Simpleware Case Study: A Novel Simulation for Detecting Iliopsoas Tendonitis After Different Hip Surgeries

Model Generation

3D models of each patient's pelvis and operative femur were generated in Simpleware software by segmenting the CT scans using a combination of manual segmentation and in-built functions. The hip resurfacing implants were imported and registered to their in-situ positions.

The iliopsoas insertion points, anterior pelvic plane (APP) landmarks (left and right anterior superior iliac spine points and the pubic symphysis), and the femoral head center were landmarked in Simpleware. Quality checks at various stages of the process confirmed accuracy of the placements of the landmarks and implants. Meshes of the bony anatomy and registered implants were then exported in their supine (CT) positions.

Simulation of Iliopsoas Impingement

A representation of the iliopsoas was generated using a novel approach to tracing the path from three superior insertion point of the iliopsoas to the inferior insertion point at the lesser trochanter (LT). The three superior insertion points are located on the lateral superior plateau and transverse process of the L5, and near the sacroiliac joint. These reproducible landmarks are present in all CT scans and accurately represent the iliopsoas’ width and location it passes over the acetabular margin. Each combination of superior and inferior insertion sites was used to create a segment for simulation, with each containing a green path (excluding the acetabular cup) and red path (including the acetabular cup) path wrapping around the pelvis, prosthetic femoral head, and operative femur. Impingement was quantified as the difference between these two path lengths, or the ‘stretch’ of the iliopsoas due to the acetabular cup.

In cases without iliopsoas impingement, the green and red path lengths are equal. However, in cases with impingement, the red path is longer than the green path. Three separate impingement values were obtained from the segments and calculated and reported as the mean and maximum of these values. The pelvis was then rotated to obtain the patient’s standing pelvic position, resulting in four iliopsoas impingement measurements per patient. Cup prominence was also measured using a method from the literature, and statistical analysis conducted using a range of metrics to determine clinical significance and other variables such as simulation sensitivity and specificity.

Results

The study demonstrated no statistically significant difference between symptomatic and asymptomatic cohorts for cup anteversion, cup inclination, cup size, femoral head size, supine pelvic tilt, or standing pelvic tilt. However, statistically significant differences were found for the symptomatic versus asymptomatic cohorts for the mean cup prominence, the average supine mean impingement, the average standing mean and standing maximum impingements, and the average supine maximum impingement. Cup prominence, standing mean impingement and standing maximum impingement significantly predicted the probability of iliopsoas tendonitis in logistic regression models.

Conclusions

The novel iliopsoas impingement detection simulation has been validated in studies of HRA and THA patients, showing it can detect iliopsoas impingement and distinguish between symptomatic and asymptomatic patients. The symptomatic HRA cohort had significantly greater impingement than the asymptomatic cohort, and the simulation predicted iliopsoas tendonitis with high sensitivity and specificity.

Interestingly, the symptomatic THA cohort showed greater impingement than the HRA cohort, contradicting the original hypothesis that more impingement would be observed in the HRA due to the use of larger cups. Current explanations of this finding suggest that iliopsoas tendonitis and impingement are three-dimensional problems, not merely related to the prominence of the cup when measured on a 2D axial slice of a CT scan. Therefore, further study is needed to understand the incidence of iliopsoas tendonitis after both types of hip arthroplasty.

The study also highlighted the multicausal nature of iliopsoas tendonitis, with two symptomatic patients showing no impingement (referred to as "false negatives"). For one of these patients, the availability of a preoperative CT scan allowed for the reconstruction of the preoperative bony anatomy in Simpleware software and the simulation of a representation of the iliopsoas. The analysis found that the iliopsoas had been lengthened by about 5 mm from preoperative to postoperative states, potentially through anteriorisation of the prosthetic femoral head, which may have been leading to pain and irritation. The sample sizes in the current studies are not large enough to draw any conclusions, and future research should explore additional factors behind iliopsoas tendonitis across larger cohorts, considering factors like age, gender, and activity level.

This study was novel for providing insight into the level of anterior cup exposure observed in HRA patients diagnosed with iliopsoas tendonitis. The simulation has the potential to be used as part of a 3D preoperative templating workflow to assist in optimizing cup orientation and positioning.