Dislocation After Total Hip Arthroplasty

Fig. 8.1

Preoperative AP hip demonstrating left hip AVN with secondary degenerative changes prior to undergoing total hip arthroplasty

He subsequently underwent a cementless left total hip arthroplasty (THA) and did well until 1 year postoperatively when he presented with the new onset of left hip pain. Radiographs showed subluxation of the femoral head (Figs. 8.2 and 8.3). The patient underwent revision surgery and at that time the liner was found to be severely worn posterosuperiorly. The cup was revised as the locking mechanism was damaged and a 36 mm femoral head was used; an abduction brace was used postoperatively (Fig. 8.4).

Fig. 8.2

AP hip demonstrating subluxation of the femoral head

Fig. 8.3

Shoot through lateral hip suggesting subluxation of the femoral head

Fig. 8.4

AP hip after revision of the acetabular component

Two weeks after the revision the patient dislocated posteriorly (Fig. 8.5). He was treated with another revision using an unconstrained tripolar construct (Fig. 8.6). Eighteen days later, he again dislocated posteriorly and was revised once again to a constrained liner (Fig. 8.7). Four months later he dislocated yet again (Fig. 8.8a, b) and was referred to one of us (GDW) for further evaluation and management.

Fig. 8.5

AP hip demonstrating a posterior dislocation

Fig. 8.6

AP hip after revision to an unconstrained tripolar construct

Fig. 8.7

AP hip after revision to a modular constrained liner

Fig. 8.8

AP hip (a) and lateral (b) demonstrating dislocation of the constrained liner

Epidemiology

Instability is a leading cause of failure of total hip arthroplasties [1]. A review of the Nationwide Inpatient Sample (NIS) that evaluated 235,857 revision THAs between October 1, 2005, and December 31, 2010, showed that instability is the leading cause of revision at 22% [2]. Additionally, US Medicare data suggest an overall 3.9% dislocation rate after total hip arthroplasty [3]. When we consider that 332,000 THAs were performed in 2010 in the United States, the need for an understanding of the etiology and corresponding treatment of the unstable total hip arthroplasty is clear.

Risk Factors for Dislocation

Classically, patient factors such as neurologic and psychological disorders, prior hip surgery, and noncompliance all increase the risk of dislocation after THA. Similarly, post-traumatic arthritis is associated with a higher risk of dislocation than primary THA for osteoarthritis [4]. Obesity has been linked to both a biomechanical predisposition to instability and higher readmission rates after primary THA [5, 6]. Likewise, alcohol abuse has been associated with higher general complication rates, including instability, after total hip arthroplasty [7]. Advanced age and an initial diagnosis of avascular necrosis are also associated with dislocation after primary THA [8].

Primary THA performed for an acute femoral neck fracture is another high-risk group with respect to instability risk [9]. The higher risk of dislocation in patients with an acute femoral neck fracture probably relates in part to having in most cases a relatively normal hip preoperatively with high range of motion, which has been shown to be a risk factor for dislocation in patients with degenerative joint disease as well [10]. Technical factors such as surgical approach, soft-tissue tension and offset, component positioning, impingement, femoral head diameter, acetabular liner profile, and surgeon experience likewise all effect the risk of dislocation [11, 12].

Revision THA carries a substantially higher risk of dislocation than primary surgery. Dislocation rates after revision THA are frequently reported to approach 10% with a history of prior dislocation, abductor deficiency, and higher Paprosky acetabular bone defects all increasing the risk [13].

Prevention

Avoidance of dislocation is one of the most important technical considerations of both primary and revision total hip arthroplasty. Surgical approach has been shown to influence dislocations rates as well. Some data suggest that anterior and anterolateral approaches are associated with lower initial dislocation rates after total hip arthroplasty [14]. However, additional complications such as fracture and sensory defects after anterior approach total hip arthroplasty may negate the stability benefit [15]. The posterior approach has been implicated in higher dislocation rates than lateral approaches but diligent repair of the short external rotators and capsule can minimize instability risk [16, 17]. Therefore, conclusive evidence that one approach is superior is not evident [18].

Femoral head size and component positions are critical components to stability. Larger femoral heads have demonstrated lower initial dislocation rates after primary total hip arthroplasty, particularly in high-risk patients [19, 20]. Appropriate implant position has been advocated to avoid dislocation and early wear of components. Lewinnek et al. described an optimal acetabular position of 15 ± 10° anteversion and lateral opening of 40 ± 10° because dislocation rates were higher outside the “safe range” [21]. However, contemporary studies have demonstrated that dislocation rates after THA are low at about 2% with more than half of the dislocations occurring in cases with acetabular orientation within the safe zone [22]. Therefore, component positioning itself is probably just one factor that lowers the risk of dislocation and careful intraoperative trialing is important to lower the risk of instability.

One of the key steps a surgeon can take to reduce the risk of dislocation is to identify patients who are at high risk as outlined above and take measures in those specific cases to reduce the risk. Surgeons for example may consider using a larger head size in a patient with known risk factors. Another recent option is the use of dual-mobility bearings (Fig. 8.9). These constructs include a mobile polyethylene insert that yields a large jump distance that resists dislocation. While these bearings have their own risks, including that of intra-prosthetic dislocation, they have been advocated by some surgeons as an alternative in patients who are known to be a high risk for instability [20].

Fig. 8.9

Operative photograph of a dual-mobility construct featuring a mobile polyethylene insert that yields a large jump distance that resists dislocation

Diagnosis

The diagnosis of prosthetic hip dislocation after total hip arthroplasty is usually made by plain radiographs, history, and physical examination. The patient will typically present with a shortened limb and external rotation with anterior instability or internal rotation with a posterior dislocation; confirmation of the direction of instability can be made with a shoot through lateral and is important to determine appropriate treatment.

Given the importance of component position on hip stability, a clear sense of the anteversion of the presently implanted components is critical to planning appropriate treatment as is an assessment of femoral offset and leg length; all of these factors are intimately connected when determining the overall stability of the hip construct. While plain radiographs can be used to assess component position (Figs. 8.10 and 8.11), we have found preoperative CT to be a more precise assessment tool. In order to accurately determine acetabular component anteversion, a CT of the entire pelvis (not just of the ipsilateral hip) is required. Further, in order to accurately determine femoral component anteversion, the CT must include a cut through the ipsilateral epicondylar axis of the knee (Fig. 8.12).

Fig. 8.10

AP pelvis demonstrating determination of cup abduction to be 41° and within the safe zone of Lewinnek. Multiple reference lines are acceptable including a trans-ischial line, trans-tear drop, or trans-obturator foramen line as depicted here

Fig. 8.11

Cross-table lateral view demonstrating 38° of anteversion with respect to the horizontal film edge as originally described by Woo and Morrey [23]

Fig. 8.12

Combined CT scan of the femoral neck and ipsilateral trans-epicondylar axis for determining femoral component anteversion. It demonstrates retroversion of the femoral prosthesis. Wera GD, Ting NT, Moric M, Sporer SM, Jacobs JJ, Paprosky WG, Della Valle CJ. The Causes and Management of Hip Instability: An algorithmic approach. Seminars in Arthroplasty 2015;26(3)131–135. Reproduced with permission from Elsevier Ltd.

Other factors that are important to consider when evaluating the patient who has dislocated include the patient’s perception of their leg length. Further, an assessment of abductor muscle strength is critical to perform, given the influence of the abductor musculature on hip stability. Finally, prior to any revision procedure, an evaluation for periprosthetic joint infection with a serum ESR and CRP, followed by an aspiration of the joint if these labs are elevated or if the clinical suspicion for infection is high, should be performed [24].

Treatment

Nonoperative treatment of the dislocated THA includes prompt closed reduction, followed by in some cases the use of a knee immobilizer (our preference based on cost) or a hip abduction orthosis, and most studies suggest that this strategy will be successful in approximately one-half to two-thirds of patients who dislocate. In a retrospective review of first-time and recurrent dislocators following THA, Dewal et al. found that abduction bracing after closed reduction of THA dislocation is ineffective in preventing re-dislocation [25]. Likewise, Murray et al. found no significant difference in the 90-day dislocation rate among patients who wore a brace compared with the non-braced group following revision THA [26].

If the instability has become recurrent, our philosophy is to identify and treat the underlying etiology of the instability. Prosthetic hip dislocation can be classified according to the primary cause of instability including (1) acetabular component malposition, (2) femoral component malposition, (3) abductor muscle deficiency, (4) soft-tissue or bony impingement, (5) late wear of a polyethylene liner, and (6) unclear etiology (Table 8.1) [27]. In our experience, identification and treatment of the primary cause of instability are associated with a success rate of 85%.

Table 8.1

Classification system for the unstable THA

Type	Acetabular component orientation	Femoral component orientation	Abductor-trochanteric complex	Impingement	Late wear	Intervention
I	Incorrect	Correct	Intact	Absent	Absent	Acetabular component revision
II	Correct	Incorrect	Intact	Absent	Absent	Femoral component revision
III	Correct	Correct	Absent	Absent	Absent	Constrained liner
IV	Correct	Correct	Intact	Present	Absent	1. Remove sources of impingement
IV	Correct	Correct	Intact	Present	Absent	2. Upsize modular head and liner
V	Correct	Correct	Intact	Absent	Present	1. Modular component exchange
V	Correct	Correct	Intact	Absent	Present	2. Upsize modular head and liner
VI	Correct	Correct	Intact	Absent	Absent	Constrained liner

Wera GD, Ting NT, Moric M, Paprosky WG, Sporer SM, Della Valle CJ. Classification and management of the unstable total hip arthroplasty. J Arthroplasty. 2012 May;27(5):710–15. Adapted with permission from Elsevier

Type I Acetabular Component Malposition

The appropriate orientation or “safe zone” of the acetabular component was described classically by Lewinnek as 40° of abduction and 15° of anteversion [21]. However, the idea that a single safe zone exists has been questioned recently because up to 58% of dislocated hips occur in patients whose acetabular component is within the safe zone [22]. Nevertheless, the position of implants in cases of unstable total hip arthroplasties should be determined. In cases where the hip is unstable and the shell is clearly malpositioned (retroversion is most common), revision of the acetabular component and placing it in the appropriate anteversion will likely correct the problem. In general, at this time we also maximize femoral head size given its protective effect against dislocation; we also consider the use of dual-mobility bearings in this population. Acetabular component malposition is a leading cause of instability after total hip arthroplasty [27]. Both plain radiographs and CT scans may be utilized to verify implant position (Figs. 8.12 and 8.13) [28].

Fig. 8.13

CT scan of the pelvis of a patient demonstrating 19.5° of acetabular retroversion

Type II Femoral Component Malposition

The incidence of femoral component malposition as the etiology of a dislocating hip prosthesis is low, with one small series suggesting a rate of 8% [27]. Interestingly, radiographic determination of femoral component anteversion can be a challenge on plain radiographs and improving this method is a current topic of research [29]. We advocate a CT scan in which the transepicondylar axis of the femur is included along with the neck of the femoral stem (Fig. 8.12). Treatment consists of revision of the femoral component which is much more challenging in most cases than revision of the acetabular component and hence the surgeon should have a clear plan preoperatively for removal of what is typically a well-fixed but inappropriately positioned stem. As with all revisions for instability, all aspects of the reconstruction must be optimized and we typically maximize femoral head size at the time of revision and once again consider the use of a dual-mobility bearing.

Only gold members can continue reading. Log In or Register to continue