Postoperative dislocation of a BHA is a relatively low-probability complication. The patients above had the first dislocation within a month postoperatively. Besides, they experienced recurrent dislocation and revision treatment. This study, on the one hand, confirms that patients with early dislocation are at high risk of recurrence. The factors contributing to dislocation in patients who had BHA are not well known. Enocson A et al. reported that the posterolateral surgical approach increases the risk of dislocation. Therefore, Therefore, the repair of short external rotator and preservation of posterior structures is an important thing. In our case, the posterior joint capsule and the abductor were repaired intraoperatively. Pala E and colleagues reported a significantly lower dislocation rate of 1.8% in the early postoperative period using a direct anterior approach (DAA). Yet, Sierra RJ et al. reported there was no significant association of dislocation with the surgical approach. A previous study reported that cognitive dysfunction was an independent risk factor associated with prosthetic dislocation. Several studies suggested that decreasing femoral offset and limb-length shortening may reduce muscle tension and induce impingement between prosthesis and acetabulum, leading to dislocation[14, 15]. It was reported that patients with low center-edge (CE) angle and femoral offset prosthesis had a higher risk for early recurrent dislocation[14, 15]. During the BHA, choose the suitable length and angle of the femoral stem neck to restore femoral offset and leg length. If the patient has a shallow acetabulum, a THA or the DAA could be considered. The mechanism leading to early dislocation in our patient may be cognitive dysfunction or the posterolateral surgical approach selection.
Routine closed reduction of a dislocated bipolar component is more difficult than that of a conventional THA because of the larger diameter of the outer cup. Closed reduction techniques should be performed with satisfactory analgesia and muscle relaxation[7, 16]. Although closed reduction was attempted in all patients, only 30 percent of patients were managed to avoid additional surgery [4, 17]. Patients with recurrent or irreducible dislocations should be treated with open reduction and component revision according to the contributing factors. Barnes CL and colleagues reported more than 50% of the patients who had redislocation eventually required operative intervention for recurrent dislocation problems. Revision surgery included such as another BHA, convert to a THA, isolated component exchange (choosing a suitable length and angle of femoral stem neck or change a bigger bipolar cup only), and so on[4, 7, 18]. Reports on the prognosis of these treatments are lacking. Some scholars reported that convert to a THA with dual mobility cups in the treatment of a hemiarthroplasty recurrent dislocation had a very good outcome. Early recurrent dislocation of BHA is a frequent indication for revision surgery. Revision method, it seems that the etiology of dislocation is affecting the eventual treatment of these recurrent dislocations.
Dislocation and dissociation of bipolar cup are closely related. Most dissociation usually occurs when the dislocation reduction fails, and dislocation may occur because of dissociated components. Varley and Parker reported that dissociations accounted for 12% of all dislocations in bipolar prostheses. Li L et al. found that 15% of all dislocations also were dissociations. The most common cause of early dissociation in the reports was manipulation for closed reduction after bipolar cup dislocation. Dissociation during attempted closed reduction is a specific implant-related complication that can occur even under general anesthesia sometimes[9, 21]. The bottle-opener mechanism is the most commonly accepted reason for early bipolar cup dissociation, and it may account for 79% of the early dissociation published cases. Another possible cause was the failure of the polyethylene locking ring. Lee YK et al. found that a single locking mechanism may be associated with an increased risk of bipolar cup-femoral head dissociation. Dual locking mechanisms may be a better recommendation in BHA. In our case, the Vario-cup had a single UHMWPE locking ring, and this may be why bipolar cup dissociation easily, even when we attempt closed reduction under general anesthesia. Three types of failure of the locking ring have been discussed. We classified the failure in our patient as a type III failure because it involved dislocation of the inner head without the detached locking ring. There are also many other reasons associated with early bipolar dissociation such us femoral head diameter, cup position, and implant design. Some scholars suggest the use of large diameter non-skirted femoral head, eccentric design of the PE liner, and small diameter polished surface femoral neck. The cause of early bipolar cup dissociation remains questionable, and further research is needed. Early recurrent dissociation of the bipolar cup is rarely reporting. The design of prosthesis may be considered one reason for recurrent dissociation. It may also be due to inappropriate treatment, to some extent, attributed to iatrogenic factors. Therefore, the management of bipolar cup dissociation is significantly important. Improper treatment may lead to recurrent dissociation or dislocation.
Rapidly increasing use of BHA may lead to a corresponding increase in the incidence of bipolar cup dissociation. Clinicians need to be aware of the risk for bipolar cup dissociation . Open reduction and revision should be prepared before a closed reduction attempt for dislocation. Once the early dislocation occurred, it always requires surgical management and bipolar component revision[8, 16, 21]. No previous studies, except for case reports, have described the strategy to manage early bipolar cup dissociation. Surgery included open reduction of the dissociation, modular components exchange (locking ring or PE liner or bipolar cup), and conversion to a THA, and another BHA (stem and bipolar components)[8, 9, 18]. We can also refer to the treatment of late bipolar cup dissociation. However, there is no standard treatment procedure, and no surgery method has shown superior results. Leonard T et.al found that mixing manufacturers when placing dual mobility articulations on well-fixed femoral stems should not increase the risk of bipolar cup instability. The isolated mobile component exchange could be used to manage early bipolar cup dissociation with well-fixed, and nondamaged implants occurring after external maneuvers[10, 16]. However, there are certain risks associated with the isolated mobile component exchange. The failure rate was 18% within 5 years, and one mode of failure was the early recurrence of intra-prosthetic dislocation. If there is a prosthesis design problem, another bipolar hemiarthroplasty with a more appropriate prosthesis mentioned above is recommended. Consequently, the management of bipolar cup dissociation must be determined according to the available historical data and cause. In the absence of components damage, a complete revision of bipolar cup, PE liner, and femoral head components is a better solution for early recurrent bipolar cup dissociation. After such difficult cases, we would recommend at least complete bipolar components and femoral head revision for early bipolar cup dissociation with well-fixed steam, even there may be a risk of failure.
There were several limitations to this study. First, the strength of our results is limited, as the study was a retrospective with a small number of subjects. Second, the follow-up periods were limited. We are aware that longer follow-up is needed to identify clinical outcomes about the treatment.