Bony Anatomy around the Watershed Line of the Distal Radius

Abstract

Background  An important surgical landmark in the distal radius is the watershed line. The watershed line is a landmark for the positioning of volar locking plates (VLP) in the distal radius. Inconsistencies remain in the literature as to the presence and dimensions of landmarks in this compact area. We studied the detailed anatomy and dimensions of the distal radius with reference to bony anatomy around the watershed line, with special attention to the area between the pronator quadratus (PQ) and radial styloid.

Materials and Methods  The distal radius regions of 31 cadavers (23 right sided and 8 left sided) were dissected and studied. The heights at the junction of the scaphoid and lunate fossa, at the radial styloid, at the midpoint in between, and the widths of the PQ line, scaphoid, and lunate fossa were measured. The angle subtended by the pronator fossa and the radial styloid was also recorded.

Results  The mean heights at the junction of the scaphoid and lunate fossa, radial styloid, and midpoint in between were 5.1, 15.7, and 8.2 mm, respectively. The widths of the PQ line, scaphoid, and lunate fossa were 27, 19.4, and 10.6 mm, respectively. The mean angulation between the pronator fossa and the radial styloid was 128.9 degrees.

Conclusion  The area between the PQ and watershed line comprises a narrow area of bone which tapers to a point at its medial extent largely below the lunate fossa, thus it can hardly contain any implant proximal to the lunate fossa. The anterior orientation of this area and the concave anatomy of the articular surface mean careful screw direction is imperative to avoid inadvertent joint penetration. An angulation exists between the pronator fossa and the radial styloid, below the scaphoid fossa.

An important surgical landmark in the distal radius is the watershed line, first described in 2005 by Nelson as a theoretical line marking the most volar aspect of the volar radius. This is distal to the pronator quadratus (PQ) line, distal limit of the PQ, and covered by the volar capsule. ¹ This watershed line was later defined by Imatani et al as corresponding to the distal margin of the pronator fossa in the lateral half of the volar radius, and a hypothetical line between distal and proximal lines in the medial half. ² The PQ line marks the highest part of the epiphysis, whereas the watershed line corresponds to the most distal edge of the epiphysis; there is variability in which is higher ³ and furthermore, differences in literature as to the presence and dimensions of landmarks in this compact area.

Volar locking plates (VLP), commonly utilized in the fixation of fractures of the distal radius, are advocated to be sited proximal to the watershed line to reduce the risk of increased contact pressures on the flexor pollicis longus (FPL) tendon. This is especially so in wrist extension, ⁴ which could lead to tenosynovitis or late rupture, ² and plates distal to the watershed line have the potential to impinge on the FPL even if fractures heal anatomically. ⁵

We set out to define the dimensions of the area of the distal radius demarcated by the PQ line and the watershed line and discern details of anatomy of this compact but anatomically complex area which may be useful for siting VLPs and screws.

Materials and Methods

The distal radius regions of 31 cadavers (23 right sided and 8 left sided) were studied. The age range of the adults from which the specimens were obtained was 44 to 92 years (average: 72.7 years). These were from 20 male and 11 female specimens. Seven of the specimens were from adults aged below 60 years (five males and two females), as distal radius fractures can occur across a broad spectrum of ages and across both genders.

All cadavers were dissected using a standard volar approach. All muscles and tendons, including that of the pronator quadratus, were dissected off the surface of the bone. With the soft tissue stripped around the distal radius, we examined the volar aspect of the distal radius macroscopically. Bony prominences were located to identify landmarks for measurements to be taken. While we recognize the anatomy of the distal radius is complex in that it has curved surfaces and a varied topography, we have made deliberate attempts to record measurements which can be surrogate guides to the surgeon while utilizing intraoperative imaging that converts the three-dimensional structure of the distal radius into a two-dimensional image.

The height of the area of interest at the junction of the scaphoid and lunate fossae (H1) was recorded; beyond this point, the surface tapered to a point and would not allow reproducible measurement between specimens. The height of the radial styloid taken from the PQ line to the tip of the styloid (H3) was recorded; a screw directed toward the radial styloid should not exceed this length. At the midpoint of these two identified landmarks, the height of the area in question was recorded as well (H2).

The transverse width across the PQ line (W _BA ) and the widths of the radioscaphoid (W _RS ) and radiolunate (W _RL ) articulations were recorded. These would be a surrogate to the relative widths of the radial and intermediate columns, as described by Rikli and Regazzoni. ⁶

The angle subtended by the planes of the pronator fossa and radial styloid, the latter taken to be representative to the plane of the area of interest between the PQ and watershed lines, was recorded with the specimens placed flat, utilizing a calibrated goniometer. This angulation has not been closely studied in literature before.

All measurements were performed by a single author, with specimens placed flat on the dissection table and utilizing the same calliper in all specimens. The measurements recorded three times each and the average of measurements recorded ( Table 1 ).

Table 1. Measurement results.

Dimension	Mean value	Standard deviation	Range
Height at medial extent H1 (mm)	5.1	0.8	3.3–6.7
Height at radial styloid H3 (mm)	15.7	2.0	12.3–20.3
Height at midpoint H2 (mm)	8.2	1.4	4.7–11
Transverse width across PQ line W BA (mm)	27	3.5	21.7–33
Radioscaphoid width W RS (mm)	19.4	2.4	14–23.7
Radiolunate width W RL (mm)	10.6	1.9	7.3–15.3
Angle subtended between pronator fossa and radial styloid (degree)	128.9	5.9	116.7–140

Abbreviations: H1, height at the junction of the scaphoid and lunate fossae; H3, height at radial styloid; H2, height equidistant between H1 and H3; PQ, pronator quadratus; WBA, transverse width of pronator quadratus line; WRL, width of radiolunate articulation; WRS, width of radioscaphoid articulation.

Results

The mean height H1 at the medial extent (at the junction of the scaphoid and lunate fossae) was 5.1 mm (range: 3.3–6.7 mm). At the radial styloid, the mean height H3 was 15.7 mm (range: 12.3–20.3 mm). At the midpoint between these two defined points, the mean height H2 was 8.2 mm (range: 4.7–11 mm). The area between the PQ and watershed lines tapers to a point sharply past the medial extent that was utilized in our study, with some specimens exhibiting very narrow heights at the ulnar-most extent ( Fig. 1 ).

Fig. 1.

Dimensions of the distal radius. H1, height at the junction of the scaphoid and lunate fossae; H3, height at radial styloid; H2, height equidistant between H1 and H3; W _BA , transverse width of pronator quadratus line; W _RL , width of radiolunate articulation; W _RS , width of radioscaphoid articulation.

The transverse width across the PQ line had a mean of 27 mm (range: 21.7–33 mm). The mean width of the boundaries along the scaphoid fossa W _RS was 19.4 mm (range: 14–23.7 mm), approximately two-thirds of the transverse width. The mean width along the lunate fossa W _RL was 10.6 mm (range: 7.3–15.3 mm), approximately one-third of the transverse width. Both the fossae have a concave anatomy with a volar angulation in the sagittal plane ( Fig. 2 ).

Fig. 2.

The angle subtended between the plane of the pronator fossa and plane of the radial styloid.

The angle subtended between the plane of the pronator fossa and plane of the radial styloid ( Fig. 2 ) was found to have a mean value of 128.9 degrees (standard deviation = 5.9 degrees).The median value was 129.7 degrees, with the range of values from 116.7 to 140 degrees. In our specimens, the area between the PQ and watershed lines are oriented anteriorly, occupying a similar plane to that of the radial styloid, though the contoured anatomy defies accurate measurement ( Fig. 3 ).

Fig. 3.

The area between the PQ and watershed lines is oriented anteriorly along the plane of the radial styloid, with the concave scaphoid and lunate fossae (yellow denotes radial styloid to midpoint; red denotes region from junction of scaphoid and lunate to midpoint). PQ, pronator quadratus.

Discussion

Nelson's original concept of the watershed line was further studied by Orbay and Touhami. ⁷ They defined the watershed line to be the ridge that is the distal limit of the volar distal radius; implants will not impinge on flexor tendons if they do not cross or project volar to this distal ridge. They observed that the most ulnar aspect of this ridge (the volar rim of the lunate fossa) is within 2 mm of the joint line and that the radial aspect of this ridge is 10- to 15-mm proximal to the joint line, albeit without details of the dimensions of the area contained in between. There are differences in literature regarding the presence of the PQ and watershed lines. In their study of 70 cadaveric specimens, Gasse et al identified these two lines on their specimens. ⁸ In contrast, Imatani et al only found that the lines were present in only 4 of the 20 cadaveric specimens that they studied. ² In our study, we found these lines were reproducibly identified, and measurements made at easily identified bony landmarks such as at the junctions of the scaphoid and lunate fossae. The values found in our cadaveric examination attest and complement the findings of that of Orbay and Touhami. In addition to this, we found that the watershed line narrows abruptly toward its ulnar aspect and will unlikely tolerate any implant placement below the lunate fossa. The width across the watershed line was a mean of 27mm in our study, confirming the findings by Kwon et al. ⁹

A study by Daniele et al involving 38 cadaveric specimens found that there was a significant difference between the mean volar tilt of the scaphoid versus the lunate, at 15.2 and 9.2 degrees, respectively, increasing from the ulnar to radial direction. ¹⁰ Theoretically, this puts the scaphoid and lunate fossae at different sagittal planes. They cautioned that lateral radiographs may therefore not provide a clear representation of the volar tilt, further complicating intraoperative visualization of screw trajectory. We measured the angle subtended by the pronator fossa and that of the radial styloid (taken to be a surrogate for the plane of the area between the PQ and watershed lines) which could correspond to the differential sagittal planes of the lunate and scaphoid fossae. This angle, as pointed out by Daniele et al, is not clearly appreciated on radiographs, due to overlapping anatomy, and is more reliably demonstrated on a cadaveric study. We found this angulation to be consistently present in all specimens, but highly variable, ranging from 116.7 to 140 degrees, with an average of 128.9 degrees. This angulation can be altered in relation to the fossae, such as during trauma, and may have implication on scapholunate biomechanics. This angulation also provides a potential point of impingement of flexor tendons and deserves further anatomical studies.

In our study, the W _RS and W _RL articulations are approximately two-thirds and one-third, respectively, with respect to the transverse width of the PQ line. Taking into context of findings by Daniele et al and the variability of the angulation between the pronator fossa and the area of interest, the risk of screw perforation is higher in the radiolunate articulations, which is contiguous with the intermediate column. ⁶ Thus, by using the division between the radial two-thirds and the ulnar one-third as a visual gauge, the distal most of screws on a plate sited against the watershed line, especially in the intermediate column, may be the best inserted at angle perpendicular or less to the plate (away from the articular surface), to minimize the risk of joint perforation. This is especially important as much in literature has focused on the challenges of visualizing this complication on an intraoperative image intensifier, in which the complex anatomy in this area overlaps on the two-dimensional image.

Imatani et al further developed their concept of the watershed line in their later study of 20 cadaveric specimens. ¹¹ At the ulnar aspect of the distal radius, they described a distal higher line and a proximal lower line forming the distal bony ridge of the pronator fossa. On the distal higher line, there is a bony prominence on the ulnar aspect. They opined that the dangerous zone for flexor impingement was on average 11 ± 1 mm radial to this bony prominence of the volar distal end of the radius, and that a plate of sufficient thickness in this region may negate the theoretical safety of this distal line. Our findings support their study. In our cadavers, the average height of the area between the PQ and watershed line at this point was 5.1 mm (standard deviation = 0.9 mm), a narrow area that may not accommodate any known existing plate design and thickness. Furthermore, the bony contour when moving ulnarwards becomes more curved and tapers to a point and siting of a plate here may inadvertently cross the watershed line. The ulnar aspect of the watershed line should thus be used as the distal limit for the siting of VLPs.

The complex curved three-dimensional anatomy of this compact area has implications on plate design, as the vast majority of commercially available VLPs utilize a flat shape. The resultant mismatch may mean the plate can potentially be more prominent, causing flexor tendon irritation. There exists special plate designs intended to spare the region of the bare area forming the floor of the FPL tendon, which in theory would allow a more distal placement, intended for very distal rim fractures or fractures of the lunate fossa, while decreasing the risk of flexor tendon impingement. We did not study the relationship of the site of VLPs and impingement on the FPL. Future studies involving comparative anatomy between computed tomography scans and cadaveric specimens with VLPs or utilizing 3-D printed models may shed further light on these issues.

Conclusion

The area between the PQ and watershed line comprises a narrow area of bone which tapers to a point at its medial extent, thus it can hardly contain any implant proximal to the radiolunate articulation. Coupled with the concave anatomy of the articular surface, careful screw direction is imperative to avoid inadvertent joint penetration. Radially, an angle is subtended between the pronator fossa and the radial styloid area. Placement of a plate distal to the PQ line, coupled together with the angulation between the pronator fossa, and the plane of the area between the PQ and watershed lines may cause inadvertent flexor tendon impingement.

Funding Statement

Funding None.