Hip Anatomy and Biomechanics





Introduction


The human hip consists of a stable but very mobile skeletal framework for the surrounding capsule, ligaments, muscles, nerves, and vasculature. Understanding the anatomy and the development of intra- and extra-articular pathologies is critical in any patient population, and careful consideration must go into the evaluation of a painful hip in the female athlete. The anatomy, biomechanics, and pathologies specific to the female athlete are explored in this chapter.


The hip is a ball-and-socket joint that is formed by the articulation of the femoral head and the acetabulum, between which there is a high level of congruence. The joint’s configuration allows for multiplanar movement; however, due to the joint being an integral part of lower limb motions such as walking, running, jumping, and kicking, a high degree of stability is also required. It is markedly stable because of not only its osseous and articular architecture but also the encompassing soft tissues. Numerous structures provide stability to the joint. The capsule is one of these essential structures, and it is reinforced by the embedded capsular ligaments. The labrum helps stabilize the joint by deepening the socket, while the ligamentum teres acts to tether the femur to the acetabulum. Furthermore, the musculature surrounding the joint provides both static and dynamic stability. This chapter explores how each of these structures contributes to the function of the hip joint and its physical implications, with a focus on the female athletic population.


Anatomy


Basic Anatomy


Bony and articular anatomy


The hip joint is a synovial joint that can be further categorized as a ball-and-socket-type of joint. The hip joint is composed of the femoral head and the acetabulum, and these two components are in high congruence with one another. The acetabulum is formed by the junction of three bones, namely, the ilium, the ischium, and the pubis, that intersect to form the triradiate zone. The ilium is a broad, fanlike bone that expands superiorly from the acetabulum. The ischium extends posteroinferiorly from the acetabulum, while the pubis does so anteroinferiorly. These bones expand peripherally during growth to give the acetabulum its depth, and the concavity of the acetabulum develops around the sphericity of the head of the femur. The acetabulum is a relatively deep cavity, which contributes significantly to the stability of the joint; its depth has been measured to be approximately 30 mm. The articular cartilage of the acetabulum varies throughout the cavity, and it typically ranges between 1.3 and 3.0 mm in thickness, with the greatest thickness in the superolateral quadrant. Although the acetabulum is commonly described as a hemisphere, it is devoid of cartilage in the central-inferior portion, with an opening at its inferior aspect spanned by the transverse acetabular ligament. The bare area is where the ligamentum teres originates.


The diameter of a native femoral head can range from 40 to 54 mm, with smaller sizes usually found in females. All areas of the femoral head that articulate with the acetabulum are covered with hyaline cartilage, therefore covering 60%–70% of the spherical head. The articular cartilage of the femoral head can be between 0.8 and 2.8 mm in thickness. The inferomedial part of the head, the fovea capitis, lacks cartilage, as that is where the ligamentum teres inserts.


Capsular and ligamentous anatomy


The capsule consists of dense fibers that are cylindrically arranged around the joint to form a sleeve; they insert proximally along the acetabular periosteum, just proximal to the labrum, and distally to the anterior aspect of the femur, along the intertrochanteric line. The capsule functions to both constrain the hip joint and maintain congruence. Posteriorly, the capsule is composed mainly of the ischiofemoral ligament proximally and the zona orbicularis (ZO) distally. The ZO forms an arched free border that partially covers the femoral neck; it attaches just medial to the intertrochanteric crest.


Most fibers of the capsule are longitudinally oriented, as is the strongest of the capsular ligaments, the iliofemoral ligament, also known as the Y-ligament of Bigelow. This ligament is located anteriorly and lies between the anterior inferior iliac spine and the ilial portion of the acetabular rim and the intertrochanteric line. It fans across the front of the joint, dividing into superior and inferior bands, taking the shape of an inverted Y. Its fibers are taut in extension and external rotation and lax in flexion, thus this ligament is thought to be essential in maintaining erect posture and reducing the requirement for active muscle contribution. ,


The ischiofemoral ligament lies posterior to the joint and is spiral in shape. The fibers of the ligament are oblique, but the ischiofemoral ligament is less defined than the pubofemoral and iliofemoral ligaments. The ischiofemoral ligament extends from the ischial rim of the acetabulum to the posterior aspect of the femoral neck, at the base of the greater trochanter. The literature describes two distinct bands in this ligament: the superior and the inferior bands. , The superior band inserts at the base of the greater trochanter, where it intermingles with the fibers of the ZO. The inferior band spreads downward to insert more posteriorly on the intertrochanteric crest. , The ischiofemoral ligament is thought to be at maximal tautness primarily during internal rotation, and also during adduction when the hip is flexed, thus limiting the extent of these actions. ,


The pubofemoral ligament is slinglike in appearance; it originates proximally at the obturator crest and the superior pubic ramus. Distally, it blends anteriorly with the inferior part of the iliofemoral ligament and wraps posteriorly to insert inferior to the ischiofemoral ligament. It works in conjunction with the iliofemoral ligament to control external rotation of the hip and has been found to be maximally taut in hip abduction and lax in adduction.


The fibers of the ZO are circular and resist distraction of the hip. It is primarily a posterior and inferior structure, forming the free border of the posterior capsule. Recent studies indicate that the ZO has a role in synovial fluid circulation within the joint; it has been postulated that it acts as a bellow to unidirectionally force fluid from the peripheral compartment to the central compartment when the hip moves in flexion and extension.


Neurovascular anatomy


Vascular anatomy (blood supply)


The primary source of blood to the hip joint is the medial femoral circumflex artery (MFCA). It arises as a branch of the deep femoral artery, and its superficial branch supplies the adductor musculature. Direct branches of the MFCA and the lateral femoral circumflex artery (LFCA), which also arises from the deep femoral artery, predominantly supply the anterior capsule; they enter from the femoral aspect and run superficially along the capsule, encircling it from distal to proximal. The superior and inferior gluteal arteries divide into supra-acetabular and acetabular branches that terminate as capsular vessels; these run from proximal to distal to form anastomoses with the MFCA and LFCA and are the major supply to the posterior capsule.


The vascular supply of the labrum is formed by branches of the medial and lateral circumflex arteries, the superior and inferior gluteal arteries, and the vascular system within the pelvis. The primary blood supply to the labrum is postulated to be the connective tissue interposed between the capsule and the capsular region of the labrum; therefore the capsular side is more vascular than the articular region ( Fig. 9.1 ). The bone adjoining the labrum is also a major contributor of blood to the structure. In one study, vascular channels originating in the osseous acetabulum were found to cross into the labrum, demonstrating that the bone-adjacent labrum has greater vascularity than its more peripheral aspects. This can have significant implications in labral healing and repair.




Fig. 9.1


Capsular blood supply ( arrow ) as shown on a high-power sagittal section.

Reprinted from Kelly BT, Shapiro GS, Digiovanni CW, Buly RL, Potter HG, Hannafin JA. Vascularity of the hip labrum: a cadaveric investigation. Arthroscopy 2005;21(1):3–11. doi-org.easyaccess2.lib.cuhk.edu.hk/10.1016/j.arthro.2004.09.016 , Copyright (2005), with permission from Elsevier.


The blood supply to the femoral head is variable. A minor branch off of the posterior division of the obturator artery supplies the ligamentum teres and thus plays a small role in vascularizing the proximal part of the head. Ascending cervical branches that arise from the extracapsular arterial ring perforate the capsule to form retinacular arteries; these are the main supply to the femoral head. The retinacular arteries have classically been divided into three main groups: posterosuperior, posteroinferior, and anterior. The posterosuperior and posteroinferior arteries are supplied primarily by the MFCA, with the anterior most commonly from the LFCA. However, Ganz et al. determined that the deep branch of the MFCA can give rise to two to four superior retinacular vessels and, occasionally, to inferior retinacular vessels ( Fig. 9.2 ). The head can be completely perfused by the superior retinacular vessels alone. This can have significant consequences in the development of avascular necrosis of the femoral head, particularly after dislocation or during surgical management of femur fractures.




Fig. 9.2


(A) The perforation of the terminal branches into the bone (right hip, posterosuperior view). The terminal subsynovial branches are located on the posterosuperior aspect of the neck of the femur and penetrate the bone 2–4 mm lateral to the bone-cartilage junction. (B) A diagram showing (1) the head of the femur, (2) the gluteus medius, (3) the deep branch of the medial femoral circumflex artery (MFCA), (4) the terminal subsynovial branches of the MFCA, (5) insertion of and the tendon of gluteus medius, (6) insertion of the tendon of piriformis, (7) the lesser trochanter with nutrient vessels, (8) the trochanteric branch, (9) the branch of the first perforating artery, and (10) the trochanteric branches.

Republished with permission of Gautier E, Ganz K, Krügel N, Ganz R. Anatomy of the medial femoral circumflex artery and its surgical implications. J Bone Joint Surg Br 2000;82-B(5):679–683; permission conveyed through Copyright Clearance Center, Inc.


Nerve anatomy


The obturator nerve is considered to be the primary source of innervation to the hip joint; however, the femoral nerve and the sciatic nerve also contribute sensory and motor innervation. The hip capsule is believed to be regionally innervated; the anterior region of the hip capsule receives innervation from the femoral and obturator nerves, while the posterior region receives innervation from the superior gluteal nerve, the nerve to the quadratus femoris, and direct branches of the sciatic nerve.


The labrocapsular complex receives innervation from the sciatic, inferior gluteal, and femoral nerves, but more substantially from the nerve to the quadratus femoris and the obturator nerve. Sensory end organs, such as Pacinian, Golgi-Mazzoni, Ruffini, and Krause corpuscles, in addition to free nerve endings, have been found in the hip labrum. Pacinian, Golgi-Mazzoni, and Ruffini corpuscles aid in proprioception, and the free nerve endings sense pain. The highest concentration of nociceptive and proprioceptive fibers is found along the attachment site of the labrum to the acetabulum. There is also a fair amount of these fibers in the center of the ligamentum teres.


As for muscular innervations, the femoral nerve provides innervation to the psoas, iliacus, pectineus, sartorius, and quadriceps muscles and provides sensory innervation to the anterior thigh via cutaneous branches. The sciatic nerve arises from spinal nerves L4 through S3 to form part of the sacral plexus and consolidates to course through the greater sciatic foramen, just inferior to the piriformis. It then divides into the tibial and the peroneal branches; the tibial branch supplies the semitendinosus, the semimembranosus, and the long head of the biceps femoris. The peroneal branch of the sciatic nerve innervates the short head of the biceps femoris. The sciatic nerve does not have any direct sensory functions.


The obturator nerve, originating from L2-4, provides innervation to the obturator externus, gracilis, and adductor muscle group. It provides sensory innervation to the inferomedial thigh via its cutaneous branches. The superior and inferior gluteal nerves arise from the ventral rami of the L4-S1 and L5-S2 sacral spinal nerves, respectively, and transverse through the greater sciatic foramen from the sacral plexus. The superior gluteal nerve exits the pelvis at or just above the piriformis and innervates the gluteus medius, gluteus minimus, and tensor fascia latae. The gluteus maximus receives its innervation from the inferior gluteal nerve, which exits the greater sciatic foramen just inferior to the piriformis muscle. Neither nerve has a sensory role.


The short external rotators, with the exception of the obturator externus, receive innervation from the sacral plexus that descends from the ventral rami of the L4-S2.


Muscular anatomy


The muscular anatomy of the hip joint can be categorized by regions. The regions include the gluteal muscles and the muscles of the anterior, posterior, and medial thigh. The gluteal region can be further divided into the superficial and deep groups. The following section provides a detailed summary of these muscle groups and individual muscles as they relate to the function and pathoanatomy of the hip.


Gluteal muscles (superficial and deep)


The superficial gluteal group is composed of the gluteus maximus, gluteus medius, gluteus minimus, and tensor fascia latae. The gluteus maximus originates from a broad area that includes the external surface of the ilium behind the posterior gluteal line, the fascia of the gluteus medius, the fascia of the erector spinae, the dorsal surface of the sacrum, the lateral margin of the coccyx, and the sacrotuberal ligament. The majority of the muscle inserts on the iliotibial (IT) band at its aponeurotic origin on the greater trochanter; the inferior segment inserts on the gluteal tuberosity of the femur. The main function of the muscle is to act as a hip extensor, primarily when the hip is in flexed position, such as when rising from a seated position. The gluteus maximus also functions as an external rotator and abductor of the hip. The gluteus medius originates from the ilium, from the anterior to the posterior gluteal lines. It then inserts onto the lateral aspect of the greater trochanter. The gluteus medius is considered the main abductor of the hip. Gluteus minimus, another abductor, originates from the external ilium, from the anterior gluteal line to the inferior gluteal line, covering the posterosuperior acetabulum. It inserts with the gluteus medius on the greater trochanter of the femur, notably on the anterior aspect. The gluteus minimus is also thought to insert onto the anterosuperior hip joint capsule; as a result, it may stabilize the femoral head in the acetabulum by tightening the capsule and thus putting pressure on the head of the femur. In addition to being an abductor, it can function as a hip flexor, internal rotator, and external rotator, depending on the position of the hip. The tensor fascia latae is the last muscle of the superficial gluteal group; it originates from the anterolateral portion of the iliac crest and the lateral aspect of the anterior superior iliac spine (ASIS). Its attachment is on the fascia lata, and continuing debate exists regarding whether this attachment is just inferior to the muscle belly where it attaches on the IT band or if the fibers of the attachment run with the IT band and the actual attachment is on Gerdy’s tubercle. This muscle primarily abducts the hip.


The deep group of the gluteal region includes the six short external rotators. These consist of the piriformis, the superior and inferior gemellus muscles, the obturator externus and internus muscles, and the quadratus femoris. The piriformis originates on the anterior surface of the second through fourth sacral vertebrae and emerges from the greater sciatic foramen of the pelvis. Its distal attachment is typically defined as anterosuperior to the trochanteric fossa. A common anatomic variant of the piriformis is for it to be split by the sciatic nerve into inferior and superior muscle bellies. However, typically, the sciatic nerve passes completely under the piriformis, through a canal formed by the piriformis and the superior gemellus. The superior gemellus, the smaller of the two gemelli, originates on the ischial spine. Inferior to that, the obturator internus arises from the medial surface of the obturator membrane as it spans the obturator foramen and the surrounding bone. The inferior gemellus originates just superior to the ischial tuberosity. The gemelli insert into the tendon of the obturator internus, creating a conjoint tendon that then inserts into the greater trochanter anteriorly. The quadratus femoris originates from the lateral aspect of the ischial tuberosity and has its muscular insertion on the posterior femur partially overlying the inferior margin of the intertrochanteric crest.


Thigh muscles (anterior, posterior, and medial compartments)


The anterior compartment of the thigh is composed of the sartorius, rectus femoris, iliopsoas, pectineus, and the lesser known iliocapsularis muscle. Together these muscles contribute to the flexion of the hip. The origin of the sartorius is on the ASIS; the muscle courses downward to insert medially to the tibial tuberosity via the pes anserinus. The rectus femoris has two heads: the anterior/direct head originates on the anterior inferior iliac spine and the posterior/indirect head originates just superior to the acetabulum. The two heads unite to form an aponeurosis that joins the muscle belly; the muscle then inserts into the base of the patella. The pectineus arises from the pectineal line of the superior ramus of the pubis; it is a flat quadrangular muscle that is the most anterior hip adductor. However, it also acts to flex the hip joint, and it is classified in the anterior compartment because of its innervation, which is primarily via the femoral nerve. It inserts onto the pectineal line of the femur, distal to the lesser trochanter. Due to its unique composition, origin, and insertion, the iliopsoas is the only periarticular hip muscle that is able to contribute to the stability of the trunk, pelvis, and leg. It has two muscle bellies that are separately innervated, allowing them to act in conjunction or separately from one another. The muscle complex originates from the transverse processes of the T12-L5 vertebrae, the anterior surface of the iliac crest, and the anterior sacrum; they merge distally to insert on the lesser trochanter. Both the iliacus and the psoas are involved in hip flexion and maximal thigh abduction; however, the iliacus can be selectively activated to control movement between the hip and pelvis, whereas the psoas is selectively activated for stabilizing the lumbar spine in standing when an axial load is applied to the contralateral hip. As a result of the psoas tendon’s position in relation to the anterior capsule, it serves as both a dynamic and a static stabilizer of the hip; it may be displaced laterally during hip flexion and medially during extension. The iliocapsularis, also called iliacus minor or iliotrochantericus, is a lesser known muscle that overlies the anterior hip capsule. It originates mostly from the anteromedial hip capsule and in part from the anterior inferior iliac spine; its insertion is located just distal to the lesser trochanter. Research suggests that contraction of the iliocapsularis results in tightening of the hip capsule, thus aiding in dynamic stabilization of the joint. Several studies have found that the iliocapsularis is hypertrophied in dysplastic hips, suggesting that it is particularly important for stabilizing the femoral head in a deficient acetabulum. ,


The posterior compartment of the thigh is composed of the hamstring muscles; these include the semitendinosus, semimembranosus, and the biceps femoris. The semitendinosus lies superficial to the semimembranosus in the posteromedial aspect of the thigh. It arises from the superomedial aspect of the ischial tuberosity and shares a common tendon with the long head of the biceps femoris. It courses distally to insert as part of the pes anserinus with the sartorius and gracilis. The semimembranosus arises on the superolateral aspect of the ischial tuberosity and inserts on the medial condyle of the tibia. The biceps femoris arises via two heads. As described earlier, the long head originates with the semitendinosus on the ischial tuberosity. The short head arises from the lateral lip of the linea aspera along the posterior femur, between the adductor magnus and vastus lateralis. It then terminates primarily on the lateral head of the fibula; however, a small, separate insertion is also found on the lateral condyle of the tibia.


The medial compartment of the thigh is made up of the adductor brevis, adductor longus, adductor magnus, and gracilis muscles; they are the primary adductors of the thigh and also contribute to flexion and internal rotation. The adductor brevis is the most superior, and it arises from the inferior pubic ramus. The adductor longus originates from the anterior aspect of the pubis, under the pubic tubercle. The origin of the adductor magnus is from the inferior pubic ramus and the ischial tuberosity; the gracilis originates solely from the inferior pubic ramus. The adductors then all insert on the medial lip of the linea aspera, with the exception of the adductor magnus, which also inserts on the adductor tubercle, and the gracilis, which inserts solely as part of the pes anserinus on the tibia.


Functional Anatomy: Central and Peripheral Compartments


Arthroscopically, the hip can be divided into two compartments: the central and the peripheral compartments ( Figs. 9.3 and 9.4 ). These compartments are demarcated by the labrum.




Fig. 9.3


The central and peripheral arthroscopic compartments of the hip.

Reprinted from Wettstein M, Dienst M. Arthroscopy of the peripheral compartment of the hip. Operat Tech Orthop 2005;15(3):225–230. doi-org.easyaccess2.lib.cuhk.edu.hk/10.1053/j.oto.2005.07.003 , Copyright (2005), with permission from Elsevier.



Fig. 9.4


Right hip magnetic resonance image, with the anatomy labeled.

Author’s original.


Central compartment


The central compartment, also known as the iliofemoral joint, includes the lunate cartilage of the acetabulum, the acetabular fossa, the ligamentum teres, and the loaded articular surface of the femoral head. , This part of the joint can only be visualized arthroscopically with hip distraction.


The labrum is a fibrocartilaginous structure composed of types I, II, and III collagen that attaches to both the perimeter of the bony acetabulum and the articular cartilage within the cavity. It courses along the acetabular rim to attach to the transverse acetabular ligament anteriorly and posteriorly, forming a contiguous structure around the acetabulum. The labrum increases the total acetabular surface area coverage by more than 25% and acetabular volume by approximately 20%, as found by Tan et al. Furthermore, stability is augmented by a vacuum force of about 120–200 N created by the labrum, which seals the joint space between the lunate cartilage and the femoral head and maintains the femoral head within the socket. ,


The type I collagen fibers of the labrum are oriented in different directions depending on the region. Anteriorly, the fibers are attached parallel to the bony edge of the acetabulum, making them susceptible to shear forces. Posteriorly, they attach perpendicular to the bony edge, allowing them to be more resistant to these forces. In the sagittal plane, the labrum appears to have a horseshoe shape. As the labrum extends peripherally from its bony attachment, the structure tapers. This gives it a triangular shape in cross-section, with the apex creating the free edge closest to the joint center. It maintains joint fluid in the central compartment, which reduces friction and supplies chondral nutrition. The labrum can be divided into two regions when observed under light microscopy: the capsular region and the articular region. The articular region attaches to the bony rim of the acetabulum through a zone of calcified cartilage, while the capsular region attaches without this zone. The thickness of the labrum varies by location; however, the average thickness is reported as approximately 5.3 mm.


Historically, the ligamentum teres was considered to be an embryonic remnant; however, with the advent of hip arthroscopy, recent investigations indicate that it may play a role in hip stabilization and may also participate in fine coordination by transmitting somatosensory afferent signals. It originates along the transverse acetabular ligament, as well as from the pubic and ischial aspects of the acetabulum. The structure then inserts onto the fovea capitis of the femur. It is enveloped in its own synovial membrane and is surrounded by a synovial fat pad, also known as the pulvinar. Additionally, the ligament carries within it an anterior branch of the posterior division of the obturator artery. Studies indicate that it may be an important stabilizer in hip adduction, flexion, and external rotation.


Peripheral compartment


The peripheral compartment of the hip includes the unloaded articular surface of the femoral head; the femoral neck; the medial, anterior, and posterolateral synovial folds; and the articular capsule with its intrinsic ligaments, including the ZO. Arthroscopically, the peripheral hip compartment can be divided into seven zones: anterior neck area, medial neck area, medial head area, anterior head area, lateral head area, lateral neck area, and posterior area. This allows for a systematic approach to view the hip so as to accurately and thoroughly view the joint space.


The retinacula of Weitbrecht (Weitbrecht ligaments) are three synovial folds that appear medial, anterior, and posterolateral in relation to the femoral neck. They overlie the retinacular branches of the femoral circumflex artery that supplies the femoral head and can be helpful landmarks during arthroscopic review of the joint. The medial synovial fold, which does not adhere to the femoral neck, can be found consistently and is a helpful landmark, especially if visibility within the peripheral compartment is poor.


The capsule inserts directly onto the acetabulum immediately proximal to the labrum, creating a capsulolabral recess that contains vascularized connective tissue and fat. After hip arthroscopy, patients who develop scarring in the region between the capsule and the labrum can have symptoms of pain and limited motion, lending to the theory that this recess is important for hip motion.


Layered Approach to the Hip


The layered approach to the hip compartmentalizes the anatomy to aid in diagnostic evaluation. In doing so, an examiner can perform a comprehensive and systematic evaluation of the hip. Four layers are described, each progressing from deep to superficial: the osteochondral layer, the inert layer, the contractile layer, and the neurokinetic layer. These are also referred to as the osseous, capsulolabral, musculotendinous, and neurovascular layers, respectively.


The Osteochondral Layer


The osteochondral layer encompasses the pelvis, acetabulum, and femur. Structural pathologies within this compartment can be further classified into three groups: static overload, dynamic impingement, and dynamic instability. Static mechanical factors occur either in standing or axially loading of the joint. Static overload can be caused by several anatomic variants, including acetabular protrusio, excessive femoral anteversion/retroversion, excessive acetabular retroversion/anteversion, lateral or anterior acetabular undercoverage, and coxa vara/valga. These deviations change the mechanics of the hip joint and thus can predispose the hip to abnormal stress and eccentric loading, leading to accelerated cartilage degeneration.


Dynamic impingement can be caused by variants such as femoroacetabular impingement (FAI), relative femoral anteversion, and coxa vara. Pain often presents during terminal hip motion as a result of these variations. FAI is becoming increasingly documented not only in the athletic population but also in the general population. It is characterized by abnormal contact between the femoral head and the acetabulum during terminal hip movement, with associated pain, and labral and articular cartilage damage. FAI can be categorized as cam type (femoral based), pincer type (acetabular based), or combined impingement, which is the most common.


When the range of motion required for normal function of the hip starts to exceed the limits of an individual’s physiologic range as set by his/her anatomy, there can be compensatory stresses and dynamic instability, in addition to the pain previously described. Instability is in the form of small, repetitive posterior hip subluxations as the femoral head shifts out of the acetabulum. As the periarticular musculature attempts to stabilize the incongruent anatomy, layers 2 and 3 are subsequently affected, as discussed later in this chapter.


The Inert Layer


The inert layer consists of the labrum, capsule, ligamentous complex, and ligamentum teres. This layer not only does provides stability to the hip joint but also serves as protection for the cartilage and as a scaffold for the vascular supply to the joint. As such, mismatched anatomy described in layer 1 has a direct effect on this layer. The underlying abnormalities can result in pathologies such as labral injury, capsular injury, ligamentum teres tears, adhesive capsulitis, and instability.


The labrum increases the intra-articular hydrostatic pressure and load distribution; therefore biomechanical studies have shown that progressive labral wear correlates with an increase in hip instability. As previously described, the labrum also contains free nerve endings that carry both nociceptive and proprioceptive fibers, which corroborates the findings of decreased proprioception and pain in athletes with torn labrums. ,


The Contractile Layer


The contractile layer is composed of the periarticular musculature, the lumbosacral musculature, and the pelvic floor. This layer plays a crucial role in balance and dynamic stability of the hip. Abnormal morphologies in layer 1, particularly dynamic impingement such as FAI, can lead to increased mechanical stresses in the sacroiliac joint, pubic symphysis, and ischium, and this strains the muscles that are attached to these structures, leading to muscle dysfunction. Patients with FAI have been found to have decreased maximal voluntary contraction levels in all the major muscle groups surrounding the hip joint, when compared with the control group. The authors of this study concluded that this anatomic pathology in layer 1 can lead to hip muscle malfunction, and as a result, the risk of compensatory injuries in these muscles is increased.


Enthesopathy, a general term for pathology in tendons or ligaments as they attach to bones, in the musculature around the joint can result in numerous muscular injuries. Because of the myriad muscles that surround the joint, injuries in this layer are categorized based on the muscles’ location relative to the hip joint; thus injuries are classified as anterior, posterior, medial, and lateral. Anterior enthesopathy includes hip flexor strains, psoas impingement, and subspine impingement. Posterior enthesopathies consist of proximal hamstring strains, piriformis injuries, and the pain syndrome known as “deep gluteal syndrome,” which involves posterior soft tissue injury and entrapment of the sciatic nerve. Medial enthesopathy is composed of adductor and rectus abdominus tendinopathies; these have been historically referred to as athletic pubalgia, or “sports hernias,” and are now often referred to as core muscle injuries.


Lateral enthesopathies include gluteus medius and minimus strains and injuries within the peritrochanteric space. Lateral enthesopathies of both gluteus medius and gluteus minimus play a role in greater trochanteric pain syndrome (GTPS), and recalcitrant GTPS has been shown to be successfully treated with surgical gluteus medius repairs. , Medius tears are much more common than minimus tears, and medius tendinopathy and tears can lead to the Trendelenburg gait and difficulty ascending stairs.


As previously described, the psoas can be displaced laterally during hip flexion and medially during extension; this motion may explain the pain that can be generated in “snapping hip.” This pathology is likely related to the psoas snapping over the femoral head or iliopectineal eminence. Fabricant et al. observed that patients with femoral anteversion greater than 25 degrees had inferior clinical outcomes when they underwent arthroscopic psoas lengthening for refractory symptomatic internal snapping hip; this further demonstrates the critical interactions between layers 1 and 3.


The hamstring tendons can tear with and without underlying layer 1 pathology. The classic mechanism of injury for a proximal hamstring tear occurs when the knee is extended and the hip is forced into flexion. However, proximal hamstring injuries can also occur during sports with rapid acceleration and deceleration. It has been hypothesized that proximal hamstring tendinopathy can also arise due to increased stress on the tendon, secondary to FAI. The decreased rotation of the hip places undue stress on the hamstring tendons, subsequently causing degeneration of the tendon. This restricted range of motion may also play a role in the development of athletic pubalgia, as well as pain at other sites, such as the lumbar spine, pubic symphysis, sacroiliac joint, and posterior acetabulum, as they increase their motion to compensate. Hammoud and colleagues reported on a series of professional athletes with recalcitrant athletic pubalgia; 32% had undergone surgery to solely address the symptoms, without undergoing treatment for the underlying FAI. After surgery to subsequently correct the underlying FAI, 95% of the athletes were finally able to return to their previous level of play, indicating the role FAI hip pathology has in developing neighboring pain syndromes.


The Neurokinetic Layer


The thoracolumbosacral nerve plexus, lumbopelvic tissue, and lower extremity structures and mechanics compose layer 4, the neurokinetic layer. From a purely reductionist view, this layer supplies blood and innervation to the joint. To expand upon this, the nerves contain nociceptive and proprioceptive receptors, which are responsible for pain and stability felt in and around the joint. This layer serves as the neuromuscular link, thus controlling how this segment of the body functions and moves, and dictates posture.


Thus the pathology seen in this layer includes neuromuscular dysfunction, nerve compression and pain syndromes, spinal radicular symptoms, and myelopathies. The more common peripheral nerve conditions around the hip include sciatic neuropathy (piriformis syndrome), superior and inferior gluteal neuropathies, lateral femoral cutaneous neuropathy (meralgia paresthetica), femoral neuropathy, obturator neuropathy, and pudendal, ilioinguinal, iliohypogastric, and genitofemoral neuropathies. The lateral femoral cutaneous nerve is a branch of the lumbar plexus, arising from the dorsal divisions on L2 and L3. It runs along the lateral edge of the psoas and then passes beneath the iliac fascia and the inguinal ligament, close to the ASIS. Owing to its location, the nerve can be subject to external compression or injury.


The sciatic nerve is commonly compressed at the level of the piriformis, resulting in a constellation of symptoms known as the piriformis syndrome. This entails buttock pain, posterior muscular tension, and radicular symptoms down the length of the leg.


The obturator nerve, as stated previously, innervates the medial muscular compartment of the leg and the inferomedial thigh via cutaneous branches; injury to the nerve has been reported from retractor placement on the transverse acetabular ligament during open approaches to the hip. Although radiculopathies and myelopathies are less common causes of hip pain, they should be kept in mind when evaluating a painful hip.


Biomechanics of the Hip


There can be significant variability in the morphologies of the femur and acetabulum, with a few that have been elucidated over the years to have substantial clinical relevance. Regarding the femur, the angle of inclination, or the neck-shaft angle, and the amount of femoral version can significantly impact the function of the joint. The neck-shaft angle is measured as the angle formed from the bisection of the line drawn through the axis of the femoral neck and the line drawn through the axis of the shaft of the femur.


The neck-shaft angle of the femur, normally approximately 125 degrees, determines the offset ( Fig. 9.5 ). The femoral offset is found by measuring the perpendicular line from the center of rotation of the femoral head to where it intersects with the line drawn down the center of the shaft. The greater the value of offset, the more lateralized the muscular attachments are, which decreases the chances of impingement and creates greater tension in the abductor mechanism and enhances stability. This occurs in femurs with decreased neck-shaft angles; these are termed coxa vara .


Aug 21, 2021 | Posted by in SPORT MEDICINE | Comments Off on Hip Anatomy and Biomechanics
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