Evaluation of multi-scale mineralized collagen-polycaprolactone composites for bone tissue engineering.

A particular challenge in biomaterial development for treating orthopedic injuries stems from the need to balance bioactive design criteria with the mechanical and geometric constraints governed by the physiological wound environment. Such trade-offs are of particular importance in large craniofacial bone defects which arise from both acute trauma and chronic conditions. Ongoing efforts in our laboratory have demonstrated a mineralized collagen biomaterial that can promote human mesenchymal stem cell osteogenesis in the absence of osteogenic media but that possesses suboptimal mechanical properties in regards to use in loaded wound sites. Here we demonstrate a multi-scale composite consisting of a highly bioactive mineralized collagen-glycosaminoglycan scaffold with micron-scale porosity and a polycaprolactone support frame (PCL) with millimeter-scale porosity. Fabrication of the composite was performed by impregnating the PCL support frame with the mineral scaffold precursor suspension prior to lyophilization. Here we evaluate the mechanical properties, permeability, and bioactivity of the resulting composite. Results indicated that the PCL support frame dominates the bulk mechanical response of the composite resulting in a 6000-fold increase in modulus compared to the mineral scaffold alone. Similarly, the incorporation of the mineral scaffold matrix into the composite resulted in a higher specific surface area compared to the PCL frame alone. The increased specific surface area in the collagen-PCL composite promoted increased initial attachment of porcine adipose derived stem cells versus the PCL construct.

Computed tomography-based tissue-engineered scaffolds in craniomaxillofacial surgery.

INTRODUCTION: Tissue engineering provides an alternative modality allowing for decreased morbidity of donor site grafting and decreased rejection of less compatible alloplastic tissues. METHODS: Using image-based design and computer software, a precisely sized and shaped scaffold for osseous tissue regeneration can be created via selective laser sintering. Polycaprolactone has been used to create a condylar ramus unit (CRU) scaffold for application in temporomandibular joint reconstruction in a Yucatan minipig animal model. Following sacrifice, micro-computed tomography and histology was used to demonstrate the efficacy of this particular scaffold design. RESULTS: A proof-of-concept surgery has demonstrated cartilaginous tissue regeneration along the articulating surface with exuberant osseous tissue formation. Bone volumes and tissue mineral density at both the 1 and 3 month time points demonstrated significant new bone growth interior and exterior to the scaffold. CONCLUSION: Computationally designed scaffolds can support masticatory function in a large animal model as well as both osseous and cartilage regeneration. Our group is continuing to evaluate multiple implant designs in both young and mature Yucatan minipig animals.

Engineering craniofacial scaffolds.

OBJECTIVE: To develop an integrated approach for engineering craniofacial scaffolds and to demonstrate that these engineered scaffolds would have mechanical properties in the range of craniofacial tissue and support bone regeneration for craniofacial reconstruction. EXPERIMENTAL VARIABLE: Scaffold architecture designed to achieve desired elasticity and permeability. Scaffold external shape designed to match craniofacial anatomy. OUTCOME MEASURE: Final fabricated biomaterial scaffolds. Compressive mechanical modulus and strength. Bone regeneration as measured by micro-CT scanning, mechanical testing and histology. SETTING: Departments of Biomedical Engineering, Oral/Maxillofacial Surgery, and Oral Medicine, Pathology and Oncology at the University of Michigan. RESULTS: Results showed that the design/fabrication approach could create scaffolds with designed porous architecture to match craniofacial anatomy. These scaffolds could be fabricated from a wide range of biomaterials, including titanium, degradable polymers, and degradable calcium phosphate ceramics. Mechanical tests showed that fabricated scaffolds had compressive modulus ranging 50 to 2900 MPa and compressive strength ranging from 2 to over 56 MPa, within the range of human craniofacial trabecular bone. In vivo testing of designed scaffolds showed that they could support bone regeneration via delivery of BMP-7 transduced human gingival fibroblasts in a mouse model. Designed hydroxyapatite scaffolds with pore diameters ranging from 400 to 1200 microns were implanted in minipig mandibular defects for 6 and 18 weeks. Results showed substantial bone ingrowth (between 40 and 50% at 6 weeks, between 70 and 80% at 18 weeks) for all scaffolds, with no significant difference based on pore diameter. CONCLUSION: Integrated image-based design and solid free-form fabrication can create scaffolds that attain desired elasticity and permeability while fitting any 3D craniofacial defect. The scaffolds could be manufactured from degradable polymers, calcium phosphate ceramics and titanium. The designed scaffolds supported significant bone regeneration for all pore sizes ranging from 300 to 1200 microns. These results suggest that designed scaffolds are clinically applicable for complex craniofacial reconstruction.

Neer Award 1999. Overuse activity injures the supraspinatus tendon in an animal model: a histologic and biomechanical study.

Overuse activity has been implicated as an etiologic factor in injury to the rotator cuff and to the supraspinatus tendon in particular. Due in part to the lack of an appropriate animal model, expex85ental studies have not addressed this issue. With the use of a rat model, we measured the effects of an overuse running regimen on 36 Sprague-Dawley rats after 4 (n = 12), 8 (n = 12), or 16 (n = 12) weeks of exercise and compared them with a control group of rats (n = 10) who were allowed normal cage activity. The histologic characteristics, the gross morphologic characteristics, and the mechanical properties of the tendon tissue were evaluated. The supraspinatus tendons in the exercised animals demonstrated significant changes as a result of overuse at all time points compared with the normal group. There was an increase in cellularity and a loss of the normal collagen fiber organization consistent with what has been seen in human tendinopathy. The tendons from the exercise groups were larger than normal in cross-sectional analysis at 4 weeks (129% of control, P < .01) and continued to increase in size with time to 16 weeks (164% of control, P = .01). The mechanical properties of the tendons deteriorated in response to overuse exercise with a decreased modulus of elasticity ranging from 52% to 61% of control (P = .07 at 4 weeks, P < .05 at 8 and 16 weeks) and a decreased maximum stress of failure ranging from 51% to 63% of control (P < .007). These findings support overuse activity as an etiologic factor in the development of supraspinatus tendinopathy and begin to describe the changes in the tendons as a result of such activity. This model can now be used to study the effect of various treatment modalities on these injuries.

The effects of overuse combined with intrinsic or extrinsic alterations in an animal model of rotator cuff tendinosis.

An in vivo animal model was used to evaluate overuse and overuse plus intrinsic tendon injury or extrinsic tendon compression in the development of rotator cuff injury. Forty-four male Sprague-Dawley rats were divided into groups of 22. Each left shoulder received an intrinsic or extrinsic injury plus overuse (treadmill running), and each right shoulder received only overuse. Eleven rats from each group were sacrificed at 4 and 8 weeks. Supraspinatus tendons were evaluated histologically or geometrically and biomechanically. Ten rats constituted a cage-activity control group. Both supraspinatus tendons of the experimental groups had increases in cellularity and collagen disorganization and changes in cell shape compared with control tendons. Tendons with injury plus overuse exhibited a worse histologic grade than those with overuse alone. The cross-sectional area of both supraspinatus tendons of the experimental rats was significantly more than in control tendons. The area of the injury plus overuse tendons was increased on average compared with overuse-alone tendons. Biomechanically, the tissue moduli of overuse/intrinsic injury tendons at 4 weeks and those of the overuse/extrinsic injury tendons at 8 weeks were significantly lower than in control tendons. Tissue moduli of the overuse/injury tendons were significantly lower than in the overuse-alone tendons at 8 weeks. This study demonstrated that damage to the supraspinatus tendon can be caused by overuse and intrinsic injury, overuse and extrinsic compression, and overuse alone.

Rotator cuff defect healing: a biomechanical and histologic analysis in an animal model.

Rotator cuff tears are one of the most common causes of pain and disability in the upper extremity. With the use of an animal model, we studied the healing response of a controlled defect in the normal supraspinatus tendon and in a tendon with a reduced intrinsic healing capacity. In 36 Sprague-Dawley rats, defects (2 mm x 2 mm) were created in the supraspinatus tendons bilaterally. To model a tendon with an intrinsically reduced capacity to heal, the tissue adjacent to the defect area in the left shoulder was treated with in situ freezing. The contralateral tendon was not frozen. After 3 (n = 12), 6 (n = 12), and 12 (n = 12) weeks, animals were killed and underwent histologic (n = 4 from each group) and biomechanical (n = 8 from each group) evaluation. An additional group of untreated animals served as a normal control group. On histologic evaluation 78% of tendons had persistent defects (defined as incomplete closure of the defect site). Over time, the tissue from both groups demonstrated an improved histologic grade but did not reach normal levels, even at 12 weeks. No histologic differences were found between defect healing in normal tendons and in those treated with in situ freezing. On biomechanical evaluation there were also no significant differences between treatment groups. Over time, an improvement occurred in tissue properties, indicating that some healing of the defects had occurred. However, these tissue properties remained an order of magnitude lower than those of normal control tendons. These findings indicate that there is an active but inadequate repair response to the defect in the rat supraspinatus tendon, which is not significantly worsened by in situ freezing of the tissue around the defect. This model has applications toward the study of techniques to improve or accelerate cuff defect healing.