Unique Biomechanical Design
The core innovation of the dual mobility implant lies in its two distinct articulation points. This system consists of three main components: a small-diameter femoral head, a large polyethylene (PE) liner, and an outer metallic acetabular shell. The first articulation occurs between the small femoral head (typically 22 or 28 millimeters) and the inner surface of the concave polyethylene liner. This initial bearing is where most of the movement takes place during daily activities, similar to a conventional total hip replacement.
The second articulation involves the outer, convex surface of the large polyethylene liner moving against the inner, highly polished surface of the metallic acetabular shell. The liner is free to rotate within the metal shell, effectively functioning as a single, large ball-and-socket joint itself. The entire construct works as two joints in tandem, which allows for an increased range of motion before the implant components physically bump into each other. The polyethylene liner’s large outer diameter creates a very large effective head size for the secondary articulation.
Addressing Dislocation Risk
The dual articulation mechanism translates directly into an increase in joint stability compared to traditional single-bearing implants. The main functional benefit is the creation of a much larger effective head size, which is a direct result of the polyethylene liner moving within the metal cup. This large head size substantially increases the “jump distance,” which is the amount of lateral movement required for the femoral head to escape the socket.
The increased jump distance provides a mechanical buffer against dislocation, particularly during extreme hip movements like deep bending or twisting. When the femoral neck approaches the rim of the polyethylene liner, instead of causing the head to dislocate, the entire liner-head complex begins to rotate within the outer metal shell. This second mobility absorbs the force and motion that would otherwise lead to an unstable hip joint in a standard prosthesis. Studies have demonstrated a reduction in the rate of post-operative dislocation, which can be as high as 1% to 6% in conventional total hip arthroplasty.
Indications for Dual Mobility Use
Surgeons typically reserve the dual mobility implant for patients who are at a high risk of post-operative hip dislocation. A primary indication is revision total hip arthroplasty, where the risk of instability is inherently greater than in a first-time surgery. The implant is also frequently selected for patients who have compromised muscle function or poor control around the hip joint.
Patients who benefit from the enhanced stability provided by this design include:
- Individuals with neuromuscular disorders, such as Parkinson’s disease, or those with underlying conditions that affect balance and coordination.
- Older patients who may have cognitive impairment or dementia, as they are less likely to adhere to strict post-operative movement precautions.
- Patients with complex anatomical issues, such as severe hip deformities or a stiff lumbar spine due to prior spinal fusion.
Potential Complications and Device Longevity
Despite their advantages, dual mobility implants carry a specific set of risks unique to their two-surface design. The most distinct complication is intra-prosthetic dislocation (IPD), a failure mode where the small femoral head separates from its polyethylene liner, leaving the liner still seated in the metal cup. This event is specific to the dual mechanism and is a different issue from the entire hip joint dislocating.
The primary causes of IPD in older designs were often linked to late polyethylene wear, which compromised the retentive rim that holds the head in place. Current-generation implants, which use improved polyethylene materials and design features, have lowered the incidence of IPD, with reported rates averaging well under 1% in primary cases.
Another theoretical concern is the potential for accelerated polyethylene wear because the implant contains two friction interfaces rather than one. Long-term studies, however, have shown that modern dual mobility cups can maintain survivorship rates comparable to standard implants, with low rates of aseptic loosening and encouraging longevity data over ten years of follow-up.