Initial stability of cementless total knee arthroplasty (TKA) tibial trays is necessary to facilitate biological fixation. Previous experimental and computational studies describe a dynamic loading micromotion test used to evaluate the initial stability of a design. Experimental tests were focused on cruciate retaining (CR) designs and walking gait loading. A FEA computational study of various constraints and activities found CR designs during walking gait experienced the greatest micromotion. This experimental study is a continuation of testing performed on CR and walking gait to include a PS design and stair descent activity. The previously described experimental method employed robotic loading informed by a custom computational model of the knee. Different TKA designs were virtually implanted into a specimen specific model of the knee. Activities were simulated using in-vivo loading profiles from instrumented tibia implants. The calculated loads on the tibia were applied in a robotic test. Anatomically designed cementless tibia components were implanted into a bone surrogate. Micromotion of the tray relative to the bone was measured using digital image correlation at 10 locations around the tray. Three PS and three CR samples were dynamically loaded with their respective femur components with force and moment profiles simulating walking gait and stair descent activities. Periods of walking and stair descent cycles were alternated for a total of 2500 walking cycles and 180 stair descent cycles. Micromotion data was collected intermittently throughout the test and the overall 3D motion during a particular cycle calculated. The data was normalized to the maximum micromotion value measured throughout the test. The experimental data was evaluated against previously reported computational finite element model of the micromotion test.Introduction
Methods
Cementless total knee arthroplasty (TKA) designs are clinically successful and allow for long term biological fixation. Utilizing morselized bone to promote biological fixation is a strategy in cementless implantation. However, it is unknown how bone debris influences the initial placement of the tray. Recent findings show that unseated tibia trays without good contact with the tibial resection experience increased motion. This current study focuses on the effect of technique and instrument design on the initial implantation of a cementless porous tibia. Specifically, can technique or instrument design influence generation of bone debris, and thereby change the forces required to fully seat a cementless tray with pegs? This bench top test measured the force-displacement curve during controlled insertion of a modern cementless tibia plate with two fixation pegs. A total of nine pairs of stripped human cadaver tibias were prepared according to the surgical technique. However, the holes for the fixation pegs were drilled intentionally shallow to isolate changes in insertion force due to the hole preparation. A first generation instrument set (Instrument 1.0) and new instrument set design (Instrument 2.0), including a new drill bit designed to remove debris from the peg hole, were used. The tibias prepared with Instrument 1.0 were either cleaned to remove bone debris from the holes or not cleaned. The tibias prepared with the Instrument 2.0 instruments were not cleaned, resulting in three groups: Instrument 1.0 (n=7), Instrument 1.0 Cleaned (n=5), and Instrument 2.0 (n=6). Following tibia resection and preparation of holes for the fixation pegs, the tibias were cut and potted in bone cement ensuring the osteotomy was horizontal. The tibial tray was mounted in a load frame (Enduratec) and the trays were inserted at a constant rate (0.169mm/sec) while recording the force. The test was concluded when the pegs were clearly past the bottom of the intentionally shallow holes.Introduction
Methods
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. 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.Introduction
Methods
Numerous studies have reported on the effects of modular insert design on stress at the tibial/femoral articular surface. However, while the insert / tibial component surface (“backside”) wear and motion have been investigated, backside stress is not well delineated. Because stress may be related to observed backside damage, this study addressed the backside stress response to insert thickness, material, and articular geometry. Twelve Natural Knee II tibial inserts (Sulzer Orthopedics Inc.) with three thicknesses (6, 12.5, and 18.5 mm), two materials (Durasul and 4150 UHMWPE), and two types of condylar geometry (congruent and ultra-congruent) were tested. Fuji film was placed between the baseplate and insert. A femoral component was loaded onto the insert in axial compression at four times Body Weight. The film was scanned into Adobe Photoshop to measure mean and peak luminosity, which was converted into stress. Analysis of Variance was performed with main effects and all two-way interactions to determine significance. The mean stress ranged from 0.61 to 3.92 MPa and the peak stress ranged from 2.17 to 10.4 MPa. Insert thickness significantly influenced both mean (p=0.001) and peak (p=0.001) backside stress. Stress for the 6 mm inserts (7.17 MPa mean, 9.91 MPa peak) were approximately 2.1 times the 12.5 mm inserts (3.47 MPa mean, 4.66 MPa peak), and were approximately 2.6 times the 18.5 mm inserts (2.74 MPa mean, 3.71 MPa peak). There was not a significant effect on mean or peak stress from material or condylar geometry. None of the interactions were significant. This study provides two important contributions. First, it establishes the backside stress magnitude during simple loading. Second, the relationship between backside stress and the insert thickness is experimentally quantified. Understanding this stress magnitude and response may be important to controlling observed in-vivo backside damage.
There are concerns that highly crosslinked polyethylenes are not appropriate for knee implants. One concern is insufficient attachment strength of the insert to the tibial baseplate. In this study, the Natural Knee II Durasul (Sulzer Orthopedics Inc.) snap feature was optimized and then tested to evaluate if the locking mechanism withstood in vivo forces. Initial testing showed that the anterior snap did not always fully engage if the conventional polyethylene design was employed. Therefore, a slight modification was made to the anterior face of the anterior snap feature. Subsequently, full engagement was consistently achieved. The optimal snaplock geometry was evaluated using size 3, 22mm thick Durasul inserts in a peel-out test. Thick inserts were employed for a worst-case scenario (greatest lever arm). The modification employed resulted in a doubling of the attachment strength. Once the optimal snap was defined, peel-out and constraint testing were conducted to determine in vivo performance characteristics the insert/baseplate attachment strength. The attachment strength of the Natural-Knee II Durasul tibial insert exceeds the maximum shear forces at the knee reported by Greenwald et al., even without an applied compressive load that would be present physiologically and would increase the attachment strength. This locking mechanism is stronger than clinically successful implants To further verify the design, a 20° shear fatigue test was conducted on 22mm Ultracongruent inserts, again, a worst-case scenario. This study evaluated the migration of the polyethylene by monitoring anterior displacement of the femur and posterior lift-off under extreme physiological loads. All knee assemblies survived 107 fatigue cycles with no adverse effects. All inserts remained firmly attached to their respective baseplates. No polyethylene cracking was detected at the tabs of the insert or in the body of the inserts. This study shows that a successful locking mechanism can be made with the Durasul material.
