April 2017

Protecting hip implants: Gait patterns help predict wear rates

In the moment: Rehabilitation

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Patient-specific implant wear rates following total hip arthroplasty (THA) are more strongly associated with gait patterns than component positioning, according to research from Chicago that suggests implant wear could be reduced with the use of predictive wear models.

By Katie Bell

“This study demonstrates the power of gait as a [mechanical] biomarker,” said study author Markus A. Wimmer, PhD, the Grainger Director of the Rush Arthritis & Orthopedics Institute and professor for research in the Department of Orthopedics at Rush University Medical Center in Chicago. “Knowing the association for a specific patient could help to define the follow-up period. Those with more wear should be seen more often. Gait modifications with the help of a physical therapist could be considered.”

The study included data on 43 men and 30 women, with an average age of 69 years and an average body mass index (BMI) of 27.3 kg/m2. All participants underwent a primary unilateral THA, performed a gait test 10 months or more after surgery, and had a series of standing radiographs taken more than one year after surgery, with a minimum of three years of clinical follow-up. All received the same implant, which featured a 28-mm metal head and a hip cup made of noncrosslinked polyethylene.

Implant wear rates were calculated based on the femoral head’s displacement relative to the cup using a computer-assisted x-ray wear-analysis suite. The investigators established three groups: Low wear was classified as less than .1 mm per year, moderate wear was between .1 and .2 mm per year, and high wear was more than .2 mm per year.

No group differences were found for positioning and gait, with the authors suggesting that different wear rates result from a combination of factors rather than single variables. The findings were epublished by Clinical Orthopaedics and Related Research in March.

A linear discriminant analysis model correctly predicted the wear level in 80% of participants with low wear, 87% of those with moderate wear, and 73% of patients with high wear. For each wear level, multiple linear and nonlinear regression showed strong associations among gait biomechanics, implant positioning, and wear rate, with the nonlinear model having a higher prediction accuracy.

Notably, flexion-extension range of motion and hip moments in the sagittal and transverse planes explained 42% to 60% of wear rate. Meanwhile, positioning factors, including cup medialization and cup inclination angle, were less predictive, explaining just 10% to 33% of wear rate.

BMI did not emerge as a wear predictor. Although patient activity level was not assessed directly, it should be reflected in the combinations of age and gait variables that were analyzed, Wimmer said.

“We believe that the results of this study are translatable to patients of all age ranges,” Wimmer said. “However, the importance of wear predictors [ie, specific gait and implant positioning variables] may differ dependent on age.”

Dana Judd, PT, DPT, PhD, an assistant professor in the Physical Therapy Program at the University of Colorado Anschutz Medical Campus in Aurora, said the postoperative gait variables associated with implant wear are likely reflective of preoperative gait abnormalities and the experience of undergoing surgery rather than a product of any particular surgical technique. The findings also are consistent with those of a different Chicago research group, in which preoperative gait variables were predictive of clinical response after THA (see “Gait and THA outcomes: Hip mechanics have predictive value,” June 2016, page 15).

However, because the trauma associated with surgery can affect muscle performance and mobility, Judd suggested that interventions to address THA wear rates be implemented postoperatively.

“Interventions might address strength training targeting the hip and abdominal musculature to stabilize the pelvis, as well as balance and neuromuscular reeducation programs to promote healthy movement and stabilization,” she said.

Sources:

Ardestani MM, Amenábar Edwards PP, Wimmer MA. Prediction of polyethylene wear rates from gait biomechanics and implant positioning in total hip replacement. Clin Orthop Relat Res 2017 Mar 2. [Epub ahead of print]

Foucher KC. Preoperative gait mechanics predict clinical response to total hip arthroplasty. J Orthop Res 2017;35(2): 366-376.

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