The acetabulum lies on the lateral aspects of the inferior pelvis and is resultant from the triradiate cartilage growth center of the pelvic innominate bone at the union of the ilium, ischium, and pubic bones. Though the acetabular recession or cup appears to have a hemispherical shape, it is only spherical in nature in the upper one-third or dome, allowing for maximal distribution of force in areas required for weight bearing during upright stance. As previously mentioned, the acetabulum inclined giving an acetabular index (slope of the acetabulum on an AP radiograph) an average of 38 degrees in males and 40 degrees in females (1) allowing for increased abduction yet decreased adduction (Fig. 11.1A). In the sagittal plane, the acetabulum is anteverted an average of 16 degrees for men and 19 degrees for women (2) allowing for increased flexion arc as well as posterior coverage and preventing escape of the head during deep flexion.

On the femoral side of the joint, the osseous structure also contributes to joint stability during both dynamic and static loading. The femoral head is semispherical in shape, ranging from hemispherical to two-thirds of a complete sphere. The head quickly tapers into the femoral neck that joins the remaining femur. The distance between the center of the femoral head and the anatomic axis of the femur represents the “head/neck offset” and is directly related to the moment arm and efficiency of the hip abductors. The orientation of the femoral head/neck complex with regard to the femoral epicondylar axis also plays a critical role in the stability of the hip during static and dynamic hip functions. Similar to the anteversion of the acetabulum, the femoral head/neck is anteverted with regard to the anatomic axis by
approximately 10 to 20 degrees in adults (Fig. 11.1B). This internal rotation of the femoral shaft in conjunction with the anteverted acetabulum allows for increased flexion arc of the hip, particularly with the leg in slight external rotation, a key component to the normal gait pattern as discussed later (3). The femoral head/neck is also angled cephalad in the coronal plane giving an overall head/neck angle of 125–140 degrees of valgus orientation (Fig. 11.1C). This allows for a more vertical transfer of the head, arm, and trunk weight through the acetabulum and down the femoral shaft while still maintaining adequate head/neck offset to allow the abductors to sufficient moment arm to function.

One of the most critical parts of the hip’s osseous structure is the articular incongruence. The acetabulum is only spherical in nature in the weight-bearing dome, whereas the femoral head is spherical in as much as two-thirds of the articular surface. This as well as the angular differences mentioned above (i.e., anteversion and inclination) results in a portion of the femoral head articular surface that is “uncovered” at any time. Maximal coverage of the femoral head, and therefore maximal congruence, is experienced in the flexed, abducted, and externally rotated position (FABER). However, during upright stance and throughout the gait cycle, the femoral head has the greatest amount of coverage during maximal force transmission (i.e., heel strike and single leg stance).

Though the inherent stability of the hip’s spheroidal design imparts a great degree of stability to the joint, additional ligamentous support, consisting of extra- and intra-articular fibrous bands, act in concert as checkrein to all six degrees of hip motion. The extra-articular ligaments consist of band-like thickenings of the capsule that allow for motion within a defined arc. Cadaveric studies have demonstrated some variability and controversy regarding the exact nature and discrete nomenclature of these bands (4), however, classically the capsule is described as having four extracapsular ligaments consisting of longitudinal fibers of the iliofemoral, the ischiofemoral, and the pubofemoral ligaments as well as the annular fibers of the zona orbicularis. These ligaments play a crucial role throughout motion and at different joint positions as evidenced by the change in motion after sectioning (Table 11.1).

Of the extracapsular ligaments, the iliofemoral ligament confers the greatest amount of static stability during erect stance. The ligament emanates as two distinct bands from a small interval between the anterior inferior iliac spine (AIIS) and the anterior superior acetabular rim and joins to the anterior femoral neck base (Fig. 11.2) (5). This Y-shaped confluence of fibers was originally described by Henry Jacob Bigelow, giving rise to the eponymous term Y-Ligament of Bigelow and is the key static stabilizer of the hip during stance primarily resisting hyperextension though they also tighten
through external rotation, adduction, and anterior femoral head translation (Table 11.1) (4,5). Biomechanical studies have further demonstrated that the iliofemoral ligament has the greatest force to failure and overall stiffness of all the capsular ligaments, corresponding to approximately 350 N and 100 N/mm, respectively (6).

Along the anterior and inferior portions, the hip capsule contains a fibrous thickening termed the pubofemoral ligament or pubocapsular ligament (Fig. 11.2A). This band originates from a wide base along the anterior aspect of the superior ramus of the pubis and along the anterior obturator crest projecting toward the anterior, inferior intertrochanteric ridge. The pubofemoral ligament is critical in resisting lateral translation, excessive abduction, and external rotation (Table 11.1) (4,5).

The ischiofemoral ligament is the main posterior ligamentous thickening of the hip capsule and originates along the entirety of the posterior, superior acetabular rim of the ischium and attaching to the medial aspect of the anterosuperior greater trochanter (Fig. 11.2B). As these fibers reach the base of the femoral neck they become confluent with the deep arcuate fibers of the zona orbicularis. A second fibrous band travels from the ischium to the posteromedial base of intertrochanteric ridge (5). In total, the ischiofemoral ligaments provide primary support to resist internal rotation throughout hip motion, while the superior bands act as secondary stabilizers to adduction with the hip in flexion (4,6).

The posterior capsule is further bolstered by a deep arch of fibers that run transversely along the base of the femoral neck
and do not cross the hip joint. These fibers are collectively termed the arcuate ligament and interconnect the superomedial aspect of the greater trochanter and the superior rim of the posterior lesser trochanter (6). Though the greatest thickening of fibers lies posteriorly, the arcuate ligament is a part of a larger annular structure termed the zona orbicularis that encircles the entire femoral neck base (7,8). This added collection creates a retaining ring that both acts to resist extremes of flexion and extension as well as distraction forces (6,7).

In addition to the extracapsular ligaments, arguably one of the most critical static components of hip stability is the labroacetabular complex. This structure consists of an intracapsular fibrocartilaginous lip that encircles all but the inferomedial aspect of the acetabular rim acting to functionally deepen the acetabular cup. The labrum consequently has a gasket effect on the overall femoroacetabular articulation, effectively sealing the joint throughout the majority of the motion arc. This has a similar effect as the labrum in the glenohumeral joint and results in a negative pressure capsule that resists deformity both during loading and unloading (9). Moreover, the rate of load and the total displacement of force applied to the hip joint itself is directly related to the presence of an intact labral seal (9). Further discussion about the role of the labrum in joint forces will be discussed in future sections.

Dynamic stabilizers of the hip joint can be divided into flexors, extensors, internal and external rotators, abductors, and adductors. Similar to the muscles of the rotator cuff, antagonist function during the extremes of motion helps to maintain the femoral head within the acetabulum. However, in the hip, the interrelationship between muscle groups is much less well understood and less clearly coupled as compared with the muscles of the rotator cup. Conceptually, it is understood that opposing muscle groups help to counter each other’s motion to create hip rotation with minimal translation as previously mentioned. For example, as the hip abducts through gluteus minimus and medius function, the adductor complex eccentrically contracts to help translate the femoral head back into the center of the acetabulum, thereby maintaining concentric and primarily rotatory motion. However, as the insertion points on hip abductors and the abductor complex are not directly opposite on the femur, the resulting difference in lever arms and moments results in a more complex intermuscular relationship than, say, that of the subscapularis and infraspinatus. Moreover, additional dynamic stabilizers also participate in direct stabilization of the hip joint by acting as physical blocks to femoral head escape. In particular, muscles that pass directly over or even attach to the hip capsule may physically bar translation of the femoral head with applied loads. One example of this is the iliopsoas muscle during erect stance and at the extremes of extension. In this position the myotendinous portions directly overlay the anterior hip capsule such that as the muscle contracts it results in hip flexion as well as anterior translation force on the femoral head. However, the taut muscle overlying the anterior hip acts to buttress the anterior capsule against translation. Similar behaviors by the gluteus minimus and the indirect head of rectus femoris may also help to contribute to hip stability in early abduction and flexion, respectively.

Though the major mechanical changes experienced by the hip are rotational in nature, most motion of the lower extremity requires a combination of both translational and rotational changes in the hip joint. The exception to this is flexion/extension in neutral abduction and external rotation. In this position and throughout this arc of motion, the center of the femoral head and acetabulum are maintained in an overlapping position by evenly balanced forces of hip flexors and adductors (3,10,11). Outside of this arc or motion, rotation of the femoral head results in associated translation that must be countered by opposing static or dynamic stabilizers to maintain a reduced joint (11).

Experimental averages for the normal range of motion of the native hip are provided in Table 11.2, however, the range of motion demanded of the hip through life’s activities is highly variable between patients. Regardless, a minimum amount of motion is required for activities of daily living such as sitting and standing, climbing stairs, or walking; and evaluation of one’s motion can allow for improved assessment of functional limitations. In general, Heller et al. (12) demonstrated that for walking unassisted on level ground the hip requires a much smaller arc of motion with a flexion/extension arc of 40 degrees, abduction/adduction arc of 10 degrees, and 10-degree rotational arc (Table 11.2). The addition of other activities such as uneven surface ambulation, running, rising from a chair, etc. require a much greater range of motion (13). In terms of where this motion, or more importantly, where the limits arise from, it is the osseous structures that determine the absolute limits of motion of the joint whereas the soft tissue components determine the realized or in vivo limits of motion.

The hip joint itself is constrained by the mechanical limitations of the bones that make up the joint in that at the extremes of motion there is osseous impingement. Although the osseous anatomy may change in response to pathology (i.e., osteoarthritis, trauma, osteonecrosis, etc.), it is the soft tissues (ligaments, muscles, labrum, etc.) that determine the
in vivo range of motion. Soft tissues act to either restrain motion or block motion by interposition and impingement. Turley et al. (3), investigated this phenomenon by comparing the extremes of motion experienced by the hips of subjects through a series of activities of daily living. They then compared these findings to the arc of motion that would be predicted by the osseous acetabulofemoral joint alone. In the overlap of these two motion arcs, they found a significantly decreased range of functional motion as compared with osseous motion particularly in extension and flexion with adduction (Fig. 11.3). Interestingly, this suggests that during extension, the hip is stabilized and limited primarily by soft tissues, not by osseous impingement as might be suggested by an anteverted acetabular cup. Conversely, Turley et al. (3) found that osseous impingement occurred in vivo at the extremes of adduction in an upright stance as well as abduction at 90 degrees of flexion corresponding to impingement of the lesser trochanter on the pubis and the greater trochanter on the lateral acetabular rim. This is particularly salient with regard to total hip replacement as implant or osseous impingement versus soft tissue impingement presumably puts the patient at greater risk of dislocation and early wear/failure (see Section “Hip Kinematics in Common Pathologies”).