What Are the Indications for Mobile-bearing Hips?
Dr. Robert Barrack spoke on the use of mobile-bearing hips in total hip arthroplasty at ICJR’s Pan Pacific Orthopaedic Congress. Following is the abstract of his presentation.
The design goals of any type of hip bearing are generally the same:
- Minimize wear
- Maximize stability and range of motion
- Maintain simplicity and flexibility in the application
One aspect of minimizing wear and decreasing complication rates is providing a wider safety margin so that bearings are tolerant of wider range of component positioning. Hard-on-hard bearings suffer from susceptibility to edge loading when components are not ideally positioned. Traditional hip replacement with metal-on-polyethylene or ceramic-on-polyethylene bearings is more tolerant of edge loading, particularly with highly cross-linked polyethylene.
A major persisting problem with traditional hip replacement, however, is dislocation, which now accounts for a very high percentage of revision procedures following primary and revision hip replacement.
Mobile-bearing hips were introduced with the goal of improving stability while maintaining low wear and increased margin for error in insertion. Potential advantages include:
- Low wear rate
- Increased range of motion
- Increased stability
- More tolerance of component malposition
This is not a new concept, having been introduced in France nearly 40 years ago. The American version of dual mobility was the bipolar hemiarthroplasty, in which there was mobility between the femoral head and the polyethylene liner, as well as an articulation between the metal outer bearing of the bipolar, which was mobile but articulated against articular cartilage. A later version of the dual mobility concept was the tripolar prosthesis configuration, in which the bipolar mated against a polyethylene liner rather than against articular cartilage.
This design potentially improved stability and reduced dislocations. However, it suffered from thin polyethylene at both interfaces and the use of conventional polyethylene in most of the small bipolar components, which were from trauma product lines and rarely, if ever, incorporated cross-linked polyethylene.
The tripolar concept was later modified by incorporating a constrained feature that was particularly useful for revisions for dislocation and in the absence of a functional abductor mechanism.
While providing stability in most cases, the constrained tripolar suffered from complications of its own:
- Decreased range of motion
- Complications of the multiple modular interfaces
The French version of the dual mobility concept was to reverse the materials involved and utilize a mobile articulating polyethylene liner against a smooth metal surface, rather than a metal head against a polyethylene surface.
While clinical results were generally good, problems included:
- Intraprosthetic failure
- Linear polyethylene wear
- Suboptimal fixation
Other concerns included lack of documentation of the wear rate of the large polyethylene mobile element against metal surface and independent documentation of the range of motion and stability of dual mobility components.
In recent years, the dual mobility hip has been introduced in the American market. Design improvements include:
- Incorporation of cross-linked polyethylene
- Use of a 28-mm head with elimination of a skirt, which should largely dispel the issue of intraprosthetic dislocation
A number of clinical studies indicate that this may well improve stability and lower the dislocation rate in challenging primary cases, as well as many revision cases prone to dislocation.
The issue of the indications for the dual mobility hip remains controversial. According to the information for use (IFU) published by the manufacturers, dual mobility hips are potentially indicated for virtually any patient who would be a candidate for hip replacement. The manufacturer does, however, list absence of an abductor mechanism or poor bone stock or skin coverage around the hip as contraindications for the procedure.
They also list a warning and a precaution regarding the use of the dual mobility polyethylene component with femoral heads from another manufacturer or articulation of the dual mobility polyethylene against metal surfaces from another manufacturer. Many revision total hips would be in one of these categories if a stem or acetabular component from a different manufacturer was retained, which is often the case.
There is strong laboratory support for improved stability due to the increased jump distance with the dual mobility hip. The major remaining question is the wear rate of the large polyethylene articulating mobile element against a metal surface.
The consensus is that the best indication currently for dual mobility hips is in patients who are at increased risk for instability in the primary or revision setting. In a primary hip, this would include:
- Older patients
- Hip fracture patients (potentially)
- Patients with other known risk factors for instability, such as neuromuscular disorders and cognitive dysfunction
Potential indications are more widely accepted in the revision scenario in which dislocation rates in double digits are often reported. Dual mobility is a more attractive option at the outset than a constrained liner, particularly if there is a functional abductor mechanism because dual mobility imparts increased range of motion and less impingement and, thus, presumably less wear than a constrained component.
It also allows for bone attachment to the acetabular shell without a tensile force, which could displace the metal shell from the pelvis before bone growth can occur.
The final advantage of a modular dual mobility is that it can be used with a versatile titanium highly porous shell with screw fixation and the full array of liner options, from large head to dual mobility to constrained. This allows for surgeon flexibility at the time of initial surgery, as well as for future revision options.
Dr. Barrack’s presentation can be found here.
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