Tibiofemoral contact stress after total knee arthroplasty: comparison of fixed and mobile-bearing inlay designs

We measured tibiofemoral contact stresses and the load-bearing contact area of fixed and mobile-bearing inlay knee prostheses under dynamic loading conditions. An electronic resistive pressure-measuring sensor was used to detect contact stresses and contact area in five cadaver knees. Stresses were measured with the tibial component aligned normally, as well as in internally- and externally-rotated positions. The average peak contact stresses measured on the fixed inlay were greater (medial 21 MPa and lateral 21 MPa) than those on the mobile inlay (medial/lateral 7.7/5.3 MPa, p = 0.04). Although the average peak contact stresses of the fixed standard inlay greatly exceeded the contact stresses of the other two inlay designs in each malrotated position tested, no statistically significant differences were seen. The data suggest that the ability of the inlay to translate on the tibial baseplate permits the inlay to align itself on the femoral component so that the contact surface area is maximized and contact stresses are reduced.     

Tibiofemoral contact stress after total knee arthroplasty

We measured tibiofemoral contact stresses and the load-bearing contact area of fixed and mobile-bearing inlay knee prostheses under dynamic loading conditions. An electronic resistive pressure-measuring sensor was used to detect contact stresses and contact area in five cadaver knees. Stresses were measured with the tibial component aligned normally, as well as in internally- and externally-rotated positions. The average peak contact stresses measured on the fixed inlay were greater (medial 21 MPa and lateral 21 MPa) than those on the mobile inlay (medial/lateral 7.7/5.3 MPa, p = 0.04). Although the average peak contact stresses of the fixed standard inlay greatly exceeded the contact stresses of the other two inlay designs in each malrotated position tested, no statistically significant differences were seen. The data suggest that the ability of the inlay to translate on the tibial baseplate permits the inlay to align itself on the femoral component so that the contact surface area is maximized and contact stresses are reduced.     

Tibiofemoral contact stress after total knee arthroplasty: Comparison of fixed and mobile-bearing inlay designs

We measured tibiofemoral contact stresses and the load-bearing contact area of fixed and mobile-bearing inlay knee prostheses under dynamic loading conditions. An electronic resistive pressuremeasuring sensor was used to detect contact stresses and contact area in five cadaver knees. Stresses were measured with the tibial component aligned normally, as well as in internally- and externally-rotated positions. The average peak contact stresses measured on the fixed inlay were greater (medial 21 MPa and lateral 21 MPa) than those on the mobile inlay (medial/lateral 7.7/5.3 MPa, p = 0.04). Although the average peak contact stresses of the fixed standard inlay greatly exceeded the contact stresses of the other two inlay designs in each malrotated position tested, no statistically significant differences were seen. The data suggest that the ability of the inlay to translate on the tibial baseplate permits the inlay to align itself on the femoral component so that the contact surface area is maximized and contact stresses are reduced.     

Polyethylene contact stresses, articular congruity, and knee alignment.

Increased conformity at the tibiofemoral articulation increases contact area and reduces contact stresses in total knee arthroplasty. Malalignment, however, can increase polyethylene contact stresses. The effect of knee alignment and articular conformity on contact stresses was evaluated in a finite element model. The polyethylene insert and femoral component were modeled in high- and low-conformity conditions. An axial tibial load of 3000 N was applied across the tibiofemoral articulation at different knee positions ranging from 0 degrees, to 90 degrees, flexion, 0 to 10 mm anteroposterior translation, 0 degrees to 10 degrees axial rotation, and coronal plane angulation (liftoff). Increased conformity significantly reduced contact stresses in neutral alignment (by 44% at 0 degrees flexion and 36% at 60 degrees and 90 degrees flexion). Liftoff significantly increased contact stresses in low- and high-conformity conditions, but to a lesser degree in the high-conformity condition. Malalignment in rotation was most detrimental especially with the high-conformity insert design. Overall, increasing articular conformity reduced stresses when the knee was well-aligned. However, malalignment in axial rotation was detrimental. Mobile-bearing knee designs with increased articular congruity may result in lower contact stresses, especially the rotating-bearing designs that theoretically minimize rotational malalignment.     

Stress analysis of the anterior tibial post in posterior stabilized knee prostheses.

Recent retrieval studies have indicated a high incidence of polyethylene wear on the anterior tibial post caused by impingement. This study investigated the influences of post-cam design features and component alignment on the stress distribution in the anterior tibial post when subjected to the impingement loading. Two three-dimensional finite element models of posterior stabilized knee prostheses were constructed, one with flat on flat (FF) and another with curve on curve (CC) contact surfaces between anterior tibial post and femoral cam. The polyethylene insert was modeled with elastoplastic properties. Nine cases, three hyperextension angles (0 degrees , 5 degrees , and 10 degrees ) combined with three axial tibial rotations (0 degrees , 2.5 degrees , and 5 degrees ) simulating different component alignments were analyzed. A vertical compressive load of 2,000 N and an extension moment of 45 Nm were applied simultaneously. The FF model had larger stress increases than the CC model in both hyperextension and tibial rotation compared with the neutral position. The maximum increase for the FF model was 68% in peak contact stress, 125% in von Mises stress, and 58% in tensile stress in the extreme case of 10 degrees of hyperextension combined with 5 degrees of axial rotation. Stress concentration was found at the anterior corner of the post in the FF model; this was not found in the CC model. The curve on curve design can reduce edge loading on the tibial post, especially during axial tibiofemoral rotation.     

The effect of conformity, thickness, and material on stresses in ultra-high molecular weight components for total joint replacement

Debris resulting from damage to the surface of polyethylene components of total joint replacements has previously been shown to contribute to long-term problems such as loosening and infection. Surface damage has been associated with fatigue processes due to stresses arising from contact between the metal and polyethylene components in these prostheses. In the present study, we used elasticity and finite-element solutions to determine these stresses for total hip replacements with head diameters of twenty-two and twenty-eight millimeters and for a condylar total knee replacement. We also examined the effect on these stresses of using carbon-fiber-reinforced polyethylene instead of plain polyethylene. Stresses associated with surface damage in the tibial component of the total knee replacement were much larger than those in the hip replacements. The analysis of contact stress as a function of thickness of the polyethylene insert for tibial components showed that a thickness of more than eight to ten millimeters should be maintained when possible. The contact stress in the tibial components was reduced most when the articulating surfaces were more conforming in the medial-lateral direction. Contact stresses were much less sensitive to changes in geometry in the anterior-posterior direction. For the hip components, the stresses were lower in the acetabular component of the twenty-eight-millimeter hip replacement than in the twenty-two-millimeter replacement. The use of carbon-fiber-reinforced polyethylene resulted in stresses that were higher by as much as 40 per cent. Because the contact area between articulating surfaces moves during flexion, portions of the surface will be subjected to cyclic stresses. The contact area for the knee replacements in flexion was smaller than for the hip replacements, and the range of the maximum principal stress was larger. Consequently, the combination of the higher stress and the moving contact area is more likely to cause surface damage due to fatigue in tibial components than in acetabular components, which is consistent with clinical observations.     

Effect of knee component alignment on tibial load distribution with clinical correlation

To determine ideal alignment and component placement of total knee prostheses, Kinematic (K) and total condylar (TC) devices were physiologically loaded and interface forces were measured. Laboratory observations were correlated with clinical (roentgenographic) findings. Asymmetric loading of the tibial component has been proposed as causing loosening and radiolucent lines. Misalignment of components is one factor that affects load sharing by bone under the medial and lateral regions of the tibial plateau. Tibial components of K and TC prostheses were inserted without cement into the cut surfaces of artificial tibiae. The mating femoral condylar components were mounted. The tibial and femoral components were individually positioned at 0 degrees (horizontal) and at certain angles of varus and valgus. Pressure-sensitive film was placed between the tibial component and the artificial tibia. A vertical load of 1500 N was used. The experiment was replicated twice. The percentages of the load on the medial and lateral regions of the tibial plateau were calculated from quantitative image analysis of the pressure patterns on the film. Roentgenograms from 532 K and 21 TC patients were examined to determine the orientations of the condylar and tibial components and the presence of radiolucent lines around the tibial component. An even distribution (ideal alignment) of load on the medial and lateral regions of the K tibial component occurred at 9 degrees of valgus tilt of the femoral component and 2 degrees of varus tilt of the tibial component and for the TC at 7 degrees valgus and 0 degrees varus. Misalignment by 5 degrees yielded a 7% change in the load distribution under the K plateau and a 40% change for the TC prosthesis; a 10 degrees misalignment produced changes of 34% and 62% for the K and TC, respectively. Small variations in clinical knee alignment produced the same percentage of radiolucent lines for each alignment group. The location of radiolucent lines was distributed among the medial, lateral, and both tibial plateaus regardless of knee alignment, although there were more medial reactions overall. The smallest incidence (8%) of radiolucent lines occurred with the K prosthesis at 7 degrees of knee valgus, the femoral component placed at 9 degrees valgus, and the tibial component at 2 degrees varus. This correlated with the ideal bench-test findings for the K device.     

Effect of the angle of the femoral and tibial tunnels in the coronal plane and incremental excision of the posterior cruciate ligament on tension of an anterior cruciate ligament graft: an in vitro study

BACKGROUND: High tension in an anterior cruciate ligament graft adversely affects both the graft and the knee; however, it is unknown why high graft tension in flexion occurs in association with a posterior femoral tunnel. The purpose of the present study was to determine the effect of the angle of the femoral and tibial tunnels in the coronal plane and incremental excision of the posterior cruciate ligament on the tension of an anterior cruciate ligament graft during passive flexion. METHODS: Eight cadaveric knees were tested. The angle of the tibial tunnel was varied to 60 degrees, 70 degrees, and 80 degrees in the coronal plane with use of three interchangeable, low-friction bushings. The femoral tunnel, with a 1-mm-thick posterior wall, was drilled through the tibial tunnel bushing with use of the transtibial technique. After the graft had been tested in all three tibial bushings with one femoral tunnel, the femoral tunnel was filled with bone cement and the tunnel combinations were tested. Lastly, the graft was replaced in the 80 degrees femoral and tibial tunnels, and the tests were repeated with excision of the lateral edge of the posterior cruciate ligament in 2-mm increments. Graft tension, the flexion angle, and anteroposterior laxity were recorded in a six-degrees-of-freedom load-application system that passively moved the knee from 0 degrees to 120 degrees of flexion. RESULTS: The graft tension at 120 degrees of flexion was affected by the angle of the femoral tunnel and by incremental excision of the posterior cruciate ligament. The highest graft tension at 120 degrees of flexion was 169 +/- 9 N, which was detected with the graft in the 80 degrees femoral and 80 degrees tibial tunnels. The lowest graft tension at 120 degrees of flexion was 76 +/- 8 N, which was detected with the graft in the 60 degrees femoral and 60 degrees tibial tunnels. The graft tension of 76 N at 120 degrees of flexion with the graft in the 60 degrees femoral and 60 degrees tibial tunnels was closer to the tension in the intact anterior cruciate ligament. Excision of the lateral edge of the posterior cruciate ligament in 2 and 4-mm increments significantly lowered the graft tension at 120 degrees of flexion without changing the anteroposterior position of the tibia. CONCLUSIONS: Placing the femoral tunnel at 60 degrees in the coronal plane lowers graft tension in flexion. Our results suggest that high graft tension in flexion is caused by impingement of the graft against the posterior cruciate ligament, which results from placing the femoral tunnel medially at the apex of the notch in the coronal plane.     

The effect of surgeon experience on component positioning in 673 Press Fit Condylar posterior cruciate-sacrificing total knee arthroplasties

Component angles of 673 Press Fit Condylar (PFC) total knee arthroplasties were measured from standard short-leg radiographs. The femoral and tibial resections were performed with intramedullary and extramedullary instrumentation. The mean coronal tibial component angle was 88.59 degrees (SD, 2.28 degrees; range, 78-98 degrees ), with 17.1% having values <87 degrees and 1.9% having values >93 degrees. The mean coronal femoral component angle was 97.43 degrees (SD, 3.44 degrees; range, 84-115 degrees ), with 9.1% having values <94 degrees and 13.1% having values >100 degrees. An ideal tibiofemoral angle of 4 degrees to 10 degrees of valgus was achieved in 75.3% of patients, being <4 degrees in 18.6% and >10 degrees in 6.1%. Alignment was not significantly different between consultant and trainee surgeons. Although varus positioning of the tibial component was the commonest error, the wide range of femoral component angles signifies problems with standard intramedullary femoral guides.     

The shapes of the tibial and femoral articular surfaces in relation to tibiofemoral movement

We report a study of the shapes of the tibial and femoral articular surfaces in sagittal, frontal and coronal planes which was performed on cadaver knees using two techniques, MRI and computer interpolation of sections of the articular surfaces acquired by a three-dimensional digitiser. The findings using MRI, confirmed in a previous study by dissection, were the same as those using the digitiser. Thus both methods appear to be valid anatomical tools. The tibial and femoral articular surfaces can be divided into anterior segments, contacting from 0 degrees to 20 +/- 10 degrees of flexion, and posterior segments, contacting from 20 +/- 10 degrees to 120 degrees of flexion. The medial and lateral compartments are asymmetrical, particularly anteriorly. Posteromedially, the femur is spherical and is located in a conforming, but partly deficient, tibial socket. Posterolaterally, it is circular only in the sagittal section and the tibia is flat centrally, sloping downwards both anteriorly and posteriorly to receive the meniscal horns. Anteromedially, the femur is convex with a sagittal radius larger than that posteriorly, while the tibia is flat sloping upwards and forwards. Anterolaterally, both the femoral and tibial surfaces are largely deficient. These shapes suggest that medially the femur can rotate on the tibia through three axes intersecting in the middle of the femoral sphere, but that the sphere can only translate anteroposteriorly and even then to a limited extent. Laterally, the femur can freely translate anteroposteriorly, but can only rotate around a transverse axis for that part of the arc, i.e., near extension, during which it comes into contact with the tibia through its flattened distal/medial surface as against its spherical posterior surface.     

Extramedullary versus intramedullary tibial cutting guides in megaprosthetic total knee replacement

Background

In a standard total knee replacement, tibial component alignment is a key factor for the long term success of the surgery. The purpose of this study is to compare the accuracy of extramedullary and intramedullary tibial cutting guides used in indigenous and imported implants respectively, in positioning of the tibial components in megaprosthetic knee replacements.

Methods

A comparative study of the accuracy of extramedullary and intramedullary tibial cutting guides was carried out in 92 megaprosthetic knee replacements for distal femoral tumors. For the proximal tibia cut for tibial component placement, an extramedullary guide was used in 65 patients and an intramedullary guide was used in 27 patients. Tibial component alignment angles were measured in postoperative X-rays with the help of CAD software.

Results

There was more varus placement in coronal plane with extramedullary cutting guide (?1.18 +/? 2.4 degrees) than the intramedullary guide (?0.34 +/? 2.31 degrees) but this did not reach statistical significance. The goal of 90 +/? 2 degrees alignment of tibial component was achieved in 54% of patients in the extramedullary group versus 67% in the intramedullary group. In terms of sagittal plane alignment, extramedullary guide showed less accurate results (2.09 +/? 2.4 degrees) than intramedullary guide (0.50 +/? 3.80 degrees) for tibial component alignment, though 78% of patients were aligned within the goal of 0–5 degrees of tibial slope angle in extramedullary group versus 63% in intramedullary group. The mean error in the measurements due to rotation of the knee during taking the X-rays was less than 0.1 degrees and distribution of the X-rays with the rotation of knee was similar in both the groups.

Conclusions

Overall, in megaprosthetic knee replacement intramedullary guides gave more accurate results in sagittal plane and exhibited similar variability as of extramedullary guides in coronal plane.     

The accuracy of image-guided knee replacement based on computed tomography

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 degrees and 1.1 degrees, respectively. The CT-based system, provided almost perfect alignment of the femoral component with less than 1 degrees of error and excellent alignment with less than 3 degrees 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.     

Three-dimensional finite element analysis of unicompartmental knee arthroplasty--the influence of tibial component inclination.

Aseptic loosening and failure of a tibial component are recognized problems in unicompartmental knee arthroplasty (UKA). Excessive stress on the supporting cancellous bone is thought to contribute to the loosening and failure. Of factors that could influence supporting cancellous bone stresses, we focused on the inclination of a unicompartmental tibial component by analyzing the effect of coronal plane and sagittal plane inclination. Detailed geometrically accurate, three-dimensional finite element models were constructed from computed tomography (CT) data of a typical adult male proximal tibia. The material properties for the models were obtained directly from the CT data to simulate the inhomogeneous distribution of cancellous bone properties. Placing the component in slight valgus inclination in the coronal plane reduced the cancellous bone stresses. Posterior inclination in the sagittal plane caused a moderate increase in the stresses. Our results suggest that slight valgus inclination of a UKA tibial component may be preferable to varus or square inclination in the coronal plane. An excessive posterior slope of a tibial component should be avoided.     

Coronal bowing of the femur and tibia in Chinese: its incidence and effects on total knee arthroplasty planning

PURPOSES: To study the incidence of femoral or tibial bowing in the coronal plane in a Chinese population, and how it affects the accuracy of bone cuts for total knee replacement when an intramedullary alignment system is used. METHODS: Standing radiographs of the entire lower limb of each patient with end-stage primary osteoarthritis of the knee were analysed. All radiographs were digitised and the extent of bowing in the coronal plane measured. A bowing was marked if an angulation was more than 2 degrees. The projected error of cutting was then calculated. RESULTS: Of 93 lower limbs, 58 (62%) of the femurs had marked bowing in the coronal plane; 41 (44%) had a mean lateral bowing of 5.3 (standard deviation [SD], 3.2) degrees; 17 (18%) had a mean medial bowing of 4.4 (SD, 1.9) degrees. Marked tibial bowing in the coronal plane was less common (30 tibias, 32%). If a cutting error of more than 2 degrees was considered unacceptable, significantly more unacceptable cuts would ensue in the groups with marked bowing (p=0.003 for femurs and p<0.001 for tibia, respectively). CONCLUSION: The incidence of femoral or tibial bowing in the coronal plane was high in a Chinese population with end-stage osteoarthritis of the knee. This phenomenon may increase bone cut errors in total knee replacement if an intramedullary alignment system is used and the extent of bowing is not recognised.     

膝关节内侧固定平台单髁假体放置位置优化的有限元分析

目的探讨股骨组件及胫骨组件冠状面位置变化对股骨及胫骨生物力学的影响。方法取1名汉族男性志愿者的左侧膝关节CT及MRI图像,建立正常膝关节三维有限元模型(finite elemental model,FEM)。设计股骨组件及胫骨组件内翻6°、内翻3°、0°、外翻3°、外翻6°,组合成25个膝内侧单髁置换FEM。沿股骨机械轴加载1000 N载荷,观察von Mises云图应力分布,测量外侧间室载荷比例,测量胫骨组件下方松质骨及内侧皮质骨、聚乙烯衬垫上表面、外侧间室股骨软骨高接触应力值。将与中立位(胫骨及股骨假体内外翻0°、胫骨假体后倾5°)比较有统计学意义的指标通过散点图标识,找出点项目密集区和稀疏区,比较两区有统计学意义的项目数量,确定股骨组件、胫骨组件优化位置。结果股骨组件0°位放置时,胫骨从内翻6°至外翻6°各组合的胫骨组件下方松质骨高接触应力差异无统计学意义;胫骨组件0°位放置时,股骨组件内翻6°、外翻6°组件下方松质骨高接触应力值与中立位比较增加(9.21±3.38)MPa和(9.08±4.13)MPa(P<0.05)。股骨、胫骨组件从内翻6°至外翻6°变化时,胫骨下方内侧皮质骨高接触应力值逐渐下降(P<0.05)。股骨组件0°位放置时,胫骨组件从内翻6°至外翻6°各组合聚乙烯衬垫上表面高接触应力值的差异无统计学意义;胫骨组件0°位放置时,股骨组件内翻6°、外翻6°组与中立位组比较分别增加(2.88±2.53)MPa和(3.47±2.86)MPa(P<0.05);股骨及胫骨组件从内翻6°至外翻6°变化时,外侧间室载荷比例及外侧间室股骨软骨高应力值逐渐下降(P<0.05)。稀疏区(股骨或胫骨从内翻3°至外翻3°的所有组合的集合)有统计学意义的指标比例(2.8%,1/36)明显小于密集区(去除稀疏区以外的所有组合的集合)的比例(57.8%,37/64),差异有统计学意义(χ^2=29.61,P<0.001)。结论在下肢力线正常、关节线不变的条件下,膝关节内侧固定平台单髁假体放置位置为股骨组件、胫骨组件内翻、外翻角度不宜超过3°。     

Three-dimensional tibiofemoral articular contact kinematics of a cruciate-retaining total knee arthroplasty

BACKGROUND: Accurate knowledge of the location of tibiofemoral articular contact following total knee arthroplasty is important in order to understand polyethylene wear and the mechanisms of component failure. The present study was performed to determine the three-dimensional tibiofemoral articular contact patterns of a posterior cruciate ligament-retaining total knee replacement during in vivo weight-bearing flexion. METHODS: Nine osteoarthritic patients who were managed with a single design of a posterior cruciate ligament-retaining total knee implant were investigated with the use of an innovative dual orthogonal fluoroscopic imaging system. The position of the components during in vivo weight-bearing flexion was measured from full extension to maximum flexion in 15 degrees intervals. Tibiofemoral articular contact was determined by the overlap of the tibiofemoral articular surfaces. The centroid of the surface intersection was used to report the point of contact location. The average tibiofemoral contact points on both the medial and lateral tibial component surfaces were reported as a function of flexion. RESULTS: The average maximum weight-bearing flexion angle was 113.3 degrees +/- 13.1 degrees (range, 96 degrees to 138 degrees ). In the anteroposterior direction, the contact location was relatively constant in the medial compartment and moved posteriorly by 5.6 mm in the lateral compartment as the knee flexed from full extension to 90 degrees of flexion. The range of the contact location in the mediolateral direction was 3.7 mm in the medial compartment and 4.8 mm in the lateral compartment. For both compartments, posterior translation of the contact point was significant from 90 degrees to maximum flexion, but the contact point at maximum flexion was not observed to reach the posterior edge of the polyethylene tibial insert articular surface. CONCLUSIONS: While the minimum anteroposterior translation of the contact point on the medial side might be interpreted as a medial pivot rotation during knee flexion, the contact point did move in the mediolateral direction with flexion. Beyond 90 degrees , both medial and lateral contact points were shown to move posteriorly but stopped before reaching the posterior edge of the polyethylene tibial insert articular surface. It seemed that the current component design did not allow the femoral condyle to roll off the polyethylene edge at high degrees of flexion because of the geometry at the posterior lip.     

Inherent laxity in total knee prostheses

The anterior-posterior and rotatory laxities of 14 total knee prosthesis designs were measured in a loading rig with compressive, shear, and torque loads representative of physiologic loads. The measured laxities covered a wide range, both greater and smaller than that of the anatomic knee. This range was mainly due to the curvature or flatness of the plastic tibial surface and conformity with the femoral component. Pressure patterns showed the corresponding contact track and area on the tibial surfaces. It is proposed that for normal function, the laxity of the device should complement the remaining anatomic structures to produce a combined laxity resembling that of the normal knee. Excessive prosthetic laxity will lead to the risk of instability, soft tissue attenuation, edge-loading on components, and high contact stresses on the plastic. Inadequate prosthetic laxity may lead to altered kinematics and excessive stresses at the interface, running the risk of long-term loosening. The authors show the laxities of many currently used devices, providing important background information for assessing the role intrinsic prosthetic constraint might play in total joint performance in clinical analyses.     

The relationship between the survival of total knee arthroplasty and postoperative coronal,sagittal and rotational alignment of knee prosthesis

Purpose

Our study sought to address four issues: (1) the relationship between postoperative overall anatomical knee alignment and the survival of total knee prostheses; (2) the relationship between postoperative coronal alignment of the femoral and tibial component and implant survival; (3) the relationship between postoperative sagittal alignment of the femoral and tibial components and implant survival; and (4) the relationship between postoperative rotational alignment of the femoral and tibial component and implant survival.

Methods

We reviewed 1,696 consecutive patients (3,048 knees). Radiographic and computed tomographic examinations were performed to determine the alignment of the femoral and tibial components. The mean duration of follow-up was 15.8 years (range, 11–18 years).

Results

Thirty (1.0 %) of the 3,048 total knee arthroplasties failed for a reason other than infection and periprosthetic fracture. Risk factors for failure of the components were: overall anatomical knee alignment less than 3° valgus, coronal alignment of the femoral component less than 2.0° valgus, flexion of the femoral component greater than 3°, coronal alignment of the tibial component less than 90°, sagittal alignment of the tibial component less than 0° or greater than 7° slope, and external rotational alignment of the femoral and tibial components less than 2°

Conclusion

In order to improve the survival rate of the knee prosthesis, we believe that a surgeon should aim to place the total knee components in the position of: overall anatomical knee alignment at an angle of 3–7.5° valgus; femoral component alignment, 2–8.0° valgus; femoral sagittal alignment, 0–3°; tibial coronal alignment, 90°; tibial sagittal alignment, 0–7°; femoral rotational alignment, 2–5° external rotation; and tibial rotational alignment, 2–5° external rotation.     

Posterior tilting of the tibial component decreases femoral rollback in posterior-substituting knee replacement: A computer simulation study

Posterior tilting of the tibial component is thought to increase the range of motion in posterior cruciate-retaining total knee replacement, but its effect on implant motion in posterior cruciate-substituting total knee replacement is unknown. This issue has become of interest recently because manufacturers have introduced instrumentation that produces a posteriorly tilted tibial cut for both implant types. The purpose of this study was to investigate how motion of posterior cruciate-substituting total knee replacement is affected when the tibial component is installed with posterior tilt. Sagittal plane implant motions were predicted from prosthesis geometry with use of a computer simulation in which the femoral condyles were assumed to sit in the bottoms of the tibial condylar wells when the knee was in extension. Rollback of the femoral component was produced by a cam-spine mechanism at higher angles of flexion. The simulations revealed that even small degrees of posterior tilt reduced rollback by limiting the interaction between the cam and spine. Tilting the component posteriorly by 5° caused the cam to contact the spine at a knee flexion angle that was 18° higher than with the untilted component. The results suggest that posterior tilting of the tibial component in posterior cruciate-substituting knee replacement may not produce the same beneficial effects that have been reported for the tilting of tibial components in posterior cruciate-retaining knee replacement.     

Effects of total knee replacement design on femoral-tibial contact conditions

Ten fresh knee specimens with prosthetic components inserted were tested in a loading rig. Compressive and shear force were applied to the femur with the tibia held fixed. The location of the femoral-tibial contact points was measured. The contact reaction forces, the shear forces, and the rocking moments transmitted to the tibial component were calculated. The variations in the test conditions were: high and low compressive force, flexion angles of 0 degree, 45 degrees, and 90 degrees, three curvatures of tibial plastic inserts, and the posterior cruciate retained or resected. When the posterior cruciate was retained, the contact points were close to the center of the component; for cruciate resection, the contacts were close to the anterior of the component. The shear forces and rocking moments were higher for cruciate resection, but the contact reaction forces were lower. There is a wide variety of knee prosthesis designs, but the amount of inherent stability between the femoral and tibial surfaces, and whether the posterior cruciate ligament is retained or sacrificed, are two of the most important design variables. This study shows that cruciate resection increases the shear forces and the rocking moments to the tibial components and that additional fixation means may be necessary to compensate. On the other hand, cruciate retention with low conformity gives higher contact forces, which may lead to more wear in the long term. Cruciate sacrificing designs with intercondylar guiding surfaces are a separate category of design and were not considered in this study.