3 A Biomechanical Assessment of Scaphoid Fracture Repair
Historically, the treatment of scaphoid fractures has appropriately focused on the tenuous blood supply of this unique bone. There have been and will continue to be arguments concerning the ideal operative approach (dorsal and proximal vs. volar and distal), the one the least injurious to the primarily retrograde blood supply to the scaphoid’s proximal pole. The biomechanical stability of the fracture is of equal importance to preservation of the blood flow to the proximal pole.
▪ Rationale and Basic Science
Scaphoid healing is influenced by the biology of fracture healing and the mechanical forces affecting the scaphoid. Because the surface of the scaphoid is composed almost entirely of cartilage, with minimal soft-tissue attachments, the scaphoid heals by primary bone healing. There is no opportunity for callus formation. Without periosteum and subsequent periosteal reaction, this intra-articular bone relies on the osteocytes to form cutting cones, which cross the fracture gap, and osteoblasts to lay down new bone, which is then strengthened through bone remodeling.1 Because of the lack of callus formation, the union site is structurally weaker early on. To achieve fracture healing in this scenario, rigid internal fixation with a compression construct would seem to be ideal.1 , 2 Primary bone healing occurs if there is adequate blood supply, local hormonal bone stimulants and modulators, and continuous viable bone contact without shearing.1 , 2
Although biology always plays a critical role in fracture healing, the biomechanics of scaphoid fracture displacement cannot be overlooked. The natural history of a scaphoid fractured through the waist, given unconstrained wrist motion, is to assume a flexed posture.3 , 4 The scaphoid is subjected to complex rotatory and bending forces throughout the wide range of normal wrist motion. This intra-articular bone is restrained proximally by the stiff scapholunate ligament and distally by stout volar ligamentous and capsular attachments. When there is an unstable break in the cortical continuity at the waist, the distal pole will be drawn into flexion while the proximal pole will remain tethered to the lunate, which naturally wants to extend. In a study of cadaveric wrists with scaphoid waist osteotomies, Smith et al found interfragmentary flexion of 22 degrees with wrist extension and 36 degrees with wrist flexion.5 These authors concluded that although rotation and compression or distraction did occur, the primary forces acting on the distal scaphoid were bending forces resulting in a displaced scaphoid fracture in a flexed and pronated position. This interfragmentary motion has also been shown to be present in scaphoid nonunions by Leventhal et al who used a three-dimensional computed tomographic (CT) registration technique to monitor carpal motion.6 Scaphoid fractures are also susceptible to shearing forces (translation) at the fracture site.
Scaphoid fractures have been proven to heal with casting, but debate continues as to the length of time and whether cast immobilization should be long- or short-arm. Biomechanical studies testing distal and proximal fracture fragment motion in short-arm thumb spica casts after a scaphoid waist osteotomy have demonstrated contradictory results with respect to interfragmentary motion.7 , 8 A clinical study has shown that stable scaphoid fractures managed in a short-arm thumb spica cast have a 90% union rate, but with slower healing times than with a long-arm thumb spica cast, which eliminates pronation and supination.9
One concern with casting as a definitive treatment is the exposure of the scaphoid fracture site’s biological bone healing front to joint fluid. Because the fracture site is subjected to repetitive micromotion in a cast, joint fluid washes across this surface, diluting local osteogenic stimulants and reducing the potential for bone union. Internal compression of the fracture fragments is one way of minimizing the impact of joint fluid at the fracture site. It is our feeling that casts should be used only to treat stable, two-part, nondisplaced fractures of the scaphoid waist and distal pole in young, healthy individuals who do not smoke.
▪ Biomechanics of Scaphoid Fracture Fixation
In unstable (or potentially unstable) scaphoid fractures, the ideal fixation would simultaneously compress the fragments, block the inherent flexion bending arm, and esist translation at the fracture site. For these reasons, the biomechanics of scaphoid fracture fixation also center on the rigidity and compression provided by the selected implant. In a seminal work on hip fracture repair, Kaufer discussed the mechanical effectiveness of internal fixation. He determined five independent variables: bone quality, fracture fragment geometry, fracture reduction, implant selection, and implant placement.10 I believe these variables apply to all fractures and their repair. Bone quality and fracture fragment geometry are variables that the surgeon cannot control. However, the surgeon certainly can control the quality of the fracture reduction, the implant selection, and the implant placement.
Fracture reduction is perhaps one of the most overlooked aspects in the treatment of a scaphoid fracture. If there is rotational deformity, there is less surface area contact for bone healing and less cancellous bone interdigitation for fracture stability. If a gap is present because of an angular malalignment, the opportunity for primary bone healing is lost. Changes in the shape of the scaphoid (e.g., a humpback deformity) have a direct impact on carpal kinematics and wrist motion.
Options for implant selection in scaphoid fracture repair primarily include Kirschner wires (K-wires, with or without threads), AO-type compression screws (with a screw head), and headless compression screws (e.g., the Herbert screw). The AO screw has been used quite successfully in the past because it generates tremendous compression.11 However, because of the screw head, it has to be placed clear of the articular surface. These screws can be used successfully if they are inserted in a retrograde manner from a volar-distal to a dorsal-proximal direction, as long as they do not impinge on the base of the trapezium.
Smooth K-wires provide stability based on their thickness and their trajectory. Increasing the number of wires improves the stability and can be used effectively, as demonstrated by Stark et al, who achieved a 97% union rate in 151 scaphoid fractures, although the average time in cast was 17 weeks (range, 8 to 33).12 Although multiple stacked K-wires may sufficiently resist bending forces, they do not compress the fracture fragments. In fact, Carter et al found parallel 0.45-mm K-wires have one-third the bending stiffness of a Herbert screw or a 3.5-mm screw.13
The original Herbert screw (Zimmer Inc., Warsaw, IN) was a variable-pitch, headless compression screw that revolutionized the operative treatment of scaphoid fractures. The essence of its success was its ability to generate significant compression at the fracture by means of a ariable pitch between the distal (or leading) threads and the proximal (or trailing) threads without the disadvantage of a prominent screw head. As the implant is screwed into bone, the wider-pitch leading threads cut into the distal fragment more rapidly than the narrower-pitch trailing threads advance into the proximal fragment, thus achieving compression at the fracture site.
There has been a significant advance in the design of the headless scaphoid screws since the first-generation Herbert screw. The screws are now cannulated and can be inserted down an accurately placed guidewire with minimal disruption of the cancellous bone. The current generation of headless compression screws come in a variety of designs with differing biomechanical properties. For example, the Herbert-Whipple screw (Zimmer Inc.) is a cannulated rendition of the traditional Herbert screw. It has a relatively wide midshaft that is smooth. The width of the screw shank assists with cantilever support, but it decreases the depth of thread purchase. The original Herbert screw (which was not cannulated) had a relatively narrow shank, and as a result, the differential between thread diameter and screw shaft diameter was relatively larger—leading to improved purchase in cancellous bone. Newer versions of the cannulated Herbert screw, such as the 3.2-mm Twin Fix compression Screw (Stryker Leibinger Inc., Kalamazoo, MI) or the 3.0-mm headless compression screw from Synthes (Paoli, PA), take advantage of increased thread purchase without increasing the diameter of the screw shaft by using a smaller-diameter guide-wire or a thinner cannulated shaft. An alternative to the smooth shaft or screw shank is a fully threaded, tapered, variable-pitch headless compression screw, which is epitomized by the Acutrak screw (Acumed, Inc., Beaverton, OR). Other designs include the Kompressor screw (Integra Life Sciences, Plainsboro, NJ).
Scaphoid screws have been extensively tested in simulated cadaveric fracture models with regard to compression and pullout strength ( Table 3.1 ). A recent study compared the compression forces of the Twin Fix screw with those of the 3.0-mm AO lag screw (with a head, but a washer that allows the screw to be buried subchondrally), the Herbert screw, and the Acutrak screw with a polyurethane saw-bone. The mean compression force of the Twin Fix screw (Stryker Orthopaedics, Mahwah, NJ) (8 ± 1 N) was significantly higher in relation to that of the AO screw (6.8 ± 1.4 N) and the Herbert screw (2 ± 1 N), but not significantly different from that of the Acutrak screw (7.6 ± 0.6 N).14 These types of research models have not addressed the actual forces that lead to fracture displacement: bending and shearing. As a result, they primarily provide information on the amount of compression obtained at the fracture interface, rather than an estimation of true fracture site stability.
To by et al15 used cyclic loading of the distal scaphoid to simulate bending and shearing forces to evaluate the rigidity of scaphoid screw constructs. They found the Herbert-Whipple and Acutrak screws required twice as many loading cycles before fracture displacement than the standard Herbert screw. Using a similar cadaveric testing model, McCallister et al demonstrated that centrally placed scaphoid screws (within the middle third of the proximal and distal poles of the scaphoid on both the anteroposterior and lateral views) improved construct stiffness significantly more than eccentrically placed scaphoid screws.16 Fixation with central placement of the screw demonstrated a 43% greater stiffness (12.7 N/mm vs. 8.9 N/mm; p < 0.01) and a 113% greater load at 2 mm of displacement (126 N vs. 59.1 N; p < 0.01) than with eccentric placement. These biomechanical studies included load-to-failure testing or cyclic loading on scaphoids in isolation, rather than on scaphoids maintained within the carpus with soft-tissue attachments preserved. Nonetheless, both studies contribute greatly to the current knowledge of scaphoid screw biomechanics.