Essential Radiographic Evaluation for Distal Radius Fractures

INTRODUCTION

∗ This adapted chapter has been previously published in Slutsky DJ, Osterman AL: Fragments and Injuries of the Distal Radius and Carpus. Philadelphia: Elsevier, 2009.

Because the interpretation of the radiographs of distal radius fractures has such a profound impact on care of these injuries, accurate assessment of standard x-rays is essential for appropriate management. X-rays are two-dimensional representations of three-dimensional structures. Subtle changes in radiographic landmarks can provide significant information that can be used to understand the pattern of fragmentation and the extent of the injury. Unfortunately, much of this information often goes unnoticed simply because the physician is not trained to recognize it. Although computed tomography (CT) and magnetic resonance imaging (MRI) can provide more detailed imaging of the fracture pattern, the extra expense and delay in treatment that are involved in obtaining a CT or MRI scan at the time of a reduction are often practical limitations curtailing their routine use.

By recognizing detailed features on standard radiographic images and identifying abnormal variations of certain key parameters, the surgeon can create a more accurate visual image of the actual osseous deformity, resulting in a more informed and rational approach to treatment. The purpose of this chapter is to provide specific guidelines for improved interpretation of x-rays in patients with distal radius fractures.

NORMAL RADIOGRAPHIC LANDMARKS

X-ray evaluation of the distal radius normally includes a posteroanterior (PA) and a lateral projection. Oblique x-rays are often included as supplemental views. As discussed subsequently, a modified lateral projection in which the beam is angled 10 degrees proximally should be a standard view to assess fracture reduction and provide more detailed visualization of the articular surface.

On the PA projection, several basic anatomic structures are easily identified ( Fig. 6-1 ). The radial styloid is seen in profile; the articular surface of the distal radius, the proximal and distal carpal rows, the distal radioulnar joint (DRUJ), and the distal ulna are also easily recognized. The articular surface of the distal radius makes a smooth, concentric arc with the proximal articular surface of the proximal carpal row. In addition, the arcs of the articular surfaces on both sides of the midcarpal joint are congruent and concentrically aligned.

On a PA projection, a transverse, radiodense line can be seen about 3 to 5 mm proximal to the distal border of the radius and is aligned with the base of the lunate and proximal pole of the scaphoid. This feature has been nicknamed the “carpal facet horizon.” In the normal radius, the carpal facet horizon is caused by the projection of the subcortical bone of the volar rim of the lunate facet ( Fig. 6-2 A). This structure is normally proximal to the distal margin of the radius, since the normal volar tilt of the articular surface of the distal radius places the volar rim more proximally than the dorsal rim. Its projection on the x-ray is radiodense because the subcortical bone of the volar rim of the lunate facet is aligned parallel with the x-ray beam.

The relationship of the articular surface of the distal radius is reversed in a fracture or malunion in which there is dorsal angulation of the distal fragment. In this circumstance, the dorsal rim of the lunate facet migrates proximally and rotates dorsally in relation to the volar rim. As a result, the subchondral bone of the dorsal rim becomes oriented parallel with the x-ray beam and creates the carpal facet horizon on the PA view ( Fig. 6-2 B). Since this single landmark can represent two different anatomic structures of the osseous geometry, it is essential to always correlate the PA view with the lateral view for evaluating displacements of the articular surface as well as for determining whether an implant is properly positioned.

This relationship also provides a method of distinguishing the dorsal and volar corners of the sigmoid notch on a PA film. Knowing this is often critical in determining whether a displaced fragment should be approached from the volar side or from the dorsal side. In addition, if the interval between the dorsal and/or volar corners of the ulnar border of the radius is widened in relation to the head of the ulna, it may suggest a displaced DRUJ. Since widening of the DRUJ removes the osseous stability from seating of the ulnar head in the sigmoid notch, this widened interval may be a contributing source of DRUJ instability.

The lateral projection is an integral part of a complete examination. Despite this, distortion of the image may occur if the arm is not positioned properly. Often, an x-ray technician positions the arm for a lateral film in an extreme position of supination or pronation of the forearm; in this circumstance, simply superimposing the radius and ulna may result in an oblique projection of the articular surface. A simple solution to this problem is to use the relative position of the pisiform to the distal pole of the scaphoid as the reference for judging the quality of the lateral projection. On a true lateral projection of the distal radius, the pisiform should overlap the distal pole of the scaphoid. If the pisiform is significantly dorsal to the distal pole of the scaphoid, the forearm is positioned in relative pronation; if the pisiform is volar to the distal pole of the scaphoid, the forearm is positioned in relative supination ( Fig. 6-3 A and B).

In a standard lateral projection, the x-ray beam is oriented perpendicular to the long axis of the radial shaft. Since the radial inclination of the ulnar two thirds of the articular surface is about 10 degrees to the long axis of the shaft, this results in an oblique projection of the joint surface on the standard lateral view. The 10-degree lateral projection positions the articular surface in profile, allowing direct visualization of any offset in the sagittal plane and accurate identification of the apical ridges of the dorsal and volar rims. This projection is performed simply by elevating the distal forearm 10 degrees from horizontal or aligning the beam 10 degrees proximally ( Fig. 6-4 A and B).

The radial styloid is visualized on the lateral projection as a V-shaped outline superimposed over the lunate with a base extending from the dorsal and palmar margins of the distal radius. Identification of the radial styloid on the lateral view is important to ensure appropriate placement of transstyloid Kirschner (K) wires or a radial column plate (see Fig. 6-3 ).

The articular surface of the distal radius normally forms a smooth, unbroken arc on the lateral view that is normally concentric with the arc of the proximal lunate; this feature is especially prominent with the 10-degree lateral projection. Normally, the lunate is located centrally within the articular surface of the distal radius and is congruent with the teardrop on the palmar side (see Fig. 6-4 ). Incongruency of the base of the lunate with the articular margin of the radius can indicate displaced intra-articular fracture elements and/or subluxation or dislocation of the radiocarpal joint ( Fig. 6-5 ).

The radius of curvature of the distal radius articular surface should match the radius of curvature of the proximal pole of the lunate. Flattening of the arc of curvature of the distal radius implies dissociation and incongruency of the articular surface across the dorsal and volar margins of the lunate facet. Occasionally, this subtle feature may be the only radiographic evidence of articular disruption ( Fig. 6-6 ).

The central axis of the lunate is normally collinear with the volar cortex of the radial shaft. Migration of the central axis of the lunate to the volar side is suggestive of significant radiocarpal instability.

The teardrop is the U-shaped outline of the volar rim of the lunate facet; it is easily identified on the lateral view and is particularly distinct on the 10-degree lateral projection (see Fig. 6-4 A and B). The relation of the lunate to the articular surface of the teardrop can be critical in defining the direction of carpal instability. In addition, dorsal rotation of the teardrop, often seen in conjunction with axial loading injuries, can produce significant articular incongruity that can be easily overlooked if radial inclination and volar tilt have been restored. Careful assessment of the teardrop should be a standard part of radiographic evaluation for distal radius fractures.

RADIOGRAPHIC PARAMETERS

Posteroanterior View

Radial inclination is well recognized as a useful measurement of the radial slope on the PA projection. Historically, this parameter has been described as the angle between the long axis of the radial shaft and a line connecting the tip of the radial styloid with the ulnar corner. Since the ulnar corner may be a different anatomic structure with volar and dorsal angulation of the distal articular surface, this measurement should use a reference point midway between the volar and dorsal ulnar corners to eliminate variation caused by dorsal angulation. This point is defined as the central reference point (CRP) ( Figs. 6-7 and 6-8 ).

A similar problem is encountered in measuring ulnar variance, defined as the difference in axial length between the ulnar corner of the distal radius and the most distal extent of the ulnar head on the PA view. For reasons described previously, the measurement of ulnar variance is more accurate and consistent when defined as the difference in axial length between the ulnar head and the central reference point. Similarly, the measurement of radial height is more accurate when defined as the difference in axial length between the tip of the radial styloid and the central reference point ( Fig. 6-9 ).