On the Horizon From the ORS: Toward an Objective Measurement of Articular Fracture Injury Severity. (2011)

The severity of an articular fracture, in particular the energy causing the injury, determines to a significant extent whether the joint progresses to disabling posttraumatic osteoarthritis (PTOA). However, fracture severity has not been amenable to objective measurement. Current articular fracture classification systems do not completely capture or quantify the severity of injury; fracture classification also is subject to poor interobserver reliability. Treatment decisions by clinicians must therefore be made based on subjective assessments of the injury severity. This approach is in contrast to decision-making done in many other clinical scenarios, in which objective measures are a fundamental part of evaluating and treating patients. For instance, physiologic and anatomic measurements (e.g., ejection fraction and percent damage of the myocardium) are considered routine in assessing a patient with a myocardial infarction. These objective measurements of the severity of the cardiac injury are critically important guides to clinical management.

A novel CT-based method has been developed to quantify the severity of articular injury. Fracture mechanics theory predicts that the fracture energy is directly proportional to the interfragmentary surface area liberated by a fracture, a theory that we have corroborated in bench-top studies of bone fracture.1 Clinically, this means that fracture comminution is proportional to fracture energy. Clinicians observe a radiograph of a severely comminuted fracture and call the injury a “high-energy fracture;” alternatively, a noncomminuted fracture, based on its radiographic appearance, is called a “low energy fracture.” What clinicians observe in the high-energy case is greater fragment surface area. Thus, what the clinician sees on radiographs can now be quantitatively assessed using CT-based image analysis techniques.

These analysis techniques have been refined to measure not only fragment surface area (i.e., comminution) but also fracture fragment displacement.2 The bone density is accounted for through a correction based on CT scan Hounsfield units. With an articular fracture of the distal tibia used as a clinical model, these metrics have been shown to closely correlate with PTOA on radiographs at a minimum of 2 years after injury.3 In addition, this research has suggested the concept of an energy threshold, above which all ankles develop radiographic osteoarthritis and below which ankles maintain their articular surface. This finding raises the possibility that the information present on the CT scan at the time of injury can be used to predict joints that are highly likely to develop PTOA despite current treatment, thereby setting the stage to consider alternative treatments soon after injury.

Currently, clinicians make treatment choices for high-energy articular fractures by intuition and clinical experience. For example, instead of reduction and fixation of a fracture, joint arthrodesis might be chosen as a treatment alternative by the clinician, based on his or her subjective assessment of the severity of articular injury. Changing from this paradigm of reliance on subjective judgment to decisions based on objective measures would be an important step in improving clinical decision making. Such a change will be even more important in the future, with the advent of biologic interventions to help preserve damaged articular surfaces. Choosing new interventions clinically, or designing clinical research studies to assess their effectiveness, requires that the degree of damage to the articular surface be meaningfully stratified. This research to measure fracture energy makes objective stratification of injury severity a reality.

In determining fracture energy from clinical CT scans, the fragment surface area is calculated, for example, slice by slice across the area of the distal tibia.2 Both limbs are scanned so that the surface area on the noninjured contralateral side can be subtracted from that of the injured side, resulting in the liberated surface area caused by the injury. When corrected for bone density and multiplied by a scalar material property, the liberated surface area provides the energy of injury. Fragment dispersal is calculated through a volumetric envelope of the injured versus noninjured side. Further work with texture analysis is under way to expedite the technique and to minimize or eliminate the need for operator intervention.

In summary, an objective articular fracture severity metric based on CT data obtained as part of routine clinical care has shown tremendous promise as an aid to clinical research and to patient care. This metric has been shown to correlate with the propensity of tibial plafond fractures to develop PTOA,3 and preliminary data suggest an energy threshold above which PTOA is practically inevitable.3 This technique is being refined for use in other anatomic areas (eg, tibial plateau). In addition to the authors’ institution, it will soon start to be used in two additional trauma centers. Further work is under way to speed the calculations of these objective measures, to translate their use from the experimental realm, and to realize their potential as a valuable tool for clinical research and, eventually, clinical care.