Technical Briefs

The Influence of Process Variation on a Cortical Bone Interference Fit Pin Connection

[+] Author and Article Information
Nathan A. Mauntler, Tony L. Schmitz

Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611

John C. Ziegert

Department of Mechanical Engineering, Clemson University, Clemson, SC 29607

J. Manuf. Sci. Eng 130(2), 024501 (Mar 24, 2008) (7 pages) doi:10.1115/1.2896103 History: Received December 15, 2006; Revised December 31, 2007; Published March 24, 2008

The interference fit is a common method for creating mechanical assemblies. When manufacturing the individual components to be assembled in this method, close dimensional control of the mating components is required in order to ensure that the amount of interference is sufficient to create a secure assembly, but not so great as to cause excessive stresses or failure of the individual components. In this work, we study interference fit connections in an assembly of human (cadaveric) cortical and cancellous bone, i.e., an allograft, used in spinal fusion surgeries. A difficulty encountered in this application is that, in addition to the machining steps, the assembly must go through subsequent sterilization and lyophilization, or freeze drying, processes that may affect the quality of the interference fit. This report examines the quality of the allograft interference fits using dimensional measurements of manufactured components at all stages of the manufacturing process, followed by examination for cracking and measurement of the pull-apart forces for assemblies. The experimental results are compared to finite element models of the interference fit and also to Monte Carlo models of the assembly using a simple thick-wall cylinder model. Experimental results show that the lyophilization process significantly affects the component dimensions, resulting in a much greater spread in interference values and likely leading to cracking and∕or loss of interference.

Copyright © 2008 by American Society of Mechanical Engineers
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Figure 1

Schematic of a graft inserted between vertebrae in a cervical spinal fusion surgery

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Figure 2

Schematic of the cortical-cancellous bone allograft examined in this study. The outer cortical bone plates and inner cancellous block are joined using two cortical bone pins.

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Figure 3

Part measurements on the CMM: (a) cortical pins, (b) cortical plates held in the manufacturer’s machining fixture, and (c) loose cortical plates

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Figure 4

Depiction of part sorting for one donor. The cortical plate hole diameters for the cortical plate-cancellous block stack ups were measured in the fixture at all locations (1–6). The stack ups in locations 1–3 were left in the fixture and later assembled. For locations 4–6, the plates and blocks were removed and the loose plates remeasured at each subsequent step in the manufacturing process.

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Figure 5

Box plots of average diametric interference data for each of ten donors as a function of production process

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Figure 6

Illustration of cortical plate cracks

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Figure 7

Pull-apart test setup: (a) load frame grips, aluminum holding blocks, and allograft; (b) schematic of aluminum blocks epoxied to allograft

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Figure 8

Pull-apart force versus predicted interference for (a) all assembled grafts and (b) grafts without cracks

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Figure 9

Reciprocating tribometer used to obtain friction coefficient of self-mated machined cortical bone

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Figure 10

Sectioned view of von Mises stress distribution predicted by the finite element analysis

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Figure 11

Contact pressure distribution over portion of a single cortical pin in contact. The z direction represents the axial direction of the pin. The inset shows the position of the plot in the allograft.

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Figure 12

One-dimensional analytical interference fit model used in the Monte Carlo simulations

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Figure 13

Statistical distributions of the output parameters from the multiple parameter Monte Carlo simulation: (a) pull-apart force; (b) von Mises stress in the plate; and (c) von Mises stress in the pin




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