A Novel Light Weight Navigation System Using the Compensation Method for Unknown Soft Tissue Thickness in THA

A Novel Light Weight Navigation System Using the Compensation Method for Unknown Soft Tissue Thickness in THA

  A CT-free navigation system that uses anatomical landmarks of the pelvis to measure the anterior pelvic plane is the standard in the determination of the orientation of the acetabular cup. However, there is a difference between the measured and actual pelvic plane due to measurement error caused by the thick soft tissue on the anatomical landmark. Thus, it is necessary to correct the error between the planned and inserted cup angle due to the pelvic inclination and thickness of the soft tissue. The goal of this work is to propose a compensation method using a statistical analysis of the thickness of this soft tissue according to the age and BMI result of the patient. A customized inclinometer made from a T-bar-shaped gauge and a goniometer is employed to measure the pelvic inclination in lying and standing positions in 33 volunteers. On average, the natural thickness of the soft tissue was reduced by 59.8±10.9%; this amount was 15.2mm under the maximum load. The residual error could then be offset using the statistical soft tissue model. With this statistical compensation method, it is possible to reduce the error in the anteversion, which results from the difference in the actual and measured pelvic planes, without the need for additional devices such as an ultrasonic probe.