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, 42 (15), 2495-500

Contact Stress Distributions on the Femoral Head of the Emu (Dromaius Novaehollandiae)

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Contact Stress Distributions on the Femoral Head of the Emu (Dromaius Novaehollandiae)

Karen L Troy et al. J Biomech.

Abstract

Osteonecrosis of the femoral head remains a challenging orthopaedic problem. The disease frequently progresses to femoral head collapse, leading to debilitating osteoarthritis in the affected hip(s). Since a major goal of pre-collapse interventions is to forestall the need for hip arthroplasty, it is important that any animal models used to develop or study such interventions also have a natural history of progression to femoral head collapse. The emu (Dromaius novaehollandiae), a large flightless bird native to Australia, consistently progresses to femoral head collapse when osteonecrosis is experimentally induced cryogenically. Full biomechanical characterization of the demands this animal places on its hip is an important consideration in future usage of this model. This study reports in vitro measurement of the contact stress distributions on the emu femoral head during stance phase of the gait cycle, using Fuji pressure-sensitive film. Applied hip loadings were based upon ground reaction forces and hip flexion angles recorded in vivo. The contact stress data showed reasonable homology with the human hip, both in terms of stress magnitude and sites of habitual loading on the femoral head.

Figures

Figure 1
Figure 1
(a) Locations of skin fiducials and force mat to measure hip joint excursion and ground reaction force (right side shown) (b) Locations of abductor and extensor muscles modeled for testing (right femur shown). From left to right, abductor muscles are: M. ilioishiofemoralis, M. iliotrochantericus medius, M. iliofemoralis, and M. iliotrochantericus cranialis.
Figure 2
Figure 2
Hip flexion angles and ground reaction forces during the stance phase of the emu gait cycle
Figure 3
Figure 3
Schematic (a) and photograph (b) of emu hip loading fixture (right femur shown). The pelvis attachment fixture (aluminum struts, plus two PMMA pots) freely moved in A–P and M-L translation, and in rotation about A–P and M-L axes; S-I translation, and rotation about the S-I and M-L axes, were constrained. The distal femur potting block was subjected to S-I force from the MTS actuator and was allowed rotational freedom along the M-L axis; all other degrees of freedom were constrained. Abductor traction was applied by a pneumatic actuator.
Figure 4
Figure 4
Percent load carried by the antitrochanter over a range of abduction angles in a single specimen.
Figure 5
Figure 5
Digitized low range (a), and super low range (b) Fuji film showing fiducial pin locations and contours deliniating contact stress isobars. The left-hand portion of (a) covers the antitrochanter and shows no stress in this recording (this portion omitted in (b) for clarity). Film (c) shows fiducial locations (F), contact patches, and major and minor ellipsoidal axes (a0 and b0).
Figure 6
Figure 6
Raw and polynomial-approximated stress distributions along major and minor ellipsoidal axes, for a typical contact patch
Figure 7
Figure 7
Idealized ellipsoidal contact patches on the right femur for 10%, 30%, 50%, 70% and 90% stance-phase increments.
Figure 8
Figure 8
Figure 1 Forces acting on the right femur during Fuji-film testing (femur is flexed away the viewer and slightly abducted, as if the animal is facing away from the viewer and standing normally). Fhip is the resultant hip force, which acts on the gray contact patch. Fabd is the resultant abductor force, shown here as separate lines of action for each muscle. Fext is the extensor force and has mainly posterior, but also medial, and superior components.

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