Introduction

Due to the high loads received by the hip joint during walking and running, the hip and its surrounding structures are prone to numerous pathologies with varying degrees of severity. In a population of participants 65 years and older, near 20% had self-reported hip pain that consequently impacted their overall health such that without these symptoms, their average general health status would be similar to people under 65 years of age. Hip pathology is especially prominent in the athletic population due to the additional forces experienced by the hip during high-impact sports-related activities. According to a recent study by Kerbel et al. athletes participating in soccer, ice hockey, and running are among the most at risk for hip injuries and disorders that can occur as a result of acute trauma or overuse or as gradual onsets.

The more widely researched conditions, and much of our understanding of hip pathologies, are associated with intra-articular pathologies and arthritic disorders. There are, however, many abnormalities that contribute to significant hip pain, which exist outside these types of lesions. An important part of treating patients with hip pain is to consider the local and surrounding factors, including nervous, osteoligamentous, tendinous, and muscular structures. Properly diagnosing and understanding hip pathology can be difficult because of its complex nature and the tendency for conditions to be concomitant and symptoms to overlap, but it is crucial nonetheless. This chapter will focus on the recognition and management of extra-articular hip pathologies.

Structural and Functional Anatomy

The hip joint plays a key role in movement, stability in facilitating weight-bearing, and dynamic support for the body. It is composed of a system of interactions between the skeleton and the surrounding soft connective tissue. A ball-and-socket joint is formed by the articulation of the femoral head and the acetabulum, which is formed by the ischium, ilium, and pubis bones. The acetabulum encompasses the entire head of the femur allowing for movement along three major axes. For support, the hip labrum and a group of superficial and deep ligaments form a fibrous capsulolabral structure, which restricts translation at the femoroacetabular articulation while still allowing for complex rotation and planar movements. ^, The movements permitted by the hip are flexion, extension, adduction, abduction, external rotation, and internal rotation. Some of the major muscles that provide these movements include the iliopsoas (hip flexion), the hamstring muscles (extension), the adductor muscles (adduction), and the gluteus maximus (extension and external rotation), minimus (abduction and internal rotation), and medius (abduction) muscles. Overall, there are 22 muscles providing stability and the forces necessary for movement, which can be divided by their anatomic positioning or their actions.

Based on the pathways of nerves through the hip and thigh regions, most intra-articular pathologies elicit pain that radiates to the anterior and medial hip (i.e., groin), whereas extra-articular conditions typically cause pain rotating to the posterior and lateral aspect of the hip. However, intra-articular pathology can certainly cause posterior and/or laterally based hip pain, and clinicians must be aware of atypical sources of hip pain when evaluating patients, particularly females.

Key anatomic structures within and around the hip and groin area include several bursae, which are fluid-filled sacs that function to cushion the joint area. These bursae include the trochanteric bursa on the lateral side of the hip as well as the iliopsoas bursa, found in the anteromedial aspect of the hip and thigh. Inflammation of these sacs, called bursitis, can be caused by several different conditions and represents a common extra-articular pathology that is a source of hip pain. In addition, injuries to the thigh muscles that generate hip movement, including strains and tears, as well as lumbar spine conditions, can often be causes of the referred hip pain. Furthermore, stress fractures about the hip and dysfunction in the surrounding joints, such as the sacroiliac (SI) joint, are all among the various nonarthritic hip pathologies that lead to hip pain. ^, Table 10.1 provides a summary of the nonarthritic and extra-articular hip pathologies to be discussed in the subsequent sections of this chapter, which are organized by anatomic location.

Anterior Hip Pathology

Anterior hip pain tends to localize to the anteromedial thigh and can result from various nonarthritic and extra-articular pathologies within the surrounding areas. Internal snapping hip is a condition found more frequently in athletes and female patients that results in a deep pain and clicking sensation within the anterior groin during hip flexion and can be caused by either acute trauma or overuse. It is characterized by the iliopsoas tendon snapping over the iliopectineal eminence, femoral head, or the lesser trochanter due to repetitive flexion and external rotation. ^, Frequently associated with snapping hip, although relatively uncommon, is iliopsoas tendonitis and iliopsoas bursitis. As the affected regions are so close, these conditions are interrelated due to the observation that inflammation of one can result in inflammation of the other. The key symptoms of iliopsoas tendonitis are anterior groin pain while climbing stairs and getting in and out of a car, and clinical indications include anterior groin pain during active hip flexion or passive hip extension. The most frequent source of this condition appears to be after a total hip arthroplasty, although the exact cause is inconclusive. The psoas and iliac muscles join to form a strong flexor and weak external rotator of the hip, and the iliopsoas tendon inserts into the lesser trochanter and is bordered by the iliopsoas bursa overlying the hip joint capsule. Along the tendon tract, prior to its insertion at the lesser trochanter, is the anterior edge of the acetabulum, the convex surface of the femoral head, and the overlying anterior capsule. As stated by O’Sullivan et al. any deviation from this system can generate inflammation and thus tendonitis, which is one plausible explanation for iliopsoas tendonitis occurring from mechanical faults during a total hip arthroplasty. The authors also suggest that anatomic variations and osteophytes along the anterior femoral neck can be related factors that may cause this condition. In addition to the iliopsoas, the rectus femoris is a muscle responsible for hip flexion and can be a cause of anterior hip pain. Notably in 2018, Kaya et al. studied the impact of extra-articular pathologies in a cohort of patients with anterior hip pain and found that tendinosis of the direct head of the rectus femoris muscle was common.

In a condition known as athletic pubalgia, also referred to as “inguinal disruption” or “sports hernia,” the pubic symphysis is disrupted producing activity-related lower abdominal, deep inguinal, and/or groin pain. ^, This condition is a common extra-articular referral source of anterior hip pain and most often occurs as a result of injuries in athletes participating in activities that require rapid changes in direction and speed, specifically twisting at the waist while running and sideways movement. ^{, ,} As described by Battaglia et al. the rectus abdominis and adductor longus tendons merge at the pubic symphysis and form an aponeurotic plate. The tearing of these tissues and the adjacent oblique aponeuroses, as well as the widening and erosion of the pubic symphysis, called osteitis pubis, produces athletic pubalgia. Furthermore, as summarized by Le and colleagues, other proposed mechanisms leading to this condition include ilioinguinal or genitofemoral nerve entrapment, musculotendinous strain of the adductor, and other various involvements of the adductor muscles. Overall, the authors concluded that it is likely a multifactorial process caused by a combination of multiple anatomic and physiologic disruptions. Overlapping with athletic pubalgia is osteitis pubis, which is a chronic inflammatory state of the pubic symphysis and the surrounding soft tissue causing debilitating anterior and medial groin pain and is common in athletes. This pain is exacerbated by rapid hip flexion from an extended position, such as kicking, or standing from a seated position. It can result from microtrauma and altered biomechanics leading to instability, as seen in overuse injuries, caused by repetitive muscle strains and stress forces on the pubis symphysis from the rectus abdominis and adductor muscles that act antagonistically. Distinguishing between athletic pubalgia and osteitis pubis can be difficult because of their many similarities, but as summarized by Dirkx and Vitale, osteitis pubis can typically be separated by pain that is prompted by direct palpation over the pubis symphysis and by direct pressure laterally on the ramus. This condition may also be referred to as pubic symphysis stress injury owing to the overuse nature of the injury.

In sports medicine, stress fractures are a common injury that are more often experienced by female patients, and a portion of these stress fractures occur in the hip region. Stress fractures that cause anteromedial hip and groin pain most frequently occur at the femoral neck and pubic rami. As with all stress fractures, these are typically overuse injuries that are caused by repeated strain without proper recovery time. More specifically, as indicated by Paluska et al. they can originate from abnormal forces on a normal bone (fatigue fracture) or from normal forces on an abnormal bone (insufficiency fracture). Femoral neck stress fractures cause pain that worsens with weight-bearing and motion, particularly internal rotation, and are common in runners. There are two types of femoral neck stress fractures: tension type and compression type. Tension-type fractures are located on the superolateral aspect of the neck and have the highest risk for a complete fracture and other complications. Compression-type fractures involve the inferomedial aspect of the femoral neck, may be managed more conservatively, and tend to be common in younger athletes. Femoral neck stress fractures can be detected by pain, which is elicited with activity or at night. Physical signs and symptoms, including swelling, may occur but are less common. These fractures must be recognized and quickly treated to avoid the disastrous outcome of a displaced femoral neck fracture. A pubic ramus stress fracture is also a low-risk fracture that occurs at the junction of the ischium and inferior pubic ramus and is most likely caused by excessive contraction of the muscles that attach at the pubis. ^, This type of stress fracture is characterized by a gradual onset and increase in pain that is worsened by activity and can lead to the pain persisting at rest. Furthermore, point tenderness over the area is common and pain with standing on the fractured leg or the inability to stand unsupported on the affected side is frequently associated with stress fractures of the pubic rami.

Anterior hip pain can also be a result of several neuropathies, with femoral nerve pathology being the most often described cause. In 2013, Martinoli and colleagues provided descriptions of peripheral nerve paths based on diagnostic imaging in studying chronic hip pain and disability caused by neuropathies. The authors noted that neuropathic conditions occur as a result of mechanical or dynamic compression of a segment of a nerve within an osteofibrous tunnel, a fibrous structure opening, or a passageway close to a ligament or muscle. The femoral nerve has both motor and sensory components and is the largest branch of the lumbar plexus, originating from L2 to L4. The nerve continues down through the psoas muscle to its lower lateral border where it passes down the iliacus muscle, exits the pelvis beneath the inguinal ligament, crosses a rigid osteofibrous tunnel next to the iliopsoas tendon, and then branches in the thigh. Femoral neuropathy causes weakness in hip flexion and knee extension related to the iliopsoas and quadriceps femoris, respectively. Less studied are neuropathies of the obturator, iliolingual, and genitofemoral nerves as causes of anterior hip pain. However, their mechanisms of producing pain can be similar to the aforementioned description provided by Martinoli et al. and are similarly related to their respective nerve tracts, with various causes typically revolved around concomitant injuries and postoperative complications.

Lateral Hip Pathology

Greater trochanteric bursitis (GTB) is a condition characterized by pain over the lateral aspect of the hip that may radiate to the lateral thigh and/or lower buttocks. It is typically an aching, chronic pain triggered by various activities including prolonged standing, running, and external rotation and abduction of the hip. It can also present as a sharp pain that is directly related to pressure over the greater trochanteric bursa. This condition and its related symptoms can be caused by irritation of the bursae surrounding the greater trochanter, particularly the three constant bursae: the gluteus minimus bursa, the subgluteus medius bursa, and the subgluteus maximus bursa. Overall, GTB can be caused by acute trauma or, more frequently, repeated microtrauma related to active use of the muscles that insert on the greater trochanter, which cause degenerative changes in tendons, muscles, and fibrous tissue. Interestingly, Zibis and colleagues reported in 2018 that females were more likely to experience GTB than males. Related to GTB is the greater trochanteric pain syndrome (GTPS), which refers more generally to pain in the lateral hip caused by various structures, in addition to bursae, such as tendons. Associated conditions include gluteus medius and minimus tendinopathy, gluteal tears, iliotibial band (ITB) syndrome, and external snapping hip. ^, In 2013, Long et al. reported that in a population of 877 patients with GTPS, 49.9% had gluteal tendinopathy, 28.5% had ITB syndrome characterized by thickening, 20.2% had trochanteric bursitis, and 0.5% had gluteal tears.

Gluteal tendinopathy refers to tendinopathy of the gluteus medius and/or minimus tendons resulting in local tenderness and pain over the greater trochanter that may radiate into the lateral thigh to the level of the knee. ^, The pain may be described as a burning or deep, dull ache that is worsened by hip abduction, prolonged sitting, side-lying positons, and climbing stairs. The most significant factors in the pathomechanics of this disorder include excessive static and dynamic hip adduction, which leads to a compressive tendon loading, combined with interactions between joint position and bone and muscle factors. The most damaging effects on the tendon are high tensile loads and excessive compression, which results in a net catabolic effect on the tendon.

As gluteal tendinopathy can coexist with other disorders and mimic the symptoms of other causes of lateral hip pain, Grimaldi et al. performed a study in 2016 to determine valid clinical tests for the diagnosis gluteal tendinopathy. The authors found that patients who experienced both pain on palpation of the greater trochanter (80% sensitivity) and lateral hip pain within 30 s of single-leg standing (100% specificity) were likely to have gluteal tendinopathy. Combining these two types of clinical tests provided the greatest accuracy in properly diagnosing gluteal tendinopathy. Gluteal tendon tears have also been reported as a cause of lateral hip pathology and have similar symptoms as the aforementioned conditions. They more commonly occur from degenerative processes, similar to the development of a rotator cuff tear in the shoulder, but can happen after trauma and result in an aching pain that radiates down the outside of the thigh with tenderness over the greater trochanter, a positive result of Trendelenburg test, and/or pain on resisted external rotation or abduction. Although less common, gluteus medius muscle (as opposed to tendon) tears can also cause lateral hip pain.

The ITB is a key component in extension, abduction, and lateral rotation of the hip. ITB syndrome is a common overuse injury that predominately causes pain in the lateral knee, but it can sometimes trigger lateral hip pain with or without snapping during activity. ^, It is also common at the lateral hip for abnormal ITB anatomy and kinematics to be associated with pain, tenderness, or weakness. Similarly, proximal ITB thickening at the greater trochanter level has been frequently observed in patients with lateral hip pain correlated to GTPS. Closely related to ITB syndrome is the external snapping hip syndrome, which occurs with the forward abrupt movement of the ITB over the greater trochanter with hip motion, particularly hip flexion or extension. ^, Often accompanying the sudden snapping, or the translocation of the ITB, is pain caused by repeated rubbing that may reflect tendonitis or bursitis. It appears that hip weakness may cause increasing friction between the ITB and the greater trochanter, and ITB thickening contributes to the snapping hip syndrome. Rarely, external snapping hip can occur via a different mechanism, when the distal gluteus maximus rolls over the greater trochanter, or from the passive movement of an adducted and internally rotated hip to flexion and external rotation.

Peripheral neuropathy, particularly that involves the lateral femoral cutaneous and the iliohypogastric nerves, can also be a cause of lateral hip pain. Entrapment of the lateral femoral cutaneous nerve at the ilioinguinal ligament is called meralgia paresthetica and typically causes burning pain, muscle aches, paresthesias, and sensory loss within the tract of the lateral femoral cutaneous nerve, over the lateral or anterolateral aspect of the thigh. ^{, ,} According to Pearce, it is distinguished from radiculopathy, as motor strength and the knee jerk reflex are preserved. Symptoms may be triggered by prolonged standing or sitting with the thigh extended due to the increased tension and angulation of the nerve. On the contrary, symptoms are improved with flexion of the thigh on the pelvis as this decreases the forces acting on the nerve. The lateral femoral cutaneous nerve originates from L2 and L3 of the lumbar spine and emerges at the lateral border of the psoas major, crosses the iliacus to the anterior superior iliac spine, and then passes under the inguinal ligament and over the sartorius muscle, where it then enters the thigh. Despite the reported variations in the course of the nerve after exiting the pelvis, the compression of the nerve observed in meralgia paresthetica most often occurs as it exits. This condition can have a diverse set of causes that have been previously summarized by Cheatham and colleagues in a literature review. They include mechanical factors, metabolic factors, and postsurgical complications after hip replacement and spine surgery. Iliohypogastric neuropathy is characterized by sensory abnormalities along the superolateral gluteal region, directly posterior to the greater trochanter, and tension occurs during extension and adduction of the hip, similar to the lateral femoral cutaneous nerve, with additional trunk extension and lateral bending. Symptoms of iliohypogastric neuropathy are similar to those of other neuropathies and can produce pain that mimics GTPS.

Posterior Hip Pathology

A common source of posterior hip pain is referred pain from pathologies of the SI joint. The SI joint and its intricate ligamentous system play a main role in stability and limiting motion in all planes of movement. The muscles that support the SI joint help deliver regional forces to the pelvic bones. More specifically, the SI joint functions as a load-transferring junction between the spine and the pelvis, transmitting and dissipating truncal loads to the lower extremities while limiting rotation. ^, Dysfunction of this joint is not entirely understood but is described as an anatomic disruption resulting in abnormal positioning and movement (hyper- or hypomobility) of the SI joint structures, often associated with inflammation. ^, This pain is most commonly located near the posterior superior iliac spine and radiates to the buttock or thigh. The mechanisms leading to SI joint dysfunction are typically idiopathic and can occur both acutely and gradually with cumulative trauma.

Another source of posterior hip pathology is sacral stress fractures, which can be either insufficiency or fatigue fractures. Insufficiency fractures are often caused by osteoporosis, while fatigue fractures are more common in athletes who participate in prolonged periods of intense training. In describing the pathophysiology of this condition, Urits et al. explained that the SI joint normally relieves the torsional stress that is created around the sacrum during a normal gait as the lower extremities alternate between flexion and extension. However, pathology of the SI joint causes this mechanical load to be transferred to the sacrum, which becomes prone to injury in cases where it is unable to withstand the offloaded stress. The symptoms of sacral stress fracture tend to overlap with those of SI joint dysfunction and other pathologies, making it a difficult condition to properly diagnose. Sacral stress fractures often present with severe buttock, low-back, hip, groin, and/or pelvic pain with limited low-back range of motion and tenderness to palpation over the sacrum, which is reportedly the trademark physical finding of this condition.

Attached at the posterior aspect of the hip are the hamstring muscles (biceps femoris, semimembranosus, and semitendinosus) responsible for extension, as well as the internal and external rotators. Tendinopathy and strains of these structures generally lead to posterior hip pain and can fall under posterior hip pathology. Specific injuries involving the hamstring are known to result in posterior hip pain, including proximal hamstring ruptures and tendinopathy. ^, Proximal hamstring injuries are among the most common muscle and tendon injuries seen in athletes. Proximal hamstring ruptures are typically caused via noncontact, high-energy injuries involving a rapid forceful hip flexion with the knee in extension and can result in one- to three-tendon avulsions. A rupture may also occur after multiple repetitive incidents of hamstring strains or tendonitis, essentially the result of cumulative microtrauma. ^, Symptoms of a proximal hamstring rupture include a sudden onset of pain with a reported tearing or popping sensation near the ischial tuberosity region with associated ecchymoses down the thigh. Furthermore, walking and sitting can be challenging due to tenderness and discomfort, and feelings of instability and muscle weakness may occur, as well as sciatica symptoms may occur in some cases. Similar but less severe is proximal hamstring tendinopathy, also referred to as “proximal hamstring syndrome” or a “recurrent hamstring tear,” which causes deep pain at the ischial tuberosity with or without sciatica radiating to the posterior thigh, accompanied by difficulty sitting for an extended period. In addition, pain may be worsened during walking when the hamstring muscles control knee extension during hip flexion, as in the swing phase of gait. Activity-related abrupt tension of the hamstring or long-term chronic strain can also lead to an avulsion fracture of the ischial tuberosity, the starting point of the hamstring muscle, in adolescent athletes. The symptoms of this injury include the sudden onset of pain in the posterior thigh and hip, an abnormal gait, and an occasional “pop” sensation in the hip, with physical presentation similar to that of the aforementioned posterior hip pathologies.

The piriformis muscle, primarily responsible for external rotation of the hip, runs laterally from the frontal surface of the sacrum and inserts at the greater trochanter. Piriformis syndrome is a condition caused by irritation of this muscle resulting in pain that radiates from the lower back to the buttock and thigh, mimicking sciatica symptoms because of its close proximity to the sciatic nerve. As outlined by Hicks et al. indications of piriformis syndrome include chronic pain in the buttock and hip area, pain when getting out of bed, inability to sit for an extended period, and pain that is worsened by hip movements. These symptoms can arise from an acute injury by forceful internal rotation of the hip, altered biomechanics due to a chronic condition, or excessive strain on the muscle from sports-related activities. ^, Several factors, including anatomic variations, may also increase the risk of this condition and compression on the sciatic nerve.

Other sources of posterior hip pain include disorders related to the ischiofemoral space (between the ischial tuberosity and lesser trochanter) and the quadratus femoris space (between the hamstring origin and lesser trochanter). Ischiofemoral impingement is a condition characterized by posterior hip pain and quadratus femoris abnormalities caused by the narrowing of the aforementioned spaces. ^, The resulting pain is localized to the deep gluteal region and is typically greatest with hip extension and external rotation, particularly a worsening of pain during running or while taking larger steps due to the added narrowing between the ischial tuberosity and the lesser trochanter during extension. Pathologies of the quadratus femoris muscle that may occur due to the impingement include edema, tears, or muscle atrophy. ^, Furthermore, symptoms related to sciatica are common as a result of the narrowing, and there can be tenderness directly over the ischiofemoral space. Although nonspecific in relation to other posterior hip pathologies, pain may occur when in a seated position because of the posterior pressure and a snapping sensation may be felt while walking with an associated antalgic gait. Interestingly, ischiofemoral impingement is considerably more common in females and occasionally occurs bilaterally. Multiple studies have attempted to determine the cause of ischiofemoral impingement and quadratus femoris muscle abnormalities, but an exact cause has not been established. For example, a greater femoral neck-shaft angle has been previously reported as a factor leading to ischiofemoral or quadratus femoris space narrowing, but Gardner et al. found that there is no correlation. On the other hand, Kheterpal and colleagues reported a higher incidence of abductor tears and abductor muscle atrophy in patients with ischiofemoral impingement compared with controls, indicating its potential pathophysiologic role.

Encompassing many of the previously mentioned conditions is a more recently recognized disorder called deep gluteal syndrome, which is caused by sciatic or pudendal nerve compression (neuropathy) as a result of pelvic lesions. In a systematic review by Park et al. the authors noted that this syndrome is generally characterized by intermittent or persistent pain and/or dysaesthesia in the posterior hip, buttock, or thigh region in which pain is increased with activity involving flexion of the hip, such as walking or sitting. Other indications of deep gluteal syndrome include sustained external rotation when in a supine position and tenderness throughout the region, noting that the precise location of tenderness may suggest the specific pathology of deep gluteal syndrome that is causing the pain and other symptoms. Based on the path of the sciatic nerve, its kinematic behavior, and tensions relative to knee and hip positioning, the authors denoted four main syndromes that impair the sciatic nerve, resulting in deep gluteal syndrome. The four conditions are piriformis syndrome, gemelli-obturator internus syndrome, ischiofemoral impingement syndrome, and proximal hamstring syndrome. The gemelli-obturator internus syndrome, the only disorder not previously discussed, is sciatic entrapment located at the gemelli-obturator internus complex. The nerve is attached by connective tissue and runs posterior to the complex and the quadratus femoris muscle after passing the piriformis. In the variable paths of the piriformis prior to its insertion on the greater trochanter, impingement of the piriformis on the sciatic nerve can occur, especially during internal rotation, in the joining of the piriformis to differing tendons related to the gemelli-obturator internus complex. Overall, posterior hip pain can be caused by impaired functioning of the structures associated with the conditions that lead to nerve entrapment, leading to deep gluteal syndrome.

Summary

Numerous nonarthritic and extra-articular pathologies exist that affect the anterior, lateral, and posterior regions of the hip. The nature of these conditions includes all structures of the hip’s complex anatomy, involving interactions between the nervous, muscular, and skeletal systems. Often times, multiple conditions occur simultaneously leading to frequent misdiagnoses and insufficient treatment management. Furthermore, separate pathologies can have overlapping symptoms, which emphasizes the importance of obtaining a wide range of understanding of various abnormalities in the structures of the hip. Ongoing research in imaging, testing, and treatment options allows clinicians to properly and adequately provide patients with the ability to recover and return to uninterrupted daily living.