Purpose: Four external fixation pin types differing in coating, design and implantation technique were tested in an animal study.

Methods: Forty tapered pins were divided into 4 Groups according to pin design type: Group A consisted of 10 standard self-tapping pins (ø5–6mm, pitch 1.75mm), Group B 10 hydroxyapatite (HA)-coated self-tapping (ø5–6mm, pitch 1.75mm), Group C 10 standard, self-drilling, self-tapping (ø5–6mm, pitch 1.25mm) and Group D 10 HA-coated, self-drilling, self-tapping (ø5–6mm, pitch 1.25mm). Four pins were randomly implanted into the femoral diaphysis of 10 sheep. The pins were implanted at 2-cm intervals apart. Pre-drilling was used for Groups A and B but not for Groups C and D. Sheep were euthanized 6 weeks after surgery.

Results: There were no major complications. Mean pin insertion torque was 3100 ± 915 Nmm in Group A, 2808 ± 852 Nmm in Group B, 2589 ± 852 Nmm in Group C and 2180 ± 652 Nmm in Group D. Mean pin extraction torque was 1570 ± 504 Nmm in Group A, 2128 ± 1159 Nmm in Group B, 1599 ± 809 Nmm in Group C and 2200 ± 914 Nmm in Group D. Insertion torque of the coated groups was lower than insertion torque of the standard groups (p < 0.05). However, extraction torque of Groups B and D was higher than Groups A and C (p < 0.05). No differences in pin fixation were found between the two coated pin groups (Group B and D). Morphologic analysis showed extensive bone to pin contact without fibrous tissue interposition in the coated pin groups and fibrous tissue interposition in the uncoated pin groups.

Conclusion/Significance: This study demonstrated that coating pins with hydroxyapatite is effective regardless of the pin design and the implantation technique.

Aim of study: The development of tissue engineering techniques evidenced that the healing of injured ligaments require the interactions of different cell types, local cellular environment and the use of devices. In order to gain new information on the complex interactions between mesenchymal stem cells (MSCs) and biodegradable scaffold, we analysed in vitro the proliferation, vitality and phenotype of MSCs grown onto a multilayered-woven-cylindric-array of Hyaff-11A8 fiber configured as ligament scaffold.

Methods: Sheep MSCs were isolated from bone marrow aspirates and grown at two different density (7,5x106/cm and 15x106/cm) in the scaffold. At different time points (2, 4, 6 days) cellular proliferation was analysed by MTT test and cellular viability by calcein-AM immunofluorescence dye and confocal microscopy analysis. Moreover, hyaluronic acid receptor (CD44) and typical matrix ligament proteins (collagen type I, III, laminin, fibronectin, actin) were evaluated by immunohistochemistry.

Results: MSCs growth was cell density-dependent and cells were uniformly distributed inside and along the scaffold. Confocal analysis showed that MSCs completely wrap the fibers at both cell concentrations analysed and were all viable both outside and inside the scaffolds only using the lower cell concentration. Moreover, MSCs expressed CD44, collagen type I, III, laminin, fibronectin and actin.

Conclusion: These data demonstrate that MSCs well survive in a hyaluronic acid-configured ligament scaffold expressing a protein important for scaffold interaction, like CD44, and proteins responsible of the functional characteristic of the ligaments.