Screw osteosynthesis is a common method to treat fractures with small and middle sized related bone fragments in an easy and effective way. After fracture reduction, the corresponding fragments are stabilized by osteosynthesis with screws made of stainless steel or Titanium. However, the remaining metal may cause unwanted discomfort such as loosening or chronical inflammation. Therefore screw removal is required when the fracture has healed. So additional morbidity for the patient due to a second surgical intervention may be a consequence. Biodegradable Mg screws have great potential for deployment in paediatric, adolescent and also adult osteosynthesis, as they achieve reasonable mechanical properties, obviate a second surgical intervention and might be able to support the healing process. However, a huge drawback of Mg implants is their fast degradation rate, which can be retarded by adding rare-earth (RE) elements for alloying. Nevertheless, they are considered to be noxious for the human body and not suitable for a growing skeleton. The development of RE-free Mg implants resulted in first Mg-Zn-Ca alloys containing 5 wt.% Zn (ZX50), which developed huge amounts of gas inside the bone and was degrading too rapid. Mg-Zn-Ca alloys containing less Zn (ZX10) were observed, showing a suitable degradation behaviour and acceptable hydrogen gas evolution in a living rat model. The aim of this study was to evaluate the degradation and gas evolution, as well as bone incorporation and bone reaction on Zn-poor Mg-Zn-Ca (ZX00) screws in an in vivo sheep model. An additional group with surface treatment (polishing) was evaluated regarding the mentioned parameters to evaluate the influence of expected surface impurities caused by the manufacturing process.