Fatigue strength testing of hip stems with statistical analysis.

Component fatigue testing, the final step in the development of total joint replacements, is performed to validate the safety of these components against fatigue failure before clinical use. Fatigue test prediction can aid the design of an efficient fatigue-testing program. The objective of this study was to perform an efficient and accurate statistical analysis of component fatigue test results, for the validation of future fatigue test predictions. Testing was performed with two aims: first, to determine the local component stress-force relationship using strain gauges; and second, to provide a statistical description of the fatigue test results. Forty-nine hip stems, in three sizes, were tested in a series of static and fatigue tests. Through effective planning and analysis, a statistical description of the component fatigue test results was determined including, 3-parameter Weibull distributions of life at two stress levels and log-Normal distributions of fatigue strength at various lives up to 5 million cycles.

Bone hardness testing in the trabecular bone of the human patella.

Bone hardness of 10 human patellas in 15 regions and at three depths was measured through in situ indentation tests. Indentation tests were performed perpendicular to the three articular surfaces (lateral and medial facets, and central ridge area) and, thus, parallel to the trabeculae. Multiple regression analysis demonstrated a significant reduction of bone hardness with depth. In addition, the lateral facet had a higher bone hardness than the medial facet, and the proximal and central regions demonstrated higher bone hardness than the distal region. Overall, the proximolateral region of the patella exhibited the highest hardness compared with other regions. As bone hardness is indicative of load bearing, these results may have important consequences for patellar resurfacing and patellar component design.

Femoral articular shape and geometry. A three-dimensional computerized analysis of the knee.

An average, three-dimensional anatomic shape and geometry of the distal femur were generated from x-ray computed tomography data of five fresh asymptomatic cadaver knees using AutoCAD (AutoDesk, Sausalito, CA), a computer-aided design and drafting software. Each femur model was graphically repositioned to a standardized orientation using a series of alignment templates and scaled to a nominal size of 85 mm in mediolateral and 73 mm in anteroposterior dimensions. An average generic shape of the distal femur was synthesized by combining these pseudosolid models and reslicing the composite structure at different elevations using clipping and smoothing techniques in interactive computer graphics. The resulting distal femoral geometry was imported into a computer-aided manufacturing system, and anatomic prototypes of the distal femur were produced. Quantitative geometric analyses of the generic femur in the coronal and transverse planes revealed definite condylar camber (3 degrees-6 degrees) and toe-in (8 degrees-10 degrees) with an oblique patellofemoral groove (15 degrees) with respect to the mechanical axis of the femur. In the sagittal plane, each condyle could be approximated by three concatenated circular arcs (anterior, distal, and posterior) with slope continuity and a single arc for the patellofemoral groove. The results of this study may have important implications in future femoral prosthesis design and clinical applications.

Improved fit by asymmetric tibial prosthesis for total knee arthroplasty.

The adequacy of the fit of the prosthesis to the tibia is an important variable in the outcome of total knee arthroplasty. A good fit covers the cancellous bone of both condyles, involves the denser bone of the tibial shell, but does not overhang beyond the bone at any point. In this research, an anatomically shaped tibial prosthetic base plate was developed from tibial measurements. A series of five template sizes, chosen to fit our patient database, was then evaluated for fit in the operating room by one surgeon. The assessment included bone exposure and the overhang of the prosthesis beyond the bone perimeter. The measurements of overhang or bone exposure were analysed. A commercial prosthesis was used as a control. An asymmetrical tibial plate conformed well to the anatomical shape of the resected upper tibia resulting in very little exposed bone and very few cases of overhang. The absence of such overhang provides better function in the surrounding stabilizing soft tissues, and so permits recovery of the maximum range of motion post-operatively.

Bone strength and histomorphometry of the distal femur.

The ultimate bone strength of the distal femur was measured radially, by indentation testing, around the transepicondylar line in 3 mm depth steps up to 12 mm below the subchondral bone plate. Specimens from 10 cadavers were used. This orientation of specimens was chosen as a way to provide measurement in a more physiologic orientation for load bearing and to standardize the assessment. Bone hardness declined sharply over the first 6 mm below the surface, tending to plateau at deeper levels. Within the top 6 mm layer the lateral condyle was softer than both the medial condyle and the central patellofemoral area (P < .05), but at deeper levels it maintained greater hardness. Of the histomorphometric parameters, those showing the greatest consistent correlation with hardness were bone volume fraction and trabecular separation. When the tibiofemoral and patellofemoral compartments were compared it was found that for a given value of bone volume fraction, condylar bone is marginally harder than patellofemoral bone. The data are relevant to the design of implants that match their geometric and material properties to the shape and strength of the underlying bone.

Relative motion at the bone-prosthesis interface.

Bone ingrowth in porous surfaces of human joint implants is a desired condition for long-term fixation in patients who are physically active (such as in sport or work). It is generally recognized that little actual bone ingrowth occurs. The best clinical results report between 10 and 20% of the total prosthetic surface in contact with bone will feature good bone ingrowth. One inhibiting factor is the relative motion of the bone with respect to the implant during load-bearing. This study investigated mathematically the interface micromotion (transverse reversible relative motion) between a flat metal tibial prosthetic surface of a prototype implant, and the bone at the resection site. The aim was to assess the effect of perimeter fixation versus midcondylar pin fixation and the effect of plate thickness and plate stiffness. Results showed that in the prototype design the largest reversible relative bone motion occurred at the tibial eminence. By design, the skirt fixation at the perimeter would prevent bone motion. A PCA (Howmedica Inc.) prosthesis has been widely used clinically and was chosen for a control because its fixation by two pegs beneath the condyles is a common variation on the general design of a relatively thick and stiff metal tibial support tray with pegs in each condylar area. The PCA tibial prosthesis showed the largest bone motion at the perimeter along the midcondylar mediolateral line, while being zero at the pegs. Maximum relative bone motion for the prototype was 37 µm and for the control was 101 µm. Averaged values showed the prototype to have 38% of the relative reversible bone motion of the control (PCA).

The physiological basis for a flexible condylar tibial plateau design.

Knee resurfacing is a successful treatment for osteo- and rheumatoid arthritis in elderly patients. The application of this treatment to younger more active and obese persons has the potential to produce premature wear, loosening, and undesirable bone remodelling. A new generation of more physiologically compatible components is required for these situations. This paper discusses the design and analysis of a prototype tibial base plate aimed at physiological load transfer. Incorporated in the design are mechanisms to alleviate lift-off phenomena, bone stress concentrations, stress shielding, and micromotion at the bone-implant interface. The design requires viable cancellous bone stock, so that the bone may respond by remodelling to the dynamic loading during normal ambulatory activities.

Failure of a knee prosthesis accelerated by shedding of beads from the porous metal surface.

A 61-year-old man who received a porous-metal-coated knee implant returned eight months later with chronic synovitis, instability, and loosening of his artificial joint. Subsequently, metal beads were detected in the joint space and soft tissues and were also embedded in the articulating surface of the tibial component. There was scoring of the tibial surface, and polyethylene wear particles were noted in the synovial and fibroconnective-tissue membranes, which had formed beneath the tibial component. Many particles were seen inside giant cells and macrophages. Failure in this case was probably accelerated by the granulomatous response in the soft tissue to wear particles. There was osteolysis rather than new bone growth at the interface with the tibial component. Analysis indicated that poor bead-bonding strength may have initiated the problem. Careful appraisal of the outcome from use of beaded porous-metal-coated devices and assurance of their adequate bonding strength are essential for further progress.

Distribution of bone strength in the proximal tibia.

Indentation tests were used to determine the ultimate strength of the proximal tibia. Measurements were made at the subchondral bone surface and on transverse planes up to 25 mm below the surface. Medial condyles were stronger than lateral condyles, and in both cases bone strength decreased abruptly with distance from the surface, especially over the first 5 mm. The mean bone strength was greater in men than in women in both condyles, especially in the harder upper layers. The areas of greatest strength on both the medial and lateral sides varied with depth. At the surface, maximum strength in the medial region was more posterior in men than in women. With increasing depth, the area shifted medially in the medial region and laterally in the lateral region. Spatial distribution of strength across planes of the tibia seems consistent with anticipated patterns of load distribution in weight bearing and with the contiguity of the trabecular bone. The data are relevant to an understanding of normal joint mechanics and to the design and placement of prostheses in total knee arthroplasty.

Geometry of the humeroulnar joint.

Clinical results with elbow prostheses have been disappointing. A detailed knowledge of elbow joint geometry and mechanics is necessary to improve prosthetic design. In this study, the humeroulnar articulation of four human cadaver elbows was examined using surface analytic methods. In this article, the location of the transverse axis of elbow flexion-extension is suggested in relation to well-defined landmarks, the medial and lateral epicondyles, and subsequently to the line connecting their most lateral points--the transepicondylar line. The geometry of the structures responsible for the carrying angle is discussed, as is the extent of cartilage-covered bearing areas of the lower humerus and upper ulna. Implications pertaining to prosthetic design and surgical technique resulting from this study are discussed.

Dynamic testing of below-knee prosthesis: assembly and components.

Prosthetic assemblies for lower limb amputees are highly engineered and consist of several components each with its own failure mechanism. This paper describes the dynamic testing of HDPE rotational moulded sockets in a specially designed machine which mimics normal gait. Thus the components are subjected to all main loadings occurring during a stride such as axial loading and A-P bending about the knee and ankle. Machine details as well as the other components of the system are described. SACH feet appear to be vulnerable by rapid wear and structural component failures at less than 100,000 cycles were observed. The sockets are much less vulnerable and stand up to simulated loading of 1350 N for approximately 400,000 cycles. Metal components such as the foot bolt may also fail in fatigue if not properly tightened. References to proposed ISO standards are also included.

Orthopedic sawblades. A case study.

Surgical stainless steel sawblades are frequently used with metal templates at implant surgery, hence there is a high potential for blade damage and suboptimal bone cuts. The authors appraised currently used operating room sawblades with a comparison of their cutting efficacy. There were numerous instances of damage to the cutting teeth surfaces. At a controlled cutting speed, such blades require significantly greater force than unused blades to cut cancellous and cortical samples of bovine bone. Microscopic assessment of cancellous bone specimens cut with damaged blades revealed torn, irregular, debris-filled surfaces. Blades should be inspected for damage after each use and discarded if their condition is in doubt.

A three-dimensional finite element analysis of the upper tibia.

A three-dimensional finite element model of the proximal tibia has been developed to provide a base line for further modeling of prosthetic resurfaced tibiae. The geometry for the model was developed by digitizing coronal and transverse sections made with the milling machine, from one fresh tibia of average size. The load is equally distributed between the medial and lateral compartments over contact areas that were reported in the literature. An indentation test has been used to measure the stiffness and the ultimate strength of cancellous bone in four cadaver tibiae. These values provided the statistical basis for characterising the inhomogeneous distribution of the cancellous bone properties in the proximal tibia. All materials in the model were assumed to be linearly elastic and isotropic. Mechanical properties for the cortical bone and cartilage have been taken from the literature. Results have been compared with strain gage tests and with a two-dimensional axisymmetric finite element model both from the literature. Qualitative comparison between trabecular alignment, and the direction of the principal compressive stresses in the cancellous bone, showed a good relationship. Maximum stresses in the cancellous bone and cortical bone, under a load which occurs near stance phase during normal gait, show safety factors of approximately eight and twelve, respectively. The load sharing between the cancellous bone and the cortical bone has been plotted for the first 40 mm distally from the tibial eminence.

Effect of muscular activity on valgus/varus laxity and stiffness of the knee.

Quantitative changes in valgus/varus knee stability with different levels of muscular activity were determined for five subjects. A specially designed machine was used to measure resistance to angulation in the frontal plane. This device held the thigh stationary, the knee straight, an cycled the leg from side to side at a constant rate between present moment limits. Resistance to this forced valgus/varus motion was measured simultaneously with torque about the knee in the sagittal plane. Muscle activity was monitored by electromyography (EMG). Direct comparison of moment-rotation characteristics allowed changes in stability to be quantified as a function of extension and flexion torque. Extension torques less than 20% of the maximum increased varus stability more than valgus stability. Flexion torques of the same relative magnitude increased valgus stability more than varus stability. Comparison with the literature suggested that prevention of opening of the lateral side of the joint under varus loading was responsible for increased varus stability with increasing torque, both with extension and flexion torques.

Resurfacing elbow prosthesis: shape and sizing of the humeral component.

The sizing and dimensioning of a new unconstrained elbow prosthesis makes use of a geometric axis for humeral articulating surface definition, an axis which is precisely positioned with respect to extra-articular anatomical landmarks. The geometry of the joint was determined by a slicing and digitization technique. It was found that for the humerus an axisymmetric surface is evident and that there exists a centroidal axis which is mostly linear except for a portion at the posterior lateral flange of the trochlea. Sizing studies were carried out on dry bone anatomical specimens and, using a standardized X-ray technique, on the elbows of volunteer subjects. Seven dimensions were chosen for statistical analysis of the joint. A multi-variate normal distribution model, using only the first principal component was found to account for 70% of the variance; components 2 and 3 explained a further 19%. From this analysis, one series of three and five sizes of prosthetic dimensions were established for 95% overall coverage of population.

One parameter model for error in instantaneous centre of rotation measurements.

Many statistical distributions have a single parameter which describes its spread. In the kinematic study of the instantaneous centre of rotation, error in data cause a non-symmetrical distribution for the centre. As an aid in describing its spread, the analytical form of the probability density function was reduced to a single-parameter model. This results in a convenient way of analysing experimental data through the use of an 'ICR Error Chart'.

Wheelchair and occupant restraint system for transportation of handicapped passengers.

The objective of research into restraining wheelchair-bound passengers of buses and vans, was to design an effective system that can be built with standard components at reasonable costs without interfering unduly with passengers and space requirements. The author consulted with rehabilitation medicine and transportation specialists and found that virtually all current systems are ineffective in either restraining wheelchair, occupant, or both under conditions of impacts at 50km/h and less. Many systems may appear technically sound, but contain weak links, thus rendering the whole system useless or making it dangerous. A system was designed that utilizes off-the-shelf components and especially designed channels. The assembly has been tested under static conditions in the laboratory by applying a pull of 6000 newtons (N), according to static test procedures standardized by the Society of Automotive Engineers (SAE) for seatbelts. Furthermore, the system was dynamically tested in a crash test at 50km/h. The system is currently used in a private van, a public van offering 5 seating positions, and a school bus designed for 17 wheelchair positions.

A quantitative method of assessing malalignment and joint space loss of the human knee.

Malalignment and joint space loss in the arthritic human knee can be measured quantitatively by employing a frame that allows for parallax correction of radiographs taken from the weight bearing lower limb. This standardized method will assist pre-operative planning for osteotomies and post-operative follow-up of patients with surgical re-aligned lower limbs. The procedure requires anatomically important points to be digitized, together with reference points built into the frame. Data are then processed automatically in a desk top computer, and the program provides for an easily understood diagram and listing of characteristic indices of malalignment.