Abstract
Introduction
In total knee arthroplasty (TKA), non-cemented implants rely on initial fixation to stabilize the implant in order to facilitate biologic fixation. The initial fixation can be affected by several different factors from type of implant surface, implant design, patient factors, and surgical technique. The initial fixation is traditionally quantified by measuring the motion between the implant and underlying bone during loading (micromotion). Extraction force has also been quantified for cementless devices. The question remains does an increase or decrease in extraction force affect micromotion based on the fact that most loading at the knee joint is in compression. The objective of this research is to investigate if there is any correlation between extraction force and implant micromotion.
Methods
The relationship between extraction force and micromotion was evaluated by performing a series of experiments using a synthetic bone analog and a tibial baseplate with hexagon pegs. Tunnels for the hexagon pegs were machined into the synthetic bone analog with different diameters, from 9.7 to 11.7 mm. The smaller diameter tunnels increase the press fit between the peg and bone.
Sixty-six implants were tested to determine maximum extraction force. The implants were extracted using an electro-mechanical testing frame at a rate of 0.4 inches / minute. Two different types of bone analogs were used for this evaluation. One was an open-cell foam with a density of 12.5 lb/ft3 and the other was a closed-cell foam with a density of 20 lb/ft3.
Twelve TKA implants were tested to determine the maximum anterior-lift off micromotion during a posterior load application. A posterior stabilized polyethylene insert and mating femoral component were used during the loading. The posterior load cycled from 90 to 900 N for 500 cycles. The micromotion was evaluated with the femur at 90 degrees of flexion. Differential Variable Reluctance Transducers (DVRTs) were located under the four corners of the implant to quantify the superior-inferior motion of the implant. A composite synthetic bone analog was used for this evaluation, with open-cell foam (12.5 lb/ft3) on the inside and closed-cell foam (50 lb/ft3) on the outside.
Results
The extraction force was higher for the denser closed-cell foam (Figure 1A). The extraction force generally increased with decreasing tunnel diameter, but there was a plateau of extraction force between 10.9 mm and 10.1 mm for the open-cell foam and peaked at 10.7mm for the closed-cell foam.
The micromotion in both posterior DVRTs were found to be similar for all tunnel diameters. The micromotion in both anterior DVRTs increased slightly when increasing tunnels diameters from 10.2 mm to 10.7 and 11.2 mm, but increased dramatically when increasing the tunnel diameter to 11.7 mm.
Discussion
In this study using a synthetic bone model, a decrease in extraction force was found to correlate with an increase in anterior lift-off micromotion (Figure 2). Next steps are to confirm these results from this simplified model in a more physiologic model with cadaveric bone and activity based loading.
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