Hip Flexors

The primary hip flexors are the rectus femoris, iliacus, psoas, iliocapsularis, and sartorius muscles. The rectus femoris muscle has two distinct origins proximally: the direct head and the reflected head. They originate at the AIIS and the anterior acetabular rim (in close proximity to the anterior hip capsule), respectively. The tendinous fibers of the rectus femoris coalesce distally and become confluent with the other quadriceps musculature in the thigh. The quadriceps consists of four distinct muscles: 1) the vastus intermedius; 2) the vastus lateralis; 3) the vastus medialis; and 4) the rectus femoris. The rectus femoris is the only quadriceps muscle that traverses both the hip and the knee joint. The rectus femoris is a powerful hip flexor, but it is largely dependent on the position of the knee and hip to assert its influence. It is most powerful when the knee is flexed, whereas significant power is lost when the knee is extended. The rectus femoris is innervated by the femoral nerve (i.e., the posterior division of L2 to L4).

The iliopsoas is another powerful hip flexor that begins in two distinct regions proximally. The iliacus has a broad origin, arising from the inner table of the iliac wing, the sacral alae, and the iliolumbar and sacroiliac ligaments. The psoas originates at the lumbar transverse processes, the intervertebral discs, and the adjacent bodies from T12 to L5, in addition to the tendinous arches between these points. Distally, the two large muscular bodies converge to become one distinct structure—the iliopsoas—and subsequently jointly insert at the lesser trochanter of the proximal femur. The nerve to the iliopsoas (i.e., the anterior division of L1 to L3) supplies the iliopsoas muscle.

The sartorius originates at the ASIS and proceeds to traverse obliquely and laterally down the thigh to eventually insert at the anterior surface of the tibia, just inferomedial to the tibial tuberosity, as part of the pes anserinus. In addition to flexing the hip and knee, the sartorius aids in the abduction of the hip. It is innervated by the femoral nerve (i.e., the posterior division of L2 and L3).

Other muscles that can be recruited to assist with hip flexion include the tensor fascia latae (TFL), the pectineus, the adductors, the gracilis, and the anterior aspects of the gluteus medius and the gluteus minimus. The contribution of these secondary hip flexors largely depends on the position of the hip at the time at which movement is initiated.

Hip Extensors

The major contributors to hip extension are the gluteus maximus, the ischiocondylar part of the adductor magnus, the semimembranosus, the semitendinosus, and the biceps femoris (i.e., the long and the short heads). The most powerful muscle of this group is the gluteus maximus, which is responsible for more than 75% of the total power output of the extensor group. The gluteus maximus originates at the posteromedial outer rim of the iliac wing (i.e., behind the posterior gluteal line), the sacrococcygeal junction, the sacrotuberous ligaments, and the aponeurosis of the gluteus medius origin. Distally, the majority of the muscle inserts at the posterior aspect of the iliotibial tract of fascia lata, whereas the remainder inserts at the gluteal tuberosity of the proximal femur. In addition to extending the flexed femur, the gluteus maximus assists with lateral rotation and abduction of the thigh, and it stabilizes both the hip and knee joints through its influence on the iliotibial tract. It is innervated by the inferior gluteal nerve (i.e., the posterior divisions of L5 to S2).

The hamstring muscles also collaboratively assist with the extension of the hip. The long head of the biceps femoris, the semitendinosus, and the semimembranosus originate at the ischial tuberosity and insert below the knee. The combined hip extensor strength of these three hamstring muscles is still significantly lower than that of the gluteus maximus. However, in maximal hip flexion, the gluteus maximus loses its mechanical advantage, and the hamstrings become the dominant hip extensors. Because the hamstrings cross the knee joint, they are also able to flex and rotate the leg at the knee. They are innervated by the sciatic nerve (i.e., the posterior divisions of L5 to S2).

Adductors

The adductors of the hip are the adductor brevis, the adductor longus, the anterior part of the adductor magnus, the pectineus, and the gracilis. The adductors originate at the inferior pubic ramus and the ischial tuberosity, whereas distally their attachments are along the linea aspera of the femur. The adductors are innervated by the obturator nerve (i.e., the anterior division of L2 to L4). The adductor longus is the most commonly damaged muscle of this group. Unlike most tendinous attachments to bone, the adductor longus attachment at the external surface of the pubic ramus is comprised of 62% muscle and only 38% tendon. It is postulated that this abnormal muscle-to-tendon ratio at the bony attachment creates a vulnerability to injury. However, given its medial location, the adductor musculature is rarely at risk during standard open or arthroscopic approaches to the hip, except in the pediatric population.

Abductors

The abductors consist of the gluteus medius and gluteus minimus muscles. Both of these muscles are innervated by the superior gluteal nerve (i.e., the posterior division of L5 to S2). The TFL and the iliotibial band also contribute to hip abduction. This action is only apparent with the hip in a flexed position, and the TFL and the iliotibial band are therefore considered secondary abductors. The gluteus medius, which is the primary hip abductor, originates at the posterior external table of the iliac wing. It is completely covered by the overlying gluteus maximus as it travels distally toward its insertion at the lateral and superoposterior facet of the greater trochanter (Figure 2-3, A). The gluteus minimus fibers run in close approximation to the lateral hip capsule, onto which some of the muscle may also insert. These fibers are often the first to be encountered during hip arthroscopy procedures when establishing the anterolateral portal. The gluteus minimus, which is responsible for 25% of abduction power, runs in the same plane deep to the gluteus medius. It inserts more anteriorly on the greater trochanter, and it has a separate long head component. Recent evidence suggests that a common cause of lateral hip pain may be tears of the hip abductor insertion and not simply trochanteric bursitis. These tears are referred to as rotator cuff tears of the hip. Anatomic restoration of the insertion of the torn gluteus medius can be achieved with standard arthroscopic technique in the recently described peritrochanteric compartment. When trochanteric bursitis does exist in isolation, it is most likely located at the posterior facet or the bald spot of the trochanter (see Figure 2-3, B).

Figure 2–3 A, Superolateral view of a cadaveric right proximal femur that shows the attachment footprints of muscles on the greater trochanter. B, Computer-generated model of the proximal femur that shows muscular insertional sites on the greater trochanter.

Functionally, the gluteus medius and gluteus minimus are critical to the gait cycle, and they assert the major stabilizing force during the end of the terminal swing phase. This force provides tension among the pelvis, the iliotibial band, and the greater trochanter; it peaks during the initial part of the stance phase, and it persists through the middle of the stance. Injury to the gluteus medius or indeed to the superior gluteal nerve can be clinically recognized by the presence of a Trendelenburg sign, which is classically described as the dropping of the pelvis on the opposite side of the pathology.

External and Internal Rotators

The external rotators include the obturator internus, the obturator externus, the superior and inferior gemelli, the quadratus femoris, and the piriformis muscles (see Figure 2-3). These small but powerful musculotendinous units act synergistically to provide the external moments that are necessary to generate lateral and rotational activities. The piriformis is the common denominator of the external rotators, and it serves as an important anatomic landmark during both the posterior approach and the surgical dislocation of the hip.

The internal rotation of the hip occurs primarily through the combined efforts of the TFL, the anterior gluteus medius, and the gluteus minimus. There are no primary internal hip rotators, and, consequently, the internal rotational moments of the hip are the weakest of all functional movements. Other secondary hip internal rotators include the hamstring muscles and the pectineus.

Vasculature of the Hip

The common iliac arteries provide the primary blood supply to the lower limbs. Each artery divides into the external and internal iliac arteries. These vessels run parallel with their venous counterparts, the internal and external iliac veins, which join to form the inferior vena cava. The external iliac artery, which travels obliquely over the psoas muscle, is particularly vulnerable to injury. Damage can occur during hip arthroplasty when accessing the acetabulum, during the placement of screws in the anterior quadrant, or, more commonly, from the aberrant placement of anterior acetabular retractors. Excessive medial reaming can also put the external iliac vessels at risk, especially during revision acetabular surgery. If iatrogenic injury does occur, the external iliac artery and vein can be accessed most easily through the ilioinguinal approach.

The obturator vessels arise from the internal iliac vessels. They pass over the quadrilateral surface of the pelvis to the upper part of the obturator foramen to emerge from the obturator canal. The obturator artery divides into anterior branch, which supplies the obturator externus and the adjacent bone, and the posterior branch, which supplies the soft tissue of the acetabular fossa. The obturator nerve mimics the course and divisions of the obturator vessels. It is responsible for the sensory cutaneous innervation of the medial thigh and the motor innervation of the adductor muscles. Overlying these neurovascular structures is a reflected portion of the parietal peritoneum and the obturator internus muscle. These structures are fairly consistent, and they are anchored firmly by the obturator membrane as they pass through the obturator foramen. Occasionally, an aberrant vessel may traverse the pelvic brim that connects the external iliac vessels and the obturator vessels. Although the obturator vessels are usually safe during arthroscopic approaches to the hip, errant passes of an inferomedially directed arthroscopic cannula can be potentially injurious. Similarly, there is little risk to the obturator vessels and nerve during open approaches for primary hip arthroplasty, but one still has to be cautious around the transverse acetabular ligament, because distal branches of the obturator vessels can be injured here. In addition, anteroinferior screw placement or excess traction on the proximal femur during an anterior approach to the hip can also be potentially harmful.

The common femoral artery is the first branch of the external iliac artery, and it traverses just anteromedial to the hip capsule as it travels distally. It is at high risk for damage during both arthroscopic and open anterior approaches to the hip. In fact, the traditional anterior arthroscopic portal is approximately 3.5 cm from the femoral neurovascular bundle (Table 2-2). During total hip arthroplasty (THA), femoral vessel injury and femoral nerve palsy have been described as arising from the incorrect placement of retractors, which can occur with all approaches to the hip. However, because the artery is a large, fairly superficial, and therefore readily palpable vascular structure, its exact location should be routinely identified and thus easily avoided.

The profundus femoris artery, which is also known as the deep femoral artery, is the first branch of the common femoral artery. It penetrates posteriorly between the pectineus, the adductor longus, and the adductor brevis, lying behind the femoral artery and vein on the medial side of the femur. The profundus femoris artery gives rise to the lateral circumflex femoral artery 90% of the time and the medial circumflex femoral artery only 30% of the time. Injuries to the profundus femoris and its branches have been described during arthroplasty approaches to the hip, but they are fairly unusual. When they do occur, it is usually as a result of anteriorly placed deep retractors or during cement extrusion in this region.

The superior gluteal vessels are branches of the internal iliac artery (i.e., the posterior branches). The vessels, along with the gluteal nerve, traverse the posterior column of the acetabulum as they exit through the sciatic notch. They emerge superior to the piriformis and then terminate in the gluteus medius and gluteus minimus muscles. The inferior gluteal and internal pudendal vessels are also branches of the internal iliac artery (i.e., the anterior branches). They exit inferior and medial to the piriformis. The inferior gluteal vessels pass through the lower part of the greater sciatic foramen. The internal pudendal vessels exit the greater sciatic notch and then reenter the pelvis via the lesser sciatic notch. Erroneous posterior screw placement can cause the disruption of these structures. Palpation of the sciatic notch and the posterior column can help to prevent the placement of proud screws and further decrease the risk of injury. However, arthroscopic approaches in the safe zones as described by Byrd and colleagues pose very little risk to these neurovascular structures.

Four sets of arteries are responsible for the arterial blood supply to the femoral head: 1) the medial circumflex artery; 2) the lateral circumflex artery; 3) the medullary artery from the shaft of the femur; and 4) the artery of the ligamentum teres. The last one provides minimal if any contribution to the vascular integrity to the femoral head, although the vessel remains patent in approximately 20% of the adult population. The exact contribution of the medullary artery to the femoral head is unknown, but it is believed that this also plays a relatively minor role in vascularization.

Therefore, the vessel that supplies the majority of the arterial supply to the head is the medial circumflex femoral artery, with varying contributions from the lateral circumflex femoral artery. These vessels branch off at the base of the femoral neck and then ascend toward the femoral head via the posterolateral and posteroinferior synovial retinacular folds (Figure 2-4, A and B). It is believed that disruption at this level (e.g., by a femoral neck fracture) poses the greatest risk for avascular necrosis. The lateral synovial folds, which contain the terminal branches of the medial circumflex femoral artery, can also be injured as a result of aggressive arthroscopic dissection (Figure 2-5) or open approaches. Therefore, they should be routinely identified and protected during peripheral compartment arthroscopy and during open joint-preserving hip surgery.

Figure 2–4 Blood supply to the femoral head. A, Photograph that shows the posterosuperior proximal femur, with terminal branches of medial femoral circumflex artery (MFCA) perforating the femur. With permission from Gautier E, Ganz K, Krugel N, Gill T, Ganz R. Anatomy of the medial femoral circumflex artery and its surgical implications. J Bone Joint Surg Br. 2000;82(5):679-683. B, Illustration of posterosuperior proximal femur that shows the following: 1) the head of the femur; 2) the gluteus medius; 3) the deep branch of the MFCA; 4) the terminal branches of the MFCA; 5) the insertion of the tendon of the gluteus medius; 6) the insertion of the tendon of the piriformis; 7) the lesser trochanter with its nutrient vessels; 8) the trochanteric branch of the MFCA; 9) the branch of the first perforating artery; and 10) the trochanteric branches.

Figure 2–5 An arthroscopic photograph of the lateral retinacular fold, which contains the ascending vessels of medial circumflex femoral artery en route to the femoral head.

Neural Supply of the Hip

The sciatic nerve is the largest nerve in the body, and it is the main branch of the sacral plexus. It contains nerve branches primarily from L4 to S3. It consists of two main peripheral nerves contained within a single sheath: the tibial nerve (anterior division) and the common fibular or peroneal nerve (posterior division). These branches enter the gluteal region just inferior to the piriformis via the greater sciatic foramen. The sciatic nerve then descends in the plane between the superficial and deep group of gluteal region muscles, crossing the posterior surfaces first of the obturator internus and associated gemellus muscles and then of the quadratus femoris muscle. It lies just deep to the gluteus maximus at the midpoint between the ischial tuberosity and the greater trochanter. At the lower margin of the quadratus femoris muscle, the sciatic nerve enters the posterior thigh. The hip surgeon should be aware that, in a small subset of the population (i.e., 10% to 12%), the sciatic nerve can bifurcate proximal to the piriformis and pass through the piriformis.