Choosing your implant - Volar locking plates – Horses for courses

1 The concept of double-tiered subchondral support (DSS) system

The volar surface of the distal radius is flat and more suitable for plate fixation. However, when a fixed angle locking plate is placed slightly proximally, the area of the volar cortex, which is distal to the plate, remains unsupported, with subsequent risk of collapse (Fig. 3a).

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Fig. 3 The concept of the double-tiered subchondral support system (DSS).

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If the plate is moved distally to avoid this, a portion of the dorsal cortex remains unsupported because of the fixed-angle screw trajectory. This puts the unsupported dorsal fragment at risk of subsequent displacement. (Fig. 3b).

This led to the development of the DSS concept, which was featured in a plate designed by Orbay et al. to overcome this problem. This concept involved a locking plate having a double row of fixed-angle, distal screws. The proximal row of screws supported the dorsal subchondral bone, while the distal row of screws supported the volar subchondral bone along with the plate (Fig. 3c). Kawasaki later popularised this design. Later modifications featured a carefully designed double row of distal screw holes, which allowed the radial styloid and the dorsal lunate fossa fragments to be held with fixed-angle pegs and screws. This allowed almost all fracture configurations to be fixed using this implant, thus reducing the need for dorsal plating.

The problem with fixed-angle screw designs was that there was no option for changing the screw trajectory. This was a disadvantage in more complex peri-articular fractures. Over a period of time, design refinements led to the development of the variable angle feature. This allowed the screw trajectory to be varied within a 1-degree to 15-degree cone in any direction. The variable angle feature is very popular and used in the current generation of locking plates. It gives a considerable advantage, particularly in peri-articular fractures. The variable angle feature allows screw trajectory to be varied to engage the middle of each articular fragment to ensure good bone purchase. It even permits an articular fragment to be secured with more than one screw by varying the screw trajectory, something not possible with fixed-angle plates.¹⁰

2 Pitfalls of locking plates and screws

Locking plates and screws come with their own set of problems. Although they offer better hold in osteoporotic bone, they do not provide any tactile feedback about screw purchase during insertion.¹¹ Locking screws do not offer any inter-fragmentary compression. If inter-fragmentary compression is required after fracture reduction, the surgeon should use a clamp or an external compression device to achieve this. Once a screw head is locked into the plate, it only functions as a positional screw. Incorrect screw length may cause longer screws to cause tendon impingement and attrition or protrude into the joint.

"Cold-welding" of the screw to the plate may occur due to overtightening of the locking screws. This makes plate removal difficult at a later stage, should implant removal be necessary. The risk of cold welding can be reduced by using manufacturer-specific torque-limiting screwdrivers to lock the screw to the plate.

3 Locking screw mechanisms

The mechanism by which the locking screws lock into the plate varies among different plates and manufacturers.¹²

An initial distal locking plate design employed non-locking screws to provide fixation as in a conventional plate. Locking of multiple screws was simultaneously achieved using a separate screw-in cover plate (Fig. 4). This, however, adds to the plate profile.

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Fig. 4 Plate locking mechanism using a cover plate.

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Some locking plates utilize a mechanism featuring a screw-hole design with a three-point wedge built into the screw hole and a screw head with multiple threads. This system thus allows for six degrees of freedom. It also allows a variation in the screw-plate angle of up to 15° during screw insertion (Fig. 5).

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Fig. 5 Locking screw hole with a three-point wedge

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Some plates employ a spherical threaded screw head screwed into the plate hole via four threaded flanges. This rack and pinion-type locking mechanism offers secure fixation (Fig. 6).

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Fig. 6 Locking screw hole with rack and pinion locking mechanism.

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Some plate designs feature a novel method of variable locking. This involves using a combination of Grade II – titanium plates and Grade V - titanium screws & pegs. Hard titanium is used to manufacture the plate. Soft titanium is used to manufacture the screws and pegs. The locking screws & pegs are designed with threads on the underside of the screw head to engage the circular "lip" within any hole on the plate, except for the oblong hole in the shaft (Fig. 7a). This permits single-step poly-axial locking within a range of 1–15°. This technology (which relies on the differential stiffness of the two titanium alloys) is used in several makes of distal radius-locking plates (Fig. 7b). This Smartlock technology has been patented by Dietmar Wolter of Hamburg, Germany.

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Fig. 7 Locking plate utilising Smartlock technology.

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4 Fragment-specific implants

In 1998, Medoff et al. proposed a fragment-specific classification of intra-articular distal radius fractures. They observed that in most fractures, there were five major components comprising 1. radial column, 2. dorsal wall, 3. dorso-ulnar corner, 4. volar rim, and 5. impacted intra-articular fragment.¹³

This concept led to the development of fracture-specific implants, including buttress plates and pin plates for styloid fragments and volar and dorsal marginal rim fractures.^13,14 (Fig. 8) Typically, these are very low-profile implants that can be applied through small incisions. They do not cause interference with tendon gliding. They rely on proximal fixation and employ a spring-like buttress effect to neutralize the deforming forces and restore the joint anatomy. They are stable enough to allow early mobilization. A fragment-specific implant is used to fix a volar lunate fossa fracture fragment.

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Fig. 8 Fragment-specific plates.

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The importance of the volar lunate facet fragment has been increasingly recognized in the past decade. The volar lunate facet fracture may occur either in isolation or, more commonly, as a component of volar shear fractures with severe comminution. The volar rim of the distal radius gives attachment to the strong volar ligaments, which play an essential role in maintaining wrist stability. Despite being a small fragment, the volar lunate fragment fracture has the potential for loss of fixation, carpal subluxation, and post-traumatic arthritis if not adequately fixed.^15,16

The teardrop angle, described initially by Robert Medoff, is used to detect articular incongruity of the lunate fossa on plain X-rays. The radiological teardrop refers to the U-shaped outline of the volar rim of the lunate facet of the distal radius. It is best seen on the 10° lateral view of the wrist. The teardrop angle is formed between the teardrop's central axis and the radial shaft's long axis (Fig. 9). An abnormal teardrop angle raises the suspicion of a volar lunate fossa fracture and prompts further investigations, like a CT scan.

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Fig. 9 Radiological teardrop angle

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A fragment-specific implant for isolated volar lunate fragment fracture is now available. This implant features a low profile, contoured plate with buttress pins at the tip (Fig. 10a). The limbs of the buttress pins can be cut to the desired length, keeping one limb slightly longer than the other to facilitate the introduction. The pilot holes for the limbs of the buttress pin plate are pre-drilled with a K wire through a jig before insertion of the buttress pin plate, which is held to the proximal fragment with a cortical screw (Fig. 10b).

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Fig. 10 Volar lunate fragment-specific pin plate.

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Another design of a volar lunate fracture fragment-specific implant features two low-profile hooks at the distal margin of the implant to hold the fragment securely and are very useful in stabilizing isolated, ulnar corner volar lunate and sigmoid notch fractures (Fig. 11). These fractures are best approached through a palmar ulnar approach instead of the usual volar Henry approach used for volar plating.

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Fig. 11 Volar lunate fragment-specific plate with distal hooks.

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5 Column-specific implants

In 1996, Rikli and Regazzoni proposed the column concept of the distal radius by longitudinally dividing the distal radius into radial, intermediate, and ulnar columns, each having a different role. The radial column (along with the attached ligaments) resists shear forces and is responsible for radio-carpal stability. The intermediate column is essential in transmitting compressive loads, and the ulnar column plays a vital role in maintaining DRUJ stability. This concept has led to an improved understanding of distal radius biomechanics and the development of column-specific implants for the radial, intermediate, and ulnar columns.

Most distal radius locking plates have plate designs that are now developed as a two-column plate with a triangular distal configuration instead of the conventional "T"-shaped design. This two-column design allows multiple screws to be inserted at different angles to hold the radial styloid and the lunate fossa fragments, thus effectively addressing the radial and intermediate columns (Fig. 12).

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Fig. 12 Dual-column locking plate with triangular configuration.

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Most current distal radius volar locking plate designs feature an elliptical hole in the stem, which allows using either a cortex or a locking screw. The provisional application of a cortex screw initially in the elliptical hole enables the surgeon to move the plate proximally or distally, as required, after viewing the plate position on the C-arm lateral view. The plate stem has additional locking screw holes for attaching the plate to the shaft of the radius. Distal row screws are locking screws available in fixed or variable-angle designs (Fig. 13).

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Fig. 13 Locking plate with proximal combi-holes and distal variable angle screw holes.

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Some designs feature a Y-shaped plate design with a central recess. This recess is thought to reduce FPL tendon irritation.¹⁷ (Fig. 14) Other volar plate designs feature a slightly offset locking hole in the stem, which allows the use of divergent locking screws (Fig. 15).

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Fig. 14 Volar plate with a central recess designed to reduce FPL irritation.

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Fig. 15 Volar plate with offset locking hole in the stem.

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In some fracture patterns, the dorsal fragments must be addressed separately via an additional dorsal approach. In an attempt to reduce the extensive dissection required for combined volar and dorsal plating and to avoid the soft tissue problems caused by dorsal implants, some distal radius locking plates feature a two-part cannulated dorsal screw system in conjunction with the volar locking plate that allows better fixation and compression of dorsal fragments, without the need for an additional dorsal plating.¹⁸ (Fig. 16)

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Fig. 16 Volar plate with two-part cannulated dorsal screw system for additional dorsal fragment fixation

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6 PEEK plates

These distal radius volar locking plates are made of carbon-fibre (CF)-reinforced PEEK (polyetheretherketone) and used with titanium screws. This material is radiolucent and offers clear visualization of the fracture site. It is highly biocompatible and avoids problems like stress shielding and cold welding, which may be a problem with conventional volar locking plates (Fig. 17). The system comes with a provisional trial implant for each plate size to enable the correct size of the plate to be chosen.¹⁹

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Fig. 17 Volar PEEK plate.

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7 Watershed line & volar rim plates

Windisch et al. initially proposed the watershed line concept in 2001.²⁰ In 2005, Nelson described the watershed line as the distal-most line of the distal radius, which was distinctly separate from the distal edge of the pronator quadratus.²¹ In a cadaveric study, Gasse et al. confirmed the presence of these two easily recognizable lines on 70 distal radius specimens.²² In another cadaveric study, Imatani et al. concluded that the watershed line was not distinct. It was thought to correspond to a hypothetical line between the distal and proximal lines in the medial half and the pronator fossa's distal margin in the lateral half of the volar radius.²³

In 2011, Soong et al. gave us a grading scale for volar locking plate placement concerning the plate's location to the watershed line, correlating plate placement with the risk of flexor tendon impingement and rupture. If an implant is placed beyond the watershed line (best visualized when viewed on lateral wrist X-rays), FPL or FDP tendon impingement is possible.²⁴

However, some distal radius fractures involve the volar rim of the distal radius. Although the fragments appear small, they give attachment to the important volar ligaments, which are responsible for stability. These fractures must be buttressed adequately to avoid the risk of carpal subluxation. This buttressing requires very distal plate placement, extending distal to the watershed line, even at the risk of flexor tendon impingement.

Most rim-plate implant designs are contoured to allow the plate to sit snugly on the volar rim of the distal radius. They are specifically designed for these fractures.²⁵ They are typically low profile and have bevelled edges to minimise tendon irritation. Some rim plates feature additional tabs extending proximally. These allow additional screws to be directed to hold the styloid or lunate fossa fragment (Fig. 18a).

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Fig. 18 Volar rim plates.

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Other designs have low-profile distal tabs, which can buttress or hold very distal fragments beyond the watershed line (Fig. 18b). Sometimes, sutures can be passed through the volar capsule and secured to the plate through the provisional K wire fixation holes at the very distal margin of these plates. If these plates are used, removing them once the fracture has united is advisable to minimise the chances of flexor tendon impingement problems.

8 Conclusion

Volar locking plates are available in many designs from various manufacturers around the globe. Often, the choice of the implant is determined by surgeon training and familiarity. However, it is desirable for any surgeon undertaking fixation of these fractures to be aware of the different available plate designs and specific design features and indications of each plate type to choose the best implant for a particular fracture pattern.

Ethical consent

Since this article simply discusses various types of volar plates for distal radius along with the implant design and usage rationale, it does not involve any clinical patient data and does not require any institutional ethical committee approval.

Funding statement

This is to state that no form of funding in any form, directly or indirectly, has been received from any source to prepare and write this article.

Guardian patient consent

Since this is an article about implant design and usage rationale, it does not involve any clinical patient data and, therefore, does not require patient/guardian consent.

CRediT authorship contribution statement

Ravi G. Bharadwaj: Conceptualization, Resourcing material-, Visualization, Writing – original draft, Writing – review & editing.