14 Dorsal Approach to Percutaneous Fixation of Scaphoid Fractures with Arthroscopic Assistance
This chapter describes the dorsal percutaneous technique for repair of scaphoid fractures using minifluoroscopy and arthroscopy ( Fig. 14.1 ).1 Stated simply, dorsal percutaneous technique requires the dorsal placement of a guide wire along the central axis of a reduced scaphoid fracture ( Fig. 14.2 ). This permits rigid fixation of the fracture by implantation with a headless screw.
The scaphoid, like other bones, heals in stages that include inflammation, repair, and remodeling. The scaphoid is covered mostly by cartilage and cannot be stabilized by an external collar of fracture callus bone (secondary bone healing). For this bone to heal, viable bone fragments with blood supply must be held in opposition during the early stages of primary bone healing as vascular channels (“osteoclastic-osteoblastic cutting cones”) cross the fracture site initiating the healing process with bridging bone. The early stages of bone healing are influenced by both local and system factors. Nicotine has been identified as a systemic factor that greatly reduces scaphoid bone healing.2 Local factors at the fracture site include mechanical forces of stress and strain.3 All unstable scaphoid fractures require rigid fixation to prevent micromotion. Micromotion at a fracture site has been determined to result in fibrous tissue formation at the fracture site rather than fracture callus.4 Open reduction of scaphoid fractures results in a high union rate5 – 7 but also a high complication rate.8 , 9
The percutaneous repair of acute scaphoid fractures, in the experienced surgeon’s hands, results in a high union rate and with minimal complications.10 – 12 Minimally invasive surgery describes the length of a surgical incision, not the goal of the surgery, which includes the reduction and rigid fixation of bone fragments. The percutaneous techniques preserve uninjured ligaments and blood supply while permitting immediate hand rehabilitation. This has resulted in an early return to work or avocation. Arthroscopy aids in the reduction of displaced scaphoid fractures, identifies occult ligament injuries, and confirms implant position.13 – 16 This chapter describes the principles of the dorsal percutaneous technique for repair of scaphoid fractures. Minimally invasive surgery requires real-time imaging using minifluoroscopy to guide arthroscopic examination, fracture reduction, and fracture fixation.
▪ Fracture Evaluation
Percutaneous or minimally invasive surgery can be used to treat unstable fractures, displaced scaphoid fractures, and complex injuries.13 , 15 , 17 , 18 To achieve successful union, a careful evaluation of the fracture for its location, fracture plane, and bone integrity must be made to determine the best implant and its position to achieve rigid fixation. The healing of acute fractures is dependent on fracture stability. During the repair phase of fracture healing, chemical and mechanical factors determine the type of bone healing. The repair phase stimulates vascular ingrowth of progenitor cells and growth factors, extending haversian canals across the fracture site. Fracture hematoma forms a collagenous extracellular matrix. During this phase, stability determines the amount of strain at the fracture site, and strain determines the type of bone healing. Strains < 2% result in primary bone healing. Strains between 2 and 10% result in secondary bone healing. Finally, with strains > 10% bone cannot heal and fibrous/granulation tissue is formed.19 The goal of fracture stabilization is to prevent or minimize shearing at the fracture site until successful healing has occurred.
Stability of fixation is dependent on the strength of a fracture fragment–implant construct. Strength is determined by five independent variables: bone quality, fragment geometry, fracture reduction, choice of implant, and implant placement.20
Rigid fixation of a fracture is influenced by the implant, the biomaterial, and finally the forces applied. The quality of bone influences the ability of the implant to hold bone fragment in opposition during healing. Bone that is fragmented or comminuted or grafts that are nonstructural provide very little stability, and fixation devices must extend beyond or circumvent these zones of weakness. Bone quality is also directly related to material density. The greater the density the stronger the fixation. The highest bone density in a scaphoid is at each pole, which suggests that the strongest fixation is provided by implants extending from pole to pole.21
Fracture geometry describes the location and orientation of the scaphoid fracture. The geometry of the fracture is greatly influenced by the forces at the fracture site. The scaphoid is a tie rod connecting the proximal and distal carpal row. Micromotion at a fracture site disrupts or delays healing. The scaphoid is a long lever arm and exerts a large force at the proximal pole. This explains both the prolonged healing time and the large number of proximal scaphoid pole nonunions. Extreme fractures of the proximal pole are difficult to rigidly fix, and provisional fixation must be used to block forces acting at the proximal pole. This is accomplished by locking the midcarpal joint and/ or bridging the scaphoid lunate joint with a compression screw to compress the fracture fragment and prevent bone shearing and nonunion ( Fig. 14.3 ). Oblique fractures may require stacking of the intramedullary canal with two screws or a screw and a K-wire to stiffen the fracture site to prevent micromotion ( Fig. 14.4 ). Fracture reduction means alignment of the scaphoid so that no fracture gaps exist, and bone fragments are opposed so that the process of primary bone healing can progress. Fracture gaps greater than 2 mm prevent bridging bone from proceeding. The most common reason for nonunion after fixation is a failure in fracture reduction and placement of a screw that maintains a fracture gap. The implant selected for fixation is chosen to prevent motion or shearing at the fracture site. The wider the implant the stiffer the fixation. A small increase in radius of an implant results in a large increase in stiffness. Cosio demonstrated that by stacking multiple K-wires in a scaphoid healing could proceed.22 A headless scaphoid screw acts much like an intramedullary femoral nail providing rigid fixation of the fracture and preventing motion within the bone canal. Finally, it has been observed that screws placed along the central axis heal faster than screws placed eccentrically. Biomechanical studies have demonstrated that the central axis is the longest straight path in a scaphoid and permits the longest screw to be placed. Studies have demonstrated that longer screws in the central axis provide greater fixation than shorter screws along the same axis. Explanations for this phenomenon include the fact that longer screws reduce forces acting at the fracture site by dispersing them a greater distance along a longer screw. Recall also, the greatest bone density of the scaphoid occurs in the proximal and distal pole.
▪ Indications for Percutaneous Fixation of Scaphoid Fractures
The indications for the operative repair of scaphoid fractures are the treatment of unstable scaphoid fractures and specific social needs, which prohibit plaster immobilization of stable scaphoid fractures. Stable scaphoid fractures are traditionally treated with plaster immobilization but require computed tomographic (CT) scans to confirm healing ( Fig. 14.5 ).9 , 7 Stable fractures may also be treated with percutaneous fixation because the union rate may be higher, the complication rate relatively low, and the difficulties of nonunion numerous.
Herbert felt that all complete fractures were unstable and recommended rigid fixation.23 Herbert classified stable scaphoid fractures as incomplete fractures and distal scaphoid tubercle fractures (type A). Indications for scaphoid fixation include displaced fractures, lateral intrascaphoid angle >35 degrees, bone loss or comminution, perilunate fracture, dorsal intercalated segmental instability (DISI) alignment, proximal pole fractures, fractures with delayed presentation (>4 weeks), and combined injuries of the scaphoid, including the distal radius or other carpal bones.24
Relative indications include stable nondisplaced scaphoid fractures desiring an early return to work or hobby. Long-term follow-up suggests a 10 to 20% failure rate with cast immobilization of presumed stable fractures.9 This group includes incomplete fractures and fractures of the distal scaphoid pole or tubercle that would be expected to unite. The data suggest a possibly higher nonunion rate for stable fractures of the scaphoid waist. Although the failure rate of stable fractures is not as high as that of the at-risk fracture patterns, one must balance the odds of fracture union against a 3- to 6-month cast immobilization treatment. This is important because scaphoid injuries typically occur in the young, active patient population, which is the least tolerant of prolonged immobilization.
Contraindications include irreducible fractures and surgeon inexperience. Disadvantages of the percutaneous technique are related mostly to the learning curve involved in perfecting the technique.12 Complications are directly related to inexperience.
▪ Preoperative Preparation
Standard radiographs are sufficient for preoperative planning unless there are questions about fracture displacement, comminution humpback deformity, or other characterizations of the fracture; then preoperative CT is required. Scaphoid vascularity is best evaluated using magnetic resonance imaging (MRI). Minifluoroscopy is the most valuable tool for preoperative and intraoperative evaluation of scaphoid alignment, bone integrity, and carpal ligament disruption. Arthroscopy is useful for confirming fracture reduction, the extent of ligament injuries, and occult injuries. Proximal pole scaphoid vascularity can also be evaluated by placing a small joint arthroscope into the base of the proximal pole after bone reaming and checking for bleeding.