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Research Papers

The Biomechanical Consequence of Insufficient Femoral Component Lateralization and Exposed Cancellous Bone in Hip Resurfacing Arthroplasty

[+] Author and Article Information
Michael Olsen1

Martin Orthopaedic Biomechanics Laboratory, St. Michael’s Hospital, 5-066 Shuter Wing, 30 Bond Street, Toronto, ON, M5B1W8 Canada; Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, ON, M5B 1W8, Canada; Orthopaedic Biomechanics Laboratory, Sunnybrook Health Sciences Centre, Orthopaedic Biomechanics Laboratory 2075 Bayview Avenue, UB 19, Toronto, ON, M4N 3M5, Canadamichaelolsen@utoronto.ca

Edward T. Davis

NHS Foundation Trust, The Royal Orthopaedic Hospital, The Woodlands Bristol Road South, Northfield, Birmingham, UK, B31 2AP

Cari M. Whyne

Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, ON, Canada; Orthopaedic Biomechanics Laboratory, Sunnybrook Health Sciences Centre, Orthopaedic Biomechanics Laboratory 2075 Bayview Avenue, UB 19, Toronto, ON, M4N 3M5, Canada; Department of Surgery, Faculty of Medicine, University of Toronto, Toronto, ON, M5B 1W8, Canada

Rad Zdero

Martin Orthopaedic Biomechanics Laboratory, St. Michael’s Hospital, 5-066 Shuter Wing, 30 Bond Street, Toronto, ON, M5B1W8 Canada

Emil H. Schemitsch

Martin Orthopaedic Biomechanics Laboratory, St. Michael’s Hospital, 5-066 Shuter Wing, 30 Bond Street, Toronto, ON, M5B1W8 Canada; Department of Surgery, Faculty of Medicine, University of Toronto, Toronto, ON, M5B 1W8, Canada

1

Corresponding author.

J Biomech Eng 132(8), 081011 (Jul 01, 2010) (7 pages) doi:10.1115/1.4001159 History: Received September 19, 2008; Revised January 15, 2010; Posted February 02, 2010; Published July 01, 2010; Online July 01, 2010

Insufficient lateralization of the femoral component coupled with exposed reamed cancellous bone has been speculated to predispose to femoral neck fracture. The current study examined the effect of mediolateral implant position and exposed cancellous bone on the strength of the resurfaced proximal femur. Composite femurs were prepared in three configurations: (1) partial, with the implant placed at the native femoral head offset of the femur, partially exposing reamed cancellous bone; (2) proud, with a medialized implant exposing a circumferential ring of cancellous bone; and (3) complete, with a lateralized implant covering all reamed cancellous bone. Specimens were loaded to failure in axial compression. A finite element model was used to further explore the effect of exposed cancellous bone, cement mantle thickness, and relative valgus orientation on the strain distributions in the resurfaced femur. The proud group (2063 N) was significantly weaker than both the partial (2974 N, p=0.004) and complete groups (5899 N, p=0.001) when tested to failure. The partial group was also significantly weaker than the complete group when tested to failure (p=0.001). The finite element model demonstrated increasing levels of strain in the superior reamed cortical-cancellous bone interface with increasing degree of exposed cancellous bone. The condition of the femoral component medialized as the result of a thick cement mantle had the greatest detrimental impact on strain level in the superior reamed cancellous bone while a valgus oriented implant provided a protective effect. This study provides biomechanical evidence that exposed reamed cancellous bone significantly reduces the load-to-failure and increases maximum strains in the resurfaced proximal femur. The perceived benefit of reconstructing the femur to its native geometry may inherently weaken the proximal femur and increase femoral neck fracture risk if the femoral component is not sufficiently lateralized to cover all unsupported reamed cancellous bone. Relative valgus orientation of the implant may help to minimize the risk of neck fracture if reamed cancellous bone remains exposed following implant impaction.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Femoral head offset measurement of intact specimen (left) and partial resurfaced specimen (right)

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Figure 2

(a) Resurfaced specimen positioned in mechanical tester and (b) FE model

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Figure 3

Mechanical testing groups: (a) partial, (b) proud, (c) complete

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Figure 4

Ultimate load-to-failure of resurfaced specimens

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Figure 5

Fracture patterns of resurfaced specimens: (a) partial, (b) proud, and (c) complete

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Figure 6

Normalized equivalent strain in superior reamed cancellous bone region for finite element models at applied loads of 3500 N and mean load-to-failure values obtained from mechanical testing

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Figure 7

Finite element model demonstrating strain pattern in complete model with 3500 N load applied. Strain was highest in the superior femoral neck adjacent the distal rim of the prosthesis.

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