Some aspects of longitudinal bone growth. An experimental study of the rabbit tibia

Hindlimb heating increases vascular access of large molecules to murine tibial growth plates measured by in vivo multiphoton imaging

Advances in understanding the molecular regulation of longitudinal growth have led to development of novel drug therapies for growth plate disorders. Despite progress, a major unmet challenge is delivering therapeutic agents to avascular-cartilage plates. Dense extracellular matrix and lack of penetrating blood vessels create a semipermeable "barrier," which hinders molecular transport at the vascular-cartilage interface. To overcome this obstacle, we used a hindlimb heating model to manipulate bone circulation in 5-wk-old female mice (n = 22). Temperatures represented a physiological range of normal human knee joints. We used in vivo multiphoton microscopy to quantify temperature-enhanced delivery of large molecules into tibial growth plates. We tested the hypothesis that increasing hindlimb temperature from 22°C to 34°C increases vascular access of large systemic molecules, modeled using 10, 40, and 70 kDa dextrans that approximate sizes of physiological regulators. Vascular access was quantified by vessel diameter, velocity, and dextran leakage from subperichondrial plexus vessels and accumulation in growth plate cartilage. Growth plate entry of 10 kDa dextrans increased >150% at 34°C. Entry of 40 and 70 kDa dextrans increased <50%, suggesting a size-dependent temperature enhancement. Total dextran levels in the plexus increased at 34°C, but relative leakage out of vessels was not temperature dependent. Blood velocity and vessel diameter increased 118% and 31%, respectively, at 34°C. These results demonstrate that heat enhances vascular carrying capacity and bioavailability of large molecules around growth plates, suggesting that temperature could be a noninvasive strategy for modulating delivery of therapeutics to impaired growth plates of children.

Thermal osteonecrosis and bone drilling parameters revisited

INTRODUCTION

During the drilling of the bone, the temperature could increase above 47 degrees C and cause irreversible osteonecrosis. The result is weakened contact of implants with bone and possible loss of rigid fixation. The aim of this study was to find an optimal condition where the increase in bone temperature during bone drilling process would be minimal.

MATERIALS AND METHODS

Influence of different drill parameters was evaluated on the increase of bone temperature. Drill diameters were 2.5, 3.2 and 4.5 mm; drill speed 188, 462, 1,140 and 1,820 rpm; feed-rate 24, 56, 84 and 196 mm/min; drill point angle 80 degrees , 100 degrees and 120 degrees and external irrigation with water of 26 degrees C.

RESULTS

Combinations of drill speed and drill diameter with the use of external irrigation produced temperatures far below critical. Without external irrigation, temperature values for the same combination of parameters ranged 31.4-55.5 degrees C. Temperatures above critical were recorded using 4.5 mm drill with higher drill speeds (1,140 and 1,820 rpm). There was no statistical significance of different drill point angles on the increase or decrease of bone temperature. The higher the feed-rate the lower the increase of bone temperature.

CONCLUSIONS

The external irrigation is the most important cooling factor. With all combinations of parameters used, external irrigation maintained the bone temperature below 47 degrees C. The increase in drill diameter and drill speed caused increase in bone temperature. The changes in drill point angle did not show significant influence in the increase of the bone temperature. With the increase in feed-rate, increase in bone temperature is lower.

Mechanics of avian fibrous periosteum: tensile and adhesion properties during growth

We report the results of direct mechanical tests of the fibrous periosteum from the tibiotarsi of white leghorn chicks at 4, 6, 8, 9, 10, 11, 12, and 14 weeks of age using a newly developed sample isolation technique. Additionally, this technique allows the determination of the apparent in vivo load on the fibrous periosteum. The periosteum has a highly nonlinear stress-strain relationship at all ages. For loading below the in vivo level, the periosteum is pliant and mean tensile modulus is 3.35 MPa (+/- 1.84 SD, n = 75). For loading above the in vivo level, tensile stiffness is nearly two orders of magnitude greater. In the region of high stiffness, mean modulus is 229.5 MPa (+/- 89.6, n = 72). In vivo, the periosteum is loaded at the transition between these two stiffness regions. We interpret this as indicating that, in vivo, the collagen fibers of the periosteum are aligned, but subject to minimal loading. Stress levels in the periosteum corresponding to in vivo conditions indicate modest loading, and mean apparent in vivo stress levels are 0.92 MPa (+/- 0.37 SD, n = 67). A second technique demonstrated that the adhesion of the periosteum in the diaphyseal region (1-6 weeks of age) is minimal, but is substantial in the metaphyseal region. The metaphyseal adhesion will affect the transmission of load between the physes. These studies suggest that growth of the fibrous periosteum follows the longitudinal growth of the bone, rather than the periosteum having a direct mechanical influence on growth plate activity. Comparison of tensile properties over the course of growth indicates a substantial increase in periosteal stiffness in the early portion of the growth period, which reaches a maximum at approximately 9 weeks posthatching. There is also a marked decline in periosteal stiffness as growth rate declines in the latest stages of growth (14 weeks). This suggests that the basic properties of periosteal collagen may undergo a transition during the course of this tissue's brief functional lifetime; that is, during long bone growth.

Quantifying the effect of ischemia on epiphyseal growth in an extremity replant model

Warm ischemia (21 degrees C) of 0, 2, 4, 6, or 8 hours was produced in a modified hindlimb preparation of 35 10-week-old Lewis rats by amputation. Subsequent microvascular anastomoses of each hindlimb to a syngeneic animal was done after which fluorochrome bone labels were administered 5 minutes after operation and on day 14 after operation. Epiphyseal plate growth (that between bone labels) was analyzed histomorphometrically and statistically. Epiphyseal plate growth was found to have a linear inverse relationship to ischemia time with a correlation coefficient (r) of -0.653 and high statistical significance (p less than 0.001). Overgrowth occurred at all ischemic periods except 8 hours, and vascular pedicle patency decreased as ischemia time progressed.