INTRODUCTION

During activities of daily living (ADL), varus moments are experienced in the knee, which can result in frontal plane rotation, or liftoff, of the lateral femoral condyle with respect to the tibial plateau. An understanding of this rotation is valuable as it could potentially lead to contact between the femoral component and polyethylene post of a total knee replacement (TKR). Therefore, the purpose of this study was 1) to assess how much frontal plane rotation was achieved due to varus moments imposed on a total stabilized (TS) TKR from the stair ascent activity, and 2) to determine whether a TS TKR could withstand the contact stresses imposed by the varus loading for 1 million cycles without the post fracturing or plastically deforming.

Introduction: Uncemented proximally filling porous-coated femoral components must be designed with an optimal level of press-fit. Excessive press-fit yields higher femoral stress which can result in periprosthetic femoral fracture (PPFx), whereas insufficient femoral stress can lead to a lack of initial mechanical stability, which “is necessary to achieve bone ingrowth into the porous surface” (Manley P.A. et. al., J Arthroplasty10:63–73, 1995) of the implant. An optimal press-fit design should also provide an accurate and repeatable femoral stem seating height in all patients.

A battery of cadaveric tests, physical “bench-top” tests, and finite element analyses (FEA) should be used in order to both quantitatively and qualitatively optimize a femoral press-fit design. In this study, a method is proposed to quantitatively rank candidate press-fit stem designs relative to successful predicates based on stem seating height and PPFx risk by recreating impact loading applied during surgery through a controlled “bench-top” model.

Methods: Three press fit candidate designs A, B & C and two clinically successful predicate proximal fit and fill stems (Secur-Fit™ Max (Fit & Fill 1) and Meridian^® TMZF^® (Fit & Fill 2), Stryker, Mahwah NJ) were evaluated. Five foam cortical shell Sawbones^® femur samples (Item# 1130, Pacific Research Laboratories, Inc., Vashon, WA) were prepared for each press-fit design. A stem impactor was attached to the stem and then the stem was hand inserted in the femur. Then the construct was mounted in the drop tower using a vice and initial drop height was set to generate approximately 5500 N of impaction force when fully seated. Each stem was serially impacted until stable then step loaded until PPFx occurred. The height above/below the medial resection plane was measured after each impaction.

Results: All press-fit designs had an initial stable seating height within the desired range without causing PPFx, using an average impaction load of 5341 N. All of the press-fit designs required, on average, roughly a 200% increase in impact load (10925 N) to cause PPFx. The press-fit deign which ranked first based on seating height accuracy, defined as the design closest to zero at stable, was Design C at −0.02 mm countersunk. Design A with a standard deviation of 0.09 mm ranked first for repeatability, defined as the design with the smallest standard deviation at stable. Finally the press-fit design which ranked first for lowest PPFx risk, defined as the design that is most countersunk prior to PPFx, was Fit & Fill 1 at 6.30 mm countersunk.

Discussion: This controlled “bench-top” impact loading model successfully showed that it can quantitatively evaluate stem seating height and PPFx risk for several different femoral press-fit designs. In order to determine the optimal design, each press-fit design was ranked with equal weight given to seating height and fracture risk. Using this test method one design alternative, press-fit Design C, ranked first as the optimal combination of seating height accuracy and consistency with a low risk of PPFx. A limitation of this impaction model is that it does not directly predict PPFx rate, it only quantifies risk of fracture. Another limitation is that this model does not simulate all of the variably that is inherent to actual patient bone types. This test is one step in a battery of tests, including cadaveric evaluation and FEA, which should be used in order to optimize a femoral press-fit design.

Introduction: Clinical experience has shown that addressing variations in bone morphology is important in the development of successful hip implant designs. Numerous studies of femoral bone morphology have been published utilizing various techniques. This study has developed a method which consistently measures large quantities of 3-dimensional digital femura geometry segmented from computed tomography (CT) scans and can accurately make anatomical measurements from these images

Methods: CT images of left femora on five hundred fifty six left femura (57% male, 43% female), consisting of 69% Caucasian, 16% Asian and 14% unknown were analyzed. The average age was 66 years, ranging from 40 to 93 years. Segmentation of the outer cortical, inner cortical, and marrow boundaries were consistently performed over all CT scans. The positions identified on the reference bone are transformed to the equivalent position on the clinical bone images, from which the dimensional data is extracted and stored. The mediolateral width (MLW), medial offset (MO) and lateral offset (LO) were measured in 10mm increments, ranging from 20mm above the lesser trochanter (LT) to 130mm below the lesser trochanter. The canal flare index was defined as a ratio of the mediolateral width at a section 20mm above the lesser trochanter to the mediolateral width at the isthmus level.

Results: The mean mediolateral width at 20mm above the lesser trochanter was 47.0 ± 4.5 (35.1–61.8; n=556). Noble reported 45.4 ± 5.3 (31.0–60.0; n=200), Husmann reported in a neck oriented study 46.3 ± 6.9 (27.6–63.6; n=310) and Laine reported 47.1 ± 4.9 (n=50). The mean medial offset at a section 20mm above the lesser trochanter was 25.1 ± 2.9 (16.7–33.4). In the study by Husmann, a mean of 25.0 ± 5.2 (9.4–45.5) was reported. The mean canal flare index was 4.49 ±.8. Noble reported a mean canal flare index of 3.80 ±.074, Husmann 3.81 ±.83 and Laine 4.3 ±.93.

Discussion: In general, the study showed minor differences to published data of proximal bone morphology. However, this more powerful study has shown that there is a higher mean canal flare index than determined by Noble and a similar mean canal flare index as determined by Laine. As reported by Laine, the canal flare index varies significantly with the placement of measurements in the canal. In this study the measurements were performed in a plane oriented by the femoral neck as a hip stem would be placed. The CFI over the isthmus width showed a greater correlation than previously shown by Noble. The novel software tool allows for anatomical measurements that can be applied to an unlimited population size enabling further applications and studies.

High tensile stress has been considered as a contributing factor to the rim fracture of polyethylene acetabular cup liner. We performed the 3 D Finite Element Analysis (FEA) to compare the stress patterns at the polyethylene liner rim as a function of polyethylene thicknesses and whether or not rim was supported by the titanium acetabular shell extension. Two 3.1 mm thick generic 52 mm titanium alloy acetabular shells with and without 2 mm high rim support extension were modelled. Six corresponding Ultra High Molecular Weight Polyethylene (UHMWPE) liners with inner bearing diameters ranging from 22 mm to 44 mm and same outer diameters, were fixed in the shells. A 2 450 N load was applied through the corresponding CoCr femoral heads to the rims of liners while the acetabular shells were fixed on the outer spherical surface. The FEA was performed in half body of the assembly. The maximum principal stresses at the rim regions of UHMWPE liners were recorded.

The results showed that in all rim supported conditions, the maximum principal stress were in compressive patterns, a preferred pattern to reduce the potential polyethylene liner fracture. In rim unsupported conditions, the stresses was in tensile on the internal bearing surface when polyethylene liner thickness was bellow 5 mm, or was bellow 9 mm if the average maximum principal stress cross the rim was considered.

We conclude that the metal rim support changes the stress pattern in the rim region of UHMWPE liner to compressive for all liner thicknesses. The stress pattern turns to tensile, or there will be a higher potential for rim fracture, if UHMWPE liner is unsupported and the polyethylene rim thickness is less than 9 mm.

Although components used this study did not include the locking details which add higher stress concentrations, the trend of stress patterns should follow the results found in this study.

Introduction: Bone resorption at the bone-implant interface is still a problem, leading to pain, poor function and the possibility of bone fracture. This loss of supporting bone tissue is due to resorption and impaired bone formation. Loosening of an implant is often not clinically or radiographically apparent for 8–10 years. It would be beneficial if these potential failures could be identified early so that revision surgery can be avoided. The aim of this study was to investigate the influence of implant material property changes and its influence on the trabecular loading patterns of the underlying supporting bone structure.

Methods: An intact and reconstructed 3D finite element (FE) model of a human femur was developed. The model was generated using PATRAN and CT scans. This was used to determine the stress, strain and interface sliding of a knee implant at heel-strike and stair climbing phases of gait. FE analysis of the model was performed using ABAQUS software. The materials properties of the bone were extracted from the CT data and applied using FORTRAN subroutines. Implant-bone interfaces were simulated using cementless fixation concepts. Sliding contact conditions were applied to simulate the immediate post-operative period.

Results: Three material property cases were analysed, with respect to the intact bone, at 100%, 25% and 2.5% of cobalt chrome’s (CoCr) Youngs modulus. At heel-strike, for the 100% case, higher stress was found at anterior flange while lower stress dominated around the pegs and intercondylar notch. For the 25% case, lower stresses were found in the intercondylar notch and higher stresses above the pegs. For the 2.5% case, stresses resembled that of intact bone, higher stresses were found above the pegs and lower stress in the intercondylar notch. In stair-climbing, for the 100% case, lower stresses were found around the pegs and in the intercondylar notch. For the 25% case, lower stresses were found in the intercondylar notch and higher stresses in areas above the pegs. For the 2.5% case, higher stresses were found at the distal condyles and lower stresses were observed in the intercondylar notch.

Discussion: The analysis presented changes in the trabecular loading and subsequently resulted in stress shielding. The general trend showed that the majority of stress shielding is occurring at the posterior flange and medial condyle while increased trabecular loading occurred at the anterior flange and lateral condyle regions. As the stiffness of the implant decreases from 100% to 25%, the differences in trabecular loading are extremely small. Both these implant material properties are very stiff in comparison to the underlying trabecular bone. However, as CoCr stiffness is decreased to 2.5% this yields a more homogenous stress distribution at the contact interfaces.

Introduction: Contact stresses, derived from navigation system and conventional TKR alignments, are compared to ideally aligned component stresses.

Methods: This study builds upon the work of previous studies, in which post-operative CT scans from 70 patients were utilized to extract knee component angular alignments from patients undergoing both navigation based and conventional TKR. Knee component (Stryker Orthopaedics Duracon^TM Condylar) FE models were oriented into specific alignment positions. Tibial insert contact stresses were computed under physiologically relevant loads at various flexion angles. FEA was also performed on ideally aligned cases for comparison purposes.

Results: At full extension, the median alignment of conventional TKR induces contact stresses 17.8% above ideal alignment conditions. Navigation based TKR alignment induces stresses 3.5% above ideal alignment conditions. At 45–90° flexion, conventional TKR alignment induces stresses 2.7% above ideal alignment conditions, while comparable navigation based TKR alignment induces stresses that match ideal alignment conditions.

Conclusion: Navigation based TKR procedures improve knee component alignment, which decreases contact stresses in UHMWPE tibial inserts. The result is a reduction in abnormal wear patterns and expected wear rates, with an increase in the structural longevity of knee system components.

The primary objective of navigation systems is to optimise component alignment to improve total knee replacement (TKR) performance. This study utilizes finite element analysis techniques to determine how component alignment affects tibial insert contact stresses. Contact stresses were derived from navigation system and conventional TKR alignments, and were compared to ideally aligned components.

This study builds upon the work of a previous study, in which post-operative CT scans from 70 patients were utilized to extract knee component angular alignments. These patients had been randomised to having either navigation based or conventional TKR.

Knee component finite element models were oriented into specific alignment positions. Tibial insert contact stresses were computed under physiologically relevant loads at various flexion angles. Finite element analysis was also performed on ideally aligned cases for comparison purposes.

At full extension, the median alignment of conventional TKR induces contact stresses 17.8% above ideal alignment conditions. Navigation based TKR alignment induces stresses 3.5% above ideal alignment conditions. At 45–90° flexion, conventional TKR alignment induces stresses 2.7% above ideal alignment conditions, while comparable navigation based TKR alignment induces stresses that match ideal alignment conditions.

Knee component alignment is improved by navigation techniques. This predictive finite element analysis study shows markedly reduced contact stresses for navigation aligned TKR compared to conventional aligned technique. The reduction in tibial insert contact pressures could reduce abnormal polyethylene wear, increasing the structural longevity of knee system components.