Polyethylene wear in total knee arthroplasty (TKA) is a complex and mutifactorial process. It is generally recognized that wear is directly related to a material wear factor, contact stress, and sliding distance. Conventional methods of predicting polyethylene wear in TKA mainly focus on peak contact stress or subsurface shear stress using finite element method analysis. By incorporating kinematics and contact stress, a new predictor for polyethylene wear in TKA (“Wear Index”) has been developed. The Wear Index was defined by multiplying deformation by femoro-tibial sliding velocity. The purpose of this study was to determine the predictive value of the Wear Index for polyethylene wear in TKA using both a numeric and an in vitro model.

Four commercially available total knee prostheses were modeled for this study. Deformation and sliding velocity were calculated based on the three-dimensional geometry of the components and the gait kinematic inputs using Hertz’s formula. One specimen of each of the four types of total knee prostheses was mounted on a custom-designed knee simulator. Vertical loads and flexion-extension uni-axial motion were simulated using computer controlled servohydraulic actuators. The same gait kinematic inputs used in the theoretical study were used in the simulation test. After the simulations, the surface of the tibial insert was examined microscopically and macroscopically and compared with the theoretically generated Wear Index.

This study showed a high correlation between the numeric model and the simulation. The depth of wear on the tibial insert correlated significantly with the Wear Index. Microscopic findings also demonstrated a good correlation between the Wear Index and observed wear patterns. Sliding velocity is an important factor for understanding wear in TKA. In conclusion, this study suggests that the Wear Index is a reliable predictor of polyethylene wear in TKA, as it incorporates both contact stress and kinematics in its calculation.

We evaluated the geometry of the resected femoral surface according to the theory for total knee arthroplasty (TKA) using three-dimensional computed tomography (3D CT).

The 3D CT scans were performed in 44 knees indicated as requiring total knee arthroplasty. The 3D images of the femurs were clipped according to the following procedures. The distal femur was cut perpendicular to the mechanical axis at 10 mm proximal from the medial condyle. Rotational alignment was fixed at 3 degrees external rotation from the posterior condylar line. The anterior condyle was resected using the anterior cortex as the reference point. The posterior condyle was cut at 10 mm anterior from the medial posterior condyle.

The medial-lateral (ML) width/anterior-posterior (AP) length was 1.58 ± 0.14 (mean ± SD). AP length of the 3D images tended to be longer than the box length of the three kinds of components provided when the ML width of the images was approximately equal to that of each component. The widths of medial and lateral posterior condyles of the images were 30.1 ± 3.8 mm and 24.8 ± 3.0 mm, respectively. In all except one case, the widths of the resected medial posterior condyles were greater than those of the medial condyles of all components when those of resected lateral posterior condyles were equal to those of the lateral condyles of the components.

The shapes of the resected femoral surface did not always match those of the components. The configuration of Japanese knee joints is different from that of American knee joints. Components with appropriate geometry should be designed for Japanese patients.

Purpose: Osseointegration is crucial for favorable outcomes after total hip arthroplasty (THA) using cement-less femoral components. Osseointegration is recognized on radiographs as the endosteal spot weld, which is the bony bridge between the implant and surrounding bone (Engh et al, CORR, 1989). The purpose of this study is to evaluate the clinical and radiographic results for patients who had hydroxyapatite (HA)-coated rough surfaced implants compared with those who had identical implants without HA-coating at three-year minimum follow-up.

Methods: Ninety-one patients, one hundred and two hips who had primary THA with titanium arc sprayed rough surfaced femoral implants were retrospectively studied. Sixty-four hips had received HA-coated implants (HA) and 38 hips had an identical component but without HA (Non-HA). Radiographical parameters analyzed included 1) endosteal spot welds, 2) radiolucent lines, 3) calcar responses, 4) pedestal formation, 5) implant loosening, 6) endosteal osteolysis.

Results: At a minimum follow up of 3 years after operation (mean, 5.5 years), the mean Harris hip score was 89.4 points in the HA group and 89.0 points in the Non-HA group. The radiographic analysis did show the significantly earlier appearance of the endosteal spot welds in HA group. The spot welds at 1 and 2 year after operation were present in 48% and 70% of HA group, while 13% and 42% of Non-HA group. There was no significance at 3 years (71% vs 66%). More than 80% of the spot welds were seen at Gruen zone 6 in the both groups. No differences were noted regarding the radiolucent lines, calcar response, pedestal formation between the groups. There were no implant loosening and osteolysis in both groups.

Conclusions: These results suggest that the use of HA-coating does provide improved fixation in the early periods and the possibility of improved durability.

Flexion after total knee arthroplasty (TKA) has recently been improved by changing implant designs, surgical techniques and early postoperative rehabilitation protocols. Especially for Asian people, deep knee flexion is essential because of their life style. Small numbers of patients can achieve full flexion after TKA, however, most current prostheses are not designed to allow deep knee flexion safely. Furthermore, the kinematics involved in knee flexion greater than 90 degrees in cases of TKA is still unknown, even though fluoroscopic studies have shown the paradoxical anterior femoral translation in posterior cruciate retaining (CR) TKA with knee flexion up to 90 degrees. The purpose of this study was to determine the femoro-tibial contact pattern in deep knee flexion.

The knee that had been operated upon was passively flexed from 90 degrees up to the maximum flexion under anesthesia soon after the surgery. Lateral roentgenograms of the knee were taken during flexion, and the three-dimensional kinematics was analyzed using image-matching techniques. Nine patients with CR type were included.

The average maximum flexion angle was 131.8 °. The contact point moved posteriorly with deep knee flexion except for one patient. Five out of nine patients showed external rotation of the femoral condyle. Two patients showed internal rotation, and the other two exhibited no rotational movement. None of the patients showed dislocation or disengagement of the components. At the maximum flexion, the edge of the posterior flange of the femoral component contacted the polyethylene insert.

This study was performed under non-weight-bearing conditions, but deep knee flexion is not usually performed in weight-bearing conditions. Most of the CR type showed posterior roll back during deep knee flexion. The design of the posterior flange of the femoral component should be changed to prevent damage to the polyethylene.

Introduction: Correct rotational alignment of the femoral component is an important factor for successful total knee arthroplasty. This study evaluated relationship between the transepicondylar axis and the posterior condylar axis in normal, varus, and valgus knees.

Methods: Thirty normal knees (mean age: 66.2 years), 30 osteoarthritic knees with varus deformity (67.9 years), and 25 osteoarthritic knees with valgus deformity (70.7 years) were evaluated using magnetic resonance imaging. Femo-rotibial angle on standing anteroposterior radiograph was 185° in the varus knees and 166.1° in the valgus knees. In the transverse view, the angle between the transepicondylar axis and the posterior condylar axis, and the angle between the line perpendicular to the anteroposterior (AP) axis and the posterior condylar axis were measured in each group.

Results: Transepicondylar line showed 6.4° of external rotation in the normal knees and 6.1 of external rotation in the varus knees relative to the posterior condylar axis. However, transepicondylar axis of the valgus knee showed 11.6° of external rotation. This angle was significantly larger than that of normal knee and varus knee (p < 0.05). The line perpendicular to the AP axis was externally rotated from the posterior condylar axis in 6.3° in the normal knees, 6.6° in the varus knees, and 8.8° in the valgus knees. The external rotational angle in the valgus knees was significantly larger than that of the normal and varus knees (p < 0.05).

Discussion and conclusion: These results suggest that there is no hypoplasia of the posterior part of the medial condyle in varus knees, however, posterior part of the lateral condyle in valgus knee is severely distorted. Based on the results of this study, 3 to 5 degrees of external rotation relative to the posterior condyles is not large enough to achieve correct rotational alignment for valgus knees.

Introduction: Analyses of the 3-D kinematics of TKA in vivo using the x-ray image matching techniques have been widely reported. However, the accuracy of those techniques has seemed not to be discussed enough. To demonstrate more accurate technique for those analyses, we developed the new calibration flame to detect the geometry of the x-ray source and more accurate image matching methods.

Materials and methods: A calibration flame was composed of four ball bearing markers. First, the optimal distributions of those ball markers were determined by computer simulations, and then, using the high-resolutional computed radiography (CR) of a metallic ball taken with the obtained optimal calibration flame, the resolving power of three degrees of freedom (DOF) translations were calculated. Next, the computer-synthesized projected images of the femoral component of TKA were calculated using the estimated x-ray source geometry and experimentally measured geometric data of the prosthesis when the full six DOF poses of the prosthesis were calculated. Matching the computer-synthesized images with the extracted and binalized 2-D CR images of the prosthesis was done automatically using computer in order to minimize the exclusive OR (XOR).

Results: The geometry of the x-ray source was estimated with accuracy of below 0.5 mm in computer simulations. The CR images of the prosthesis were matched with the computer-synthesized images until the XOR reached under one pixel and then, the accuracy of below 1.0 mm translations and 1.0 degrees rotation were recognized for the resolving power of six DOF poses of the prosthesis.

Discussion: The more accurate measurement of full six DOF poses is indispensable to estimate not only the 3-D kinematics but also the contact stresses or predicted polyethylene wear on TKA in vivo. The new calibration flame and the image matching technique we developed appear to be effective for analyses of TKA in vivo.

Our study evaluated the accuracy of an image-guided total knee replacement system based on CT with regard to preparation of the femoral and tibial bone using nine limbs from five cadavers. The accuracy was assessed by direct measurement using an extramedullary alignment rod without radiographs.

The mean angular errors of the femur and tibia, which represent angular gaps from the real mechanical axis in the coronal plane, were 0.3° and 1.1°, respectively. The CT-based system, provided almost perfect alignment of the femoral component with less than 1° of error and excellent alignment with less than 3° of error for the tibial component. Our results suggest that standardisation of knee replacement by the use of this system will lead to improved long-term survival of total knee arthroplasty.

Introduction: The purpose of this study was to investigate the effects of combination treatments with anticoagulant (warfarin) and a lipid-lowering agent (probucol) on the prevention of steroid-associated osteonecrosis (ON) in rabbits.

Materials and Methods: Male adult Japanese white rabbits were intramuscularly injected once with 20mg/kg body weight of methylprednisolone acetate into the right gluteus medius muscle. These rabbits were divided into three groups: a warfarin plus probucol treatment group (WP Group, n=25), a probucol treatment group (PR Group, n=30), and a non-prophylactic treatment group (NP Group, n=20). Two weeks after the cortico-steroid injection, both femora and humeri were histopathologically examined for the presence of ON, and the sizes of bone marrow fat cells were morphologically examined.

Results: The incidence of ON in the WP Group (5%) was significantly lower than that in the NP Group (70%) (p < 0.0001). The incidence of ON in the PR Group (37%) was significantly lower than that in the NP Group (p < 0.05), but it was significantly higher than that in the WP Group (p < 0.01). The mean size of the bone marrow fat cells was significantly smaller in the WP Group (53.5 ± 4.1μm) than that in the NP Group (60.0 ± 4.0μm) (p < 0.0001). There were no significant differences in the size of bone marrow fat cells between the WP and the PR Groups (52.0 ± 5.0μm).

Discussion: This study experimentally confirmed that anticoagulant plus lipid-lowering agent treatment has a preventative effect on steroid-associated ON in rabbits.

Introduction: When osteonecrosis is located in the medial portion of the femoral head, transtrochanteric curved varus osteotomy (varus), in which the lateral intact area is transposed to the weight-bearing portion, is indicated. The purpose of this study was to evaluate the clinical outcomes of this procedure.

Materials and Methods: Cases consisted of 60 hips in 52 patients with osteonecrosis of the femoral head who had a varus osteotomy from 1981 to 1998. Fifty-five hips out of 60 were followed (follow-up rate: 92%; 5 hips dropped out). The underlying associated factors were alcohol (5), trauma (2), and corticosteroids (40); 8 hips were from patients without a known factor (idiopathic). Nineteen were male and 36 were female. The average age was 34 years at the time of surgery. Forty-three hips were classified as ARCO Stage III-A, 11 in Stage III-B, and 1 in Stage IV.

Results: The average follow-up was 8.1 years (range, 0.8 to 20 years). The average preoperative Harris Hip Score of 51 points improved to an average of 81 at the latest follow-up. Radiographically, osteonecrosis in 46 hips (84%) healed or had no progression of collapse. Nine hips (16%) showed osteoarthritic changes, including progression of collapse, in which 4 cases had undergone conversion to THA. The post-operative intact area ratio in these 4 cases was 16%, while that in the other 51 cases was 70% (p< 0.005).

Discussion: When the intact area remains at the lateral portion of the femoral head, varus osteotomy is useful not only for healing of the necrotic lesion but also for the prevention of osteoarthritis. If the intact area ratio is over 34 % with hip abduction, varus osteotomy is indicated.

Our study describes the mid-term clinical results of the use of transtrochanteric valgus osteotomy (TVO) for the treatment of osteoarthritis of the hip secondary to acetabular dysplasia. The operation included valgus displacement at the level of the lesser trochanter, and lateral displacement of the greater trochanter by inserting a wedge of bone. We reviewed 70 hips. The mean age of the patients at operation was 44 years (14 to 59). Most (90%) had advanced osteoarthritis.

The scores for pain and gait had improved significantly at a mean follow-up of 9.4 years. The rate of survival until an endpoint of a further operation during a follow-up of ten years was 82%. The survival rate was 95% in patients with unilateral involvement who were less than 50 years of age at operation. TVO is a useful form of treatment for advanced osteoarthritis of the hip, particularly in young patients with unilateral disease.

We have studied the correlation between the prevention of progressive collapse and the ratio of the intact articular surface of the femoral head, after transtrochanteric rotational osteotomy for osteonecrosis. We used probit analysis on 125 hips in order to assess the ratio necessary to prevent progressive radiological collapse over a ten-year period. The results show that a minimum postoperative intact ratio of 34% was required. This critical ratio may be useful for surgical planning and in assessing the natural history of the condition.

We have studied the effect of hydroxyapatite (HA) coating in 15 ovariectomised and 15 normal rats which had had a sham procedure. Twenty-four weeks after operation, HA-coated implants were inserted into the intramedullary canal of the right femur and uncoated implants into the left femur. The prostheses were removed four weeks after implantation. Twelve specimens in each group had mechanical push-out tests. Sagittal sections of the other three were evaluated by SEM.

The bone mineral density (BMD) of the dissected left tibia was measured by dual-energy x-ray absorptiometry. The difference in BMD between the control and ovariectomised tibiae was 35.01 mg/cm² (95% CI, 26.60 to 43.42). The push-out strength of the HA-coated implants was higher than that of the uncoated implants in both groups (p < 0.0001), but the HA-coated implants of the ovariectomised group had a reduction in push-out strength of 40.3% compared with the control group (p < 0.0001).

Our findings suggest that HA-coated implants may improve the fixation of a cementless total hip prosthesis but that the presence of osteoporosis may limit the magnitude of this benefit.