Contact Mechanics in

Hip ProsthesisDan Flavin

Background• Prosthetic hips a common

replacement joint in the US.

• Artificial ball and socket to replace worn biological parts.

• Hip joint formed of two parts: o Femoral implant with head

(monobloc or modular)o Acetabular cup (monobloc or

modular)

• Lubricated with synovial fluid (non-Newtonian)

Types• Three major categories:

o Metal on Polymer (MOP)• Standard 1970s , common to present• Metal femur head, UHMWPE acetabular cup

o Metal on Metal (MOM)• Both head and cup metal (316L or CoCr)• Common 1990s-2010 (recall)

o Ceramic on Ceramic (COC)• Modular metal femur with ceramic head, ceramic liner on cup• Common 1990s to present

o Other combinations (COP, MOC) also availableo All available in varying head sizes, clearances

• Larger head less likely to dislocate, but has higher friction and inertia

• MOP and MOM tend to shed debris, COC can fracture

• All have similar lifespans

The Problem• To address wear and debris problems, need to

determine contact area and loading.• This data can then be combined with lab

experiments to build full understanding of action at joint.

• Complicated by convexity – methods include “flattening” into Hertzian problem.

• Apply FEA with COMSOL to determine contact area, to allow further analsis.

The Model

The Good Results (MOM, COC)

The Not So Good Results (MOP)

Contact Area Charts

0.01 0.012 0.014 0.016 0.018 0.02 0.022 0.024 0.0260.00%

0.50%

1.00%

1.50%

2.00%

2.50%

COC Surface Area Use(by joint clearance, in µm)

COC (50) COC (100) COC (150)

Head Diameter

Conta

ct

Perc

ent

0.01 0.0120.0140.0160.018 0.02 0.0220.0240.0260.00%

5.00%

10.00%

15.00%

20.00%

25.00%

30.00%

35.00%

MOP Surface Area Use(by joint clearance, in

µm)

MOP (50) MOP (100) MOP (150)

Head Diameter

Conta

ct

Perc

ent

Conclusion• Segmentation of graph suggests finer mesh

required near contact point• Complete FUBAR of MOP results suggests failure

in model for larger displacements. o Use load stepping or larger displacement plugin to allow better control

of model