Tendon Irritation or Rupture from Pastpointing of Distal Screws

The earliest complication reported and the most common is tendon irritation or outright rupture resulting from screws extending beyond the dorsal cortex, or pastpointing.

The dorsal cortex of the distal radius can be thought of as being composed of two surfaces, one radial and one ulnar to Lister’s tubercle. In the MR image shown in Figure 16-1 the two surfaces form a dihedral angle of approximately 140 degrees (see also Fig. 16-3), neither of which is parallel to the plane of a lateral radiograph. Therefore, the lateral radiograph will not profile either of the dorsal distal radial surfaces. Even if the profile of Lister’s tubercle is ignored, a lateral view of the distal radius will not be able to determine if the screw is extending beyond the dorsal cortex. This brings up several additional points.

FIGURE 16-1 MR image of the distal radius, indicating the dihedral angle of the dorsal surfaces of the radius. The two surfaces are angled about 40 degrees, and neither of them is in line with the lateral radiograph. Note the close apposition of the tendons to the dorsal surface of the radius.

First, the classic AO teaching has included the concept of “bicortical purchase,” that is, surgeons attempted to obtain fixation into the proximal and distal cortices of the bone being plated. A new approach is needed, however, with locked volar plates for the distal radius. The “proximal cortex” that needs to be engaged by the screw with a locked plate is the plate, not the volar cortex. The “distal cortex” in the distal radius is not the dorsal cortex, which is actually very thin.

Indeed, the thinness of the dorsal cortex is easily appreciated during surgery when using the dorsal approach. In addition, together with the common finding of a high degree of comminution in distal radius fractures, this makes the dorsal cortex an unsuitable point of fixation for the screws. The true source of stability in locked volar plating of the distal radius is support of the subchondral bone. As originally pointed out by McQueen, the subchondral bone is always thick and strong, even in severely osteoporotic patients. If the distal screws are in contact with the subchondral bone, the articular surface will be supported. If the distal screws are placed even a few millimeters proximal to the subchondral bone, subsidence is possible. Therefore, in the context of locked volar plating, the “distal cortex” is actually the subchondral bone, not the dorsal cortex.

The second point is that extensor tendons are less than 1 mm away from the dorsal cortex, separated from it by only a thin layer of periosteum. Pastpointing by only 1 mm will bring the screw into contact with the tendons. The earliest locked volar plates used standard orthopaedic screws. Their cutting flutes are on the order of 2 to 3 mm and are particularly dangerous. Many of the current generation of locked volar plates use custom distal locking screws, with a taper on the leading end on the order of 1 mm and much smaller cutting flutes. Traditional orthopaedic practice has been to have the screw pastpoint beyond the cutting flutes and/or screw tip taper to obtain optimal purchase on the far cortex. This practice in locked volar plating will bring the screw tip into contact with the extensor tendons. Even screws designed without a formal cutting flute will have a tapering thread, to start the thread into the bone. This leading edge of the thread can be problematic if it comes in contact with the tendons, which are forced against the dorsal cortex whenever the patient performs simultaneous wrist flexion and finger extension.

The third point, returning to an analysis of the distal radial anatomy and the lateral radiograph, is that even oblique views of the distal radius, such as obtained with intraoperative mini-fluoroscopy, cannot easily define the location of the dorsal cortex, because some tendons, particularly the extensor pollicis longus (EPL), lie in a slight indentation in the dorsal cortex (Fig. 16-2).

FIGURE 16-2 A, Dorsal view of a distal radius (prepared specimen). Note the groove for the EPL tendon, just ulnar to Lister’s tubercle. B, End view of a distal radius (prepared specimen). Note the prominence of Lister’s tubercle and the depth of the EPL groove compared with the surrounding bone.

Therefore, even fluoroscopic views may not precisely determine the location of the tip of the screw with respect to the dorsal cortex and the adjacent tendon. Screw tips should stop short of the dorsal cortex by 2 to 4 mm to be sure that they do not past-point and come in contact with the extensor tendons.

A fourth point is that there is no need to attempt to place the screws beyond or even close to the dorsal cortex. As observed earlier, the “distal cortex” is the subchondral bone. An examination of the lateral radiograph (see Fig. 16-8, later) shows that the subchondral bone stops several millimeters shy of the dorsal cortex. A screw can fully support the subchondral bone and yet stop 2 to 4 mm short of the dorsal cortex.

Case Examples

A 65-year-old woman fell and sustained a distal radius fracture. A locked volar plate was placed and intraoperative fluoroscopy indicated proper distal screw placement: just below the subchondral bone, out of the joint, and screw tips just below the dorsal cortex. Plain films were taken (Fig. 16-3A) and demonstrated a slight ulnar placement of the plate but good purchase on the dorsal ulnar corner and adequate purchase of the radial styloid on the posteroanterior view and good placement of the distal screws—just below the subchondral bone, out of the joint, and screw tips just below the dorsal cortex—on the lateral facet view.

FIGURE 16-3 A, Posteroanterior and lateral facet views of a distal radius treated with a volar plate. Screws do not project beyond the dorsal cortex. B, Close-up view of lateral radiograph in same patient with slight rotation. Screws are at or beyond the dorsal cortex. C, CT of the distal radius showing screws are definitely beyond the dorsal cortex.

Follow-up plain films in the office, probably with a slightly different rotation, indicated that the screw tips were just penetrating the dorsal cortex but seemed appropriate at the time on review by several orthopaedic surgeons (see Fig. 16-3B).

However, the patient complained about wrist stiffness and pain more than most patients. There was absolutely no localizing tenderness along the dorsal radius to suggest screw prominence, and there was no irritation with finger range of motion. After several visits without improvement in symptoms or change in the physical examination to suggest screw prominence, computed tomography (CT) was performed (see Fig. 16-3C).

The CT scan clearly shows that the screws are extending beyond the dorsal cortex. The screws are 2.7 mm in diameter, so they are extending about 3 mm dorsal to the radius. Note the dihedral angle of the two dorsal surfaces, as well as the fact that the tendons seem to be lying in a depression such that fluoroscopic views might not reveal their prominence.

The patient was taken to the operating room, where the screws were found to be just a few millimeters beyond the dorsal cortex.

In a different case (Fig. 16-4A), note how much tendon damage was done by a screw in precisely the wrong place—directly below the EPL tendon, with only 1 mm of pastpointing (see Fig. 16-4B). Even minimal pastpointing can be disastrous if screws are positioned in the wrong place!

FIGURE 16-4 A, Dorsal intraoperative view shows significant injury to extensor tendons. B, Magnified dorsal view of same patient with tendons retracted. Tip of screw in the groove for the EPL shows that only 1 mm of pastpointing caused the tendon injuries noted in A. C, Lateral radiograph of distal radius. Note the appearance of screw penetration of the joint. D, Facet lateral to distal radius. Note the screws are actually not in the joint.

Each screw in the distal row of the locked volar plate requires unique consideration. The radial styloid screw is at the greatest angle, with its axis oriented radially. The length of this screw requires a fluoroscopic view of the dorsoradial surface of the radius, not the dorsal surface, and is always the shortest screw. The screw in the most ulnar position has the greatest requirement for accuracy of placement, owing to the nature of the dorsal ulnar corner fragment. The central screw holes require the most accurate determination of length compared with the dorsal cortex, unless one is attempting rigid internal fixation of the EPL tendon. One should bear in mind these unique characteristics as the fluoroscopic evaluation of the distal row is performed while pronating and supinating the forearm.

To summarize, in locked volar plating for distal radius fractures, in terms of “bicortical purchase,” the “proximal cortex” is the plate and the “distal cortex” is the subchondral bone. There is (1) no need to engage the dorsal cortex, (2) the tendons are in close apposition to the dorsal cortex (<1 mm), (3) it is difficult to determine the precise location of the dorsal cortex and the tendons in relationship to the tip of the screws, and (4) slight pastpointing (~1 mm) can lead to tendon injury. It is recommended to have the screw tips 2 to 4 mm short of the dorsal cortex on the lateral radiograph.

Placement of the Distal Screws into the Radiocarpal Joint