**7. Biomechanics of the acetabulum and applied mechanics of fracture fixation**

#### **7.1 Normal mechanics of the hip joint**

Of the many joints in the human body, the hip joint has been the one which has attracted the most attention from investigators [26, 27]. The reasons are; first, in normal activity this joint carries the greatest load, load intensity fluctuating between zero and its maximum during each cycle of activity; secondly, probably because of this loading, mechanical failures of the hip joint and of the neighboring bony structure, particularly the upper femoral region, constitute a large proportion of the problems confronting the orthopedic surgeon.

Mechanical forces acting within the normal hip joint are complex and difficult to quantify precisely. During locomotion, large forces occur across the hip joint in which each leg alternately supports the weight of the body. During mid-stance, little acceleration and relatively constant force are applied across the joint, making midstance ideal for a static loading model of investigation. Forces across the joint itself are greatest during midstance and are derived from two primary sources:


*Body weight* is centered just anterior to S2 vertebra and exerts a force on the hip joint, which acts to rotate the pelvis about the femoral head toward the center of gravity. Counteracting this force is the *abductor moment*, this act to rotate the pelvis in the opposite direction. During single-leg stance, these two forces cancel each other out and, therefore, the pelvis remains upright.

Because both of these forces have magnitude and direction, they can be expressed as vectors on a free body diagram. The Abd is greater than BW, owing to a shorter moment arm, so that in the steady state.

$$(\mathbf{B}\mathbf{W} \times \mathbf{a}) = (\mathbf{A}\mathbf{b}\mathbf{d} \times \mathbf{b})^2 \tag{1}$$

The joint reactive force is the compressive force experienced at the femoroacetabular articulation, and it is the result of the need to balance the moment arms of the body weight with the pull of the hip abductors at the greater trochanter to maintain a level pelvis (**Figure 13**).

The primary contributions to the joint reactive force are the muscular forces generated to level the pelvis during standing and gait, with a smaller contribution from body weight. The magnitude of this force varies with activities such as the single leg stance phase of gait and it has been found to be as much as 2–4 times the body weight during level walking and stair ascent and slightly higher during stair descent [27, 28].

Smooth gait relies on a well-synchronized series of concentric and eccentric muscular contractions to facilitate a balanced stride. A complete neuromuscular loop exists that maintains the appropriate position between the femoral head and acetabulum with balanced muscular regulation achieved at both the voluntary and involuntary level.

The weight-bearing portion of the hip has been found to vary with position of the femur in relation to the acetabulum and the amount of load placed through the articulation. During normal loading of a nonarthritic joint during activities such as walking, majority of the articular surface participates in weight bearing. This involves the anterior, superior and posterior parts of the femoral head and forms two columns of force that are transmitted within the acetabular margin, joining at the superior aspect of the acetabular fossa. The geometric orientation of the articular cartilage is also optimized for load transfer, because the thickest portions are at the areas of the acetabulum and femoral head most frequently loaded during gait.

#### **7.2 Biomechanical consequences of acetabular fracture**

A number of studies have focused on the biomechanical consequences of acetabular fracture [28–32]. These studies can be divided into those focusing on.

**19**

arthritis.

**Figure 13.**

*Surgical Anatomy of Acetabulum and Biomechanics DOI: http://dx.doi.org/10.5772/intechopen.92330*

1.Intra-articular contact area and pressure.

3.Instability or loss of congruence after fracture.

The studies that focus on contact area and pressures argue that increased joint stress from incongruity or altered loading characteristics eventually will lead to degenerative posttraumatic arthritis through repetitive cartilage damage. The guiding hypothesis is that increased stresses within the cartilage exceed the capacity of the tissue to adapt, initiating a cascade of degenerative changes that ultimately leads to arthritis in the joint. It showed that increased peak pressures, especially in the superior region of the acetabulum, do lead to degenerative

*A line drawing of hip joint loading. The joint reaction force vector of the hip is the result of the moments around the hip caused by two opposing surface. The body weight (BW) is centered in front of S2 and is distance away from the center of the femoral head. The force of the abductor muscle (Abd), centered from the midiliac wing to the greater trochanter is distance b away from the femoral head. During single-leg stance, the product of BW × distance a will equal the force of the Abd × distance b. Because distance b is much shorter* 

Clinically, attempts to define the weight-bearing portion of the acetabulum have used *the roof arc measurement*, which represents the angle formed between a vertical line drawn to the geometric center of the acetabulum and a tangential line drawn from the geometric center to the point at which the fracture line enters the joint on antero-posterior and Judet view radiographs. When measured on standard anteroposterior and 45° oblique radiographs, the roof arc measurement gives an estima-

tion of the amount of articular surface remaining intact (**Figure 14**).

2.Rigidity of fracture fixation.

*than a, the force of the abductor mechanism is greater.*

*Surgical Anatomy of Acetabulum and Biomechanics DOI: http://dx.doi.org/10.5772/intechopen.92330*

#### **Figure 13.**

*Essentials in Hip and Ankle*

primary sources:

• Body weight (BW)

• Abductor moment (Abd)

maintain a level pelvis (**Figure 13**).

descent [27, 28].

involuntary level.

loaded during gait.

**7.2 Biomechanical consequences of acetabular fracture**

other out and, therefore, the pelvis remains upright.

shorter moment arm, so that in the steady state.

Mechanical forces acting within the normal hip joint are complex and difficult to quantify precisely. During locomotion, large forces occur across the hip joint in which each leg alternately supports the weight of the body. During mid-stance, little acceleration and relatively constant force are applied across the joint, making midstance ideal for a static loading model of investigation. Forces across the joint itself are greatest during midstance and are derived from two

*Body weight* is centered just anterior to S2 vertebra and exerts a force on the hip joint, which acts to rotate the pelvis about the femoral head toward the center of gravity. Counteracting this force is the *abductor moment*, this act to rotate the pelvis in the opposite direction. During single-leg stance, these two forces cancel each

(BW × a) = (Abd × b)<sup>2</sup> (1)

Because both of these forces have magnitude and direction, they can be expressed as vectors on a free body diagram. The Abd is greater than BW, owing to a

The joint reactive force is the compressive force experienced at the femoroacetabular articulation, and it is the result of the need to balance the moment arms of the body weight with the pull of the hip abductors at the greater trochanter to

The primary contributions to the joint reactive force are the muscular forces generated to level the pelvis during standing and gait, with a smaller contribution from body weight. The magnitude of this force varies with activities such as the single leg stance phase of gait and it has been found to be as much as 2–4 times the body weight during level walking and stair ascent and slightly higher during stair

Smooth gait relies on a well-synchronized series of concentric and eccentric muscular contractions to facilitate a balanced stride. A complete neuromuscular loop exists that maintains the appropriate position between the femoral head and acetabulum with balanced muscular regulation achieved at both the voluntary and

The weight-bearing portion of the hip has been found to vary with position of the femur in relation to the acetabulum and the amount of load placed through the articulation. During normal loading of a nonarthritic joint during activities such as walking, majority of the articular surface participates in weight bearing. This involves the anterior, superior and posterior parts of the femoral head and forms two columns of force that are transmitted within the acetabular margin, joining at the superior aspect of the acetabular fossa. The geometric orientation of the articular cartilage is also optimized for load transfer, because the thickest portions are at the areas of the acetabulum and femoral head most frequently

A number of studies have focused on the biomechanical consequences of acetabular fracture [28–32]. These studies can be divided into those focusing on.

**18**

*A line drawing of hip joint loading. The joint reaction force vector of the hip is the result of the moments around the hip caused by two opposing surface. The body weight (BW) is centered in front of S2 and is distance away from the center of the femoral head. The force of the abductor muscle (Abd), centered from the midiliac wing to the greater trochanter is distance b away from the femoral head. During single-leg stance, the product of BW × distance a will equal the force of the Abd × distance b. Because distance b is much shorter than a, the force of the abductor mechanism is greater.*


The studies that focus on contact area and pressures argue that increased joint stress from incongruity or altered loading characteristics eventually will lead to degenerative posttraumatic arthritis through repetitive cartilage damage. The guiding hypothesis is that increased stresses within the cartilage exceed the capacity of the tissue to adapt, initiating a cascade of degenerative changes that ultimately leads to arthritis in the joint. It showed that increased peak pressures, especially in the superior region of the acetabulum, do lead to degenerative arthritis.

Clinically, attempts to define the weight-bearing portion of the acetabulum have used *the roof arc measurement*, which represents the angle formed between a vertical line drawn to the geometric center of the acetabulum and a tangential line drawn from the geometric center to the point at which the fracture line enters the joint on antero-posterior and Judet view radiographs. When measured on standard anteroposterior and 45° oblique radiographs, the roof arc measurement gives an estimation of the amount of articular surface remaining intact (**Figure 14**).

**Figure 14.** *Medial, anterior and posterior roof arc angle.*
