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Saturday, March 18, 2000

Total ankle replacement devices are successful

Minimally constrained total ankle replacement devices have recently shown higher success rates. Since these total ankle designs replace the articulating surfaces of the ankle joint with less artificial geometries, the ankle must rely to a greater extent on the restraints provided by the ligaments, say researchers of a study in poster exhibit 250.

To provide the required stability, they say the ligaments must be taut and, at the same time, not restrict the required flexion and extension of the joint. Their study examines the effect of varying degrees of ligamentous tightness on ankle kinematics.

Specific parameters were measured during testing on four cadaveric ankles before and after total ankle replacement. The STAR mobile-bearing total ankle replacement was used. Ligament tightening was varied by the use of different inserts, varying in thickness from 6 mm to 10 mm, as used intraoperatively. The measured parameters included path of rotation axis during flexion, energy required for flexion and torsional resistance to manually induced rotation.

Observations also were noted regarding the relative motion of the total ankle com-ponents. Flexion was induced using an ankle simulator incorporating passive loading. Each ankle was moved through a full range of flexion while lightly loaded axially. The simulator did not constrain the location of the rotation axis, but allowed the ankle to follow its path of least resistance.

An electromagnetic motion tracking system recorded displacement and rotation of the talus relative to the tibia. A six degree-of-freedom load cell measured forces applied for all tests.

Increasing ligament tight-ening resulted in a more constrained ankle, demonstrated by less variation in the rotational axis orientation, a greater energy requirement for flexion and increased stiffness to torsion. In comparison to the normal ankle, the in situ total ankle replacement, at any ligament tightness, exhibited less variation in rotational axis position and orientation, required more en-ergy for flexion and provided less resistance to torsion. The axis of rotation of the STAR ankle depended highly on the orientation of the talar component to the talus, and, following prescribed methods of implantation, resulted in an axis that was aligned more horizontally and closer in line with the coronal plane than the normal ankle joint axis.

The researchers concluded that mobile bearing designs might offer the advantages of simultaneous sliding and fully congruent rotation compared to two-component designs. However, the axis of rotation is highly dependent on positioning of the talar component. Increasing the mobile bearing thickness restricts motion, limits axis, excursion and increases stiffness to torsion.

Co-authors of the study, all of the Orthopaedic Biomechanics Laboratory, University of Iowa, are Terence E. McIff, PhD, research fellow, orthopaedic surgery; Charles L. Saltzman, MD, associate professor, orthopaedic surgery and associate professor Biomedical Engineering; and Thomas D. Brown, PhD, professor, orthopaedic surgery and professor biomedical engineering.

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Last modified 18/March/2000 by IS