Impact of the Femoral Head Position on Moment Arms in Total Hip Arthroplasty: A Parametric Finite Element Study.
BACKGROUND
Although the importance of accurate femoral reconstruction to achieve a good functional outcome is well documented, quantitative data on the effects of a displacement of the femoral center of rotation on moment arms are scarce. The purpose of this study was to calculate moment arms after nonanatomical femoral reconstruction.


METHODS
Finite element models of 15 patients including the pelvis, the femur, and the gluteal muscles were developed. Moment arms were calculated within the native anatomy and compared to distinct displacement of the femoral center of rotation (leg lengthening of 10 mm, loss of femoral offset of 20%, anteversion ±10°, and fixed anteversion at 15°). Calculations were performed within the range of motion observed during a normal gait cycle.


RESULTS
Although with all evaluated displacements of the femoral center of rotation, the abductor moment arm remained positive, some fibers initially contributing to extension became antagonists (flexors) and vice versa. A loss of 20% of femoral offset led to an average decrease of 15% of abductor moment. Femoral lengthening and changes in femoral anteversion (±10°, fixed at 15°) led to minimal changes in abductor moment arms (maximum change of 5%). Native femoral anteversion correlated with the changes in moment arms induced by the 5 variations of reconstruction.


CONCLUSIONS
Accurate reconstruction of offset is important to maintaining abductor moment arms, while changes of femoral rotation had minimal effects. Patients with larger native femoral anteversion appear to be more susceptible to femoral head displacements.
Department of Orthopaedics and Traumatology, Centre Hospitalier Universitaire Vaudois, Lausanne, Switzerland; Department of Orthopaedic Surgery, Schulthess Clinic, Zurich, Switzerland.
Hannes A
HA
Rüdiger
Department of Orthopaedics and Traumatology, Centre Hospitalier Universitaire Vaudois, Lausanne, Switzerland; Laboratory of Biomechanical Orthopedics, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland.
Valérie
V
Parvex
Laboratory of Biomechanical Orthopedics, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland.
Alexandre
A
Terrier
D000328:Adult; D000368:Aged; D019644:Arthroplasty, Replacement, Hip; D003198:Computer Simulation; D005260:Female; D005269:Femur / Q000502:physiology* / Q000601:surgery; D005270:Femur Head / Q000601:surgery; D020342:Finite Element Analysis; D006622:Hip Prosthesis; D006801:Humans; D007719:Knee Joint / Q000000981:diagnostic imaging / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000502:physiology / Q000601:surgery; D011859:Radiography; D016059:Range of Motion, Articular / Q000502:physiology; D012399:Rotation
arthroplasty; biomechanics; center of rotation; hip; moment arm; muscle