Right-Left Differences in Knee Extension Stiffness for the Normal Rat Knee: In Vitro Measurements Using a New Testing Apparatus

Knee stiffness following joint injury or immobilization is a common clinical problem, and the rat has been used as a model for studies related to joint stiffness and limitation of motion. Knee stiffness measurements have been reported for the anesthetized rat, but it is difficult to separate the contributions of muscular and ligamentous restraints to the recorded values. in vitro testing of isolated rat knees devoid of musculature allows measurement of joint structural properties alone. In order to measure the effects of therapeutic or surgical interventions designed to alter joint stiffness, the opposite extremity is often used as a control. However, right-left stiffness differences for the normal rat knee have not been reported in the literature. If stiffness changes observed for a treatment group are within the normal right-left variation, validity of the results could be questioned. The objectives of this study were to utilize a new testing apparatus to measure right-left stiffness differences during knee extension in a population of normal rat knees and to document repeatability of the stiffness measurements on successive testing days. Moment versus rotation curves were recorded for 15 right-left pairs of normal rat knees on three consecutive days, with overnight specimen storage in a refrigerator. Each knee was subjected to ten loading-unloading cycles, with the last loading curve used for analysis. Angular rotation (AR), defined here as the change in flexion-extension angle from a specified applied joint moment, is commonly used as a measure of overall joint stiffness. For these tests, ARs were measured from the recorded test curves with a maximum applied extension moment of 100 g cm. Mean rotations for testing days 2 and 3 were 0.81-1.25 deg lower (p < 0.001) than for day 1, but were not significantly different from each other. For each testing day, mean rotations for right knees were 1.12-1.30 deg greater (p < 0.001) than left knees. These right-left stiffness differences should be considered when interpreting the results of knee treatment studies designed to alter knee stiffness when using the opposite extremity as a control

Differences in the Radius of Curvature Between Femoral Condyles: Implications for Osteochondral Allograft Matching

BACKGROUND: The radius of curvature (ROC) is an important variable related to potential cartilage incongruities in the transplantation of a large femoral osteochondral allograft. The anterior-posterior length (APL) of a condyle is used as a criterion for donor-graft acceptance. We hypothesized that there would be a linear correlation between the ROC and APL of a condyle, that the ROC and APL would differ significantly between the medial femoral condyle (MFC) and the lateral femoral condyle (LFC), and that a donor graft from the LFC would be suitable for an MFC defect. METHODS: Knee magnetic resonance imaging scans of 147 patients with no cartilage defects were analyzed. Best-fit circles in the sagittal plane were determined at standardized locations on each condyle. Assuming the use of a 20-mm graft that was flush to the edges of the native cartilage, the central graft prominence was calculated for potential donor-host differences in the ROC. RESULTS: There was a linear correlation between the ROC and APL. There were significant differences in the mean ROC and APL between the MFC and LFC. Based on calculations of the central graft prominence among all ROC combinations within the patient group, 100% of potential medial-to-medial, 97.8% of lateral-to-lateral, and 92.5% of lateral-to-medial transplantations would produce a central graft prominence of \u3c1 mm. On average, an allograft harvested from an LFC (mean ROC, 25.7 mm; mean APL, 69.8 mm) implanted into an MFC defect site (mean ROC, 31.9 mm; mean APL, 66.6 mm) would have a central graft prominence of 0.4 +/- 0.3 mm. CONCLUSIONS: Assuming a maximum central graft prominence tolerance of +1 mm, our findings demonstrate that matching the ROC or APL would not be necessary for potential medial-to-medial or lateral-to-lateral allograft transplants within this patient group. Implantation of an LFC donor allograft into an MFC defect is also supported by our findings

Effects of Proud Large Osteochondral Plugs on Contact Forces and Knee Kinematics: A Robotic Study

BACKGROUND: Osteochondral allograft (OCA) transplantation is used to treat large focal femoral condylar articular cartilage defects. A proud plug could affect graft survival by altering contact forces (CFs) and knee kinematics. HYPOTHESIS: A proud OCA plug will significantly increase CF and significantly alter knee kinematics throughout controlled knee flexion. STUDY DESIGN: Controlled laboratory study. METHODS: Human cadaver knees had miniature load cells, each with a 20-mm-diameter cylinder of native bone/cartilage attached at its exact anatomic position, installed in both femoral condyles at standardized locations representative of clinical defects. Spacers were inserted to create proud plug conditions of +0.5, +1.0, and +1.5 mm. CFs and knee kinematics were recorded as a robot flexed the knee continuously from 0 degrees to 50 degrees under 1000 N of tibiofemoral compression. RESULTS: CFs were increased significantly (vs flush) for all proudness conditions between 0 degrees and 45 degrees of flexion (medial) and 0 degrees to 50 degrees of flexion (lateral). At 20 degrees , the average increases in medial CF for +0.5-mm, +1-mm, and +1.5-mm proudness were +80 N (+36%), +155 N (+70%), and +193 N (+87%), respectively. Corresponding increases with proud lateral plugs were +44 N (+14%), +90 N (+29%), and +118 N (+38%). CF increases for medial plugs at 20 degrees of flexion were significantly greater than those for lateral plugs at all proudness conditions. At 50 degrees , a 1-mm proud lateral plug significantly decreased internal tibial rotation by 15.4 degrees and decreased valgus rotation by 2.5 degrees . CONCLUSION: A proud medial or lateral plug significantly increased CF between 0 degrees and 45 degrees of flexion. Our results suggest that a medial plug at 20 degrees may be more sensitive to graft incongruity than a lateral plug. The changes in rotational kinematics with proud lateral plugs were attributed to earlier contact between the proud plug\u27s surface and the lateral meniscus, leading to rim impingement with decreased tibial rotation. CLINICAL RELEVANCE: Increased CF and altered knee kinematics from a proud femoral plug could affect graft viability. Plug proudness of only 0.5 mm produced significant changes in CF and knee kinematics, and the clinically accepted 1-mm tolerance may need to be reexamined in view of our findings