INTRODUCTION

As computer navigated surgery continues to progress to the forefront of orthopedic care, the application of a navigated total shoulder arthroplasty has yet to appear. However, the accuracy of these systems is debated, as well as the dilemma of placing an accurate tool in an inaccurate hand. Often times a system's accuracy is claimed or validated based on postoperative imaging, but the true positioning is difficult to verify. In this study, a navigation system was used to preoperatively plan, guide, and implant surrogate shoulder glenoid implants and fiducials in nine cadaveric shoulders. A novel method to validate the position of these implants and accuracy of the system was performed using pre and post operative high resolution CT scans, in conjunction with barium sulfate impregnated PEEK surrogate implants.

One possible mechanism by which metal-on-metal hip resurfacing (MOMHR) may be associated with prosthesis loosening, periprosthetic fracture, and femoral neck narrowing is through an increase in bone resorption by osteoclast cells. Whilst it is known that metal ions such as cobalt (Co) and chromium (Cr) ions (that are elevated locally and systemically after MOMHR), may affect osteoblast and macrophage activity in-vitro, little is known about the effect of these ions on osteoclasts. We examined whether these ions have an adverse effect on human peripheral blood derived osteoclasts at levels that are clinically relevant after MOMHR. Peripheral blood mononuclear cells from healthy donors were seeded onto dentine wafers, and treated to transform them into osteoclasts using standard techniques in the presence of various clinically relevant concentrations of Co2+, Cr3+, and Cr6+. After 3 weeks of culture osteoclast number and resorption pit formation was quantified using histological techniques. All 3 metal ions had a dose-dependent effect on both osteoclast formation and resorption activity. At ion levels found in serum after MOMHR, an increase in osteoclast formation and bone resorption was found, but at higher levels found in synovial fluid, osteoclast cell proliferation and resorption activity was decreased, likely due to a direct toxic effect of the ions on the cells (Figure 1). Cr6+ was more toxic than the other ions at higher concentrations. Our data suggest that metal ion release following MOMHR may increase osteoclast activity systemically that might have a deleterious effect on general and local bone health, and may contribute to the observed bone related complications of MOMHR.

Carbon nanotubes are an exciting new type of material and have extraordinary properties (1). A special category of carbon nanotubes (multiwalled or MWNT) is flexible yet have tensile strengths 200 times stronger than traditional carbon fibers (2). Because of their extremely large surface area-to-volume ratio, theory suggests that MWNTs can bond more strongly to polymethylmethacrylate (PMMA) than any other material tested (2). The combination of large tensile strength and strong interfacial (PMMA matrix) bonding suggests that when added to bone cement, MWNTs could bridge and arrest fatigue or impact cracks and thereby favorably improve the clinical performance of bone cement.

The objective of this study was to determine the validity of this hypothesis and whether MWNTs can significantly improve the tensile properties of PMMA. Methods MWNTs (20–30 nanometers in diameter, 20–100 microns long) were grown on a fused quartz substrate by the thermal decomposition of xylene in the presence of a metal catalyst. They are formed in well-aligned mats and grow perpendicular to the walls of a tubular reactor. As a first approach MWNTs were separated and dispersed through the liquid monomer component of PMMA by using an ultrasonic probe. The remaining polymer component was then mixed with this dispersion and the product was used to prepare specimens by casting in molds. Since prior work in other polymer systems (3) indicated that small concentrations of MWNTs could significantly affect a polymer’s physical properties, only fractions (1/16, ¼ and ½) of 1% of MWNTs (by weight) were used to prepare tensile test specimens. Control (0% MWNTs) and experimental (MWNT containing) groups of PMMA specimens were cured in air at room temperature for 7 days and then pulled to failure at 6 mm/min in a protocol conforming to ASTM D638. Maximum load, strength, results a total of 41 specimens have been prepared and tested: 13 controls, 9 with 0.063%, 10 with 0.25%, and 9 with 0.5% (by weight) of MWNTs. Carbon nanotubes improved the tensile load bearing properties of all experimental groups from 17% to 24%, and these values were significant (p=0.01 and p=0.02) for the 0.25% and 0.5% concentrations. The lowest concentration of MWNTs made the smallest improvement (17%) and this was not significant (p=0.07). Scanning electron microscopic examination of the fractured surface revealed nanotubes that were well distributed throughout the matrix.

Discussion: These preliminary results clearly demonstrate that carbon nanotubes can significantly improve the mechanical performance of bone cement. This result is especially encouraging because the MWNTs were added only to the monomer component. Additional performance enhancement may be expected from ongoing work using higher concentrations of MWNTs and their dispersion into the polymer component of bone cement in addition to the monomer component.

Since MWNTs are also electrically conducting and have magnetic properties, MWNTs may also help dissipate the heat generated by polymerization or permit bone cement with an “engineered” mechanical anisotropy. Although static tensile tests are an incomplete measure of bone cement, these preliminary results are very encouraging and motivate continuing study of the more clinically relevant (impact resistance, fatigue properties, etc.) measures of the mechanical performance of MWNT augmented bone cement.

A chance observation of asymmetrical bone ages in a child with spastic hemiplegia stimulated a prospective gathering of bilateral hand radiographs in 33 hemiplegic patients, and on a single occasion in a control group of 23 patients with leg length discrepancy in the absence of neurological disorder. The bone age assessments according to Greulich and Pyle, which by convention has used the left hand only, were done by a single expert observer blinded to the clinical details.

13 hemiplegic patients (39%) had delayed bone ages of 6 months or more. When present it was always delayed on the hemiplegic side. The mean delay for the whole group was 2.5 months, whereas there was no mean difference in the control group (p = 0.001). The oldest bone age with asymmetry was 14.5 years in males and 12 years in females, indicating that when present the delay “catches up” in the last 2-3 years of growth.

In hemiplegia the percentage leg length discrepancy also tends to decrease during later growth, and after 80% of growth the hemiplegic side outgrows the normal leg by a mean of 0.3cm/year. No correlation could be found between the delay of bone age and the severity of either the neurological abnormality or the actual discrepancy of length. The implications for clinical management will be discussed.