Bipolar hemiarthroplasty is commonly used in the treatment of hip fractures [7, 9, 10, 14]. To obtain two articulating surfaces, the bipolar cup has an articulation for the femoral head within an articulation for acetabulum [1, 2, 5, 13, 14]. Because of this, there is a risk of dissociation of the femoral head from the bipolar cup [8, 12, 15, 16]. Although bipolar cup dissociation is rare, it cannot be treated with closed reduction and so typically results in open surgery [3, 8, 12, 15, 16, 18]. Dissociation of the bipolar cup may occur late or early postoperatively. Late dissociations are caused by eccentric wear of the polyethylene liner and sometimes fracture of the locking ring, which allows for the femoral head to impinge and function as a lever to dissociate the head from the liner . However, sometimes the locking ring can fracture in the absence of polyethylene wear, and that same “bottle-opener effect” can cause early dissociation of the bipolar cup during attempts at closed reduction of a dislocated bipolar hemiarthroplasty [8, 15]. During the closed reduction, the posterior acetabular rim can act as a fulcrum to pivot the bipolar cup and separate it from the femoral head.
Various mechanisms are currently used for locking the bipolar cup and the femoral head component. The locking mechanism can be categorized into two types according to the number of locking mechanisms. One is a single locking mechanism of the polyethylene ring (Fig. 1) and the other is a dual locking mechanism of the outer metal ring lock and the inner polyethylene ring lock (Fig. 2). Although some case reports have demonstrated dissociation of the bipolar cup during closed reduction of dislocated bipolar hemiarthroplasty [3, 12, 15, 16, 18], there has been no study to our knowledge on the frequency of and factors associated with early dissociation of bipolar hemiarthroplasty.
Therefore, the purposes of this study were (1) to evaluate the frequency of dissociation of the femoral head bipolar cup during manual reduction of a dislocated bipolar hemiarthroplasty; and (2) to explore in a preliminary fashion any prosthesis or patient factors that may be associated with dissociation.
Patients and Methods
We reviewed the medical records and radiographs of 60 patients (60 hips) who were treated within 3 years of a bipolar hemiarthroplasty for prosthetic dislocation. The time period of this study was May 2003 to August 2016, and it included records from three tertiary care referral hospitals. Fifty-two patients underwent hemiarthroplasty at our institutions and eight had their index procedures performed elsewhere. We excluded five hips (five patients) because the locking mechanisms of the bipolar cups could not be identified. This left 55 hips (of which 52 had the index hemiarthroplasty performed at one of the three participating centers) for evaluation in this retrospective study. Because the index hemiarthroplasties were not performed exclusively at the three participating centers, we cannot comment on the indications for prosthesis selection in these patients (such as why some patients may have received components with single rather than double locking mechanisms). Choice of bipolar cup design depended on the manufacturers of femoral stems. There were 13 men and 42 women. Mean age at the time of dislocation was 79 years (range, 66–95 years). Dislocation occurred 2 weeks to 33 months (mean, 4.4 months) after the hemiarthroplasty.
The bipolar cups in these patients included the Multipolar Cup® (Zimmer, Warsaw, IN, USA), which was used in 26 hips; the Bencox Bipolar Cup® (Corentec, Cheoan, South Korea) used in 20 hips; the Bipolar Head® (Aesculap, Tuttlingen, Germany) used in five hips; the RINGLOC Cup® (Biomet, Warsaw, IN, USA) used in three hips; and the Self-Centering Cup® (DePuy, Warsaw, IN, USA), which was used in one hip. A single locking mechanism was used in 26 hips (Bencox Bipolar Cup®, Bipolar Head®, and Self-Centering Cup®) and a dual locking mechanism in 29 hips (Multipolar Cup® and RINGLOC Cup®).
The outer diameter of the bipolar cup was 42 to 53 mm (median, 45 mm). An alumina femoral head was used in 45 hips and a metallic head in 10. The diameter of the femoral head was 28 mm in 53 hips and 22 mm in two hips. A short-neck head was used in 28 hips, a medium-neck head in 19, and a long-neck head in eight hips.
Cemented stems were used in 12 hips and cementless stems in 43. The femoral stems were the CLS (Zimmer) in 13 hips, the MS-30 (Zimmer) in eight, the Bencox II Stem (Corentec) in six, the Bencox Stem (Corentec) in four, the Benfix Long Stem (Corentec) in four, the Bencox ID stem (Corentec) in three, the Bencox M stem (Corentec) in three, the Bicontact® (Aesculap) in five, the ML Taper Stem® (Zimmer) in four, the TaperLoc® (Biomet) in three, the Versys® (Zimmer) in one hip, and the KAR Stem® (DePuy) in one hip.
Manual reduction was attempted in all 55 dislocations at 0 to 17 days after the dislocation; the procedure was successful in 34 hips. The remaining 21 hips underwent open reduction and revision of the bipolar components and femoral heads after the unsuccessful closed reduction; of those, 14 hips had persistent dislocations after the attempted closed reduction, and seven had dissociation of the bipolar cup caused by the attempted closed reduction.
Demographic and clinical data, including gender, age, body mass index, the time to dislocation after the hemiarthroplasty, the time to the attempt of manual reduction after the dislocation, and type of locking mechanism of the bipolar cup, were compared between the dissociation group and the nondissociation group. The locking mechanism of the bipolar cup was categorized as either a single locking (Fig. 1) or a dual locking mechanism (Fig. 2).
Statistical analysis was performed with use of the Mann-Whitney U test and the Fisher’s exact test. Differences were considered significant at the p < 0.05 level.
This study was approved by our institutional review board, and informed consent was waived.
Among 55 dislocated bipolar hemiarthroplasties, dissociation of the bipolar cup from the femoral head occurred in seven hips (13%) during manual reduction of the dislocated bipolar hemiarthroplasty (Fig. 3).
Among the patient and prosthesis factors we considered, the only one associated with an increased risk of dissociation during attempts at closed reduction was the presence of a single internal polyethylene ring rather than a double locking mechanism. Six of the seven dissociated hips had an internal polyethylene ring for the locking mechanism, whereas 20 of 48 (42%) of the hips that dislocated but did not dissociate had only a polyethylene ring locking mechanism (p = 0.044). Otherwise, with the numbers available, there were no differences in terms of patients’ demographic and prosthetic factors between the group with dissociation (n = 7) and those that did not dissociate (n = 48; Table 1). However, because of the small number of patients with dissociation, we caution the reader that we may be underpowered on some comparisons, and we could not perform a multivariate analysis to control for potential confounding variables. Our findings on this research question, therefore, must be viewed as preliminary.
Dislocation of bipolar endoprostheses is an uncommon complication of patients treated for hip fractures [3, 12, 15]. Dissociation during attempted closed reduction is a complication that can occur in this vulnerable subset of patients [15, 16, 18]. We found that this occurred in 13% (seven of 55) of our patients, and, although our study is small and must be considered preliminary, the only factor we found that was associated with prosthesis dissociation during closed reduction was the presence of a single polyethylene locking mechanism rather than a dual locking mechanism.
This study has limitations. Because not all of the patients in this series had the index procedure performed at our center, we cannot comment on whether selection bias might have influenced the findings; specifically, we do not know what specific factors, if any, informed the prosthesis selection process (five bipolar types were represented in this report). With only seven events of interest (dissociations), we were quite limited in terms of the analyses we could perform; for example, multivariate analysis could not be performed, and so we cannot determine whether confounding variables had a role to play here. Larger, multicenter studies will be needed to determine whether other factors influence both the likelihood of use of a single locking mechanism and the risk of prosthesis dissociation; however, no such factors come readily to mind, and so we believe our finding is likely robust. It certainly has face validity in that one might reasonably expect a double locking mechanism to resist dissociation better than a single locking mechanism. Also, with only a few events of interest, we may be underpowered on some of the other patient and prosthesis factors we considered. Future, larger studies will need to see whether, for example, femoral head size or femoral head material is associated with a higher likelihood of prosthesis dissociation. Our study may have been too small to make some of these comparisons effectively.
We found that the risk of dissociation during closed reduction of a bipolar endoprosthesis was 13% (seven of 55 hips). To our knowledge, this is the first study to report the frequency of bipolar cup dissociation during manual reduction of dislocated bipolar hemiarthroplasty, although there have been several case reports on bipolar cup dissociation [3, 16, 17]. In patients with dissociation of a bipolar cup, we replaced new bipolar cups and femoral heads. Although conversion to a constrained type of THA may be another option, we did not use it, because we were concerned about failures of constrained liners  and because of the cost associated with these implants. Bipolar cup dissociation can be classified as either early or late. Excessive wear of the polyethylene liner seems to contribute to late dissociations . Hasegawa et al. reported late dissociation of bipolar cups in six patients (seven hips). The mean time to failure after these arthroplasties was 7.5 years (range, 4.8–9.2 years). In their retrieval study, severe polyethylene wear at the rim and failure of the locking ring appeared to be the cause of dissociation. Once the locking ring detached, the inner head dislocated from the outer head, which moved into a varus position inside the acetabulum. In that study, the proportion of patients experiencing late dissociations was 11% .
The only factor we found that was associated with the risk of dissociation during closed reduction was prosthesis design, specifically a single polyethylene locking mechanism rather than a dual locking mechanism. The locking mechanisms of the bipolar cup and the femoral head component can be categorized as a single locking mechanism or a dual locking mechanism. In the single locking mechanism, the femoral head is assembled in the body of the polyethylene liner within a metal shell. A slotted polyethylene ring is placed and expands to lock into a groove at the peripheral lip of the polyethylene liner (Fig. 1). The dual locking mechanism has an outer metal ring inside a bipolar metal shell and a slotted polyethylene ring. The femoral head is assembled within the polyethylene liner with a slotted polyethylene ring. The inner ring expands to capture the femoral head. The polyethylene liner and captured femoral head are assembled in the metal shell. The outer metal ring locks the polyethylene liner within the metal shell (Fig. 2). In early dissociations, the so-called “bottle-opener effect” and subsequent failure of the locking mechanism can result in dissociation of the bipolar cup during closed reduction of dislocated bipolar hemiarthroplasty [8, 15]. As a result, a weak bipolar cup locking mechanism may be an important risk factor for dissociation. Implants that use eccentric offset of the bearing center (components in which the center of the outer cup is located distal to the center of the inner bearing) have been developed to reduce the risk of impingement between the metal neck and the internal locking polyethylene ring after bipolar hemiarthroplasty ; future studies will need to determine whether they also reduce the risk of dislocation or dissociation. In the present study, a single locking mechanism was associated with a higher risk of bipolar cup dissociation than the dual locking mechanism. Considering that we had so few patients with dissociation, a well-designed study with larger numbers of patients should be performed. It is likely that this will require more than three centers to achieve; it may require a national registry to achieve sufficient numbers.
In summary, this study is the first to report the frequency of bipolar cup dissociation and the role of the locking mechanism in bipolar cup dissociation. We found that a single polyethylene locking mechanism (rather than a dual locking mechanism) may be associated with the risk of bipolar cup dissociation during manual reduction of a dislocated bipolar cup, although bipolar cup dissociations were relatively rare. Even so, because dual locking mechanisms are available and have no known disadvantages, we cautiously recommend their use over single locking mechanisms in bipolar hemiarthroplasty unless future studies refute our preliminary findings.
1. Bhattacharyya T, Koval KJ. Unipolar versus bipolar hemiarthroplasty for femoral neck fractures: is there a difference? J Orthop Trauma. 2009;23:426–427.
2. Cornell CN, Levine D, O'Doherty J, Lyden J. Unipolar versus bipolar hemiarthroplasty for the treatment of femoral neck fractures in the elderly. Clin Orthop Relat Res. 1998;348:67–71.
3. Georgiou G, Siapkara A, Dimitrakopoulou A, Provelengios S, Dounis E. Dissociation of bipolar hemiarthroplasty of the hip after dislocation. A report of five different cases and review of literature. Injury. 2006;37:162–168.
4. Hasegawa M, Sudo A, Uchida A. Disassembly of bipolar cup with self-centering system: a report of seven cases. Clin Orthop Relat Res. 2004;425:163–167.
5. Healy WL, Iorio R. Total hip arthroplasty: optimal treatment for displaced femoral neck fractures in elderly patients. Clin Orthop Relat Res. 2004;429:43–48.
6. Krein SW, Chao EY. Biomechanics of bipolar hip endoprostheses. J Orthop Res. 1984;2:356–368.
7. Lee YK, Ha YC, Chang BK, Kim KC, Kim TY, Koo KH. Cementless bipolar hemiarthroplasty using a hydroxyapatite-coated long stem for osteoporotic unstable intertrochanteric fractures. J Arthroplasty. 2011;26:626–632.
8. Lee YK, Ha YC, Koo KH. In reply: 'Bottle-opener' effect in bipolar hip prostheses dislocations. J Arthroplasty. 2015;30:895.
9. Lee YK, Joung HY, Kim SH, Ha YC, Koo KH. Cementless bipolar hemiarthroplasty using a micro-arc oxidation coated stem in patients with displaced femoral neck fractures. J Arthroplasty. 2014;29:2388–2392.
10. Lee YK, Kim KC, Yoon BH, Ha YC, Koo KH. Current trends of stem use in hemiarthroplasty for femoral neck fracture in South Korea. Clin Orthop Surg. 2014;6:285–289.
11. Lewis PL, Graves SE, de Steiger RN, Cuthbert AR. Constrained acetabular components used in revision total hip arthroplasty: a registry analysis. J Arthroplasty. 2017;32:3102–3107.
12. Moriarity A, Ellanti P, Talha S, McKenna J. Dislocation and dissociation of bipolar hip hemiarthroplasty. BMJ Case Rep. 2015;2015.
13. Ng DZ, Lee KB. Unipolar versus bipolar hemiarthroplasty for displaced femoral neck fractures in the elderly: is there a difference? Ann Acad Med Singapore. 2015;44:197–201.
14. Rogmark C, Leonardsson O. Hip arthroplasty for the treatment of displaced fractures of the femoral neck in elderly patients. Bone Joint J. 2016;98:291–297.
15. Salvi AE. 'Bottle-opener' effect in bipolar hip prostheses dislocations. J Arthroplasty. 2014;29:2053–2054.
16. Uruc V, Ozden R, Duman IG, Kalaci A, Li L, Ren J, Liu J, Wang H, Sang Q, Liu Z, Sun T. Five cases of early dissociation between the bipolar hip endoprosthesis cup components; either spontaneously or during reduction maneuvers. Acta Orthop Traumatol Turc. 2017;51:172–176.
17. Yuenyongviwat V, Iamthanaporn K, Hongnaparak T. Spontaneous dissociation of bipolar hip hemiarthroplasty in a patient with nerve palsy: a case report and review of the literature. Int J Surg Case Rep. 2015;15:112–115.
18. Yun HH, Park JH, Park JW, Lee JW. Femoral stem displacement during closed reduction of a dislocated bipolar hemiarthroplasty of the hip. Orthopedics. 2010;33:118–121.