Dual Mobility for Chronic Hip Instability: A Solution Option

Abstract

A dual-mobility acetabular component consists of a large, fixed, porous-coated acetabular component and a bipolar femoral component. These components are often called tripolar components. This configuration provides a stable, well-fixed implant platform against bone and 2 articular interfaces, a large polyethylene surface directly apposed to the highly polished metal shell, and a standard sized (28 mm, 32 mm) femoral head captured within polyethylene.

The dual-mobility cup was designed to reduce the incidence of dislocations in patients at increased risk of instability (eg, patients undergoing revision). The cup appears to offer a safe, effective, durable solution to hip instability. The concept has extensive laboratory and clinical support. Although the long-term durability of dual-mobility cups, particularly in young, active, large patients, is not known, the tested wear rates of the dual-mobility design with the current generation of highly cross-linked polyethylene are significantly lower than any previously reported wear rates. The recently released anatomic dual-mobility cup seeks to reduce the potential for iliopsoas impingement while retaining the stability and wear characteristics of the original dual-mobility design.

A dual-mobility acetabular component consists of a large, fixed, porous-coated acetabular component and a bipolar femoral component (Figure 1). These components are often called tripolar components. This configuration provides a stable, well-fixed implant platform against bone and 2 articular interfaces, a large polyethylene surface directly apposed to the highly polished metal shell, and a standard sized (28 mm, 32 mm) femoral head captured within polyethylene. This concept was conceived, developed, and described by French orthopedic surgeon Gilles Bousquet in 1976.^1,2

Figure 1: Fixed porous shell and bipolar femoral component.

Figure 2: Preoperative radiograph of a patient with the potential to ambulate independently with a neuromuscular disorder, limited cognitive function, and a dysplastic acetabulum (A). One year postoperatively, the patient ambulated independently and was pain free (B).

The dual-mobility implant was developed to reduce the incidence of dislocation in patients at risk of instability who required primary or revision total hip arthroplasty (THA) surgery. The patients at increased risk of dislocation after primary THA include: (1) patients older than 75 years; (2) women older than 70 years; (3) patients who have had prior hip surgery; (4) patients with neuromuscular diseases; (5) patients with cognitive dysfunction; and (6) patients with an American Society of Anesthesiologists (ASA) score >3 (Figure 2).

The dual-mobility acetabular component potentially provides substantially greater range of motion than conventional articulations with large femoral heads (Figure 3). This theoretical protection in stability has been confirmed in a number of studies (Table 1).¹ The dual-mobility cup has also been used successfully in revision THA where the dislocation rate using conventional, nonconstrained implants may be 10% to 20% (Table 2).² This high rate of stability achieved with the dual-mobility cup in patients at increased risk of dislocation has been accompanied by a survival rate that compares favorably with THA using conventional, unconstrained articular surfaces (Table 3).²

Figure 3: Potential range of motion (ROM) of the dual-mobility cup (ADM; Stryker, Mahwah, New Jersey) vs conventional articulations with large femoral heads (Trident; Stryker).

The dual-mobility design using first-generation cross-linked polyethylene has been shown to reduce polyethylene wear to rates below those reported for conventional articulations using conventional polyethylene. Moreover, when the dual-mobility concept is paired with a current highly crossed-linked polyethylene, the wear rates are 97% lower than those obtained with the first-generation cross-linked polyethylene in dual-mobility cups (Figure 4).

Figure 4: The impact of the dual-mobility design on polyethylene wear (Trident and ADM; Stryker).

The original dual-mobility shell, which was designed to accommodate a large (>48 mm) femoral head, was hemispherical. This shape had a tendency to overhang the bony rim of the acetabulum, particularly in the region of the iliopsoas tendon (Figure 5). An anatomically shaped dual-mobility cup has recently been developed to prevent impingement of the iliopsoas tendon on the shell (Figure 6). This shell is inserted with instrumentation that orients the iliopsoas cutout on the shell with the iliopsoas notch on the bony acetabulum.

Figure 5: Hemispherical dual-mobility cup associated with iliopsoas impingement (A, B). Anatomically shaped dual-mobility cup with iliopsoas cutout (C, D).

	Figure 6: Anatomic dual-mobility acetabular trial with serrations denoting the iliopsoas fossa (A). Anatomic dual-mobility shell on inserter with positioning mark visible superiorly (B). Anatomic dual-mobility shell inserted in acetabulum with positioning mark visible superiorly, assuring that iliopsoas cutout on shell is correctly oriented (C).

Conclusion

The dual-mobility cup appears to offer a safe, effective, durable solution to hip instability. The concept has extensive laboratory and clinical support. Although the long-term durability of these implants is not known, the tested wear rates of a dual-mobility design with the current generation of highly cross-linked polyethylene are significantly lower than any previously reported wear rates.

References

1. Guyen O, Pibarot V, Vaz G, Chevillotte C, Béjui-Hugues J. Use of a dual mobility socket to manage total hip arthroplasty instability. Clin Orthop Relat Res. 2009; 467(2):465-472.
2. Guyen O, Pibarot V, Vaz G, Chevillotte C, Carret JP, Bejui-Hugues J. Unconstrained tripolar implants for primary total hip arthroplasty in patients at risk for dislocation. J Arthroplasty. 2007; 22(6):849-858.

Author

Dr Stulberg is from Northwestern University, Chicago, Illinois.

Dr Stulberg is a consultant for Stryker.

Presented at Current Concepts in Joint Replacement 2009 Winter Meeting; December 9-12, 2009; Orlando, Florida.

Correspondence should be addressed to: S. David Stulberg, MD, Northwestern University, 680 N Lake Shore Dr, Ste 1028, Chicago, IL 60611 (jointsurg@northwestern.edu).

doi: 10.3928/01477447-20100722-51