What, Exactly, Are Hip Dysplasia’s Biomechanics?

A multi-disciplinary team from Washington University School of Medicine (St. Louis, Missouri) was able to use a “well-defined, robust set of patient data that included 3D descriptions of bony anatomy, muscle anatomy, muscle function, and biomechanics” to really put excruciating detail into defining and describing the biomechanics of hip dysplasia.

Members of the team were pulled from a variety of relevant disciplines including physical therapy, orthopedic surgery, mechanical engineering and materials science, mechanical engineering, and radiology.

The full report (“The biomechanical disadvantage of dysplastic hips”) of their excellent work is available in the June 2022 edition of the Journal of Orthopaedic Research.

Michael D. Harris, Ph.D. assistant professor at the Wash U in St. Louis and co-author explained both the purpose and design of the study to OTW, “Awareness of the bony abnormalities associated with hip dysplasia has been advancing for many years. Along the way, there have been suggestions that the surrounding muscles may be affected.”

“The time was right for our study because we finally had a well-defined, robust set of patient data that included 3D descriptions of bony anatomy, muscle anatomy, muscle function, and biomechanics. We could then thoroughly investigate the consequences of dysplastic bone on the muscle, and the implications for joint damage.”

The Wash U team compared skeletal geometry, muscle volumes, intramuscular fatty infiltration, moment arms, and isometric strength in 20 patients with developmental dysplasia of the hip to 15 healthy controls.

They found evidence that femoral coverage was significantly less in patients, femoral neck-shaft angles were larger, and hip joint centers were more lateral. They also determined that these skeletal abnormalities were associated with smaller abductor muscle moment arms in patients with developmental dysplasia of the hip and that these patients also had larger gluteus medius volumes, but no differences in fatty infiltration, as compared to those in the control group.

Too Much Demand on Certain Muscles

“We confirmed that skeletal deformities of the acetabulum and femur are associated with hip joint center lateralization and reduced abductor muscle moment arms, which results in a phenomenon we call a biomechanical disadvantage,” Dr. Harris explained to OTW.

“Meaning, because of the reduced muscle moment arms, the hip abductor muscles must generate larger-than-normal force to meet the torque demands of hip stabilization and movement. The chronically increased demand on abductor muscles likely led to the gluteus medius hypertrophy we found, and to previously reported higher-than-normal medial and superior joint reaction forces during walking. The abnormal muscle geometry and function, in context with abnormal skeletal geometry, are likely strong, but underappreciated, contributors to damaging loads in dysplastic hips.”

“By more completely identifying the contributors to patients’ symptoms and the mechanisms of joint damage, we can inform both more complete treatment and evaluation of treatment. We hope that as physicians and physical therapists become more aware of the changes to muscle in cases of hip dysplasia, those changes will be on their minds as they piece together a patient’s prognosis and intervention strategy.”