[Spongiosa formation in plate osteosynthesis--a comparative animal experiment study of current and auto-compression plates using the Zespol principle].

Osteosynthesis with modern self-contracting plates has a few fundamental drawbacks apart from its undisputably good points. Among these drawbacks we have the pressure strain on the bone surface; the traction strain exercised by the screw thread in the bone; and, chiefly, spongiotisation and bone atrophy due to so-called stress protection. A new modification of this process, called Zespol, in which the plate is no longer fastened direct to the bone but is fastened floating over the bone surface to so-called platform screws, tries to solve these problems. In a comparative animal experiment tibial transverse osteotomies in the rabbit were treated by osteosynthesis using small fragmentary ACP and Zespol plates. In respect of healing of the fracture none of the two methods showed any advantage. Histological evaluation of the plated bone sections were characterised in both plate systems by increasing spongiotisation and bone atrophy that increased with the length of application. Degradation processes were about equal with both systems. A special feature with the Zespol animals was that the cleft originally existing between the plate and the bone was filled up with newly formed spongy and later cortical bone. Due to this process the original cortex layer lost its function and atrophied severely. After 12 to 18 weeks the bone surface had reached the lower side of the Zespol plate and there was contact by touch. This restored similar biomechanical conditions as those existing in conventional plates after this period.