Three-dimensional (3D) weight-bearing alignment of the lower extremity is crucial for understanding biomechanics of the normal and pathological functions at the hip, knee, and ankle joints. In addition, implant position with reference to bone is a critical factor affecting the long-term survival of artificial joints. The purpose of this study was to develop a biplanar system using a slot-scan radiography (SSR) for assessing weight-bearing alignment of the lower extremity and for assessing implant positioning with respect to bone. A SSR system (Sonial Vision Safire 17, Shimadzu, Kyoto, Japan) with a custom-made rotation table was used to capture x-ray images at 0 deg and 60 deg relative to the optical axis of an x-ray source [Fig.1]. The SSR system uses collimated fan beam x-rays synchronized with the movement of a flat-panel detector. This system allows to obtain a full length x-ray image of the body with reduced dose and small image distortion compared with conventional x-ray systems. Camera calibration was performed beforehand using an acrylic reference frame with 72 radiopaque markers to determine the 3D positions of the x-ray source and the image plane in the coordinate system embedded in the reference frame. Sawbone femur and tibia and femoral components of the Advance total knee system (Wright Medical Technology, Arlington, TN, USA) were used. Computed tomography of the sawbone femur and tibia was performed to allow the reconstruction of the 3D surface models. For the component, the computer aided design (CAD) model provided by the manufacturer was used. Local coordinate system of each surface model was defined based on central coordinates of 3 reference markers attached to each model. The sawbone femur and tibia were immobilized at extension, axial rotation, and varus deformity and were imaged using the biplanar SSR system. The 3D positions of the femur and tibia were recovered using an interactive 2D to 3D image registration method [Fig.2]. Then, the femoral component was installed to the sawbone femur. The 3D positions of the femur and femoral component were recovered using the above-mentioned image registration method. Overall, the largest estimation errors were 1.1 mm in translation and 0.9 deg in rotation for assessing the alignment, and within 1 mm in translation and 1 deg in rotation for assessing the implant position, demonstrating that this method has an adequate accuracy for the clinical usage.

Progression of osteoarthritis (OA) of the knee is related to alignment of the lower extremity. Postoperative lower extremity alignment is commonly regarded as an important factor in determining favourable kinematics to achieve success in total knee arthroplasty (TKA) and high tibial osteotomy (HTO). An automated image-matching technique is presented to assess three-dimensional (3D) alignment of the entire lower extremity for natural and implanted knees and the positioning of implants with respect to bone.

Sawbone femur and tibia and femoral and tibial components of a TKA system were used. Three spherical markers were attached to each sawbone and each component to define the local coordinate system. Outlines of the 3D bone models and the component computer-aided design models were projected onto extracted contours of the femur, tibia, and implants in frontal and oblique X-ray images. Threedimensional position of each model was recovered by minimizing the difference between the projected outline and the contour. The relative positions were recovered within −0.3 ± 0.5 mm and −0.5 ± 1.1° for the femur with respect to the tibia, −0.9 ± 0.4 mm and 0.4 ± 0.4° for the femoral component with respect to the tibial component, −0.8 ± 0.2 mm and 0.8 ±0.3° for the femoral component with respect to the femur, and −0.3 ± 0.2 mm and −0.5 ± 0.4° for the tibial component with respect to the tibia.

Clinical applications were performed on 12 knees in 10 OA patients (mean age, 72.5 years; range, 62–87 years) to check change in the 3D mechanical axis alignment before and after TKA and to measure position of the implant with regard to bone. The femorotibial angle significantly decreased from 187.8° (SD 10.5) to 175.6° (SD 3.0) (p=0.01). The 3D weight-bearing axis was drawn from the centre of the femoral head to the centre of the ankle joint. It intersected significantly medial (p=0.01) and posterior (p=0.023) point at the proximal tibia before TKA. The femoral component rotation was 3.8° (SD 3.3) internally and the tibial component rotation was 14.1° (SD 9.9) internally. Compared with a CT-based navigation system using pre-and post-operative CT for planning and assessment, the benefit to patients of our method is that the post-operative CT scan can be eliminated.

In order to understand the actual weight-bearing condition of lower extremity, the three dimensional (3D) mechanical axis of lower limb was compared with the loading direction of ground reaction force (GRF) in standing posture.

Three normal subjects (male, 23–39 yo) participated in the study. A bi-planar radiograph system with a rotation table was used to take frontal and oblique images of entire lower limb. Each subject’s lower limb was CT scanned to create 3D digital models of the femur and tibia. The contours of the femur and tibia in both radiographs and the projected outlines of the 3D digital femur and tibia models were matched to recover six-degree of freedom parameters of each bone. The 3D mechanical axis was a line drawn from the centre of the femoral head to the centre of the ankle. A surface proximity map was created between the distal femoral articular surface and the proximal tibial articular surface. A force plate was positioned on the rotation table to measure GRF during biplanar X-ray exposure. Each subject put one’s foot measured on the force plate and the other on the shield. Bi-planar radiographs were taken in double-limb standing, double-limb standing with toe up in the leg measured, and single-limb standing. The anterior and medical deviations of the loading direction of GRF from the 3D mechanical axis were determined at the proximal tibia and normalized by the joint width in anteroposterior direction and by the joint width in lateral direction.

For all subjects the passing points of the 3D mechanical axis at the proximal tibia were almost in the middle of the joint width in lateral direction. Compared to the 3D mechanical axis, the loading direction of GRF passed through the anterior region in double-limb standing and single-limb standing, and anteromedial region in single-limb standing. The normalized medial deviation was significantly greater in singlelimb standing than in double-limb standing (p=0.023). The separation distance tended to decrease in the medial compartment in single-limb standing, and to increase in toe up in the entire region.

Deviation of the loading direction of GRF from the 3D mechanical axis at the proximal tibia varied among standing postures, relating to the change in weightbearing condition as indicated in the separation distance map. These results provide the mechanical perspective related to the causes and progression of knee OA and may contribute to the improvement of surgical treatments such as arthroplasty and osteotomy.

We investigated the clinical outcome of a reconstructive procedure of the medial patellofemoral ligament for the treatment of habitual or recurrent dislocation of the patella in four children (6 knees), with a minimum follow-up of four years. The technique involves transfer of the tendon of semitendinosus to the patella using the posterior one-third of the femoral insertion of the medial collateral ligament as a pulley.

There was no recurrence of dislocation after surgery. The mean Kujala score at follow-up was 96.3 points. Radiological assessment showed that the congruence angle, the tilt angle and the lateral shift radio were restored to normal. The lateral and medial stress shift ratios and the Insall-Salvati ratio remained abnormal.

We conclude that this technique can be recommended for the treatment of habitual or recurrent patellar dislocation in children, although hypermobility and patella alta are not fully corrected.