H.W. Wevers

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.

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.

Universal bone cutting device for precision knee replacement arthroplasty and osteotomy

Since malplacement contributes most to loosening of total knee replacements (TKR), a jig was devised, aligned to and mounted on the tibia, with a 3 degrees of freedom sliding saw. A central distractor, attached to the jig, positions and aligns the knee at 0 degrees or 90 degrees. The femur is then rigidly linked to the jig for bone cutting. Resurfacing designs (Cloutier and Townley) have been regularly implanted, aligned +/- 1 degree. This precision should minimize loosening and improve function.

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.

Three-dimensional reconstruction of joint surfaces using a microcomputer.

A microcomputer was used to analyse the surface characteristics and geometry of articulating joints. Both hardware configuration and software organisation were described. Data used in this analysis were obtained by sequential resection of entire joints (elbows, metatarsophalangeal joints and knees) secured in an embedding medium. The exposed joint profile after each resection in a bone milling machine was recorded photographically. Each record of freshly cut profile was manually digitised and automatically processed with a desktop microcomputer. The complete structure of these articulating surfaces was reconstructed in three dimensions to be displayed in any desired orientation as a series of parallel, consecutive and uniformly spaced sections. These data have been used to derive information on cartilage thickness, underlying bone structure, orientation and anatomical shape of the joint surfaces. The stored surface geometry may be retrieved at any time for related studies of joint kinematics, joint sizing and prosthetic joint design.

Application of Bench-Mounted Saws for Precision Replacement Arthroplasty of the Arthritic Knee — The Questor Systems

Prosthetic replacement arthroplasty has revolutionised surgical treatment for arthritis of hip and knee joints, but as time passes, the number of failures, (mainly) due to loosening of the implant, increases. Although poor design and material features have a recognised importance in loosening, a malpositioned prosthesis must carry the greatest responsibility [1–4]. The precision involved in the manufacturing process of the implant is set to a level of tolerance ±0. 2 mm. This is, in all likelihood, a factor of over ten times greater precision than current bone cutting/placement techniques. The geometric accuracy employed in the construction of the implants sets a standard to be emulated in the orientation and accurate placement of the prosthesis; it recommends a comparably high level of placemealignment accuracy for the bone cuts.

Behaviour of an instron 1122 under cyclic testing over a small displacement range

Abstract When an INSTRON machine is used for cyclic testing over a small displacement range, load vs deformation curves may contain relatively large errors due to the dynamic characteristics of the machine.

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.