Adjustable Stiffness, External Fixator for the Rat Femur Osteotomy and Segmental Bone Defect Models

Summary

One constraint of preclinical research in the field of bone repair is the lack of experimental control over the local mechanical environment within a healing bone lesion. We report the design and use of an external fixator for bone repair with the ability to change fixator stiffness in vivo.

Abstract

The mechanical environment around the healing of broken bone is very important as it determines the way the fracture will heal. Over the past decade there has been great clinical interest in improving bone healing by altering the mechanical environment through the fixation stability around the lesion. One constraint of preclinical animal research in this area is the lack of experimental control over the local mechanical environment within a large segmental defect as well as osteotomies as they heal. In this paper we report on the design and use of an external fixator to study the healing of large segmental bone defects or osteotomies. This device not only allows for controlled axial stiffness on the bone lesion as it heals, but it also enables the change of stiffness during the healing process in vivo. The conducted experiments have shown that the fixators were able to maintain a 5 mm femoral defect gap in rats in vivo during unrestricted cage activity for at least 8 weeks. Likewise, we observed no distortion or infections, including pin infections during the entire healing period. These results demonstrate that our newly developed external fixator was able to achieve reproducible and standardized stabilization, and the alteration of the mechanical environment of in vivo rat large bone defects and various size osteotomies. This confirms that the external fixation device is well suited for preclinical research investigations using a rat model in the field of bone regeneration and repair.

Introduction

A number of studies have improved our understanding of the biologic mechanisms involved in bone tissue repair^1-6. The effects of mechanical conditions on bone repair such as axial, shear and interfragmentary movements (IFMs) have been studied extensively^7-15. In the past several years, more and more studies started to emerge describing the influence of mechanical environment on bone healing using fracture, osteotomy and large segmental bone defect in vivo models. Therefore, reliable fixation methods are needed to get reproducible and reliable study outcomes.

The mechanical environment around the healing fractu....

Protocol

Animal care and experimental protocols were followed in accordance with NIH guidelines and approved by the Beth Israel Deaconess Medical Center Institutional Animal Care and Use Committee, Boston, MA. (Protocol Number: 098-2009)

1. Preparation of Surgical Materials and Instruments

1. Sterilize all surgical materials and instruments used to perform surgery prior to use. Pack needed materials, with or without an instrument tray, inside a folded cloth or wrapped paper and seal with autoclave tape for steam sterilization. The temperature of the autoclave should be at 125-135 ºC for 20-25 min of sterilization time, and then 10-15 m....

Results

Design specifications

Stabilization of the rat femur with the external fixation system enables the creation of osteotomies from 0.5 to 5 mm. The external fixator system is a locked external fixator made of polyether ether ketone (PEEK - [the main body]) and titanium-aluminium-niobium alloy (TAN - [the mounting pins]), which offers a simple, reproducible and adjustable design, and is available in four different stiffnesses: 10, 40, 70 and 100% (100% being the standard, most rigid fixator (.......

Discussion

The most critical steps of a surgical procedure to create a large bone defect are: 1) choosing the appropriate body weight of the rat to match the size of the external fixator; 2) maintaining a sterile environment during the procedure; and 3) following the surgical procedure protocol.

The main goals of this study were to design, manufacture and characterize a new, variable stiffness external fixator for the rat femoral large defect model, and to use this fixator in determining the interplay be.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
RatExFix simple 100%	RISystem AG Davos, Switzerland	RIS.612.120
RatExFix simple 70%	RISystem AG Davos, Switzerland	RIS.612.123
RatExFix simple 40%	RISystem AG Davos, Switzerland	RIS.612.121
RatExFix simple 10%	RISystem AG Davos, Switzerland	RIS.612.122
RatExFix Connection element 100%	RISystem AG Davos, Switzerland	RIS.612.130
RatExFix Connection element 70%	RISystem AG Davos, Switzerland	RIS.612.131
RatExFix Connection element 40%	RISystem AG Davos, Switzerland	RIS.612.132
RatExFix Connection element 10%	RISystem AG Davos, Switzerland	RIS.612.133
RatExFix Main body	RISystem AG Davos, Switzerland	RIS.611.101
RatExFix InterlockingScrew	RISystem AG Davos, Switzerland	RIS.412.110
RatExFix Mounting pin 0.85 mm	RISystem AG Davos, Switzerland	RIS.412.100
RatExFix Saw Guide 100% 5 mm	RISystem AG Davos, Switzerland	RIS.312.100
Accu Pen 6V+	RISystem AG Davos, Switzerland	RIS.390.211
HandDrill	RISystem AG Davos, Switzerland	RIS.390.130
Drill Bit 0.79 mm	RISystem AG Davos, Switzerland	RIS.593.203
Gigly wire saw 0.22 mm	RISystem AG Davos, Switzerland	RIS.590.100
Square box wrench 0.70 mm	RISystem AG Davos, Switzerland	RIS.590.112
Square box wrench 0.50 mm	RISystem AG Davos, Switzerland	RIS.590.111
Centering bit 1.00 mm	RISystem AG Davos, Switzerland	RIS.592.205
Scalpel Blade handle	Fine Science tools
Scalpel Blade (Size 15)	Fisher Scientific
Tissue Forceps	Fine Science tools
Scissors	Fine Science tools
Retractor	Fine Science tools
Needle Holder	Fine Science tools
Henahan Elevator	Fine Science tools
S-shape curved dissecting and ligature forceps	Fine Science tools		2
Dressing Forceps	Fine Science tools		2
Sterile Fenestrated drape	Fisher Scientific		for surgery
Sterile gauze	Fisher Scientific		for surgery
5 ml syringe	Fisher Scientific		for irrigation of defect
24-27G needle	Fisher Scientific		for irrigation of defect
1 cc Insulin syringes	Fisher Scientific		for drug injections
sterile saline	Fisher Scientific		for bone defect irrigation
sterile gloves	Fisher Scientific		to perform surgeries
chlorohezadine	Fisher Scientific		disinfecting solution for surgical site
Vicryl suture 4-0 with SH-1	Fisher Scientific		to suture muscle
Ethibond suture 3-0	Fisher Scientific		to suture skin
Isofluorine	Sigma-Aldrich		for anesthesia
Buprenorphine	Sigma-Aldrich		analgesia during and after the surgery
Cefazolin	Sigma-Aldrich		antibiotic during and after the surgery
Sprague-Dawley Rats or any other strain	Charles River Laboratories International, Inc. (Wilmington, MA USA)