W3 Biomechanics and Biology of Plate Fixation of Distal Radius Fractures
Anatomical or near-anatomical restoration, fracture stabilization, “atraumatic” surgery, pain control, and early progressive joint mobilization constitute the basic principles of the operative management of fractures that cannot be satisfactorily reduced or maintained within acceptable parameters by nonoperative methods.1,2 Although perfect anatomical restoration is the ideal, good to excellent functional results may be achieved with near-anatomical reduction, owing to some tolerance of mild deformity.
Loss of length may be linear along the axis of the radius, or may be caused or accentuated by loss of dorsal articular tilt or displacement of the fracture. The distal radius may accommodate 2 to 3 mm of shortening and a loss of 15 to 20 degrees of articular angulation in the frontal and sagittal planes with little functional loss.3–10 Dorsal translation of the lunate in the distal radial fossa with secondary midcarpal collapse owing to loss of dorsal angulation may be an indication for fracture restitution.11 Up to 1 to 2 mm of distal radial articular surface incongruity may not appreciably increase the risk of later post-traumatic arthritis.12,13 Patients with fractures not meeting these criteria may be candidates for open reduction and plate stabilization.
Open reduction and internal plate fixation may be used to achieve fracture restoration and stability throughout the entire healing process without protruding wires or pins and to allow early intensive rehabilitation. These advantages may offset the disadvantages, which include additional operative trauma, fragment devascularization, and some additional risk of wrist stiffness; occasional tendon rupture; and, at times, plate removal. Plates must be matched to the fracture configuration and selected according to the surgeon’s judgment and skills. Bone grafting of defects enhances distal radius fracture stability and healing capacity. We discuss the operative approaches and plate designs that have evolved in an effort to achieve and maintain sufficient biomechanical fracture support, while improving plate biocompatibility with fracture healing.
Approaches
Sharp low-energy incisions are designed to approach the fracture directly in a single plane and to minimize bone fragment devascularization and scar tissue formation. Nevertheless, open reduction, periosteal stripping, and internal fixation convert a closed fracture into a complex open fracture. The opportunity for good fracture reduction and adequate stability to allow otherwise unimpaired fracture healing and early wrist motion may warrant this method of treatment.
Traditionally, dorsally displaced distal radial plates have been approached dorsally, and palmarly displaced fractures have been approached from the palmar side, allowing reliable “buttressing” of the metaphyseal fragments. A plate applied adjacent to the comminuted side and opposite to a side with cortical contact is substantially stronger than the opposite configuration.14 Conversely, the high frequency of postoperative symptoms and complications resulting after dorsal plating has led to the development of smaller “low-profile” plates, “fragment-specific” plates and operative approaches, and plates that may be inserted on the palmar side of the fracture for all fracture configurations.
Dorsal Approach
Dorsal plate application may result in overlying wrist extensor tendon adhesions, irritation, attrition, and occasional extensor tendon rupture; skin irritation from the plate; and wrist stiffness. Nevertheless, some distal radius fracture configurations may be more reliably stabilized and bone graft or substitute applied under direct vision using the dorsal approach. Dorsally applied plates may more reliably prevent redisplacement in some instances of dorsally displaced metaphyseal fragments than palmar plating.
Plate spurs caused by trimming some of the holes of the plate stem or bar may be a substantive source of extensor tendon irritation, attrition, or rupture. Subcutaneous transposition of the extensor pollicis longus to the radial side of the wrist and repair of the third dorsal compartment retinaculum over the plate may limit these complications. Although extensor pollicis longus transposition may restrict the extremes of thumb extension, this is rarely a significant problem. Newer plates have beveled edges, tapered ends, and smooth nonadherent surfaces to prevent soft tissue irritation and adhesions. Despite these precautions and a variety of lower profile design changes, plate removal may be necessary more frequently for dorsally applied plates than for plates positioned on the palmar side.
Palmar Approach
Tendon problems may be more inherent to the dorsal approach than attributable to specific plate characteristics.15,16 There is less space available between bone and tendons on the dorsal side of the radius than on the palmar side. The palmar side of the radius, owing to the increased space available between the bone and the flexor tendons, also may accommodate a thicker and stronger plate to accommodate the increased plate loads generated using this configuration.
Plates inserted on the palmar side of the distal radius may be partially covered by the pronator quadratus, providing additional protection to the extrinsic flexor tendons. Extrinsic flexor tendon attrition or rupture is rare with palmar plating. Proper rotational fracture fragment realignment may be facilitated more easily on the palmar side of the radius, owing to the flat palmar surface of the radius compared with its rounded dorsal side.
Conversely, a palmar incision requires elevation of the pronator quadratus to approach the fracture and placement of a portion of the plate between the pronator quadratus and the bone, diminishing the palmar blood supply of distal radial fragments and impairing their revascularization from this source.17 Occasionally, the distal muscular portion of the flexor pollicis longus must be raised to fit the plate stem on the bone. Although early discomfort on the palmar radial side of the distal forearm and thumb stiffness may result, symptoms are usually transient. Although the “extended flexor carpi radialis approach” preserves the important ulnar circulation to the distal radius, it substantially extends the dissection and compromises the blood supply of a second side of the fracture, the lateral radius.18 Bone grafting dorsal defects from the palmar side of the radius may be more problematic even with the extended approach, owing to difficulty in visualizing the defect.
Biomechanics of Fracture Reduction
Traction, ligamentotaxis, “periosteotaxis,” and manipulation are the mainstays of fracture reduction. The brachioradialis is the only muscle attached to the distal radial fracture fragment. Sarmiento and colleagues19 recognized the resistance and deforming force of the brachioradialis on the distal radial metaphyseal or styloid fragment during the wrist flexion and forearm pronation maneuvers of classically applied closed reduction techniques. The brachioradialis also may remain a deforming force after closed fracture reduction. Sarmiento and colleagues19 reported and advocated fracture reduction, positioning, and cast bracing with the forearm in a supinated position to relax brachioradialis tension during and after fracture reduction.
Orbay and coworkers18 popularized brachioradialis tendon insertion release or lengthening to achieve the same effect, facilitating the biomechanics of open reduction of extra-articular distal radius metaphyseal or intra-articular styloid fragments. The brachioradialis tendon is approached by incising the radial septum, the fascia separating the flexor and extensor compartments of the forearm proximally and containing the insertion of the brachioradialis tendon and the tendons of the first extensor compartment distally over the radial styloid. Brachioradialis release is especially helpful in established fractures and nascent malunions.
The “rule of the majority,” also known as the “vassal rule,” may be helpful in assembling the fracture fragments. This rule states that the major fragments should be realigned, and that the smaller or “vassal” fragments follow the major fragments into position. Replacement of each of the articular fragment components before definitive plate fixation may avoid some of the difficulties that may be encountered in reducing ulnar “die-punch” fragments after radial styloid fixation. Fluoroscopy or arthroscopy or both may be useful in achieving fracture and articular alignment. Kirschner wires may be used for provisional fixation before plate insertion.
Plate Biomechanics
Plate strength is proportionate to the cube of its thickness and inversely proportionate to the cube of its length.20 Screws enhance plate strength and holding power at the plate-bone interface. Wider spacing of screws in the stem increases the bending strength of plate-screw-bone fixation. The torsional strength of plate stem fixation is independent of screw spacing and is proportionate to the number of screws holding the stem.21
Evolution of Distal Radius Plates
Basic Distal Radius T-Plate
In 1973, Mathys designed metallic T-shaped small fragment plates (Synthes, Paoli, PA) that were proportionate to the size of the distal radius (Fig. W3-1).1,2 These plates were among the first generation of modern plates manufactured specifically for distal radius fracture fixation. Distal radius plates support or buttress distal radius fractures. Their stem is affixed to the diaphyseal fragment, and a bar supports and incorporates the metaphyseal fragment or fragments. These plates may be bent and contoured to coapt to the bone.
FIGURE W3-1 A, AO/ASIF small fragment distal radius T-plate (long stem). Arrow 1 points to conventional 3.5-mm round plate hole. Arrow 2 points to elliptical 3.5-mm conventional plate stem hole; this hole allows minor plate position adjustments after initial screw insertion. B, Undersurface of conventional distal radius T-plate (short stem).
(Courtesy of Synthes, Paoli, PA.)
Low-Profile, Low-Contact Plates
Plate design has continually evolved toward smaller, “lower profile” implants in an effort to maintain biomechanical fracture stability and improve biocompatibility.22–26 “Relieved” areas of the plate stem, bar, and undersurfaces decrease the amount of plate surface in contact with the bone and facilitate revascularization of the bone segment under the plate.27 The “pi” plate and its T-plate analogue (Synthes, Paoli, PA) were among the early plates of lower profile design (Figs. W3-2 and W3-3). The use of the “pi plate” in distal radial fractures has not shown any appreciable decrease, however, of extensor tendon irritation, attrition, and rupture compared with its predecessors.22
FIGURE W3-2 A, Lower profile dorsal distal radius “pi” plate. Arrow 1 points to round 2.4-mm threaded plate hole for conventional screw or locking peg. Arrow 2 points to round 2.7-mm conventional plate-stem hole. Arrow 3 points to notch to accommodate Lister’s tubercle. B, Undersurface of “pi” plate. Arrow 4 points to lateral area of plate-stem relief.
(Courtesy of Synthes, Paoli, PA.)
FIGURE W3-3 A, Low-profile T-plate. Arrow 1 points to round 2.4-mm threaded plate hole for conventional screw or locking peg. Arrow 2 points to elliptical 2.7-mm conventional plate-stem hole; this hole allows minor plate position adjustments after initial screw insertion. Arrow 3 points to conventional 2.7-mm round plate-stem hole. B, Undersurface of low-profile distal radius T-plate. Arrow 4 points to lateral area of plate-stem relief.
(Courtesy of Synthes, Paoli, PA.)
Fragment-Specific Fixation
Geissler and Fernandez12 first reported “fragment-specific fixation” in 1991 using a mini-fragment T-plate for palmar “die punch” intra-articular fragments (Fig. W3-4). Leslie and Medoff25 and Barrie and Wolfe26 expanded the concept of “fragment-specific fixation,” introducing “pin plates” (TriMed, Valencia, CA) designed to fit the lateral contour of radial styloid fragments and for volar and dorsal “die-punch” and marginal lip fragments (Fig. W3-5). These smaller plates may sometimes be inserted through smaller incisions, but when more than one plate is used, one or two additional incisions may be required.
