Tornetta Rockwood Adults 9781975137298 FINAL VERSION

11

CHAPTER 1 • Biomechanics of Fractures and Fracture Fixation

to 5 Nm with lifting a half gallon of milk. 129 Pronation and supi- nation produce only small rotational moments in the range of 1 to 2 Nm. 129,154 In the postoperative period, most elbow tasks are limited to the light to moderate level, generating compressive forces as high as 1.0 × BW. Lower Extremity During walking activities, a combination of ground reaction forces (GRF) and muscle forces generates loads in the lower extremity. GRF are the upward-directed loads generated as the foot strikes the ground during locomotion. They can be directly measured using a force plate or a force sensor placed in the shoe. The GRF during normal gait is a double-peak force with the first peak occurring in the early stance phase at contralateral toe off and the second occurring at the late stance phase corre- sponding to contralateral heel strike. 67 Forces in the hip and knee during gait are a combination of the GRF and compressive forces resulting from muscle contrac- tion. Using instrumented prostheses to measure the joint force and a force plate to measure the GRF, the effect of muscle forces on the joint loads becomes clear. The measured GRF during gait is 1.0 to 1.1 × BW. The forces measured in the knee and hip show a similar double peak distribution, but are substantially higher. The peak forces in the knee are 2.0 to 2.8 × BW, while the hip forces are 2.2 to 2.6 × BW. 21,67,74,83,90,110 Rising from a chair and climbing stairs generate higher forces than level walk- ing. Forces in the knee during chair rise range between 2.5 and 2.8 × BW and up to 3.5 × BW when ascending and descend- ing stairs. 20,67,90 In the hip, maximum compressive loads of up to 2.8 × BW and significant torsional loads are seen with stair climbing. 19,21,24 Hip loads during physical therapy exercises are also of interest following fracture surgery. In general, loads measured in instrumented hip prostheses during therapy are lower than those during weight bearing. 191 Weight bearing exercises with closed kinetic chains (lifting the pelvis with one or both feet on the bed) generated the greatest forces in the hip, approaching or exceeding the forces generated in unrestricted weight bearing (2.7 × BW) in this study. Isometric contractions of the gluteus and abductors generated forces greater than 50% of unrestricted weight bearing, with the absolute magnitude dependent on the force of contraction. Exercises with long lever arms, including straight leg raises and hip abduction, generated forces between 40% and 70% of unrestricted weight bearing. Dynamic non– weight-bearing exercises, including heel slides and pelvis tilt exercises, generated the lowest forces of 38% of unrestricted weight bearing. Crutch Ambulation Abnormal gait patterns following injury or surgery can increase the forces on the hip considerably relative to normal walking. 23 Assistive devices such as crutches are often used to unload the injured extremity and prevent fixation failure. The effect of walking aids in reducing loads in the lower extremity var- ies between patients, and loads may increase if the technique is not performed properly. 68 Resultant force reductions in the hip using three-point (two crutches and the contralateral leg)

and four-point (two crutches and both legs) gaits range from 12% to 30%. Bending moments are reduced by 20% to 30%, and torsional moments are reduced by 11% to 33%. 68 These modest decreases in joint forces have led authors to suggest that the main function of walking aids is to normalize the gait pattern and the prevention of extreme forces that occur during stumbling caused by the loss of balance rather than specifically unloading a joint. 23,68 It may not be possible for patients to adhere to recommended partial or non–weight-bearing instructions. 50,66,211 Written and verbal instructions to remain non-weight bearing resulted in a 27% noncompliance rate in a group of lower extremity fracture patients. 51 In the case of partial weight bearing, no patients in a cohort of postoperative and healthy subjects could reliably maintain a 200 N weight bearing restriction, despite training with a therapist. 211 Loading at the shoulder joint during crutch ambulation is greater than 1.0 × BW and can be as high as 1.7 × BW, relative to normal loads during daily activities of 1.0 to 1.2 × BW. 219 It is accepted that early weight bearing with crutches after plate or intramedullary nail fixation of the humeral shaft is safe, although forces on the fixation construct may exceed failure loads depend- ing on the patient size and the type of implant used. 49,162 OSTEOPOROSIS Osteoporosis presents two important biomechanical challenges for the orthopedic surgeon. First, the risk of fracture from daily activities and low energy falls increases. Second, attempts at surgical fixation of the resulting fractures have a higher rate of failure because of poor anchorage of implants in the weak bone. An understanding of the biomechanical considerations surrounding osteoporosis is required for proper treatment of these difficult fractures. Fracture Risk Nearly 90% of osteoporotic proximal femur fractures occur as a result of a fall with the final 10% occurring as spontaneous fractures during ambulation. 72,115,161,198 The risk of proximal femur fracture in osteoporosis is predicated by two mechanical factors: the local bone density and the structural integrity of the proximal femur. 40,158 The fracture resistance of osteoporotic bone is a function of the third power of the bone mineral density (BMD), 43 mean- ing that a small change in bone density results in an exponen- tially large increase in fracture risk. Clinically, BMD is measured by dual-energy x-ray absorptiometry (DXA). In the proximal femur, BMD as measured by DXA is a poor predictor of fracture, with only a 60% correlation with fracture risk. DXA is thought to be a poor predictor of proximal femur fracture because it is a two-dimensional average of the bone density 124 and does not account for the mineral composition and structural integrity of the region. Quantitative CT (qCT) can detect local changes in BMD and reveals that bone is preserved in the inferior neck where walking loads are greatest. 124 Using qCT, local bone density is decreased in the posterosuperior neck, which is the area responsible for transmitting loads from falls. 146 This cor- relates with the observation that most proximal femur fractures

Copyright © 2020 Wolters Kluwer Health, Inc. Unauthorized reproduction of this content is prohibited.

LWBK1698-C01_p001-042.indd 11

05/12/18 8:38 PM