British Journal of Radiology (2006) 79, 216-220
© 2006 British Institute of Radiology
doi: 10.1259/bjr/31965396
The Bristol Hip View: a new hypothetical radiographic projection for femoral neck fractures
M Bradley, FRCR,
M Shaw, FRCR and
D Fox, FRCR
Department of Radiology, Southmead Hospital, North Bristol Trust, Westbury on Trym, Bristol BS10 5NB, UK
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Abstract
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This experimental study is to evaluate a modified radiographic view of the femoral neck in the assessment of femoral fractures. A dry femur and pelvis was set up in a rig to simulate the positioning of a routine anteroposterior (AP) pelvis X-ray view. Films were exposed to create a routine AP pelvis, AP hip and two views with external tube angulation of 15° and 30°. Observers were asked to evaluate the films using a visual analogue score on two separate occasions. The same films were performed on a further fractured femoral neck to assess the fracture clarity. There was good intraobserver and interobserver correlation. Observers ranked the 15° and 30° angled films as showing the femoral neck most clearly, over and above the traditional views (p<0.001). The fracture was best demonstrated on the 30° angled film (p<0.001). The 30° angled view appears to demonstrate the femoral neck anatomy more clearly than the traditional views but also showed increased fracture sharpness. The authors are proceeding to a clinical trial to assess this in trauma practice.
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Introduction
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Our standard departmental policy for radiographs for the patient with a suspected femoral neck fracture is an anteroposterior (AP) pelvis with lateral hip of the symptomatic side. The geometry of the AP film means that the angle of incidence of the central beam to the femoral neck is in the order of 70°. An AP hip view centred on the head then reaches approximately 75°. Ideally a 90° angle should be obtained for the optimum visualization of the femoral neck.
It has been observed that when the diagnosis is in doubt due to difficulty with identifying the fracture, a view, similar to the Judet obturator oblique view, can be useful in delineating the fracture. In order to assess this observation an experimental study was set up with cadaveric bones. The study compared four different views, two representing the AP pelvis and AP hip and two new angled views to obtain angles of incidence of 90° and 105° to the femoral neck (Figure 1
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Figure 1. Diagrammatic representation of the typical beam incident angles for a routine anteroposterior(AP) pelvis (70° to femoral neck), AP hip, (75° to femoral neck), beam angle of 15° (90° to femoral neck), and 30° beam angle (105° to femoral neck).
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Femoral neck fractures may result in varying degrees of external rotation of the lower limb due to unopposed action of the gluteus maximus, piriformis, obturator internus and gemelli muscles on the femur. The second aim of our study was to assess the affect of changes in external rotation of the lower limb on the femoral neck angle relative to a base line of the anterior inferior iliac spine (AIIS). This external rotation could have a direct affect on the angle of incidence of the X-ray beam to the femoral neck.
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Methods
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Ethics committee approval was granted. This study was largely performed experimentally.
CT was used to measure femoral neck angles on patients who were undergoing CT for valid clinical reasons. Angles of internal and external foot rotation were studied.
A disarticulated femur and pelvis was assembled in a rig to closely simulate the AP pelvis with feet in-turned. The femoral neck angle was set according to the mean data measured from the CT.
Four films were then exposed to create the standard AP pelvis ( = D), AP hip ( = B), 15° angled beam towards femoral head ( = A), 30° angled beam towards femoral head ( = C) (A and C were centred on the femoral head) (Figure 2).
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Figure 2. (a) Film of femoral neck obtained using a 15° angled beam. (b) Film of femoral neck simulating an anteroposterior (AP) hip. (c) Film of femoral neck obtained using a 30° angled beam. (d) Film of femoral neck obtained simulating an AP pelvis.
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A second femur was fractured at right angles through the mid femoral neck using an osteotome and then glued together anatomically. The rig was set up in the same fashion with CT confirming the same femoral neck angle. The same four films were then exposed to demonstrate the neck and fracture (Figure 3). These were randomly labelled W, X, Y, Z (Table 1).
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Figure 3. (a) Film (Y) of fracture with least sharpness (view equivalent to an anteroposterior (AP) pelvis). (b) Film Z showing the greatest fracture sharpness (30° angulation to the femoral head).
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Blinded observers were asked to fill in a questionnaire based on the four X-ray views randomly displayed for both rigs using a visual analogue scoring scale. The same observers repeated the process a month later to show intraobserver consistency. A variety of observers were asked including; radiologists, orthopaedic surgeons (both consultant and SPR), accident and emergency consultants and senior radiographers.
The observers were asked to assess the clarity of visualization of the femoral neck (sub-capital, mid neck and intratrochanteric) and the sharpness of the fracture.
The CT data measured femoral neck angles relative to the AIIS with internal and external rotation. This was to evaluate whether a correction angle was needed to be added to the new views to ensure consistency of 90° beam incidence to the femoral neck when patients with neck fractures present with limb shortening and external rotation.
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Results
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46 observers were randomly shown the two sets of films; 10 radiology consultants, 8 specialist registrars, 10 orthopaedic consultants, 9 middle graders, 8 senior radiographers and 1 consultant emergency physician.
Analysis showed no statistically significant differences (Kappa) between the two occasions of observation (p<0.001) or between grade/speciality of observer; i.e. excellent intraobserver and interobserver correlation. 71% of observers ranked A and C as best.
The questionnaire tried to differentiate between the sub-capital, mid-cervical and intratrochanteric areas to see if any particular film out-performed in any one area.
Pairwise comparisons of the means, using the Bonferroni correction for multiple comparisons, revealed the following:
Sub-capital
A significantly out-performed B, C and D (p<0.001) and B, C, D were not significantly different (p<0.05).
Mid-cervical
A performed similarly to C (p<0.13) and both were significantly better than B and D (p<0.001).
Intratrochanteric
All films performed similarly with no statistical variation (Figure 4). The fracture sharpness was better demonstrated on W and Z than X and Y (p<0.001). Y represented the traditional AP pelvis performed most commonly (Figure 5). Z out-performed W by a similar statistical difference (p<0.001).
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Figure 4. Graphic representation of the radiographs A, B, C, D showing the observers' results by region. This shows increased performance of A and C.
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Figure 5. Graphic representation of the fracture sharpness. This shows increased clarity of the fracture in film Z, out performing the traditional views X and Y(p<0.001).
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The CT data for femoral neck angles relative to the AIIS baseline showed wide variation and overlap with no statistical relationship for foot external rotation. A random sample of scans was re-measured showing agreement in the measurements.
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Discussion
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The reported incidence of occult femoral neck fractures on plain radiographs is approximately 4% [1]. There is very little in the recent literature regarding optimizing plain radiography to decrease the incidence of occult femoral neck fractures. The authors hope that by including this further radiographic view it will decrease the numbers of patients requiring further investigation.
When plain radiographs are negative, and there is a high index of suspicion, MRI has been shown to be sensitive and specific in diagnosis of occult femoral fractures.
Studies have shown that in radiographic negative cases, where clinical concern is high, MRI showed femoral neck fractures in 23–50% [1–3, 6]. Fractures other than those of the femoral neck were demonstrated in 11–33% of cases [2, 3, 6]. Most commonly these were insufficiency fractures of the pubic rami or sacrum.
A further modality for diagnosis is radionuclide bone scans. The sensitivity has been reported as 93–100% [4, 5], the specificity as 96%, and the positive predictive value as 97%. This was regardless of patient age, and time from presentation to scanning [4]. However, there have been reported cases of a negative bone scan in a fractured neck of femur [7], and false positive results due to ligamentous avulsion and periosteal injury [8].
Fluoroscopy has been used with success. By gently internally rotating the femur and obtaining high quality exposures the diagnosis of femoral fractures was made in 8 out of 16 patients in whom the initial radiographs were negative [9]. Internal rotation elongates the femoral neck and hence improves visualization of fractures. Our study used the same principle, having the X-ray beam closer to 90° to the femoral neck making the fracture line more obvious.
The observers ranked A and C as the preferred choice for anatomy in 71%. C (Z), however, was significantly better for fracture clarity than A (W), and both were superior to the standard views. C (Z) tended to elongate the femoral neck, for the same reasons as to the 40° angled scaphoid view now widely used routinely for trauma, i.e. the central ray is no longer at right angles to the bone, creating geometric distortion. The observers were not used to looking at the femoral neck with this elongated appearance and so this may explain why A was ranked higher than C for the anatomical demonstration. The 40° angled scaphoid view is a good corollary as to why the authors expect the angled hip view to out perform the normal view for a fracture at right angles to the femoral neck. It is recognized, however, that not all femoral neck fractures will lie at 90°, but it is proposed that it is these fractures that are difficult to see on standard views and therefore may be better demonstrated on the new view.
Specialist investigations are both expensive and time consuming, and if there is a quick and cheap method of obtaining the diagnosis when the initial radiographs are negative, then this will undoubtedly benefit both the patient and institution.
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Conclusion
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Suspected fractured femoral necks are common clinical problems. We have demonstrated that radiographs angled at 15° and 30° towards the femoral head show greater clarity of both the subcapital and the midcervical areas than the standard views used in current clinical practice. The femoral neck fracture was also better demonstrated using these two views, but best on C (30° angulation).
The authors now intend to conduct a prospective trial to evaluate this in clinical practice, to evaluate whether in equivocal cases a radiograph angled 30° to the femoral head (the Bristol view) should be considered to aid the diagnosis of fracture.
Received for publication May 6, 2005.
Revision received June 28, 2005.
Accepted for publication July 15, 2005.
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References
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Abstract
Introduction
