Ronald Mark Gillies

A resorbable porous ceramic composite bone graft substitute in a rabbit metaphyseal defect model.

The success of converted corals as a bone graft substitute relies on a complex sequence of events of vascular ingrowth, differentiation of osteoprogenitor cells, bone remodeling and graft resorption occurring together with host bone ingrowth into and onto the porous coralline microstructure or voids left behind during resorption. This study examined the resorption rates and bone infiltration into a family of resorbable porous ceramic placed bilaterally in critical sized defects in the tibial metaphyseal–diaphyseal of rabbits. The ceramics are made resorbable by partially converting the calcium carbonate of corals to form a hydroxyapatite (HA) layer on all surfaces. Attempts have been made to control the resorption rate of the implant by varying the HA thickness. New bone was observed at the periosteal and endosteal cortices, which flowed into the centre of the defect supporting the osteoconductive nature of partially converted corals. The combination of an HA layer and calcium carbonate core provides a composite bone graft substitute for new tissue integration. The HA‐calcium carbonate composite demonstrated an initial resorption of the inner calcium carbonate phase but the overall implant resorption and bone ingrowth behaviour did not differ with HA thickness.

A comparison of the thermal properties of 2- and 3-fluted drills and the effects on bone cell viability and screw pull-out strength in an ovine model

BACKGROUND Drilling of bone is associated with an increase in temperature of the surrounding bone which may result in osteonecrosis. METHODS In this study, cutting efficiency and thermal properties of one 2-fluted drill and two 3-fluted drills were determined in vitro using a porcine model. Drills were then used to create pilot holes in an in vivo ovine model to facilitate implantation of pedicle screws. The effect of the characteristic thermal profiles of each drill on cortical bone cell viability and screw pull-out strength was then assessed. FINDINGS Cutting efficiencies of both 3-fluted designs were found to be greater than that of the 2-fluted drill, but this did not translate into a decrease in the maximum temperatures during drilling for both drills. Histologically, no empty osteocyte lacunae were seen at 2 or 4 weeks, suggesting that temperatures were not sufficiently high enough to induce thermonecrosis in the ovine tibia. No differences were found in the pull-out strength of the screws. INTERPRETATION Both 2- and 3-fluted drills are currently in clinical use. Despite the theoretical advantage that 3-fluted drills possess over their 2-fluted counterparts, there is a lack of evidence in the literature in support of their use. In this study the observed increases in cutting efficiency of the 3-fluted drills tested did not translate into a reduction in heat generation or improvement in bone healing or screw fixation.

The influence of design parameters on cortical strain distribution of a cementless titanium femoral stem

The strain distribution imposed on a femur following a total joint replacement is an important factor, in proximal bone loss due to stress shielding, and long term clinical success. This study investigated how five different design parameters of a cementless titanium femoral prosthesis influenced cortical strains. Test loads were applied and strains were measured with and without an abductor force simulation, using six human cadaveric femora. The cementless design used demonstrated significant calcar loading proximally and a similar strain distribution to the intact femur distally. Implant gross geometry was the major factor in determining the cortical strain distributions under abductor simulation in both axial and torsional loading.

Plating of Metacarpal Fractures: Unicortical or Bicortical Screws?

Mid-shaft transverse osteotomies were performed in 18 cadaveric metacarpals and randomly divided into two groups. Using dorsally applied plates for repair, one group was secured using 6 mm unicortical screws, while bicortical screws were used in the second group. The metacarpals were tested to failure with a four-point bending protocol using a servo-hydraulic testing machine and a 1 kN load cell. The mean load to failure was 596 N (SD=142) for the unicortical and 541 N (SD=171) for the bicortical group. The stiffness was 333 N/mm (SD=116) for the unicortical and 458 N/mm (SD=158) for the bicortical group. Both load to failure and stiffness were not statistically significant between the two groups. Failure occurred by fracture at the screw-bone interface in all specimens: no screw pull-out was observed. No biomechanical advantage was found when using bicortical screws in metacarpal fracture plating.

Spinal fusion using an autologous growth factor gel and a porous resorbable ceramic

Augmenting healing through a single application of an exogenous growth factor or bone morphogenetic protein is not a new concept. The use of autologous growth factors through platelet isolation and concentration provides multiple endogenous growth factors to the healing site. A posterolateral fusion model in aged sheep (5- to 6-year-old ewes) was used to examine the effects of the addition of growth factors through autologous platelet isolation on the biomechanic and histologic properties of the fusion using a resorbable coral bone graft substitute. At 6 months the combination of autologous growth factors to the Pro Osteon 500R plus aspirated bone marrow resulted in the greatest bending stiffness but not ultimate load. Autologous growth factors can be isolated from platelets and concentrated to provide multiple growth factors to the fusion site to aid in spinal fusion.

In vitro assessment of proximal polyethylene contact surface areas and stresses in mobile bearing knees

Wear of the polyethylene (PE) insert in total knee arthroplasty remains a significant problem. The generation of biologically active wear particles may ultimately affect implant longevity through osteolysis or premature/catastrophic PE failure. The rate and pattern of wear is influenced by many factors, including component geometry and individual loading conditions, which determine the contact surface area and kinematics of the reconstructed knee. Contact areas and stresses at the proximal femoral-PE insert interface and distal PE-tibial interface contact surface areas were measured in nine mobile bearing total knee designs at 0, 30, 60, 90 and 110 degrees of flexion at 3600 N (5 x body weight) using a standardized test method. Proximal and, to a lesser degree, distal interface contact area footprints decreased significantly with increasing flexion angle based on the conformity of the designs, resulting in a corresponding increase in the mean and peak stresses.

Cell Structure and Biology of Bone and Cartilage

Bone and cartilage are highly specialized connective tissues that are engineered by nature to perform a variety of specialized tasks. As a result, these tissues have unique cellular constituents and ultrastructural organization that help optimize the biochemical demands and biomechanical loads in vivo. Our understanding of these connective tissues has increased over the past few centuries, and progress continues as techniques are defined and improved or new ones are developed. The understanding of the ultrastructural and mechanical properties of bone and cartilage is a dynamic area that is constantly evolving with new understanding. This chapter departs from traditional overviews of bone and cartilage, and presents an examination of cellular constituents and ultrastructural organization of these tissues in the context of recent experimental and theoretical studies. It is the hope of the authors to bring to light many new and exciting findings.

In vitro study of shear force on interbody implants

Objective: The lordosis of the lumbar spine and body weight result in significant shear forces through the lumbosacral disc spaces. These forces result in translational motion across the disc space, which is resisted but not completely abolished by pedicle screw stabilization. It is postulated that this motion may be a factor in the development of nonunion of lumbar interbody fusions. An in vitro study of the micromotion of porcine specimens implanted with serrated or smooth interbody spacers and subjected to shear forces under compressive preload was conducted to determine whether the surface serrations on vertebral interbody implants significantly resist shear forces and resulting sagittal translation. Methods: Measurements of anterior vertebral translation were recorded on porcine cervical spine segments, subjected to 25 N of anteroposterior shear load while under a 300-N compressive preload. Baseline testing was performed on intact specimens and partially destabilized specimens (facet joints removed). Following partial discectomy, the specimens were divided into two groups for testing: one using smooth-surfaced and one using serrated interbody spacers. Results: Under 25-N shear load, the specimens tested with the serrated spacers showed anterior vertebral translation of 0.046 ± 0.013 mm, whereas those tested with the smooth spacers measured 0.152 ± 0.075 mm (P < 0.01). Conclusions: The presence of surface serrations on the interbody implants significantly increased the resistance to shear forces in this model. In the clinical setting, we postulate that micromotion at interbody fusion sites will be substantially less when serrated implants are used and may help reduce the incidence of nonunion.