Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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SECTION ONE • General Principles

Figure 1-8.  Fracture energy is calculated based on the length of the fracture lines and the fracture surface area. Articular surface involvement can be quantified by measuring the length of the frac- ture line on the joint surface ( dashed line ). (From Dibbern K, Kempton LB, Higgins TF, et al. Frac- tures of the tibial plateau involve similar energies as the tibial pilon but greater articular surface involvement. J Orthop Res. 2016;35(3):618–624. Copyright © 2016 American Association of Phys- icists in Medicine. Reprinted by permission of John Wiley & Sons, Inc.)

magnitude of the loads seen in the hip or knee. 219 Investigations using an instrumented shoulder prosthesis found that activities near the limits of the range of motion such as combing one’s hair (0.8 × BW) and motions with long moment arms such as lifting a weight with the outstretched arm (1.2 × BW) generate the highest forces. 22,219 Because of the limited range of motion, the gleno- humeral loads in the immediate postoperative period are much lower. In the first 2 months after surgery, the measured forces were in the range of 0.2 × BW, increasing to a maximum of 0.4 × BW as the shoulder reached the limits of range of motion. 22 Because many muscles cross the elbow, significant compres- sive loads can be generated during flexion and extension activi- ties. When a weight is placed in the hand, these loads increase significantly. Peak ulnohumeral compressive forces range from 0.5 to 3.1 times BW, depending on the activity. 129 Loads at the elbow joint can be categorized by the intensity of activity, with each level generating increasing compressive forces (Table 1-4). 129 Varus and valgus moments are small (0.2–2.75 Nm) 5 but increase

require a large number of movements with greater degrees of freedom. 154 In general, upper extremity loads are the greatest when the arm is moved to the limits of the range of motion or when large rotational moments are generated by holding objects away from the body. The clavicle is responsible for transmitting compressive, tor- sional, and tensile forces from the thorax to the upper extrem- ity. Shoulder abduction results in the highest compressive and torsional loads on the clavicle, while external rotation generates high tensile loads. 118 The arm motions required to bring the hand from the side of the body to the mouth, such as those necessary for eating, were computationally simulated in the setting of a midshaft clavicle fracture. These motions transmit high axial compression and bending forces through the frac- ture, resulting in a downward and posterior displacement of the lateral fracture fragment. 206 Forces in the glenohumeral joint during daily activities are in the range of 1 times body weight (BW), roughly one-third the

TABLE 1-4. Loads Across the Elbow for Common Activities of Daily Living

ADL Category Type of Activity

Assumed Weight in Hand Peak JRF (N)

70–350 (0.1–0.5 × BW)

Light

Eating, dressing, personal hygiene

0.5–2.3 kg (1–5 lb)

419–698 (0.6–1 × BW)

Moderate

Opening a door, lifting a small bag or gallon of milk

2.7–4.5 kg (6–10 lb)

768–1396 (1.1–2.1 × BW)

Strenuous

Manual labor with tool in hand, lift a small child

4.9–9.1 kg (11–20 lb)

1466–2094 (2.2–3.12 × BW)

Extreme

Maximal (isometric) flexion efforts

9.5–13.6 kg (21–30 lb)

JRF, joint reaction force; BW, body weight. Data from Kincaid BL, An K-N. Elbow joint biomechanics for preclinical evaluation of total elbow prostheses. J Biomech . 2013;46(14):2331–2341.

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