➢ Atraumatic hip instability is an increasingly recognized source of pain and hip dysfunction. It can result from numerous causes, including femoroacetabular impingement, prior trauma, injury to the capsuloligamentous structures, and idiopathic etiologies.
➢ Occult hip instability can be a challenging diagnosis that requires careful attention to, and interpretation of, history, physical examination, and radiographic imaging findings.
➢ Iatrogenic hip instability is a potential complication of both open and arthroscopic hip-preserving surgical procedures that can have catastrophic results.
➢ Atraumatic hip instability is a pathologic entity that can be successfully addressed with open and arthroscopic procedures.
Hip instability is typically associated with high-energy trauma, hip arthroplasty, and developmental dysplasia1. However, recent advances in the knowledge of hip anatomy and pathomechanics have led to an emerging focus on atraumatic causes of instability2-4. As concepts and innovations in hip-preservation surgical procedures continue to expand, appreciation of the importance of the hip capsule and periarticular soft tissues has grown2. This is reflected in several recent reports of iatrogenic instability following hip-preserving procedures5-13. As a result, there has been heightened interest in better defining the anatomic contributions to hip stability and methods of optimizing the integrity of dynamic and static structures that impart hip stability.
The etiology of atraumatic hip instability is likely multifactorial and may be associated with an anatomic predisposition resulting from abnormal osseous morphology, generalized hyperlaxity, repetitive microtrauma, and iatrogenic injury. Atraumatic hip instability can present in several forms, including frank dislocation, subluxation, or, more subtly, microinstability. Atraumatic hip instability remains an evolving concept, and its prevalence is unknown. However, it is likely an underappreciated entity. In a series of thirty-seven revision hip arthroscopies for persistent pain, thirteen cases (35%) had unaddressed instability14. An emerging body of evidence provides support that atraumatic hip instability is a true pathologic entity and may be successfully addressed using hip-preserving techniques. However, there is considerable controversy regarding its etiology, clinical relevance, and treatment.
Anatomy and Biomechanics
Under normal physiologic conditions, the hip is an inherently stable joint. Although the hip’s osseous architecture contributes greatly to its stability, soft tissues about the hip play an important role in maintaining static and dynamic restraint to optimize arthrokinematics and to prevent pathologic joint motion2. The hip joint capsule comprises the iliofemoral, ischiofemoral, and pubofemoral ligaments and the zona orbicularis. Together, these form a strong capsuloligamentous complex that limits hip rotation, translation, and distraction15,16. Injury to the hip capsule can result from an acute traumatic injury (subluxation or dislocation), iatrogenic causes (arthroscopic or open surgical violation), or repetitive microtrauma. Myers et al. studied the relative contribution of the acetabular labrum and hip joint capsule in a sectioning model. The authors found that isolated sectioning of the iliofemoral ligament resulted in significantly greater external rotation compared with the intact state (p < 0.0001)17. Additionally, the authors found that labral repair alone did not return the hip to the normal stability state, but capsular repair resulted in normalization of rotation and translation. In their cadaveric study on the effect of capsulotomy on hip instability, Bayne et al. also observed that transverse capsulotomy resulted in both increased hip translation and rotation18.
The acetabular labrum is a fibrocartilaginous ring that surrounds the acetabular rim and creates a negative-pressure seal around the femoral head, limiting cartilage consolidation and maintaining fluid dynamics within the femoroacetabular articulation19. The importance of the labrum as a static stabilizer has been demonstrated in several biomechanical studies17,20. Venting the labrum and disrupting its suction seal have been shown to propagate translational and axial instability21. Creation of a labral tear, in addition to disruption of the seal, yields even greater instability21. Nepple et al. demonstrated that disrupting the labral seal results in axial instability and that labral reconstruction restores stability to the normal state22.
The ligamentum teres originates from the ischial and pubic portions of the acetabular notch and inserts onto the fovea capitis of the femoral head. Despite recent studies exploring its role, its overall contribution to hip stability remains a subject of debate23. However, emerging evidence suggests that it may impart rotational stability and may serve a more crucial role in hips with compromised osseous stability24. The ligamentum teres has been shown to reach an end point at an average of 100° of flexion and 20° of abduction and to prevent anteroinferior subluxation in this position. Several studies have demonstrated that the ligamentum teres plays a role in stabilizing the hip in deep flexion25-27. This suggests that the ligamentum teres may form an inferior sling that supports the femoral head in the squatting position28. Although the exact mechanism by which the ligamentum teres confers hip stability continues to be defined, there is an emerging consensus that it plays a role in imparting hip stability29.
Atraumatic hip instability presents as a spectrum ranging from obvious cases involving frank dislocation to subtle forms of occult instability that can be challenging to diagnose. Evaluation should begin with a thorough history. Patients will rarely present reporting actual instability. They may describe giving-way episodes. Popping and clicking are commonly associated with chondrolabral pathology but can also indicate subtle instability. Snapping of the iliopsoas tendon can result from chronic attenuation of the anterior capsule and fatigue overload, due to insufficient osseous containment and increased translational forces. Periarticular muscular compensations are frequently associated with atraumatic hip instability and can include snapping or contracture of the iliotibial band, osteitis pubis, or abductor tendinitis. Although an association between these problems and hip instability has been reported, the prevalence of instability in patients presenting with these symptoms is unknown30. It is important to inquire about previous surgical procedures and prior trauma, including instability episodes. For some patients, pain may be the only presenting symptom31.
The examination of the patient should begin with an evaluation of gait, hip range of motion, and strength testing. Several examination maneuvers can be helpful in diagnosing instability, including anterior and posterior apprehension signs, the external rotation dial test, and the axial distraction test. Posterior laxity may be suspected when there is apprehension with flexion, adduction, and internal rotation. Anterior apprehension may occur with abduction, extension, and external rotation of the hip (Video 1). Anterior capsular laxity is evaluated using the external rotation dial test (Video 2)32. The patient’s lower extremity is maximally externally rotated and is then released. A positive test occurs when there is no firm end point and no internal rotation recoil. Axial distraction may be increased and may cause apprehension or an audible pop. Beighton signs of hypermobility, including elbow hyperextension, ability to touch the thumb to the volar aspect of the forearm, and skin elasticity, should be checked33.
A variety of imaging studies may assist in the diagnosis of atraumatic instability. However, because radiographic markers of femoroacetabular impingement are common in asymptomatic individuals34, imaging findings must be carefully interpreted within the context of the patient history and physical examination. A well-centered anteroposterior radiograph of the pelvis as well as anteroposterior, false profile, and lateral views of the hip should be made. These imaging studies may demonstrate focal or global acetabular undercoverage, radiographic markers of femoroacetabular impingement, disparity of congruence, malalignment of the proximal part of the femur or the acetabulum, or sequelae of a previous injury or surgical procedure. Magnetic resonance imaging (MRI), combined with arthrography, can be useful to identify injury to the labrum and capsuloligamentous structures of the hip. It may also show thinning and attenuation of the capsular ligaments35. An axial traction test under anesthesia may demonstrate inferior subluxation of the femoral head and a vacuum sign on fluoroscopy (Fig. 1)36. Although such a finding is highly indicative of instability, a negative traction test does not rule it out. Arthroscopic signs of instability include a ligamentum teres tear, a positive drive-through sign, and a redundant, patulous, and sometimes attenuated capsule (Video 3).
Mechanisms and Treatment
Hip Instability Associated with Femoroacetabular Impingement
In the past decade, an association between femoroacetabular impingement and hip instability has been increasingly recognized. In a population of professional dancers, advanced imaging techniques have revealed that impingement and subluxation occur frequently at extremes of hip range of motion37. This suggests that osseous conflict at supraphysiologic positions causes a levering effect of the femoral head from the acetabulum37-39. In addition, real-time, dynamic, three-dimensional computed tomography (CT) has been used to demonstrate that considerable anterior and posterior subluxation occurs in patients with femoroacetabular impingement morphology40.
The relationship between femoroacetabular impingement and symptomatic hip instability has also been reported, and several authors have reported success arthroscopically addressing femoroacetabular impingement in the setting of hip instability41-43. Berkes et al. reported on a series of three patients with femoroacetabular impingement and posterior hip instability following low-energy trauma43. Philippon et al. also reported on a series of professional athletes who sustained traumatic hip dislocations during competition41. Of fourteen athletes who suffered dislocations, nine were found to have radiographic markers of femoroacetabular impingement. In a separate series of twenty-two patients with acute posterior hip instability episodes, Krych et al. noted that the mechanism of instability was typically due to low-energy trauma, and eighteen of twenty-two patients had structural abnormalities consistent with femoroacetabular impingement42. Radiographic markers of femoroacetabular impingement are common even in asymptomatic patients, which makes these associations difficult to interpret34. However, in a comparative study, Steppacher et al. demonstrated a higher prevalence of radiographic markers associated with femoroacetabular impingement in patients who sustained posterior hip dislocations compared with a group without instability44. These findings suggest that femoroacetabular impingement may introduce an anatomic predisposition to posterior translation and subluxation, rendering these patients more vulnerable to hip instability42. Acetabular retroversion, a common finding in patients with femoroacetabular impingement, can further contribute to instability by decreasing the distance that the femoral head must be translated before it is unstable.
Microinstability and Ligamentum Teres Reconstruction
Hip microinstability refers to a subtle form of hip instability that can result from a number of etiologies. These include repetitive injury resulting in attenuation of the hip capsule and ligaments, inherent ligamentous laxity, ligamentum teres injuries, and osseous abnormalities including subtle dysplasia. Treatment initially takes the form of nonoperative management, including physical therapy and activity modification. If this fails, microinstability can be addressed surgically.
Microinstability from subtle dysplasia can be treated arthroscopically. However, caution must be taken as hip arthroscopy in the setting of dysplasia has been associated with serious complications, including iatrogenic instability and rapid progressive joint degeneration8,45. Patients with severe dysplasia (lateral center-edge angle of <15°) are best treated with periacetabular osteotomy, which is often combined with concomitant hip arthroscopy46. Those with borderline dysplasia (lateral center-edge angle of 20° to 25°) may be treated with arthroscopy alone. Domb et al. performed arthroscopic labral repair and capsular plication on twenty-two patients with microinstability related to borderline dysplasia (lateral center-edge angle of 18° to 25°) and reported significant improvements in all outcome scores at the two-year follow-up (p < 0.0001)47.
The contribution of the ligamentum teres to the overall stability of the hip joint is controversial and remains a subject of debate. Surgical hip dislocation employs routine sectioning of the ligamentum teres to gain full, circumferential access to the femoral head and central joint space. To date, iatrogenic instability has been reported only rarely as a consequence of planned ligamentum teres disruption during surgical hip dislocation48. Nevertheless, several recent publications have explored the association of hip microinstability and ligamentum teres deficiency24. In a small case series, Oh et al. performed an arthroscopic hip surgical procedure on two patients with occult hip instability and noted partial ligamentum teres tears in both49. Several authors have reported treating persistent hip pain and instability in ligamentum teres-deficient hips with ligamentum teres reconstructions using a variety of grafts, with promising results36,50-53. However, in all publications describing techniques or outcomes related to ligamentum teres reconstruction, authors have concurred that ligamentum teres reconstruction should only be performed on a select group of high-demand patients with persistent symptomatic instability in whom index hip arthroscopy has previously failed to adequately address symptoms.
Although tears of the ligamentum teres may be associated with instability, they more frequently present as pain and can usually be successfully managed with debridement alone31. In many cases, damage to the ligamentum teres occurs in the presence of additional intra-articular pathology, such as labral injuries, chondral injuries, or more global degenerative changes. These findings suggest that, for many patients, the ligamentum teres does not function as a primary stabilizer in a normally functioning hip. Biomechanical studies have demonstrated that the ligamentum teres is most important at relative extremes of hip motion25,27,28. Therefore, it may have a more important functional role in high-demand athletes36,53.
Iatrogenic Hip Instability
Strategic capsulotomies and focal capsulectomies are routinely performed during hip arthroscopy to visualize anatomy and to facilitate instrumentation of the joint. In rare instances, this can result in iatrogenic hip instability. McCormick et al. reported a high prevalence of capsular defects seen in patients undergoing revision hip arthroscopy54. A growing body of case reports supports the notion that hip instability can directly result from techniques used during arthroscopic or open hip preservation surgical procedures and highlights the importance of the stabilizing effect of the hip capsule (Table I)5-13.
Several authors have reported devastating complications after performing hip arthroscopy in the setting of moderate to severe hip dysplasia. Matsuda and Khatod described rapidly progressive osteoarthritis and superior femoral subluxation after performing hip arthroscopy with labral repair in a patient with dysplasia55. The authors concluded that, in the setting of moderate to severe dysplasia, hip arthroscopy and labral repair should be combined with periacetabular osteotomy. Mei-Dan et al. reported a similar experience with labral repair in a patient with dysplasia who developed superior femoral subluxation and rapidly progressive osteoarthritis eventually requiring total hip arthroplasty8. Benali and Katthagen performed anterolateral labral resection and osteophyte removal in a patient with dysplasia who also developed femoral head subluxation and rapid progression of osteoarthritis that required total hip arthroplasty for salvage5. These reports highlighted the importance of labral preservation in patients with structural instability, but also cautioned that, despite restoration of labral integrity, arthroscopic hip surgical procedures can result in iatrogenic instability and end-stage arthrosis when injudicious techniques are applied. However, with careful attention to maintaining capsular integrity, patients with borderline dysplasia can be successfully managed with arthroscopic labral repair and capsular plication47.
Iatrogenic instability can also occur as the result of overly aggressive acetabular rim resection. Souza et al. reported an anterior hip dislocation on the first postoperative day following arthroscopic pincer decompression, which they attributed to an excessive anterior acetabular rim resection11. Larson and Stone described two patients with structurally unstable acetabular rim fractures that were contributing to pincer impingement56. Because complete excision would result in acetabular deficiency and potential hip instability, the authors proposed a novel technique in which arthroscopic partial excision of the rim fragment was performed, followed by internal fixation of the base of the rim fragment. The authors reasoned that this approach allowed for adequate osseous decompression while maintaining overall structural stability. In a cohort of patients undergoing revision hip arthroscopy, Domb et al. described six patients with hip instability57. Three of these patients had undergone over-resection of cam deformities, and two had poor outcomes despite revision procedures.
Iatrogenic hip instability following a hip-preservation surgical procedure is not exclusive to arthroscopic procedures; instability following surgical hip dislocation has also been described. Nepple et al. described a patient who presented with iatrogenic hip subluxation after undergoing a surgical hip dislocation with acetabuloplasty, labral refixation, and femoral osteochondroplasty for pincer impingement58. Due to ongoing symptoms and subluxation, the patient was successfully treated with periacetabular osteotomy.
Posttraumatic Hip Instability
Hip instability associated with musculoskeletal trauma typically results from high-energy mechanisms. When accompanied by osseous injury to the femoral head or acetabulum, such injuries may require closed or open reduction and occasionally internal fixation, depending on specific fracture characteristics. Recurrent hip instability in the setting of normal osseous restraints has been considered rare59. However, recent evidence suggests that attenuation of the hip’s soft-tissue stabilizers often accompanies osseous injuries of the hip and can lead to the development of microinstability. Moorman et al. described a series of eight professional football players who sustained traumatic posterior hip subluxations and found that all of them demonstrated characteristic MRI findings of posterior acetabular lip fracture and iliofemoral ligament disruption60. All were successfully treated nonoperatively; however, two players developed osteonecrosis and subsequently underwent total hip arthroplasty. Disruption of the hip’s soft-tissue restraints can compromise the dynamic stability of the hip because of loss of force coupling and may lead to persistent atraumatic instability, despite the maintenance or reestablishment of normal acetabular anatomy (Table II)41,61-69.
Idiopathic Frank Hip Instability
Frank instability resulting from idiopathic etiologies is rare but has been reported. Provenzano et al. described the case of a twenty-five-year-old woman with no osseous abnormalities or ligamentous laxity who experienced a hip dislocation while she was sleeping and sustained three subsequent recurrent dislocations70. An arthrogram demonstrated a voluminous capsule and the patient was successfully treated with open capsular imbrication. McGrory et al. reported a case of a twenty-three-year-old woman with a history of congenital hypotonia, dysplasia, and generalized ligamentous laxity who experienced recurrent posterior hip subluxation with flexion past 90°71. The authors attributed her instability to a combination of acetabular dysplasia and soft-tissue laxity. They hypothesized that congenital hypotonia may have played a role but were unable to define it. Philippon performed thermal capsulorrhaphy in six patients for idiopathic hip instability, with resolution of instability and improved symptoms in all patients67.
Corticosteroid use has also been associated with atraumatic hip instability. Trousdale reported on a single patient with a history of lupus, treated with systemic corticosteroids, who experienced a hip dislocation after a ground-level fall68. She later experienced multiple episodes of recurrent instability and a failed femoral derotational osteotomy and ultimately required a periacetabular osteotomy. Fischer et al. reported on a case of a fifty-four-year-old patient with asthma and prolonged corticosteroid use who experienced recurrent bilateral atraumatic hip dislocations72. An open capsular plication was performed but failed on postoperative day 3. At the time of the surgical procedure, the capsule was noted to be lax and the authors postulated that long-term corticosteroid use may have contributed to the pathophysiology of the disease state.
Subtle forms of idiopathic hip instability have also been reported. Blakey et al. performed dynamic MRI on a series of eleven hips with signs and symptoms of laxity and found attenuation of the iliofemoral ligament35. They concluded that repetitive microtrauma to the capsule may lead to chronic attenuation and subsequent destabilization. Bellabarba et al. reported on a series of five patients with painful coxa saltans who were found to have subluxation with the presence of a vacuum sign during simple manual traction under fluoroscopy. This finding was not present on the contralateral, asymptomatic side. They postulated that the popping sensation occurs as the suction seal of the hip is broken and suggested that idiopathic dynamic hip instability may cause a painful snapping hip30.
A comprehensive review of the literature suggests that atraumatic hip instability is a true pathologic condition with objective findings that correspond to clinical symptoms. Despite improvements in diagnostic imaging, dynamic testing, and overall awareness, further study is necessary to better understand this challenging and often subtle condition. Multiple factors often intertwine to produce atraumatic hip instability, including femoroacetabular impingement, dysplasia, generalized laxity, repetitive microtrauma, iatrogenic injury, and posttraumatic soft-tissue injury. There is ample evidence supporting the concept that the capsule, labrum, ligamentum teres, and periarticular myotendinous envelope all play complementary roles maintaining hip stability. As technical innovations in hip preservation continue to grow, surgeons have gained powerful new tools to address the wide spectrum of pathologies contributing to hip disorders. However, caution must be taken when employing these tools in patients who have, or are at risk for, atraumatic instability. Although hip-preserving procedures can benefit these patients, severe complications can occur when these procedures are not performed appropriately. As the popularity and scope of hip-preservation surgical procedures increase, atraumatic hip instability should be recognized and should be carefully managed so that potentially catastrophic complications can be avoided and patient outcomes can be optimized.
Investigation performed at the Department of Orthopaedics, Sports Medicine and Hip Preservation, University of Rochester, Rochester, New York
Disclosure: No external source of funding was used for this investigation. On the Disclosure of Potential Conflicts of Interest forms, which are provided with the online version of the article, one or more of the authors checked “yes” to indicate that the author had a relevant financial relationship in the biomedical arena outside the submitted work and “yes” to indicate that the author had other relationships or activities that could be perceived to influence, or have the potential to influence, what was written in this work.
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