Total Hip Arthroplasty Instability

Epidemiology

Dislocation subsequent to total hip arthroplasty (THA) represents the most common cause for revision surgery after total hip arthroplasty (rTHA) in the United States. Rates of instability cited in the literature range from 0.5% to 10% after primary THA and from 10% to 25% subsequent to rTHA. In fact, the cumulative risk of dislocation does not remain constant over time; rather, it increases as a result of trauma, polyethylene wear, increased capsular laxity, and deterioration of abductor muscle strength with aging. Furthermore, over 60% of patients having sustained a dislocation encounter repeat events, and over half require revision surgery.

Although the incidence of dislocation has decreased over the past several decades consequent to improvements in implant design, durability, and surgical technique, the overall number of primary THAs being performed has been steadily increasing over time. As such, a net increase in the volume of rTHAs being performed for instability has been noted and is likely to continue along a similar trend in the coming years.

Dislocation events can broadly be classified in a temporal fashion: early versus late. Early and late dislocations are defined as less than 2 years versus greater than 2 years from the index procedure, respectively. More useful, however, are classification systems attempting to identify the causative process that may then translate to treatment targeting the source of instability. A traditional method to determine the pathology responsible for THA instability is to consider patient factors, implant factors, and surgical factors. Along this thought process, Dorr et al. described instability as a result of hip position, soft-tissue imbalance, and component malposition. Similarly, Wera et al. devised a classification system based on six etiologies ranging from component malposition to unexplained etiology, and provided an algorithmic approach to management ( Table 19.1 ). In this chapter, risk factors for instability will be outlined. Additionally, a systematic approach to the diagnosis of instability and subsequent management will be presented.

Risk Factors and Prevention

Prior to both primary and revision THA, careful consideration of patient and surgical factors that could compromise final construct stability is imperative. Patient factors associated with increased risk of instability can represent an inherent baseline characteristic or result from acquired disease ( Table 19.2 ).

Inherent patient risks for instability include the following: extremes of age, obesity, ^, alcohol abuse, lumbosacral pathology, ^, and prior hip surgery. Although advanced age has consistently demonstrated an association with increased risk for instability, a definite cutoff has not been clearly defined and ranges from 70 to 85 years of age. ^{, ,} Moreover, a recent study by Esposito et al. retrospectively assessing 22,097 THAs suggested instead a bimodal distribution of age for risk of dislocation after THA, with individuals <50 and ≥70 years of age at increased risk. Similarly, although a definite body mass index cutoff defining increased risk remains a matter of debate, several large studies have cited an increased risk associated with a body mass index >30 to 35 kg/m ² . ^, Within the last decade, there has been increasing evidence outlining the influence of sagittal balance and lumbosacral mobility on the functional version and inclination of the acetabulum. ^, It is imperative that the treating surgeon identify patients with poor spinopelvic mobility and adjust the preoperative plan to prevent potential impingement resulting in dislocation. Patients with lumbosacral fusion appear to be especially at risk for instability consequent to the resultant spinopelvic immobility ( Fig. 19.1 ). In a study by Buckland et al., the authors compared 14,747 patients with spinal fusion having undergone THA to a control group of 839,004 individuals. Dislocation in the spinal fusion group was noted to be proportionally greater in the spinal fusion cohort according to the number of spinal levels fused: 1.5% in controls, 2.96% for 1- to 2-level fusion, and 4.12% for ≥3-level fusion. Lastly, rTHA carries a significantly greater risk of postoperative instability compared with primary THA. In fact, rates of dislocation after rTHA are frequently reported to near 10% with a history of prior dislocation, abductor deficiency, and significant acetabular bone loss all contributing to such an elevated risk.

Similarly, several acquired risk factors have been described as they relate to both the pathologic process affecting the hip and disease or dysfunction impairing the individual. Specific pathologic processes with increased risk for instability after THA include osteonecrosis of the femoral head ^, and femoral neck fracture. Early work by Woo and Morrey demonstrated that compared with THA performed for degenerative arthritis, the dislocation rate was double for avascular necrosis, triple for congenital dislocation, and fourfold higher for fractures. Several recent studies, however, negate such an association with high-grade congenital hip dysplasia and suggest that satisfactory results with low rates of instability can be achieved. Similarly, acquired disease processes such as neuromuscular disorders, ^, abductor dysfunction, and dementia have been associated with an increased risk of THA instability. A multidisciplinary approach to the care of patients with neuromuscular dysfunction is necessary due to the abnormal muscular tone surrounding the THA. Common conditions that may present with hip pain requiring THA include those with spinal cord injury or history of stroke, poliomyelitis, cerebral palsy (CP), multiple sclerosis (MS), and Parkinson disease (PD). ^, However, data from the Scottish National arthroplasty nonvoluntary registry assessing 1399 THAs performed between 1996 and 2004 in patients with a history of cerebrovascular accident has demonstrated annual rates of dislocation ranging from 0.0 to 0.3, suggesting that low rates of instability can be achieved. Similarly, a recent study by DeDeugd et al. comparing revision rates of the affected versus unaffected limb in 51 patients with a history of poliomyelitis having undergone THA demonstrated similar survivorship and complication rates to reported results for patients undergoing THA for osteoarthritis. On the other hand, multiple studies have demonstrated substantial rates of postoperative instability in patients undergoing THA using conventional implants. Similarly, a recent study assessing 207,285 THAs from the Nationwide Readmission Database between 2012 and 2014 demonstrated an odds ratio for dislocation of 1.6 for patients with PD and 1.9 for patients with dementia when compared with unaffected controls.

Also important is the consideration of surgical factors associated with instability after THA. These risk factors should be especially important to the treating physician, as they may potentially be more readily modifiable intraoperatively. Traditionally, surgical factors associated with increased risk of postoperative instability after THA have included the following: posterior surgical approach, lack of capsular repair, implant malpositioning, component impingement, small femoral head-neck ratio, and limited surgeon experience. Surgical approach and its relationship to instability has historically been a topic of heated debate. Evidence discussing approach-related dislocation risk emerged from the large series by Woo and Morrey, demonstrating a 5.8% versus 2.3% rate of dislocation after THA with the posterior and anterolateral approaches, respectively ( P < .01). However, subgroup analysis demonstrated similar dislocation rates with the posterior approach to other approaches when performed with larger heads (32 mm vs. 22 and 28 mm). Multiple subsequent studies comparing dislocation rates between approaches have reported markedly low dislocation rates (<1%) when attentive posterior soft-tissue repair is performed. ^, As such, differences in rates of instability may not be of significant importance when employing commonly used 32- to 36-mm head sizes and attention is paid to adequate soft-tissue repair. Lastly, THA instability can result from improper acetabular or femoral component placement. Popularized by Lewinnek et al., the authors described a “safe zone” for acetabular component placement, 15 ± 10 degrees of anteversion and 40 ± 10 degrees of abduction, with the goal of minimizing postoperative instability. Recently, however, such a fixed target has been questioned. ^, In fact, in a retrospective analysis of 206 THAs that subsequently dislocated, Abdel et al. observed that the majority (58%) had an acetabular socket within the Lewinnek safe zone. As a result, recent literature has instead focused on a more individualized approach to component placement, with an emphasis on functional component anteversion in the context of spinopelvic alignment. ^,

Diagnosis

The diagnosis of THA dislocation is made through a combination of patient history, physical examination, and radiographic assessment. Adequate history collection is important to identify the mechanism of dislocation that can occur with minimal force or can be the result of more substantial trauma, such as a fall. The former scenario suggests a high degree of instability, whereas the latter can be concerning for concomitant fracture. Moreover, identification of leg position resulting in THA dislocation can be useful to identify “at-risk” positioning. Additionally, clarification of the operative date and any previous episodes of dislocation should be noted. Lastly, any history suggestive of potential infection should be elicited, and erythrocyte sedimentation rate and C-reactive protein should be drawn. If there is any suspicion for potential concomitant infection on history and/or laboratory assessment, aspiration of the hip joint should be performed prior to any consideration for revision surgery.

Classically, patients with THA dislocation present with a shortened and internally rotated limb in the context of posterior instability or external rotation of the extremity in the presence of anterior instability. Special attention to a detailed neurovascular examination of the leg with a focus on sciatic nerve function, both before and after any reduction attempt, is important and should be clearly documented. Moreover, assessment of abductor mechanism strength is essentially due to the influence of the abductor musculature on dynamic hip stability. Lastly, in the context of a fall, a thorough exam to exclude associated injuries to other areas of the body is required.

Plain radiographic examination consisting of anteroposterior and lateral views help confirm both the diagnosis and direction of dislocation, and help exclude associated fractures. Repeat images after closed reduction can help identify potential sources of instability and guide eventual treatment. Radiographic assessment of leg lengths and femoral offset provides insight into the appropriateness of soft-tissue tensioning surrounding the prosthetic joint. Identification of displaced fractures of the greater trochanter must be identified as a potential source of ongoing instability consequent to the dynamic stability conferred by the abductor mechanism of the hip. Additionally, component position, especially anteversion, is an important element to assess as a potential cause of instability that should be addressed. At our center, we have found that in the context of THA instability, evaluation through computed tomography is of great benefit to determine component version. Important to consider, however, is that inclusion of the entire pelvis is required to properly assess acetabular component version. Similarly, to correctly determine femoral component version, an ipsilateral cut of the distal femur is required to determine the knee’s transepicondylar axis.