This study has been reviewed and approved by the Institutional Review Board (IRB) of the Third Hospital of Hebei Medical University and that it conformed to the provisions of the Declaration of Helsinki.
1.Specimen Preparation
Fourteen adult cadaver specimens that were preserved with formalin were included; samples from those who had rheumatism, tuberculosis, tumors, osteoporosis and other pathological or anatomical changes were excluded. All samples were required to be without visible deformities of the knee joint; have articular cartilage integrity; have unrestricted knee flexion and extension; have retained the joint capsule, ligaments, patella retinacular tissue, femoral head four tendon and patellar tendon; and have all muscular tissue removed. Before the experiment, all the structures of each specimen, especially the knee joint, were examined by X-ray to determine the uniformity of the knee joint, confirm that there were no imaging abnormalities, and confirm that there was a smooth joint surface. Then, the upper part of the femur was dissected, and the soft tissue was removed with a scalpel. The femur was generally kept at 25 cm, and the tibia and fibula were kept at 25 cm. The specimens were maintained in standby packages to prevent dehydration, kept at -20 °C for cryopreservation.
2.experimental method
Thawed the cadaver specimens at room temperature 12 hours before the experiment began. Then, middle and upper tibial fracture models were made, along with the internal rotation deformity (5 degrees, 10 degrees, 15 degrees) and the external rotation deformity (5 degrees, 10 degrees, 15degrees). Finally, along the lower edge of the femur, horizontal incisions of approximately 3–4 cm were made into the ligaments on both sides of the patellofemoral ligament and at the level of the joint gap. The subcutaneous fat was separated, the bursa was cut, and the joint space was exposed. The anterior and posterior cruciate ligaments and the medial and lateral menisci were preserved so as not to affect the normal distribution of contact stress in the joint.
The specimen was then erected and the proximal end of the femur was embedded with denture base resin (type II self-setting dental powder) and denture base resin solution (type II self-setting dental tray water), and fixed perpendicularly to the homemade iron groove, as illustrated in Fig. 1. After the dental tray powder has solidified, fix the tibial end of the same specimen in the same way. Then attach the two-end clamp to the BOSE Electroforce 3520-AT biomechanical testing machine (Fig. 2), adjust the fixed position of the femoral and tibia stumps, so that the lower limb mechanical axis is close to the position when standing naturally. Then, an ultra-low-sensitivity pressure-sensitive tablet (0.5 to 2.5 MPa) was used to measure the contact pressure. The pressure-sensitive sheet is trimmed into a suitable shape and sealed in a polyethylene film bag. The total thickness of the pressure-sensitive sheet and the polyethylene film bag is controlled to 250 µm. Carefully place the pressure-sensitive film under the knee meniscus[13] separately at the medial and lateral knee incisions, while trying to keep the depth and position of each insertion, and tightly sutured to the open joint capsule is shown in Fig. 2. Finally ,the specimen was pressurized to 200 N at 10 N/s to eliminate creep. After stabilization, a vertical load was applied to the specimen at a rate of 10 N/s to 400 N for 2 min; then, the pressure-sensitive film was carefully removed from the knee joint. The measurement for each lower limb specimen was repeated 3 times.
The pressure values were read with the FPD-305E densitometer and FPD-306E pressure transducer. The stress values were uniformly and accurately read, the stress area of each pressure-sensitive film was divided into four quadrants (front outer, front inner, rear inner and rear outer), five values were read in each quadrant for a total of 20 values, and finally, the average was calculated.
3.Selection and use of pressure-sensitive film
This experiment used a double-page ultralow pressure-sensitive film (LLW type). Precompression was performed in the knee joint before performing biomechanical experiments. First, a template was cut from thin cardboard according to the shape of the articular surface of the tibial plateau, and then the ultralow pressure-sensitive film was cut according to the shape of the template. The pressure-sensitive film was trimmed into a suitable shape by the color distribution of the pressure-sensitive sheet and sealed in a polyethylene film bag. Since the color of the pressure-sensitive film material varies with humidity and temperature, humidifiers and air conditioners were used during the experiment to keep the indoor temperature at 25 ~ 30 °C and the relative humidity at 35% RH ~ 80% RH.
4.Statistical processing
The experimental data were processed by SPSS 21.0 statistical software. To ensure that the conditions of the parametric test are met, the normality of the measured variables is verified using the Shapiro-Wilk test, and the uniformity of the variance is verified using the Levene test. Calculate the mean stress and standard deviation of the tibia at different rotation angles. The analysis of variance was used to compare the stress of different rotation angles under the action of vertical load, and the SNK (Student-Newman-Keuls) test was used to make a pairwise comparison between the multiple sample means. P < 0.05 was considered statistically significant.