[Mathematical kinematic methods in joint prosthetics]

Establishment of a Knee-Joint Coordinate System From Helical Axes Analysis—A Kinematic Approach Without Anatomical Referencing

This study establishes a functional knee-joint coordinate system (FCS) derived from active motion. The scale invariant properties of helical axes were used in order to avoid inter-observer errors associated with the traditional anatomical referencing techniques. The algorithm was tested with six cadaveric specimens in a knee-joint motion and loading apparatus. To determine the FCS sensitivity to variable loading, rotational moments were applied to the tibia while extending and flexing the knee. Each derived FCS was compared with the clinically derived anatomical coordinate system (ACS). The FCS was reproducible when the loading condition was the same. Changing the rotational moments from internal to external affected the orientations and the positions of the FCS. The largest displacement of 20.8 mm in average occurred in the medio/lateral direction. The FCS corresponded with the ACS for all specimens and loading conditions. The origin was always located within the femur along the transepicondylar line. The orientations differed less than 16.6 degrees in average, thus allowing the use of clinical terminology. These findings suggest that the FCS might improve the ability to clinically assess kinematic alterations provided that the reference motion is reproducible.

Anterior cruciate ligament rupture translates the axes of motion within the knee

OBJECTIVE

To quantify the dynamic effects of anterior cruciate ligament deficiency on human knee joint motion.

DESIGN

Three-dimensional motion was assessed by measuring the kinematics of intact and anterior cruciate ligament deficient knee joint specimens during simulated flexion-extension cycles.

BACKGROUND

It is known that the anterior cruciate ligament plays an important role in controlling three-dimensional knee joint motion. Nevertheless, dynamic effects of deficiency are not fully understood.

METHODS

Six cadaveric knees were tested in a knee joint motion and loading apparatus prior to and after anterior cruciate ligament resection. To determine if the kinematic results depended on additional loading, internal and external rotation moments of 3.4 Nm were axially applied to the tibia. The kinematics were analysed in terms of finite helical axes.

RESULTS

Sectioning the anterior cruciate ligament had little effect on the orientations of the finite helical axes. However, applying moments did affect the axes orientation. In contrast, relative translations of the axes were significantly affected by the deficiency for all rotational moments applied. Referring to the individual knee anatomy the largest translation of 12.5 mm (median) occurred in medial/lateral direction.

CONCLUSIONS

Anterior cruciate ligament rupture primarily causes a translation of the finite helical axes in medial/lateral direction. Consequently, increased anterior excursion of the tibia occurs (subluxation) and therefore dynamic instability.