Functional Anatomy of the Hip Joint

$\mathrm{M}=\mathrm{W}\times \mathrm{B}/\mathrm{A}$

$\mathrm{Since}\kern0.24em \mathrm{B}=3\times \mathrm{A},$

$\mathrm{We}\kern0.24em \mathrm{find}\kern0.24em 13\kern0.24em \mathrm{M}=\mathrm{W}\times 3\times \mathrm{A}/\mathrm{A}=3\mathrm{W}$

$\mathrm{Since}\kern0.24em \mathrm{R}=\mathrm{M}+\mathrm{W},\kern0.24em \mathrm{then}\kern0.24em \mathrm{M}=3\mathrm{W}\kern0.24em \mathrm{and}\kern0.24em \mathrm{R}=4\mathrm{W}$

where R = 4 × 5/6 body weight =20/6 body weight. As noted above, the sum of weight affecting a single hip is more than three folds the body weight. Besides, motions such as climbing, running, and jumping might apply on the hip a load that is approximately as much as ten folds the body weight [6–9, 12].

Center of gravity of the body runs posterior to the joint axis and causes sagittal inflexion posterior to the stem in a hip with prosthesis. Forces having such effects will increase in a hip with prosthesis especially when the hip is flexed, when climbing up and down the stairs, or sitting on chair. Forces affecting the stem in coronal and sagittal planes will cause torsional effect. During a walk cycle, forces running toward the femoral head in a hip with prosthesis will arrive creating a 15–25° angle to anterior on sagittal plane. In cases of motions such as climbing up a stairs and straight leg raise, the forces will further come forward. The load to apply on prosthesis during straight leg raise will be more than the body weight. This is quite significant in patient management [4, 5, 12]. These forces will cause retroversion of femoral component or forcing it to posterior. Due to that, stem fractures usually start on the anterolateral surface of the stem. Increasing the width of the proximal part of the stem will better fill the femoral metaphysis and increase the torsional stability of the femoral component [12].

Hip Joint Muscles

We can group the muscles affecting the hip joint as follows according to their functions:

Hip Joint Extensors

These are M. Gluteus Maximus, long head of M. Biceps Femoris, M. semitendinosus, M. semimembranosus, back fibers of M. Adductor Magnus and M. piriformis.

M. Gluteus Maximus is the strongest extensor of the hip. Its deep layer plays a role in the extension of the pelvis when flexed on two sides while the femur is static, and in extension, internal rotation and medial flexion of pelvis when flexed on one side. The surface layer of the gluteus maximus plays a role in extension, external rotation, and abduction of the femur [11].

Hip Joint Flexors

Psoas, iliacus, rectus femoris, sartorius, pectineus, adductor longus, adductor brevis, and gracilis muscles affect the iliacus and pelvis when the femur is static, and affect psoas major and lumbar spine. As they have the same tendon and cause the femur to do the same motion, iliacus and psoas are named as a single muscle, iliopsoas [11].

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