Influence of surface morphology and internal structure on the mechanical properties and tribological response of Boa Red Tail and Python Regius snake skin.

The understanding of the tribological behavior of natural structures has been used as inspiration to design and optimize surfaces for diverse applications in engineering. In the present work, morphological, microstructural, mechanical and tribological characterization of the shed skin of two snake species, namely Boa Red Tail and Python Regius was carried out. Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) analyses showed the existence of deterministic patterns, i.e., ordered arrays of geometrical features at the surface, while Transmission Electron Microscopy (TEM) allowed studying the internal structure and chemical composition of the skin sheds. Nanoindentation measurements showed significant variations in hardness and elastic modulus from the surface to the inner layers of the skin, and pin-on-disc tests revealed anisotropic behavior of the friction coefficient (COF) as a function of the sliding direction against balsa wood in dry conditions. Correlations between the friction data, nano-indentation mechanical properties and subsurface skin structure were established for both species taking into account the ways in which the skins' deterministic patterns influence the tribological performance.

Reconstruction after resection of tumors around the knee: role of the free vascularized fibular graft.

We present our experience with reconstruction after resection of tumors around the knee, using free vascularized fibular grafting. The study included 23 patients. The lower femur was involved in 17 cases, the upper tibia in 6. The cases included giant cell tumor of the lower femur (2 patients), giant cell tumor of the upper tibia (1 patient), malignant fibrous histiocytoma of the lower femur (1 patient), parosteal osteosarcoma (1 patient), and periosteal osteosarcoma (1 patient). The remaining patients suffered from conventional osteogenic sarcomas. The size of the defect ranged from 12 to 16 cm in length. Skin flap necrosis after tumor resection was the most common complication encountered. Other complications included peroneal nerve involvement in one case and rupture of the arterial anastomosis in another. All transferred fibulas progressed to union within 7-9 months. Union time of both upper and lower ends of the fibula and time of appearance of periosteal reaction were identical. In evaluating periosteal hypertrophy of the fibula, the hypertrophy (de Boer) index (de Boer HD, Wood MB, J Bone Joint Surg 1989;71B:374-378) proved unreliable. False positive results are obtained, when callus formation around the lower end of the femur is far more abundant than at the upper end of the fibula. For this reason, we introduced the graft index. The latter is the ratio between the diameter of the graft at its thinnest portion at latest follow-up to its diameter on the day of operation, as calculated on plain radiographs. Two of the viable fibulas developed stress fractures after plate removal. Functional and quality-of -life results were satisfactory. It is concluded that the free vascularized fibular graft remains a valuable reconstruction option after the resection of tumors around the knee, provided certain precautions are followed. First, before closure of the wound, the skin flaps should be assessed for their viability. Necrotic parts should be excised. Stable fixation is a necessary prerequisite at the time of operation. Removal of the fixation device should not be guided by union or periosteal hypertrophy, but by true widening of the medullary canal.