Occult Bone Fractures: Classification And Early Diagnosis Using Helical CT Versus MRI

Objective To investigate the rational classification of occult bone fractures and helical CT, MRI findings of its every subtype; To compare the efficacy of helical CT and MRI in early diagnosis of occult bone fractures using receiver operating characteristic (ROC) analysis;To preliminarily investigate serum level and its diagnostic value of alkaline phosphatase (ALP) and bone alkaline phosphatase (BAP) in patients with stress fracture.Materials and Methods 152 patients with negative or little nonspecific features of bone on conventional radiography, which included 74 cases without and 78 with definite history of trauma, were prospectively evaluated with helical CT and MRI due to high clinical suspicion of occult bone fractures based on a combination of medical history and correlated examinations. The final diagnosis were confirmed on the basis of imaging findings combined with clinical features or follow-up and were divided into 4'types as fatigue fracture, insufficiency fracture, occult traumatic fracture and occult intraosseous fracture in terms of mechanism of injury and quality of bone affected, and the imaging findings were analyzed.The sensitivity, specificity, and accuracy of helical CT and MRI for early diagnosis were compared using ROC analysis. In 14 patients with fatigue fracture and 11 with insufficiency fracture, serum level of ALP and BAP at the time of pain and pain relief were compared.Results The main CT findings of fatigue fracture and insufficiency fracture were obscure fracture line without obvious transposition, while negative in some cases. On MRI, it mainly manifested as linear low, slightly high, and high signal intensity on T1WI, T2WI, and STIR, respectively, with or without bone marrow edema. The main CT features of occult traumatic fracture were sharp fracture line without transposition involved bone cortex, while on MRI linear with or without peripheral patchy areas of low and high signal intensity on T1WI and T2WI respectively were usually seen which represented fracture line and bone marrow edema respectively. CT scan is usually negative in most cases of occult intraosseous fracture, but MRI always demonstrated intraosseous bandlike or amorphic low on T1WI and high signal intensity on T2WI with normal signal intensity of bone cortex, accompanied with high frequency of injury of ligaments or other soft tissues.Areas under ROC curve of CT and MRI for each type of occult fracture were 0.88 and 0.93,0.95 and 0.91,0.93 and 0.95,0.67 and 0.95 for radiologist 1,and 0.86/ and 0.91,0.94 and 0.89,0.92 and 0.93,0.66 and 0.95 for radiologist 2,respectively. MRI was significantly superior to helical CT for early diagnosis of fatigue fracture and occult intraosseous fracture (P<0.05).The sensitivity of CT and MRI for early diagnosis was 74%/91%,84%/84%,84%/94%,13%/89% for radiologist 1,and 83%/87%,84%/89%, 87%/97%,18%/92% for radiologist 2, respectively. For radiologist 1,the specificity was 93%/79%,94%/81%,96%/92%,92%/92%, and 86%/79%,88%/75%,92%/88%, 92%/88% for radiologist 2.MRI had a significantly better sensitivity than CT in early diagnosis of occult intraosseous fracture (P<0.05).Interobserver agreement of CT and MRI for each type of occult fracture was 0:86/0.71,0.85/0.66,0.80/0.74, and 0.52/0.81. Mean level of serum ALP and BAP in 14 patients with fatigue fracture was 259U/L and 156U/L at the time of pain,159U/L and 85U/L during the period of pain relief. There was a statistically significant difference between serum level of the two periods for both ALP and BAP (P<0.05). In 11 patients with insufficiency fracture, a statistically significant difference was also found between mean level of serum ALP and BAP of the two periods (P<0.05), which was 430U/L and 231U/L for ALP,276U/L and 118U/L for BAP, respectively.Conclusion 1.It is clinically rational to divide occult bone fractures into 4 types, ie, fatigue fracture, insufficiency fracture, occult traumatic fracture, and occult intraosseous fracture in terms of mechanism of injury and quality of bone affected. Accurate diagnosis of subtype should be made using imaging modalities, which is helpful for the clinical treatment.2. With a lower specificity, MRI is more sensitive than CT for early diagnosis of fatigue fracture and insufficiency fracture. However, results of ROC analysis indicate that CT has a higher accuracy than MRI for early diagnosis of insufficiency fracture while MRI is superior to CT for fatigue fracture. MRI is the first choice of modality for early diagnosis of stress fracture, while CT is valuable in patients with atypical MRI features.3. Although ROC analysis shows that MRI is slightly more accurate than CT, they are both useful for early diagnosis of occult traumatic fracture. MRI should be used for early diagnosis of occult intraosseous fracture because CT is of little value.4. Serum level of ALP and BAP in patients with fatigue and insufficiency fracture is elevated at the time of pain and decrease with fracture healing. ALP and BAP may be used as a biochemical maker for early screening of patients with stress fracture and dynamic measurements of ALP and BAP may be useful to differentiate stress fracture from other disease.