Research Papers

Compact Two Degrees-of-Freedom External Fixator System for Correction of Persistent Clubfoot Deformity

[+] Author and Article Information
Ying Ying Wu

Department of Biomedical Engineering,
Carnegie Mellon University,
5000 Forbes Avenue,
Pittsburgh, PA 15213
e-mail: yingyinw@alumni.cmu.edu

Anton Plakseychuk

Bone and Joint Center,
Magee Womens Hospital,
University of Pittsburgh Medical Center,
300 Halket Street,
Pittsburgh, PA 15213
e-mail: plakap@upmc.edu

Kenji Shimada

Department of Mechanical Engineering,
Carnegie Mellon University,
5000 Forbes Avenue,
Pittsburgh, PA 15213
e-mail: shimada@cmu.edu

1Corresponding author.

Manuscript received August 20, 2018; final manuscript received February 16, 2019; published online April 4, 2019. Assoc. Editor: Rita M. Patterson.

J. Med. Devices 13(2), 021005 (Apr 04, 2019) (6 pages) Paper No: MED-18-1138; doi: 10.1115/1.4043109 History: Received August 20, 2018; Revised February 16, 2019

Bone deformities are often complex three-dimensional (3D) deformities, and correcting them is difficult. To correct persistent clubfoot deformity in adolescents or adults, an external fixator is sometimes used to encourage tissue growth and preserve healthy tissues. However, it is difficult to set up, resulting in long surgeries and steep learning curves for surgeons. It is also bulky and obstructs patient mobility. In this paper, we introduce a new approach of defining clubfoot deformity correction as a six degrees-of-freedom (6DOF) correction, and then reducing it to just two degrees-of-freedom (2DOF) using the axis-angle representation. Therefore, only two physical trajectory joints are needed, which in turn enables a more compact fixator design. A computer planner was developed to minimize the bulk of the external fixator, and to optimize the distraction schedule to avoid overstretching the soft tissues. This reduces the learning curve for surgeons and shortens surgery time. To validate the system, a patient-specific clubfoot simulator was developed, and four experiments were performed on the clubfoot simulator. The accuracy of midfoot correction was 11 mm and 3.5 deg without loading, and 41 mm and 11.7 deg with loading. While the external fixator has to be more rigid to overcome resistance against correction, the surgical system itself was able to achieve accurate correction in less than 2 h. This is an improvement from the current method, which takes 2.5–4.5 h.

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Grahic Jump Location
Fig. 1

Simplification of clubfoot correction to midfoot deformity and heel deformity

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Fig. 2

Setup of mock surgery for clubfoot correction

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Fig. 3

The ankle axis was located (green circles) using two ball bearings attached to the hinge joints

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Fig. 4

Photo of representative fixator setup showing new D-shaped plate and trajectory joints (labeled)

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Fig. 5

Clubfoot deformity correction: (a) clubfoot simulator before correction and (b) clubfoot simulator after correction

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Fig. 6

Plot of cost function across bounding volume for one particular ankle joint axis and position. Other ankle joint axes and positions yield similar plots.

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Fig. 7

Results of distraction schedule optimization in experiment 1: (a) control points of B-spline, (b) distraction length per day of correction, (c) schedule of joint values, and (d) schedule of joint increment every day



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