CORR Insights®: A Modeling Study of a Patient-specific Safe Zone for THA: Calculation, Validation, and Key Factors Based on Standing and Sitting Sagittal Pelvic Tilt

Abstract

Where Are We Now? One of John Charnley’s priorities when he designed the first successful THA more than 50 years ago was to minimize friction at the joint bearing surfaces. To accomplish this, he used a 22-mm femoral head. Advances since Charnley’s time in developing wear-resistant polyethylene [11] facilitated the use of large head sizes, with the goal of increasing hip ROM and decreasing the risk of dislocation [8, 9]. Despite these improvements, dislocation remains one of the leading complications of THA, along with loosening and infection. A recent meta-analysis that included more than 4.6 million primary THAs reported a pooled incidence of dislocation of 2.1% at a mean 6-year follow-up [9]. Among other factors, the risk of dislocation was higher in those with BMI of 30 kg/m2 and those with previous surgery including spinal fusion, and lower in those with larger femoral heads, dual mobility cups, and cemented fixation.[9] Following the pioneering work of Lazennec et al. [10], investigations have analyzed the interaction between the kinematics of the spine and the hip, and its relation to acetabular cup positioning. This, along with the position of the femoral component, can influence the risk of dislocation. Currently, three distinct challenges in THA research are ongoing to avoid impingement and dislocation. The first challenge is to screen and identify the patients at high risk of dislocation. This would involve examining spinal deformity or spinal stiffness, typically by measurements from lateral radiographs, which have been found by research investigators to be valuable for preoperative planning [6]. The second challenge is to calculate the effect of functional pelvic tilt on the position of the acetabular cup [12]. Studies have used radiographic and/or CT data to provide guidelines for cup positioning [14], while others have used the radiographic measurements to establish an analytical solution for cup positioning. For example, Snijders et al. developed a trigonometric solution, published online as a tool (www.3D-Hip.com) [13]. The third challenge is to estimate a safe, impingement-free target zone for acetabular component and femoral stem positioning. This can be addressed as a purely geometric problem. In one study [16], this problem was addressed using a three-dimensional (3-D) CAD model to perform a kinematic analysis as a function of design and implantation parameters of both components. Arguing that computerized simulations may be time consuming and impractical, one study [5] addressed this problem using analytical geometry. The authors developed patient-specific algorithms to calculate all impingement-free cup and stem positions by incorporating pelvic tilt, the prothesis ROM, and prosthetic design [5]. The current study by Tang et al. [15] used the same general approach to calculate the patient-specific zone; however, they incorporated measurements taken from standing and sitting lateral radiographs, and imposed a ≤ 45° inclination angle. They applied this algorithm in 10 robotic-assisted THA surgeries, and constructed patient-specific 3-D models to test the validity. The algorithm provided in the present study offers an efficient method to calculate impingement-free ROM, taking into account both standing and sitting pelvic tilt. If offered as a software tool, the method has the potential of providing key information for planning THA surgeries and predicting the risk of dislocation in patient-specific situations. Such a software tool, therefore, can provide key input parameters to surgical navigation or robotic systems, which in turn may be able to deliver implant positioning accurately and precisely. Where Do We Need To Go? The next frontier in minimizing the risk of THA dislocations is objective evaluation and cost-benefit analysis of all existing tools and methods. A wide variety of navigation and robotic tools are available to assist with precise positioning of components within a designated target zone. T