Impact of patient, surgical, and implant design factors on predicted tray-bone interface micromotions in cementless total knee arthroplasty.

Micromotion magnitudes exceeding 150 µm may prevent bone formation and limit fixation after cementless total knee arthroplasty (TKA). Many factors influence the tray-bone interface micromotion but the critical parameters and sensitivities are less clear. In this study, we assessed the impacts of surgical (tray alignment, tibial coverage, and resection surface preparation), patient (bone properties and tibiofemoral kinematics), and implant design (tray feature and surface friction) factors on tray-bone interface micromotions during a series of activities of daily living. Micromotion was estimated via three previously validated implant-bone finite element models and tested under gait, deep knee bending, and stair descent loads. Overall, the average micromotion across the tray-bone cementless contact interface ranged from 9.3 to 111.4 µm, and peak micromotion was consistently found along the anterior tray edge. Maximizing tibial coverage above a properly sized tibial tray (an average of 12.3% additional area) had minimal impact on micromotion. A 1 mm anterior tray alignment change reduced the average micromotion by an average of 16.1%. Two-degree tibial angular resection errors reduced the area for bone ingrowth up to 48.1%. Differences on average micromotion from ±25% changes in bone moduli were up to 75.5%. A more posterior tibiofemoral contact due to additional 100 N posterior force resulted in an average of 79.3% increase on average micromotion. Overall, careful surgical technique, patient selection, and controlling kinematics through articular design all contribute meaningfully to minimizing micromotion in cementless TKA, with centralizing the load transfer to minimize the resulting moment at the anterior tray perimeter a consistent theme.

Drivers of initial stability in cementless TKA: Isolating effects of tibiofemoral conformity and fixation features.

The initial fixation of cementless tibial trays after total knee arthroplasty is critical to ensure bony ingrowth and long-term fixation. Various fixed-bearing implant designs that utilize different fixation features, surface coatings, and bony preparations to facilitate this initial stability are currently used clinically. However, the role of tibiofemoral conformity and the effect of different tray fixation features on initial stability are still unclear. This study assessed the implant stability of two TKA designs during a series of simulated daily activities including experimental testing and corresponding computational models. Tray-bone interface micromotions and the porous area ideal for bone ingrowth were investigated computationally and compared between the two designs. The isolated effect of femoral-insert conformity and fixation features on the micromotion was examined separately by virtually exchanging design features. The peak interface micromotions predicted were at least 47% different for the two designs, which was a combined result of different femoral-insert conformity (contributed 79% of the micromotion difference) and fixation features (21%). A more posterior femoral-insert contact due to lower tibiofemoral conformity in a force-controlled simulation significantly increased the micromotion and reduced the surface area ideal for bone ingrowth. The maximum difference in peak micromotions caused by only changing the fixation features was up to 33%. Overall, the moment arm from the insert articular contact point to the anterolateral tray perimeter was the primary factor correlated to peak and average micromotion. Our results indicated that tray-bone micromotion could be minimized by centralizing the load transfer and optimizing the fixation features.

Impact of surgical alignment, tray material, PCL condition, and patient anatomy on tibial strains after TKA.

Bone remodeling after total knee arthroplasty is regulated by the changes in strain energy density (SED), however, the critical parameters influencing post-operative SED distributions are not fully understood. This study aimed to investigate the impact of surgical alignment, tray material properties, posterior cruciate ligament (PCL) balance, tray posterior slope, and patient anatomy on SED distributions in the proximal tibia. Finite element models of two tibiae (different anatomy) with configurations of two implant materials, two surgical alignments, two posterior slopes, and two PCL conditions were developed. The models were tested under the peak loading conditions during gait, deep knee bending, and stair descent. For each configuration, the contact forces and locations and soft-tissue loads of interest were taken into consideration. SED in the proximal tibia was predicted and the changes in strain distributions were compared for each of the factors studied. Tibial anatomy had the most impact on the proximal bone SED distributions, followed by PCL balancing, surgical alignment, and posterior slope. In addition, the thickness of the remaining cortical wall after implantation was also a significant consideration when evaluating tibial anatomy. The resulting SED changes for alignment, posterior slope, and PCL factors were mainly due to the differences in joint and soft-tissue loading conditions. A lower modulus tray material did result in changes in the post-operative strain state, however, these were almost negligible compared to that seen for the other factors.

Validation and sensitivity of model-predicted proximal tibial displacement and tray micromotion in cementless total knee arthroplasty under physiological loading conditions.

The initial fixation of cementless tibial trays after total knee arthroplasty is crucial to bony ingrowth onto the porous surface of the implants, as micromotion magnitudes exceeding 150 µm may inhibit bone formations and limit fixation. Experimental measurement of the interface micromotions is still very challenging. Thus, previous studies investigated micromotions at the bone-tray interface via finite element methods, but few performed direct validation via in vitro cadaveric testing under physiological loading conditions. Additionally, previous models were validated by solely considering relative displacements of the marker couples placed around the tray-bone interface. In this paper, we present an experimental-computational validation framework for investigating micromotions at the tray-bone interface under physiological conditions. Three cadaveric specimens were implanted with cementless rotating-platform implants and tested under gait, deep knee bending, and stair descent loads. Corresponding subject-specific finite element models were developed and used to predict the marker (tray-bone) relative displacements and tibial surface displacements. Experimental measurements were used to validate model estimations. Subsequent sensitivity analyses were performed on implantation and friction parameters to represent model uncertainties. The models appropriately differentiated between locations, activities, and specimens. The average root-mean-square (RMS) differences and correlations between measured marker relative displacements and predictions from the 'best-matching' models were 13.1 µm and 0.86. RMS differences and correlations between measured surface displacements and predictions were 78.9 µm and 0.84. Full-field interface micromotions were investigated and compared with predicted marker relative displacements. The marker relative displacements underestimated the actual interface micromotions. Initial tray-bone alignment in anterior-posterior, flexion-extension, and varus-valgus degrees of freedom have a considerable impact on the interface micromotions. The validated cadaveric models can be further used for pre-clinical assessments of new TKR tray design. The outcomes of the sensitivity analyses provide further insights into reducing interface micromotions via clinical techniques.