Small Random Angular Variations in Pelvic Tilt and Lower Extremity Can Cause Error in Static Image-based Preoperative Hip Arthroplasty Planning: A Computer Modeling Study

BACKGROUND

Many THA simulation models rely on a limited set of preoperative static radiographs to replicate sagittal pelvic tilt during functional positions and to recommend an implant orientation that minimizes the risk of prosthetic impingement. However, possible random changes in pelvic or lower extremity angular motions and the effect of coronal and axial pelvic tilt are not included in these preoperative models.

QUESTIONS/PURPOSES

(1) Can prosthetic impingement occur if the pelvic tilt or lower extremity alignment randomly varies up to ± 5° from what is measured on a single preoperative static radiographic image? (2) Do changes in coronal and axial pelvic tilt or lower extremity alignment angles have a similar effect on the risk of prosthetic impingement?

METHODS

A de-identified pelvis and lower-body CT image of a male patient without previous THA or lower extremity surgery was used to import the pelvis, femur, and tibia into a verified MATLAB computer model. The motions of standing, pivoting, sitting, sit-to-stand, squatting, and bending forward were simulated. THA implant components included a full hemispherical acetabular cup without an elevated rim, polyethylene liner without an elevated rim, femoral head (diameter: 28 mm, 32 mm, 36 mm, or 40 mm), and a triple-taper cementless stem with three different neck shaft angles (127°, 132°, or 135°) with a trapezoidal neck were used in this model. A static model (cup anatomical abduction 40°, cup anatomical anteversion 20°, stem anatomical anteversion 10°) with a predefined range of sagittal pelvic tilt and hip alignment (0° coronal or axial tilt, without random ± 5° change) was used to simulate each motion. We then randomly varied pelvic tilt in three different pelvic planes and hip alignments (flexion, extension, abduction, adduction, rotation) up to ± 5° and assessed the same motions without changing the implant's anatomical orientation. Prosthetic impingement as the endpoint was defined as mechanical abutment between the prosthetic neck and polyethylene liner. Multiple logistic regression was used to investigate the effect of variation in pelvic tilt and hip alignment (predictors) on prosthetic impingement (primary outcome).

RESULTS

The static-based model without the random variation did not result in any prosthetic impingement under any conditions. However, with up to ± 5° of random variation in the pelvic tilt and hip alignment angles, prosthetic impingement occurred in pivoting (18 possible combinations), sit-to-stand (106 possible combinations), and squatting (one possible combination) when a 28-mm or a 32-mm head was used. Variation in sagittal tilt (odds ratio 4.09 [95% CI 3.11 to 5.37]; p < 0.001), axial tilt (OR 3.87 [95% CI 2.96 to 5.07]; p < 0.001), and coronal tilt (OR 2.39 [95% CI 2.03 to 2.83]; p < 0.001) affected the risk of prosthetic impingement. Variation in hip flexion had a strong impact on the risk of prosthetic impingement (OR 4.11 [95% CI 3.38 to 4.99]; p < 0.001).

CONCLUSION

The combined effect of 2° to 3° of change in multiple pelvic tilt or hip alignment angles relative to what is measured on a single static radiographic image can result in prosthetic impingement. Relying on a few preoperative static radiographic images to minimize the risk of prosthetic impingement, without including femoral implant orientation, axial and coronal pelvic tilt, and random angular variation in pelvis and lower extremity alignment, may not be adequate and may fail to predict prosthetic impingement-free ROM.

CLINICAL RELEVANCE

Determining a safe zone for THA implant positioning with respect to impingement may require a dynamic computer simulation model to fully capture the range of possible impingement conditions. Future work should concentrate on devising simple and easily available methods for dynamic motion analysis instead of using a few static radiographs for preoperative planning.

Evaluation of Extreme Hip Joint Movement after Total Hip Arthroplasty: A Retrospective Study

Introductions: The number of total hip arthroplasty (THA) is increased every year in our country. Mobility ofthe hip joint after THA is important to achieve. Subject’s expectation to perform daily activity, moreoverextreme hip movement are crucial for some particular populations. This study aimed to evaluate patient’s functional outcome and ability to peform extreme hip joint movement. Methods: This is a retrospective study of patients who received total hip arthroplasty in our center. Evaluated factors Include age, gender, body mass Index (BMI), socio-economic status, diagnosis, physical therapy, radiographic acetabular inclination, acetabular cup diameter, and head diameter. Clinical evaluation performed with Harris hip score (HHS). Extreme hip joint movement evaluation performed with patient’s ability to perform squatting and Salat (Muslim prayer). The chi-square and mann-whitney tests were used for data analysis. Results: We collected data from a total of 22 subjects. All patients (100%) could perform squatting postoperatively. Among the subjects, therewere 11 patients (11/22, 50%) could perform Salat activity in excellent manner, while the rest (11 patients)need some modification of movement during Salat activity. All patients have significant improvement of Harris hip score compared to preoperative condition. Further, we tried to compare factors between the two group for possible difference. No significant difference in age, gender, BMI, comorbidities, preoperative diagnosis between two group of patients. Conclusions: Subject receiving THA can be expected to perform extreme hip joint movement post-operatively. However, some patients still could not perform Salat activity in excellent manner.  Age, gender, BMI, and comorbidities seem to be had no effect on the patient’s ability to performextreme hip movement post-operatively.