Development of a Simplified Ankle Distractor

1. Background

Osteoarthritis (OA), the joint pain and dysfunction caused by deterioration of synovial joints, is the most common joint disease. It is among the most important causes of pain, disability, and economic loss in all populations. The physical impairment caused by OA of a single lower extremity joint is comparable to that reported for major life-altering disorders such as end-stage kidney disease and heart failure. At present there is no intervention that has been proven to prevent the development and progression of OA. (Buckwalter, et al) [1]

Ankle distraction arthroplasty is a technique shown to greatly decrease pain due to end-stage ankle arthritis. Unlike arthrodesis (fusion of the joint), distraction maintains the joint’s natural movement and it is far less complicated than joint replacement. There is a considerable body of research supporting the idea that distraction of an end-stage arthritic joint (most of the work thus far has been done on ankles, although there has also been some investigation of the efficacy of the treatment for knee arthritis) for a period of weeks allows the growth of new tissue in the joint. Although this tissue is likely not true articular cartilage, its presence has been shown to significantly decrease pain in the majority of cases and to be a long lasting remedy for a condition that would otherwise commonly be treated with arthrodesis. [2]

Devices currently available for this procedure are generally complicated to use because they are designed to build frameworks that perform a wide range of functions related to fracture treatment. This versatility also tends to make those devices larger and more expensive, and their aggressively mechanical appearance makes potential joint distraction patients hesitant to use them. While fracture patients do not have a choice when treated with these devices, elective patients are instinctively resistant to their use, even when assured that the end result will most likely be a significant improvement in the quality of their lives.

2. Methods

The design and development process began with discussions with a small group of foot and ankle surgeons at the University of Iowa Hospitals and Clinics about the need for improvements in the area of ankle distraction. We then went on to develop a series of 3D CAD sketches for discussion. Following a cycle of further discussion and design iteration, we moved on to prototyping.

Earlier research in our laboratory showed that the most commonly used system, the Ilizarov fixator, produces inconsistent results when used in ankle distraction due to variability in the surgical procedure and the inherent flexibility of the wires used to support the foot. Based on that study [3], we decided to use pins for all bone fixation and to minimize pin flexure by shortening the distance between the pin insertion point and the frame.

FE model of distractor

The geometry of the distractor is based on the geometry of the ankle. Unlike the traditional approach in which the hinge axis is positioned after the frame is installed, the joint is aligned to the axis under fluoroscopy before it is fixed to the device. The concept here is a method that is both simpler and quicker without any loss of functionality.

The basic fixation scheme is a footplate to which the foot is fixed with two pins, one transfixion pin passing through the posterior calcaneus and attaching to both sides of the foot plate, and a second half pin from the lateral side into the distal process of the calcaneus. The clamps that attach the pins to the footplate are moveable to allow for holding screws that are variably placed within the calcaneus to optimize bony purchase. The tibia is then fixed by two or three half pins inserted bicortically from the anterior side to mounts on a lead screw on a frame that pivots about the foot plate on an axis approximating the ankle joint. The lead screw is advanced to achieve distraction of the joint.

We built a series of physical prototypes that we progressively refined to deal with issues that arose in testing. Initial prototypes were evaluated with plastic bone surrogates (Sawbones, Vashon, WA). We then moved on to cadaveric testing with the application of the distractor performed by a skilled orthopedic trauma surgeon. This allowed us to better evaluate its suitability for application to actual patients. At the same time, we developed a finite element model of the device that allowed manipulation of the configuration and materials of the distractor to maximize its rigidity while minimizing its weight.

FE model of distractor

3. Results

This invention is designed specifically for one procedure, ankle distraction, and it is more compact and simpler to use than other devices which may have a wider range of application at the sacrifice of simplicity.

This new device is less intimidating than other products. It is also designed to be less cumbersome to wear, as it does not extend behind the leg as other devices do.

The device includes a hinge on the axoid of the ankle’s natural rotation, which can be locked as desired. An important feature of the hinge is that the medial side is a ring which allows medial/lateral radiographic imaging of the joint itself.

While designed specifically for ankle distraction arthroplasty, it may also be used post-operatively to apply traction between the foot and the leg to unload the ankle joint surfaces, creating space within the joint to improve healing of the cartilage and forestalling the development of osteoarthritis following an injury.

A series of prototypes was iterated to correct problems as they arose. Prototypes tested with plastic bone surrogates identified the clamps to the footplate required revision. Cadaveric testing with the orthopedic trauma surgeon revealed further challenges with the foot pin clamps. Encouragingly, fluoroscopy was able to verify that the tibiotalar joint was in fact being progressively distracted as the lead screw was advanced during our cadaveric tests.

Cadaver limb in distractor

4. Interpretation

Initial trials on cadaver limbs have been successful enough to encourage us to modify the prototype further based on the comments of the surgeon.

The distractor is now protected by a provisional patent and we are in discussion with possible industry sponsors who are considering its commercial viability. Given the increasing acceptance of distraction arthroplasty, it is reasonable to expect that a device that makes the procedure simpler and more “patient friendly” may well find success in the marketplace.

We would like to move into a larger series of cadaveric testing to further validate the FE model and confirm device function during a variety of activities of daily living before proposing testing in OA patients.