Interpretations of the Term “Watershed Line” Used as Reference for Volar Plating

Abstract

Objective  The objective of this systematic review is to provide an overview of all interpretations of the definition of the watershed line used in literature.

Methods  The Preferred Reporting Items for Systematic Reviews and Meta-analysis (PRISMA) guidelines were followed for this review. A comprehensive search was performed for definitions of the watershed line given in literature. A total of 32 studies giving an explicit interpretation of the definition of the watershed line or anatomical reference for plate positioning in writing and/or imaging were included.

Results  In 32 studies, we found eight different landmarks used to refer to the watershed line or correct plate positioning. Five studies used two different soft tissue landmarks. Six different bony landmarks were described in 24 studies. These could further be subdivided into three anatomical interpretations, described in seven studies, in which the term “watershed line” is explained as a distinguishable anatomical line, and two surgical interpretations, described in 15 studies, which are purely reflecting the optimal location of the volar plate. One interpretation of the watershed line described in two studies combined both anatomical and surgical landmarks.

Conclusion  The (mis)interpretation of the definition of the term “watershed line” as described by Orbay is subject to the type of landmarks and purpose used: soft tissue or bony landmarks and an anatomical or a surgical purpose. A clear distinction can be made between interpretations using bony landmarks, as the true watershed line is defined and definitions using soft tissue landmarks, which might represent the reference points surgeons use in clinical practice.

Since their introduction by Orbay, in 2000, ¹ the popularity of volar plates for distal radius fractures has continued to rise. ^{2 3 4 5 6 7 8} Volar plates are less likely to violate soft tissue structures, especially tendons, compared to the traditionally used dorsal plates, ^{7 8 9 10} as there is more physical space between the volar side of the distal radius and the flexor tendons. However, potential iatrogenic flexor tendon rupture after volar plating is an infrequent but avoidable complication due to malpositioning of the plate. ^{7 11 12 13 14} Retrospective studies report flexor tendon ruptures in up to 12% of patients after volar plating of the distal radius. ^{13 14 15 16 17 18} This significant percentage of flexor tendon injuries, combined with the ongoing popularity of volar plating, will result in a marked increase in incidence of flexor tendon problems after volar plating of distal radius fractures.

Orbay proposed the term “watershed line,” defined as “the transverse ridge that limits the concave surface of the volar radius,” as a reference point for safe plate placement to avoid iatrogenic flexor tendon tenosynovitis and rupture. ¹⁹ Later, Nelson and Orbay coined the watershed line as “a theoretical line marking the most volar aspect of the volar margin of the radius ²⁰ ( Fig. 1 ). Subsequently, several articles describe the watershed line as “… closest to the flexor tendons.” ^{21 22 23 24 25}

Fig. 1.

Nelson et al and Orbay coined the term “watershed line” (WS) as “a theoretical line marking the most volar aspect of the volar margin of the radius,” to serve as the distal margin for volar plating to minimize these tendon injuries. ^{19 20 26} PQ, pronator quadratus line or PQ Line; VR, volar radial ridge; WS, watershed line; X, volar radial tuberosity (with kind permission from http://eradius.com/AnatomyOfDistalRadius.htm ²⁰ ).

In 2011, Soong et al showed that Orbay's definition is indeed clinically relevant, as they found that plates prominent at the watershed line increase the risk of tendon injury. In the control group with plates with more proximal positioning, no ruptures were seen. ¹⁴ Soong et al recommended that surgeons avoid implant prominence at the watershed line. ¹⁴ Soong defined the watershed line as “the most prominent part of the volar surface of the distal radius.” ¹¹ Cadaveric studies show it remains a challenge to place the plate correctly in relation to the watershed line, as the watershed line may not be an easy identifiable distinct line intraoperatively. ^{27 28} Moreover, as both the Orbay and Soong definitions already illustrate, the interpretation of the term “watershed line” may vary. Remarkably, both the initial definition and this subsequent interpretation of the watershed line refer to bony anatomical landmarks, while upon the volar approach, especially in fractured distal radii, these bony landmarks are hard or even impossible to identify. In clinical practice, surgeons may rely on the soft tissue anatomy to orientate themselves to correct placement of the plate which might influence their interpretation of the watershed line.

This review aims to provide an overview of the interpretations of the definition of the watershed line of the distal radius as provided by Orbay and its anatomical landmarks made in current literature. This will help to identify anatomical landmarks that may assist in proper placement of a volar plate when fixing a distal radius fracture to avoid flexor tendon injury. We hypothesize the interpretation of the term “watershed line” is not consistent throughout the orthopaedic literature, and may be categorized into “soft tissue” and “bony referenced” references.

Methods

Our systematic review was conducted and reported in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-analysis (PRISMA) guidelines. ²⁹ Keywords included, but were not limited to, “distal radius fractures,” “volar plating,” “flexor tendon rupture,” “prominence,” and “watershed line.” The full search strategy is included in Appendix 1 . In collaboration with a clinical librarian, a search was performed in the electronic medical databases PubMed, MEDLINE, Scopus, and Cochrane (inception to March 2018) to identify relevant studies using the search terms included in Appendix 1 . We supplemented the electronic search with manual searches. The references list of each selected article was checked to identify additional studies missed at the electronic search. EndNote X8 (Boston, MA) software was used to manage the search. All studies published in English with full-text availability were included. There were no limitations for year of publication.

Appendix 1. Search strategy (for example, MEDLINE).

S. No.	Searches
1	Radius/radius fractures
2	Distal*.tw,kf. and 1
3	(Distal* adj4 (radial or radius or pole or edge or fracture*)).tw,kf.
4	2 or 3
5	Volar.tw,kf.
6	(watershed or water shed or watershedline or line or placement* or position* or locat* or prominence or zone*).tw,kf.
7	((flexor tendon* or flexor policis longus or FPL or pronator quadratus or flexor digitorum or digitorum profundus) adj5 (ruptur* or patholog* or irritat* or tendosynovitis)).mp.
8	6 or 7
9	plate* or plating or fixation*).tw,kf.
9	4 and 5 and 8 and 9
10	limit 10 to English language

Study Selection

First, titles and abstracts were screened on relevance. We included all studies that described volar plating surgical techniques (as part of prospective and retrospective case series), volar plate placement, anatomy of the volar distal radius, and/or the relation between volar plating and flexor tendon pathologies for full-text analysis. The full texts of these studies were screened for a description or characterization of the watershed line and/or anatomical landmarks for placement, location, or positioning of a volar plate. All studies giving an explicit definition of the watershed line or anatomical reference for plate positioning in imaging and/or writing were included. Studies depicting the watershed line in one or more of their figures were also included. Discrepancies were resolved by consensus after discussion between the senior authors.

Data Collection

We collected all interpretations and images of the watershed line or anatomical reference for plate positioning. These were categorized by consensus between all authors. Included studies are described in Table 1 and Fig. 2 .

Table 1. Characterizations of the watershed line.

Number	Description
1	A theoretical line marking the most volar aspect of the distal radius 11 20 34 35
2	The distal radial physeal scar 36 37
3	The most distal edge of the epiphysis 28
4	The ridge that limits the concave profile of the pronator fossa or transverse ridge 17 19 21 23 24 26 38 39
5	The ulnar prominence 40
6	Distal margin of the pronator fossa on the radial side and a hypothetical line between the distal higher rand the proximal lower lines in the ulnar aspect 27 41
7	The distal border of the pronator quadratus muscle 30 31 32 33
8	The origin of the volar carpal ligaments 22

Note: Numbers correspond to Fig. 2 .

Fig. 2.

Interpretations of the term watershed line. Colors correspond to characterizations in Table 1 . The most proximal landmark used to characterize the watershed line was “the distal border of the pronator quadratus” ^{30 31 32 33} (line 7) and the most distal was “the most distal edge of the epiphysis” ²⁸ (line 3).

Results

Study Selection

The search yielded 343 unique citations. The manual search resulted in seven additional manuscripts, two of which were included in the study after full-text review. A total of 32 studies were eventually included. All studies described or characterized the watershed line and 19 of these 32 studies depicted the watershed line in a figure as well.

Study Characteristics

Four publications were review articles, two analyzing (tendon) complications after volar plate fixation for distal radius fractures ^{7 15} and two studies reviewing the anatomy and morphology of the volar distal radius. ^{28 41}

The following eight publications were cadaveric studies: two focusing on the anatomy of the distal radius, ^{27 36} two analyzing the fit of volar plates, ^{23 40} and four studies evaluating the relation between volar plate and tendons. ^{18 30 38 42}

Sixteen of the included publications were performed in a clinical setting. Four of these were case series describing tendon injury. ^{21 34 35 43} Five studies were imaging studies, ^{22 31 36 44 45} four analyzing the anatomy of the distal radius, and one analyzing the relation between the distal radius and plate fit. Two retrospective studies and one prospective study analyzed the outcome after volar plating for distal radius fractures. ^{17 32 39} Four clinical studies directly analyzed the relation between volar plates and flexor tendons. ^{11 33 46}

Four publications were expert opinions on the anatomy of the distal radius or volar plating of distal radius fractures. ^{19 20 24 26}

Outcomes were interpretations of the watershed line and references for plate positioning. Thirty-two articles were found describing the watershed line. Several of these interpretations of the term “watershed line” were identical or shared the same anatomical components. Based on consensus amongst the four senior authors experienced in volar plating for distal radius fractures, all characterizations were grouped according to the flowchart in Fig. 3 .

Fig. 3.

Flowchart of eight different definitions of the watershed line/plate position. They are separated based on their soft tissue or bony landmarks, anatomical or surgical relevance. Three papers were excluded due to no clear or contradictory definition.

In general, after comparing all descriptions, “the watershed line” could be divided into the following categories: (1) interpretations that are based on soft tissue landmarks ^{22 30 31 32 33} and (2) interpretations using bony landmarks as the main anatomic reference. ^{11 17 19 20 21 23 24 26 27 28 34 35 36 37 38 39 40 41}

Within these two categories, we found eight different interpretations of the term “watershed line.” Two descriptions made use of soft tissue landmarks which were (7) “the distal border of the pronator quadratus” ^{30 31 32 33} and (8) “the origin of the volar carpal ligaments.” ²² The descriptions using bony landmarks could further be subdivided into anatomical characterizations (i.e., descriptions in which the watershed line is interpreted as a distinguishable anatomical line) ^{11 20 28 34 35 36 37} and surgical characterizations (i.e., descriptions which are purely reflecting the optimal location of the volar plate). ^{17 19 21 23 24 26 38 39 40}

We found three different anatomical descriptions and two different surgical descriptions using bony landmarks. One characterization combined both the anatomical and the surgical description. The three anatomical descriptions using bony landmarks were (1) “a theoretical line marking the most volar aspect of the distal radius,” ^{11 20 34 35} (2) “the distal radial physeal scar,” ^{36 37} and (3) “the most distal edge of the epiphysis” ²⁸ . The two surgical descriptions using bony landmarks were (4) “the ridge that distally limits the concave profile of the pronator fossa” ^{17 19 21 23 24 26 38 39} and (5) “the ulnar prominence.” ⁴⁰ The one characterization that combined both anatomical and surgical definitions was (6) “the distal margin of the pronator fossa on the radial side and a hypothetical line between the distal higher and proximal lower lines on the ulnar aspect.” ^{27 41} All characterizations are written out and shown in Table 1 and Fig. 2 . Three descriptions did not fall under one of these subgroups, either because they were contradictive in itself or failed to present any landmark ^{42 45 46} ( Appendix 2 ). The most proximal interpretation of the term “watershed line” was “the distal border of the pronator quadratus” ^{30 31 32 33} and the most distal interpretation was “the most distal edge of the epiphysis” ²⁸ ( Fig. 2 ).

Appendix 2. Excluded definitions.

Definition	Reference (year)
The pronator fossa ends distally at the watershed line, which is a bony prominence that is not covered by the muscle belly of the pronator quadratus	Matityahu et al (2013) 42
the most ulnar aspect of the watershed line is close (2 mm) to the joint line, and the radial aspect of the watershed line is proximal (10–15 mm) to the joint line as it courses along the base of the styloid process	Komura et al (2012) 46
… watershed line, which Orbay called the distal ridge of the pronator fossa on the palmar surface	Yoneda et al (2014) 45

Interestingly, we found articles giving a very similar written description of the watershed line but showing great differences between their illustrated versions of these respective lines in their included figures. Additionally, several references 42 45 46 gave contradictive descriptions. For example, Komura et al only described the distance Orbay proposed to be situated between the joint line and the watershed line, while missing the essential definition of the line itself. ⁴⁶

Discussion

The aim of this systematic review was to provide an overview of the different interpretations of the term “watershed line” as described by Orbay used in literature, to answer the clinically relevant question: “What are the anatomical landmarks of the distal radius used for proper placement of a volar plate when fixing a distal radius fracture to avoid flexor tendon injury?”

We found 32 articles giving a description of the watershed line, referring to both soft tissue, as well as bony landmarks, which could be further reduced to a total of eight different descriptions when grouping these 32 interpretations according to the used landmarks. Interpretations of the watershed line, and thus landmarks for correct placement of volar plates for distal radius fractures, are not consistent throughout the orthopaedic literature.

Orbay defined the term “the watershed line” as an anatomical reference for placement of a volar plate for distal radius fractures. ¹⁹ As often occurs with new terms, over time alternations to the initial meaning can develop and might lead to incorrect use or understanding of the term. ⁴⁷ In the index reference of the term “watershed line,” the first definition Orbay used in 2005 was “the transverse ridge that distally limits the concave surface of the volar radius.” ¹⁹ Later, on the website of the Eradius in 2008, Orbay in collaboration with Nelson, further defined the watershed line, but this time rephrased it as “a theoretical line marking the most volar aspect of the volar margin of the radius.” ²⁰ The first characterization is defined by a line that is only distinguishable on the bare bone and thus not visible upon surgery. The latter characterization (the end of the concavity) is a surgical definition; it defines the most distal limit where a volar plate can be placed to keep the volar flexor tendons safe. Also, as the design of most volar plates follows the concave profile, this surgical definition fits the use of the volar plate best.

Most articles explaining the term “watershed line” indeed refer to the definition of Orbay. ^{7 11 17 18 19 21 22 31 33 35 36 38 43 45 46} His surgical definition is mostly used, which may be explained by the fact that this reference is an actual article rather than a website, ²⁰ which is easier to find and cite. However, even when naming and referencing Orbay, a wide range of interpretations of the watershed line are given.

Even though the difference between the more anatomical characterization of Orbay ¹⁹ and the more surgical characterization of Nelson and Orbay ²⁰ is only a few millimeters, these millimeters may exactly be the difference between safe plate placement and plate placement in a prominent position where it can interfere with the flexor tendons and cause iatrogenic tendon rupture.

According to Imatani et al, who performed an anatomical study on 20 cadaveric specimens, the watershed line may not be an easy identifiable distinct line intraoperatively, as it corresponds to the distal margin of the pronator fossa in the lateral half of the volar radius and to a hypothetical line between the distal and proximal lines in the medial ulnar half ²⁷ ( Fig. 2 , dark green line). By combining Orbay's anatomical and surgical characterizations in a cadaveric study, the authors do not give a clear answer as to where the watershed line is located. Although, the authors do argue the radial and ulnar bony prominence should be key structures for volar plate placement intraoperatively.

The previous description by Imatani et al ²⁷ was taken one step further by Opperman et al, who described the ulnar prominence “as a good landmark of the distal limit for safe plate positioning.” ⁴⁰ The authors agree that this is the most volar part of the volar surface and, therefore, located on the watershed line according to the index definition, as it protrudes more than the radial prominence and is palpable upon surgery, might therefore be the most useful landmark for volar plate placement. Future research should point out how safe the placement is clinically when related to this prominence.

Several other articles referred to soft tissue landmarks, mostly the distal border of the pronator quadratus, as a reference of plate position. Not only is the pronator quadratus situated well below both the end of the concave surface of the pronator fossa and the most volar surface of the distal radius, soft tissue structures, especially the pronator quadratus, are often either disrupted by the fracture itself, dissected upon the volar surgical approach when the plate is placed, or both. They may, therefore, not be the most reliable landmark to guide a surgeon to safe plate placement.

The fact that we found several articles giving a very similar written descriptions of the watershed line but showing great differences in their illustrated interpretations of the term “watershed line,” and articles giving contradictive descriptions further adds to our impression that the term “watershed line” is subject to a great intra- and interobserver variability and is not easily uniformly defined.

Conclusion

The results of this review have demonstrated that the definition of the watershed line provided by Orbay, “a theoretical line marking the most volar aspect of the volar margin of the radius,” has been misinterpreted consistently in current literature. This resulted in several descriptions of this definition characterized by soft tissue or bony landmarks that are related to the initial finding. Soong et al validated the concepts of the watershed line and defined its consequences in volar plating for distal radius fractures. Opperman et al highlighted the ulnar most volar bony prominence “as a good landmark of the distal limit for safe plate positioning” ⁴⁰ which is indeed located on the true watershed line.

Funding Statement

Funding None.