The Effect of External Fixator Pin Geometry and Dynamic Loading on Bone Remodelling at the Pin-Bone Interface (PBI), in an In Vivo Sheep Tibia Model

Abstract

SummaryThe purpose of this study was to study the effects of pin design and dynamic loading on the biological response of bone at the pin-bone interface (PBI). An electro-pneumatic pin-loading device was used to mimic the forces that would be experienced by external fixator pins, during patient ambulation following repair of a fracture with external skeletal fixation. The remodelling response was assessed histologically with fluorescence microscopy of plasticembedded undecalcified pin-bone sections, and surface-stained histomorphometry.There were no significant differences between the AO Schanz and Howmedica Apex 5.0 mm diameter self-tapping pin designs when they were inserted in 4.5 mm diameter predrilled pilot holes. When pin type was controlled for, significant differences (p<0.05) existed between loaded and control groups as well as between entry (near) and exit (far) cortices. Less bone resorption was observed in the non-loaded group in comparison to the dynamically loaded group, and bone resorption in the near cortex consistently exceeded the amount of bone resorption in the far cortex for both treatment groups.The purpose of this study was to examine the effects of pin design and dynamic pin loading on the remodelling response of cortical bone at the pin-bone interface (PBI) of intact sheep tibiae. No significant differences were demonstrated between pin designs; however, significant differences (p<0.05) existed between loaded versus control pins, as well as between entry and exit cortices.

Microdamage at the pin-bone interface of external skeletal fixation pins after short-term cyclical loading ex-vivo

“…Short-term cyclical loading of the external fixation pin constructs did not have a significant effect on the formation of microdamage within the bone adjacent to the fixation pin, and no regional effects within compression cortices (8) were detected. In an earlier in vivo study using a similar design and model (12), pin loosening was a common complication after the application of much lower cyclical bending moments for a total of 10,000 cycles, at 500 cycles every other day for 20 days. Taken together, this suggests that microdamage, caused by pin insertion, is the major factor that will modulate reparative adapative remodeling in the early period after surgery.…”

“…Our hypotheses were tested using the ovine tibia. The ovine tibia is a standard model, which has been commonly used for studies of external skeletal fixation (4,5,10,12,13).…”

In vitro comparison of two centrally threaded, positive-profile transfixation pin designs for use in third metacarpal bones in horses

“…23,24 To avoid increased pin loosening caused by osteolysis, it is important that the implant be well anchored in the bone. Forces that frequently change direction and magnitude result in local bone resorption in areas that have contact with the pin.…”

“…23 The Schanz pins have the same trocar tip and thread de-sign as the STTP but no real cutting flute. This finding leads to the assumption that the cutting or tapping flutes of the STTP might be too short so bone debris cannot be sufficiently eliminated during insertion of the pin.…”