Dual Mobility Hips: Biomechanics, Indications, and Risks

Dislocation is the Achilles' heel of Total Hip Arthroplasty (THA). It is the most common reason for revision in the USA. While large femoral heads (36mm, 40mm) have reduced the risk, the Dual Mobility (DM) cup has emerged as the ultimate solution for the unstable hip.

Originally designed by Gilles Bousquet in France in 1974, the concept was initially met with skepticism due to wear concerns. However, with modern highly cross-linked polyethylene (HXLPE), DM has seen a massive resurgence globally.

The Biomechanics: How It Works

The Dual Mobility construct is unique because it has two articulations (hence "Dual").

1. Small Articulation (Inner): A small femoral head (usually 22mm or 28mm) snaps into a large polyethylene liner. This articulation moves during routine daily activities (walking, sitting). It has low friction (small radius).
2. Large Articulation (Outer): The large polyethylene liner acts as a giant femoral head, articulating inside the polished metal acetabular shell. This articulation engages only at the extremes of motion (deep flexion, cross-legged sitting).

The "Jump Distance" Revolution

The primary determinant of hip stability is the Jump Distance—the vertical distance the femoral head must travel to climb out of the cup.

• Jump Distance Formula: $JD = r(1 - \cos \theta)$
  • $r$ = Radius of the head.
  • $\theta$ = Angle of the cup coverage.
• Standard 32mm Head: Small radius = Small jump distance. Easy to dislocate.
• Dual Mobility: The effective head size is the outer diameter of the liner (e.g., 48mm). Large radius = Massive jump distance. It is mechanically incredibly difficult to dislocate a DM cup.

Visual Element: A side-by-side comparison diagram. Left: Standard 32mm THA showing the head climbing the rim. Right: DM construct showing the massive excursion required to dislocate the large liner.

Indications: Who Needs It?

We don't use DM for everyone because of cost and potential wear/modular risks. It is reserved for the "High Risk" patient.

1. The "Stiff Spine" (Spinopelvic Imbalance): Patients with lumbar fusions cannot flex their spine when sitting. Their pelvis remains retroverted (open), leading to posterior impingement and anterior dislocation. DM provides the ROM forgiveness they need.
2. Neuromuscular Disorders: Parkinson’s, Cerebral Palsy, Dementia. These patients cannot follow "hip precautions."
3. Revision Surgery: Any revision for instability is a mandatory indication. Also used in cases of abductor deficiency (where the muscle tension is poor).
4. Femoral Neck Fractures: In elderly, cognitive-impaired patients undergoing THA for fracture, DM reduces the dislocation rate from ~10% to <1%.
5. Hyperlaxity: Ehlers-Danlos syndrome.

The Unique Complication: Intra-Prosthetic Dislocation (IPD)

Dual Mobility introduces a complication that does not exist in standard THA: Intra-Prosthetic Dislocation (IPD).

Mechanism

This is a failure of the retentive mechanism.

1. The polyethylene liner gets stuck (jammed) at the rim of the metal shell (impingement).
2. The patient forces the leg.
3. The small inner metal head levers out of the polyethylene liner.
4. Result: The small metal head is now rattling around inside the large metal shell ("Metal on Metal" effect), while the poly liner is floating free.

The "Bubble Sign"

On X-ray, the head appears eccentrically placed within the shell. If you look closely, you might see the crescent-shaped shadow of the dislocated poly liner in the soft tissues. This is the "Bubble Sign."

Management

IPD is a surgical emergency. The metal head will destroy the metal shell rapidly (metallosis). It requires revision of the liner and often the head.

Conclusion

Dual Mobility is a powerful tool in the arthroplasty surgeon's arsenal. It essentially solves the problem of dislocation for high-risk patients. However, it requires precise surgical technique—impingement of the liner neck against the cup rim must be avoided to prevent wear and IPD. With modern materials, the "French Paradox" (low wear despite large surface area) seems to hold true, making DM a safe long-term choice.

#Arthroplasty #HipReplacement #DualMobility #Biomechanics #OrthoImplants #RevisionHip #OrthoVellum #PatientSafety