Abstract
Total hip replacement (THR) is one of the most successful orthopedic operations, yet it continues to be plagued with problems despite the many advances in the procedure. Inconsistent placement of the acetabular cup persists even in the hands of most experienced surgeons, leading to early and late failure including instability, impingement, polyethylene wear, osteolysis, and component loosening. Cup mal-position is the single greatest cause of early instability and late polyethylene wear. Despite advent of recent technology including navigation and use of fluoroscopy cup mal-alignment persists. Several studies show 50% of experienced surgeons missing the target ranges using Lewinnnk desired safe zones. The act of impaction of the cup with a mallet is a crude and unreliable process. The surgeon's mallet imparts large and uncontrolled forces on the impaction rod creating variable torques, leading to inconsistent cup placement. Navigation and Fluoroscopy add precision to the operation however that level of precision is not maintained throughout the course of the operation. There is a market need for a tool that helps maintain “precision tolerance” through out the course of the operation.
A new device is theoretically proposed and prototyped for this process (Patent Pending). The new paradigm involves elimination of impaction forces created by unpredictable blows of the mallet. A low energy and high frequency device is utilized to insert and position the acetabular cup without the use of the mallet. The cup is inserted (not impacted) with significantly less force than the typical 2000N forces created with a mallet. The cup is also simultaneously positioned to the desired alignment while the device is active with the surgeon effectively feeling minimal haptic resistance to the movement of the cup. The system therefore proposes to eliminate cup mal-alignment for all surgeons, removing the primary cause of hip dislocations as well as factors contributing to late failure. In addition the idea allows the academic surgeon to better study the relationship of the position of the cup and clinical outcomes eliminating the need to use “safe zone ranges”. As well, this process completely eliminates acetabular fractures as a complication of this operation.
Two devices were prototyped with use of electrical and pneumatic energy. Both devices proved the concept. Both devices allowed modulation of the applied force and “effective” disarming of the frictional forces involved in cup impaction, allowing insertion and positioning of the acetbular cup to occur with smooth haptic control and without the use of violent force. The device can be used individually, with navigation and fluoroscopy, with robots and/or with any other intra-operative measurement device and can be a significant adjunct for THR.
Cup Mal-Alignment is an unsolved problem in THR surgery causing poor outcomes for patients, anxiety and a sense of failure for the surgeons, and a great cost to society in general. A new device is described to solve this problem. The science involved is proposed and described in detail and primarily involves understanding and utilizing the mechanical properties of bone/pelvis and understanding and manipulating the complex frictional forces at play.
