Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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CHAPTER 1 • Biomechanics of Fractures and Fracture Fixation

during loading, 108 increasing the fracture risk. In the case of locked lateral plates, a distance of less than 8 cm between the top of the plate and the femoral stem significantly decreases the force required to cause a fracture. 138 In both locked and nonlocked constructs, extending the plate beyond the tip of the implant sig- nificantly reduces the bone strain and fracture risk. 108,138 Interprosthetic Fractures The use of a stemmed total knee implant below a total hip implant produces an uninstrumented segment of bone in the midshaft of the femur that is at risk for an interprosthetic fracture. The risk of fracture between two well-fixed stems depends on multiple biomechanical factors, including the distance between the implants and the quality of cortical bone. 119,142,199,217 Even in weak bone, normal walking does not generate forces capable of causing an interprosthetic fracture. 199 There is some evidence that an interprosthetic distance less than 110 mm may increase the risk of fracture in weak bone under bending loads. 199 Other studies found no correlation with interprosthetic distance but a significant correlation with cortical thickness. 119,217 In the set- ting of a loose arthroplasty implant or an intramedullary nail, the risk of interprosthetic fracture increases as the distance between the implants decreases. 119,142 In the upper extremity, the risk of interprosthetic fractures was similarly found not be related to the interprosthetic distance between the stems of well-fixed total shoulder and total elbow prostheses. 170 End Screw Fractures Periprosthetic fractures also occur at the end of plate constructs. Rates of fractures near the end screw are 1% to 3% for non- locking screws 16,49,209 and 2.6% for locking screws. 200 Plates are extramedullary load-bearing implants and present a differ- ent mechanical environment than a well-fixed intramedullary implant. A plate on the surface of the bone is not aligned with the neutral axis. Bending forces induce a stress riser at the screw hole at the end of the plate, increasing the risk of peri-implant fracture. Classic descriptions of conventional plating techniques recommended the placement of a unicortical screw at the end of the plate to allow a more gradual stress distribution. This recommendation was abandoned after several biomechanical studies showed that the use of a unicortical end screw resulted in lower strength than a conventional bicortical screw. 16,69 The advent of locking screws changed the biomechanical environment around the end screw. In locking screw constructs, the load transferred from the cortex to the fixed-angle locking screws results in highly concentrated stress around the end screw. Although locked plates are stronger than compression plates in axial loading, stress concentration increases the risk for fracture around the end screw in torsion and bending. 31 The use of a nonlocking screw at the end of an otherwise all-locked construct was shown to significantly decrease the risk of frac- ture without affecting axial strength. 31 Using an angled screw in both locking and nonlocking applications was shown to decrease pullout strength and end screw fracture risk in strong bone models. 164,205 Inserting the end screw in an oblique manner places the near and far cortex screw holes at different levels, presumably decreasing the stress

Figure 1-10.  Loads required to cause a femur fracture. The presence of a hip implant significantly lowers the load to failure. An intramedullary nail below a hip stem further increases the risk of fracture. 143

the greater trochanter, analogous to a Vancouver A fracture. Lateral bending loads in the presence of a stable hip implant cause a stress riser at the tip of the stem that decreases the strength by 32% relative to an uninstrumented femur. 182 The resulting fracture at the tip of the prosthesis resembles a Van- couver B fracture. Under anterior bending loads, the presence of a hip implant does not affect the forces required to cause a fracture. 142,182 Anterior bending results in a Vancouver C supra- condylar femur fracture. The Vancouver classification of peri- prosthetic fractures is discussed in Chapter 59. In the setting of a loose implant, the risk of periprosthetic fracture increases considerably. 25,145 Most fractures around loose implants occur with low energy falls rather than high energy trauma. Fractures may occur secondary to any loading direction, but spiral fractures secondary to torsional loading are common in the setting of a loose implant. 183,225 A biomechanical study found that a femur with a loose cemented implant is 58% weaker in tor- sion than a femur with a well-fixed implant. 107 The resulting spi- ral fractures are more proximal than those in well-fixed implants. Periprosthetic fractures in the femur distant from the implant can be stabilized with an intramedullary nail or a plate. The use of an intramedullary nail to stabilize a distal femur fracture below a total hip prosthesis leaves an uninstrumented segment of bone that increases the risk of a peri-implant fracture. 143 If a plate is used to stabilize the fracture, the fracture risk increases when a gap exists between the plate and the prosthetic stem. As the dis- tance between the end of the fixation plate and the hip or knee stem decreases, the strain in the uninstrumented bone increases

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