David Siu

A quantitative approach to radiography of the lower limb. Principles and applications

A method is described which provides standardised reproducible radiographic images of the lower limb. Anteroposterior and lateral radiographs are digitised and processed by computer to provide graphic/numeric displays of angles and linear measurements, relating the centre points of the hip, knee, and ankle. Two cases illustrate how surgical planning is facilitated when standardised data are available. These data confirm the close relationship between postoperative limb alignment and positioning of prosthetic elements.

Surgical implications of varus deformity of the knee with obliquity of joint surfaces

Some arthritic knees with varus deformity show excessive valgus angulation of the femoral joint surface with proximal tibia vara. This causes a downward and medial inclination of the articular surfaces in the coronal plane. The patients we studied had a medial shift of the standing load-bearing axis, and arthritic changes mainly in the medial compartment. Some also had lateral tibial subluxation with twisting of the distal femur and proximal tibia in opposite directions. We assessed the articular geometry by precise radiographic analysis, and compared the results with those in normal volunteers and a group of osteoarthritic patients. The prevalence of this type of deformity in our osteoarthritic patients was 11.5%; its recognition allows the use of specific operative correction that may include double osteotomy or the precise orientation of prosthetic components.

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.

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.

The development of a surface arthroplasty for the elbow.

Complex kinematics, anatomical features, and load distribution have contributed to the poor function of constrained and semiconstrained cemented arthroplasties of the elbow. Resurfacing by porouscoated components has the potential, by reproduction of normal joint geometry and restoration of ligament balance, to recreate relatively normal kinematics and load-bearing and provide relief of pain. A method was developed to provide information on the geometry of the lower humeral joint surface and olecranon fossa. The information gained was used to design components to resurface the trochlea, capitellum, and olecranon fossa. A technique was also developed to remove a minimal amount of subchondral bone from the ulna and humerus in a precisely directed fashion for exact fit of the porouscoated components.

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.

Alignment of the first metatarsal-phalangeal joint: important criteria for a new joint replacement

Joint replacement is one of many options for the treatment of the first metatarsophalangeal (MTP) joint. Studies of the geometry of that joint have shown that it consists of two distinct articulations, the metatarsophalangeal and the MT-sesamoidal. Both are important, but the MT-sesamoidal tolerates only small deviations from an ideal alignment. The aim of this study was to investigate the alignment of the first MTP joint of a potential patient population, in order to design an optimal surface replacement. One measurement, the extension angle between the MT and the proximal phalanx was found to be the most important alignment criterion for the successful design of an implant and the necessary instruments. This angle controls the delicate interplay between the metatarsal head and the sesamoids and tolerates only small deviations from the normal range before the chance of sesamoidal subluxation increases significantly. The pre-operative knowledge of this and other alignment criteria is important for the ideal placement of an anatomical implant.

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.