Long-term effects of plate osteosynthesis: Comparison of four different plates

Various phenomena have been observed subsequent to plate osteosynthesis, fo r example, refracture after plate removal. Experimental research has shown that changes in the cortex occur within the first three months after platin g and again several months later. These changes are independent of the frac ture and take the form of porosis under the plate and excessive bone growth around the plate. Porosis under the plate was regarded until recently as b eing due to unloading of the bane by the plate, also known as stress shield ing. Investigations of the relationship between bone porosis and the change s in periosteal blood supply due to its compression by the plate, however, have been neglected. In this study, the effect of plate properties such as structural stiffness ('unloading'), implant material, and plate contact surface (altered periost eal blood supply) on bone after osteosynthesis were investigated. This was done by comparative histomorphometry of the altered bone in sheep after app lication of four plates differing in the above-mentioned properties. After plating the sheep tibia with a trapezoid plate with narrow contact su rface, significantly larger bone cross sections were observed one year afte r the operation and considerable bone growth around the plate. The area of early temporary porosis in the cortex under the plate as observed in the fi rst nine weeks and after one year was not significantly different for any p articular plate, all of which were applied subperiosteally. The trapezoid p lates were easier to remove, thus causing less damage to the bone lamellae along the sides of the plate. The marked increase in bone cross section after one year and the larger are as of bone growth around the trapezoid plates with smaller contact surface can be attributed to the larger grooves cut along the sides of the plates. Cortical porosis was mainly the result of impaired periosteal blood supply which was of equal size in all groups as a result of careful periosteal str ipping and subperiosteal plate insertion. It was assumed that applying the plate onto the periosteum would be associated with effects on periosteal bl ood supply directly related to the plate and consequently cortical porosis. Plate related stress shielding and the implant material had no significant effect on the extent of cortical porosis.