Abstract: This study was carried out to compare femoral component rotation of 18 knees from 18 patients who suffered from lateral flexion instability after total knee arthroplasty (Western Ontario and McMaster University Osteoarthritis [WOMAC], 6.4 points; International Knee Society [IKS] score, 119 points) with 10 asymptomatic controls (WOMAC, 0.1 points; IKS score, 182 points) after total knee arthroplasty. The symptomatic patients showed increased lateral joint laxity as determined by fluoroscopic stress radiography. Femoral component rotation was determined by computed tomography scans. The femoral component rotation was more internally rotated in symptomatic patients (5.5 degrees ) than in controls (1.0 degrees ) (P = .04). Varus laxity in flexion was higher in symptomatic patients (11.0 degrees ) than in controls (7.0 degrees ) (P < .001). Increased lateral flexion laxity is associated with increased internal femoral component rotation and a less favorable clinical outcome.

Abstract: BACKGROUND: It is known that in vivo kinematics after total knee replacement is influenced by the design of the implant. The goal of this study was to show that the sagittal curvature of two different knee prostheses differing in geometric design predicts their in vivo motion behavior. METHODS: Three-dimensional tibio-femoral displacements of two prosthesis designs (single radius vs. dual radius) were measured during knee extension under weight bearing conditions by in vivo video fluoroscopy. Finite helical axes were computed to represent the tibio-femoral motions. Angular deviation alpha and the spatial localization deviation delta were used to characterize the motions. Angular deviation is the angle between each incremental finite helical axis and the medio-lateral axis of the femoral component of the prosthesis. The spatial localization deviation is the distance between each finite helical axis and the center of the femoral component of the prosthesis. Statistical comparisons were performed using the median and the interquartile range of the angular deviation and the spatial localization deviation. FINDINGS: The single-radius design showed finite helical axes concentrated at a single axis near to the medio-lateral axis of the femoral component. The angular and spatial localization deviation of the dual radius design were larger compared to the single radius design, exhibiting finite helical axes varying between two axes. INTERPRETATION: Video fluoroscopy in combination with finite helical axis analysis proved to be suitable methods to evaluate the in vivo kinematical behavior of total knee arthroplasty, which can be useful for implant designers. Knowledge of in vivo kinematics can also provide surgeons with more background information about the total knee arthroplasty models they implant.

Abstract: BACKGROUND: Malrotation of the tibial and femoral components has been recognized to be a clinical complication affecting the performance and durability of total knee arthroplasty. This study used a novel strain acquisition technique to determine the effect of tibio-femoral component malrotation on tibial torque and strain distribution of the proximal tibial cortex with a cemented fixed-bearing posterior-stabilized knee. METHODS: Using electronic speckle pattern interferometry, strain on the proximal tibia of human cadaveric knees was obtained in response to 1500N axial loading for neutrally aligned tibial and femoral components, and for 10 degrees internal and external malrotation between the tibial and femoral components. Local strain gage measurements were combined with full-field optical strain measurements to quantify effects on tibial cortex strain and strain distributions caused by the 10 degrees malrotations. In addition, tibial torque was measured for incremental degrees of tibio-femoral malrotation. FINDINGS: Tibio-femoral malrotations as small as 2 degrees caused tibial torque in excess of 4 Nm. At 10 degrees malrotation, tibial torque significantly increased to over 8 Nm (P<0.001) as compared to neutrally aligned components. Local strain gage results significantly increased from 500 muepsilon to 632 muepsilon compressive strain in response to 10 degrees external malrotation, and to 1000 muepsilon compressive strain in response to 10 degrees internal malrotation. Full-field optical strain reports yielded the highest strain of 2153 muepsilon for 10 degrees internal malrotation 30 mm below the joint line. INTERPRETATION: Laser-based strain measurement technology provides novel capabilities to capture cortex strain fields. The sensitivity of cortex strain and torsion to small amounts of tibio-femoral malrotation may explain factors contributing to aseptic implant loosening of the tibial component.

Abstract: Femoral component malalignment after total knee arthroplasty is known to cause clinical symptoms, such as anterior knee pain. For intraoperative referencing, several anatomical landmarks are used by surgeons. One frequently used landmark is the transepicondylar axis, yet the accuracy and reproducibility of defining this axis have not been established. In 6 human cadavers, 4 different experienced orthopedic surgeons performed selections of the most prominent points of the medial and lateral epicondyle. Each individual position was digitized and recorded by an accurate optical navigation system. In addition, the most prominent points of the medial and lateral epicondyle were defined on a computed tomography image. After transforming the cadaver points in the computed tomography coordinate system, distances to the epicondyles were measured. The overall distribution of selected points was located in an area of 278 mm2 upon the medial epicondyle and 298 mm2 of the lateral.

Abstract: A complete cement mantle is important for the longevity of a total hip replacement. In the minimally-invasive direct anterior approach used at the Innsbruck University hospital, the femoral component has to be inserted into the femoral canal by an angulated movement. In a cadaver study, the quality and the extent of the cement mantle surrounding 13 Exeter femoral components implanted straight through a standard anterolateral transgluteal approach were compared with those of 13 similar femoral components implanted in an angulated fashion through a direct anterior approach. A third-generation cementing technique was used. The inner and outer contours of the cement mantles was traced from CT scans and the thickness and cross-sectional area determined. In no case was the cement mantle incomplete. The total mean thickness of the cement mantle was 3.62 mm (95% confidence interval 3.59 to 3.65). The mean thickness in the group using the minimally-invasive approach was 0.16 mm less than that in the anterolateral group. The distribution of the thickness was similar in the two groups. The mean thickness was less on the anteromedial and anterolateral aspect than on the posterior aspect of the femur. There is no evidence that the angulated introduction of Exeter femoral components in the direct anterior approach in cadavers compromises the quality, extent or thickness of the cement mantle.