Dual Mobility Acetabular Cup Utilization in Total Hip Arthroplasty: Mitigating Instability Risks

Abstract

Background

With the increase in life expectancy and the rising number of total hip arthroplasty (THA) cases, the rate of complications is also expected to increase. One of the most challenging complications is dislocation, which is the leading cause of revision surgery within the first year after THA. This study aimed to assess hip instability rates in high-risk patients who underwent dual mobility cup (DMC) implantation.

Methods

This retrospective study reviewed all patients who underwent THA with DMC at Rasul-e-Akram Hospital between 2014 and 2021. DMC was used in high-risk patients for dislocation, including those with neuromuscular diseases (e.g., Parkinson’s disease, poliomyelitis) and intracapsular femoral neck fractures (FNF), instead of the standard cup. Clinical outcomes, instability, and other complications were recorded. All data was analyzed using SPSS software version 27.0.1. The chi-squared test was used to compare binary variables, and the student’s t-test was used to compare numerical variables after checking for normal distribution.

Results

A total of 163 patients (168 hips) underwent THA with DMC, with a mean age of 58 ± 5.3 years. Five patients had bilateral total hip surgery. The average follow-up time was 48 ± 5.8 months. The average BMI of patients was 28.3 ± 3.1. The preoperative Harris Hip Score (HHS) was 49 ± 8.5, while the postoperative HHS at the four-year follow-up was 89 ± 2.4. None of the patients experienced a dislocation requiring revision, and One case of severe wear on the polyethylene's inner surface and intraprosthetic dislocation, without a dislocated metal head, was identified and revised. No cases of component loosening or significant osteolysis were observed.

Conclusion

The use of DMC can significantly reduce the dislocation rate in patients with risk factors such as femoral neck fractures while improving patient satisfaction and restoring near-normal function. We recommend greater consideration of this cup in patients at risk of dislocation.

↑What is “already known” in this topic:

Dislocation is a key complication after THA, especially in high-risk patients with neuromuscular diseases, femoral neck fractures, or advanced age. Dual mobility cups (DMC) reduce dislocation rates through a dual-bearing design but raise concerns about polyethylene wear and intraprosthetic dislocation. Standard cups have higher dislocation rates, particularly in patients needing greater hip motion for cultural or religious practices.

→What this article adds:

This study shows DMC’s effectiveness in high-risk patients. Aging is a progressive risk factor for dislocation, suggesting potential prophylactic DMC use. Further studies are needed.

Introduction

Total hip arthroplasty, as one of the most successful procedures, has significantly reduced pain and improved patient function (1). With the increasing number of joint replacement cases and the extended life expectancy of patients, the rate of complications is also expected to rise. Hip joint replacement procedures can be associated with complications such as infection, dislocation, loosening, and periprosthetic fractures (2). One of the most challenging complications is postoperative dislocation, which leads to patient morbidity and imposes a significant burden on the healthcare system (3).

Dislocation and instability are among the most critical challenges in hip joint replacement, especially in cultures or religious settings where patients require a greater range of motion, thereby increasing the risk. Dislocation is the most common cause of revision surgery within the first year postoperatively, with a prevalence of 4% in primary joint replacement and approximately 25% in revision cases (1, 4).

The risk factors for dislocation can be categorized into patient-related and procedure-related factors. Patient-related risk factors include: Neuromuscular diseases (NMD) (5), Obesity (6), Spinopelvic disorders (7), Cognitive impairment (6), Advanced age (5), THA following femoral neck fracture or hip osteonecrosis (6) and Rheumatoid arthritis (7).

Procedure-related factors include: Posterior surgical approach, Component malposition, Small femoral head size, and inadequate soft tissue tensioning (1).

Insufficient patient education before and after surgery can also contribute to dislocation risk. A comprehensive perioperative evaluation of possible risk factors is essential to minimize the likelihood of dislocation. For example, posterior soft tissue-preserving approaches (such as lateral or anterior approaches) or proper repair of the joint capsule and short external rotator muscles during the posterior approach can help mitigate approach-related complications (8). Selecting an appropriate prosthesis design and constraint level is also crucial for preventing dislocation. This includes choosing a larger femoral head (9) or increasing constraint in patients with soft tissue deficiencies (1).

In terms of constraint, prostheses were traditionally classified as constrained or non-constrained. The low-friction Charnley’s concept applies to non-constrained prostheses (standard) (10). In constrained prostheses, the femoral head is locked inside the polyethylene using a ring, and the polyethylene liner is fixed to the metal shell (11).

To address instability after THA, prosthesis designs with two levels of mobility have been developed, such as tripolar and dual mobility cups (DMCs). In a tripolar prosthesis, a bipolar head (40 mm) is placed inside a standard polyethylene liner with a 40 mm inner diameter, which is one example (12). The dual mobility concept, first introduced in France in 1974, can be considered an evolution of the tripolar concept (13). In this design, a smaller metal head (typically 28 mm) is housed within a larger polyethylene head, which, in turn, is placed inside the metal acetabular shell (14).

The level of constraint provided by DMCs is intermediate between non-constrained and fully constrained prostheses, leading to their classification as “semi-constrained”. Due to two bearing surfaces, an effective increase in femoral head size, and a higher head-neck ratio and jump distance, DMCs enhance prosthetic stability and range of motion. However, the increase in bearing surface area raises concerns about potential long-term polyethylene wear (15, 16). Specific complications associated with DMCs include intraprosthetic dislocation (17) and failure in the polyethylene head.

Since the primary advantage of DMCs is their ability to reduce dislocation rates in both primary and revision THA (18), this study aimed to evaluate the mid-term outcomes of high-risk patients who underwent DMC implantation, with a particular focus on postoperative dislocation rates.

Methods

In this retrospective study, we included patients who underwent hip joint replacement with a dual-mobility prosthesis in the orthopedic departments of Rasoul Akram and Moheb Mehr Hospitals between 2014 and 2021. We began using dual-mobility prostheses in 2012, initially for patients with neuromuscular conditions such as Parkinson’s disease and poliomyelitis. Since 2014, we have also used this prosthesis for patients with intracapsular femoral neck fractures (FNF) instead of the standard cup (Figure 1). Dislocation is one of the most significant complications in FNF.

Figure 1.

83 years old male with Femur neck fracture. A) Preoperative radiography B) 6 years post-operative radiography

All patients underwent surgery under general anesthesia using a direct lateral approach in the lateral position. The implant used was the first-generation DMC (the SYMBOL brand, manufactured by DEDIENNE SANTÉ, Nimes, France). No cages were used in any patients. Most stems were cementless, except in patients with osteoporosis or weak bones, where a cemented stem was used.

A closed suction drain was not used in any patients. Pelvic X-rays were taken immediately after surgery. On the first postoperative day, all patients began ambulation with partial weight-bearing using a walker under the supervision of a physiotherapist. We extracted demographic information, diagnosis, underlying conditions, surgical approach, cup type, anteversion angle (based on the clinical angle recorded by the surgeon), operation time, estimated blood loss, and perioperative complications from medical charts.

All patients who received a dual-mobility cup were included in the study. The exclusion criteria were patients with a follow-up period of less than 12 months. In 12 cases, the opposite hip had previously undergone surgery with a standard cup, and these hips were not included in the study. The last pelvic radiographs of the patients were reviewed.

Patient-reported outcomes were assessed using the Harris Hip Score (HHS) before surgery and at the last follow-up visit (4). The maximum possible score is 100, with results categorized into four groups: excellent (90–100), good (80–90), fair (70–80), and poor (<70) (19). In bilateral cases, each hip was evaluated separately. The average follow-up period was determined based on both the last X-ray and patient questionnaires. Additionally, we used the Forgotten Hip Score (FHS), a Persian-validated patient-reported outcome measure (PROM) designed to assess prosthetic awareness during daily activities after total hip arthroplasty (THA) (20, 21).

Statistical Analysis

All data were collected in Excel and analyzed using SPSS software version 27.0.1.

A P-value of less than 0.05 was considered statistically significant. The chi-squared test was used to compare binary variables, and the Student’s t-test was applied to numerical variables after confirming normal distribution.

Results

Five out of 168 patients who underwent surgery with a dual-mobility system were excluded from the study due to a follow-up period of less than 12 months. The remaining 163 patients (168 hips) had an average age of 58 years at the time of surgery. Of these, 90 patients were female and 73 were male. Joint replacement was performed using the lateral approach in the lateral position in 85 right hips and 78 left hips. The average BMI of the patients was 28.3. A history of smoking was present in 16% of patients, and 8% had a history of drug abuse. The indications for joint replacement are listed separately in (Table 1). All patients had at least one risk factor in their history or examination, as shown in (Table 2).

Table 1. Etiology of total hip replacement with DMC.

Etiology	Prevalence	Percentage(%)
Idiopathic OA	48	29.4
ON	20	12.2
FNF	68	41.7
Dysplasia	24	14.7
Instability	1	0.6
AS	1	0.6
Coxaprofounda	1	0.6

Table 2. Probably Risk factors for Dislocation.

Risk factor			Patient Number	Percentage (%)
1	NMD	CVA	8	4.9
		CP	4	2.4
		polio	2	1.2
		Dysplasia	24	14.7
		Myopathy	1	0.6
		Abductor deficiency	9	5.5
2	Spine disease		44	26.9
3	FNF		68	41.7
4	ON		20	1.2
5	HO preop.		4	2.4
6	HO postop.		5	3.0
7	Obesity (BMI>35)		2	1.2
8	2 risk factors		24	14.7

Twenty-four patients had two simultaneous risk factors. The average follow-up period was 48 months (range: 12–72 months). A total of 131 patients received cementless stems, while 37 received cemented stems. A 22 mm metal head was used in 35 patients, while the remaining 133 hips received a 28 mm head.

The average Harris Hip Score (HHS) before surgery was 49, which increased to 89 at the last follow-up visit (P < 0.001). The results were categorized as excellent in 34 patients (20.2%), good in 50% of patients, fair in 20.0%, and poor in 3.0%. Only one dislocation (0.6%) was reported two months after surgery; it was successfully reduced in a closed procedure under general anesthesia. One patient in the cohort experienced an intraprosthetic dislocation, though they did not have a conventional dislocation. This resulted in a snapping sensation and a feeling of instability, leading to the patient becoming a candidate for revision surgery (Figure 2).

Figure 2.

Bilateral FHON 48 years old male. A) Preoperative radiography, B) 7 years post-operative radiography with sever polyethylene wear, C) polyeth-ylene wear, D) post-operative radiography, after head & polyethylene exchange (dry revision), stem & cup were well fixed.

No cases of prosthesis loosening were reported. One patient (0.6%) sustained a periprosthetic fracture three years after the index surgery following a fall, which required internal fixation. Another patient developed persistent wound discharge, which improved with frequent dressing changes and relative immobilization without leading to periprosthetic joint infection (PJI).

A total of 114 patients (69.9%) returned to their previous activity levels, and 89 patients (54.6%) reported that they "forgot" their joint was ever affected by "forgotten hip". Postoperative X-ray evaluations showed no progressive radiolucency around the acetabular cup nor any migration or positional changes of the cup.

Sixteen patients (9.8%) exhibited mild stress shielding around the femoral stem in the proximal metaphysis (Gruen’s zones I, VII), but there were no cases of stem loosening or need for revision. Before surgery, four patients (2.4%) had heterotopic ossification (HO) classified as Brooker Class III or IV. During surgery, any heterotopic bone accessible via the surgical approach or causing impingement was removed, and Indomethacin was administered for 35 days. In three patients (1.7%), significant abductor muscle damage occurred during the resection of heterotopic bone, necessitating the use of a dual-mobility cup to address the issue. After surgery, five patients (2.9%) developed new HO (two classified as Brooker Class I and three as Brooker Class II), but these cases did not cause any complications during follow-up, and no specific therapeutic interventions were required.

Discussion

In this study, age was not identified as an independent risk factor; however, the dislocation rate was observed to be higher among younger individuals with standard cups. Our findings indicate that dual-mobility cups (DMC) can achieve a low dislocation rate (0.6%) in patients with existing risk factors.

The results of this study demonstrate that the dual-mobility cup is a suitable option for patients at risk of dislocation. Dislocation is one of the most challenging complications after hip replacement. The reported revision rate of total hip replacement due to dislocation ranges from 2% to 26% in the literature, leading to disability, high healthcare costs, reduced patient quality of life, and poorer outcomes. Additionally, with increasing life expectancy and advancing age, spinal conditions and spinopelvic imbalance become more prevalent, further raising dislocation rates (22).

A study by Hailer et al. (23) using data from the Swedish Hip Arthroplasty Registry identified several dislocation risk factors, including a femoral head size of 22 mm, female sex, the posterior approach, small incisions in minimally invasive surgery, and femoral head osteonecrosis (FHON). Similarly, Rowan et al. (24) demonstrated a significant difference in dislocation rates between standard and dual-mobility cups. Studies also indicate that in total hip replacement following FHON and femoral neck fractures (FNF) using a standard cup, the dislocation rate is relatively high (25-27).

Regarding FNF, various studies report different dislocation rates. In four studies (28-31), the dislocation rate was below 2%, while Nich et al. (32) reported a rate of 6.6%. In a meta-analysis by Lorio et al. (33) examining standard cups in total joint replacement after FNF, the dislocation rate was reported to be below 10.6%.

Approximately 69% of dislocation-related revisions occur within the first year postoperatively, with another 15% occurring in the second year (23). This means that 85% of cases happen within the first two years. Given that our study’s average follow-up period was four years, our results are reliable and applicable in clinical practice.

In Hailer et al.’s study (23), factors such as the posterolateral approach, gender, femoral head size (22 mm), and ASA score did not significantly impact dislocation rates. In our study, the overall revision rate was 1.2%, which is comparable to other studies and lower than the revision rate associated with standard cups. Notably, the revision rate due to dislocation alone was 0.6%.

One concern regarding the dual-mobility cup is increased volumetric wear. Laboratory studies (34) have shown higher wear rates compared to standard cups. However, Laura et al. (35) suggest that with advancements in polyethylene materials and dual-mobility prosthesis design, this issue may be mitigated.

For patients with severe wear, revision surgery may be required to prevent wear particle-induced osteolysis and hip instability (36). In our study, the patient with an intraprosthetic dislocation was young, highly active, and had undergone bilateral hip replacement with DMC. This patient underwent revision surgery, during which the metal head and polyethylene head were exchanged while the acetabular shell and stem remained well-fixed and free of osteolysis (Figure 3). The retrieved large polyethylene head was analyzed in the lab, revealing significant wear in three areas (Figure 3). The outer diameter of the polyethylene head was 49 mm before surgery and 47.68 mm after surgery, indicating 1.32 mm of wear over seven years (Figure 3C).

Figure 3.

Different potential source of wear in Dual mobility polyethylene head

AB) inner surface wear “packet wear”

C) outer surface wear

D) mouth wear

The inner diameter was 28 mm before surgery and 29.96 mm after surgery, showing 0.98 mm of wear over the same period (Figure 3A, Figure 3B). Examination of the opening of the polyethylene head (Figure 3D) revealed significant corrosion, likely due to contact between the stem neck and the polyethylene head. A notable accumulation of debris was observed at the opening, which could contribute to the production of small polyethylene particles, “mouth wear”, in addition to the previous two wear sources.

In our case, there was no evidence of osteolysis or loosening after seven years. However, a longer follow-up is needed for a more definitive assessment.

A dual-mobility cup was also used in patients with severe brooker HO, where resection was necessary due to abductor muscle damage (37), increasing the future risk of dislocation. Although HO itself does not elevate dislocation risk due to relative joint stiffness, HO resection can damage the abductor muscles, leading to abductor deficiency. This justifies the use of dual-mobility cups in such cases.

Patients with developmental dysplasia of the hip (DDH) or neuromuscular disorders (NMD), such as poliomyelitis, often have weakened abductor muscles due to biomechanical disturbances of the hip joint and impaired muscle development (38, 39). In cases of unilateral or asymmetric dislocation, these patients may also have pelvic obliquity and spinal deformities, further increasing instability risks after total hip replacement. Therefore, higher constraint or dual-mobility cups are recommended for these patients (Figure 4).

Figure 4.

Right hip OA with NMD (poliomyelitis left lower extremity domi-nant) in 62 years old female. A) Preoperative radiography with pelvic obliquity, B) 2 years post-operative radiography, correct pelvic obliquity.

One challenge of using monoblock dual-mobility cups is the lack of ancillary screw fixation, which has been addressed by the development of new modular cups. However, this improvement comes with an increased risk of metallosis, necessitating further research.

In our country, cultural factors influence patient satisfaction following hip replacement. Activities such as sitting on the floor, cross-legged sitting, the Seiza position (for prayer or gatherings), and floor dining are important to many patients. If they can return to these activities postoperatively, their satisfaction improves. We used HHS to assess patient satisfaction (19), but this scale does not account for such cultural activities. This is a limitation in patient evaluation, and we recommend incorporating regional activity-based criteria into scoring systems.

Most patients undergoing hip joint replacement are elderly. Even those without current dislocation risk factors may develop them over time due to aging-related conditions such as spinal arthritis, stiffness, and reduced lumbopelvic mobility, increasing the likelihood of prosthesis dislocation. Prophylactic use of DMC may help reduce dislocation rates in this patient population, but further randomized studies are needed to confirm this.

Study Limitations

Our study has several limitations. It is retrospective in nature, and no power analysis was conducted to determine the sample size. Additionally, since all patients with risk factors were treated with dual-mobility cups at our center, we did not have a control group for comparison. Future studies with larger sample sizes and multiple patient-reported outcome measures (PROs) would provide more robust findings.

Conclusion

The Dual Mobility Cup is associated with a low dislocation rate in patients with a high baseline risk of instability. We suggest placing greater emphasis on using this cup in patients who may develop risk factors for dislocation.

Ethical Considerations

All subjects gave their informed consent for inclusion before they participated in the study. The study was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by the Ethics Committee of IR.IUMS.REC.1402.247.

Conflict of Interests

The authors declare that they have no competing interests.

Cite this article as : Khaje Mozafari J, Aminian A, Yeganeh A, Abolghasemian M. Dual Mobility Acetabular Cup Utilization in Total Hip Arthroplasty: Mitigating Instability Risks. Med J Islam Repub Iran. 2025 (7 Apr);39:51. https://doi.org/10.47176/mjiri.39.51