Background and aim: Mesenchymal stem cells (MSCs) are multipotent cells capable of differentiation into osteogenic and chondrogenic pathways. MSCs are among the key repair cells in fracture healing and implant osseointegration. They are also an attractive tool of cell therapy in reconstruction procedures of bone. Minipigs are a large-animal model recommended for preclinical studies of orthopaedic bone implants. Minipigs are claimed to have bone physiology close to humans, but their MSC characteristics are poorly defined. The aim of this study was to isolate and characterize minipig bone marrow and peripheral blood derived MSCs in comparison of human MSCs.

Methods: Five male minipigs (weight 36.2 ± 2.2 kg) were subjected for experimental femoral osteotomy, which was fixed with either compression plate or intra-medullary nailing. Before surgery, bone marrow (BM) sample (2–4 ml) was aspirated from the posterior iliac crest and a peripheral blood (PB) sample (20 ml) was also collected. Mononuclear cells (MNC) were isolated by Ficoll gradient centrifugation. MSCs were cultured and selected by plastic adherence. Cell morphology was evaluated during the whole culture period and proliferation capacity was examined by determining the number of population doublings (PDs) at the end of each passage. Osteoblastic differentiation capacity was investigated by culturing MSCs in the presence of beta-glycerophosphate, dexamethasone and ascorbic acid. The lineage phenotype was studied by alkaline phosphatase and von Kossa staining.

Results: MNC were successfully isolated from all BM and PB samples. Plastic adherent cells obtained fibroblast-like morphology and proliferated over time in culture. The maximum PDs were 3.4 ± 0.7 and 4.3 ± 0.5 for BM- and PB-derived cells, respectively. The maximum PD capacity of PB-derived cells was significantly higher than that of BM-derived cells (p=0.027). However, when cultured in osteoblastic induction medium, only BM-derived cells were capable of differentiating into alkaline phosphatase positive osteoblasts with an occasional presence of von Kossa-stained mineralized bone nodules. The maximum PDs of minipig BM-derived MSCs were similar to those of human BM-MSCs isolated from young adult fracture patients.

Conclusion: We successfully isolated plastic adherent MSCs from minipig bone marrow samples, which proliferated and differentiated into cells of osteoblastic lineage. BM-derived porcine MSCs had similar morphology to human MSCs. There were marked inter-individual variations in the proliferation and differentiation capacity of minipig MSCs, resembling the observations in humans. No circulating MSCs could be detected in minipigs before surgery and this confirmed our previous observation in humans.

Introduction: In postmenopausal female patients with hip osteoarthritis, osteoporosis as well as altered local trabecular bone architecture may lead to a increased migration of uncemented hip prostheses.^1,2 The aim of this study was to determine whether 3D bone architecture and mechanical properties of intertrochanteric cancellous bone in the proximal femur predict RSA migration of uncemented femoral stems.

Materials and Methods: The study population consisted of 61 female patients with primary hip osteoarthritis. Informed consent was obtained prior to any study-related procedures. The Ethical Committee approved the study protocol.

All patients underwent a total hip replacement (ABG II, Stryker) with uncemented proximally hydroxyapatite-coated femoral stem with tantalum RSA markers. Ceramicceramic bearing surfaces were used. A uniplanar type of RSA setup was applied. The RSA examinations were performed postoperatively and at 3, 6, and 12 months.

During surgery, cancellous bone biopsy was taken from the proximal femur from the site of stem implantation. The specimens were scanned with micro-CT. 3D analysis of trabecular bone geometry and BMD was performed using CTAn software. After micro-CT imaging, the trochanteric cancellous bone specimens were subjected to a destructive compression test. Maximum force and stiffness were calculated. Linear regression analysis was applied to study correlations between different parameters investigated.

Results: The patients exhibited major differences in the density and structural quality of intertrochanteric cancellous bone. Significant correlations were found between the measured biomechanical parameters and the structural parameters calculated from micro-CT data.

Unexpectedly, the quality of intertrochanteric cancellous bone did not predict RSA migration of the femoral stems. The femoral stems reached high stability within 3 months and there were no significant differences in the axial and rotational migration of the femoral stems between the patients with normal or poor quality of the intertrochanteric cancellous bone.

Discussion: The 3D bone architecture, mineral density and mechanical properties of the local intertrochanteric cancellous bone do not seem to serve as predictors of femoral stem migration. The observation suggests that the significance of cancellous bone quality for the initial stability of uncemented femoral stems has been over-emphasized.