Hip injuries are becoming an increasing problem in the population in general, and they have had a significant impact on the military with its young, soldier athletes. The physical and tactical training requirements of military personnel cause tremendous amounts of both force and torque to travel through the hip joint. These forces generated during military training are analogous to those experienced by high-level athletes during intense training and competition. The ground reaction forces transferred through the body from these activities have been linked to musculoskeletal injuries [1]. During normal walking and running, the hip experiences loads 6–8 times the body weight [2]. Recent advances in the understanding of injuries around the hip and their treatment have created the potential for individuals to return to an active lifestyle. The importance of hip pain evaluation and treatment has gained growing importance in the active, athletic population.

Several studies have been performed looking at the impact of various types of injuries it has on various populations [3–6]. The impact of these injuries results in significant strain on the patient and economy. The military population is required to perform duties and activities that place greater strain on the hip joint than the average population. These injuries, along with others, result in loss of man-hours, depletion of manpower for deployment, and increased health-care costs [7–10].

There are several conditions that cause hip pain, some of which are only now becoming better understood. Hip pain can be classified into intra-articular, ­extra-articular, or mimickers. Historically, many injuries would be treated with ­prolonged activity restriction and therapy; however, recent advancements in imaging and understanding of anatomy have enabled physicians to better diagnose and select new and emerging treatment options for these complex patients.

The bony anatomy of the hip joint is comprised of the acetabulum and the femoral head. They articulate as a true ball and socket joint allowing motion in multiple planes. A capsulolabral complex that includes the labrum, capsule, and ligaments provides stability and support during normal motion [11, 12]. While the bony anatomy and capsulolabral complex set the permitted motion of the hip joint, the surrounding musculature is responsible for providing the motion.

Knowledge of the muscular anatomy surrounding the hip is essential to understanding, diagnosing, and treating hip injuries. The iliopsoas and rectus femoris muscles act as the primary hip flexors with secondary flexors including the pectineus, sartorius, and tensor fascia lata muscles [13]. The gluteus maximus and hamstrings are responsible for hip extension. The three adductors (adductor longus, adductor brevis, and adductor magnus) with gracilis facilitate hip adduction, while the gluteus medius and minimus are responsible for abduction. Only the gluteus minimus and the tensor fascia lata in a minor way stimulate internal hip rotation. The muscles assisting external rotation include the gluteus maximus and multiple small external rotators (superior and inferior gemellus, obturator internus and externus, piriformis, and quadratus) .

Labral Tears

Tears of the labrum are one of the most common causes for subspecialty referral for hip pain. Degenerative tears of the labrum were first identified in dysplastic and arthritic hips as a result of abnormally increased loads about the labrum [14]. Labral tears have now been associated with multiple pathologic states that result in increased strain on the acetabular labrum, including trauma and femoroacetabular impingement [15–17]. As our understanding of hip injuries has evolved, we have recognized that activities that require repetitive pivoting, or twisting, and hip flexion result in an increased incidence of labral tears [18].

Patients will typically present with a gradual onset of pain in the anterior aspect of the hip or groin that may be related to a particular event. This pain is typically exacerbated by activity, especially sports, and prolonged sitting. Some patients describe mechanical symptoms associated with particular movements and may walk with a subtle Trendelenburg gait [15]. The most predictive physical exam finding is a positive impingement test, placing the hip in flexion, adduction, and internal rotation [15].

Diagnosis of labral tears is difficult, and patients often go extended periods of time before achieving a correct diagnosis [15, 19]. In asymptomatic active duty service members, labral tears can be found in over 80 % of the individuals with magnetic resonance imaging [20]. With such a large number of labral tears present in subjects without symptoms of hip pain, it seems reasonable to conclude that the prevalence of hip labral tears would only increase with the appearance of symptoms. However, studies have demonstrated that 22 % of the athletes with groin pain and just over half of individuals with mechanical symptoms have labral tears on advanced imaging or arthroscopy [21–23]. Clinically, it is difficult to determine whether a labral tear is the cause of hip dysfunction or if it is present simply as a distracter to the true underlying pathology .

Femoroacetabular Impingement

Femoroacetabular impingement (FAI) typically presents as pain located in the groin . This pain routinely increases with prolonged sitting and activity, especially those activities involving repetitive hip flexion and cutting movements [24]. Most patients describe a gradual onset of pain and increasing limitations. FAI is the result of an anatomic variation of the acetabulum, femoral head-neck junction, or both that causes abnormal contact forces [25, 26]. In a young, active, military population, the prevalence of radiographic evidence of FAI in those who present with complaints of hip pain is over 85 % [27].

Patients with hip impingement have reproduction of pain when the hip is brought into flexion, adduction, and internal rotation. They typically have internal rotation of less than 20° and when placed into a figure-of-four position, the affected side will have an increase in the distance from the table to the lateral side of the knee compared to the asymptomatic contralateral side.

FAI is not just a primary cause for injury to the hip in active duty service members. The presence of this anatomic variation may be associated with other problems for the hip as it is subjected to the rigors of military service. Studies evaluating active duty patients with femoral neck stress fractures have found that greater than 50 % of the individuals had at least one radiographic finding consistent with FAI [28, 29]. It may also cause athletic pubalgia and sports hernias in high-performance athletes as a result of the abnormal motion in the hemi-pelvis with incidence ranging from 15 to 40 % [30, 31]. As this information was collected from nonmilitary, high-performance athletes, the translation of this to the military population may represent an under- or overestimate. More research is needed regarding this topic to determine the prevalence of these types of injuries in a military population .

Osteoarthritis

Hip arthritis is the result of progressive joint degeneration that results in significant pain and dysfunction. It has been reported that arthritis affects over 27 million Americans with a direct yearly cost ranging from $2650 to 5700 per person [32, 33]. The majority of these costs are productivity-based secondary to work time lost. The prevalence of hip osteoarthritis (OA) in the general population ranges from 2.7 to 25 % [34, 35]. OA of the hip results in associated comorbidities and a higher mortality rate when compared to non-arthritic individuals [36, 37].

A patient with an arthritic hip will complain of the gradual, usually atraumatic onset of pain. The hip is painful and stiff in the morning with improvement in symptoms after beginning activity. This pain worsens again in the afternoon and with periods of prolonged standing or activity. Typical radiographic findings include joint space narrowing, subchondral sclerosis and cysts, and osteophyte formation. Despite the uniformly good results of total joint arthroplasty for hip arthritis, it results in significant lifestyle and activity limitations for the relatively young military members treated in this way.

The incidence of hip arthritis in the military may be lower than in the civilian population, estimated at 35/100,000 person-years compared to 56–88/100,000 person-years, respectively [7]. Branch of service (particularly Army, Navy, and Marines), sex (female), age (> 40), and race (black) were associated with increased adjusted incidence rate ratios for the development of arthritis [7]. While the overall incidence of hip OA in the military is lower than the general population, this is likely secondary to the large percentage of young individuals that make up the military. An incidence of 140 per 100,000 person-years in service members over 40 years old is much larger than the incidence in general population [7].

Stress Fractures

Stress fractures are another common cause of hip pain in active duty service members. These injuries can occur in the femoral neck, acetabulum, or pubic rami. Stress-related injuries are not unique to a military population but do occur at a higher rate given the requirements of rigorous training particularly among initial entry trainees. Stress fractures of all anatomic regions have been reported to occur in up to 30 % of the trainees, with pelvic or acetabular stress fractures representing the smallest fraction between 1–10 % of all stress injuries [38–40]. Stress fractures result from a sudden increase in loads placed on healthy or compromised bone. The repetitive stress causes a normal response of bone remodeling with resorption and new bone formation. There is an imbalance in the normal remodeling process that occurs resulting in the reparative process being overwhelmed. Nutrition, endocrine, and other mechanical factors can significantly affect this process.

Secondary to the nature of military service and entry training, these other factors play a significant role in increasing the prevalence of this injury. The prevalence of femoral neck stress fractures in military trainees has been reported at 12 in 10,000 recruits [41]. It has been noted that 40 % of the individuals who sustain femoral neck stress fractures during military training were medically discharged from service [41].