Constrained liners, dual mobility or large diameter heads to avoid dislocation in THA.

Dislocation remains a common cause of failure after total hip arthroplasty. The limitations of existing approaches to address instability have led to the development of powerfull options: constrained liners, dual mobility and large heads. These implant-related options have proven to be very efficient, but have raised concerns.With constrained liners, restricted range of motion (ROM) is responsible for impingement leading to high likelihood of failure, depending on the design, with various failure modes.Improvements of the bearing materials have addressed some of the concerns about increased volumetric wear of conventional polyethylene and offer an option to reduce instability: large diameter heads have the advantage of increased ROM before impingement, increased head-neck ratio, and jump distance. Highly cross-linked polyethylene helps address the risk for increased wear, and also large heads provide improved stability without the risk of mechanical failures observed with constrained liners. However, the increase of the head size remains limited as reducing the thickness of the liner may lead to fractures. In addition, the jump distance decreases as the cup abduction increases.The dual mobility concept simultaneously attempts to address head-neck ratio, constraint, and jump distance. Despite the need for longer follow-up, concerns raised about potential increased wear and intra-prosthetic dislocation with first generation implants have been addressed with modern designs.With a dramatic increase of the head-neck ratio whilst reducing the risk of mechanical failure or excessive wear, dual mobility THA outperforms large diameter heads and constrained liners at 10 years follow-up. For these reasons, dual mobility continues to gain interest worldwide and is becoming the most popular option to manage instability. Cite this article: Guyen O. Constrained liners, dual mobility or large diameter heads to avoid dislocation in THA. EFORT Open Rev 2016;1:197-204. DOI: 10.1302/2058-5241.1.000054.

Introduction

Despite the fact that total hip arthroplasty (THA) is commonly reported with successful results, instability remains a disappointing complication and one of the most common reasons for revision. Prevalence of instability has been reported ranging from 0.2% to 7% after primary procedures and can reach 10% and even more after revision surgery.[1]

Despite numerous surgical options which have been proposed, treatment of instability remains a challenge, and highly variable success rates have been reported. Re-operation for instability is known to carry the highest likelihood of failure of any re-operation after THA[2] with re-dislocation rates ranging from 20% to 40%.[3,4]

With better understanding of the causes of dislocation and development of improved and powerful technologies over the last decade, improved rates of prevention of instability or restoration of stability are anticipated. Selection of the implant is one of the critical steps to prevent unstable THA. Three main options that provide some protection against dislocation have emerged: constrained liners, dual mobility implants, and use of large diameter femoral heads.

A literature review of each of these options follows with particular attention to the clinical results, and advantages and disadvantages are identified.

Constrained liners

The use of constrained liners has been reported with encouraging results in restoring stability in revision THA for recurrent dislocation. Therefore, constrained implants gained interest in the late 1990s and have become one of the most popular options worldwide to treat instability. Many manufacturers have produced various commercial implants, but there are two prevailing designs of constrained liners. In the first, the liner extends beyond a hemisphere with polyethylene extended around the rim and with an inner diameter of the opening that is smaller than the prosthetic head. Reduction of the head within the liner is achieved through mechanical expansion of the inner diameter of the rim. An external metal ring is locked to the liner to prevent it from re-expanding (Fig. 1).

Fig. 1

An example of a constrained system using an external metal ring locked to the liner in order to avoid mechanical re-expansion of the liner once the head is within the socket.

In the second design, the constrained tripolar implant (Stryker Howmedica Osteonics, Rutherford, NJ) consists of a bipolar component locked into an outer polyethylene liner during the manufacturing process. The opening of the liner has an embedded metallic locking ring. The bipolar component consists of a 22 mm, 28 mm, or 32 mm prosthetic head that snaps into a polyethylene shell with a polished cobalt-chrome backing. It is free to rotate, but is locked in place by a second inner retaining ring (Fig. 2).

Fig. 2

The complex design of the constrained tripolar implant (Stryker Howmedica Osteonics, Rutherford, NJ), involving numerous parts, is shown. The bipolar component (22, 28 or 32 mm head snapped into a polyethylene shell with a polished cobalt-chrome backing and locked in place by a retaining ring) is locked into an outer polyethylene liner with an embedded metallic locking ring during the manufacturing process.

A literature review of the use of constrained implants reported a mean rate of dislocation of 10%, and a mean re-operation rate, for reasons other than dislocation, of 4%, at a mean follow-up of 51 months (range 24 to 124 months).[5]

Efficacy of constrained devices has been shown to be design-dependent. Dislocation rates have been reported ranging between 4.5% and 29% with the S-ROM cup.[6,7] The tripolar constrained design provided better short-term results with dislocation rates ranging from 2.9% to 3.5%,[8] and 6% to 7% at ten years.[9,10]

Berend et al[11] reported a ten-year follow-up of 667 constrained THAs with an overall dislocation rate of 17.5%.

Limitations of constrained implants

Both designs of constrained implants result in a restricted range of motion (ROM) and have a greater prevalence of impingement of the femoral neck on the cup. Impingement is responsible for high stress transmission to multiple interfaces, leading to liner damage, locking mechanism failure, dislocation and loosening (Fig. 3).

Fig. 3

An example of a locking mechanism failure of a constrained tripolar implant.

Unacceptably high failure rates have been reported with the use of constrained devices. Berend et al[11] reported a long-term failure rate of 42.1% with the tripolar constrained implant, while Labek et al[12] reported a 100% failure rate using the Duraloc constrained inlay (Depuy/Johnson and Johnson, Warsaw, IN). Survival at 10.2 years was 90% with the tripolar constrained implant using component failure as the end-point, and 68% for all modes of failure.[10] A review of the failed tripolar constrained implants at the Mayo Clinic found an average time to failure of only 28.4 months, with a total failure rate of 11%.[13] Five different modes of failure had been identified. Because of the complexity of the design, mechanical failures were found at most of the device interfaces. Impingement was involved in the occurrence of all types of mechanical failure.

A modified design of constrained device has been proposed, with strategically positioned polyethylene cut-outs to provide significant improvements in ROM in flexion, internal rotation, extension and external rotation.[14] A 12% re-dislocation rate and a 3% rate of revision for cup aseptic loosening at only 1.8-year follow-up has been reported in a series of patients treated for recurrent dislocation with this device.[15]

Dual mobility

Dual mobility is a concept first introduced by Bousquet in the late 1970s. Such a system combines both the large head articulation and low friction arthroplasty concepts. In a dual mobility articulation, the interposition of a mobile ultra high molecular weight polyethylene (UHMWPE) component between the prosthetic head and the highly polished inner surface of an outer metal shell provides two bearings (Fig. 4).

Fig. 4

The dual mobility concept: exploded view (a) and assembled view (b). The mobile polyethylene component between the prosthetic head and the inner surface of the metal cup provides two bearings (inner between the prosthetic head and the polyethylene component, and outer between the polyethylene component and the outer metal shell).

In so doing, the dual mobility system provides a greater effective head size and improved head-to-neck ratio (Fig. 5). Dual mobility is therefore expected to improve the ROM to impingement and joint stability. Laboratory studies have confirmed this assumption and have emphasised the advantages of dual mobility over conventional implants.[16,17] Computer simulation studies have demonstrated greater posterior jump distance with dual mobility implants than with standard hemispherical fixed bearings.[18]

Fig. 5

The dual mobility concept : the prosthetic head (22.2 mm or 28 mm) is snapped into the mobile UHMWPE component and is free to rotate, and the outer surface of the mobile component articulates against the outer metal shell. Therefore the head-to-neck ratio is increased as the mobile polyethylene component increases the effective head size which actually corresponds to the outer diameter of the mobile polyethylene component.

With dual mobility systems, in vitro motion preferentially occurs at the inner bearing and the outer bearing engages at the extremes of motion. Analysis of retrieved implants has shown wear patterns at the outer surface of the mobile UHMWPE component, confirming that mobility in vivo occurs at the two bearings.[19,20] Better understanding of the biomechanics of dual mobility has led to improvements of the original design and modern, newer-generation implants have become available, with enhanced cementless cup fixation, optimised geometry, and improved bearing materials (Fig. 6).

Fig. 6

Evolution of the design of dual mobility systems: from the original Bousquet’s design (a) to modern design (b) with optimised neck/chamfer and cup geometry. Note the thin and highly polished neck of the femoral component to limit wear at the third joint.

The use of either a dual layer coating of hydroxyapatite and titanium plasma spray, or a porous metal coating with modern dual mobility implants (stainless steel or cobalt-chromium outer shell) has improved mid-term survivorship.[21] Long-term survivorship of modern dual mobility implants are promising but are not yet available.

In addition, specific designs have been developed in order to secure the press-fit fixation in cases with limited bone stock (Fig. 7). In cases with severe bone loss, the use of cemented dual mobility implants into a cage is a reliable option to consider.[22]

Fig. 7

Examples of specific designs of dual mobility systems for revision cases are shown. According to the bony conditions, (a) the press-fit fixation of the dual mobility implant can be improved with the use of pegs and supra-acetabular screws, or (b) with a hook and flanges possibly combined with a modular cup for screw fixation and a Cobalt-Chrome liner.

Various newer geometries with subtle modifications of the original cylindrospherical design of the cup have been offered by manufacturers during recent years: hemispherical, subhemispherical, or anatomical cups are currently available in order to improve the prosthetic ROM free of impingement, and to avoid psoas tendon-to-cup impingement.

As dual mobility systems have raised concerns regarding the potential for increased polyethylene wear, highly cross-linked polyethylene, has recently been introduced. Results of in vitro wear tests support the use of highly cross-linked polethylene, with a significant reduction in wear of at least 85% under adverse conditions and over 97% under pristine conditions when compared with a single articulation hip with conventional polyethylene.[23] Even in cases with excessive cup abduction, in vitro wear patterns of dual mobility implants using highly cross-linked polyethylene compare favourably with conventional implants.[24] Clinically, encouraging reports with the use of highly cross-linked polyethylene are emerging; however, follow-up remains limited to date.[25-27]

Clinical reports on the use of first-generation dual mobility implants have emphasised the efficiency of the concept in preventing instability.[28,29]

Over the past ten years, the number of clinical reports with the use of modern designs of dual mobility implants both for primary and revision procedures has risen. Most of these studies are observational. In primary procedures, the use of dual mobility systems has been reported with low dislocation rates, ranging from 0% to 4.6% both in patients at risk for dislocation and in non-selected patients (Table 1). Unlike conventional implants, the cumulative risk for dislocation does not increase with time with dual mobility systems.[29]

Table 1.

Dual mobility implants in primary total hip arthroplasty

Year	Authors	Study	No. hips	Patients	Follow-up	Dislocation rate	Survivorship
2015	Vigdorchik et al.[25]	Multicentre retrospective	485	Unselected	Min 2yrs	0%	–
2015	Epinette et al.[27]	Prospective comparative	143	Unselected	2-6 yrs	0%	100% at 4.1 yrs
2014	Caton et al.[30]	Retrospective comparative	105	Unselected	Min 10 yrs	0.9%	97.9% at 10 yrs
2014	Epinette et al.[26]	Prospective multicentre	437	Unselected	2-5 yrs	0%	99.7% at 4 yrs
2014	Bensen et al.[31]	Retrospective	175	At risk for dislocation	–	4.6%	–
2014	Vasukutty et al.[32]	Retrospective	65	At risk for dislocation	Mean 5 yrs	0%	100% at 5 yrs
2013	Sanders et al.[33]	Retrospective	10	At risk for dislocation	Mean 3.2 yrs	0%	–
2013	Leclercq et al.[34]	Multicentre prospective	200	Unselected	10-13 yrs	0%	99% at 10 yrs
2013	Prudhon et al.[35]	Retrospective	105	At risk for dislocation	Mean 7.5 yrs	0.9%	95% at 10 yrs
2013	Combes et al.[36]	Retrospective multicentre	2480	Unselected	Min 7 yrs	0.6%(0.28% IPD*)	93% at 10 yrs
2012	Hamadouche et al.[37]	Retrospective multicentre	168	Unselected	5-8 yrs	2% (IPD*)	94.2% at 7 yrs
2008	Bauchu et al.[38]	Retrospective multicentre	150	Unselected	Mean 6.2 yrs	0%	97.4% at 7.1 yrs
2007	Guyen et al.[39]	Retrospective	167	At risk for dislocation	Mean 3.4 yrs	0%	96.4% at 5 yrs

IPD: intra-prosthetic dislocation.

Dual mobility has also been reported with remarkably low dislocation rates in revision THA, ranging from 0% to 1.4% at short- to mid-term follow-up.[40-43] In the challenging situation of unstable THA, revision is known to carry a high probability of failure.[2] Reports on the use of modern dual mobility systems in such situations have demonstrated the efficacy of dual mobility to restore stability, with short- to mid-term re-dislocation rates ranging from 0% to 5.5% (Table 2).

Table 2.

Dual mobility implants in revision procedures for instability

Year	Authors	Study	No. hips	Follow-up	Dislocation rate
2015	Van Heumen et al.[44]	Retrospective	50	2.5 yrs	0%
2014	Jakobsen et al.[45]	Retrospective	56	3.6 yrs	1.8%
2013	Saragaglia et al.[46]	Retrospective	29	3.8 yrs	3.4%
2012	Mertl et al.[47]	Retrospective multicentre	180	7.7 yrs	4.8%
2012	Hailer et al.[48]	Retrospective multicentre	228	2 yrs	2%
2011	Leiber-Wackenheim et al.[49]	Retrospective	59	8 yrs	1.7%
2010	Hamadouche et al.[50]	Retrospective	51	4.3 yrs	4.3%
2009	Guyen et al.[51]	Retrospective	54	Mean 4 yrs	5.5% (2 IPD*)

IPD: intra-prosthetic dislocation.

Limitations of dual mobility systems

With an additional bearing, dual mobility systems have raised concerns of whether or not wear might be increased compared to a conventional bearing. Clinical reports on the use of the first generation of dual mobility implants have shown encouraging results, with global survival rates as high as 81% at 15 years, 75% at 20 years, and 74% at 22 years.[29] In addition, wear measurements from retrieved first-generation dual mobility implants have confirmed low wear rates.[20] With improved designs of modern dual mobility systems, six-year survivorship has been reported as high as 100%, but no long-term survivorship data are yet available.[52]

As described previously, highly cross-linked polyethylene has been introduced to optimise wear resistance in dual mobility. However, to date, despite encouraging early results, the long-term benefit of highly cross-linked polyethylene has not been clinically demonstrated.

Intra-prosthetic dislocation (IPD) is another potential limitation of dual mobility systems.[53] This specific failure mode has been revealed with the experience of the first generation of implants, and occurs when the prosthetic head dislodges from the mobile polyethylene component. An incidence of 2% to 4% of such a complication with the first generation of implants had been reported.[28,29] As the main mechanism for such a complication results from polyethylene wear at the retentive rim of the polyethylene component, IPD is typically a long-term complication. With substantial improvements of the head/neck geometry, recent reports have demonstrated a dramatic decrease of the incidence of IPD ranging from 0% at 6 years[52] to 0.28% at 10 years[36] with the newer generation of implants using conventional polyethylene.

The recent introduction of highly cross-linked polyethylene with modern designs has raised concerns regarding the possibility of increased risk of IPD, because of the potentially reduced mechanical properties and fatigue strength of irradiated polyethylene.[54] Damage in the retentive area may lead to IPD. For this reason, despite encouraging early clinical results at between two and five years’ follow-up,[26] and because sporadic reports of early IPD have recently emerged,[55,56] the long-term benefit of highly cross-linked polyethylene in dual mobility systems is not yet clinically demonstrated.

Large diameter heads

Although dislocation is multi-factorial, head size has been recognised to have a strong influence on stability. Increasing the head size results in an increase of the head-to-neck ratio, improvement of the range of motion to impingement, and an increase in the amount of displacement required before the head dislocates.[57] This has led to an increasing use of large diameter prosthetic heads over the last decade.[58] Multiple studies have emphasised the benefit of large heads in reducing dislocation rates. However, the use of larger head sizes has raised concerns regarding wear. Advances in bearings (hard-on-hard bearings, and highly cross-linked polyethylene) with improved wear properties have led to renewed interest in the use of large heads, and have expanded prosthetic head options from the traditional sizes of 22 mm, 28 mm and 32 mm to diameters as large as 60 mm. Crowninshield et al.[59] have demonstrated an almost linear increase in the prosthetic ROM free of impingement with an increase in the femoral head diameter from 22 mm to 40 mm. In addition, the displacement required for dislocation substantially increased with the head size. However, increasing cup abduction greatly reduces the stability advantage of larger femoral heads, and may lead to increased tensile stress at the periphery of the polyethylene, material deformation, implant failure and dislocation. Despite an increase in ROM to impingement with increasing head size from 22 mm to 38 mm, Burroughs et al[60] did not observe a significant benefit going from 38 mm to 44 mm in terms of prosthetic impingement.

Other clinical studies with large femoral head sizes have been reported with reduced dislocation rates. Lombardi et al[61] reported a dislocation rate as low as 0.05% in a series of 2020 THAs using greater than 36 mm heads. Stroh et al[62] reported a significantly higher rate of dislocation with small diameter heads (1.8%) compared to the large diameter heads (0% with 36 mm or 40 mm diameters).

In a prospective randomised clinical trial comparing dislocation rates between revision THAs using 36 mm and 40 mm head diameters on one hand, with 32 mm head diameter on the other, Garbuz et al[63] reported a significantly reduced dislocation rate with the larger heads (1.1% versus 8.7%).

Lachiewicz et al[64] reported a 4% rate of early dislocation using 36 mm and 40 mm diameter heads in a series of 122 primary hip arthroplasties performed in patients presumed at high risk for dislocation.

Available larger diameter material combinations include metal or ceramic on highly cross-linked polyethylene and metal-on-metal. The use of large head metal-on-metal bearings has been largely abandoned as national joint registries have shown significantly higher failure rates.[65,66]

Large ceramic heads are available commercially in sizes up to 48 mm. Ceramic-on-ceramic bearings have extremely low wear rates, and ceramic-on-polyethylene bearings are also reported with attractive low wear rates.

Limitations of big heads

Despite the finding that large femoral heads have clearly decreased the risk of instability, they have raised some concerns regarding potential downsides. Larger diameter heads are responsible for increased volumetric wear. Reduced thickness of the polyethylene with larger head size may also lead to early failure, because of increased stress within the material. Development of highly cross-linked polyethylene has partly diminished these concerns with significant improvement of wear resistance.[67] However, concerns regarding the potential for mechanical failures and fractures with thinner polyethylene liners remain as sporadic cases have been reported.[68,69]

In addition, large metal heads have raised concerns regarding the potential adverse local tissue reactions (ALTR) secondary to corrosion and metal release at the head-neck taper junction.[70] Increasing the head size generates large torsional forces at the trunnion-head junction, and significantly increases the maximal principal stress in the neck medial area, regardless of the material used for the head (Cobalt-Chrome or Alumina).[71] These torsional forces potentiate tribocorrosion[72] and probably lead to ALTR.

The use of large heads has also been reported with potential anterior hip pain and groin pain secondary to impingement against the iliopsoas muscle or tendon.[73,74] The recent introduction of antomically-contoured heads to address this potential disadvantage of big heads has not been evaluated as yet.

Summary

Constrained liners, dual mobility and large diameter femoral heads are powerful and efficient options to prevent or to treat THA instability. Before choosing one implant, the arthroplasty surgeon should be aware of the design concept, the advantages, disadvantages and outcome data. He also has to keep in mind that the surgical techniques remain critical whatever the selected implant.

Constrained liners have been reported not only with inconsistent results on stability related to the design of the constraining device but also with a high risk for mechanical failure because of high stress transmission. Large diameter heads require the use of alternate bearings such as highly cross-linked polyethylene or ceramic in order to address the concern about increased wear. However, thickness of the liner and risk for fracture of polyethylene or ceramic remains a concern. The dual mobility concept simultaneously attempts to address head-neck ratio, constraint, and jump distance. Unlike those reports regarding the use of constrained devices and large heads, recent reports show no evidence of increased wear nor risk for mechanical failure with modern designs of dual mobility at 10 years’ follow-up. Dual mobility therefore continues to gain interest worldwide and is becoming one of the most popular current options to manage unstable THA.