Introduction: Total knee arthroplasty (TKA) has become a successful clinical treatment for patients in regard to relief of pain, correction of deformity and restoration of function with promising long term behaviour [Pradhan et al. 2006].

In TKA the generation of polyethylene wear debris is mainly affected by the factors design of the articulating bearing, contact stresses, kinematics, implant material and surface finish [McEwen et al. 2005].

The objective of our study was to evaluate the in vitro wear behaviour of fixed bearing knee designs in comprehension to the contact mechanics and resultant kinematics for different degrees of congruency.

Material and Methods: Wear simulator testing on 12 TKA devices has been performed according to ISO 14243-1 under load control. The knee replacements were tested in the fixed bearing configurations LC, CR and DD with different degrees of tibio-femoral congruency.

For gravimetric wear assessment the protocol described in ISO 14243-2 has been used, followed by a kinematic analysis of the single test stations.

The articulating contact and subsurface stresses have been investigated in a finite element analysis.

Results: The contact areas are increasing from Search® Evolution LC (144 mm2) to Columbus® CR (235 mm2) and Columbus® DD (279 mm2), whereas the peak surface contact stresses are decreasing from Search® Evolution LC (34.4 MPa) to Columbus® CR (20.9 MPa) and Columbus® DD (18.1 MPa). The estimated amount of wear has decreased from Search® Evolution LC (21.4 mg/million cycles) to Columbus® CR (8.9 mg/million cycles) and Columbus® DD (2.2 mg/million cycles).

The wear rates between the knee design configurations differ substantially and statistically analysis demonstrates a significant difference (p< 0.01) between the test groups in correlation with congruency.

Conclusion: The present study demonstrates the influence of different bearing types on contact stresses, abrasive wear and kinematics for three different degrees of tibio-femoral congruency under elimination of production, material and sterilization parameters.

Corresponding author: Dr.-Ing. Thomas M. Grupp

Research and Development: e-mail: thomas.grupp@aesculap.de

Introduction: Degenerative disc desease is one of the most frequently encountered spinal disorders. The intervertebral disc is a complex anatomic and functional structure, which makes the development of an efficient artificial disc a challenge [1].

Based on the complexity of the anatomical structures and the nearly unknown loading conditions at the moment only contradictory knowledge exists about the kinematics after TDA and in particular the location of the center of rotation in the human lumbar spine [2].

The objective of our study was to evaluate the kinematics of the human lumbar spine and the ability of TDA to restore the native conditions in regard to range of motion (ROM), neutral zone (NZ) and center of rotation (COR).

Material and Methods: In-vitro flexibility testing on functional spinal units (FSU) out of 12 fresh frozen lumbar spines has been performed. The FSU (L2/L3 and L4/L5) were tested first in the native condition, followed by nucleotomy and partial annulus resection and also after TDA with activ L (lumbar artificial disc, Aesculap Germany).

Therefore a spinal simulator has been customized, applying pure moments for flexion/extension, lateral bending and axial rotation (+/−7.5Nm) and axial preload (FP=400N) with a defined velocity (1°/s). The instantaneous COR has been calculated based on the velocity pole method using a 3D ultrasonic motion analysis system, measuring the twelve components of motion.

Results: The TDA with activ L leads to a good restoration of ROM and NZ in all loading directions under in-vitro flexibility testing. The instantaneous COR is exemplary described for the native condition under flexion/ extension in the sagittal plane. For the native condition the COR is located in the center of the inferior vertebral endplate. After nucleotomy the COR shifts dorsally into the region of the spinal cord and a significant grade of instability has been measured.

After insertion of the lumbar artificial disc the instability can be reduced to the native grade of motion and the COR is located again in the main axis of the spinal column in the upper third of the inferior vertebra.

Conclusion: The instantaneous COR has been estimated in-vitro for the different loading situations in the human lumbar spine before and after TDA. Based on the newly introduced method further optimizations of TDA devices can be undergone in regard to the particular aspect of physiological kinematics.