CHAPTER 7 Distal Radius Fractures Treated with the CPX System
There are many ways to treat distal radius fractures. Over the years, many techniques and instruments have evolved in this frequently changing landscape. The basic principles of fracture treatment, however—fracture reduction, restoration of joint congruency, maintenance of reduction throughout healing, early mobilization, and the resumption of activities of daily living—remain the same.1
This ubiquitous fracture has become very important to our aging population. Life expectancy has increased dramatically, and people are more active as they age.2,3 A cavalier attitude in treating these fractures can lead to unhappy patients who are unwilling to accept either the deformity or the functional compromise of such injuries despite their age.
Deforming angular, compressive, and torsional forces affect fractures of the distal radius.4,5 Such forces are generated by musculotendinous units, gravity, and patient activity, and can lead to collapse of the fracture, especially in elderly patients with osteoporotic bone.2,3 This collapse occurs in four radiological dimensions—radial height, radial inclination, palmar tilt, and ulnar variance—resulting in deformity and functional loss to the patient.5–8
To counteract these deforming forces, many forms of immobilization are used. The traditional method of cast immobilization still has a place in the treatment of distal radius fractures.6–10 In unstable intra-articular fractures, the treatment becomes challenging. Casting alone or in combination with the application of percutaneous Kirschner wires (K-wires), pins in plaster, or external fixators has not solved the problems associated with these fractures.7,11–13 Although dorsal plating and, more recently, volar plating techniques provide stable fixation, they require extensive soft tissue dissection.14–19
Biomechanical Concepts of Internal Fixation
A review of the literature provides an understanding of the basic biomechanical principles of internal fixation applied to distal radius fractures. A single wire (pin) through a fracture fragment allows rotation and translation along the axis of the wire.4,20,21 A second wire through the fragment provides stability between the two fragments as long as the second wire is not parallel to the first.4,21,22 If the second wire is parallel to the first, the fragment can still translate along the pin axis; complete stability is not insured.4 In an experimental cadaver model, Graham and Louis found that as the number of pins was increased from one to four, the construct became more stable.20 In this model, the pins that crossed each other at different angles through the ulnar shaft provided the greatest stability.
Rogge and colleagues,4 in a three-dimensional finite element model, showed that two pins traversing the fracture, as in cross-pinning, are more stable than two parallel pins. Naidu and coworkers23 reported similar findings. Crossed K-wires capture the larger fragments and buttress the smaller fragments, preventing gross articular collapse. In the wrist, where loads of 100 to 180 lb are anticipated in the postoperative phase, it is beneficial to distribute the load over multiple crossed K-wires in the Kapandji fashion.24,25 This configuration also allows the surgeon to decrease the diameter of the pins significantly, from 3.5 to 1.6 mm.
Cross pin fixation has been in use for a long time.26 Cross pin fixation alone does not hold an unstable distal radius fracture, however, unless another means of further support is provided, either by a cast or an external fixator.26,27 The application of a cast or a bridging external fixator carries specific inherent problems that are well known within the orthopaedic community.28–34 The CPX system was conceived as a hybrid of cross pin fixation and a nonbridging external fixator. The nonbridging external fixator provides stability to the cross pin construct, holding reduction of the distal radius fracture without compromising wrist mobilization.
The CPX system is a minimally invasive technique using closed reduction and internal fixation with percutaneous cross pin fixation and a nonbridging external fixator. The CPX device consists of an adjustable two-part aluminum sliding bar (11.5 to 14.5 cm) (Fig. 7-1). Two screws adjust the length of the bar, and at each end of the bar there is a head with three adjustable K-wire fixators. Each K-wire fixator has a guide hole, allowing freedom to angle the K-wire 10 degrees from center, and two screws, one to control the angle of the K-wire insertion and the other to lock the K-wire to the fixator (Fig. 7-2). The CPX system is indicated for treatment of displaced reducible extra-articular and nondisplaced and displaced reducible intra-articular distal radius fractures.
CPX System
Biomechanics of CPX System
To achieve successful outcomes, it is important to understand how this system works biomechanically and how it differs from other systems available today. It has been shown that the stability of the fixation with K-wires is greatly enhanced by increasing the number of K-wires.20 As noted, ensuring that these wires are not parallel to each other provides three-dimensional stability.21 To minimize motion of the fragments and prevent articular step-off or deformity, it is important to achieve multiplanar (three-dimensional) stability of all major fragments (Fig. 7-3).7 The CPX system, with a minimum of four wires (two distally and two proximally) crossing each other at different angles, greatly enhances the stability of the construct.
In traditional bridging and nonbridging external fixators, the pins are perpendicular to the long axis of the bone. This configuration unloads the fracture.5 Comparatively, the pins of the CPX system are more longitudinally oriented and do not unload the fracture (Fig. 7-4). Wires with small diameters (1.6 mm), which flex when the construct is loaded and allow load sharing across the fracture fragments, facilitate callus formation27 and reduce the risk of nonunion because of stress shielding. Also, the cross positioning of K-wires fixes the larger fracture fragments while buttressing the smaller fragments, helping to maintain joint congruency (see Figs. 7-3 and 4).
The cross pin configuration works similar to reinforced concrete (i.e., the pin is similar to steel, and the bone is similar to cement), and the external device acts like a pillar giving rigidity to the whole system (resisting compressive, torsional, and angular forces) (see Fig. 7-4). Among external fixators, bilateral and three-dimensional frames give more stability because of their multiplanar configuration compared with unilateral frames.34,35 Conversely, the CPX system is lightweight (41 g, with the pins), and despite its unilateral frame, achieves three-dimensional stability because of its multiplanar K-wire configuration (see Fig. 7-3).
Indications and Contraindications
Using AO classification, the CPX system is indicated for extra-articular A2 and A3 fractures; simple articular B1 fractures; nondisplaced or reducible and minimally displaced three-part fractures; and C1 and comminuted C2.1, C2.2, and C3.1 fractures. Although, I have successfully treated dorsal shear B2.2 and volar shear B3.3 fractures (see Illustrated Cases later), further investigation and clinical studies are needed to recommend these fractures and C2.3, C3.2, and C3.3 fractures further as indications for the CPX system. Osteoporosis and an unstable distal radioulnar joint are not contraindications. A small distal fragment (<1 cm) is not an absolute contraindication, as long as it is reducible and stable. This system is not recommended for patients with massive swelling, an unstable soft tissue envelope, open fractures, dementia, or advanced Parkinson’s disease, or for patients who are not willing to commit to the postoperative protocol.
Surgical Technique
The fracture is reduced by using the classic maneuver, palmar flexion and ulnar deviation.36 If this maneuver fails, finger trap traction can be used. Finger traps are applied to the thumb and index finger and sometimes to the middle finger, with 10 lb of traction or more, if needed. If greater radial inclination is desired, only the thumb is placed in a finger trap. The only drawback to longitudinal traction is that palmar tilt is not restored.37 This problem can be overcome by applying dorsal pressure to the distal fragment, pushing in the volar direction to maintain palmar tilt until the first K-wire is in place.
After reduction, it is important to FluoroScan and assess the fracture in the anteroposterior, lateral, and oblique planes for restoration of radial inclination, radial height, palmar tilt, ulnar variance, and articular joint congruency. The tissue protector is placed against the radial styloid between the first and second dorsal compartment, and FluoroScan is done to determine placement of the first K-wire (Fig. 7-5A). The tissue protector is removed, and a small stab wound is made in its place (Fig. 7-5B
