General
This study was performed in accordance with the Declaration of Helsinki and approved by the local Ethics Committee (Ethikkommission der Medizinischen Fakultät Heidelberg, reference S-351/2018). The tissue samples were obtained from Science Care (Phoenix, AZ, USA), which is accredited by the American Association of Tissue Banks. All donors and/or their legal guardian(s) provided informed consent prior to sample acquisition.
In 15 pairs of fresh frozen human legs the Attune total knee replacement system (DePuy Synthes, Warsaw, IN, USA) was implanted by a surgeon experienced in the surgical technique. Preservation of biomechanical properties prior to the experimental period was ensured by frozen storage [26]. In a randomized manner, two different cementation techniques (Groups A and B) were used for the implantation of the femoral component of the Attune system.
Group A consisted of specimens in which the articular surfaces of the femoral components and of the femoral condyles all had conventional cement application using a cement gun. Both surfaces were covered with cement, as this has been shown to provide the best results [13]. Also, the cement gun has been shown to provide superior cementation of the bone to finger packing [14, 23, 24].
Group B had the cement applied to the distal femur with a pressurizing nozzle added to the cement gun. The cement was applied to the femoral component in the conventional manner as in group A. (Fig. 1).
More details on the cementation technique are provided below.
The right and left sides of the 15 leg pairs were randomly allocated to group A or B by means of a computer-generated list (Randlist 1.2; Datinf GmbH, Tübingen, Germany). The mean donor data showed a mean age of 68.3 ± 11.5 years, a mean height of 174.4 ± 10.9 cm, a mean weight of 75.1 ± 16.4 kg, and a mean body mass index of 24.6 ± 4.7 kg/m2.
The bone mineral density (BMD) was assessed for both groups to improve the comparability. Franck et al. showed a high correlation between standard dual-energy absorptiometry (DXA) at the hip and various locations such as the extremities [27]. Therefore, we measured bone mineral density using DXA with standard hip parameters (Hologic QDR-2000, Marlborough, Massachusetts, USA). For all 30 knee joints, native radiographs in anterior–posterior (a.p.) and lateral projections were obtained to exclude bone pathology that would preclude a knee prosthesis and to determine prosthesis size using TraumaCad software (Voyant Health, Ltd., Brainlab AG, Munich, Germany). The same prosthesis size was planned and implanted on the right and left side of each leg pair. The following prosthesis sizes were used: 5 × size 5, 3 × size 6, 3 × size 7, 4 × size 8. Postoperative radiographs were performed to verify the implantation result and to exclude intraoperative fractures.
Cementing procedure
Prior to surgery, the human legs were thawed to room temperature. To standardize the experimental conditions and the surgical steps, all adjustments and resection measurements were documented and repeated on the contralateral side. Bone stock preparation and implantations were performed according to the prosthesis manufacturer's surgical instructions. The entire prosthesis was implanted and the femur and tibia were subsequently separated for testing. Prior to cementation, the cancellous bone was cleaned of lipid deposits, blood and bone debris using the OptiLavage system (Zimmer Biomet Holdings, Warsaw, Indiana, USA) and superficially dried with a compress until immediate cement application. The implantation of the femoral components for both Groups A and B was performed with a vacuum mixed high viscosity bone cement (Optipac 40 Refobacin Bone Cement R, Zimmer Biomet Holdings, Warsaw, Indiana, USA). The cement was applied early (in other words directly after the waiting phase) using cement timing for vacuum mixed cement at a room temperature of 21.2 ± 0.2 °C. We applied the bone cement to the non-articulating surface of the femoral components in Groups A and B 80 s after starting the mixing process. In the next step for Group A, the cement was applied to the prepared bone at 110 s using the above described cementing technique. In Group B, a cement gun with cement cartridge was also used, but a pressurizer nozzle was attached to the conventional nozzle to apply the cement to the bone in a no-touch technique (no manual manipulation of the cement after application) at 110 s (Fig. 2).
In group A, a homogeneous and uniform layer of PMMA cement was applied to the femoral bone stock with a cement gun (Optigun, Zimmer Biomet, Warsaw, Indiana, USA) medial and lateral from anterior to posterior for complete coverage. In addition, a homogeneous and uniform layer of cement was applied to the entire inner surface of the femoral knee component medially, laterally, and anteriorly transversely with the cement gun. On both the component and the bone, manual modelling with clean medical gloves was performed to ensure even coverage. In group B the cement gun was modified to deliver cement at an increased pressure by the attachment of a pressurizing nozzle with 23-degree angled tip. The cement was applied to the femoral surface under pressure, and the amount of cement was standardized to assess the influence of the cementing technique. An identical amount of cement was used in Groups A and B. The cement was applied to the femoral component in the same way in both groups [12].
The impaction of the femoral component was performed 140 s after start of mixing. The femoral component was impacted until the edges of the cement pockets were in contact with the distal bony resection surface. Excess cement was removed, and the trial liner insert was placed on the previously implanted tibial component. Subsequently, the leg was placed in extension position at 240 s after start of mixing, where the cement was allowed to harden.
Load simulation and determination of relative motion
After the cementing procedure, the tibia and femur were separated, and the soft tissues removed. Afterwards, the specimens were cast in a mold using synthetic resin (Rencast FC 53, Huntsman Advanced Materials GmbH, Germany), in order to secure the specimens into the material testing machine. For the assessment of implant stability, an incremental dynamic load was applied at 1 Hz for the axial force with simultaneous extension-flexion between 20° and 50°, as had been done in a prior study [28]. The load maxima occurred at the time of extension and flexion, respectively [29]. A force representing daily stair climbing [30,31,32] was applied using a servo-hydraulic testing machine (MTS 858 Mini Bionix II, MTS Systems Corporation, Eden Prairie, USA) (Fig. 3). A preload of 200 N was applied before cyclic loading was started with the four load levels 1200 N, 1500 N, 1800 N, and 2100 N. The maximum load level corresponded to the force exerted on the knee of a person with a body weight of 75 kg during stair climbing [32]. The selected body weight for the load simulation corresponded to the average donor body weight. Optical markers were placed on the bone and the adjacent implanted component as shown in Fig. 3. The determination of the three-dimensional relative motion between the femoral component and bone was performed using an optical, camera-based system (PONTOS- GOM – Gesellschaft für Optische Messtechnik mbH, Braunschweig, Germany). Figure 3 shows the implant and bone markers (A: anterior, B: distal, C: posterior) of the three analyzed zones. The system is calibrated to a measurement volume of 250 × 200 mm2. The markers on the object to be measured were located in the center of this defined volume. Each of the markers were detected in greyscale by a stereo camera system, and a 3D point triangulation was done to calculate the 3D marker position and displacement vector in the defined coordinate system. The relative motion was calculated from the corresponding implant and bone markers. The two cameras of the stereo system operate each with a resolution of 2448 × 2050 pixels and a measuring accuracy of 1 µm according to the manufacturer's specifications [33]. However, the measuring accuracy of an optical measuring system depends strongly on the environmental conditions. Under laboratory conditions, we achieved a measuring accuracy of ± 4.9 µm for the test setup used. All measurements were done at the medial side. The calculated results of the resultant maximum relative motion were normalized to the right femur for maximum extension and flexion (20°, 50°).
Statistical analysis
Prior to the start of the experimental study, a sample size calculation was performed using G*Power 3.1 (University of Kiel, Germany) [34] based on the reported data by Schwarze et al. [12]. The sample size differed (7, 9, 11, 16, 12 per group) depending on the cement penetration zone analyzed. The calculation of the sample size estimation with the largest number of cases per group was based on the following input parameters, tails: two, effect size d: 1.33, α err prob: 0.05 and power (1-β err prob): 0.95. This results in the output parameters sample size of 16 for each group. A sample size of 15 paired fresh frozen human legs was chosen for feasibility reasons. The data were evaluated descriptively using the arithmetic mean, standard deviation, minimum and maximum. Prior to analysis, the normal distribution of the data was evaluated using a Shapiro–Wilk-test and the homogeneity of variance was verified using the Levene-test. We conducted a two tailed t-test for independent samples to assess effects between both groups on the parameters BMD and relative motion within each load level, flexion angle and fixation zone. All data were analyzed using SPSS 25 (IBM, Armonk, NY, USA) with a significance level of p < 0.05.