Instability is one of the most common complications after total hip arthroplasty (THA), with an overall incidence of approximately 2%.¹ Furthermore, instability is currently the most common reason for THA revision.^2,3 Because the etiology and treatment of instability are diverse, a systematic approach to evaluating and treating these patients is critical. Fundamental to the management of instability is an adequate understanding of the specific underlying cause. Wera et al⁴ proposed classifying patients with instability after THA into six different categories of etiology (Rush types): I, a malpositioned acetabular component; II, a malpositioned femoral component; III, an abductor deficiency; IV, impingement; V, late polyethylene wear; and VI, unknown etiology. In their series of 75 patients revised for instability, 69% were type I or III. Although there are certainly some potential weaknesses with the Rush classification, especially given our improved understanding of the implications of spinopelvic abnormalities, it can still serve as a useful tool in categorizing patients and forming subsequent treatment plans. In this chapter, we discuss the strategic assessment of instability and how best to use the direct anterior approach (DAA) to both prevent and manage this complication.

In this endeavor of establishing etiology, there are several important steps in the evaluation of patients with instability after THA. First, a thorough history must be taken, which should focus on the timeline and nature of instability events.⁵ If the patient is able to elaborate on the specific movement or position that led to dislocation, this may be helpful in determining the direction of instability as well as the potential etiology. Furthermore, differentiating early versus late instability after THA is crucial in elucidating the underlying cause.

For those patients who have experienced multiple instability events soon after primary THA, component malposition should be the primary consideration. As such, basic and advanced imaging becomes critical in evaluating component position, as will be detailed later. For those patients with late-onset instability who previously had a well-functioning THA, infection, component wear, and metallosis (in metal-on-metal or metal-on-polyethylene bearings) must be considered as potential causes. These patients may require, in addition to imaging studies, inflammatory laboratory values (erythrocyte sedimentation rate and C-reactive protein) as well as metal ions depending on the components.

Physical examination of these patients should evaluate for positions of instability, range of motion, limb length, and strength (with a particular focus on abductor strength). The overlying skin should be evaluated for stigmata of infection as well as the position of previous incisions. Limb length, both objective and subjective, should be thoroughly discussed and evaluated preoperatively because it may be necessary to increase the functional length of the affected limb to achieve stability in the revision setting.⁵

The initial evaluation of patients with instability after THA should begin with standing anteroposterior (AP) pelvis and lateral radiographs. The importance of using standing radiographs in the evaluation of the acetabular component lies in the variation of the functional position of the acetabulum with standing pelvic tilt.⁶ It has been demonstrated that, for every degree of posterior pelvic tilt, there is a 0.7° to 0.8° increase in functional anteversion.^7,8,9 Therefore, a supine AP pelvic radiograph can be deceptive in patients with abnormal pelvic tilt. When there is concern for spinopelvic immobility and abnormal pelvic tilt, which can easily be noted on the AP or lateral standing radiographs, there may be a role for standing/sitting lateral pelvic radiographs.^6,10 In addition to evaluating cup position with AP pelvis imaging, offset and limb length restoration should be evaluated. Relative shortening or inadequate offset restoration of the hip can lead to instability through poor soft tissue tensioning and increased femoral-pelvic bony impingement.

Although several methods of measuring acetabular anteversion on AP pelvic radiographs have been described,^11,12 the gold standard for evaluating both acetabular and femoral version is a computed tomographic (CT) scan that includes the full femur as well as the pelvis.
A CT scan of the femur allows for evaluation of femoral version by comparing the epicondylar axis at the knee to the femoral stem position in the axial cuts.^5,13 Acetabular anteversion can also be evaluated via axial cuts on a CT pelvis, comparing the cup position to the sagittal plane of the pelvis. This can be determined using a line perpendicular to the line formed between both anterior superior iliac spines. The CT pelvis can allow for additional assessment of polyethylene wear and any associated osteolysis as well. If there is clinical suspicion for abductor deficiency or other soft tissue destruction due to metallosis or infection, then metal artifact reduction sequencing magnetic resonance imaging can be obtained.

As demonstrated by Wera et al,⁴ in cases of component malposition, there is a much higher incidence of acetabular cup malposition than femoral malposition. There are likely several reasons for this. First, there is much greater potential for cup malposition than femoral malposition given that a nonmodular femoral stem generally follows the native anatomy of the proximal femur and does not allow for excessive changes to native version. Furthermore, as several more recent studies have demonstrated, acetabular version and inclination are heavily influenced by pelvic tilt, which can be difficult to account for.^7,8 Therefore, evaluation of the cup position is paramount to any instability workup.

Historically, an adequate cup position was defined by whether the cup was placed in safe zones as described by Lewinnek et al¹⁴ These safe zones of 40° ± 10° of inclination and 15° ± 10° of anteversion were based on a retrospective study of 300 THA patients. A 6.1% dislocation rate was observed with cups placed outside the safe zones compared with a 1.5% dislocation rate within the safe zones. However, several recent studies have demonstrated that the majority of dislocations take place in patients with cups in the traditional Lewinnek safe zones.^15,16 Therefore, it has been suggested that either the true safe zones are narrower or the traditional safe zones do not properly account for abnormal native anatomy due to pelvic tilt and/or spinopelvic immobility.¹⁷

The effect of spinopelvic immobility and pelvic tilt on cup position has been an area of increasing interest in the literature, and is more comprehensively addressed in another chapter within this textbook. In individuals with normal spinopelvic mobility, there is compensatory movement through the lumbar spine when going from standing to sitting positions to allow for posterior pelvic tilt. This allows for greater flexion through the hip joint without anterior impingement.^10,18,19,20 This normal mechanism of compensatory pelvic tilt can be deranged in two different ways: (1) a patient’s standing pelvic tilt may have substantial anterior or posterior pelvic tilt compared with a neutral pelvis and (2) a patient may have decreased spinopelvic mobility such that pelvic tilt does not significantly change (<10°) when going from the standing to sitting position.^6,21

The consequences of the presence of these derangements in a patient are two-fold. Misunderstanding of this in the initial surgery can lead to cup malposition, and these patients are at a substantially higher risk of instability in patients with sagittal spinal deformity and/or lumbar instrumented fusions.^22,23,24 Esposito et al¹⁸ also showed that there is a higher risk of dislocation in all patients who had spinopelvic stiffness on sit-to-stand radiographs. This may be somewhat approach dependent. We have shown that the DAA can help mitigate the increased dislocation rate in these patients with stiff spinopelvic junctions.²⁵ However, anterior THAs are not immune to instability, especially in the stiff spine patient.