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.

Introduction

We investigated the usefulness of flap surgery for Gustilo type IIIB and C severe open fracture of the tibia, for which treatment is difficult.

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.

In revision total hip arthroplasty (THA), it is essential to cope with the bone stock loss. The acetabular bone loss is reconstructed by bulk bone grafts, bone chips, bone cement or jumbo cup. The impaction bone-grafting (IBG) technique is a technique that can restore acetabular bone loss, while enough bone allografts are not easy to obtain and the quality is not always sufficient. Thus we mixed hydroxyapatite (HA) granules into bone chips to supplement the volume and the mechanical strength of allografts. To investigate the dynamic migration of cemented cup fixed with IBG, we made acetabular bone defect models and the migration of the cup was traced by a high-speed photography camera.

Composite test blocks were used as synthetic acetabulum models. A hemisphere defect of 60mm in diameter was made. We tested 4 different bone/HA ratio; 100%/0%, 75%/25%, 50%/50% and 0%/100%. Each group consisted of 6 specimens. The grafted materials were impacted using impactors. Then, a 46 mm polyethylene cup was fixed with bone cement. The specimens were clamped to the MTS mechanical tester at an angle of 20 degrees. A dynamic load of 150 N to 1500 N with a frequency of 1 Hz was applied for 15 minutes, followed by a dynamic load of 300 N to 3000 N for the same time period. Then the load was released for 15 minutes. The cup migration was traced by the camera during loading and releasing. This camera captures 15 images per second thus it enables us to trace the migration of the cup during cyclic loading. The cup migration at the end of 3000N loading was measured. Elastic recoil was defined as the difference between the migration at the end of 3000N loading and that when the load reached to 0N. Visco-elastic recoil was defined as the difference between the migration at the release of loading and that after 15 minutes. Data were investigated by Pearson’s correlation coefficient test.

A strong negative correlation (r = −0.71) was observed significantly between the amount of the migration and bone/HA ratio. In elastic recoil, statistically significant correlation was (r = −0.55) observed. In visco-elastic recoil, there is no correlation between the amounts of the visco-elastic recoil and bone/HA ratio.

In the reconstruction of bone defects, initial stability of the cup is a first step to expect the long term survival. The initial stability depends on the mechanical properties of the grafted materials. To supplement the volume and mechanical strength of bone allografts, we added HA granules to the bone chips. In the current study, the cup migration was smaller by adding HA granules. Elastic recoil was affected, while visco-elastic recoil was not affected. These results indicated that the mixture of HA granules to bone chips stabilized the cup during loading period and load releasing period.

Highly closslinked polyethylene has been developed to reduce polyethylene wear and to expect the longevity of THA. In daily activity of patients, total hip prostheses repeatedly suffer impact loading. However, the mechanical properties, especially impact load transmission, are not well investigated and the viscoelasticity might influence the loosening of cemented all-polyethylene cup. In this study, the impact load transmission through the complex of polyethylene cup, ceramic ball and metallic femoral stem was investigated.

Impact compressive tests on the complex were performed using Hopkinson pressure bar apparatus. Conventional and highly crosslinked polyethylene cups of three different sizes (40mm, 50mm, 56mm)were compared. The impact load was applied either from the cup or from the stem. The impact load transmission ratio (ILTR) i.e. the ratio of the magnitude of transmitted load to that of incident one was investigated. The loading pulse profiles were theoretically calculated based on the one dimensional elastic wave propagation theory and were compared with experimental results.

The ILTR was independent of the cup size in all experimental conditions. When the impact load was applied from the cup, the ILTR was not different between two types of polyethylene. On the other hand, when the impact load was applied from the stem, the ratio was greater than the previous loading condition, while the ratio of crosslinked polyethylene was significantly lower than that of conventional one (p < 0.05). The theoretically predicted stress pulse profiles were well correlated to the experimental ones.

The lower ILTR in highly crosslinked polyethylene is considered to be due to lower stiffness. These mechanical properties increase the deformity of the cup and may cause the loosening of the cup. These results indicated that the ILTR was not different among cup sizes, while the ratio was significantly affected by the loading conditions and the type of polyethylene.

Change in the joint line in TKA has been recognized as an important parameter in association with post-operative soft tissue tension, range of motion, and knee kinematics. In general, the joint line has been assessed only in tibial side based on the bony reference point of tibia. However, the joint line should also be assessed in the femoral side. This is because a replaced femoral condyle often does not accurately restore the geometry of the original condyle, depending on the alignment, the size, or the design of the component. This discrepancy, especially in the geometry of the distal and posterior condyle will greatly affect the knee kinetics in association with the soft tissue tension. Objective of this study was to investigate how joint line was changed in femoral and tibial condyle by TKA.

We have developed a method to assess the femoral-joint line and the tibial joint line three-dimensionally and quantitatively by the 3D model image matching to biplanar computed radiography. Twenty-knees underwent TKA and 3D joint line examination.

Most of the knees demonstrated the significant proximal movement of the medial joint line in tibia, while the lateral joint line was restored. The significant distal movement of the distal femoral joint line was demonstrated in most of the knees, and it was demonstrated more frequently in medial condyle. Most of the knees demonstrated the significant anterior movement of posterior femoral joint line while no knee demonstrated the significant posterior movement.

From the results of this report, it was proved that the joint line can be changed by TKA procedure not only in tibial condyle but also in distal and posterior femoral condyles with considerable variations. In addition, it was also proved that there can be a difference in the change in the joint line between medial and lateral condyle.

Award for the best student biomaterials paper (US$ 2,000); a proper certificate

Single plane 2D-3D image matching procedure using fluoroscopic images with CAD data of components has been a gold standard of the in-vivo knee kinematics analysis after total knee arthroplasty (TKA). Numerous literatures have highlighted the “Condylar lift-off” (CLO) phenomenon that is thought to be the cause of eccentric polyethylene wear. However, these reports have not taken account of the 3D geometry of tibial polyethylene insert (TPI).

We have developed a system for analyzing static 3D relationship between femoral and tibial component after TKA accurately utilizing the biplanar computed radiography. By applying this system to fluoroscopic knee motion analysis, it has been possible to analyze the 3Dbehavior of femoral component on the TPI by reducing the error in determining the out of plane translation and rotation. Four knees underwent TKA and postoperative knee motion analysis. Knee kinematics was analyzed by translation of medial and lateral estimated contact points of femoral component on TPI. CLO was defined as the separation of femoral component from TPI by more than 1 mm.

All 4 knees showed the “tilting” of femoral condyle relativeto tibial base plate in coronal plane (this phenomenon has been generally recognized as CLO) resulted from that one femoral condyle contacted with the lower potion in convex geometry of the TPI while the other contacted with the higher potion. This was occurred by a rotation of femoral condyle. However, no CLO was demonstrated in this series. This might be because that recorded knee motions were relatively slow and supported by examiners.

From the results of this report, it was proved that a tilting of femoral component relative to tibial base plate in coronal plane not always indicates CLO. For detailed analysis of knee kinematics after TKA, it was thought to be necessary to take account of the geometry of TPI.

Highly crosslinked polyethylene has been developed to reduce polyethylene wear and to expect the longevity of THA. In daily activity of patients, total hip prostheses repeatedly suffer impact loading. However, the mechanical properties, especially impact load transmission, are not well investigated and the viscoelasticity might influence the loosening of cemented all-polyethylene cup. In this study, the impact load transmission through the complex of polyethylene cup, ceramic ball and metallic femoral stem was investigated.

Impact compressive tests on the complex were performed using Hopkinson pressure bar apparatus. Conventional and highly crosslinked polyethylene cups of three different sizes (40mm, 50mm, 56mm) were compared. The impact load was applied either from the cup or from the stem. The impact load transmission ratio (ILTR) i.e. the ratio of the magnitude of transmitted load to that of incident one was investigated. The loading pulse profiles were theoretically calculated based on the one dimensional elastic wave propagation theory and were compared with experimental results.

The ILTR was independent of the cup size in all experimental conditions. When the impact load was applied from the cup, the ILTR was not different between two types of polyethylene. On the other hand, when the impact load was applied from the stem, the ratio was greater than the previous loading condition, while the ratio of crosslinked polyethylene was significantly lower than that of conventional one (p < 0.05). The theoretically predicted stress pulse profiles were well correlated to the experimental ones.

The lower ILTR in highly cross linked polyethylene is considered to be due to lower stiffness. These mechanical properties increase the deformity of the cup and may cause the loosening of the cup. These results indicated that the ILTR was not different among cup sizes, while the ratio was significantly affected by the loading conditions and the type of polyethylene.

Impaction allografting is one of the techniques for reconstruction of femur during revision total hip arthroplasties. The initial stability of the stem fixed with impacted morsellized allogtafts and cement depends on multiple factors. The aim of this study was to investigate the stability of stem in reference to the size of bone chips, femoral bone defect and implant design.

Morsellized grafts of human femoral heads were prepared using a reciprocating type bone mill or a rotating type bone mill. Femoral bone defect was created at proximal medial cortex. Two types of polished stem were tested; CPT stem and VerSys CT stem (Zimmer Inc.). The cross section of the stem was relatively rectangular in CPT stem, while round in VerSys CT stem. Morsellized grafts were impacted into an over-reamed plastic bone and the stem was fixed with PMMA bone cement. Cyclic compression test and torsional test were performed using an Instron type machanical tester. Bone chips prepared by a reciprocating type bone mill contained large chips with broad size distribution, which represented high stiffness in compression test and high maximum torque in torsional test. Femoral bone defect and implant geometry did not affect the axial stability of stem, while large bone defect and round shape stem showed significantly lower maximum torque.

These results indicated that the size of bone chips, femoral bone defect and implant geometry affected the initial stability of the stem. Impaction grafting seems to be a technically demanding procedure, however several factors can be controlled to obtain secure implant stability.