Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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SECTION ONE • General Principles

Figure 1-9.  Loads on the proximal femur during stand- ing ( green ) and from a fall causing a fracture ( red ). The direction and magnitude differ substantially. Peak loads on the trochanter during fall from standing height can be as high as 11 times body weight. 146

occur from a low energy fall rather than spontaneously during walking. 72,115,161,198 It also explains the common valgus fracture pattern seen in femoral neck fractures as the superior neck com- presses and the inferior neck length is preserved. The loads experienced by the proximal femur during a fall from standing height are different in direction and magnitude from those of normal ambulation (Fig. 1-9). A fall causes a lat- eral impact on the greater trochanter, resulting in a proximal femur load directed perpendicular to the long axis of the femo- ral shaft. The load magnitude is determined by the impact force, which can be decreased by fat overlying the point of impact and increased by muscle loads generated by a protective response to a fall. 179 Depending on these factors, loads on the proximal femur can reach 10 to 17 × BW during a ground level fall. 146,179 Fixation Failure in Osteoporotic Bone Early ambulation in patients with osteoporotic fractures requires the use of implants with sufficient fixation strength to support weight bearing. Failure of implants in osteoporotic bone occurs by loss of fixation in bone rather than by break- age of the implant. 88,194 The strength of fixation in both cortical and cancellous bone is important in the fixation of osteoporotic fractures. In cortical bone, osteoporosis causes cortical thin- ning. Screw fixation strength is highly dependent on cortical thickness. A 1-mm loss in cortical thickness can result in a 50% decrease in the strength of screw fixation. 193 The use of bicorti- cal fixed angle locking screws in thin cortical bone improves the ability of a plate to resist loading, 94,194,228 although locked screws may increase the risk of periprosthetic fractures in bending. 31 The strength of fixation in cancellous bone is especially important in the care of osteoporotic fractures. Common oste- oporotic fracture locations include the proximal femur, verte- brae, and distal radius. Fixation of these fractures requires stable anchorage of implants in cancellous bone. Especially in the lower extremity, it is often not possible for the patient to limit loads postoperatively, 132 making the need for stable fixation even more important. The risk of implant fixation failure is directly related to the bone density. The minimum BMD below which fixation is expected to fail has been defined for several osteoporotic frac- ture locations (Table 1-5). The absolute values for the proximal femur and vertebral body are similar, while those for the proxi- mal humerus and pedicles are considerably lower.

Osteoporotic bone exhibits regional differences in trabecular bone density. 158 For example, in the proximal humerus the high- est bone density is in the medial and dorsal region of the humeral head, with the lowest density in the anterior superior head. 144,208 Implants that direct screws into locations with the greatest local bone density improve mechanical fixation and lessen the risk of failure. 144,188,194 Fixation strategies that emphasize placement of screws in the regions of strongest trabecular bone result in supe- rior biomechanical and clinical results. 96,173,188 The concept of a stress riser relates to relative weakness of a segment of bone secondary to a defect in the bone or the sud- den change in stiffness from an instrumented to an uninstru- mented segment of bone. For example, cantilever bending of a femur after application of a transverse load causes an elevation in the local stress in the bone near the stem of the prosthesis, increasing the risk of periprosthetic fracture (Fig. 1-10). 143 The following discussion will examine the mechanical implications of implant-related stress risers. Periprosthetic Fractures Similar to osteoporotic fractures, normal walking loads do not result in periprosthetic fractures around well-fixed implants. Periprosthetic fractures usually occur after a low energy fall, which imparts torsional and bending loads to the femur. The morphology of the resulting fracture is dependent on the type of load applied. Torsional loads typically cause fractures around PERIPROSTHETIC, INTERPROSTHETIC, AND END SCREW FRACTURES

TABLE 1-5. Minimum Bone Mineral Density for Fixation Failure

Minimum BMD for Implant Stability (mg/cm 3 )

Location

250 133

Proximal femur

95 135

Proximal humerus

95–150 222

Vertebral body (pedicle)

220 163

Vertebral body

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