Tracy E. Orr

Influences of mechanical stress on prenatal and postnatal skeletal development.

A new theory is introduced to describe some of the influences of mechanical stresses on chondroosseoubiology. It is proposed that degeneration and ossification is a normal process for all cartilage in the appendicular skeleton, which is (1) accelerated by intermittently applied shear stresses (or strain energy), and (2) inhibited or prevented by intermittently applied hydrostatic pressure. These concepts were applied using finite element computer models in an effort to predict the ossification pattern of the prenatal and postnatal femoral anlage. The theoretical calculations successfully predicted the key features of skeletal morphogenesis including the development of (1) the primary ossification site, (2) a tubular diaphysis and marrow cavity, (3) meta-phys-al and epiphyseal trabecular bone, (4) the location and geometry of the growth plate, (5) the appearance and location of the secondary ossific nucleus, and (6) the existence and thickness distribution of articular cartilage. The results suggest that degenerative joint disease in immobilized or non-load-bearing mature joints may be a manifestation of the final stage in the ossification of the anlage. In nonfunctional joints, the absence or reduction of intermittent hydrostatic pressure in the articular cartilage permits cartilage degeneration and the progressive advance of the ossification front toward the joint surface until the articular cartilage has been ossified.

Computer predictions of bone remodeling around porous-coated implants

Computer simulations of bone remodeling in response to mechanical stresses can be used to understand normal growth and development of the skeleton or to predict the remodeling of bone in response to prosthetic devices. Using a previously derived bone maintenance theory, a technique for computing bone density distributions was applied to the proximal femur and tibia using two-dimensional, multiple-loading finite element models. The models initially represented solid, homogeneous structures. Using an iterative bone remodeling technique that relates bone apparent density to loading history, the internal distributions of apparent density and elastic modulus for the normal bones were predicted. The finite element models were then modified to represent bones in which porous-coated femoral surface replacements and tibial tray components had been implanted. The same iterative remodeling method was then applied to predict the distribution of bone around these components. The predicted bone density distributions for the natural femur and tibia agree with previously documented normal bone morphology. The predicted bone density distributions around various implanted prostheses were characteristic of the component under investigation and were consistent with clinical and experimental findings of other investigators. In the femoral head, stress shielding occurred underneath the metal surface replacement cup, resulting in lower densities in the femoral head. The addition of a central femoral cup fixation peg caused bone hypertrophy around the peg. In the tibia, the stress concentrations around the pegs also resulted in denser bone, with a concomitant decrease in bone density at more peripheral locations underneath the prosthetic tray. This remodeling technique has the potential to be an important tool in predicting the possible remodeling consequences of new implant design features.

Stress analyses of glenoid component designs

Metal backing of glenoid components for total shoulder replacements and the use of bony ingrowth surfaces on these components have recently been introduced. In this study, finite element analyses were performed to determine the stress fields in the natural glenoid and to calculate the change in bone stresses after implantation of glenoid components of various designs. The effects of metal backing, keel geometry, and superior constraint on bone stresses indicate that stress distributions on the natural glenoid corresponded to bone morphology. Metal-backing the glenoid component may cause slight improvement in stress transfer to cortical bone. Altered fin geometry better stabilized the glenoid component. Superior restraints on the component intending to prevent subluxation increase stresses and may cause earlier loosening than encountered with unconstrained components.

The Incorporation of Friction Interfaces in a Non-Linear, Finite Element Model of a Plated Long Bone

In recent years much attention has been focused on the problem of refracture after the removal of internal fixation plates. Two concepts that have been implicated as contributory causes of cortical bone osteopenia or thinning which may result in refracture are: 1) mechanical stress shielding; and 2) vascular insufficiency. The first of these two possibilities has received considerable attention and is the focus of the present study.

Control of chondro-osseous skeletal biology by mechanical energy

The growth and maturation of the chondro-osseous skeleton is accomplished by the proliferation, maturation, degeneration, and ossification of cartilage. Bone tissue which replaces cartilage during development is modeled to efficiently resist the loads to which it is exposed. Osteoarthritis in the aged involves the degeneration and destruction of the remaining cartilage on the joint surface and osteophyte formation. All of these biological processes are greatly influenced by the mechanical loading. The role of mechanical stresses in chondro-osseous biology is, however, incompletely understood.