Proximal femoral fractures are a major cause of morbidity and mortality in the ever-increasing elderly population, and represent an increasing financial burden on healthcare systems1). In the majority of patients presenting with a hip fracture, there is a clear fracture line on standard anteroposterior and/or lateral hip radiographs. However, in a small number of patients in which there is clinical suspicion but inconclusive radiographs, further imaging with a magnetic resonance imaging (MRI) scan is recommended. According to National Institute for Health and Clinical Excellence (NICE) guidelines, if MRI is contraindicated or not available within 24 hours, computed tomography (CT) scanning may be performed2).

Although there is an abundance of literature on hip fracture outcomes in general, much less is known about the outcomes of occult hip fractures diagnosed on MRI^3,45). Therefore, the purpose of the present study was to evaluate the demographics, injury characteristics, management and clinical outcome of patients diagnosed with occult femoral neck fractures on MRI.

From 2005 to 2014, 64 patients (23 males and 41 females) were hospitalised with occult hip fractures diagnosed on an MRI scan. They were further subclassified according to their anatomical location (Fig. 1). There were 33 intracapsular hip fractures and 31 extracapsular fractures. The mean time from a negative hip radiograph to MRI investigation was three days (range, 1–5 days). Thirteen patients were managed non-operatively, comprising of isolated greater trochanter and incomplete inter-trochanter fractures. Following MRI diagnosis, two-thirds of patients undergoing surgery had their procedure within 48 hours (mean, 2.06 days; range, 0–16 days). Fracture types, surgical procedures performed and mortality are detailed in Table 3.

The mean duration of hospitalization was 16 days (range, 3–29 days). Two-thirds of patients returned to their pre-existing residence after being discharged, while one third went to rehabilitation centres. Two patients developed a post-operative superficial wound infection which was successfully treated with antibiotics. There were no known cases of metalwork failure. Thirty- and ninety-day mortality rates were 3% and 10%, respectively. The ninety-day mortality rates of intra-capsular versus extra-capsular fracture patients were not significantly different (P=0.122). However, patients who underwent internal fixation had a lower mortality rate compared to those who underwent joint replacement procedures, although this was not statistically significant (P=0.056).

The prevalence of occult hip fractures is approximately 3%2). In the presence of clinical suspicion for hip fractures but negative radiographs, NICE guidelines recommend further imaging with an MRI scan. If MRI is contraindicated or not available within 24 hours, CT scanning may be performed2). MRI has been shown to detect occult fractures more accurately than CT scans and the T1 sequence is believed to be 100% sensitive in highlighting occult femoral neck fractures^6,78). Occult hip fractures missed by 64 slice CT scan have been accurately detected by MRI9).

To our knowledge, this is the single largest study specifically reviewing outcomes of patients with occult hip fractures diagnosed on MRI scan. The results suggest that patients with occult neck femur fractures have a shorter length of stay, lower mortality rates and are more likely to be discharged to their pre-existing residence when compared with outcomes of the NHFD10). The thirty- and ninety-day mortality rates were 3% and 10%, respectively, values which compare favorably to results from the NHFD which reveal an overall thirty-day mortality rate of 6.7%^10,1112).

In our study, 66% of patients were discharged to their pre-existing residence, significantly better compared to NICE-commissioning guides where only 45% returned to their home13). The improved outcome seen in patients who sustain occult neck of femur fractures may be because these injuries represent lower energy injuries compared to those with displaced fractures. Therefore the associated morbidity is lower compared to patients sustaining a displaced neck femur fracture where more significant tissue disruption and internal hemorrhage probably occurs with the primary injury. Similarly, in those managed with internal fixation, the surgical procedure is technically less challenging compared to patients with a displaced fracture where fracture reduction must be achieved before a definitive fixation. These factors are likely to explain the reduced length of stay, lower mortality rates and higher discharge to preinjury residence in patients with occult hip fractures.

MRI investigation can be very useful in diagnosing incomplete inter-trochanteric fracture, as illustrated in Fig. 2 and 3. There is some debate about the need to fix incomplete inter-trochanteric fractures. In a review of eight patients with occult inter-trochanteric fractures diagnosed by MRI, Alam et al.3) reported successful outcomes for five patients managed non-operatively. However a recent systematic review by Kim et al.14) reported that of the 61 incomplete inter-trochanteric fractures treated with surgical fixation, no complications were reported and they concluded that due to the risks of incomplete non-displaced fractures progressing to complete displaced fractures, conservative management should be undertaken with caution. Given that the aim of treating hip fractures is to enable early mobilization, we would also advocate for early fixation of occult intertrochanteric hip fractures.

Patients treated with internal fixation had a lower mortality rate compared to those who underwent joint replacement procedures (P=0.056). This may be because sliding hip screw fixation in an undisplaced hip fracture is a less invasive procedure compared to a replacement procedure.

Pihlajamäki et al.5) reported on the long-term outcomes of 66 conservatively managed undisplaced femoral neck fractures followed up for a mean of 18.3 years. Although they reported favorable outcomes with conservative management with no cases of fracture displacement or avascular necrosis, these were fatigue fractures of the femoral neck in young military recruits and so represent a different injury compared with occult femoral neck fractures seen in the elderly.

Beloosesky et al.15) reported on a smaller cohort of 29 patients with occult hip fractures diagnosed on bone scan and compared the results to a randomly chosen group of 94 patients diagnosed with a hip fracture by plain radiographs. In this study, those with hip fractures diagnosed on plain radiographs had a significantly increased rate of complications. There was no significant difference in inpatient length of stay or 30-day mortality between groups. The authors also reported that there were no significant differences in survival between the groups during 13 years of follow-up.

Our study has certain limitations. We were unable to collect detailed information about the body mass index, or American Society of Anesthesiologists (ASA) score for our patients. Due to these limitations, a multivariate analysis could not be performed; however, we performed a univariate analysis on the available data. All of our patients were ambulant at the time of injury and were mobilized before discharge to their usual residence. This was our primary clinical outcome of interest; however, we did not quantify it by performing any outcome measure scoring.

Occult hip fractures represent a unique subset of proximal femoral fractures. This study has shown a reduced length of stay, reduced need for rehabilitation following discharge and reduced short-term mortality compared to national data. This injury-specific information may help facilitate a more informed discussion with patients sustaining occult hip fractures.