Mechanism of injury, injury patterns and associated injuries in patients operated for chest wall trauma

Thoracic trauma, encompassing trauma to the chest wall, lungs and cardiovascular system, accounts for approximately 23‒28% of trauma-associated mortality [1, 2]. Rib fractures are a common injury [3], especially in blunt trauma, and occur in approximately 40% of patients with thoracic trauma [4]. An increased number of rib fractures is associated with an increase in morbidity and there is a correlation between the number of ribs fractured, injury severity score (ISS) [5] and mortality [3, 6]. Multiple rib fractures are highly associated with pneumothorax, haemothorax and pulmonary injuries [4] and can lead to chest wall instability and flail chest [7] with subsequent respiratory insufficiency, a requirement for ventilator support and a high mortality [8]. Multiple rib fractures and flail chest are generally associated with high-energy trauma, such as road traffic accidents [4]. In the elderly, however, multiple rib fractures can occur after low-energy impact, such as falls that cause multiple, comminute and displaced rib fractures [9]. Elderly patients with rib fractures often present with a significantly less severe injury status and with lower ISS than younger patients, but have a higher rate of mortality [10, 11].

Rib fractures have previously been reported in 10% of trauma patients [3]. They are, however, frequently under-diagnosed because chest radiography fails to diagnose the majority of fractures [12, 13] and the true incidence may be underestimated. Computed tomography (CT) has a higher sensitivity for diagnosing rib fractures than radiography, with the added advantage of evaluating intra-thoracic injuries [13, 14]. Nevertheless, CT may also fail to diagnose rib fractures, particularly if these are non-displaced, chondral or anterior fractures [15].

Unstable thoracic cage injuries or flail chest occur in approximately 6% of patients with rib fractures [4] and have been described in 22% of patients with major chest trauma requiring intensive care [16], although the definition of major chest trauma was unclear in this study. The diagnosis of flail chest as anatomical or physiological can be disputed. Currently, and according to AIS 2008, the widely used definition is that flail chest is an anatomical diagnosis requiring three or more adjacent ribs fractured in more than one location [7]. This definition does not, however, take into account all injuries in the thoracic cage since sternal fractures in combination with rib fractures are not included in the definition. There is also lack of consensus as to what constitutes an anterior, lateral or posterior fracture, or an upper and lower rib fracture. These definitions are important because lower rib fractures are associated with increasing risk of intra-abdominal injuries [17, 18].

To our knowledge, no study has previously investigated an association between MOI and chest wall injury patterns. The aim of this study is to describe the MOIs, patterns of chest wall injuries, and associated injuries in patients who undergo open reduction internal fixation (ORIF) of chest wall injuries.

In this retrospective study of 211 trauma patients who underwent ORIF due to acute chest wall injuries, we found that falls and traffic accidents were equally common and constituted the mechanism of injury in the majority (88%) of cases. This contradicts previous studies, which show that traffic accidents are a more common MOI in thoracic trauma [3, 4]. One possible explanation for the dissimilarity is the older age of the patients in this study (mean age 58 years). Another explanation may be the lower incidence of traffic accidents in Sweden compared to other countries [22]. We found an age-related variance with traffic accidents dominating the younger population compared to the older population where falls dominate. This does concur with previous studies that have shown an increased prevalence of fall accidents in older patients [23]. Previous studies suggest that elderly patients have lower ISS despite higher mortality [11, 3, 23]. Injury severity scores, ISS and NISS, were higher in traffic accidents, especially in MVC with other vehicle(s) and in PVA. We found that ISS was related to MOI rather than the age group of patients. Interestingly, there was no significant difference in ISS and NISS between falls from the same level and falls from height.

The indication for chest wall stabilization was in 87% of cases flail chest. Patients had at least one dislocated rib fracture. The majority of patients had unilateral rib fractures, although one-third had bilateral rib fractures. Bilateral rib fractures were more common in traffic accidents and associated with more rib fractures, although the proportion of fractures operated was less than when compared to unilateral injuries. More studies are needed to determine the optimal percentage of rib fractures to stabilize in flail chest, as stabilizing too few may not prevent deformity, and stabilizing too many may cause rigidity [24]. There was no difference in the number of rib fractures between men and women despite elderly women having a higher risk of osteoporosis [25]. Only 12% of patients had isolated thoracic trauma, which was associated with falls, whereas 64% of patients had injuries to at least three body regions, associated with traffic accidents. Traffic accidents were associated with abdominal organ injury, pelvic fracture, extremity fracture, and spinal fracture. In fact, 96% of patients in frontal collision MVC had spinal fractures and 94% in PVA had extremity fractures. Motor vehicle collision with frontal collisions was associated with a greater extent with injuries in at least three body regions and abdominal organ injury, pelvic fracture, extremity fracture and spinal fracture, than side collisions. Previous studies have also shown that collision type has a greater impact on injury severity than speed [26].

In our study, flail chest was based on anatomical criteria. We found signs of a “floating segment”, such as paradoxical breathing, in 25% of patients with flail chest. This was recorded in 43% of cases in patients with bilateral rib fractures and was associated with sternal and anterior flail segments. It is understandable that paradoxical breathing was more commonly seen in bilateral injuries as these have more frequent sternal and anterior flail segments, which are easier to discover. It is equally understandable that posterior flail segments do not produce a visible floating segment considering the overlying thick musculature. Whilst the occurrence of paradoxical breathing may have been underreported considering that this was a retrospective study, others have also reported a low prevalence of paradoxical breathing despite flail chest [27].

Patients had at least one rib fracture in zone 2, whereas zone 1 rib fractures were more common in patients with bilateral injuries and associated with anterior and lateral flail segments. Zone 3 rib fractures were associated with posterior flail segments. Other studies have similarly defined rib fractures as upper and lower or divided them into different zones than our study [28]. An international consensus as to how chest wall injury patterns should be defined would be beneficial. We developed the commonly used definition of flail chest to involve the sternum. We found lateral and posterior flail segments to be the most common injury patterns. Sternal fractures and anterior rib fractures were more common in patients with bilateral injuries, leading to sternal and anterior flail segments, whereas lateral and posterior rib fractures leading to posterior flail segments were more common in patients with unilateral injuries. We found an association between collision type and type of flail segment. In particular, sternal and anterior flail segments were found in frontal collision MVC, while posterior flail segments were found in side collision MVC. We had no information as to whether or not our MVC patients had been belted. Previous studies have shown a different injury pattern with a higher distribution of injuries in unbelted patients [29]. Flail chest was found in all patients with PVA and crush injury, included in the study. Lateral flail segments were the most common in PVA and motorcycle accidents, whereas posterior flail segments were more common in falls.

In our study, a high percentage (76%) of patients underwent thoracotomy. It is debatable as to whether or not thoracotomy or video-assisted thoracoscopic surgery (VATS) should be performed in conjunction with ORIF of rib fractures. The procedure allowed us to diagnose intra-thoracic injuries to a greater extent. We found that pre-operative CT failed to diagnose pulmonary contusions, lung lacerations, and diaphragmatic injuries in several patients. Ultrasound has been shown to be a valuable adjunct in the initial assessment of trauma patients [30] and is more sensitive than chest radiographs in diagnosing rib fractures [14], although the examination may be more time-consuming, be difficult in the presence of subcutaneous emphysema, be inaccessible to subscapular rib fractures and painful [31]. However, ultrasound can serve as a complement to CT with the added advantage of possible repetition without radiation, but it is still less sensitive than CT, despite having a high specificity concerning diagnosing intra-thoracic injuries [32]. At our hospital, ultrasound was used as a bedside tool in the resuscitation of our patients but we had not implemented a standardized protocol for assessing fractures in the sternum, cartilage or ribs, and therefore this modality was not used in our study. Also, pulmonary contusions often develop over time and it is possible that these could not be diagnosed on initial CT or ultrasound [33]. We found a low sensitivity for CT in diagnosing diaphragmatic injuries, which seem to be more prevalent than previously described. Zarour et al. reported a low incidence (0.9%) of traumatic diaphragmatic injuries, which mainly occurred in penetrating trauma [34]. There is a difference, however, between what has traditionally been described as traumatic diaphragmatic rupture secondary to blunt trauma and the diaphragmatic injuries seen in conjunction with rib fractures that more resemble lacerations after stab injuries. These are more common than previously known and we found them in 18% of patients undergoing thoracotomy in this study. It is important to diagnose diaphragmatic injuries as these can lead to late complications with high mortality [35, 36].

Even though CT was used in all our patients and is known to be the diagnostic tool of choice since chest X-rays fail to diagnose 75% of rib fractures seen on CT [13], we may have underestimated the number of rib fractures. Moreover, we have not described chondral injuries separately in this study. These can be difficult to diagnose and are frequently underestimated despite contributing to the development of sternal and anterior flail segments. Another limitation of our study is the variation of operative techniques since all patients did not undergo thoracotomy, and this may be a potential bias when assessing the number of patients with missed diaphragmatic injury.

The retrospective nature of the study is a limitation since a prospective study could have yielded additional information. The number of included patients was low when divided into the different mechanism of injury categories, which limited the comparative analyses. Notably, this is a study of patients undergoing ORIF and may not, therefore, be representative of all patients with multiple rib fractures and flail chest. Nevertheless, our study is a unique, in-depth analysis of the association of rib fractures and different types of chest wall injury patterns. It is important to recognize that patients with chest wall trauma have different injury patterns as this influences the operative approach in patients undergoing ORIF and may help raise suspicion for associated injuries.

The MOI associated with multiple rib fractures and flail chest differs according to age group, and is associated with different chest wall injury patterns and risks of extra-thoracic injuries. Diaphragmatic injuries secondary to penetrating ribs are more common than previously thought. Traffic accidents and head-on collisions specifically are associated with polytrauma and severe injuries.