Anatomy Notes and Their Clinical Significance for the Volar Approach

By David L. Nelson, MD

The volar approach has become popular for the fixation of distal radius fractures. Our experience with this technique raised questions about details of anatomy and prompted a cadaveric study. Three investigators, both individually and jointly, dissected eight cadavers and examined ten dried bones. Several anatomic features were identified in each specimen that may be useful to surgeons when placing a volar plate. The results have been confirmed in greater than 100 subsequent surgeries and in many cadaver volar plate labs. Names are proposed for these features to better facilitate communication between surgeons.

Volar Radial Tuberosity

A prominence exists (marked "x" in Figure 1) on the radial margin of the pronator quadratus (PQ) fossa. It is easily palpable clinically volar and proximal to the radial styloid, feels like a single bump, and as such, may be termed a â€œtuberosityâ€. Clinical Relevance: Plates that are placed too radial will overlie this tuberosity, will be pronated, and will not lie flat on the radius. In addition, these plates may become palpable to the patient, requiring subsequent removal. We propose the name "volar radial tuberosity" for this structure.

Volar Radial Ridge

A ridge of varying prominence (marked "VR" in Figure 1) extends proximally from the volar radial tuberosity and marks the radial limit of the pronator quadratus fossa. Clinical Relevance: Although the ridge is not usually in the way of volar plates, it could pronate a plate placed proximally and radially. We propose the name "volar radial ridge" for this structure.

Clinical Relevance: Although not usually in the way of volar plates, the ridge could pronate a plate placed too proximally and radially. To avoid this, some surgeons routinely rongeur the ridge to provide a flat surface. Proper plate placement should avoid the necessity to rongeur the ridge.

Fibrous Transition Zone

There is a distinct and consistent region of adherent fibrous tissue that exists between the distal margin of the pronator quadratus and the mobile volar wrist capsule. The proximal margin of this fibrous tissue does not move with the volar wrist capsule as the wrist is flexed and extended, is firmly attached to the radius over a broad area, and extends further proximally than what would normally be called the origin of the volar wrist capsule. We propose the name "fibrous transition zone" (FTZ) to describe this region, and the name "pronator quadratus line" (PQ line) to describe the linear boundary between the distal margin of the muscle and the proximal extent of the volar fibrous tissue.

Clinical Relevance: Leaving a rim of this fibrous tissue attached to the pronator quadratus will facilitate repair of this muscle. The fascia of the pronator quadratus is so thin that the standard incision and suturing technique does not provide a secure repair, and is more an exercise in imagination than reconstruction. On the other hand, incising the FTZ just distal to the PQ line by two millimeters or so provides a strong edge to the pronator for suture repair and allows a secure closure. This in turn allows the pronator muscle to form a thick and reliable interposition between the distal edge of the plate and overlying flexor tendons. Incidentally, incising the PQ just lightly into the fibrous edge of the first dorsal compartment will also allow a secure closure of the radial border of the pronator quadratus. While the demarcation of the FTZ and the volar wrist capsule is not distinct, we feel that distinguishing the FTZ from the volar capsule is important. We do not believe that dissection of the FTZ causes instability or capsular scarring and stiffness. On the other hand, dissection of the volar capsule proper can cause both instability and scarring of the mobile fibrous tissue of the capsule. In addition, elevation of the FTZ will facilitate visualization of the volar fracture fragments, yet not injure the volar wrist capsular ligaments.

Watershed Line

There is a theoretical line marking the most volar aspect of the volar margin of the radius, which is distal to the PQ line and is covered by the volar capsule (indicated by arrow in Photograph 2). We propose the name "watershed line" for this imaginary line (this name was first propose by Dr. Orbay).

Clinical Relevance:
Any hardware placed volar to this line would function as a fulcrum for the flexor tendons, especially in power grip (wrist extension), potentially risking tenosynovitis or rupture. We feel it is important to ensure that the volar plate does not project above this line. Clinically, it does not appear necessary to do so, as small volar rim fragments can be supported by a K-wire technique.

Lunate Facet Buttress

The lunate facet’s volar rim protrudes volarly more than the volar rim scaphoid facet. Proximally, this volar rim is supported by a buttress of a varying size (in Photograph 1, this structure would be where the initials “PQ” are located). We propose the name “lunate facet buttress” for this structure.

Clinical Relevance:
In some patients, the lunate facet buttress is only a millimeter or so higher than the rest of the volar radius, presenting a relatively flat surface for plate placement. In other patients, the buttress can be several millimeters high. It has been our experience that patients with a high lunate facet buttress usually also have a prominent volar radial tuberosity. When this occurs, the volar surface of the radius is not flat, but has a pronounced rolling contour. Flat volar plates do not fit well in these patients, do not support smaller fragments properly, and are more prominent than properly contoured plates. Prominence of the plate due to the size of the lunate facet buttress can risk tendon irritation and the plate can be more palpable to the patient.

Lunate facet size

Dr. Orbay has found that about 85% of lunate facets are small and about 15% are large, ie, the volar lip of the lunate facet is relatively prominent volarly. These are relative terms, obviously, and we are working to quantify them. The large facets are found in patients who also have a large volar radial tuberosity.

Clinical Relevance: Patients with a relatively prominent volar radial tuberosity and relatively prominent volar lip need to have a relatively curved plate and/or more attention must be paid to how the plate is positioned, to be sure that it properly suppports the radius and is not palpable to the patient.

Figure 1
PQ = Pronator Quadratus Line, or PQ Line, WS = Watershed Line,
X = Volar Radial tuberosity, VR = Volar Radial Ridge

Figure 2
The arrow marks the most volar portion of the radius, called the Watershed Line

PQ line
Figure 3
This clinical photograph shows the PQ line (dotted). This is a volar
view of a left radius, with the fingers to the right and the elbow to the left.
The PQ Line is located at the junction of the distal magin (to the right) of
the pronator quadratus muscle and the proximal margin of the fibrous
tissue just distal to the PQ muscle.