Navigation and hip impingement simulation software has been available for over 10 years.

Although hip implant retrieval analysis supports a clinical need for hip navigation, the current commercially available systems fail to provide the level of accuracy, cost, ease of use, and intraoperative functionality to be widely accepted. With the addition of highly accurate hip motion simulations that model all possible combined hip motions, it is felt hip navigation can become both simpler and more robust. A new navigation system is proposed that should operate with an increased level of accuracy, ease of use, and functionality.

Materials and Methods: Simulations of native and implant hip motions to impingement were modeled in HipNav a noncommercial validated hip navigation and simulation program. Implant simulations and simulations based on 30 normal hip high resolution CT scans were performed. Further, 30 normal cadaver hips were studied by optical tracking navigation technologies with modified BrainLab Vectorvision software. The results were graphic represented in 3-D data graphs representing all normal combined hip motions. Flexion/extension was graphed on one axis, abduction/adduction and internal rotation/external rotation on the other two axes. The graphs were modeled in Maple software and then for ease of presentation converted to 3D StudioMax for clarity. The simulations were also performed on CAD implant models all implantation positions within 2 standard deviations of implantation error. This was based on current CT based literature on hip implantation variability. Modeling of augmented cup liners and liner positions as well as modeling of 28mm, 32mm, and 36 mm femoral head sizes were also performed. Graphic representations of all hip motion studies were superimposed on other normal motion graphs clearly demonstrating any range of motion deficiencies. A proposed “gizmo” femoral head neck trial device (with a unique neck geometry that forces impingement) was modeled in all optimal and sub-optimal orientations. Ranges of motion simulations to impingement were then recorded. This created unusually shaped “tube graphs” of a unique character. These unique data graphs provide a “fingerprint” of the relative orientation of the femoral and acetabular components. These data sets are then stored as a reference. When performing a range of motion to impingement intra-operatively with the “gizmo” trial head/neck device, this “fingerprint” can be kinematicly obtained and matched to one of the stored reference files. This method allows the determination of implant orientations without a registration step.

Results: Many questions regarding implant positioning, head size, use and placement of augmented liners were readily and clearly demonstrated with this methodology. The 28 mm head implants could not recreate the normal hip motions that were recorded from the cadaver studies regardless of implant position. Augmented liners increased impingement to a dramatic degree when place in the posterior or superior positions. The loss of motion to impingement due to these augmented liners was equivalent to over-anteverting the cup 30 degrees. The “gizmo” device due to its large neck and unique geometry is able to force impingement resulting in the creation of unique data sets that can determine combined anteversion and cup abduction without CT, or any registration of the pelvic plane and with the patient in any position on the OR table. The use of statistical shape modeling of individual patient X-rays will further improve accuracy of this methodology.

Discussion: This method of combining the strengths of navigation and simulation creates a powerful new tool that may allow intra-operative hip navigation to become commonplace and improve the ability of the surgeon to provide a more successful and predictable surgical result for his patients The problems of ease of use, accuracy and intra-operative functionality are significantly improved with this new proposed method. The addition of this technology to existing optical tracking systems is not difficult and the additional hardware and software required to implement this solution is readily available. The ability of this method not to require a preoperative CT or intra-operative pelvic plane registration eliminates all registration errors as a contributing cause of overall combined implantation error. Although inherently reasonable, this proposed method is not yet commercially available and has not been clinically proven to reduce dislocations and impingements in patients. It remains a work in progress.

HipNav, a validated CT-based computer simulation software program, was used to calculate prosthetic and native hip ROM using collision detection. High resolution CT scans and CAD models of THA implants were used to create the simulations. Point cloud graphs were developed to graphically represent three-dimensional hip ROM graphs for all combinations of potential motion within maximal ROM parameters based on ligamentous restraints. A total of 27 normal hips were selected from a group of computer assisted total hip patients having surgery on the opposite side. The hips were then segmented and hip motion simulated inside the accepted limits of 50 degrees abduction, 30 degrees of adduction, 45 degrees internal and external rotation, 120 degrees of flexion and 40 degrees of extension

Point cloud graphs of the normal hips provided the baseline for minimal acceptable available motion. Recent literature indicates that acetabular cup placement is quite variable using traditional methods. One thousand five hundred different acetabular component positions (abduction from 30–60 degrees and 0–50 degrees of anteversion) were analyzed based on this data and their corresponding point cloud graphs were overlaid and compared to the native hip point cloud graph. The femoral component was set at 15 degrees of anteversion.

When simulating a THA with a 28mm femoral head and non-augmented liner, regardless of acetabular component positioning, native hip ROM could not be duplicated. Further, many positions inside two standard deviations of reported cup placement accuracy had substantial impingement. This technique provides a graphical tool that will help evaluate THA range of motion and clearly demonstrates how implantation accuracy affects hip ROM and impingement.