Anatomy and Biomechanics, Evaluation, Clinical Examination, and Imaging of the Hip



Anatomy and Biomechanics, Evaluation, Clinical Examination, and Imaging of the Hip


Mitchell C. Weiser, MD, MEng, FAAOS

Ferdinand J. Chan, MD, FAAOS


Dr. Weiser or an immediate family member serves as a board member, owner, officer, or committee member of the American Academy of Orthopaedic Surgeons. Neither Dr. Chan nor any immediate family member has received anything of value from or has stock or stock options held in a commercial company or institution related directly or indirectly to the subject of this chapter.





ABSTRACT

The hip is a complex diarthrodial ball-and-socket joint, allowing for multiaxial rotatory movement in flexion-extension, abduction-adduction, and internal-external rotation. Knowledge of the anatomy, ossification, and biomechanical principles of the native hip joint is critical to understanding the pathologic basis of hip injuries and disorders in both the pediatric and adult patient population. This knowledge empowers the physician to appropriately apply and interpret physical examination maneuvers and imaging studies to aid in diagnosing common hip maladies.

Keywords: hip anatomy; hip joint; hip physical examination; imaging


Introduction

The hip joint is composed of the femoral head and the acetabulum, which articulate as a ball-in-socket diarthrodial joint capable of multiaxial rotatory motion. In addition to the bony anatomy, the hip joint is supported by soft tissue, including the acetabular labrum and hip capsule, which contribute to the primary stabilization of the articulation of the femoral head in the acetabulum. This is further reinforced with the hip musculature serving as dynamic secondary stabilizers. It is an oversimplification to understand the hip as a pure ball-and-socket joint because the femoral head is not a true sphere in shape and the socket of the acetabulum is not a true hemisphere (it is horseshoe shaped). Anatomic variations in acetabular inclination, version, and dome coverage combined with variations in proximal femoral architecture and version add further complexity to the idealized hip joint articulation. Another level of complexity to this articulation is again added when considering dynamic pelvic positioning through the hip-spine relationship. Understanding the normal hip joint anatomy and its variants, both benign and pathologic, is fundamental to the practicing orthopaedic surgeon.


Osseous and Ligamentous Anatomy

The process of hip-joint formation begins early in embryonic development, beginning during the fourth week of embryonic gestation. By the sixth week of embryonic development, chondroblasts appear and by the end of the eighth week, cartilage templates of the femur and acetabulum are complete, as are microscopic precursors of the acetabular labrum, ligamentum teres, and transverse acetabular ligament. Ossification of the femur is complete up to the lesser trochanter by the 16th week, and the primary ossification centers of the ilium, pubis, and ischium also have appeared.1

The acetabulum is formed from both the triradiate cartilage and the acetabular cartilage complex, which is the fusion of the ilium, ischium, and pubis. The triradiate cartilage forms the nonarticular aspect of the medial wall of the acetabulum and contributes to 70% of the overall depth of the socket.2 The acetabular cartilage complex forms the articular surface of the acetabulum and is composed mainly of hyaline cartilage. Three secondary ossification centers of the acetabulum appear between 8 and 9 years of age along the rim of the acetabulum: anteriorly, superiorly, and posteriorly. Fusion of these centers occurs by adulthood and contributes to the development of the acetabular rim. The
superior secondary ossification center is the most common of the three to fail to fuse, which leads to the radiographic appearance of an os acetabuli. Radiographic closure of the triradiate cartilage occurs at the median age of 13.9 years.3 The normal versional orientation of the acetabulum, with the pelvis in neutral position while supine with respect to the sagittal plane, is anteverted, that is, the opening of the acetabulum facing anteriorly. The degree of normal anteversion occurs on a spectrum and is generally believed to be between 15° and 20°4 with a mean of 17.6°.5 Functional acetabular anteversion is a dynamic parameter and varies according to pelvic positioning. In patients with normal spine mobility, posterior pelvic rollback (or tilting), such as that which occurs with sitting, increases the functional anteversion of the acetabulum, whereas anterior tilting, such as when standing, decreases the functional anteversion. The normal relationship between pelvic tilt and functional acetabular anteversion is a change of 1° in acetabular anteversion for every 0.8° change in pelvic tilt.6 Changes in spinal-pelvic mobility can change the nature of this relationship because of pathologic conditions (such as lumbar spine stiffness from degenerative diseases) or prior lumbar fusion. Understanding the nature of the normal and pathologic hip-spine relationship is critical, particularly when considering implant positioning targets during total hip arthroplasty. Acetabular coverage of the femoral head is directly related to the depth of the acetabulum and proper formation of the acetabular rim. This can be measured radiographically on a supine AP pelvis radiograph using the center-edge angle of Wiberg. Normal acetabular superolateral coverage is thought to be between 25° and 35° with an angle less than 25° representing borderline dysplasia and an angle greater than 40° representing overcoverage.4

The primary ossification center of the femur appears in the seventh week of gestation, and ossification occurs proximally to the greater trochanter and femoral neck by birth. Secondary ossification centers at the greater trochanter, and femoral head appears between 4 and 7 months of age. These ossification centers are connected by a small isthmus that is responsible for the overall growth and diameter of the femoral neck. The secondary ossification center of the femoral head contains the physis, which is responsible for 30% of length of the femur and 13% of the overall length of the limb.7 The median age of radiographic closure of the secondary ossification center of the femoral head occurs at age of 15.4 years and that of the greater trochanter at 16.8 years.3 The bony trabecular architecture of the proximal femur is oriented in a pattern designed to resist the mechanical forces placed across it. The formation of these trabecular patterns is done via bone remodeling in response to the Wolff law. These trabecular patterns are oriented into primary and secondary tensile and compressive groups. This trabecular ultrastructure leads to the formation of a small triangular area in the inferior femoral neck femoral neck relatively devoid of trabeculae. This area is known as the Ward triangle and is bounded by the principal compressive trabeculae, secondary compressive trabeculae, and primary tensile trabeculae. The Ward triangle serves as a weak point in the femoral neck and osteoporotic femoral neck fractures occur commonly through this area in elderly patients.8 Proximal femoral orientation is also determined by application of the Wolff law and progressive bone remodeling during development. At the time of birth, the mean anteversion of the proximal femur is 45°, decreasing to 31.1° by 1 year of age, and finally to 15.4° by 16 years of age.9 Similarly, the neck-shaft angle of the proximal femur also changes over time, with the mean neck-shaft angle at 1 year of age being 132.6° and decreasing to 127.3° by age 18 years.9 Disruptions in the application of physiologic mechanical forces to both the proximal femur and acetabulum during development lead to pathologic conditions of the hip, such as developmental dysplasia.

Understanding the blood supply to the femoral head is critical when considering surgical intervention for hip preservation or trauma procedures. A silicone injection study demonstrated that the dominant blood supply to the femoral head comes from superior retinacular vessels via the medial femoral circumflex artery in most of the anatomic specimens (29 of 36 hips), although there are some individuals in whom the inferior gluteal artery may serve as the dominant blood supply (6 of 36 hips).10 Additionally, some individuals may have a substantial anastomosis between the medial femoral circumflex artery and the inferior gluteal artery adjacent to the tendon of the obturator externus.11

The hip joint is stabilized by a soft-tissue envelope consisting of the joint capsule and acetabular labrum. The joint capsule is divided into internal and external fibers. The internal fibers form the zona orbicularis, which is the capsular ring that attaches to the base of the femoral head. This structure provides added stability to the hip joint through a screw-home mechanism in extension and external rotation and can serve as a checkrein to limit distraction of the femoral head.12 It is an important arthroscopic landmark that divides the hip joint into the central and peripheral compartments. The zona orbicularis is also thought to play a contributory role in the circulation of synovial fluid within the hip joint.12 The external fibers of the capsule run longitudinally and are divided into three main
ligaments: pubofemoral, ischiofemoral, and iliofemoral. The pubofemoral ligament originates on the iliopectineal eminence of the superior ramus, wraps around the iliofemoral ligament, and blends into the insertion of the ischiofemoral ligament along the inferior femoral neck just distal to the acetabular rim. The pubofemoral ligament helps control excessive abduction and external rotation during hip extension.12 The ischiofemoral ligament originates from the root of the ischial ramus and traverses the femoral neck superolaterally to insert at the base of the greater trochanter. It serves to reinforce the capsule in internal rotation with neutral position of the hip and restrict combined flexion-adduction positions.12 The iliofemoral ligament is the largest of the three and is divided into two bands: medial and lateral, the Y ligament of Bigelow. The ligament originates from a broad base along the anterior inferior iliac spine and anterior rim of the acetabulum. The medial limb passes anteriorly along the femoral neck in a vertical orientation and inserts along the anterior femur at the level of the lesser trochanter along the intertrochanteric line. The lateral limb traverses the anterior hip in an oblique orientation and inserts superior to the intertrochanteric line at the greater trochanteric crest. The iliofemoral ligament serves to restrict internal rotation in hip flexion and reinforce the capsule in external rotation and hip extension.12 The soft-tissue envelope surrounding the hip joint is richly innervated with proprioceptive and nociceptive fibers, allowing these structures to serve as important sources of hip pain when damaged. The hip capsule receives innervation from all the major nerves surrounding the hip including branches of the obturator, femoral, sciatic, and superior gluteal nerves, and the nerve to the quadratus femoris.13 The complexity of this innervation can lead to hip pain being felt in the groin, thigh, buttock, knee, and as far distal as the foot.14

The acetabular rim blends into the acetabular labrum, a fibroconnective tissue that extends from the rim of the acetabulum that contributes to the suction seal of the femoral head within the acetabular cavity. The height of the labrum is variable along the acetabular rim and has been found to be between 4 and 8 mm on average.15 A 2020 study correlated the height of the labrum with the strength of the suction seal to resist distractive forces, with labral height less than 6 mm having reduced capability to resist distractive forces on the femoral head.16 The labrum consists of type I and III collagen along its capsular-facing surface, and fibrocartilage along its articular-facing surface. Most of these collagen fibers have been found to be circumferential in alignment.17 The articular surface of the labrum is contiguous with the articular cartilage of the acetabulum.18 The inferior aspect of the acetabulum is bounded by the transverse acetabular ligament, which connects the anterior and posterior inferior rims of the acetabulum. As such, it has been reported as a reliable intraoperative landmark to gauge acetabular anteversion during total hip arthroplasty.19 The acetabular labrum is also richly innervated and contains several different types of sensory nerve organs along its articular-facing side, including nociceptors and mechanoreceptors.20 The main blood supply to the acetabular labrum was shown in a silicone injection study to come from a periacetabular periosteal vascular ring that traverses the osteolabral junction of the labrum flowing outward to the free edge of the labrum. There is no significant contribution to this vascular supply from the hip capsule, osseous labral rim, or synovial lining.21

The ligamentum teres is a soft-tissue structure that connects the femoral head and acetabulum directly. It is composed of two separate bands, approximately 30 to 35 mm in length, which connect the fovea capitis of the femoral head to the transverse acetabular ligament inferiorly and pubic and ischial acetabular margins medially. It comprises type I, III, and V collagen fibers and contains an anterior branch of the posterior division of the obturator artery.12 The innervation of the ligamentum teres is controversial, but a 2019 study states there has been histologic evidence for the presence of free nerve endings and Pacini corpuscles contained within it, suggesting that it can be a source of pain and play a role in proprioception.22


Muscular Anatomy

The thigh has three separate muscular compartments: anterior, lateral, and medial. There are 22 separate muscles that cross the hip joint, and 29 muscles about the hip joint and proximal thigh that play a role providing motor function for the joint. The muscles as presented here are grouped by their functional actions. A full list of the muscles about the hip joint grouped by their anatomic location and information on their origins and insertions, innervation, and function is provided in Table 1.


Anatomic Compartments

There are three anatomic compartments of the thigh: anterior, posterior, and medial. The gluteal region contains four distinct compartments: (1) the tensor compartment, which contains the tensor fascia lata and a branch of the superior gluteal nerve; (2) the medius-minimus compartment, which contains the gluteus


medius and minimus and is supplied by the superior gluteal nerve and vessels; (3) the deep gluteal compartment, which contains the short external rotators; and (4) the maximus compartment, which contains the gluteus maximus and is supplied by the inferior gluteal nerve and vessels. Although a rare entity, compartment syndrome of each gluteal compartment has been reported.23







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May 1, 2023 | Posted by in ORTHOPEDIC | Comments Off on Anatomy and Biomechanics, Evaluation, Clinical Examination, and Imaging of the Hip

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