Hip Microinstability—Now We Know What It Is, But What Do We Do About It?

In recent years, more and more evidence has been published showing hip microinstability as a true and important diagnosis for hip preservation surgeons to recognize.1, 2 The recent Delphi consensus defines microinstability as persistent and excessive hip motion that becomes symptomatic.3 Systematic reviews now include hip microinstability—alongside femoroacetabular impingement, acetabular dysplasia, and labral pathology—as a recognized cause of nonarthritic hip pain.1, 4 In mechanical terms, microinstability is described as supraphysiologic femoroacetabular translation arising from the interplay of soft-tissue abnormalities.5 Its hallmark is symptomatic excessive motion in the absence of frank macroinstability, which denotes gross hip instability characterized by loss of joint congruence due to substantial capsuloligamentous disruption, traumatic dislocation, or iatrogenic injury.6 As our community has gained recognition of microinstability, it is important to define the appropriate treatment for patients with this diagnosis. First, it is essential to distinguish hip microinstability from borderline hip dysplasia (BHD). BHD simply is defined based on a lateral center-edge angle (LCEA) between 20 and 25 degrees or, in some studies, 18 and 25 degrees.7 Some patients with BHD exhibit no additional signs of hip instability and therefore may do well with hip arthroscopy alone to address femoroacetabular impingement and labral pathology; others, who do show signs of hip instability, should undergo bony stabilization in the form of a periacetabular osteotomy (PAO) and/or derotational femoral osteotomy (DFO).8, 9 Hip microinstability, on the other hand, represents a collection of signs/symptoms which point to soft tissue instability of the hip joint.3 Therefore, in hips with microinstability but normal acetabular bony coverage, it is important to stabilize the soft tissues during hip arthroscopy. In particular, the capsule plays a predominant role. Arthroscopic capsular plication has emerged as a key technique for restoring hip stability. The procedure involves imbricating and tightening the capsule—typically by taking larger bites of tissue or suturing at an oblique angle—to reduce capsular redundancy and constrain excessive motion.10 A recent series reports significant functional improvement,11 and isolated suture plication without concomitant femoroplasty or acetabuloplasty has been shown to improve pain and function at ≥1 year.12 Reviews of management strategies continue to emphasize capsular plication as the mainstay in hips with laxity,13 while biomechanical and surgical technique articles explain how imbrication of the capsule augments the screw-home mechanism to resist translation.14 Patients undergoing revision hip arthroscopy with an unhealed capsulotomy should undergo capsular plication when feasible or capsular reconstruction if the defect is too large to repair primarily. Even in well-executed primary arthroscopy, some hips fail. A leading cause of such failure is inadequate capsular healing: in a case series of 194 patients undergoing hip arthroscopy with interportal capsulotomy and routine capsular closure, approximately 9% of capsules failed to heal, with nonhealing strongly correlated with worse outcomes.15 When revision hip arthroscopy is considered, intraoperative assessment of the capsule is critical. If the remnant capsule is viable, direct repair or augmentation may suffice; but when capsular tissue is deficient or scarred, capsular reconstruction using grafts (e.g., iliotibial band, dermal) is increasingly utilized.16 Beyond the capsule, the labrum plays a suction-seal role to resist femoral head translation.17 Thus, appropriate treatment of labral pathology, either repair or reconstruction, is important to restore this suction-seal mechanism. In hips with microinstability, an everted labrum is a finding that deserves attention. In these cases, the labrum does not make concentric contact with the femoral head with the hip off traction, thereby disrupting the suction seal and permitting micro-motion.18 Thus, it is essential to evaluate the labrum off traction during hip arthroscopy to ensure that the labrum contacts the femoral head. Repair, advancement, or reconstruction of the everted labrum restores the seal and enhances stability.19 Recent reports support combined labral and capsular intervention in microinstability scenarios. In the revision hip arthroscopy context, labral failure must be addressed. Systematic reviews show that labral reconstruction in revision settings yields significant improvements in patient-reported outcomes, often comparable with repair when repair is feasible.20 In complex revision cases, surgeons may need to perform combined capsular reconstruction and labral reconstruction in order to restore both capsular stability and labral suction seal. When patients present with persistent or recurrent symptoms after primary hip arthroscopy, it is essential to identify the underlying reason for failure.21 If the labrum is deemed nonviable or irreparable, labral reconstruction should be considered. However, persistent symptoms may also reflect unrecognized structural abnormalities such as acetabular dysplasia, generalized instability, or femoral torsion abnormalities. In these cases, PAO and/or DFO should be considered to comprehensively address the underlying hip pathology. In patients presenting with both hip microinstability and BHD, it is important to recognize that these patients have both bony and soft tissue instability. Therefore, we recommend combined hip arthroscopy and PAO (either in a simultaneous or staged fashion) to address both the soft tissue instability (through capsular plication and appropriate treatment of labral pathology) and bony instability by improving acetabular bony coverage. Some of these patients may be better suited for a DFO rather than a PAO. For example, in a patient with a lateral center edge angle of 25° and adequate anterior wall coverage but a femoral torsion of 35°, bony instability comes from femoral head malpositioning more so than insufficient acetabular coverage. Therefore, a retrotorsing DFO would be most appropriate in this scenario. Patients with both hip microinstability and BHD are unlikely to have optimal outcomes with hip arthroscopy alone as they represent the subset of borderline dysplastic patients with additional signs of hip instability.8 In sum, microinstability must be treated with a tailored, pathology-driven approach (Table 1). In hips with inadequate bony coverage, PAO and/or DFO should be strongly considered. In hips with normal acetabular coverage, arthroscopic capsular plication and appropriate treatment of labral pathology remain the mainstay. In revision settings, addressing capsular insufficiency through plication or reconstruction is critical. Recognizing the everted labrum and capsular deficiency intraoperatively allows for comprehensive restoration of stability. The author (M.J.K.) declares the following financial interests/personal relationships which may be considered as potential competing interests: M.J.K. reports consulting with Smith & Nephew and Atheneum Consulting; paid speaker for Vericel Corporation; and Editorial Board for Arthroscopy. The other authors (D.P., J.D.) declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article.