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

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. The safety of this zone is dependent not only on concerns for impingement and dislocation, but also fixation and bony coverage of the acetabular component. Selection from a wide variety of available tools may prove difficult. A recent review [3] summarized the accepted guidelines for defining a patient-specific safe target zone for acetabular cup placement and categorized the available surgical guidance systems based on the level of surgical control. Non-robotic navigation provides the highest level of control and flexibility for the surgeon, by allowing changes to preoperative plans based on intraoperative findings. Other systems include imageless, image-based, haptic, and robotic-assisted systems, which differ in the level and type of surgeon interaction and control, with active robots providing nearly full automation and minimal surgeon interaction [3]. The authors stated that while these tools provide superior precision (reproducibility), their true benefit in clinical outcome has not yet been established, with most studies presenting only short-term follow-up [3]. Clearly, future studies should evaluate the costs, tradeoffs, and potential benefits of each type of system.

In the absence of long-term clinical follow-up studies, investigators have relied on other approaches to test the performance of their tools. In the current study, the accuracy of the impingement-free ROM prediction was considered validated by testing the results in a patient-specific 3-D computer-aided design model, which was used to evaluate ROM in 10 patients who underwent robotic-assisted THA [15]. This raises a question regarding terminology. What is implied to the community when a method or approach is presented as validated? One study proposed three distinct terms to establish biometric monitoring technologies: verification, analytical validation, and clinical validation [4]. The standardization of terminology proposed is perhaps just as necessary for the surgical tools discussed in this commentary. Specifically, Goldsack et al. [4] stated “the term ‘clinically validated’ is often found in marketing literature for digital medicine tools, but, currently, its meaning is not clear. A standardization framework is needed to bring meaning to this term.” They argue that technologies should provide a body of evidence to support their quality and safety, as well as their efficacy. Specifically, pertaining to surgical navigation and guidance systems for THA implant positioning, long-term clinical follow-up studies should be demanded that assess outcome measures that a patient can perceive, including improved function, decreased pain, or better implant longevity, before considering an approach and/or product validated and safe for clinical use.

How Do We Get There?

A common finding among different studies of impingement-free ROM is that, for certain patients, the range of safe positions for the cup and stem is narrow. Therefore, implantation of the components requires high precision and accuracy in surgical technique, challenging surgeon capabilities, and pushing the limits of sophisticated and highly capable guidance systems. As indicated, concerns for fixation and bony coverage of the cup may further complicate component placement.

The calculation of a patient-specific impingement-free zone can be feasible; however, implantation of THA components within this safe zone requires the use of expensive technology like navigation or robotic systems. Moreover, validation studies are required to provide evidence of improved implant positioning and decreased impingement. Finally, long-term follow-up studies are required to provide evidence of mitigating the incidence of dislocation. Clinical trials are expensive, difficult, and cannot always be controlled or randomized. Ultimately, this approach is an expensive proposition that benefits a small percentage of THA patients (2.1% after 6 years of follow-up), while increasing costs for all.

One alternative solution that merits serious consideration is using dual-mobility acetabular components. Modern generation dual-mobility cup designs have addressed many of the complications associated with earlier generations, which were in large part related to polyethylene wear. Specifically, the use of highly cross-linked polyethylene has substantially reduced the resulting complications with dual-mobility cups. However, intraprosthetic dislocation and modular interface corrosion may still remain as potential problems. Nevertheless, with promising results, a wider range of THA candidates may be considered for dual-mobility cups, even if they are not found to be at high risk for dislocation. In a recent review, Blakeney et al. [1] proposed consideration of dual-mobility cups for all THA candidates, albeit with reservations concerning the lack of long-term follow-up information. Moreover, as suggested previously, further in vitro testing may be necessary to study the potential complications with these devices [2]. Specifically, laboratory studies can evaluate implant designs for dislocation, polyethylene damage or wear, or fretting wear and corrosion at the modular component interfaces.

Regardless of the technology used, it is important for the surgeon to have a thorough understanding of the kinematics and interactions among the hip, pelvis, and spine. The increasing availability of navigation and guidance tools carries the danger of promoting a “thought-free” process in which the algorithms or machines play a substantial role in the decision-making process. Ultimately, no technological system can provide answers with full certainty. When a mathematical formula is used to predict the future, it cannot account for the numerous variables that might affect the outcome. Uncertainty remains. This principle is well established in theoretical physics [7], but also applies perfectly to the present situation, considering all the uncertainties involved in physiological systems. Clinical validation should be considered necessary before mathematical algorithms are applied in practice.