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Research Papers

# Repair of Periprosthetic Pelvis Defects With Porous Metal Implants: A Finite Element Study

[+] Author and Article Information
Danny L. Levine1

Orthopaedic Implant Division, Zimmer, Inc., Warsaw, IN 46580danny.levine@zimmer.com

Mehul A. Dharia, Dale A. Degroff, Douglas H. Wentz

Orthopaedic Implant Division, Zimmer, Inc., Warsaw, IN 46580

Eik Siggelkow

Orthopaedic Implant Division, Zimmer GmbH, Winterthur CH-4804, Switzerland

Roy D. Crowninshield

Crowninshield, Inc., Asheville, NC 28803

1

Corresponding author. Present address: Zimmer, Inc., P.O. Box 708, 1800 West Center Street, Warsaw, IN 46580.

J Biomech Eng 132(2), 021006 (Jan 29, 2010) (9 pages) doi:10.1115/1.4000853 History: Received April 24, 2008; Revised October 15, 2009; Posted December 17, 2009; Published January 29, 2010; Online January 29, 2010

## Abstract

Periacetabular osteolysis is a potentially difficult surgical challenge, which can often drive the choice of reconstruction methods used in revision hip replacement. For smaller defects, impaction of bone grafts may be sufficient, but larger defects can require filler materials that provide structural support in addition to filling a void. This study utilized finite element analysis (FEA) to examine the state of stress in periprosthetic pelvic bone when subjected to a stair-climbing load and in the presence of two simulated defects, to show the effect of implanting a defect repair implant fabricated from Trabecular Metal™ . Even a small medial bone defect showed a local stress elevation of $4×$ compared with that seen with an acetabular implant supported by intact periacetabular bone. Local bone stress was much greater ($8×$ the baseline level) for a defect case in which the loss of bone superior to the acetabular implant permitted significant migration. FEA results showed that a repair of the small defect with a Trabecular Metal™ restrictor lowered periprosthetic bone stress to a level comparable to that in the case of a primary implant. For the larger defect case, the use of a Trabecular Metal™ augment provides structural stabilization and helps to restore the THR head center. However, stress in the adjacent periprosthetic bone is lower than that observed in the defect-free acetabulum. In the augment case, the load path between the femoral head and the pelvis now passes through the augment as the superior rim of the acetabulum has been replaced. Contact-induced stress in the augment is similar in magnitude to that seen in the superior rim of the baseline case, although the stress pattern in the augment is noticeably different from that in intact bone.

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## Figures

Figure 1

Overview of the pelvis model with the stair-climbing load. The coordinate system origin is located at the center of the head; +X is the anterior, +Y is the medial, and +Z is the superior.

Figure 2

Series of models used in the restrictor study, including (left) a baseline model with no defect, (middle) a model with an unfilled lens-shaped defect, and (right) a defect filled with 26 mm restrictor

Figure 3

FE models used in the augment study including (left) baseline, (middle) unfilled superior defect, and (right) defect filled with augment and bone cement. The cup components are: UHMWPE liner, titanium alloy shell, and TM coating. The model at the right also includes a TM augment and bone cement.

Figure 4

Augment model. Highlighted surface faces the acetabular cup.

Figure 5

von Mises equivalent stress (MPa) in pelvis for (left) baseline, (middle) unfilled defect, and (right) defect filled with 26 mm restrictor (restrictor not shown)

Figure 6

Normalized results for the restrictor case

Figure 7

Displacement (mm) plot for a fully loaded baseline model (left), a no-augment model (middle), and a model with augment (right)

Figure 8

von Mises stress (MPa) for a baseline model (left), a model with superior defect (middle), and a model with augment (right)

Figure 9

von Mises stress (MPa) in an augment case featuring stress in augment and bone cement. The sharp discontinuity in stress pattern at the middle of the augment is due to the augment’s fenestrations (see Fig. 4).

Figure 10

Normalized peak von Mises stress results in a periprosthetic cancellous bone for the augment case

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