip dislocation had a 66.7% incidence of femoral head fracture, while posterior dislocation had a 71.9% incidence. Acetabular fractures displaced by more than 5 mm had a 26.9% incidence of femoral head fracture, while acetabular fracture displaced less than 5 mm had only 4.2% incidence of femoral head fracture. CONCLUSION: Femoral head fractures are a very common associated finding in patients presenting with acetabular fractures. In patients with a posterior wall component of the fracture or associated hip dislocation, a femoral head fracture is more likely than not present. Conversely, in acetabular fractures with less than 5 mm displacement or anterior column fractures without posterior acetabular involvement, femoral head fractures are very unlikely in the absence of a dislocation event.