Retention of the posterior cruciate ligament stabilizes the medial femoral condyle during kneeling using a tibial insert with ball-in-socket medial conformity.

PURPOSE: Resecting the posterior cruciate ligament (PCL) increases posterior laxity and increases the flexion gap more than the extension gap in the native (i.e. healthy) knee. These two effects could lead to significant anterior displacement of the medial femoral condyle in kneeling following total knee arthroplasty even when using a tibial insert with a high degree of medial conformity. Using an insert with ball-in-socket medial conformity and a flat lateral articular surface, the primary purpose was to determine whether the medial femoral condyle remained stable with and without PCL retention during kneeling. METHODS: Two groups of patients were studied, one with PCL retention (22 patients) and the other with PCL resection (25 patients), while kneeling at 90º flexion. Following 3D model-to-2D image registration, A-P displacements of both femoral condyles were determined relative to the dwell point of the medial socket. RESULTS: With PCL resection versus PCL retention, the medial femoral condyle was 5.1 ± 3.7 mm versus 0.8 ± 2.1 mm anterior of the dwell point (p < 0.0001). Patient-reported function scores were comparable (p ≥ 0.1610) despite a significantly shorter follow-up of 7.8 ± 0.9 months with PCL retention than 19.6 ± 4.9 months with PCL resection (p < 0.0001). Range of motion was 126 ± 8° versus 122 ± 6° with and without PCL retention, respectively (p = 0.057). CONCLUSION: Surgeons that use a highly conforming tibial insert design can stabilize the medial femoral condyle during kneeling by retaining the PCL. In patients with PCL resection, the 9 mm high anterior lip of the insert with ball-in-socket medial conformity was insufficient to prevent significant anterior displacement of the medial femoral condyle when weight-bearing on the anterior tibia.

The peak force to push a trial tibial insert into position cannot be used to select the correct thickness in total knee arthroplasty.

An objective of a total knee arthroplasty (TKA) is to restore native (i.e. healthy) function, and a crucial step is determining the correct insert thickness for each patient. If the insert is too thick, then stiffness results, and if too thin, then instability results. Two methods to determine the insert thickness are by manually assessing the joint laxity and by using a trial insert with goniometric markings that measures the internal-external rotation of the trial with respect to a mark on the femoral component. The former is qualitative and depends on the surgeon's experience and 'feel' and while the latter is quantitative, it can be used only with an insert with medial ball-in-socket conformity. An unexplored method is to measure the force required to push a trial insert into position. To determine whether this method has merit, the push force was measured in 30 patients undergoing unrestricted kinematically aligned TKA using an insert with ball-in-socket medial conformity, a flat lateral surface, and retention of the posterior cruciate ligament. During surgery, the surgeon determined three appropriate thicknesses to test from a selection ranging from 10 mm to 14 mm in 1 mm increments. The peak push forces going from an insert 1 mm thinner than the correct thickness as determined by an insert goniometer and from the correct thickness to 1 mm thicker were measured. Mean peak forces for the different insert thicknesses were 127 ± 104 N, 127 ± 95 N, and 144 ± 96 N for 1 mm thinner, correct, and 1 mm thicker, respectively, and did not differ (p = 0.3210). As a result, measurement of peak force during trial positioning of a tibial insert cannot be used to identify the correct thickness for all insert designs.

Finding Order in Chaos: Quantitative Predictors of Chaos Terrain Morphology on Europa.

The mechanisms for chaos terrain formation on Europa have long been a source of debate in the scientific community. There exist numerous theoretical and numerical models for chaos formation, but to date there has been a lack of quantifiable observations that can be used to constrain models and permit comparison to the outputs of these chaos models. Here, we use mapping and statistical analysis to develop a quantitative description of chaos terrain and their observed morphologies. For nine chaos features, we map every block, or region of pre-existing terrain within disrupted matrix. We demonstrate that chaos terrains follow a continuous spectrum of morphologies between two endmembers, platy and knobby. We find that any given chaos terrain's morphology can be quantified by means of the linearized exponential slope of its cumulative block area distribution. This quantitative metric provides a new diagnostic parameter in future studies of chaos terrain formation and comparison.

Anterior laxity and patient-reported outcomes 7 years after ACL reconstruction with a fresh-frozen tibialis allograft.

PURPOSE: After reconstructing a torn ACL with a soft tissue allograft, the long-term healing process of graft maturation following the short-term healing process of graft incorporation into the bone tunnels might lead to recurring instability and concomitant decreases in the activity level, function, and patient satisfaction. Relying on roentgen stereophotogrammetric analysis (RSA), the primary purpose was to determine whether anterior laxity increased and whether patient-reported outcomes declined between 1 and 7 years for a particular graft construct, surgical technique, and rehabilitation programme. METHODS: Eighteen of 19 patients, who participated in an earlier RSA study which extended to 1 year after the surgical procedure, were contacted 7 years after the surgical procedure. An examiner, different from the treating surgeon, measured anterior laxity under 150 N of anterior force using RSA in 16 patients and obtained outcome scores in 17 patients. One patient moved abroad and could not be contacted. One patient reinjured his reconstructed ACL and was excluded. RESULTS: The average increase in anterior laxity of 1.5 ± 2.1 mm between 1 and 7 years after surgery was not significant (p = 0.08), and the average increase in anterior laxity of 2.7 ± 2.3 mm between the day of surgery and 7 years was significant (p < 0.001). There were no significant declines in activity (median Tegner score, 6 at 1 year, 6 at 7 years), function (average Lysholm score, 94 at 1 year, 91 at 7 years), and subjective satisfaction (average International Knee Documentation Committee score, 90 at 1 year, 87 at 7 years) between 1 and 7 years after surgery. CONCLUSION: In demonstrating that the ACL graft construct remains functional in the long term, this study supports the use of a fresh-frozen tibialis allograft in patients with an average age of 37 years at the time of surgery when used in conjunction with a surgical technique which avoids roof and PCL impingement, uses slippage-resistant fixation devices, and allows brace-free, self-paced rehabilitation. LEVEL OF EVIDENCE: IV.

Design, calibration and validation of a novel 3D printed instrumented spatial linkage that measures changes in the rotational axes of the tibiofemoral joint.

An accurate axis-finding technique is required to measure any changes from normal caused by total knee arthroplasty in the flexion-extension (F-E) and longitudinal rotation (LR) axes of the tibiofemoral joint. In a previous paper, we computationally determined how best to design and use an instrumented spatial linkage (ISL) to locate the F-E and LR axes such that rotational and translational errors were minimized. However, the ISL was not built and consequently was not calibrated; thus the errors in locating these axes were not quantified on an actual ISL. Moreover, previous methods to calibrate an ISL used calibration devices with accuracies that were either undocumented or insufficient for the device to serve as a gold-standard. Accordingly, the objectives were to (1) construct an ISL using the previously established guidelines,(2) calibrate the ISL using an improved method, and (3) quantify the error in measuring changes in the F-E and LR axes. A 3D printed ISL was constructed and calibrated using a coordinate measuring machine, which served as a gold standard. Validation was performed using a fixture that represented the tibiofemoral joint with an adjustable F-E axis and the errors in measuring changes to the positions and orientations of the F-E and LR axes were quantified. The resulting root mean squared errors (RMSEs) of the calibration residuals using the new calibration method were 0.24, 0.33, and 0.15 mm for the anterior-posterior, medial-lateral, and proximal-distal positions, respectively, and 0.11, 0.10, and 0.09 deg for varus-valgus, flexion-extension, and internal-external orientations, respectively. All RMSEs were below 0.29% of the respective full-scale range. When measuring changes to the F-E or LR axes, each orientation error was below 0.5 deg; when measuring changes in the F-E axis, each position error was below 1.0 mm. The largest position RMSE was when measuring a medial-lateral change in the LR axis (1.2 mm). Despite the large size of the ISL, these calibration residuals were better than those for previously published ISLs, particularly when measuring orientations, indicating that using a more accurate gold standard was beneficial in limiting the calibration residuals. The validation method demonstrated that this ISL is capable of accurately measuring clinically important changes (i.e. 1 mm and 1 deg) in the F-E and LR axes.

Optimized design of an instrumented spatial linkage that minimizes errors in locating the rotational axes of the tibiofemoral joint: a computational analysis.

An accurate method to locate of the flexion-extension (F-E) axis and longitudinal rotation (LR) axis of the tibiofemoral joint is required to accurately characterize tibiofemoral kinematics. A method was recently developed to locate these axes using an instrumented spatial linkage (ISL) (2012, "On the Estimate of the Two Dominant Axes of the Knee Using an Instrumented Spatial Linkage," J. Appl. Biomech., 28(2), pp. 200-209). However, a more comprehensive error analysis is needed to optimize the design and characterize the limitations of the device before using it experimentally. To better understand the errors in the use of an ISL in finding the F-E and LR axes, our objectives were to (1) develop a method to computationally determine the orientation and position errors in locating the F-E and LR axes due to transducer nonlinearity and hysteresis, ISL size and attachment position, and the pattern of applied tibiofemoral motion, (2) determine the optimal size and attachment position of an ISL to minimize these errors, (3) determine the best pattern of pattern of applied motion to minimize these errors, and (4) examine the sensitivity of the errors to range of flexion and internal-external (I-E) rotation. A mathematical model was created that consisted of a virtual "elbow-type" ISL that measured motion across a virtual tibiofemoral joint. Two orientation and two position errors were computed for each axis by simulating the axis-finding method for 200 iterations while adding transducer errors to the revolute joints of the virtual ISL. The ISL size and position that minimized these errors were determined from 1080 different combinations. The errors in locating the axes using the optimal ISL were calculated for each of three patterns of motion applied to the tibiofemoral joint, consisting of a sequential pattern of discrete tibiofemoral positions, a random pattern of discrete tibiofemoral positions, and a sequential pattern of continuous tibiofemoral positions. Finally, errors as a function of range of flexion and I-E rotation were determined using the optimal pattern of applied motion. An ISL that was attached to the anterior aspect of the knee with 300-mm link lengths had the lowest maximum error without colliding with the anatomy of the joint. A sequential pattern of discrete tibiofemoral positions limited the largest orientation or position error without displaying large bias error. Finally, the minimum range of applied motion that ensured all errors were below 1 deg or 1 mm was 30 deg flexion with ±15 deg I-E rotation. Thus a method for comprehensive analysis of error when using this axis-finding method has been established, and was used to determine the optimal ISL and range of applied motion; this method of analysis could be used to determine the errors for any ISL size and position, any applied motion, and potentially any anatomical joint.

Posterior cruciate ligament retention with medial ball-in-socket conformity promotes internal tibial rotation and knee flexion while providing high clinical outcome scores.

BACKGROUND: Although retaining the posterior cruciate ligament (PCL) is advantageous in unrestricted kinematically aligned TKA, it is often excised with a medial stabilized implant. The primary objectives were to determine whether PCL retention using an insert with ball-in-socket (B-in-S) medial conformity to maximize A-P stability promotes internal tibial rotation and flexion while providing high patient-reported outcome scores. METHODS: Two cohorts of 25 patients each were treated with unrestricted kinematically aligned (KA) TKA using a tibial insert with B-in-S medial conformity and a flat lateral articular surface. One cohort retained the PCL; the other had it excised. Patients performed deep knee bend and step-up exercises during fluoroscopic imaging. Following 3D model-to-2D image registration, anterior-posterior (A-P) positions of the femoral condyles and tibial rotation were determined. RESULTS: For deep knee bend, mean internal tibial rotation with PCL retention was significantly greater at maximum flexion (17.7° ± 5.7° versus 10.4° ± 6.5°, p < 0.001) and significantly greater at 30°, 60°, and 90° flexion as well (p ≤ 0.0283). For step-up, mean internal tibial rotation with PCL retention was significantly greater at at 15°, 30°, and 45° flexion (p ≤ 0.0049) but was marginally not significantly greater at 60° (i.e. maximum) flexion (12.3° ± 4.4° versus 10.1° ± 5.4°, p = 0.0794). Mean flexion during active knee flexion with PCL retention was significantly greater (127° ± 8° versus 122° ± 6°, p = 0.0400). Both cohorts had high median Oxford Knee, WOMAC, and Forgotten Joint Scores that were not significantly different (p = 0.0918, 0.1448, and 0.0855, respectively) CONCLUSION: Surgeons that perform unrestricted KA TKA should retain the PCL with an insert that has B-in-S medial conformity, as this maintains extension and flexion gaps while also promoting internal tibial rotation and knee flexion as well as providing high clinical outcome scores.

Validation of a new method for finding the rotational axes of the knee using both marker-based roentgen stereophotogrammetric analysis and 3D video-based motion analysis for kinematic measurements.

In a previous paper, we reported the virtual axis finder, which is a new method for finding the rotational axes of the knee. The virtual axis finder was validated through simulations that were subject to limitations. Hence, the objective of the present study was to perform a mechanical validation with two measurement modalities: 3D video-based motion analysis and marker-based roentgen stereophotogrammetric analysis (RSA). A two rotational axis mechanism was developed, which simulated internal-external (or longitudinal) and flexion-extension (FE) rotations. The actual axes of rotation were known with respect to motion analysis and RSA markers within ± 0.0006 deg and ± 0.036 mm and ± 0.0001 deg and ± 0.016 mm, respectively. The orientation and position root mean squared errors for identifying the longitudinal rotation (LR) and FE axes with video-based motion analysis (0.26 deg, 0.28 m, 0.36 deg, and 0.25 mm, respectively) were smaller than with RSA (1.04 deg, 0.84 mm, 0.82 deg, and 0.32 mm, respectively). The random error or precision in the orientation and position was significantly better (p=0.01 and p=0.02, respectively) in identifying the LR axis with video-based motion analysis (0.23 deg and 0.24 mm) than with RSA (0.95 deg and 0.76 mm). There was no significant difference in the bias errors between measurement modalities. In comparing the mechanical validations to virtual validations, the virtual validations produced comparable errors to those of the mechanical validation. The only significant difference between the errors of the mechanical and virtual validations was the precision in the position of the LR axis while simulating video-based motion analysis (0.24 mm and 0.78 mm, p=0.019). These results indicate that video-based motion analysis with the equipment used in this study is the superior measurement modality for use with the virtual axis finder but both measurement modalities produce satisfactory results. The lack of significant differences between validation techniques suggests that the virtual sensitivity analysis previously performed was appropriately modeled. Thus, the virtual axis finder can be applied with a thorough understanding of its errors in a variety of test conditions.

Virtual axis finder: a new method to determine the two kinematic axes of rotation for the tibio-femoral joint.

The tibio-femoral joint has been mechanically approximated with two fixed kinematic axes of rotation, the longitudinal rotational (LR) axis in the tibia and the flexion-extension (FE) axis in the femur. The mechanical axis finder developed by Hollister et al. (1993, "The Axes of Rotation of the Knee," Clin. Orthop. Relat. Res., 290, pp. 259-268) identified the two fixed axes but the visual-based alignment introduced errors in the method. Therefore, the objectives were to develop and validate a new axis finding method to identify the LR and FE axes which improves on the error of the mechanical axis finder. The virtual axis finder retained the concepts of the mechanical axis finder but utilized a mathematical optimization to identify the axes. Thus, the axes are identified in a two-step process: First, the LR axis is identified from pure internal-external rotation of the tibia and the FE axis is identified after the LR axis is known. The validation used virtual simulations of 3D video-based motion analysis to create relative motion between the femur and tibia during pure internal-external rotation, and flexion-extension with coupled internal-external rotation. The simulations modeled tibio-femoral joint kinematics and incorporated 1 mm of random measurement error. The root mean squared errors (RMSEs) in identifying the position and orientation of the LR and FE axes with the virtual axis finder were 0.45 mm and 0.20 deg, and 0.11 mm and 0.20 deg, respectively. These errors are at least two times better in position and seven times better in orientation than those of the mechanical axis finder. Variables, which were considered a potential source of variation between joints and/or measurement systems, were tested for their sensitivity to the RMSE of identifying the axes. Changes in either the position or orientation of a rotational axis resulted in high sensitivity to translational RMSE (6.8 mm of RMSE per mm of translation) and rotational RMSE (1.38 deg of RMSE per degree of rotation), respectively. Notwithstanding these high sensitivities, corresponding errors can be reduced by segmenting the range of motion into regions where changes in either position or orientation are small. The virtual axis finder successfully increased the accuracy of the mechanical axis finder when the axes of motion are fixed with respect to the bones, but must be used judiciously in applications which do not have fixed axes of rotation.

Does graft construct lengthening at the fixations cause an increase in anterior laxity following anterior cruciate ligament reconstruction in vivo?

A millimeter-for-millimeter relation between an increase in length of an anterior cruciate ligament graft construct and an increase in anterior laxity has been demonstrated in multiple in vitro studies. Based on this relation, a 3 mm increase in length of the graft construct following surgery could manifest as a 3 mm increase in anterior laxity in vivo, which is considered clinically unstable. Hence, the two primary objectives were to determine whether the millimeter-for-millimeter relation exists in vivo for slippage-resistant fixation of a soft-tissue graft and, if it does not exist, then to what extent the increase in stiffness caused by biologic healing of the graft to the bone tunnel offsets the potential increase in anterior laxity resulting from lengthening at the sites of fixation. Sixteen subjects were treated with a fresh-frozen, nonirradiated, nonchemically processed tibialis allograft. Tantalum markers were injected into the graft, fixation devices, and bones. On the day of surgery and at 1, 2, 3, and 4 months, Roentgen stereophotogrammetric analysis was used to compute anterior laxity at 150 N of anterior force and the total slippage from both sites of fixation. A simple linear regression was performed to determine whether the millimeter-for-millimeter relation existed and a springs-in-series model of the graft construct was used to determine the extent to which the increase in stiffness caused by biological healing of the graft to the bone tunnel offset the increase in anterior laxity resulting from lengthening at the sites of fixation. There was no correlation between lengthening at the sites of fixation and the increase in anterior laxity at 1 month (R(2)=0.0, slope=0.2). Also, the increase in stiffness of the graft construct caused by biologic healing of the graft to the bone tunnel offset 0.7 mm of the 1.5 mm potential increase in anterior laxity resulting from lengthening at the sites of fixation. This relatively large offset of nearly 50% occurred because lengthening at the sites of fixation was small.

Do varus or valgus outliers have higher forces in the medial or lateral compartments than those which are in-range after a kinematically aligned total knee arthroplasty? limb and joint line alignment after kinematically aligned total knee arthroplasty.

AIMS: The aims of this study were to determine the proportion of patients with outlier varus or valgus alignment in kinematically aligned total knee arthroplasty (TKA), whether those with outlier varus or valgus alignment have higher forces in the medial or lateral compartments of the knee than those with in-range alignment and whether measurements of the alignment of the limb, knee and components predict compartment forces. PATIENTS AND METHODS: The intra-operative forces in the medial and lateral compartments were measured with an instrumented tibial insert in 67 patients who underwent a kinematically aligned TKA during passive movement. The mean of the forces at full extension, 45° and 90° of flexion determined the force in the medial and lateral compartments. Measurements of the alignment of the limb and the components included the hip-knee-ankle (HKA) angle, proximal medial tibial angle (PMTA), and distal lateral femoral angle (DLFA). Measurements of the alignment of the knee and the components included the tibiofemoral angle (TFA), tibial component angle (TCA) and femoral component angle (FCA). Alignment was measured on post-operative, non-weight-bearing anteroposterior (AP) scanograms and categorised as varus or valgus outlier or in-range in relation to mechanically aligned criteria. RESULTS: The proportion of patients with outlier varus or valgus alignment was 16%/24% for the HKA angle, 55%/0% for the PMTA, 0%/57% for the DLFA, 25%/12% for the TFA, 100%/0% for the TCA, and 0%/64% for the FCA. In general, the forces in the medial and lateral compartments of those with outlier alignment were not different from those with in-range alignment except for the TFA, in which patients with outlier varus alignment had a mean paradoxical force which was 6 lb higher in the lateral compartment than those with in-range alignment. None of the measurements of alignment of the limb, knee and components predicted the force in the medial or lateral compartment. CONCLUSION: Although kinematically aligned TKA has a high proportion of varus or valgus outliers using mechanically aligned criteria, the intra-operative forces in the medial and lateral compartments of patients with outlier alignment were comparable with those with in-range alignment, with no evidence of overload of the medial or lateral compartment of the knee. Cite this article: Bone Joint J 2017;99-B:1319-28.

Does a tensioning device pinned to the tibia improve knee anterior-posterior load-displacement compared to manual tensioning of the graft following anterior cruciate ligament reconstruction? A cadaveric study of two tibial fixation devices.

Devices that are pinned to the tibia to tension an anterior cruciate ligament (ACL) graft produce joint reaction loads that in turn can affect the maintenance of graft initial tension after tibial fixation and hence knee anterior-posterior (AP) load-displacement. However, the effect of these devices on AP load-displacement is unknown. Our objectives were to determine whether tensioning by device versus tensioning by hand causes differences in AP load-displacement and intraarticular graft tension for two commonly used tibial fixation devices: a bioresorbable interference screw and a WasherLoc. AP load-displacement and intraarticular graft tension were measured in 20 cadaveric knees using a custom arthrometer. An initial tension of 110 N was applied to a double-looped tendon graft with the knee at extension using a tensioning device pinned to the tibia and a simulated method of tensioning by hand. After inserting the tibial fixation device, the 134 N anterior limit (i.e., anterior position of the tibia with respect to the femur with a 134 N anterior force applied to the tibia) and 0 N posterior limit (i.e., AP position of the tibia relative to the femur with a 0 N force applied to the tibia) were measured with the knee in 25 degrees flexion. Intraarticular graft tension was measured at extension. These limits and intraarticular graft tension were also measured after cyclically loading the knee 300 times. Compared to a simulated method of tensioning by hand, tensioning with a device pinned to the tibia did not decrease the 134 N anterior limit and did not cause posterior tibial translation. However, intraarticular graft tension was maintained better with a tensioning device pinned to the tibia for the Washerloc, but not the interference screw. For two commonly used tibial fixation devices, a tensioning device pinned to the tibia does not improve AP load-displacement at 25 degrees flexion over tensioning by hand when the graft is tensioned at full extension, but does improve the maintenance of intraarticular graft tension for the Washerloc.

Tension in a double loop tendon anterior cruciate graft during a simulated open chain knee extension exercise.

Concerns exist regarding the tension developed in a reconstructed anterior cruciate ligament (ACL) during open chain knee extension exercises used to rehabilitate the knee. Therefore, the primary objective was to measure tension in an ACL graft during a simulated open chain knee extension exercise as a function of ankle weight. A secondary objective was to determine whether the graft tension was reduced with relatively high stiffness fixation. The open chain exercise was simulated in seven cadaveric specimens in which the ACL had been reconstructed with double loop tendon grafts. Graft tension was measured at 15 degrees of flexion as the effective ankle weight was increased from 22.5 to 67.5 and then to 112.5 N for three different fixation stiffnesses (25, 125, and 225 N/mm). The initial tension was set to restore the 225 N anterior limit of motion to that of the intact knee at 30 degrees of flexion. Increasing the ankle weight caused the graft tension to increase significantly (p<0.0001), but the increase with the highest ankle weight was only 62 N on average. Increasing the fixation stiffness caused the graft tension to decrease significantly (p<0.0001) because the initial tension decreased by 107 N as the fixation stiffness increased. Because the graft tension with the highest ankle weight was limited to 112 N on average, high stiffness fixation methods, which are also resistant to lengthening in the region of the fixation, may reduce the risk of graft construct lengthening during open chain knee extension exercises.

A new tibial coordinate system improves the precision of anterior-posterior knee laxity measurements: a cadaveric study using Roentgen stereophotogrammetric analysis.

Roentgen stereophotogrammetric analysis (RSA) can be used to measure changes in anterior-posterior (A-P) knee laxity after anterior cruciate ligament (ACL) reconstruction. Previous measurements of A-P knee laxity using RSA have employed a tibial coordinate system with the origin placed midway between the tips of the tibial-eminences. However, the precision in measuring A-P knee laxity might be improved if the origin was placed on the flexion-extension axis of rotation of the knee. The purpose of this study was to determine whether a center-of-rotation tibial coordinate system with the origin placed midway between the centers of the posterior femoral condyles, which closely approximates the flexion-extension center-of-rotation of the knee, improves the precision in measuring A-P knee laxity compared to the tibial-eminence-based coordinate system. A-P knee laxity was measured using each coordinate system six times in three human cadaveric knees implanted with 0.8-mm diameter tantalum markers. For each laxity measurement, the knee was placed in a custom loading apparatus and biplanar radiographs were obtained while the knee resisted a 44 N posterior shear force and 136 N anterior shear force. A-P knee laxity was determined from the change in position of the tibia, with respect to the femur, resulting from the posterior and anterior shear forces. The precision for each coordinate system was calculated as the pooled standard deviation of A-P knee laxity measurements. The precision of the center-of-rotation coordinate system was 0.33 mm, which was about a factor of 2 better than the 0.62 mm precision of the tibial-eminence coordinate system (p=0.006). The 0.33 mm precision with the center-of-rotation coordinate system suggests that an observed change of either 0.56 mm (i.e. 1.7 standard deviations) or greater in A-P knee laxity over time is a real change and not due to measurement error when the new tibial coordinate system is used and other factors contributing to variability are controlled as was done in this study. Accordingly, clinicians and researchers should consider the use of this alternate tibial coordinate system when making serial measurements of A-P knee laxity using RSA because the improved precision allows for the observation of smaller differences.

Effect of intravenous furosemide on the renal excretion of digoxin.

The effect of an 80-mg intravenous dose of furosemide on the urinary excretion of digoxin was determined in three adult men with normal renal function, each of whom was taking 0.25 mg digoxin daily on a chronic basis. On two separate days, serum samples were taken and urine was collected every 2 hours over an 8-hour period for determination of digoxin, creatinine, calcium, and sodium concentrations. On the first day of study, a saline bolus was given intravenously, and on the second day, furosemide was given. In all subjects, urinary digoxin excretion increased after furosemide in direct proportion to the increase in urine volume. No consistent correlation was seen between digoxin excretion and creatinine, calcium, or sodium output. No significant changes in serum digoxin were found in this active study. These results are consistent with the hypothesis that increasing glomerular filtration rate or total urine volume increases the renal excretion of digoxin and may result in increased total urinary output of this glycoside.

Is the circumferential tensile modulus within a human medial meniscus affected by the test sample location and cross-sectional area?

Quantifying the material properties of the human menisci is paramount to understanding their biomechanical functions within the knee. One important intrinsic material property governing the biomechanical functions of the meniscus is the circumferential tensile modulus. The purpose of this study was to determine if the circumferential tensile modulus of the human medial meniscus depends on the location and thickness of the sample tested. The following three hypotheses were tested: (a) the circumferential location (anterior, central, and posterior) does not significantly affect the tensile modulus, (b) the radial location (inner to outer) significantly affects the tensile modulus, and (c) the thickness (cross-sectional area) significantly affects the tensile modulus. Test samples, whose length was oriented in parallel with the circumferential collagen fibers, were collected from different circumferential and radial locations throughout 30 human medial menisci. Samples of three different thicknesses (0.5, 1.5, and 3.0 mm) were taken from three equal groups of 10 menisci (i.e., one thickness per group). The circumferential tensile modulus was measured under quasi-static loading. Statistical analysis showed no significant effect of the circumferential or radial location of the sample on the circumferential tensile modulus. This indicates that an overall circumferential tensile modulus may be calculated for the human medial meniscus by averaging the values determined at the various locations. However, the thickness of the test sample had a significant effect on the measured circumferential tensile modulus; the modulus varied inversely with the thickness. Thus, moduli determined from test samples that are too small in cross-sectional area overestimate the effective modulus of the tissue on the whole, and the cross-sectional area of the sample must be considered when determining a representative circumferential tensile modulus for the medial meniscus in a human knee.

How the fixation method stiffness and initial tension affect anterior load-displacement of the knee and tension in anterior cruciate ligament grafts: a study in cadaveric knees using a double-loop hamstrings graft.

There were two objectives to this study. The first was to investigate the relationship of graft fixation stiffness and graft initial tension on the anterior load-displacement behavior of knees reconstructed with a double-loop hamstrings tendon graft. The second was to determine the corresponding graft tensions at 225 N of anterior force applied to the knee. To satisfy these objectives, the anterior-load displacement curves were measured for seven cadaveric knees with the ACL intact at flexion angles ranging from 0 degrees to 90 degrees. The ACL was reconstructed in the same knees using a double-loop hamstrings graft. A/P load-displacement curves of the knee and graft tension were measured as the fixation method stiffness and the initial tension applied at full extension were varied (25-326 N/mm and 25-300 N). The 0 N posterior limit (unloaded position of tibia) and the anterior laxity (difference between the 0 N posterior limit and 225 N anterior limit) were computed to characterize the A/P load-displacement of the intact and reconstructed knees. The key results were that the 0 N posterior limit of the tibia was insensitive to changes in stiffness (p>0.6503) but that increasing initial tension caused increasing posterior subluxation of the tibia with respect to the femur (p=0.0001). The tibia was subluxed posteriorly by 5-6 mm on average at high levels of initial tension. Both initial tension and stiffness significantly affected the anterior laxity (p=0.0001 for both factors). Anterior laxity was restored closely to normal (i.e. <1 mm difference) by relatively high initial tension of 200 N in combination with low stiffness of 25 N/mm and by low initial tension of 25 N in combination with higher stiffness ranging between 94 and 326 N/mm. When anterior laxity is restored to normal using a high initial tension-low stiffness combination however, the tibia undergoes a large posterior subluxation with respect to the femur in the unloaded state (approximately 5 mm) and a relatively high graft tension of 275 N is developed at 225 N of anterior force. Both the tibial subluxation and graft tension are reduced substantially with low initial tension-higher stiffness combinations because the amount of initial tension required to restore anterior laxity to normal is reduced by about 200 N.

In vivo tensile behavior of a four-bundle hamstring graft as a replacement for the anterior cruciate ligament.

The purpose of this study was to measure the in vivo tensile behavior of a double-looped semitendinosus and gracilis graft used to reconstruct a torn anterior cruciate ligament in the human knee. In 14 subjects, intraoperative tension was measured for each of the four graft bundles during passive motion from 0 to 90 degrees of flexion. Two hypotheses were tested: (a) the peak tension carried by each of the four bundles was equal during passive motion, and (b) the mechanics of the bundles mimicked the functional bands of the native anterior cruciate ligament. The total tension was also calculated and used to determine strength requirements for fixation devices. The peak tensions of the four bundles during passive motion were not equal; however, enough tension was present in each bundle that load-sharing occurred between bundles. The pattern of tension between the anterior and posterior bundles mimicked the reciprocating load-sharing behavior of the functional bands of the native anterior cruciate ligament. Reciprocal tensile behavior was consistently achieved with the use of a single femoral tunnel centered on the most isometric line without the need for two separate femoral sockets. The maximum total tension was 296 N; this was nearly equal to the strength of one commonly used fixation device.

Contact pressure and tension in anterior cruciate ligament grafts subjected to roof impingement during passive extension.

Contact between an anterior cruciate ligament graft and the intercondylar roof has been termed roof impingement. Grafts with impingement sustain permanent damage, and if the injury is extensive enough, then the graft may fail, causing recurrent instability. This study evaluated two mechanical factors that could be responsible for the graft injury associated with roof impingement: an increase in graft tension or elevated pressures between the graft and the roof, or both. An anterior cruciate ligament reconstruction was performed using an Achilles tendon graft in five fresh-frozen cadaveric knees. Using a six-degree-of-freedom load application system, the anterior displacement of the knee with the native anterior cruciate ligament was restored in the reconstructed knee at a flexion angle of 30 degrees and with an anterior force of 200 N applied. Pressure between the graft and intercondylar roof, graft tension, and flexion angle were measured during passive knee extension for three tibial tunnel placements (anterior, center, and posterior). Intercondylar roof impingement increased the contact pressure between the graft and the roof but had no significant effect on graft tension. Therefore, during passive knee extension, the contact pressure between the anterior cruciate ligament graft and the intercondylar roof is a more likely cause of graft damage than increased graft tension.

Quadriceps load aggravates and roofplasty mitigates active impingement of anterior cruciate ligament grafts against the intercondylar roof.

Because of the complications of impingement of anterior cruciate ligament grafts on the intercondylar roof and because current surgical procedures locate the tibial tunnel such that impingement is avoided during passive but not active extension, the objectives of this study were to determine if (a) active extension precipitates and aggravates roof impingement, and (b) a roofplasty mitigates the effects of impingement. The tibial translation, flexion angle defining the onset of roof impingement, graft-roof contact pressure, and graft tension were measured for six cadaveric specimens. In each specimen, two tibial tunnel positions were studied: one customized for the slope of the intercondylar roof, and the other translated 6 mm anteriorly from the customized position. For a quadriceps load of 1,500 N, the flexion angle defining the onset of impingement, the peak contact pressure, and the graft tension increased significantly for both tunnel positions. The increases occurred because of the anterior tibial translation caused by the active load. Although a roofplasty decreased the onset of the angle of impingement, the graft tension remained unaffected. Thus, to mitigate the effect of impingement during active rehabilitative knee extension exercises, the position of the tibial tunnel must be customized to the angle of the intercondylar roof and a roofplasty must be performed. The extent of bone removed must be customized as well and can be determined by removing bone from the intercondylar roof in excess of that required to freely pass a rod, the same diameter of the graft, through the tibial tunnel into the intercondylar notch with the knee in full passive extension.