12 Anterior Rim Fractures

10.1055/b-0039-169252

12 Anterior Rim Fractures

Jorge L. Orbay, Gabriel Pertierra

Abstract

The volar marginal fragment (VMF) originates from the volar rim of the radius’s lunate fossa. Though recorded incidence is low, the VMF is often missed during radiographic evaluation, leading to a lack of surgical planning. However, the VMF is essential to restoration of wrist function. Failed reduction of the small, avascular, and sometimes displaced fragment can yield catastrophic results (e.g., fragment resorption and carpal subluxation) that lead to wrist arthrodesis. Current surgical techniques that supplement volar plating for VMF management may include Kirschner wires (K-wire) or screw fixation, tension band wiring, fragment-specific plating, and modular plate extensions. In any case, the most important factor in VMF management is timeliness—identification and effective management upon index treatment. In order to effectively treat the VMF, the authors of this chapter recommend use of the extended FCR (flexor carpi radialis) approach for optimal operative visualization, and use of a volar hook plate extension.

12.1 Introduction

12.1.1 Background

An anterior rim fragment, also known as a palmar rim, or volar marginal fragment (VMF), is a small fragment occurring at the volar rim of the lunate fossa, which complicates the management of distal radius fractures. They are difficult to fix because of their small size and distal location. They are uncommon, and their true incidence is unknown, but estimated to occur in 1 to 5% of distal radius fractures. ¹ ^, ² Despite its small size, the volar rim of the lunate fossa is a key part of the radiocarpal articulation and is essential in maintaining joint stability. Failure to properly address a VMF can lead to complications, particularly wrist subluxation (▶Fig. 12.1a, b). ² ^, ³ ^, ⁴ After the advent of volar plating, it soon became apparent that volar fixed angle plates often failed to stabilize volar rim fragments. ³ ^, ⁴ The last few years have led to an increased awareness and understanding of this problem and have seen the development of new techniques for treatment.

**Fig. 12.1** Carpal subluxation resulting from failure to manage the volar marginal fragment (VMF) successfully. **(a)** Postoperative fixation with volar plating. **(b)** Failure of VMF fixation results in resorption, collapse, and palmar subluxation.

12.1.2 Classification

VMFs are classified according to the primary distal radius fracture type and the timing of presentation. They can be present in dorsal- or palmar-directed fractures as extensions of articular comminution and in dorsal fracture-dislocations as avulsion injuries (▶Fig. 12.2a, b). ² ^, ⁵ ^, ⁶ ^, ⁷ The presence of a VMF potentiates the instability of a palmar-directed fracture. This combination is particularly difficult to stabilize and may require extended buttressing support and/or direct fragment purchase. In dorsal-directed fractures or in dorsal fracture-dislocations, the pull of the short radiolunate ligament rotates the VMF into extension, requiring effective fragment purchase for reduction. ² ^, ⁵ ^, ⁶ ^, ⁷ Often, volar marginal fractures are missed during initial presentation and treatment, only to be recognized when carpal subluxation is apparent (▶Fig. 12.1a, b). Fractures picked up more than 4 weeks after the index fracture are considered to have a late presentation; those diagnosed and treated early have a better prognosis than fractures diagnosed late or after failure of initial treatment. ³ ^, ⁸ ^, ⁹

**Fig. 12.2** Dorsal fracture-dislocation with a rotated volar marginal fragment (VMF). **(a)** Preoperative. **(b)** After volar plating and hook plate extension fixation. The *red line* represents the volar surface.

12.1.3 Anatomy and Biomechanics

The scaphoid and lunate fossae function differently. The geometric center of the scaphoid fossa is in line with the centerline of the radial shaft in the sagittal plane, and has two well-developed columns of trabecular bone transmitting articular loads to both the dorsal and palmar radial cortices. ¹⁰ The centroid of force application by the scaphoid coincides with the geometrical center of the fossa. ¹¹ This renders an articular fracture fragment consisting mostly of the scaphoid fossa relatively stable and easy to fix with internal fixation. On the other hand, the geometric center of the lunate fossa is offset in the sagittal plane with respect to the radial centerline and has only one well-developed column of trabecular bone that transmits all articular loads to the palmar cortex. The centroid of lunate force application is located toward the palmar aspect of its articular surface and does not coincide with the geometric center. This arrangement renders lunate fossa palmar fragments relatively unstable, particularly in the case of palmar rim fragments. These have limited bony support. Therefore, articular loading results in shearing forces at the fracture surface ⁵ ^, ⁷ ^, ¹⁰ (▶Fig. 12.3a–c).

**Fig. 12.3** **(a)** Centroids of joint reaction force. **(b)** Trabecular columns of subchondral plate support result in **(c)** different load and fracture patterns for the radius scaphoid fossa (left) and lunate fossa (right).

The scaphoid fossa concavity is directed in a more palmar direction (palmar tilt) than the lunate fossa, making the latter a more important restraint to palmar translation. ⁵ Also, because of the palmar location of the centroid of lunate force application, fracture of the palmar rim of the lunate fossa often results in palmar carpal dislocation. ⁵ ^, ⁷

When visualizing the radius from its distal end (▶Fig. 12.4), the lunate fossa extends palmarly relative to the scaphoid fossa. The interfossa sulcus is a longitudinal groove on the palmar surface of the radius located between the scaphoid and lunate fossae. It limits the lateral or radial edge of the lunate fossa, narrowing its base and making it prone to fracture. Fracture lines that create a volar marginal fracture extend from the interfossa sulcus in a medial or ulnar direction, separating the palmar rim from the distal radius.

**Fig. 12.4** *Blue zones* indicate the presence of ligaments. *Yellow dashed lines* indicate typical fracture lines that divide the radial scaphoid fragment (RSF), posterior lunate fragment (PLF), and anterior medial fragment (AMF). The *red dashed line* represents the zone from which volar marginal fragments (VMFs) may originate.

The palmar surface of the distal radius presents a transverse ridge called the watershed line (▶Fig. 12.5a). ⁹ During wrist extension, it acts as a pulley for the digital flexors and extends in a generally transverse direction from the volar radial tubercle to the volar rim of the lunate fossa. ¹² This ridge runs close to the joint line (2–3 mm proximal) over the palmar rim of the lunate fossa, and is incorporated in palmar rim fragments.

**Fig. 12.5** **(a)** The watershed line (WL), transitional fibrous zone (TFZ), and pronator quadratus (PQ) are labeled on a cadaveric specimen. **(b)** Intraoperative view of dorsally displaced fracture with rupture between both structures.

Blood supply to the volar rim of the lunate fossa depends on intraosseous vessels in the radius. The palmar rim fragment is small, covered distally by hyaline cartilage and palmarly by the volar wrist capsule and the origin of the short radiolunate ligament. The fractured volar rim fragment is often avascular, as often there is no retrograde vessel traversing proximally through its remaining soft tissue attachments. When not stabilized, the lack of blood supply to the fragment makes it prone to nonunion and resorption. When adequately stabilized, revascularization by primary bone healing or creeping substitution can allow it to heal.

After volar locked plating, rigidity of main fragment fixation by locking screws or pegs focuses stress at the volar rim fracture. Buttressing support by the plate surface is sufficient to stabilize some volar fragments of moderate or large size but insufficient to stabilize small fragments. The presence of the watershed line limits plate placement as more distal placement endangers flexor tendons. ¹³ Plates must be located at least 2 to 3 mm proximal to the watershed line in order to prevent flexor tendon injury. ⁹ Therefore, other fixation strategies besides buttressing support are used when managing small palmar rim fragments. Surgeons have used K-wires, individual screws, tension band fixation, extended buttressing, and hook plate extensions for this purpose. Hook plate extensions are different from fragment-specific hook plates; these are stand-alone implants intended for larger volar ulnar fragments.

Fixation of the VMF must be stable and rigid enough to allow healing, and be of low profile in order to prevent flexor tendon injury. K-wires and screws are not rigid enough unless they rest against the distal edge of the volar plate to increase stability, and they are prone to back out and irritate tendons. Tension banding of small VMFs to the volar plate is difficult due to fragment size. This technique also lacks rigidity, as purchase is often into capsular tissue or ligament, and the line of pull does not provide support against axial loads (▶Fig. 12.1a, b). Extending the plate’s buttressing surface distal to the watershed line increases the risk of tendon injury, does not resist axial loads, and does not provide the rigidity needed for small or comminuted volar rim fragments. Direct purchase on the fragment is possible with a hook plate extension (Skeletal Dynamics, LLC; Miami, Florida, United States). This device is a modular extension of the volar plate that provides stability by skewering the VMF with two rigid tines, and allows effective resistance to axial loads (▶Fig. 12.6a–d). It is attached to the volar plate by an adjustable mechanism and a setscrew. Unfortunately, it must cross the watershed line to reach and fix the volar rim. The risk of flexor tendon injury is ever-present and these cases must be followed closely to detect flexor tendon irritation early, and remove the hardware after fracture healing, if necessary.