Comparative analysis of functional outcomes and complications in dual mobility versus large femoral heads in total hip arthroplasty

Abstract

Background

Total hip arthroplasty (THA) is a common intervention for severe hip disorders. However, postoperative instability and dislocation continue to present significant challenges. To address these issues, dual mobility (DM) cups and large femoral heads (LFH) have been employed, each offering unique biomechanical benefits.

Methods

This prospective study involved 120 patients undergoing THA, with participants evenly split into two groups: Dual Mobility (DM, n = 60) and large femoral heads (LFH, n = 60). Assessments were performed using the Harris Hip Score (HHS) and a comprehensive range of motion (ROM) evaluation at 6, 12, and 24 months. The statistical analysis utilised t-tests, chi-square tests, and Mann–Whitney U tests, establishing significance at p < 0.05.

Results

The DM group showed better postoperative range of motion (ROM) across all measures, achieving statistically significant enhancements (p < 0.001) when compared to the other group. The average HHS increased by 47.8 points in the DM group, while the LFH group saw an improvement of 42.53 points. The DM group experienced no dislocations but had two instances of intraprosthetic dislocation, whereas the LFH group experienced four dislocations. The overall complication rates were comparable (p = 0.595), though the types of complications differed between the two groups.

Conclusion

Dual mobility total hip arthroplasty (THA) offers enhanced functional recovery, improved range of motion (ROM), and a lower risk of dislocation compared to LFH designs. The biomechanical stability of dual mobility implants makes them a favoured option in THA, suggesting a need for further multicenter research to confirm these findings.

Graphical abstract

1. Introduction

Total hip arthroplasty (THA) effectively alleviates pain and restores mobility in patients with advanced hip arthritis unresponsive to conservative treatments. Its goals include restoring joint mechanics, increasing mobility, and improving stability.¹ However, postoperative dislocation and instability are major complications. Dislocation rates range from 0.2 % to 10 % for primary hip replacements, increasing to 28 % in revision surgeries, influenced by patient factors, follow-up duration, and implant type.²

In 1974, G. Bousquet introduced the dual mobility (DM) socket, designed to reduce dislocation and enhance joint stability. It features a large polyethylene liner (38–54 mm) attached to a metallic acetabular component, holding a smaller femoral head (22–32 mm) for dual bearing. The primary motion occurs at the inner articulation, with the outer engaging during extreme motion.³ The DM features a mobile femoral head within a polyethylene liner inside a metallic shell, creating a soft-on-hard articulation that minimises wear and boosts stability by enlarging the effective head diameter and reducing impingement.⁴ DM implants prevent and manage hip instability through two articulation points: between the femoral head and liner, and liner and shell, unlike traditional THA's single contact. This improves motion and stability, lowering dislocation risk.

DM components are increasingly utilised in both primary and revision total hip arthroplasty (THA), enhancing stability while maintaining patient outcomes and implant longevity.⁵ Specific complications linked to this dual mobility system include intra-prosthetic dislocation and retentive failure, in which the femoral head fails to stay securely locked with the polyethylene liner, often due to wear in the liner's retaining mechanism. Additionally, this metal-on-metal contact can hasten the wear on the femoral head. A common indication of retentive failure is the “bubble sign,” which suggests liner dislocation.⁶

Despite improvements in implant design, dislocation remains a significant issue in total hip arthroplasty (THA). Two strategies—dual mobility cups and large femoral heads—aim to improve stability, each with benefits and downsides. However, comparative clinical data on their effectiveness are limited. This study compares functional results and complications to guide implant selection in THA.

2. Materials and methods

This study prospectively analysed 120 patients who underwent total hip arthroplasty at the Department of Orthopaedics of a tertiary care postgraduate institute. Each group comprised 60 patients: Group A received dual-mobility total hip arthroplasty (THA) [Fig. 1], while Group B underwent large-size femoral head single-mobility THA. Patients were assigned to Dual Mobility (DM) and Large Femoral Head (LFH) groups based on informed consent, reflecting ethical standards and clinical practice. Preoperative evaluations included range of motion and the Harris hip score,⁷ alongside demographic details such as age, sex, and the Basal Metabolic Index. These clinical parameters (range of motion and Harris Hip Score) were reevaluated during follow-up visits at 6, 12, and 24 months.

Fig. 1.

Diagrammatic illustration of dual mobility System⁸ demonstrating articulation between the acetabular component and liner, and between the liner and the femoral head. Both of these articulations allow mobility at each interface.

All patients requiring total hip replacement due to primary or secondary hip arthritis, femoral neck fractures, or hip implant failures were considered for inclusion. In contrast, patients with bleeding disorders, cancers, limb ischemia, or local/systemic infections were not included. The pre-operative assessment involved obtaining informed consent, conducting routine blood tests, taking pelvic radiographs with bilateral hip views (anterior-posterior and lateral), and performing an MRI of the affected hip when necessary.

All participants provided written informed consent to participate in the study, in accordance with the ethical guidelines established by the institutional committee, with approval number. BREC/Th/19/ortho05, dated 30 December 2019. Each participant underwent a comprehensive evaluation of their medical history and a physical examination to assess their bone and mineral status, while pain severity was measured using the VAS Scale. Data collection was conducted via a structured proforma that incorporated anthropometric measurements. Data were organised in Excel and analysed with SPSS v20.0. Chi-square tests compared categorical variables; z-tests assessed proportions where appropriate. For normally distributed data, t-tests were used; otherwise, Mann-Whitney U tests. Significance was set at 5 %, with 80 % power.

Patients were positioned laterally on a standard table. The pelvis was stabilised with a padded pegboard and four padded posts at the anterior superior iliac spine and near the sacrum. The operative hip was mobilised pre-draping for movement and stability. The limb was prepared and draped freely to allow dislocation and manipulation during surgery.

Using the Southern Moore⁹ technique, a curved incision was made approximately 5 cm distal to the greater trochanter, extending proximally along its posterior border and curving toward the posterior superior iliac spine (5–7 cm). Following the incision of the skin and subcutaneous tissue, the fascia lata and iliotibial band were split along the fibres of the gluteus maximus. A Charnley retractor was employed to maintain exposure. With the hip in internal rotation, the piriformis and short external rotators were identified and detached at their insertions, then reflected using stay sutures to protect the sciatic nerve and expose the capsule. A T-shaped capsulotomy unveiled the femoral head and neck, and after internal rotation, flexion, adduction, and gentle traction, the hip was dislocated. Releasing the gluteus maximus insertion, the inferior capsule, and the rectus femoris helped in exposing contracted hips.

A femoral neck osteotomy was performed according to the pre-operative plan, utilising the lesser trochanter as a reference point. Retractors were employed to expose the acetabulum. The labrum, pulvinar, and loose tissue were removed; acetabular reaming was carried out using landmarks (transverse ligament, cup guides) to ensure proper version and inclination. Sequential reaming is performed to achieve acetabular congruity while safeguarding the subchondral bone. The metallic acetabular shell was inserted with or without supplemental screw fixation, generally targeting a 40° inclination and 20° anteversion. The mobile polyethylene liner is snapped or locked into place within the acetabular shell [Fig. 2]. This liner features dual articulation, allowing it to rotate within the metal shell and the femoral head to rotate within it.

Fig. 2.

An illustration depicting the trail of the acetabular component, the placement of the final acetabular cup, and the positioning of the femoral liner inside the cup.

After inserting the cup, the leg was rotated internally, flexed, and adducted to elevate the femur. Soft tissue near the piriformis fossa was cleared. Femoral preparation and broaching were completed, using the vertical tibia as a guide for femoral version. Serial reaming allowed for proper femoral stem placement with correct anteversion. A small metal head (22–28 mm) was fixed to the stem. The liner was then engaged over the head, manually or with instruments, before reduction. Precautions were taken to ensure the head locked into the liner, avoiding intraprosthetic dislocation (IPD). After confirming the fit with trial components, the final implant, head, and liner were inserted into the acetabular shell and checked for stability and range of motion without impingement. Leg length was then reassessed, and soft tissue tension was evaluated.

Debris was removed, and the hip was aligned correctly, ensuring that no soft tissue remained trapped. The posterior capsule and short external rotators were repaired, haemostasis was achieved, followed by a layered closure of the fascia lata, iliotibial band, gluteus maximus, subcutaneous tissue, and skin, with a negative suction drain in place.

3. Results

Our study evaluates the functional outcomes and complication profiles of dual mobility implants compared to large femoral head constructs in patients undergoing primary total hip arthroplasty (THA), consistent with the study objectives. Each group consisted of sixty patients; approximately two-thirds (66.7 %) of those receiving single mobility total hip arthroplasty were male, while around half (53.3 %) of the dual mobility patients were male.

The mean age of patients who underwent total hip arthroplasty with dual mobility was 45 ± 10 years, while it was 52 ± 7.5 years for those who received single mobility. In the dual mobility group, the mean ± SD for weight, height, and BMI were 80 ± 9.7 kg, 1.64 ± 0.09 m, and 29.6 ± 2.2 kg/m2, respectively; in the single mobility group, the corresponding values were 81.8 ± 4.4 kg, 1.69 ± 0.07 m, and 28.6 ± 1.7 kg/m2, respectively [Table 1].

Table 1.

A comparison between pre-operative and post-operative range of motion.

Variables	Single mobility			Dual mobility
	Pre-operative (N = 60) Mean ± SD	Post-operative (N = 60) Mean ± SD	p-value	Pre-operative (N = 60) Mean ± SD	Post-operative (N = 60) Mean ± SD	p-value
Flexion	80.3 ± 4.0	88.3 ± 8.3	0.001	81.7 ± 4.3	98.7 ± 7.0	<0.001
Adduction	12.8 ± 2.9	19.7 ± 4.1	<0.001	15.1 ± 2.6	26.9 ± 3.9	<0.001
Abduction	19.3 ± 2.9	26.7 ± 4.1	<0.001	20.5 ± 3.4	35.4 ± 3.4	<0.001
External rotation in hip flexion	15.2 ± 5.0	25.0 ± 4.1	<0.001	16.4 ± 4.7	28.8 ± 3.1	<0.001
External rotation in hip extension	14.1 ± 4.5	24.5 ± 4.2	<0.001	14.9 ± 4.8	28.4 ± 3.3	<0.001
Internal rotation in hip flexion	11.6 ± 3.7	12.6 ± 1.9	0.369	11.1 ± 2.7	17.5 ± 2.6	<0.001
Internal rotation in hip extension	10.3 ± 1.7	12.1 ± 2.0	0.006	9.2 ± 1.9	17.1 ± 2.6	<0.001

In patients undergoing total hip arthroplasty with single mobility, flexion improved (mean ± SD) by 8 ± 4.3°, while those with dual mobility saw a greater increase to 17 ± 2.7°. Post-operative adduction (mean ± SD) also increased by 6.9 ± 1.2° in single mobility patients, compared to a rise of 11.8 ± 1.3° for dual mobility patients. Additionally, post-operative abduction (mean ± SD) improved by 7.4 ± 1.2° in single-mobility patients, whereas dual-mobility patients experienced a more significant increase of 14.9°.

Post-operative external rotation during hip flexion improved by 9.8 ± 0.9° in patients with single mobility after total hip arthroplasty, compared to a 12.4 ± 1.6-degree increase observed in those with dual mobility. In the case of external rotation during hip extension, single-mobility patients showed an increase of 10.4 ± 0.3°, while dual-mobility patients experienced a rise of 13.5 ± 1.5°. For internal rotation in hip flexion, patients with single mobility had a change of 1 ± 1.8°, while those with dual mobility noted an improvement of 6.4 ± 0.1°. Regarding internal rotation during hip extension, single mobility demonstrated an increase of 1.8 ± 0.3°, whereas dual mobility [Fig. 3] revealed an enhancement of 7.9 ± 0.7°.

Fig. 3.

Showcasing the post-operative range of motion (measured in degrees) in the study group.

Statistically significant postoperative changes were noted in both groups, except for internal rotation during hip flexion in patients who underwent total hip arthroplasty with a single-mobility approach. These results suggest that patients undergoing total hip arthroplasty with dual mobility showed a marked improvement in their range of motion (ROM), which was significantly higher in the DM-THA group across all measures (p < 0.001). The exception remains internal rotation during flexion in the SM-THA group, which was not statistically significant compared to those treated with single mobility.

Patients treated with total hip arthroplasty using dual mobility experienced a postoperative gain of 47.8 points, compared to a gain of 42.53 points in those using single mobility. Consequently, the results were excellent in the dual mobility group and good in the single mobility group. Although the postoperative Harris Hip Score (HHS) [Fig. 4] was higher in the DM-THA group, the difference was not statistically significant.

Fig. 4.

Comparison between the pre-operative and post-operative Harris Hip scores.

Both groups reported four cases of Deep Vein Thrombosis, and an additional four cases of hip dislocation were observed among patients who underwent single mobility total hip replacement. The overall complication rates were not statistically different (p = 0.595). However, differences in the types of complications were evident: the DM-THA group had no dislocations, while the SM-THA group experienced four [Fig. 5]. The DM-THA group also had two instances of intra-prosthetic dislocation, both involving ceramic head fractures. According to the Clavien-Dindo classification,¹⁰ most complications in both groups were Grade II, such as deep vein thrombosis. Notably, the dual mobility group had two Grade IIIb complications due to intra-prosthetic dislocations requiring surgical revision. Conversely, the LFH group experienced three Grade IIIa dislocations and one additional Grade IIIb complication.

Fig. 5.

Post-operative complications observed in dual mobility THR compared to single mobility THR.

4. Discussion

We critically analysed and interpreted the clinical and functional outcomes observed in the present study, contrasting them with existing literature. This emphasises the relative benefits, drawbacks, and complication profiles of dual mobility versus large femoral head implants in primary total hip arthroplasty.

According to the research by Rowan et al.,¹¹ which indicated that the average age of patients with dual mobility is 48.4 years and for those with fixed bearing total hip arthroplasties is 48.5 years, our study revealed that the mean age of patients receiving total hip arthroplasty with dual mobility was younger at 45 ± 10 years compared to 52 ± 7.5 years for those undergoing total hip arthroplasty with single mobility. Additionally, a prospective cohort analysis by Gonzalez AI et al.^12, which included 316 patients from a hospital registry, demonstrated that individuals who had Dual Mobility Total Hip Arthroplasty exhibited a higher mean age.

The dual mobility group's Harris Hip Score (HHS) was 43.87 ± 7.10 pre-surgery, rising to 91.67 ± 5.20 post-surgery. In the Single Mobility group, the pre-operative HHS was 45.93 ± 6.64, which increased to 88.46 ± 7.78 after the procedure. Consequently, patients who underwent Dual Mobility Total Hip Arthroplasty experienced a gain of 47.8 points, while those in the Single Mobility group gained 42.53 points. Similar findings were reported in previous studies [Table 2].

Table 2.

Comparative analysis of pre- and post-operative Harris hip scores (HHS) across studies.

Study Name	No. of Patients	Type of Implant	Pre-op HHS	Post-op HHS	HHS (Improvement)	Magnitude of change
Present Study	120	DM vs SM	44	92	+48	Large
Mahmoud MM et al.13	26	DM	12	79	+67	Very large
Carulli C et al.14	31	DM	62	76	+14	Moderate
Chughtai M et al.15	410	DM	NA	94	NA	NA
Fahad S et al.16	27	DM	74	77	+3	Small
Ukaj S et al.17	94	DM	NA	88	NA	NA
Heifner JJ et al.18	1048	DM	49.1	93	+43.9	Large

Although both groups showed a statistically significant increase in the Harris Hip Score (HHS), assessing its clinical importance requires examining the Minimal Clinically Important Difference (MCID). Previous research has demonstrated improvements in MCID following total hip arthroplasty for HHS, with results varying according to patient groups and settings (Heintzman et al.¹⁹; Clement et al.²⁰). In our study, the DM-THA group's average increase was 47.8 points, compared to 42.53 points for the SM-THA group. This indicates not only statistical significance but also meaningful clinical gains, implying that both treatments significantly improved patient function. The greater increase observed in the DM group may also suggest a potential clinical benefit in real-world recovery.

In patients who underwent Single Mobility Total Hip Arthroplasty, the mean improvements in flexion, adduction, abduction, and both internal and external rotation during hip flexion and extension were 10°, 6.9°, 7.4°, 9.8°, 10.4°, 1°, and 1.8°, respectively. Conversely, those receiving Dual Mobility Total Hip Arthroplasty exhibited average improvements of 17°, 11.8°, 14.9°, 12.4°, 13.5°, 6.4°, and 7.9°, respectively. This notable increase in range of motion is consistent with Acker et al.,²¹ who reported superior range of motion and gait parameters in patients with dual mobility cups, especially among frail elderly individuals in the same age group. Another study by Agarwala et al.²² indicated that the average range of motion for the Dual Mobility Total Hip Arthroplasty cohort significantly surpassed that of the conventional Total Hip Arthroplasty group. The enhanced range of motion in the dual mobility group is attributed to the inner head of the dual mobility cup, which supports more efficient movement and minimises the risk of impingement, thus further improving the range of motion. Furthermore, patients were divided into two age groups: ≤50 years and >50 years [Table 3].

Table 3.

Stratified Analysis by Age: Dual Mobility vs Large Femoral Head THA.

Age Group	Implant Type	n	Δ HHS (Mean ± SD)	Δ Flexion (°)	Δ Abduction (°)
≤50 years	DM	35	48.5 ± 6.2	17 ± 2.5	15 ± 2.8
≤50 years	LFH	20	44.0 ± 7.1	9 ± 3.1	8 ± 2.9
>50 years	DM	25	47.0 ± 5.9	16 ± 2.9	14 ± 2.7
>50 years	LFH	40	41.5 ± 6.8	8 ± 2.7	7 ± 2.4

In each group, the DM group demonstrated greater improvements in HHS and ROM compared to the LFH group. The benefits were similar across both age categories, indicating that the functional advantages of DM implants are not solely related to the younger average age of the DM group.

In our study, four cases of hip dislocation were identified among patients with Single Mobility Total Hip Arthroplasty. Conversely, the Dual Mobility group showed no dislocations during a 24-month follow-up. However, this group experienced two intra-prosthetic dislocations (IPDS) [Fig. 6], along with ceramic head fractures. These fractures, combined with IPDS, are severe complications that require revision surgery and can adversely affect patient morbidity and implant longevity more than standard dislocations. A retrospective analysis by Caton et al.,²³ involving 320 patients who underwent total hip replacement surgery with a minimum follow-up of 10 years, revealed that dual mobility total hip arthroplasty (THA) significantly reduced dislocation rates to 0.9 %, compared to 12.9 % for conventional total hip arthroplasty (THA). Similarly, Makeen et al.²⁴ reported one dislocation in the Dual Mobility Total Hip Arthroplasty group and another in the conventional Total Hip Arthroplasty group. Similarly, Achudan et al.²⁵ reported that four dislocations (4.6 %) occurred in the conventional THA group, while no complications were reported in dual mobility Total hip arthroplasty. Zagorov et al.²⁶ found that the dislocation rate was zero in the Dual Mobility group, compared to 11.1 % in the Fixed-bearing group and 3.1 % in the bipolar hemiarthroplasty group. Romagnoli et al.²⁷ concluded that dual mobility acetabular components help reduce the risk of post-operative complications in high-risk patients undergoing either primary or revision hip arthroplasty. Graversen et al.²⁸ observed no dislocations or revision surgeries in patients who received Dual Mobility Total Hip Arthroplasty for acute displaced femoral neck fractures. Although dual mobility (DM) implants typically provide better stability, the risk of severe IPDs and ceramic fractures underscores the need for careful implant selection, such as choosing alternatives to ceramic heads. Factors contributing to these issues may include the brittleness of ceramic heads, wear of the locking mechanism, and suboptimal assembly. Alternative head materials, such as cobalt–chromium or oxidised zirconium, could provide greater toughness and fracture resistance while maintaining low wear rates. Future studies should systematically compare different head materials in DM constructs to evaluate their effects on IPD incidence, long-term wear behaviour, and the longevity of implants.

Fig. 6.

X-ray showing intraprosthetic dislocation in a Dual Mobile Hip. The clinical image reveals notable polyethylene wear and a broken ceramic head, which was treated with a metal femoral head and a new polyethylene liner.

However, in our study, the benefits of DM are somewhat limited by certain constraints. The small sample size hampers the detection of rare complications. Conducted at a single tertiary care centre, the study may also be susceptible to selection bias. The twenty-month follow-up mainly addresses early to mid-term outcomes; longer-term data are needed to evaluate implant survival, wear—particularly of the polyethylene liner in DM implants—late dislocations, osteolysis, and revision rates. Moreover, the prospective cohort did not include randomisation or blinding, which increases the likelihood of performance and detection biases due to uncontrolled patient variables. Future studies should focus on multicentric research with larger participant cohorts, extended follow-up periods, and stratified analysis based on age and pre-existing health conditions to provide more comprehensive and universally applicable findings.

5. Conclusion

Dual mobility total hip arthroplasty provides superior functional outcomes and a notably enhanced range of motion compared to conventional single-mobility implants, while also effectively minimising the risk of postoperative dislocation. Despite a comparable rate of general complications between groups, the absence of dislocations in the dual mobility cohort highlights its potential as a more stable and dependable option for primary total hip arthroplasty. These findings confirm the biomechanical advantages offered by dual mobility constructs, particularly in optimising joint stability and mobility. However, the occurrence of intra-prosthetic dislocations and ceramic head fractures warrants further investigation into implant longevity and design refinement.

Patient consent

All procedures followed were in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and with the Helsinki Declaration of 1975, as revised in 2008 (5). Informed consent was obtained from all patients to be included in the study.

Author contributions

Primary Surgeon - Author 3 and Author 1.

Author 1 was primarily responsible for conceptualisation, study design, defining the intellectual content, conducting the literature search, overseeing clinical studies, acquiring and analysing data, performing statistical analysis, preparing the initial manuscript draft, editing, and final review.

Authors 2 and 4 made significant contributions to the study design, literature review, clinical implementation, and data acquisition.

Authors 3 and 5 participated in clinical studies, assisted in data collection and statistical analysis, and contributed to manuscript editing.

Authors 6 and 7 were involved in data analysis, helped in manuscript preparation and editing, and performed critical manuscript review. All authors have read and approved the final version of the manuscript.

Ethical statement

All participants provided written informed consent to participate in the study, in accordance with the ethical guidelines established by the institutional committee, with approval No. BREC/Th/19/ortho05, dated 30 December 2019.

Funding/sponsorship

This study is not funded or sponsored by any individual, group or society/institution.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.