Design Innovation

A Novel In Vivo Joint Loading System to Investigate the Effect of Daily Mechanical Load on a Healing Anterior Cruciate Ligament Reconstruction

[+] Author and Article Information
Mark Stasiak M. Eng1

Laboratory for Soft Tissue Research, Hospital for Special Surgery, 535 East 70th Street, New York, NY 10021; Department of Biomedical Engineering, City College of New York, New York, NYstasiakm@hss.edu

Carl Imhauser, Jonathan Packer, Asheesh Bedi, Robert Brophy, David Kovacevic, Kent Jackson, Xiang-Hua Deng, Scott Rodeo

Laboratory for Soft Tissue Research, Hospital for Special Surgery, 535 East 70th Street, New York, NY 10021

Peter Torzilli

Laboratory for Soft Tissue Research, Hospital for Special Surgery, 535 East 70th Street, New York, NY 10021; Department of Biomedical Engineering, City College of New York, New York, NY 10031


Corresponding author.

J. Med. Devices 4(1), 015003 (Mar 26, 2010) (7 pages) doi:10.1115/1.4001158 History: Received July 16, 2007; Revised December 14, 2009; Published March 26, 2010; Online March 26, 2010

We designed and validated a novel knee joint fixation/distraction system to study tendon–to-bone healing in an in vivo rat model of anterior cruciate ligament (ACL) reconstruction. The system uses an external fixator to apply a cyclic distraction of the knee joint while monitoring the resultant force developed across the joint, thus providing a temporal indication of structural changes during the healing process of the bone-tendon-bone reconstruction. The validation was performed using an optical kinematic tracking system to determine the local displacement of the knee. The average system compliance was determined to be 42.4±8.8μm/N with a coefficient of variation of 20.7%. The compliance was used to obtain a best fit correction factor which brought the total root mean square error of knee joint distraction to within 179μm (16.1%) of the applied distraction. We performed a pilot study using 15 rats that had ACL reconstructions using a flexor digitorum longus tendon autograft and found that the animals tolerated the indwelling fixator and daily anesthesia over a 10 day loading protocol. Our knee joint fixation/distraction system provides a valuable tool to study how mechanical stimuli affect in vivo bone-tendon-bone healing.

Copyright © 2010 by American Society of Mechanical Engineers
Topics: Stress , Bone , Displacement
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Figure 1

Illustration of an ACL reconstruction using a tendon graft pulled through drilled bone tunnels in the femur and tibia. The graft is secured with sutures to the periosteum at the tunnel exits outside of the joint in our animal model.

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Figure 2

Illustration of ex-fix placement immediately before ACL reconstruction. Mounting jigs are temporarily supported on the bone tunnel drill bit. These jigs hold the ex-fix parallel to the tunnel axes while two pins each, (0.9 mm threaded k-wires) are drilled into the femur and tibia. After the ex-fix pin clamps are tightened down onto these pins, the drill bit and mounting jigs are removed. The graft tendon can now be pulled through the tunnels and secured with sutures to the periosteum at the tunnel exits, completing the ACL reconstruction.

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Figure 3

Anesthetized rat during loading in the CDM with bilateral support of bone pins (schematic on top with photo on bottom)

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Figure 4

The kinematic tracking system measured the 3D spatial position of reflective markers glued to the femur and tibia, subsequently, displacement across the joint space was calculated. These data were used to develop a correction factor to account for the compliance of the ex-fix.

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Figure 5

Displacement-load responses of ten unilateral (left) and eight bilateral (right) external fixators obtained from our in vitro loading experiments. The least squares linear approximation (solid lines) for unilateral and bilateral fixation yielded a best fit compliance of 143.4 μm/N and 41.3 μm/N with R2 values of 0.72 and 0.92, respectively. The 95% confidence bands and the 95% prediction bands of the linear approximation are designated with long dashes (inner lines) and short dashes (outer lines), respectively.

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Figure 6

Sample data (specimen 6) comparing the knee displacements obtained from motion analysis (knee) to: (1) displacements measured by the LVDT, (2) predictions of knee displacement obtained using the specific compliance correction factor for an individual animal (specific), and (3) predictions of knee displacement obtained using a least squares fit to all the data

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Figure 7

Load displacement curves from 50 cycles of knee joint distraction in the CDM as recorded by the custom program from a single representative animal on day 9 of the pilot study. The curves show no evidence of loosening of the mechanical components or failure of the soft tissue, which would be indicated by sudden drops in load.



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