Urs P. Wyss

Knee and hip kinetics during normal stair climbing

Understanding joint kinetics during activities of daily living furthers our understanding of the factors involved in joint pathology and the effects of treatment. In this study, we examined hip and knee joint kinetics during stair climbing in 35 young healthy subjects using a subject-specific knee model to estimate bone-on-bone tibiofemoral and patello-femoral joint contact forces. The net knee forces were below one body weight while the peak posterior-anterior contact force was close to one body weight. The peak distal-proximal contact force was on average 3 times body weight and could be as high as 6 times body weight. These contact forces occurred at a high degree of knee flexion where there is a smaller joint contact area resulting in high contact stresses. The peak knee adduction moment was 0.42 (0.15) Nm/kg while the flexion moment was 1.16 (0.24) Nm/kg. Similar peak moment values, but different curve profiles, were found for the hip. The hip and knee posterior-anterior shear forces and the knee flexion moment were higher during stair climbing than during level walking. The most striking difference between stair ascent and level walking was that the peak patello-femoral contact force was 8 times higher during stair ascent. These data can be used as baseline measures in pathology studies, as input to theoretical joint models, and as input to mechanical joint simulators.

Biodegradable Controlled Antibiotic Release Devices for Osteomyelitis: Optimization of Release Properties

Abstract— Controlled antibiotic release films, melt‐extruded cylinders, and suspension‐extruded/coated cylinders were manufactured from biodegradable poly(d, l‐lactide) (PDLLA) and poly(d, l‐lactide‐co‐∈‐caprolactone). These devices have potential application in the treatment of osteomyelitis. The in‐vitro release properties of the devices were examined with drug loadings varying from 16 to 50%. Gentamicin sulphate films and melt‐extruded gentamicin/PDLLA cylinders demonstrated a large initial burst and incomplete release. The films and melt‐extruded cylinders made from poly(d, l‐lactide‐co‐∈‐caprolactone), low mol. wt poly(d, l‐lactide), and a mixture of d, l‐lactic acid oligomer and high mol. wt poly(d, l‐lactide), did not remain intact during the entire release period. While this is undesirable, these materials do have the advantage of not requiring a processing temperature of greater than 110°C. Antibiotic release from high mol. wt PDLLA‐coated gentamicin/PDLLA cylinders, with 40 and 50% loading, was very rapid. The antibiotic could only diffuse out through the open ends of the cylinder. Coated gentamicin sulphate cylinders with 20 and 30% drug loading gave the most promising properties in terms of a small initial burst, and a gradual and sustained release. The release rate and duration from the coated cylinders could be adjusted by cutting the cylinder into different lengths; the time required for 90% of the entrapped gentamicin to be released into water from 30% loaded PDLLA‐coated cylinders 0·2, 0·4, 0·7 and 1 cm in length was 1000, 1700, 2300, and 2800 h, respectively. This offers a convenient method to adjust the release to meet the specific antibiotic requirement of different patients. Cephazolin and benzylpenicillin were found to be unsuitable for sustained release longer than 300 h due to the hydrolytic instability of the drugs in water.

Axial lower-limb alignment: comparison of knee geometry in normal volunteers and osteoarthritis patients

Osteoarthritis of the knee is associated with deformities of the lower limb and malalignment of the limb segments. Pathogenetic relationships between the two are poorly understood. Alignment was studied by standardized radiography in 167 symptomatic Canadian osteoarthritis patients, and compared with 119 healthy adult volunteers. In healthy adults overall alignment (hip-knee-ankle angle) was principally determined by distal femoral valgus (condylar hip angle) and proximal tibial-plateau varus (plateau-ankle angle): the angle between the joint surfaces (condylar plateau) was relatively constant. In osteoarthritis, disease-associated differences included condylar-plateau angles that were divergent: accentuated medial convergence in varus osteoarthritis and lateral convergence in valgus osteoarthritis. This was interpreted as change arising from focal loss of cartilage in the medial (varus osteoarthritis) or lateral (valgus osteoarthritis) compartments of the knee. The changes would contribute to increasing limb malalignment during disease progression. But differences of limb geometry also contributed to malalignment. These were the average trends: in varus osteoarthritis there was abnormal femoral geometry (lesser femoral condylar valgus), but tibial surface geometry was the same. In valgus osteoarthritis, the opposite was true: abnormal tibial geometry (lesser plateau varus), but normal femoral geometry. A possible explanation is that these abnormal knee geometries pre-exist and predispose to osteoarthritis, although it is not impossible that they (like condylar-plateau angle) change as disease progresses. Further approaches to population studies are discussed based on these findings, along with their implications for knee surgery.

An Investigation of Poly(lactic acid) Degradation

To elucidate the degradation mechanism of poly(lactic acid), the decrease in the intrinsic viscosity of poly(D,L-lactide) in a homogeneous water/ acetone solution was investigated. The hydrolysis of poly(D,L-lactic acid) in water/acetone solution can be catalyzed by protons. The molecular weight degradation of solid poly(D,L-lactic acid) in water was primarily affected by the degree of polymer purity. Polymerization conditions such as initiator concen tration, temperature and time did not have an obvious effect on the molecular weight degradation. In the case of polymer samples with low purity (i.e., directly polymerized or containing solvent or oligomer), degradation was ini tially very rapid. On the other hand, initial degradation of purified polymer was very slow before accelerating.

Loosening performance of cemented glenoid prosthesis design pairs.

OBJECTIVE The purpose of this pilot study (n=3) was to compare the loosening performance of glenoid prosthesis design pairs where only one design variable differed. DESIGN Glenoids were subjected to dynamic edge loading in a biaxial test setup. BACKGROUND Glenoid component loosening is the primary concern in total shoulder arthroplasty. METHODS After the humeral head was cycled 100,000 times to the superior and inferior edges of the glenoid, the tensile edge displacements were measured under superior and inferior off-center loading. RESULTS AND CONCLUSIONS Based on this study, a rough-backed design had dramatically better loosening performance than a smooth-backed; curved-backed was superior to flat-backed; a less-constrained articular surface was better than a more-constrained articular surface; pegs outperformed a keel; threaded pegs were marginally preferable to cylindrical pegs; and an all-polyethylene design rocked slightly less than a metal-mesh-backed design. RELEVANCE A comparison of the laboratory loosening behavior of glenoid prostheses may lead to improved designs, subsequently leading to a reduction in the incidence of clinical loosening.

Trabecular microstructure in the medial condyle of the proximal tibia of patients with knee osteoarthritis

The microstructural characteristics of osteoarthritic subchondral bone in the medial tibial condyle are clearly different from normal age-matched bone. Subchondral sclerosis in osteoarthritis indicates not only an increase in bone volume fraction but also alteration in other microstructural characteristics. Eleven medial tibial condyles were collected from ten subjects during arthroplastic surgery for knee oseoarthritis. They were compared to four medial tibial condyles from four age-matched controls with no history of any bone or joint disorder. Six sections from anterior to posterior and three levels from proximal to distal were evaluated in each medial condyle. Five histomorphometric parameters were measured: bone volume fraction (BVf), trabecular thickness (Tb.Th), trabecular number (Tb.N), trabecular separation (Tb.S), and trabecular connectivity (Tb.C). In general, the osteoarthritic subchondral bone had a higher bone volume fraction than control bone but the microstructure was characterized by fewer, widely spaced, thicker than normal trabeculae. There were also highly localized regional differences by depth from the articular surface and from anterior to posterior across the medial condyle. These variations in OA subchondral bone microstructure may significantly affect biomechanical competence of bone in a way not predictable by bone volume fraction measurements alone.

An investigation of the synthesis and thermal stability of poly(dl-lactide)

SummaryThe synthesis and thermal degradability of poly (DL-lactide) were investigated. Key factors affecting the polymer molecular weight were found to be monomer recrystallization, initiator concentration and the vacuum level during drying/sealing of the polymerization reaction ampoule. It was found that poly (DL-lactide) is thermally unstable above its melting temperature. Monomer recrystallization, polymer precipitation and a low initiator content of the polymer significantly inhibited the rate and extent of thermal degradation.

Glenoid cancellous bone strength and modulus

The objectives of this study were to determine the strength and modulus of glenoid cancellous bone, including regional variations. The motivations were: to select a suitable bone substitute for standardized testing of glenoid prosthesis loosening, to assist in shoulder prosthesis design and to provide input data for finite element analyses. Ten glenoids from eight cadavers (mean age, 81) were tested by in situ indentation. Mean strength ranged from 6.7 to 17 MPa for the ten glenoids, the overall mean being 10.3 MPa. Mean E moduli ranged from 67 to 171 MPa for the individual glenoids, the overall mean being 99 MPa. These values are likely at the lower end of what would be expected for normal bone since strength and modulus decrease with age and the available specimens were older. These values may be appropriate for prosthesis design, however, since mechanical properties are reduced in rheumatoid arthritic bone. Regional trends were very similar for modulus and strength. The strongest region was postero-superior. The central column, correlating with the keel position in many glenoid components, was weaker than both the anterior and posterior regions but deeper. A large drop in strength and modulus below the subchondral layer emphasizes the importance of maintaining this layer during prosthetic replacement.

Glenohumeral contact forces

Abstract Glenohumeral contact forces have only been calculated previously either for simple abduction or for athletic activities. The objective of this study was to determine the glenohumeral contact forces for tasks which are demanding of the shoulder but which would commonly be performed by older people. The functional tasks chosen were using the arms to stand up from and sit down into a chair, walking with a cane, lifting a 5 kg box to shoulder height with both hands, and lifting a 10 kg suitcase. The trunk angles, arm angles and hand loads of six healthy subjects, average age 55 years, were recorded. This information was input into a biomechanical computer model which optimized the muscle force distribution by minimizing the sum of squared muscle stresses subject to constraints on the maximum muscle forces and maintaining the direction of the resultant force within the glenoid fossa. Average contact forces ranged from 1.3 to 2.4 times body weight (930-1720 N), the highest force being for lifting a suitcase. This latter value would be even higher if lifting either a greater load or to a greater height. Thus, contact forces at the shoulder should not be underestimated. This study provides functionally relevant contact forces which can be used for mechanical testing or finite element modelling of shoulder prostheses.

A PROCEDURE TO VALIDATE THREE-DIMENSIONAL MOTION ASSESSMENT SYSTEMS

The automation provided by computer-assisted motion-tracking systems allows for three-dimensional motion and force analysis. These systems combined with mathematical modelling are able to analyse quickly the intricate dynamics of human movement. Understanding the limitations of human motion analysis as performed by the present measurement techniques is essential for proper application of the results. It is necessary to validate the analysis system prior to subject testing. This paper provides a validation of an optoelectric motion-tracking system used in a dynamic knee assessment study. While the validation is shown with one particular system only, it is suggested that all systems used in two- or three-dimensional motion analysis should be tested similarly in the actual configuration used. Three simple mechanical representations of the human knee have been used in this validation. The first model provided an understanding of the source and behaviour of the error introduced to the accuracy of defining a vector between the recorded coordinates of two markers. The other two models investigated the effect of processing methods specific to the knee analysis project. Separating the markers by at least 180 mm is recommended to produce stable vectors. Relative joint angles could be calculated in all three planes of rotation. The error in calculating flexion and longitudinal rotation was less than 2.0 degrees, while calculating adduction introduced errors of 4.0 degrees. Force calculations were found to be within 8%. The system behaviour was found to be consistent within the calibrated volume about the force platform. Simple mechanical models combined with straightforward procedures can provide validation in terms of clinically relevant parameters.