In this review, the recent literature evaluating the anatomic considerations, etiology, and management options for athletes with hip instability are investigated. Studies on the osseous, chondrolabral capsuloligamentous, and dynamic muscular contributions to hip stability are highlighted. Microinstability, iatrogenic instability, and femoroacetabular impingement-induced instability are discussed with a focus on demographic and outcomes research in athletes. Surgical techniques including both open and arthroscopic approaches are additionally evaluated.
Key points
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Hip instability may be a source of significant hip dysfunction and pain.
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The etiology of hip instability may be traumatic or atraumatic, including microinstability, iatrogenic instability, and femoroacetabular impingement-induced instability.
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Surgical options for the management of instability include open and arthroscopic approaches and are targeted toward correction of the individual pathology.
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Early results suggest significant improvement in patient reported outcomes after surgical management of hip instability.
Introduction
Hip instability is becoming increasingly recognized as a source of significant pain and functional impairment. Historically, instability was thought to occur secondary to direct trauma, although more recently atraumatic causes have been described. Atraumatic instability may be further defined as macroinstability, microinstability, iatrogenic instability, or femoroacetabular impingement (FAI)-induced instability. Treatment encompasses both operative and nonoperative treatment modalities directed toward the underlying etiology. In this review, we outline the anatomic factors that contribute to hip stability and summarize the clinical presentation of hip instability in athletes. We particularly evaluate the recent literature regarding traumatic and atraumatic instability, focusing on surgical treatment options and outcomes.
Anatomic considerations for hip stability
The normal native hip joint is an intrinsically stable structure owing to contributions from both static and dynamic stabilizers. Static stabilizers include osseous, chondrolabral, and ligamentous structures, whereas dynamic stabilization is provided by the surrounding musculature ( Fig. 1 ). The osseous morphology includes a highly congruent acetabulum and femoral head. The acetabulum provides an average of 170° of head coverage and is oriented with 48° of coronal abduction and 21° of anteversion, whereas the femur has an average of 10° of anteversion and a neck-shaft angle of 130°. Variations in the osseous development of the acetabulum or femur, including undercoverage, overcoverage, or changes in orientation, may significantly affect hip stability. Depending on the degree of acetabular coverage, the orientation of the acetabulum and/or proximal femur and the articular geometry may affect the osseous contribution to the static stabilizing structures.
The chondrolabral junction has recently been highlighted as a significant contributor to hip stability through the suction seal. Ferguson and colleagues initially described the importance of the labrum in maintaining a pressurized fluid environment in the hip joint. Cadet and colleagues further described the negative pressure effect of the fluid seal, showing that the suction seal was lost in conditions of labral tear and resection. Suppauksorn and colleagues recently showed restoration of the suction seal after labral repair. Subsequent biomechanical studies have validated the contributions of the fluid seal to hip stability, demonstrating increased femoral head rotation and translation with loss of the chondrolabral suction seal.
Other major contributors to hip stability include the capsuloligamentous structures surrounding the hip joint. The iliofemoral ligament is the largest and strongest ligament, resisting anterior hip translation as well as rotation. , The ischiofemoral and pubofemoral ligaments also provide significant resistance to internal and external rotation, respectively. Other smaller ligamentous structures, including the zona orbicularis and ligamentum teres, additionally provide a component of stability to the hip joint. , Together, these static structures work with the dynamic musculature to control hip stability throughout motion.
Clinical evaluation
An evaluation of hip instability encompasses the typical sequence of history, physical examination, and imaging workup. Particular questioning should be performed to elucidate the nature of pain and mechanical symptoms, as well as the avoidance of particular motions. Patients may describe mechanical symptoms or apprehension in certain positions. A thorough assessment of any prior hip surgery with operative data should be performed. The medical history, particularly any familial collagen disorders, should be obtained. The physical examination should include an assessment of the Beighton criteria, posterior impingement with extension, the hip dial test, and the axial distraction test. Imaging usually involves a standard hip series of radiographs (anteroposterior pelvis, false profile view, and Dunn lateral). Specific radiographic findings, including the vacuum sign on splits radiographs and the femoral head cliff sign, have been described as diagnostic tools for instability. MRI with or without MR arthrography may additionally be used to better assess soft tissue pathology, particularly of the labrum and capsuloligamentous structures. A 3-dimensional computed tomography scan may also be obtained to identify any relevant bony morphology.
Discussion
Hip Macroinstability
Hip macroinstability or traumatic hip instability is the result of a relatively high-energy impact to the hip joint resulting in gross hip instability. The spectrum of injury may include hip subluxation to dislocation along with damage to the surrounding capsuloligamentous structures, labrum, and cartilage ( Fig. 2 ). , Simultaneous fractures may also occur in the acetabulum, femur, or both. The mechanism of injury in athletes is typically a direct posterior force to a flexed and adducted hip, most commonly occurring in sports such as football, rugby, soccer, and gymnastics. Although posterior dislocations occur most commonly, anterior dislocations have also been described.
In cases of acute dislocation, management involves urgent reduction with a subsequent computed tomography scan to confirm a concentric reduction and rule out any concomitant fractures or intra-articular loose bodies. Isolated dislocations with confirmed concentric reduction, although rare, may be treated nonoperatively. Dislocations with acetabular fractures of the posterior wall are typically examined for instability under anesthesia, with fixation recommended in cases of posterior instability. Large acetabular fractures, femoral fractures involving the weight-bearing region of the head or neck, or intra-articular loose bodies require surgical intervention for fixation. There should be a high suspicion for loose bodies after hip dislocation; 1 study involving patients who underwent arthroscopy found loose bodies in 92% of patients, with 78% of those having negative imaging preoperatively. Postinjury and postoperative weight-bearing restrictions and rehabilitation vary depending on pathology, although they usually include initial hip precautions to prevent recurrent dislocation. Additionally, MRI should be considered 6 weeks after injury to rule out the development of avascular necrosis.
Although recurrence rates are low after an initial traumatic dislocation, there is a high rate of long-term sequelae associated with injury. Dreinhofer and colleagues reported on 50 hip dislocations all reduced within 3 hours of injury and still found complication rates of 4% for osteonecrosis, 18% for post-traumatic arthritis, and 8% for heterotopic ossification, with 38% of patients reporting fair or poor results. In a similar study, Sahin and colleagues found a 16.1% rate of post-traumatic arthritis and a 9.6% rate of osteonecrosis with 71% of patients reporting very good to medium satisfaction. Bhandari and colleagues reported on 109 hip dislocations with acetabular fractures managed operatively and found anatomic reduction was essential to limit post-traumatic arthritis, but still found a rate of post-traumatic arthritis of 25.5%, even with anatomic reduction. Reported factors that may influence prognosis include the direction of the dislocation, the extent of the injury, and the time to reduction. A recognition of the relatively high rates of these sequelae are important for discussing overall outcomes and return to sport goals in the athletic population.
Recurrent dislocation, subluxation, or continued symptomatic instability after trauma, although rare, requires a further workup for an assessment of any soft tissue pathology. A traumatic injury to the ligamentous capsule, labrum, or cartilage has been described previously. , Although the initial treatment involves formal therapy for dynamic stabilization, refractory instability may necessitate a surgical intervention. Surgical management may be approached via an open or arthroscopic route to address labral repair, capsular repair, plication, or reconstruction, or cartilage restoration. , The literature regarding the outcomes of surgical intervention in recurrent traumatic instability is limited to a few case reports and thus further research is needed to delineate specific results.
Hip Microinstability
The concept of hip microinstability emerged more recently as a clinical entity characterized by extraphysiologic motion resulting in hip pain or dysfunction with or without gross symptomatic instability. , A diagnosis of instability may be challenging, because there are no objective criteria that are universally accepted for microinstability. In contrast with traumatic instability, patients with microinstability typically do not define a discrete mechanism of injury. Provocative examination maneuvers, however, may be similarly present in both traumatic instability and microinstability.
The pathology is thought to be due to repetitive microtrauma in the presence of underlying osseous and soft tissue abnormalities. Numerous etiologies have been described causing microinstability, typically classified based on the following 6 groups: osseous abnormalities, connective tissue disorders, traumatic injuries, repetitive microtrauma, iatrogenic injury, and idiopathic ( Fig. 3 ). Common bony abnormalities occur with developmental dysplasia of the hip, including a flattened aspherical femoral head, a shortened femoral beck, a shallow acetabulum, loss of anterolateral coverage, and excessive anteversion of the acetabulum and femur. , Connective tissue disorders such as Ehlers–Danlos syndrome, Marfan syndrome, and Down syndrome, as well as joint hypermobility syndrome, can also contribute to microinstability. , , Both traumatic and microtraumatic injuries may occur in athletes from a direct event or repetitive impact, respectively. Iatrogenic sources are further discussed elsewhere in this article. The identification of the underlying pathology, when present, is crucial for appropriate management.
The management of microinstability includes both operative and nonoperative treatments. As a newer diagnosis, there are no primary publications regarding the nonoperative management of microinstability. Nonetheless, most experts agree that traditional nonoperative modalities, such as activity modification, physical therapy, anti-inflammatory medications, and injections, may be used as a first-line treatment or before surgery for in-season athletes. , Targeted neuromuscular rehabilitation with sport-specific strengthening may address any functional deficits or allow satisfactory compensation. In refractory cases or those with severe pathology, open or arthroscopic surgery may be indicated.
The surgical treatment options are directed toward correcting the underlying pathologic etiology. In cases of severe dysplasia, correction with an osteotomy may be necessary. However, in athletes with borderline hip dysplasia, capsular plication has been shown to be sufficient for stabilization and improved clinical outcomes. , Capsular plication may also be used for patients with connective tissue disease, , with capsular reconstruction reserved for extreme ligamentous insufficiency. Thermal capsulorraphy has also been proposed as a means of reducing capsular volume, although concerns related to intra-articular thermal injury have limited its widespread acceptance. A labral pathology is typically treated similar to FAI, with repair, reconstruction, and augmentation as options depending on the labral integrity. , The management of ligamentum teres pathology remains controversial in the literature, although options for reconstruction have been described.
The literature on surgical outcomes for treatment of microinstability are limited, but early studies have supported improved patient-reported outcomes. A recent study by Cancienne and colleagues evaluating arthroscopic management of borderline hip dysplasia showed significant improvements in the modified Harris Hip Score (mHHS), Hip Outcome Score–Sport-Specific Subscale, and Hip Outcome Score–Activity of Daily Living without significant differences between patients with borderline hip dysplasia and those without borderline hip dysplasia. A subgroup analysis also showed that patients with borderline hip dysplasia were as likely to reach the minimal clinically important difference for patient-reported outcomes as patients without borderline hip dysplasia. Domb and colleagues also found significant improvements in the mHHS, Hip Outcome Score–Activity of Daily Living, Hip Outcome Score–Sport-Specific Subscale, Non-Arthritic Hip Score, and visual analog scale for pain in patients with hip instability and underlying borderline hip dysplasia at a mean of 2 years postoperatively. Similarly, Kalisvaart and Safran found significant improvements in the mHHS and the International Hip Outcome Tool score at a minimum of 12 months postoperatively. They also showed that 9 of the 11 collegiate or professional athletes were able to return to sport. Notably, poor surgical prognostic factors for patients with dysplastic microinstability include a broken Shenton’s line, a femoral neck-shaft angle of greater than 140°, a lateral center-edge angle of less than 19°, and a body mass index of greater than 23 kg/m 2 . In patients with Ehler-Danlos syndrome and hip instability, Larson and colleagues reported significant improvements for the mHHS, 12-Item Short Form Health Survey, and visual analog scale pain score at a mean of 45 months. Unfortunately, long-term outcomes and larger studies on sports performance remain to be determined for athletes with microinstability.
Iatrogenic Hip Instability
Iatrogenic hip instability is a newly recognized cause of hip pain and dysfunction in patients that may involve gross instability or microinstability. Multiple case reports of anterior, posterior, and superolateral instability have been documented after hip arthroscopy. , , Iatrogenic injury can occur in any of the stabilizing structures of the hip or may involve multiple structures resulting in significant symptomatic instability. Common sources include excessive cam or pincer resection, iliopsoas release, excessive labral debridement, and capsular injury.
Because prior studies have identified the hip capsule as a major stabilizing structure, the surgical management of the capsule is recognized as an integral component of operative treatment. Additionally, biomechanical studies have demonstrated that capsulotomy size inversely affects the force required for hip distraction and directly affects hip stability. , Evidence of capsular defects has been reported after capsulotomy during hip arthroscopy. In fact, capsular insufficiently has been shown to be one of the most common indications for revision hip arthroscopy. Accordingly, the literature on iatrogenic instability has focused on the identification of the capsular status and the treatment of capsular defects to restore stability and hip function ( Fig. 4 ).
