The goal of total knee arthroplasty (TKA) is to relieve pain and restore the function of the knee joint. Recently the number of TKA cases in Korea has increased considerably with increase in elderly population and change in life style. Accordingly, demand for TKA design that is capable of better accommodating anatomical dimensions and life styles of Koreans is also on the rise. During the prototype design process for the Korean-TKA, different stem and keel designs of the tibial base plate have been attempted to improve fixation and longevity of the implant. In this study, we conducted a biomechanical analysis of the tibial base plate using finite element analysis (FEA). Specifically, biomechanical effects of insert positioning in the tibia were assessed to investigate the likelihood of tibial fracture and implant loosening due to mal-positioning of the implant.

A 3-D finite element(FE) models of the left femur, patella, and tibia were developed from computed tomography (CT) scan data (a normal Korean male, 27 years of age, 70 kg). 2-D truss elements were chosen to represent ligamentous structures such as lateral & medial collateral ligament, posterior cruciate ligament, patella tendon and patella ligament. Nonlinear elastic materal properties for the soft-tissue structures were also adopted from literatures. The surgical model was then constructed after inserting Korean-TKA prototype in the intact model. Here, the implant was the posterior cruciate ligament retaining type (CR) with the fixed bearing system. To simulate loading on the knee joint in heel strike and toe off positions, 15° and 45° flexions of the femur orientation were simulated under the compressive load of 3.8 and 5.7 times of body weight (BW= 700N), respectively, in a uniform pressure at the horizontal section of the femur. The tibia was assumed to be completely constrained. The surgical position of the tibial insert was varied from the center either to the medial or to the lateral direction by 3-mm. The peak von mises stresses (PVMS) at the stem and the keel regions of the tibial insert were assessed.

With respect to the central positioning the lateral shift of the tibial plate resulted in higher PVMS than the medial. Particularly, increases of 24.5 %, 29.8%, and 28.4% were observed at the stem, the lateral keel, and the medial keel, respectively, due to lateral mal-positioning of the implant. With the medial shift, on the other hand, PVMS increase remained at around 6% level at the stem and the lateral keel. A decrease of 4.5 % was noted at the medial keel region.

In this study, a computational approach was used to evaluate biomechanical effect of tibial plate positioning on the stress distribution within the implant. The lateral mal-positioning showed more stress concentration than the medial. This may be due to the fact that body weight is transmitted more to the lateral portion of the tibia (5.5:4.5) that is smaller and thinner than its counterpart. These results suggest that the lateral deviation of the implant can be more likely cause TKR loosening and tibial fracture.

The total disc replacement (TDR) devices are gaining popularity because of their capability of allowing joint motion at the index level. Studies have shown that motion preservation can reduce the likelihood of further degeneration at the adjacent level with better surgical outcome. Current lumbar TDR devices require an anterior approach for implantation. However, it is known that its clinical outcome may depend on implant insertion and placement during surgery. Only limited number of biomechanical studies regarding the effect of placement orientation on the clinical outcome is currently available. The purpose of this study was to investigate effects of various surgical placement of a lumbar TDR on the kinematics and load-sharing characteristics using finite element method (FEM).

A previously-validated 3-D nonlinear FE model of the intact lumbar motion segment (L3-S1) based on computer tomography (CT) images of a cadaveric specimen (male, age 56, no pathologies) was used as the baseline FE model. Then, implantation of ProDisc-L (Spine Solutions, Inc., Synthes, Paoli, PA, USA) was simulated into the L4–L5 disc space through anterior approach with removal of the nucleus, anterior longitudinal ligament, and the anterior part of the annulus. The location of lumbar TDR was varied in the sagittal and the coronal planes. In the sagittal plane, the implants were placed anteriorly at 3-mm (S-3), 5-mm (S-5), and 7-mm (S-7) offset from the posterior margin of the endplate. In the coronal plane, the devices were shifted from the baseline position laterally to the right by 1-mm (C-1), 2-mm (C-2), and 3-mm (C-3) from the mid-sagittal line along the lower endplate. All of the models were subject to 150N compressive pre-load and flexion/extension moments of 10Nm at the superior endplate L4, while the inferior endplate of L5 was fully constrained. Changes in motion (ROM) and facet loads at the index and adjacent levels were assessed at different implant position.

Results showed that deviation from the central placement (from S-3 to S-7 and from C-1 to C-3) decreases ROM while increasing facet load at the index level. The effect was more pronounced in the sagittal plane than in the coronal plane:10% decrease in ROM and 1% increase in facet load in the sagittal plane vs. no significant change in the coronal plane. As expected, changes were more evident during extension than in flexion. While the kinematics of the spine was restored to the pre-operative stage at the index level (L4-5), the ROM decreased at the adjacent level (L5-S1) in a compensating manner. The overloading of the facet seemed to indicate mal-alignment of the implant can further trigger facet degeneration, which may require unwanted revision or additional surgical treatment.