2DOF External Fixator and Computer-Assisted Surgical System for 6DOF Orthopedic Correction Using Axis-Angle Representation

Orthopedic deformities are often complex three-dimensional (3D) deformities, and the reconstruction of the original or normal geometry is difficult. In this thesis, the use of external fixators were investigate for long bone deformity correction and clubfoot correction. An external fixator works by attaching to bones or bone fragments and moving them to the target geometry. Its key advantages are that it encourages tissue growth and preserves healthy tissues. However, current six degrees of freedom (6DOF) external fixators are difficult to set up, resulting in long surgeries and steep learning curves for surgeons. They are also bulky and obstruct patient mobility. The integration of computational methods and surgical assistive device to the surgery to improve the accuracy of external fixation was proposed. A new method of defining orthopedic deformity correction was developed, and the 6DOF correction problem was reduced to just 2DOF using axis-angle representation. Therefore, only two physical trajectory joints are needed so the fixator can be more compact. The planner minimizes the bulk of the external fixator, and optimizes the distraction schedule to avoid overstretching the soft tissues. The surgical assistive device is a passive positioning linkage that assists the surgeon in building an accurate external fixator that can achieve complete correction. It is not actuated but has brakes to hold its end effector pose. The planner and linkage is expected to reduce the learning curve for surgeons and shorten surgery time. To validate the system, a patient-specific clubfoot model was developed. This model has a 3D printed rigid skeletal structure with an outer layer of gel that mimics human muscles. Thus, it can support bone pin insertions while still maintaining the flexibility to demonstrate the correction. Four experiments were performed on the foot model. 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 two hours. This is an improvement from the current method which takes 2.5 to 4.5 hours.