Background: Many unlinked total elbow replacement designs with
radically differing articular geometries exist, suggesting that there is no
consensus regarding an optimal design. A feature inherent to the articular
design is the intrinsic constraint afforded to the joint by the implant. Our
aim was to compare the intrinsic constraints of unlinked implants with that of
the normal ulnotrochlear joint.
Methods: We tested twelve cadaveric ulnotrochlear joints with a
custom-made multiple-axis materials testing machine. With compressive loads
ranging from 10 to 100 N, the joints were moved in either valgus or varus
directions at 90° of flexion. The ulnotrochlear components from a single
example of five medium-sized unlinked elbow replacements (Ewald, Kudo,
Pritchard ERS, Sorbie-Questor, and Souter-Strathclyde) were also tested. The
recorded measurements included the torques and forces, angular displacement,
and axial displacement of the humerus relative to the ulna.
Results: In general, the peak torque and the constraint ratio
significantly increased with increasing compressive load for the implants as
well as for the normal elbow. In valgus displacement, the Souter-Strathclyde
implant had the highest and the Sorbie-Questor had the smallest peak torque
and the Souter-Strathclyde had the highest and the Ewald had the smallest
constraint ratio. In varus displacement, the Kudo had the highest and the
Ewald had the smallest peak torque and constraint ratio.
Conclusions: The constraint ratio is a characteristic that is useful
for describing elbow joint behavior and for comparing the behavior of implants
with that of the human elbow. Of the unlinked implants tested, the
Souter-Strathclyde and Kudo prostheses most closely approximated the behavior
of the human elbow joint. Implants that resemble the human elbow in appearance
do not replicate normal behavior consistently, whereas other implants that do
not resemble the human elbow closely do not deviate markedly from human
behavior. Thus, much basic information about elbow form and function is needed
to improve the performance of total elbow prostheses.