Instability after Total Hip Arthroplasty: Issues, Avoidance, and Treatment

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CHAPTER SYNOPSIS

Instability after total hip arthroplasty is a serious complication that results in patient dissatisfaction and frequently necessitates revision surgery. This chapter discusses the various causes of instability and evaluation of the unstable hip and presents a treatment algorithm.

IMPORTANT POINTS

1

Closed reduction can be successful in approximately 60% of patients after their first dislocation if it occurs early.
2

Patients who have multiple dislocations frequently will need revision surgery.
3

Determination of the cause of instability is essential for a successful revision.

CLINICAL/SURGICAL PEARLS

1

Use modular components to ease the adjustment of soft tissue tensioning in the setting of well-fixed and well-positioned components.
2

If component positioning or clinical track record is poor, revision of the components should be considered even when the components are well fixed.

CLINICAL/SURGICAL PITFALLS

1

A definitive surgical plan is essential before a revision operation.
2

Soft tissue tension and competency should be thoroughly evaluated during revision.
3

Adequate exposure during index surgery is essential to avoid impingement and component malposition as an underlying cause of the instability.
4

Use modern techniques for well-fixed femoral (extended trochanteric osteotomy) and acetabular (explant systems) component removal when necessary.

INTRODUCTION

Dislocation is second only to aseptic loosening as the most common cause of failure of total hip arthroplasty (THA). The incidence of dislocation after THA has been reported to be approximately 2% (Scottish and Kaiser Registries) after primary THA and 5% to 20% after revision THA. The causes of instability after THA often are multifactorial and include malposition of components, impingement, inappropriate soft tissue tensioning, and patient noncompliance. In some patients the exact cause of dislocation is not ascertainable, and in many patients the causes are multifactorial. The key to optimizing outcomes after revision THA for recurrent dislocation is the precise preoperative determination of the cause of dislocation so that appropriate corrective measures can be undertaken intraoperatively.

THA is one of the most successful, cost-effective, and predictably good surgeries available. With widespread success and increased familiarity in the general public, it has become one of the most common elective orthopedic surgeries. Since the procedure was developed the 1960s, most currently practicing orthopedic surgeons are trained in the procedure during residency, and many general orthopedists perform this procedure. Nonetheless, the experience of the surgeon performing the procedure has been shown to have a measurable impact on the rate of dislocations. Hedlundh et al looked at the dislocation rates at three large orthopedic centers and found a significantly higher rate of dislocation in patients whose surgeon performed fewer than 30 THAs per year. For every 10 THAs performed annually, the risk of dislocation decreased by 50%. The rate of dislocations was not found to be significantly different in patients with surgeons who performed 30 or more THAs annually. This illustrates the point that although the procedure has become commonplace, it is still a technically demanding procedure. This also suggests that surgeons who do not perform the surgery regularly may, in fact, be doing a disservice to their patients by undertaking the procedure. Similarly, revision in hips with instability should not be done by surgeons who perform few primary THAs or who have limited experience in revision THA.

In examining the incidence of hip instability, the timing of dislocations can be arbitrarily divided into early and late. Woo and Morrey defined “early” as any dislocation occurring during the first 3 postoperative months. In their large series of patients they found that 59% of their THA patients with instability had their first dislocation within 3 months of the procedure. At 1-year post-index THA this number increased to 77%, and by 5 years 94% of their patients with instability had had at least one dislocation. Ali Khan et al also discussed the timing of dislocation, arbitrarily defining “early” as any dislocation within the first 5 weeks after THA. He reported 94 early dislocations with 61% of these patients having only one dislocation. In 48 patients with dislocation after 5 weeks, 42% had only one dislocation. The significance of this finding is that early dislocations occur before scar tissue formation and subsequent healing of the capsule (if preserved) or formation of the pseudocapsule. It follows that if the cause of a THA dislocation was noncompliance (and not related to malposition of the components) closed reduction of the hip and prevention of recurrent dislocation should lead to appropriate scarring and stability of the hip in the long run. Of course, for the patient who received a capsular repair during closure after THA, early dislocation resulted in a capsular injury that is not completely reversed through closed reduction. In cases of late dislocation, scar tissue attenuation and impairment of muscle function can lead to an incremental loss of soft tissue tension that can, in turn, lead to recurrent THA dislocation. Closed reduction of these hips would not necessarily be expected to lead to continued stability because the soft tissue tension is not balanced. In summary, early THA dislocations showed better results with conservative treatment (closed reduction and bracing) compared with late dislocations, and single dislocations showed better results with conservative treatment compared with patients with recurrent dislocation.

In a study of THA revisions done exclusively for instability, outcomes of revision surgery were better in patients who had dislocation early. The authors believed that this was because hips with incorrect soft tissue tensioning or malposition of components would be more likely to dislocate early. They also found that patients with an identifiable cause for instability had better outcomes after revision surgery. Thus the patients with obvious causes of instability had earlier revisions and better outcomes. This should not suggest that revisions for instability be done earlier rather than later. Rather, it illustrates that knowledge of the causative factors leads to better outcomes of revision surgery. In cases of late instability, the soft tissue tension and the placement of the components were adequate enough for the patient to maintain stability in the early period, making it less likely that these two factors alone are responsible for the dislocation. In cases in which no clear cause is found for dislocation, revision surgery has a significantly lower success rate.

When presented with an unstable THA, a thorough history, physical examination, and radiographic examination should be performed. The history of present illness should include how the current dislocation occurred (the activity being performed in relation to the position of the limb) as well as an inquiry into other potential previous episodes of subluxation or dislocation. The examiner should inquire regarding the presence of infectious symptoms such as fevers, chills, pain, and night sweats. Other helpful historic information can be obtained from the operative notes. These include the surgical approach, what soft tissues were (or were not) repaired, and what type of components were used. Adequate information about the implanted prostheses often cannot be assessed by radiographic silhouettes and the operative report alone. In these instances a surgeon considering revision must obtain the hospital records (related to the index procedure), which should contain the implant sticker details with manufacturer, size, material type, lot number, and so forth. These details contain essential information about the pending revision, which will allow the most efficient and safe THA revision for the patient. At times the operative report and implant stickers contain information that may not be immediately obvious from the radiographs alone: (1) presence, absence, or number of screws through the acetabular component; (2) trunion taper dimensions; (3) use of bone cement (not visible on radiographs) to secure a liner within a shell; and (4) outer diameter of the acetabular component. Physical examination should include both lower extremities, noting gait, range of motion, strength (particularly of the abductor musculature), neurovascular status, leg length, position of previous incisions, and leg position on presentation. Radiographic examination is crucial to determine component alignment and should include an anteroposterior pelvis as well as an anteroposterior and cross-table lateral of the affected hip. If component alignment is still unclear, computed tomography should be considered. Blood tests for complete blood count with differential, erythrocyte sedimentation rate, and C-reactive protein levels also should be routinely obtained because indolent infections may not show outward symptoms in the history or the physical examination but still be the causative factor for the instability.

PATIENT RISK FACTORS

Patient-related risk factors must be considered despite the fact that they are beyond the control of the surgeon. Numerous authors have studied the incidence of dislocation related to a patient’s age, gender, and body habitus. Consensus exists in the literature that height and weight are not independent predictors of dislocation risk. The two largest published series on gender differences report that women dislocate their THAs approximately twice as often as men. This disparity is believed to be attributable to differences in muscle mass and strength and the compliance and elasticity of the soft tissues as a result of genetic and hormonal differences between the sexes. Although a difference exists in the incidence of THA dislocation between sexes, no difference has been found in the success of revision surgery for instability. Daly and Morrey described a series of patients who underwent revision for instability and found no difference in the outcomes between men and women.

Box 28-1

RELATIVE RISK FACTORS

Age

Female sex

PATIENT RISK FACTORS

Cognitive dysfunction

Alcoholism

Neuromuscular disease

History of lumbosacral fusion

History of prior ipsilateral hip surgery

SURGICAL RISK FACTORS

Index THA surgical approach

Absence of soft tissue repair in posterior approach

Component malposition

Incorrect soft tissue tension

Impingement by osteophytes or heterotopic ossification

Risk Factors

Age as an independent risk factor for dislocation after THA is more controversial. Most large series of patients report that age is not an independent predictor of instability, but Woolson and Rahimtoola reported a slight trend toward higher incidence in older patients ( P = .9). This would seem to make sense because patients of advanced age (older than 80 years) have increased frailty, loss of muscular tone, and inability to follow postoperative restrictions, all of which increase their susceptibility to dislocation. These patients also have a higher incidence of cognitive problems, which have been linked to increased dislocation rates.

Another important patient factor to consider is a history of prior hip surgery. If a patient who has dislocated his or her THA has had any ipsilateral hip surgery before the primary arthroplasty, the surgeon should consider the fact that more complex soft tissue issues probably were present at the time of primary arthroplasty, which also could be a causative factor in the instability. In their series of 10,500 patients Woo and Morrey found a 2.4% incidence of instability in hips without previous surgery. The incidence of instability doubled to 4.8% in hips that had undergone any prior surgical intervention.

Patients with neuromuscular disorders have been shown to have an increased incidence of instability after THA. Any neuromuscular condition that causes weakness in muscles can lead to problems with soft tissue tensioning. Diseases inherent to the musculature, such as muscular dystrophy or myasthenia gravis, directly cause muscle weakness, which can increase dislocation incidence. Other neurologic conditions also can cause similar abductor weakness, such as Parkinson’s disease, multiple sclerosis, and severe spinal stenosis. Suspicion for neurologic weakness of the abductors should be considered in any patient who has undergone spinal fusion and demonstrates any residual neurologic symptoms. If the abductor muscle function is in question after physical examination, electromyography can be performed preoperatively to determine whether the muscles have been denervated. In cases in which the abductor function is questionable, the surgeon may want to have constrained liners available. The patient who has undergone a previous spinal fusion (particularly a long, solid fusion segment, including the sacrum) poses a risk for post-THA dislocation for reasons that do not necessarily include abductor muscular weakness. Although the finding has not been extensively studied, the authors have observed a high correlation between THA instability and previous lumbosacral fusion. The reason for this likely is because people with supple lumbosacral junctions have mobile pelvises that accommodate into lumbosacral kyphosis as the individual leans forward, flexes his or her hips, and rises from a seated to a standing position. The THA recipient with a normal spine perhaps flexes the hips (relative to the pelvis position) to approximately 100 degrees, whereas a person with a long, rigid spine-sacrum fusion may need to flex his or her hips more than 130 degrees to perform the same action ( Fig. 28-1 ). This extreme degree of hip flexion will be magnified for the patient with an excessive lordotic position of the lumbosacral fusion.

Cognitive disorders are more prevalent with advanced age and are associated with higher risk for THA dislocation. Age therefore is generally associated with a higher likelihood of noncompliance with postoperative restrictions. Alzheimer’s disease or any other age-related senility is one reason why some older patients cannot follow postoperative restrictions. Older patients who have a long history of excessive alcohol consumption also have similar decreases in cognitive abilities. Cognitive dysfunction has been shown to be an independent risk factor for hip instability after arthroplasty. Woolson and Rahimtoola looked at a series of 315 consecutive THAs performed in identical fashion by a single surgeon using the same components for all cases in an attempt to remove all nonpatient variables. They had an overall incidence of dislocation of 4%. They looked at age, height, weight, surgical time, sex, presence of cerebral dysfunction, cup diameter, neck length, femoral fixation type, and angles of acetabular abduction and anteversion. The only one of the 11 factors that showed any statistical significance was cerebral dysfunction. Six dislocations (13%) occurred in 47 patients with cerebral dysfunction compared with eight dislocations (3%) in 268 cognitively normal patients. They considered patients to have cerebral dysfunction if they had any episodes of confusion during the hospital stay, reported a history of excessive alcohol intake, or both. Some authors recommend that patients with a history of Alzheimer’s dementia or Parkinson’s disease should undergo hip arthroplasty through an anterolateral or direct lateral approach because the increased incidence of dislocations in these populations.

Although the surgeon cannot change any patient-related risks, these factors should be taken into account. In high-risk patients the surgeon should make both the patient and the family aware of any reason why the patient may be more likely to have continued problems. A realization of how many patients with dislocation will continue to have instability is important, as illustrated by the experience of Daly and Morrey in which 39% of patients who underwent revision for instability continued to have dislocations. Patients also should understand that every time a patient undergoes another procedure, complications are more likely. By informing patients and families of the risks involved, they may be more compliant with essential postoperative instructions.

SURGICAL RISK FACTORS

The incidence of dislocation as it relates to surgical approach has been studied extensively. Woo and Morrey reported a dislocation rate of 5.8% for hips operated on through the posterior approach and 2.3% for hips operated on through the anterolateral approach in their series of patients. Another study, a large meta-analysis of dislocation rate related to surgical approach, was performed by Masonis and Bourne. They examined 260 clinical studies and found 14 studies that met their inclusion criteria. The total number of patients in the meta-analysis was 13,203. They found a dislocation rate of 3.23% for posterior approaches (3.95% in patients without posterior repair and 2.03% in patients with posterior repair), a 2.18% rate for the anterolateral approach, 1.27% for the transtrochanteric approach, and 0.55% for the direct lateral approach.

Even though the posterior approach has the highest dislocation rate of the four approaches, it continues to be one of the two most popular approaches among surgeons because no violation of the abductor muscles occurs, as in the anterolateral or direct lateral approaches, and no osteotomy of the greater trochanter is necessary, as in the transtrochanteric approach. Both the abductor violating approaches have a significant incidence of postoperative limp from abductor dysfunction. The transtrochanteric approach was originally popularized by Charnley. It has a low dislocation rate when the trochanter is repaired with appropriate soft tissue tension and heals correctly. But modern surgeons have less experience with this approach, and the dislocation rate skyrockets to 17.6% when nonunion of the trochanter with proximal migration occurs. Alberton et al found that 2.3% of patients who had trochanteric osteotomy had nonunion, and 78% of these patients had further instability.

The higher dislocation rate of the posterior approach described by Woo and Morrey occurred in patients who did not have repair of the posterior capsule. Most of the patients in their series had repair of the piriformis tendon and partial posterior capsulectomy. In a meta-analysis Masonis and Bourne found a nearly twofold increase in dislocation rates for patients who did not have posterior repair compared with those who did (3.95% vs. 2.03%).

In 1998 Pellicci et al described an enhanced posterior soft tissue repair (EPSTR). They studied the dislocation rate in THAs performed through the posterior approach with and without their soft tissue repair. The authors operated on 519 patients after they had started using the EPSTR and compared these patients with a historic cohort of 555 patients who had surgery before implementation of the capsular repair. They had one dislocation in the 519 patients who had repair of the posterior structures (0.2%) with a minimum of 6 months of follow-up. The historic cohort of 555 patients had a total of 26 dislocations (4.7%) with no specification of the follow-up period. These results were highly statistically significant. Although the follow-up period for patients with soft tissue repair was short, the majority of patients who had dislocations occurred within the first 3 months. Even under the assumption that twice as many patients would have had a dislocation given a longer follow-up, formal repair of the posterior structures with their EPSTR would still lead to a 10-fold reduction in the incidence of instability.

Other surgical factors involved in dislocation include malposition of components, impingement (prosthetic, cement, or bony), and improper soft tissue tensioning. Impingement can be caused by osteophytes that were not resected at the time of the primary surgical procedure, capsular scar tissue, or heterotopic ossification. Anterior femoral or acetabular osteophytes can impinge when the hip is flexed, so removal of these during primary THA is important. Heterotopic ossification is first assessed by using plain radiography, but acetabular osteophytes can be difficult to assess on plain films. Computed tomography can clearly delineate acetabular osteophytes, but routine use for preoperative evaluation of planned revisions is not recommended because impingement can be directly evaluated during the procedure. Skirted necks should be avoided and high head/neck ratios should be sought to decrease the risk of prosthetic impingement. Cement must be cleared from all unwanted areas (especially near the femoral neck, which would have an adverse effect on head/neck ratio).

The most common cause of instability is malpositioning of components. Cases of anterior instability may occur in patients with excessively anteverted cups ( Fig. 28-2 ). This may be worse in patients who have had surgery through an anterolateral approach. They typically demonstrate instability during extension and external rotation of the hip. Underanteverted acetabular components can lead to posterior dislocation, particularly when the posterior approach was used because of impingement anteriorly or the loss of posterior soft tissue support ( Fig. 28-3 ). Malposition also can lead to instability through other mechanisms. If the cup is overly abducted (i.e., theta angle more than 50 degrees), the superior edge of the liner will experience increased stresses, which can lead to accelerated wear. As the polyethylene thins or fractures, instability can develop. Simply exchanging a worn liner without repositioning an overly abducted cup may solve the instability issue but may still lead to decreased longevity of the revision implants and thus recurrence of instability. In an examination of a large series of revised hips, Daly and Morrey found that six of seven patients who underwent revision surgery for multiple causes without correction of acetabular component malposition continued to have instability. They also found that the largest number of successful revision surgeries occurred in patients who underwent repositioning of the acetabular component, underscoring the importance of evaluating cup position. Because acetabular component positioning is harder to ascertain at the time of surgery, a greater number of unstable hips will require revision of the socket rather than the femoral component. Another recent addition to the armamentarium of tools to combat dislocation is the use of primary hip stems with a modular neck. These stems are relatively new and need to be clinically proven.