Autodistraction with Force-Feedback

Investigators:

• William G. Mackenzie, MD
• Tariq Rahman, Ph.D.
• Jinyong Wee, Ph.D.
• Rahamim Seliktar, Ph.D., Drexel University
  
• Robert E. Akins, Ph.D.
• Dean Richardson, DVM, University of PA
  
• David Levine, DVM, University of PA

Sometimes children develop with one leg or one arm shorter than the other. To correct this problem, orthopedic surgeons can perform a procedure called distraction osteogenesis to lengthen the bone in the shorter limb. This limb lengthening is commonly performed in skeletally immature children and relies on distraction osteogenesis to produce new bone. The typical method is to perform an osteotomy and to mechanically distract the divided bone segments gradually so that new tissue forms to bridge the gap. The distraction rate is constant throughout lengthening (about 1 mm/day). This rate of lengthening is empirical and does not consider variations in patients. As a result, high forces are often imparted to the patient, causing pain and damage to soft tissue, articular surfaces, and growth plates. Currently, there is no procedure in which the amount of force dissipated in the limb is considered when adjusting the lengthening rates. The intent of this proposal is to develop a method of measuring the forces in the distraction zone and controlling the distraction rate with a motorized distractor based on the measured forces.

We are investigating the use of an automatic device that detects the force being applied to the limb during lengthening. It is expected that adjusting the rate of limb lengthening to match the forces in the limb will lead to better results with fewer problems and less discomfort for the patient than current practice. In particular, this project concerns the development of a sensor and controller to allow the distraction rate to be varied based on measuring the force developed during distraction. This is in contrast to the fixed-displacement-driven distraction approach used at present.

Fig 1. Autodistractor mounted on the unilateral fixator.The system includes a force sensor, motor controller and data logger.

Fig 2. System attached to sheep hind leg

We have used the newly developed system mounted on the distractor, shown in figure 1, to lengthen six sheep legs to date, figure 2. Four of these sheep were lengthened with a variable rate that was controlled by the force measured in the distraction zone. Figure 3 below shows the force profile for one of the sheep. The force was continuously measured and stored on a data logger for the duration of the lengthening. We also took radiographs periodically to assess the callus formation. The radiographs are shown below, figure 4.

Fig 3. The raw force data was filtered using a moving average with 1600 hundred points. The smoothed data is shown over the raw. The stepped line at the bottom is the distraction rate, which starts at 1mm/day and goes up to 1.75mm/day then descends to 1.5 mm/day. The arrows indicate the time of radiographs.

We have found that we can change the distraction resisting forces by varying the rate of distraction. This is seen in the force graph as a decrease in the force in two locations. Our goal is to change the force profile earlier during the distraction process before the muscles get too stiff. A key to achieving this is to be able to differentiate between soft tissue forces and callus forces. This is currently an area of investigation in the lab.

a)day 2	b)day 11	c)day 18	d)day 25	e)day 30

Fig 4. Radiographs of the callus formation during limb lengthening.

Publications

Younger ASE, Mackenzie WG, Morrison JB. Femoral forces during limb lengthening in children. Clin Orthop 1994; 301: 55-63

Wee J., Rahman T, Mackenzie W, Akins R, Levine D, Richardson DW, Dodge GR, Seliktar R. Optimizing Limb Lengthening Using an Autodistractor and Force Measurements Proceedings of the “9th Biennial ASME Conference on Engineering Systems Design and Analysis. Haifa, Israel, July 7-9 2008

Wee J., Rahman T, Mackenzie W, Akins R, Seliktar R, Dodge GR, Levine D, Richardson DW, Thabet A. Using Force Measurement to Drive Autodistraction in Limb Lengthening. The 18th Annual Scientific Meeting of the Limb Lengthening and Reconstruction Society: ASAMI-North America, Albuquerque NM. Jul 11-13, 2008