Hip Implant Contact Mechanics

Patient & Gait

Body weight 80 kg

Gait cycle duration 1.10 s

Gait phase (scrub when paused) 0%

Bearing Couple

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Joint force

— N

% body weight

—%

Peak contact pressure

— MPa

Contact radius

— mm

heel-striketoe-offswing

How it works

Walking loads the hip joint far more than static body weight — muscle forces stabilising the pelvis add to the ground-reaction force. The classic (Bergmann/Paul) in-vivo curve has two peaks: loading response just after heel-strike (~210–230% body weight) and pre-swing near toe-off (~185–200%), separated by a mid-stance dip, then a low baseline during swing:

F(t) ≈ BW·[0.55 + 2.05·G(t,0.13,0.055) + 1.85·G(t,0.50,0.05)]·dip(t)   in stance
F(t) ≈ 0.15·BW                                                        in swing

The femoral head and the acetabular liner are modelled as two curved elastic bodies in Hertzian contact. For force F, effective radius R and effective modulus E*:

1/E* = (1-ν₁²)/E₁ + (1-ν₂²)/E₂        (combines both materials)
a  = ( 3·F·R / (4·E*) )^(1/3)          contact-patch radius
p₀ = 3F / (2·π·a²)                     peak contact pressure
p(r) = p₀·√(1 − (r/a)²)                 pressure across the patch

UHMWPE liner — very low E* (~1 GPa): large, conforming contact patch, low peak pressure (a few MPa) but the polymer creeps and wears under repeated cycling.
Metal-on-metal — high E* (~125 GPa): small stiff contact, higher peak pressure (~100+ MPa), but the alloy tolerates it elastically — the trade-off is metal ion release, not fracture.
Ceramic-on-ceramic — highest E* (~200 GPa): the smallest, highest-pressure contact of the three, but the hardest and most wear-resistant surface — the trade-off is brittle fracture risk, not wear.
This is a simplified elastic half-space model — real UHMWPE deforms plastically at these pressures, which is exactly why the material trade-off exists.

Femoral head / stem Joint reaction force Contact-pressure sensors

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Hip Implant Contact Mechanics: Gait-Driven Bearing Stress