This study investigated a conservative femoral component for hip replacement surgery. Re-surfacing implants with two hydroxyapatite-coated pins were inserted into the right femoral necks of eight adult goats for one year. Radiographic analysis revealed bone loss directly beneath the prosthetic cup with increased amounts of bone at the distal end of the pins. Microscopic analysis revealed localised areas of bone formation adjacent to the implant. Oxytetracycline bone markers demonstrated significantly decreased bone formation rates (p<0.05) in bone adjacent to the implant in the operated compared to the control hips. Image analysis results demonstrated an average bone attachment of 30.94% to the implant surface. Attachment varied regionally, with greatest bone attachment at the end of the pins (78.99%) contributing 22% of total bone attachment to the implant. Least bone attachment (13.82%) was measured beneath the prosthetic cup and adjacent to the medial aspect of the pin. This contributed 4% of total bone attachment. Image analysis results demonstrated that in four of five comparable regions of operated and control hips, bone area was greater in control hips. Values were significant in three regions adjacent to the implant (p=0.002, p=0.008, p<0.05). Results revealed no significant correlation between bone attachment to the implant pins and bone area beneath the prosthetic cup. However, these findings do suggest that this conservative design led to stress shielding resulting in unfavourable bone remodelling despite the osteoconductive hydroxyapatite coating.

In order to determine the effect of improvements in design and materials on wear of total knee replacement, a valid test method is required. This test method should reproduce the wear patterns seen on retrieved knee replacements. We have tested various designs of implants using a force-control knee simulator and analysed the wear on these implants. The tests were conducted in 50 per cent bovine serum for up to ten million cycles. On condylar replacements, wear was associated with scratching and burnishing. Wear rates between different designs could be discriminated. Delamination wear occurred only in those samples which had been artificially aged. Tests showed that wear on a meniscal knee design equalled that, or was greater than that, observed on a condylar design. On meniscal designs wear was associated mainly with the meniscal bearing surface.

The kinematics of a mobile bearing knee, which allowed [plusmn]20[deg ] of rotation and 4.5 mm of anteroposterior translation, was measured for ascending and descending a step, deep-knee bend, normal walking, and twisting. A fluoroscopic technique was used, analyzed by 2 different methods. The rotations and displacements during the activities were similar to those of moderate-to-high constrained fixed bearing knees. The motion patterns were variable among test subjects and in general did not reproduce normal knee motion. Because of the freedom of anteroposterior translation and rotation in the design, however, each knee could determine its own neutral position and its own axis of internal-external rotation, depending on the activity.

Validation of a wear simulator requires that the device produce a similar type and amount of wear and particles of a comparable morphology as occurs clinically. Using techniques previously established to compare polyethylene particles from hip simulators to those from retrieved tissues, particles isolated from six revised posterior stabilized knee replacements were characterized and compared to particles generated from the same knee design worn in a knee simulator. The particles produced in the knee simulator were of comparable size but had less variability in their form factor compared to the particles produced in vivo. Comparable wear features were seen on the articulating surfaces in both groups. These results indicate that this knee joint simulator is able to reproduce a baseline type of wear that is similar to that in vivo and should encourage further use of this device to better understand knee component wear and function.

The performance of Constrained Condylar and Rotating Hinge types of total knee replacement was compared by mechanical testing, by using the Knee Society Clinical and Radiographic evaluations, and by a self-assessment questionnaire. In mechanical tests to evaluate varus strength, Constrained Condylars developed at least 6 degrees of varus rotation, but there was minimal varus rotation in the Rotating Hinges. This was reflected in the stability scores in the Knee Society evaluation. The shorter stems used in the Constrained Condylars resulted in a higher variation in femoral-tibial angles, and a smaller than ideal valgus angle. Radiographically, the only radiolucent lines observed were those adjacent to the joint itself, and there was no difference between the two types of total knee replacement. From the questionnaire, there was a high correlation in the performances between the operated and non-operated knees in the Rotating Hinge group. This indicated the Hinges were capable of matching their performance to a required level, thereby producing a better overall clinical result. In summary, the study indicated that a Rotating Hinge type should be given greater consideration, particularly if the disadvantages of larger bone resection and longer stems could be overcome.

When the normal knee is flexed from 0 to maximum, the lateral femoral condyle displaces posteriorly, but the medial condyle remains almost in the same position. There is anteroposterior and rotational laxity about this neutral path. The hypothesis of this study was that this motion could be provided in a total knee arthroplasty by specially shaped bearing surfaces, in particular, where the lateral femoral condyle converged inward around the periphery from extension to full flexion. Tibial surfaces were generated with such femoral condyles moving through a neutral motion path. Displacing the femur in anteroposterior or rotation about the neutral position at a given angle of flexion intersected a volume of the tibial surface in both directions, showing that the femur was "at the bottom of the tibial dish." The volumes were comparable with those for conventional condylar replacement total knee arthroplasties. These data supported the general concept of converging femoral condyles, which may be present in the natural knee itself to guide motion.

Two distal femoral replacements were instrumented to measure axial force, torque and bending moments in the prosthesis shaft. Data are reported up to 2.5 years for the following activities: uni- and bi-lateral standing, walking, stair climbing and descending, treadmill walking, jogging and jumping. In the first subject the greatest averaged peak shaft forces found were: jogging 3.6 Bodyweight (BW), stair descending 3.1 BW, walking 2.8 BW, treadmill walking 2.75 BW, and stair ascending 2.8 BW. Bending moments about the antero-posterior axis (varus–valgus) and medio-lateral axis (flexion–extension) peaked in the range 8.5–9.8 and 4.7–7.6 BW cm respectively, over the follow-up period. Axial torques peaked in the range 0.2–1.3 BW cm, outwardly directed. At most follow-up sessions, forces and moments during jogging were generally greater than those for other gait activities. In the second subject forces and moments were generally only 45–70% of those in the first subject, due to inadequate musculature around the knee. The data can be applied to the design and testing of distal femoral replacements and even to total knee replacements, and contributes to the knowledge of forces acting in the distal femur during activity.

The ability to climb a steep step or rise from a low chair after total knee replacement may be enhanced if the required force in the quadriceps muscle is reduced. This can potentially be achieved if the total knee produces a large lever arm measured from the femoral-tibial contact point to the patellar ligament. A reduced quadriceps force would also reduce the patello-femoral force and the femoral-tibial contact force. The contact point location is likely to be a function of the geometry of the femoral and tibial components in the sagittal plane, including the relative distal and posterior radii of the femoral profile, the location of the bottom-of-the-dish of the tibial surface, the radius of the tibial surface, and the presence or absence of the posterior cruciate ligament. A three-dimensional model of the knee was developed including the quadriceps and various ligaments. In the study, the motion was confined to flexion extension and displacement in the sagittal plane. The quadriceps was assumed to be the only muscle acting. A standard software package (Pro/Mechanica) was used for the analysis. For a femoral component with a smaller distal radius, there was 12% reduction in the quadriceps muscle force and up to 11% reduction in the patello-femoral force from about 100 up to 60 degrees flexion. However, apart from that, there were less than 10% differences in all the forces as a function of all of the design variables studied. This was attributed to the relatively small changes in the lever arm of the patella tendon, since the tendon moves in an anterior-posterior direction along with the femur. An additional factor explaining the results was the change in the anterior-posterior contact point as controlled by the forces in the patella tendon and in the soft tissues. The results imply that for a standard condylar replacement knee, the muscle and contact forces are not greatly affected by the geometrical design variables.

It is generally assumed that the wear rates in knee replacements are reduced as the contact area is increased. Hence, fixed bearing or mobile bearing designs with large contact areas throughout the full range of flexion wear less than partially conforming fixed-bearing designs. This hypothesis was investigated in an experimental model, where flat-ended ultra high molecular weight polyethylene pins of varying diameters were reciprocated and rotated on polished metal plates under a constant load with serum lubrication. The pin diameters ranged from 8-23 mm, giving nominal contact pressures from 23.9-2.8 MPa, covering the range associated with a wide spectrum of total knees including mobile-bearings. For pin diameters of 8-12 mm, the mean wear rates were in the range of 5.0-16.0 E-10 g/cycle. For diameters of 17 and 23 mm, the mean wear rates were approximately 1.0 E-10 g/cycle. The latter wear rates were significantly less than the former. Scanning electron microscopy indicated milder wear processes with the larger diameters, while the smaller diameters exhibited transverse ripples and cracks and detachment of thin layers from the surface. The percentages of granules (mostly submicron), fibrils and flakes, and the sizes of these particle types were similar among all pin diameters, except that, for the 23 mm pin diameter, the percentage of fibrils increased and of flakes decreased. This work supports the hypothesis that larger contact areas, up to the maximum area tested in our study, produce lower wear rates, and suggests that there is no disadvantage regarding particle type or size associated with the larger areas of contact.

Total knee replacements using intercondylar cams, such as posterior stabilized types, have been in use for many years. In a previous study, software was written to analyze an alternative shape of the intercondylar cams. The goal of the current study was to investigate in a more general way the potential of intercondylar cams, or guide surfaces, for reproducing the anterior-posterior motion of the natural knee throughout the flexion range. Typical sagittal outlines for the femoral and tibial bearing surfaces were defined, and a parametrized shape for the femoral guide surface was defined to produce a wide range of shapes. Software was written in which the femoral component was flexed in increments, with the posterior translation defined as a function of the flexion angle. The shape of the tibial guide surface was derived from the locus of the femoral guide surface at its multiple flexion positions. By iterating methodically through possible shapes of femoral guide surfaces, several types of total knee replacement components in common use today were identified, as well as other configurations of potential interest. For quantification of a given design, the software calculated the anterior and posterior laxity at each flexion angle. Laxity was defined as the motion before the femoral guide surface impacted the tibial guide surface or until the contact point of the bearing surfaces reached a specified slope. Convex femoral and concave saddle-shaped tibial guide surfaces produced small laxities in both directions over most of the flexion range. A saddle design with small laxities in the first half of flexion, combined with a posterior stabilized feature, was an interesting combination. Potential improvements to the currently used designs were shown in this study, and new shapes of intercondylar guide surfaces were derived that could be considered for application.

This article begins to address the validation requirements of wear testing on total knee replacements in a knee simulator. The knee simulator has four stations. The axial force is variable but reaches a maximum of 2.3 kN. Physiologic anteroposterior shear force and rotational torques are supplied to the knee. The forces and displacements are timed to coincide with those of a typical gait cycle. Kinematics of the simulator are dependent on the type of knee being tested. Tests of designs with well known clinical histories were done to 10 million cycles. The relative amounts and types of wear shown by the designs were similar to that found in their clinical histories. Wear tracks on more conforming designs were larger, and the penetration into the plastic appeared to be less. This did not necessarily mean that wear, as measured by loss of material, was reduced on conforming designs. Delamination of the plastic was achieved only after aging the tibial components. Wear particles isolated from the lubricating fluid were similar in size and shape to those isolated from in vivo specimens. However, the relative amounts of wear particle shapes were different depending on the design. At the start of the tests, all of the flexibly mounted tibial components showed more motion than after 5 million cycles, indicating that the surface of the plastic became more conforming. This study showed that knee wear similar to wear observed in vivo can be reproduced in the laboratory. The parameters and methods elucidated in this introductory study should form the basis for use in preclinical wear tests of total knee replacements.

There are possible advantages of using uncemented fixation in total knee replacement. In this prospective randomised multi-centre study, a comparison was made between cemented and two types of uncemented fixation for the Kinemax design. There were 12-14 cases in each group. Beads were inserted in the bones from which component migration was measured at time intervals up to 2 years. The axial migrations were significantly less for cemented and HA-coating, compared with press-fit, at all time intervals. The clinical data showed no differences at 2 years except for more cases of pain in the uncemented groups. Radiographically, the cemented interfaces showed the least change, press-fit showed a radiolucent line and a radiodense line, and HA showed a diffuse radiodensity adjacent to the components. It was concluded that for the Kinemax design of tibial component, press-fit was inferior to cemented, but that there was the potential for designing a special component for uncemented fixation for which HA-coating would be an advantage.

Although many designs of total hip replacement have high success rates at followup times of 10 years and more, new designs continue to be introduced. Some of these designs are similar to what is available already, but may offer advantages such as additional sizes or improved materials. In such cases, a key question is whether the new implant will produce the best possible results for routine use. Rather than simply relying on long-term followup data, which would eliminate the widespread use of a new device for a long time, it is suggested that the vast experience and knowledge of the orthopaedic community should be used. The proposal is that an authoritative group of individuals formulate a specification of the design features of total hip replacement which produce successful results. A second key question is how to be reasonably certain that a device with significantly new features, materials or techniques, is better and whether and when it should be used, either routinely or for specific indications. The proposal is that federal funding should be available to the research community for the specification and development of extensively researched and validated test methods. A point which is applicable to all innovations is that rather than relying on only one test or on a limited number of tests to validate the device, the Design Method must be used. This method involves the formulation of a methodical series of tests, preclinical and clinical, covering all combinations of use of the device. There are no simple answers to validating a design, but there are positive signs that new innovations are being introduced more carefully than in the past, and that many of these innovations will lead to improved long-term results.

To combat the high incidence of aseptic loosening for young patients and for patients with failed implants after resection for bone tumors, intramedullary cementless fixation of massive tumor implants was investigated. These implants consist of a hydroxyapatite coated titanium stem. To date, 47 of these prostheses have been inserted for the treatment of primary bone tumors. Radiographs indicate that the stems are osseointegrated. Radiolucent lines have not been seen between the implant and the bone. Bone remodeling changes have been observed. In several cases in which the implant was not seated properly on the transaction site, bone grew to the shoulder of the implant. Bone remodeling was particularly evident in stems that were coated over their entire surface. In these cases, the implant induced local bone resorption so that the bone around the midstem region became thinner, with resorption of cortical bone on the periosteal surface and maintenance of bone on the endosteal surface adjacent to the stem. This effect was attributed to stress shielding, and a three-dimensional finite element model using loading data obtained from a telemetry study indicated that, where the stem was bonded to the bone over the entire surface, stresses in the outer cortex became reduced. In the finite element model, reducing the region of hydroxyapatite coating to approximately 1/3 of the stem length reduced the extent of the low-stress area in the outer cortex. Subsequently, prostheses have been coated with hydroxyapatite over only approximately 1/3 of their stem. This method of fixing the massive endoprosthesis to the bone is thought to be successful in the short-term and offers an alternative to cemented fixation.

The starting point of this article is a general design criterion applicable to all types of total knee replacement. This criterion is then expanded upon to provide more specifics of the required kinematics, and the forces which the total knee must sustain. A characteristic which differentiates total knees is the amount of constraint which is required, and whether the constraint is translational or rotational. The different forms of total knee replacement are described in terms of these constraints, starting with the least constrained unicompartments to the almost fully constrained fixed and rotating hinges. Much attention is given to the range of designs in between these two extreme types, because they constitute by far the largest in usage. This category includes condylar replacements where the cruciate ligaments are preserved or resected, posterior cruciate substituting designs and mobile bearing knees. A new term, ‘guided motion knees’, is applied to the growing number of designs which control the kinematics by the use of intercondylar cams or specially shaped and even additional bearing surfaces. The final section deals with the selection of an appropriate design of total knee for specific indications based on the design characteristics.

A computational model has been developed using a current generation computer-aided engineering (CAE) package to predict total knee replacement (TKR) kinematic in the sagittal plane. The model includes friction and soft tissue restraint varying according to the flexion angle. The model was validated by comparing the outcomes of anterior-posterior (A-P) laxity tests of two contemporary knee replacements against data obtained from a knee simulating machine. It was also validated against predictions from a computer model reported in the literature. Results show good agreement in terms of A-P displacements. Further tests were performed to determined the influence of the soft tissue restraints varying with flexion angle. This work represents the first attempt to use a sophisticated commercial CAE package to predict TKR motions and the advantages of the modelling procedure chosen are discussed.

This study investigated the concept of using plates to attach endoprostheses to bone after segmental resection for bone tumours in an animal model. Titanium alloy plates integrated with the prosthesis and coated with hydroxyapatite were attached to bone by screws. This type of uncemented fixation relied on the induction of periosteal bone formation into and around the plates to secure the implant to bone. Two, three, and six-slotted plate designs were investigated. On retrieval, each plate was securely fixed by new bone. Bone apposition on the external surface of the plates occurred through a combination of periosteal bone production, invasion of bone through slots in the plate, and bone growth over the ends of the plates. Most plates became incorporated into a remodelled cortex. Higher bone turnover rates (microm day(-1)) were seen in bone in the slots of the plate compared with normal cortical bone turnover (p < 0.05). Significantly higher rates of turnover were measured beneath slotted parts of the plates compared with regions below the unslotted parts (p < 0.05). The cross-sectional area of bone surrounding the six-plate implant design was significantly higher than that of the three-plate (p < 0.05) and two-plate (p < 0.05) designs. In addition, significantly more bone formed adjacent to the six-plated implant design compared with that in the contralateral limb (p = 0.002). However, no significant difference was found when the total cortical area around the three-plated design was compared with that of the contralateral limb (p = 0.63). In contrast, significantly less bone was measured adjacent to the two-plate design than in the untreated limb (p = 0.001). Image analysis also demonstrated increased cortical porosity adjacent to the six-plate design compared with the three-plate (p = 0.004) and two-plate (p < 0.05) designs. Finite element analysis demonstrated that the six and three-plate designs increased the second moment of area compared with that in the left tibia (p = 0.003 and 0.066, respectively). However, the attachment of the more flexible two-plate design did not significantly increase the second moment of area compared with that in the contralateral limb (p = 0.235). It was concluded that due to both mechanical and biological effects, the hydroxyapatite-coated plate designs generated new bone that enhanced fixation and encouraged plate integration into the load-bearing structure of the cortex. This method of fixation may be an alternative to the use of intramedullary cemented stems in patients requiring bone tumour implants and may be the only way to preserve the joint in difficult cases where only short segments of bone remain.

The experimental evaluation of any total knee replacement (TKR) design should include the pre-implantation quantification of its mechanical performance during tests that simulate the common activities of daily living. To date, few dynamic TKR simulation studies have been conducted before implantation. Once in vivo, the accurate and reproducible assessment of TKR design mechanics is exceedingly difficult, with the secondary variables of the patient and the surgical technique hindering research. The current study utilizes a 6-degree-of-freedom force-controlled knee simulator to quantify the effect of TKR design alone on TKR mechanics during a simulated walking cycle. Results show that all eight TKR designs tested elicited statistically different measures of tibial/femoral kinematics, simulated soft tissue loading, and implant geometric restraint loading during an identical simulated gait cycle, and that these differences were a direct result of TKR design alone. Maximum ranges of tibial kinematics over the eight designs tested were from 0.8 mm anterior to 6.4 mm posterior tibial displacement, and 14.1° internal to 6.0° external tibial rotation during the walking cycle. Soft tissue and implant reaction forces ranged from 106 and 222 N anteriorly to 19 and 127 N posteriorly, and from 1.6 and 1.8 Nm internally to 3.5 and 5.9 Nm externally, respectively. These measures provide valuable experimental insight into the effect of TKR design alone on simulated in vivo TKR kinematics, bone interface loading and soft tissue loading. Future studies utilizing this methodology should investigate the effect of experimentally controlled variations in surgical and patient factors on TKR performance during simulated dynamic activity.

Bearing surfaces of total condylar knees which are designed with a high degree of conformity to produce low stresses in the polyethylene tibial insert may be overconstrained. This study determines femoral and tibial bearing surface geometries which will induce the least destructive fatigue mechanisms in the polyethylene whilst conserving the laxity of the natural knee. Sixteen knee designs were generated by varying four parameters systematically to cover the range of contemporary knee designs. The parameters were the femoral frontal radius (30 or 70mm), the difference between the femoral and tibial frontal radii (2 or 10mm), the tibial sagittal radius (56 or 80mm) and the posterior–distal transition angle (−8 or −20°), which is the angle at which the small posterior arc of the sagittal profile transfers to the larger distal arc. Rigid body analyses determined the anterior–posterior and rotational motions as well as the contact points during the stance phase of gait for the different designs. In addition, a damage function which accumulated the fluctuating maximum shear stresses was used to predict the susceptibility to delamination wear of the polyethylene (damage score). This study predicted that of the 16 designs, the knee with a frontal radius of 70mm, a difference in femoral and tibial frontal radii of 2mm, a tibial sagittal radius of 80mm and a posterior distal transition angle of −20° would satisfy the conflicting needs of both resistance to delamination wear and natural kinematics.