Shin Kai

Automatic construction of an anatomical coordinate system for three-dimensional bone models of the lower extremities - Pelvis, femur, and tibia

Automated methods for constructing patient-specific anatomical coordinate systems (ACSs) for the pelvis, femur and tibia were developed based on the bony geometry of each, derived from computed tomography (CT). The methods used principal axes of inertia, principal component analysis (PCA), cross-sectional area, and spherical and ellipsoidal surface fitting to eliminate the influence of raters bias on reference landmark selection. Automatic ACSs for the pelvis, femur, and tibia were successfully constructed on each 3D bone model using the developed algorithm. All constructions were performed within 30s; furthermore, between- and within- rater errors were zero for a given CT-based 3D bone model, owing to the automated nature of the algorithm. ACSs recommended by the International Society of Biomechanics (ISB) were compared with the automatically constructed ACS, to evaluate the potential differences caused by the selection of the coordinate system. The pelvis ACSs constructed using the ISB-recommended system were tilted significantly more anteriorly than those constructed automatically (range, 9.6-18.8°). There were no significant differences between the two methods for the femur. For the tibia, significant differences were found in the direction of the anteroposterior axis; the anteroposterior axes identified by ISB were more external than those in the automatic ACS (range, 17.5-25.0°).

The Quadriceps Vector is Most Parallel to the Spherical Axis With Minimal Difference for Gender or Ethnicity

BACKGROUND The extensor mechanism may be the key to kinematic alignment of total knee arthroplasty. The purpose of this study was to determine any differences in the direction of the quadriceps vector based on gender or ethnicity and to determine which anatomically derived alignment axis is closest to the quadriceps vector. METHODS Computed tomography scans and patient records for 14 Caucasians (9 men and 5 women) and 40 Japanese (19 men and 21 women) were evaluated. Three axes of alignment-anatomic, mechanical, and spherical-were identified, measured, and compared to the quadriceps vector in each case. Principal component analysis was used to determine the quadriceps vector by using 3-dimensional models of muscles on computed tomography scans. RESULTS No statistically significant differences in the orientation of the quadriceps vector were found based on gender or ethnicity, and the quadriceps vector was most closely aligned with the spherical axis. CONCLUSION Because the quadriceps is the primary knee extensor, the spherical axis therefore may be a ubiquitous guide to alignment of the arthroplasty knee based on motion.

The Vector of Quadriceps Pull is Directed From the Patella to the Femoral Neck

BackgroundThe quadriceps is the primary extensor of the knee. Its vector, which is perpendicular to the flexion axis of the knee, is important in understanding knee function and properly aligning total knee components. Three-dimensional (3-D) imaging enables evaluation using a 3-D model of each quadriceps component.Questions/purposesWe calculated the direction and magnitude of the quadriceps vector (QV) and the precision of the measurement, and asked whether the QV bears a constant relationship to the femur and is aligned with an anatomically based axis on the femur.MethodsUsing CT data of 14 subjects, we created a 3-D solid model of each quadriceps muscle component. Vectors (3-D direction and length) for each quadriceps component were determined using principal component analysis for muscle direction and volume for magnitude; vector addition established the directional vector of the combined muscle. The combined vector originating in the center of the patella was compared with the shaft, mechanical, and spherical (center femoral head to center medial side of the knee) axes.ResultsThe QV passed from the patella center proximally crossing the femoral neck between the femoral head and greater trochanter and was most closely aligned with the spherical axis.ConclusionsThe QV axis may be an important reference for alignment of total knee components.Clinical RelevanceThe spherical axis can be used in aligning total knee components to the flexion axis of the knee.